Spal t
srt

FOR THE PEOPLE
FOR EDVCATION

FOR SCIENCE

LIBRARY

OF

THE AMERICAN MUSEUM
OF

NATURAL HISTORY

GEOLOGY OF MONGOLIA

I ALVId

PLATE I.

THE FLAMING CLIFFS OF DJADOKHTA.
(Painted by C. Lester Morgan after field sketch in water color by Frederick K. Morris.)

CENTRAL ASIATIC EXPEDITIONS
ROY CHAPMAN ANDREWS, Leader

PeEOrOGY OF MONGOLIA

A RECONNAISSANCE REPORT
BASED ON THE INVESTIGATIONS OF THE YEARS 1922-1923

BY
CHAREES” P.”BERKEY

CHIEF GEOLOGIST

PROFESSOR OF GEOLOGY, COLUMBIA UNIVERSITY

FREDERICK kK. MORRIS
GEOLOGIST

AMERICAN MUSEUM OF NATURAL HISTORY

With 44 “Plates and 161 Illustrations in the ext
Six eMaps in Pocket at End

NATURAL HISTORY OF CENTRAL ASIA
VOE. ik

THE AMERICAN MUSEUM OF NATURAL HISTORY
HENRY FAIRFIELD OSBORN, ‘President
NEW YORK

1927

Copyright, 1927
by
The American Museum of Natural History

G. P. Putnam’s Sons, Publishers
New York and London.

Made in the United States of America

PREFACE

THE series of volumes of which this one forms a part is intended to cover
a wide range of investigations conducted by expeditions into Central Asia
under the scientific supervision of the American Museum of Natural History.
The Third Asiatic Expedition of 1922-23 was one of these, operating in Mon-
golia under the leadership of Roy Chapman Andrews, and the volume now
presented grew out of the observations made at that time. The succeeding
volumes will include investigations in all of the branches of science repre-
sented, and will cover the results of many years of field exploration and re-
lated study by different groups of men. The whole effort is directed to a
better scientific understanding of the continent of Asia, especially its central
and less readily accessible portions. Field observations and the collecting
of material have already occupied several years, and the exploratory side of
the work is still in progress. Doubtless it will take a much longer time to
describe and interpret the collections and field notes.

This volume is devoted entirely to geology. It is confined to reconnais-
sance studies, especially the traverses of the Third Asiatic Expedition in the
season, 1922, when both geologists were in the field together. The results
were used in 1923 when the junior author had additional opportunity to
revisit certain districts and see new ground. Since the manuscript of this
volume was first written, the Expedition of 1925 gave further opportunity
to both geologists to carry the investigations forward, but the results are not
included in this volume. The reconnaissance observations of 1925, together
with a series of special studies, are to form the next volume.

The traverses furnishing the material for this volume amount to about
5000 miles of travel in central Mongolia. Observations are distributed from
the vicinity of Kalgan on the southeast, to Urga on the north, and, on the
west, to Sain Noin in the Khangai Mountains and Baga Bogdo of the Altai
Ranges. The area has been crossed and recrossed with little duplication of
trails; nevertheless, there are wide spaces untouched and large areas whose
geologic details are quite undetermined. The general geologic structure,
however, has proved to be readable; the major units have been recognized so
many times and at so many different places that it is believed the general

iv;

vi PREFACE

geologic features and history are now reasonably well established. The area
to which these studies apply amounts to not less than 200,000 square miles.
It is an enormous area to cover in so short a time, and if it were not for a
certain simplicity of geologic history the results could not be presented with
any confidence. Doubtless an immense amount of detail will be added as
the work proceeds, but it is believed that no large item in the geologic state-
ment is likely to be greatly changed.

This reconnaissance is put forward as a step in the unraveling of the
geology of Central Asia. We recognize its incompleteness and its many
unsolved problems, yet we are confident that it is based upon true lines with
respect to structure and historic succession, and that it ought to serve a use-
ful purpose as a background for additional exploratory investigations.

Many problems have presented themselves as the work has proceeded.
It is hoped that some of them may be given further study. Data already
in hand bear on problems that have not been discussed in this report but
are expected to yield results on laboratory investigation. There are yet
other problems which cannot be solved except by additional field work of a
detailed and extended nature. In so far as future work makes it possible,
we hope to present these in succeeding volumes as opportunity may arise.

The four parts of this volume exhibit very different range and have dif-
ferent purposes.

Part I is essentially an introduction to the field and methods of
work.

Part II covers the traverses and records observations along the route.

Part III includes a series of locality studies where more careful and
detailed work could be done.

Part IV is an attempt to summarize in geologic terms the meaning
of the mass of data assembled from these studies.

Readers not primarily concerned about the geological record will find cer-
tain portions of more general interest. To them we recommend the introduc-
tions under each Part as well as certain chapters in each Part, as illustrative
of the nature of the investigations. Chapters I, II], X11, XIV, XXI and
XXII present a series of pictures to the general reader, and we commend
them as of broad enough interest to be of service to the layman.

We are indebted to many organizations and many more individuals for
helpful suggestions and for assistance in assembling and presenting this work.
Due recognition is given some of them in the body of the report.

To the generous friends of scientific exploration who have contributed
the funds that have made this work possible we owe a special debt of grati-

PREFACE vil

tude. Without their support neither the field investigations nor the sub-
sequent research could have been carried out. The sponsors have also borne
the expense of publication; thus enabling the Museum to offer the volume
at cost. Their interest in pure science and their faith in this venture have
helped to materialize visions that otherwise must have vanished as they
came, as many others before have vanished.

It is fitting that special mention should be made in this place of the helpful
coéperation of the Geological Survey of China, and that we should acknow-
ledge the interest of individual members of the staff, particularly Dr. V. K.
Ting, former Director, and Dr. Wenhow Wong, the present Director. To
_Dr. A. W. Grabau, Chief Stratigrapher and Palzontologist, we have turned
repeatedly for helpful suggestions. Dr. Grabau has undertaken the identi-
fication of the invertebrate fossils collected by the Expedition and will pub-
lish his results in a subsequent volume of this series.

In the preparation of this material for publication, still more vital assist-
ance has been rendered by members of the staff of the American Museum of
Natural History, whose close connection with the work of the Expedition has
given special acquaintance with and insight into its scientific problems.

It is a pleasure to express special obligation to President Henry Fairfield
Osborn whose prophetic vision is being fulfilled and whose guiding hand in
these Asiatic explorations has been felt from the beginning.

We are deeply indebted to Acting Director George H. Sherwood on whose
shoulders has fallen the home administration.

To Dr. W. D. Matthew, whose wide knowledge of paleontology and
Cenozoic history make him an authority, we have referred many questions,
particularly as to the correlation of a number of the Tertiary formations.

The heavy labor of editorial work has been borne by Dr. Chester A.
Reeds, assisted by Miss C. M. Beale and Mrs. Mary V. Forster. It is an espe-
cial pleasure to record our gratitude to Dr. Reeds, whose constructive sug-
gestions have been exceedingly helpful. Dr. E. W. Gudger has aided in the
preparation of the bibliography.

Among the institutions whose services should be acknowledged is Colum-
bia University, from whose faculty ranks the chief geologist came, on an as-
signment to field investigation. This service was rendered at a sacrifice made
by the staff of the Department of Geology, since an absence of this kind
thrust additional responsibility on every remaining member. Special recog-
nition should be accorded to Professors James F. Kemp, Roy J. Colony, and
Douglas W. Johnson, who together shared the chief burden left to the Depart-
ment. Professor Johnson has read portions of the manuscript and has given
generously the benefit of his keen and scholarly criticism. Contributions to
such a piece of work, therefore, are made by many whose actual participation

vill PREFACE

is not apparent, and it would have been much more difficult to carry it to
completion without their help.

Professor V. A. Obruchev, of the Academy of Sciences at Moscow, has
kindly contributed a list of references to Russian geological publications which
proved a most helpful guide in reviewing the literature on Central Asia.
Professor Obruchev has also aided in the preparation of the map showing the
routes of Russian explorers (Plate II).

To Mrs. Florence Eddowes Morris, Mrs. Flora Cook Parks, Miss Helen
R. Fairbanks, and Miss Agnes Molloy, we are indebted for valuable help on
the manuscript. The illustrations are the work of many hands, some of
which are acknowledged in the text. Messrs. Andrews, Granger, and Shack-
elford have furnished some of the photographs. Professor George B. Bar-
bour, of Yenching University, Peking, contributed the photographs appearing
as Plates VIII A, XLI A and B. Mr. Edward J. Alexander drafted the geo-
logic cross-sections of the first ten chapters, chiefly from the field notebooks
of the senior geologist; Mr. Erwin J. Raisz drafted the base maps, and Mr.
Leon B. Hills made a number of the diagrams.

A place of highest honor for contributory service must be reserved for
the members of the field staff of the Expedition with whom we worked day
after day amid the privations and perplexities of strange and uncharted re-
gions. With these men we endured the hardships that months of isolation
always bring. With them we worked in surroundings of seemingly boundless
desolation where only the unaccountable enthusiasm of the explorer, or the
zeal of the scientific investigator, or the lure of the confirmed wanderer could
have made the drudgery of those days appear to be worth what it cost. With
their codperation and encouragement the secrets of the desert were slowly
searched out. And, now that we can look back on those toilsome days that
stretched out almost endlessly month after month, with never a glimmer from
the outside world, and with self-appointed tasks that taxed endurance to the
last dregs, all seems like an entrancing dream that slowly takes on the glamour
of a golden age.

These men we came to know intimately and, as our problems were dis-
cussed together, we learned to rely on them for daily help and encouragement.
Sympathetic and helpful companions they were through all the confusions and
discouragements that in the nature of the case belong to the borders of the
unknown. Their service cannot be measured. We can only say that we
appreciate it all, and realize full well that except for their fine spirit of codper-
ation the results must have been very different indeed.

To the leader of the Expedition, Dr. Roy Chapman Andrews, whose
humanness and sanity under disappointments and successes alike helped to
stabilize what might have become an ineffective effort, is due a kind of credit

PREFACE ix

unlike that of any other. Except for his genius the Expedition would never
have been formed; except for his success in interesting public support and
inspiring faith in the undertaking, the work could never have been kept going,
and, except for his confident leadership and good fortune in the face of moun-
tains of difficulty, calamity might easily have overtaken the venture.

To Walter Granger, Chief Palzeontologist, whose patient quiet manner
and scientific competence inspired general confidence, we owe another spe-
cial acknowledgment. The work of the geologist and of the paleontologist
interlock and overlap. Both use the same data. Some of the records that
both search for in the field are to be found in the same strata, and the history
that both strive to unravel is closely interwoven. Their stories supplement
each other. Their discoveries are often made together or are made one for
the other and must be freely contributed to the general good. Therefore,
it is vital to the success of both that the fullest coéperation and the frankest
relations should be maintained. With such a man as Granger the task is
easy.

To J. B. Shackelford, the photographer, whose artistic work speaks for
itself, we owe an equal debt for other qualities. His ingenuity and practical
knowledge in lines quite beyond his own special field made him the general
advisor of the camp, and his genial nature and wonderful gift of humor never
failed.

During the field season of 1925, Major L. B. Roberts, assisted by Lieu-
tenant F. B. Butler and Lieutenant H. O. Robinson, made a series of -route
maps from Kalgan to Orok Nor, about 865 miles. These maps will be pub-
lished in a later volume of this series, but the altitudes, which were deter-
mined by means of a large Gurley transit, have been used to correct the ane-
roid readings of the earlier traverses covered in the present volume. The out-
line of Tsagan Nor (Plates XX1X and XXX) has been corrected also to agree
with Major Roberts’ special map of that lake.

Each member of the whole staff has his own special claim to recognition
and acknowledgment. In a kind of work where the chauffeur of one day
may become a discoverer the next, or where a camp helper of one season
may develop into an efficient scientific aid the next, and where competence
and reliability in any single task make for so much greater efficiency in the
whole organization, it is not possible to single out every individual for his full
share of credit. Sometimes even our lives were in the hands of our com-
panions, and that may have been true more times than we knew. If Bayard
Colgate, in charge of transportation, had been less efficient or less accommo-
dating; if Merin, the leader of the caravan, had been incompetent or untrue;
if the supporting staff had failed in any major service, the whole story of
accomplishment must have been very different.

x PREFACE

So we recall with gratitude and appreciation the services of F. A. Lar-
sen, the interpreter, and of the native Mongol guides, Sirimpil, Bato, Aioshi,
and Tserin; of Merin’s caravaneers, Banjien, Bato, Okher, Otoburun, and
Sanjarav; of Colgate’s transportation staff, Wang Hung Ping and Ah Sah;
of the taxidermists, Chi Shou Lun and Hsia Wen Chiang; of Granger’s staff
of collectors, George Olsen, Peter C. Kaisen, and Albert F. Johnson; and
those men whose constancy in their daily service about the camp made our
own work possible, Loh and his staff, Chang Kwei Mo, Hwei Hsiu Yen and
Kan Ch’uen Pao.

There have been scores of others, of lesser influence only because of fewer
opportunities, but whose interest has been keen and whose sympathetic sup-
port and encouragement have been a spur to such accomplishment as is rep-
resented. We do not forget the importance of their service although it is not
practicable to enter here all of the names that might properly claim a place.

To them all we offer this tribute of appreciation. That one of our num-
ber deserves most credit who executed his own task the best and thereby
helped to make the whole interdependent organization so much the more
efficient. Such results as have been attained in any field are only in part the
simple product of the individuals directly responsible; in much larger part
than is readily indicated they are a joint accomplishment. This is true also
of the two authors of this volume. The interpretations here presented have
grown up under continuous unreserved codperative contribution of observa-
tion, interpretative understanding and experience. Certain parts of the
volume took form originally as individual contributions, but all have been
subjected to drastic revision by both authors. The work has grown in our
hands so gradually and under such intimate discussion, and has been modi-
fied so many times that we ourselves are not able to separate clearly the ele-
ments that belong to each. The actual labor of preparation has fallen most
heavily on the junior author; in the field, the larger experience of the senior
author may have counted more heavily in coérdinating and interpreting obser-
vations; but the result as presented is a joint product on which we have agreed,
both as to form and substance.

CHARLES P. BERKEY,
FREDERICK K. Morris,
Geologists.
New York Clty,
October 25, 1926.

CONTENTS

PREFACE . : : : : i : : A C = 2 :
List oF PLATES 2 , : a = . : r - A A
List oF FIGURES : é : : , ri : A ; :
PART I.—GENERAL INTRODUCTION ‘ 5 3

CHAPTER
I.—GEOLOGICAL OBJECTIVES AND METHODS OF THE EXPEDITION WITH A
REVIEW OF FORMER EXPLORATIONS . : - . : 5

INTRODUCTION

THE APPROACH TO MONGOLIA

THE GOBI REGION

PREHISTORIC ADVENTURE

HISTORIC TRAVEL AND MODERN EXPLORATION

EARLIER STUDIES OF SPECIAL SIGNIFICANCE é * . .

II.—BOUNDARIES OF THE GOBI REGION

INTRODUCTION

THE NORTHERN BOUNDARY
Northern boundary—western section
Northern boundary—central section
Northern boundary—eastern section .
Summary of the Transbaikal border

THE EASTERN BOUNDARY
Summary of the eastern boundary . 5 : x :

THE SOUTHERN BOUNDARY . : E : 3 :
Southern boundary—eastern section . ; ¢ - 5
Southern boundary—central section . : : c
Southern boundary—western section . : ¢ 3 ¢

THE WESTERN BOUNDARY 3 ; 3 , : = é 3

xii CONTENTS

CHAPTER PAGE
PART II.—ROUTE STUDIES OR ITINERARY . - A 3 ee kei
INTRODUCTION . ; ; : : : : , ; : AQ
THE TASK OF A GEOLOGICAL RECONNAISSANCE EXPEDITION ; = - 39
RECORDING OBSERVATIONS. ‘ : ; : : : ea
IIL.—FROM KALGAN TO IREN DABASU . ; : : 5 c - : 45
KALGAN TO WAN Cu’UAN Pass é : : 5 : : Br ls

The lower pass : : : : ; - - . eat

Wan Ch’uan basin and the upper pass : 6 é : AS

THE PACIFIC DIVIDE TO P’ANG KIANG : : 5 ; 5 = 49

The granite hills of Chakhar . : : 3 : : ay BAS

The P’ang Kiang hollow . : : : F ‘ ‘ ies

P’aANG KIANG TO IREN DABASU , : : 5 5 e 21055

The first fossils : : j é A 5 - 4 - 59

IV.—FROM IREN DABASU TO URGA : 3 5 : ° A . 4 60
IREN DABASU TO CAMP JURASSIC . : ; 5 - - - 60

The structural relations at Camp Jurassic . ; : : Gy,

Camp Jurassic TO Mount TUERIN ; : : < é = go

The granites of Mount Tuerin . ; ? : ‘ : Syed

Mount TuUERIN TO BOLKUK GoL .. . ‘ ; : : = a7 6

The Arctic divide . : , F ‘ : ‘ : 5 Ge
OBSERVATIONS IN THE VICINITY OF CAMP BOLKUK GOL. : ar 83

Course of the Bolkuk Gol , : 2 : : : .. 85

Glacial evidence in the Gangin Daba : : : ‘ 5 4245)

Groundice . : 5 : : : : ; : Se roy

Local culture . : ; j d : : ; 5 5 he

Side traverse toward Urga ; : é ; - ; 5 teks)

Mineral resources . F 4 : A s ‘ - 5 ute)

V.—FROM URGA TO TSETSENWAN 4 f “ A 5 > : 5 go
TRAVERSE ALONG THE TOLA RIVER ‘ : c ‘ ‘ - 90

THE VICINITY OF FIVE ANTELOPE CAMP . 3 : 2 6 3 £93)

FROM THE TOLA RIVER TO TSETSENWAN ‘ c - , = | 204

THE VICINITY OF TSETSENWAN : é ; : . : 1 97

A side traverse north of Tsetsenwan . ‘ 5 é ‘ - 98

CONTENTS
CHAPTER
The serpent-form dikes : ; 7 5 é
Contact effects of the granite margin . 5 4
Jurassic structural relations . : ; 5 :

A side traverse south of Tsetsenwan . 5 : ‘

VI.—FROM TSETSENWAN TO SAIN NOIN AND THE ARCTIC DIVIDE .

WESTWARD FROM TSETSENWAN

THE VICINITY OF CAMP CANYON BROOK .

From Camp CANYON BROOK TO THE ONGIN GOL
Rock-bound hollows
Continuation of the traverse . ; : : :
The Ongin Gol

FROM THE ONGIN GOL TO SAIN NOIN
The vicinity of Sain Noin

From Rainy GULCH TO THE ARcTIC DIVIDE . : 5

A step across the divide . : ¢

VII.—FROM THE HOT SPRINGS OF SAIN NOIN TO MOUNT USKUK
ARISHAN TO GORIDA
The Gorida basin
The old Uliassutai trail . : 3 F 5
OLD ULIASSUTAI TRAIL TO Mount Uskuk
The saltpan of Guchu Burt
ONDAI SAIR

VIII.—THE RETURN JOURNEY FROM TSAGAN NOR TO ARTSA BOGDO
CHOOSING A ROUTE
TsSAGAN NoR TO THE VOLCANIC CLIFFS
Desert hollows ; :
VoLcANIc CLirrs TO ARTSA BOGDO
NORTH MARGIN OF ARTSA BoGbDo . : : ;

Picture writings ‘ ‘ : ; : : 4

IX.—FROM ARTSA BOGDO TO SAIR USU - . 6
ArtsA Bocpo To DJADOKHTA i :
The flaming cliffs of Djadokhta :

xiii
PAGE

98
102
103

105

xiv CONTENTS
CHAPTER
DJADOKHTA TO ONGIN GOL IN SUMU
Ongin Gol in Sumu .
ONGIN GOL TO Sarr Usu A 2

X.—FROM SAIR USU TO KALGAN .
PALAOZOIC STRATA e 5

CONTINUATION OF TRAVERSE TO ARDYN OBO

Sediments of Ardyn Obo .

ArpyN Oso To SHARA MuruN
Paleozoic strata of Jisu Honguer
Tertiary sediments of Shara Murun

SHARA MURUN TO KALGAN
Chinese settlements .

The return to Kalgan

PART III.—SPECIAL AND LOCALITY STUDIES

INTRODUCTION
Method of Mapping

XI.—IREN DABASU AND IRDIN MANHA .
INTRODUCTION

GENERAL ASPECT AND LARGER RELATIONS OF THE BASIN

Origin of the hollow at Iren Dabasu .

GENERAL ROCK STRUCTURE
The Iren Dabasu formation
Variegated Tertiary beds .

The Houldjin formation

The Arshanto and Irdin Manha formations

Shara Murun .

AGE AND CORRELATION .

XII—ARISHAN, THE SACRED MOUNTAIN OF SAIN NOIN

INTRODUCTION
FEATURES OF THE LOCALITY
GEOLOGICAL FORMATIONS

Graywacke series

CONTENTS

CHAPTER

The granite . j : : 5 5
The Jurassic formation . : - :
Special products ‘ ‘ , c -
STRUCTURAL FEATURES . : : ; :
THE SPRING WATERS AND THEIR ORIGIN . :

XIII.—MOUNT USKUK AND THE TSAGAN NOR BASIN

PRINCIPAL STRUCTURAL FEATURES i ; -
THE ROCK FORMATIONS : : : : 5
ROCKS OF THE ANCIENT FLOOR : - 5
The crystalline metamorphics . : 9

The graywacke series § f : 6

Bathylithic granite

Jurassic strata : ‘ é : -
Post-Jurassic intrusives . : ; :
ROCKS OF THE SEDIMENTARY COVER A é
Cretaceous strata. : : :
Fossil content . : : s 5
Tertiary strata : 5 : - ‘

Fossil content . j : : a
Former extent of later sediments

Evidence of the faulted margins

Evidence of the sedimentary remnants .

Evidence of the basalt remnants

Evidence of planation .
EROSIONAL HISTORY OF THE SOUTH BLOCK OF USKUK
DEFORMATION OF THE Mount USKUK REGION .
THE SALTPAN OF GUCHU BURT

TsAGAN Nor 5 ‘ ‘ 5 5 .

XIV.—THE GURBUN SAIKHAN RANGES

THE JOURNEY F : é 4 : 4
A desert well . é : , : :
The return journey . ‘ : é 5

GEOLOGICAL OBSERVATIONS AT ARTSA BOGDO .
The south basin P , ; s =

xvi CONTENTS
CHAPTER
APPROACH TO THE GURBUN SAIKHAN
THE FORMATIONS OF THE GURBUN SAIKHAN
Ancient rocks .
Limestones
Serpentine .
Graywacke-slate series : : ;

Topographic character of the Gurbun Saikhan

XV.—ARTSA BOGDO AND OSHIH
THE ArtsA Bocpo MounTAIN BLOCK
The marginal shelf
History of the shelf
Rock formations of the mountain
Ancient meta-crystallines and intrusives
Folded sediments and associated eruptives
Later igneous rocks
Tectonic features
Traces of former sedimentary cover
Topographic features
Stream pattern
Glaciation .
THE OsHIH HOLLOW
Features of the locality

The geologic column

XVI.—PROBLEMS AND AREAS DESERVING SPECIAL STUDY
INTRODUCTION

SEDIMENTARY BASINS AND POTENTIAL FOSSIL FIELDS .

The area of the great pass
P’ang Kiang ;
The paper-shales of mile 299
The flaming cliffs of Uskuk
Oshih (Ashile) basin
Djadokhta

Ardyn Obo

Shara Murun . -

CONTENTS
CHAPTER

Ulan Nor F 3 : : - 5
The badlands of the Ongin Gol . :

STRUCTURAL AND HISTORICAL PROBLEMS .
The Camp Jurassic area : :
The crystalline upland east of the Ongin Gol
The transition country of Gangin Daba

Sair Usu
Losin Sumu . ; 0 ; : .
Jisu Honguer . ‘ ‘ : a 5
GENERAL PROBLEMS. A : 5
PART IV.—SUMMARIES AND DISCUSSIONS . 3
INTRODUCTION

XVII.—STRUCTURAL ELEMENTS OF THE OLDROCK FLOOR
Two MAJOR DIVISIONS
SUBDIVISION OF THE OLDROCK FLOOR
Mesozoic intrusives . ; : : 2
Jurassic sediments
Paleozoic strata
The great Mongolian bathylith .
The Khangai graywacke series .

THE ANCIENT CRYSTALLINE COMPLEX j .
The Wu T’ai system
The T’ai Shan complex . : : 5

XVIII.—SURFACE FEATURES AND THEIR ORIGIN
BasINs AND MouNTAINS

THE BASIN OF MONGOLIA
Deserts in general
The shaping processes

MAJOR SUBDIVISIONS OF THE MONGOLIAN BASIN
Talas of the eastern and southern province
The western faulted talas .

Mountains : 3 : :
Fault-block mountains of the Altai type

XViii CONTENTS

CHAPTER

Ikhe Bogdo : - - “ P ° “ .
Baga Bogdo : : - ° : - 5 :
Artsa Bogdo ‘ ‘ : c = A 5
Gurbun Saikhan - - ~ - :
The mountains of the eastern province . 5 -
Volcanic mountains . , : 5 : -
THE STRUCTURE LINES OF MONGOLIA . ‘ - - ;
XIX.—SURFACE FEATURES AND THEIR ORIGIN (continued) . .
PLATEAUS, PLAINS AND FLATLANDS : ; é : . 5 :
INTRODUCTION—THE LEVEL LANDS OF THE DESERT
AGENCIES WHICH CARVE THE EROSION PLANE . é ;
Erosion by wind F , é c : Spats
Rainfall . ; ; : : ‘ A 0 : 5 -
Drainage : ; ; : ; : s :

Immigrant streams
Native streams
Erosive work of running water .
THE PIEDMONT SLOPE
THE GOBI EROSION PLANE
Original slopes of the erosion plane
Deformation of the Gobi surface
Deposits on the Gobi upland
Redissection of the Gobi erosion plane
Summary of the Gobi erosion plane
DESERT HOLLOWS
THE OLDER PENEPLANES
The pre-Cretaceous peneplane
The Mongolian peneplane
The Khangai peneplane
Relations of the Khangai and Mongolian peneplanes
Relations of the Mongolian peneplane and the Gobi erosion plane
Relations between the Gobi erosion plane and the lowlands of the
P’ang Kiang stage
COMPARISON OF PENEPLANES WITH THOSE OF SURROUNDING REGIONS .
LAKES

HUMMOCKS AND DUNES .

PAGE
310
3II
313
314
317
318
319

323
323
323
324
325
326
327
327
328
329
330
332
333
333
333
335
336
336
341
341
342
343
343
345

347
347
348
350

CONTENTS
CHAPTER
XX.—STRATIGRAPHY OF THE LATER SEDIMENTS
THE LATE MESOZOIC ERA

CRETACEOUS DEPOSITS

The Ondai Sair formation

The Oshih (Ashile) formation

The Iren Dabasu formation

The Wan Ch’uan formation
UPPER (?) CRETACEOUS DEPOSITS

The Djadokhta formation
COMMENTS ON CRETACEOUS SEDIMENTS

THE CENOZOIC ERA

PALEOCENE PERIOD

The Gashato formation
EOCENE PERIOD

The Arshanto formation

The Tukhum formation

The Irdin Manha formation

The Shara Murun formation
OLIGOCENE PERIOD :

The Ardyn Obo formation

The Houldjin formation

The Hsanda Gol formation
MIOCENE PERIOD .

The Loh formation .
PLIOCENE PERIOD . :

The Hung Kureh formation

SUMMARY OF THE CRETACEOUS AND TERTIARY DEPOSITS

XXI.—SUCCESSIVE CLIMATES OF MONGOLIA
INTRODUCTION .
PAST CLIMATES OF MONGOLIA
LATE PRE-CAMBRIAN OR SINIAN PERIOD
PALEOZOIC ERA
MESOZOIC ERA

Lower Jurassic period

xx CONTENTS

CHAPTER

Cretaceous period . : : - S
Oshih and Ondai Sair time . “ 5

Tren Dabasu time P : - :
Djadokhta time . : ~ 3
CENOZOIC ERA. ‘ - - - -

Eocene period .
Gashato time
Arshanto time
Irdin Manha time
Shara Murun time
Oligocene period
Ardyn Obo time .
Houldjin time
Hsanda Gol time
Miocene period
Pliocene period
Hung Kureh time
Ertemte time
Pleistocene period
RECENT CHANGES OF CLIMATE
EVIDENCE BEARING ON CLIMATIC CHANGE
Dominant denudation
Double dissection ‘ ; : :
The choked gorge of Wan Ch’uan Hsien Pass
Desert trees
The forest margin
Tree growth at Tiger Canyon
Ancient stream courses
Terraced alluvial fans
Minor scarps in erosion hollows
Lakes :
Ancient lake beaches of Tsagan Nor .
Glaciation
A vanished race
An abandoned dam
MEANING OF THE EVIDENCE
PRESENT CLIMATE

PAGE

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378

379

379
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393
393
395
395
395

CONTENTS
CHAPTER
XXII.—SUMMARY OF GEOLOGIC HISTORY .
ANCIENT ERAS CHARACTERIZED BY METAMORPHISM. F :
The T’ai Shan complex . : ; : F s :
The Wu T’ai system
The Khangai series .
The granite bathylith : 5
PALAOZOIC MARINE HISTORY

Mip-MESOZOIC SEDIMENTATION INITIATING THE CONTINENTAL HISTORY
OF CENTRAL ASIA

DEVELOPMENT OF BASIN STRUCTURE WITH COVERING OF LATER SEDI-
MENTS . .

A SEDIMENTATION SUMMARY
DEVELOPMENT OF VOLCANISM
Deep-seated magmas
Serpent-form dikes .
The brittle porphyries : : : ; :
Basaltic and other lavas and pyroclastics
DEFORMATION HISTORY .
Epeirogenic movements
Orogenic disturbances
Warping : : : : : :
Faulting : ; : : ;
Metamorphic deformation : : 3
Grouping of deformation products
Epochs of erosion
Disconformities
ORIGIN OF PRESENT TOPOGRAPHY . : : . ‘ ‘
RELATION OF GEOLOGIC HISTORY TO THE DEVELOPMENT OF MAN .

BIBLIOGRAPHY . é . : .
INDEX . . . . . . . . . : .

Xxi
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397
398
398
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400

402

403
495
408
408
409
409
410
410
410
411
411
412
413
413
414
416
417
418

421

449

AT

ab viyye

PLATES

FACING

PLATE PAGE
I.—The Flaming Cliffs of Djadokhta (colored) ‘ ; . Frontispiece
II.—Map of the routes of Russian geologists in Central Asia (colored) . 6
III.—Map of the routes of non-Russian geologists in Central Asia (colored) . 17
IV.—Location map of the borders of Mongolia . : ; ; : 5 oP
V.—(A) The first mountain barrier Bt : : : : ‘ Be ee
(B) The Great Wall above Nan K’ou Pass P F : : ees
VI.—Conventional symbols used in the geologic sections. : j me 45
VII.—Conventional symbols used in the geologic sections (continued) . 46
VIII.—(A) View from head of the Wan Ch’uan Pass. : f ‘ 45
(B) Cultivated fields of Chinese farmers . : : : ; t. 748
IX.—(A) The upland as seen from the second camp . ‘ : 3 5 lg)
(B) Camp in the granite hills of Chakhar . é , : : ; ) 349
X.—(A) Dissected dinosaur beds at Iren Dabasu : 3 5 :) 60
(B) Residuary granite bowlders : F é : : : 5 (eo)
XI.—(A) Updragged sandstone beds at Camp Jurassic : : é PeeoL
(B) Granite residuals on the slopes of Mount Tuerin . : : = OI
XI1.—(A) Peneplaned granite floor along the Urga trail : : j 1K)
(B) Residuals of the granite bathylith, west of Tsetsenwan 3 SLO
XIII.—(A) The aggraded trench at Canyon Brook : ; : c Bo, a
(B) Rock terrace at Rainy Gulch. : : : ' ; = Te
XIV.—(A) The Arctic Divide in the Khangai Mountains . : : 830

(B) Sain Noin Khan and the Khangai range . : : : 5 8%)

Xxiii

XxXiv PLATES

PLATE

XV.—Mongol lama and his small daughter at Sain Noin
XVI.—Mongol herdsman and his son at Sain Noin

XVII.—(A) Block disintegration in the granite at Arishan
(B) The salt pan of Guchu Burt

XVIII.—(A) The sand dune belt of Tsagan Nor
(B) The lava-capped mesa at Oshih .

XIX.—Wind-scoured cliffs at Djadokhta

XX.—(A) The great temple and lamasery, Ongin Gol in Sumu
(B) A nest of dinosaur eggs at Djadokhta

XXI.—(A) The scarp of Oligocene beds at Ardyn Obo .
(B) Structural detail of the scarp at Ardyn Obo

XXII.—(A) The beveled slate floor of Bilgoho
(B) Badlands of the Shara Murun formation at Ula Usu

XXIII.—Geologic map of the Iren Dabasu area (colored) .
XXIV.—The Sacred Mountain of Arishan (colored)

XXV.—(A) Guardians of the Sacred Mountain
(B) Xenolith of graywacke capping a granite hill

XXVI.—Geologic map of the Arishan area (colored)
XXVII.—Geologic map of the Mount Uskuk area (colored)
XXVIII.—Geologic map of the Hsanda Gol area (colored) .
XXIX.—Geologic map of part of the Tsagan Nor basin (colored)
XXX.—Geologic map of the Hung Kureh area (colored)

XXXI.—(A) The Hsanda Gol -
(B) The southern margin of the south Uskuk block
XXXITI.—(A) The Uskuk block from Dzun Hsir ‘
(B) Sedimentary remnant in the south Uskuk block .

XXXIII.—(A) The flaming badlands south of Mount Uskuk
(B) Dissected sand deposits in Tsagan Nor basin

FACING
PAGE

183
In pocket
213

218
218

In pocket
In pocket
In pocket
In pocket
In pocket
230
230
231
231
248
248

XXXIV.—(A) The intermont basin between Artsa Bogdo and the Gurbun Saikhan 249

(B) Typical morning scene at a desert well

249

PLATES

PLATE

XXXV.—(A) A fault zone in the Oshih basin .
(B) The badlands at Oshih Nuru

XXXVI.—(A) The tilted Lower Cretaceous strata at Oshih Nuru
(B) Cannonball Canyon .
XXXVII.—Residuary stack of jaspery silica at Oshih

XXXVIII.—(A) Beveled surface of upturned phyllites at Gorida
(B) Shattered graywacke near Jisu Honguer

XXXIX.—(A) Deformed jasperite in the graywacke series
(B) The phyllites of Artsa Bogdo

XL.—(A) An outpost forest at the Arctic Divide
(B) The terraced alluvial fan at Tiger Canyon .

XLI.—(A) Two stages of dissection at Wan Ch’uan
(B) The choked gorge at Wan Ch’uan Pass

XLII.—(A) Shrinkage cracks in the floor of a playa near Tsagan Nor

(B) The abandoned beaches of Tsagan Nor

XLIII.—(A) A pre-Mongol grave . : : : 3 ,
(B) Gap through the Great Wall at Wan Ch’uan Pass

XLIV.—An ancient monolith

XXV

FACING
PAGE

268
268

269
269
272
302
302
303
303
388
388
389
389
396
396
397
397
418

FIGURES

FIGURE

1.—Generalized geologic section from Tientsin to Wan Chu’an Pass

2.—Two cross profiles of the Gobi basin

3.—Profile and section illustrating methods of recording observations

4.—A general location and route map
5.-Geologic section, Miles 0-30, Wan Ch’uan Pass
6.—Geologic section, Miles 30-60, Miao T’an
7.—Geologic section, Miles 60-90, Ertemte
8.—Geologic section, Miles 90-120, Chap Ser
9.—Geologic section, Miles 120-150, the Chakhar Hills
10.—Geologic section, Miles 150-180, P’ang Kiang
11.—The P’ang Kiang basin
12.—Geologic section, Miles 180-210 i
13.—Geologic section, Miles 210-240, Irdin Manha
14.—Geologic section, Miles 240-260, Iren Dabasu
15.—Geologic section, Miles 260-290
16.—Geologic section, Miles 290-320
17.—Geologic section, Miles 320-350, Ude
18.—Geologic section, Miles 350-380, Camp Jurassic
19.—Geologic section, Miles 380-410
20.—Geologic section, Miles 410-440
21.—Geologic section, Miles 440-470
22.—Geologic section, Miles 470-500
23.—Geologic section, Miles 500-530, Mount Tuerin
24.—Geologic section, Miles 530-560
25.—Geologic section, Miles 560-590
26.—Geologic section, Miles 590-620 F :
27.—Geologic section, Miles 620-650, Continental Divide

XXVii

.

XXVili FIGURES

FIGURE

28.—Geologic section, Miles 650-670, Bolkuk Gol
29.—Glacial cirque, Bolkuk Gol . “ 4 < - ;
30.—Bowl-shaped valley heads in the Gangin Daba Mountains
31.—Geologic section, Miles 668-688, traverse to Urga .
32.—Geologic section, Miles 670-700, to the Tola River
33-—Geologic section, Miles 700-730, Tola River Valley
34.—Geologic section, Miles 730-760, Tola River Camp
35.—Geologic section, Miles 760-790, Five Antelope Camp
36A.—The fault-block at Tsetsenwan .

B.—The fault-block at Tsetsenwan (continued)
37.—Geologic section, Miles 790-820, Tsetsenwan
38.—Geologic section, Miles 0-20, north of Tsetsenwan
39.—Margin of a roof-pendant
40.—Geologic section, Miles 0-30, south of Tsetsenwan
41.—Geologic section, Miles 30-45, south of Tsetsenwan
42.—Route map, Miles 820-850 .
43.—Geologic section, Miles 820-850 ieee
44.—Geologic section, Miles o-8.3, north of Canyon Brook
45.—Rock drawings near Canyon Brook
46.—Route map, Miles 850-880 . ; :
47.—Geologic section, Miles 850-880, Canyon Brook
48.—Route map, Miles 880-910 . ‘
49.—Geologic section, Miles 880-910, oan Gol .
50.—Route map, Miles 910-940 .
51.—Geologic section, Miles 910-940, popes Gulch
52.—Sketch map of Sain Noin
53-—Route map, Miles 940-961 .
54.—Geologic section, Miles 940-961, Forest ee
55.—Route map, Miles 960-990 . :
56.—Geologic section, Miles 961-990, Arishan
57-—Geologic section, Miles 990-1020, Gorida
58.—Route map, Miles 990-1020
59.—The ragged mountains of Gorida
60.—Route map, Miles 1020-1051
61.—Geologic section, Miles 1020-1051, Uskuk

PAGE

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83

86

89

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95

95

98

99
IOI
IOI
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107
107
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121
121
123
123
126
129
129
133
134
134
137
138
140
140

FIGURES
FIGURE
62.—Geologic section, Miles 0-30, Tsagan Nor
63.—Geologic section, Miles 30-60, Volcanic Cliffs
64.—Geologic section, Miles 60-90, Artsa Bogdo .
65.—Geologic section, Miles o-30, Gun Usu
66.—Geologic section, Miles 30-60, Dubshih
67.—Geologic section, Miles 60-90, Djadokhta
68.—The Khurul Obo : j
69.—Geologic section, Miles 90-120, Ulan Nor
70.—Geologic section, Miles 120-150, Ongin Gol .
71.—Geologic section, Miles 150-180
72.—Geologic section, Miles 180-210
73.—The margin of a sediment basin at Los in Sumu
74.—Geologic section, Miles 210-240, Sair Usu Trail

75.—Geologic section, Miles 240-270, Porphyry Camp .

76.—Geologic section, Miles 270-300
77.—Geologic section, Miles 300-330, Sair Usu
78.—Geologic section, Miles 330-360, Paleozoic inlier
79.—Geologic section, Miles 360-390
80.—Geologic section, Miles 390-420
81.—Geologic section, Miles 420-450

82.—Geologic section, Miles 450-480 :
83.—Geologic section, Miles 480-510, joie Obo .
84.—Map of Ardyn Obo
85.—Dissection of the scarp at pee Obo
86.—The geologic column of Ardyn Obo
87.—Geologic section, Miles 510-540, Taliu in Sumu
88.—Geologic section, Miles 540-570, ‘‘Choppy Sea”’
89.—Geologic section, Miles 570-600, Jisu Honguer
90.—Geologic section, Miles 600-630, Shara Murun
91.—Geologic section, Miles 630-660, Boltai Urtu
92.—Geologic section, Miles 660-690, Shara Khata
93.—Geologic section, Miles 690-720

94.—Geologic section, Miles 720-750, Chinese cultivation

95.—Geologic section, Miles 750-780
96.—Geologic section, Miles 780-810

Xxix
PAGE
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145
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159
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176
176
177
179
179
180
180
185
185
186
186
188
188

XXX FIGURES
FIGURE
97-—Geologic section, Miles 810-827 . : : 5 - 2 -
98.—Location map of the eastern Altai region : 3 2
99.—Sketch map of the region between Iren Dabasu, Shara ue and P’ang Kiang
100.—Block diagram of the area between Iren Dabasu, Shara Murun, and P’ang Kiang
101.—The scarp at Irdin Manha
102.—Generalized geologic section between Iren Dabasu and Irdin Manha
103.—The stratigraphic section at Iren Dabasu
104.—The scarp at Holostai
105.—Location map of Arishan
106A.—Field sketch of the Baga Bogdo front
B.—Field sketch of the Baga Bogdo front (continued)
107.—Generalized geologic section across the Tsagan Nor basin ;
108.—Domed and dissected planation surface of the Mount Uskuk area .
109.—Field sketch of Baron Shiliuta
110.—Field sketch of the eastern margin of the Mount Uskuk block
111.—Gobi plantation surface beveling tilted strata ;
112.— Generalized geologic section from Artsa Bogdo to the Gurbun Saikhan
113.—Generalized geologic section of the Gurbun Saikhan
114.—Generalized structure section of the piedmont shelf at Artsa Bogdo
115.—Generalized structure section of the eastern end of Artsa Bogdo
116.—Analytical profile of the upland of Artsa Bogdo
117A.—The upland meadows of Artsa Bogdo
B.—The upland meadows of Artsa Bogdo (continued)
118.—The mesa and stripped inliers of the Oshih basin .

119.—Two fault zones in the Oshih basin. , : 5 - A“
120.—Location map of proposed studies : é : c - :
121.—Section of the Flaming Badlands of Uskuk .

122.—Dome at Camp Jurassic . ‘ ; ‘ - F ; ‘

123.—Typical margin of a sediment basin <

124.—Geology of the oldrock floor along the route b 5 - : :
125.—Typical structure section of the oldrock floor

126.—Geologic section north of Tsetsenwan .

127.—Geologic section of an area of marine Permian beds

128.—Typical geologic section of the Khangai series

129.—Typical geologic section of the pre-Cambrian complex .

PAGE

189
194
198
199
200
201
203
209
214
224
225
229
239
240
242
245
253
257
261
263
264
266
267
269
270
275
277
281
289
290
291
293
295
297
299

FIGURES

FIGURE
130.—Summary table of the formations of the oldrock floor .
131.—Sketch map of Mongolia ; ,
132.—A generalized block diagram of a faulted basin
133.—Four stages of sedimentation at Gurbun Saikhan
134.—View of Oshigo Ola from the south
135.—Tectonic map of Transbaikalia (after Obruchev) .
136.—General structure lines of Asia
137.—The Gobi upland at Ardyn Obo .
138.—The Gobi upland at Irdin Manha
139.—The piedmont slopes of Los Ola
140.—Diagram showing origin of residuary gravels
141.—The P’ang Kiang lowland .
142.—The short gullies at Ardyn Obo .
143.—Re-dissection of a smooth scarp at Shara Murun .
144.—Early stage in the development of a reversed scarp
145.—Late stage in the development of a reversed scarp
146.—Reversed scarps at Shara Murun F
147.—The Mongolian upland in the Chakhar Mountains
148.—The Tola River terraces
149A.—The Arctic Divide . :

B.—The Arctic Divide (continued) .
150A and B.—Alternative correlations of the older erosion surfaces
151A and B.—Alternative explanations of the Mongolian peneplane
152.—The zolian crossbedding at Djadokhta

153.—Successive stages in the development of a gobi basin _

154 and 155.—Two diagrams to illustrate the relation of warping to ee in the

formation of basins
156.—Columnar diagram of the later sediments

157.-—Glacial map of Eurasia

158.—Field sketch of a bowl-shaped ee head in the on Daba Mountains

159.—Drawings cut upon a basalt block at Artsa Bogdo

160.—Sketch map of central Mongolia, showing geologic units along the traverses

161.—Generalized columnar section of the oldrock floor

XXXi
PAGE
301
304
310
316
318
320
321
324
324
330
334
337
338
339
339
340
340
342
342
344
345
345
346
356
367

369
370
383
384
394
404
415

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4

VOL. II—I

A GEOLOGICAL RECONNAISSANCE
OF THE

GOBI DESERT REGION OF CENTRAL ASIA

PART I
GENERAL INTRODUCTION

PART I—GENERAL INTRODUCTION

CHAPTER I—GEOLOGICAL OBJECTIVES AND METHODS OF THE EXPEDITION, WITH A REVIEW
OF FORMER EXPLORATIONS

CHAPTER II—BOUNDARIES OF THE GOBI REGION

CHAPTER I

GEOLOGICAL OBJECTIVES AND METHODS OF THE EXPEDITION,

WITH A REVIEW OF FORMER EXPLORATIONS

INTRODUCTION

THIS volume is confined to the geological reconnaissance of the seasons of
1922 and 1923. In it are recorded observations made in the course of an
extended itinerary across the desert regions of Mongolia. The major traverses
include the following:

I.

From Kalgan to Urga, a distance of six hundred and seventy
miles, through Wan Ch’uan Hsien Pass over the Pacific divide,
across the Gobi basin to the Arctic slopes of the Tola River
region.

From Urga southwestward along the Tola River, through the
Tsetsenwan district across the Ongin Gol, a distance of four
hundred miles, to Sain Noin Khan on the Arctic divide.

From Sain Noin southwestward and southward three hundred
miles to Tsagan Nor at the base of Baga Bogdo, one of the Altai
mountain ranges.

From Tsagan Nor in the basin at the foot of Baga Bogdo, a hun-
dred miles and more eastward to the Artsa Bogdo and Gurbun
Saikhan ranges.

>
From the Gurbun Saikhan northward across the desert to the
Sair Usu Trail, returning on that trail six hundred miles to Kal-
gan.

These traverses, together with side trips from certain base camps, gave
an itinerary of more than three thousand miles—most of it in desert or semi-
desert country, part of it in the mountainous country of the Arctic divide,
and a much smaller part in the isolated mountain groups of the easternmost

3

4 GEOLOGY OF MONGOLIA

ranges of the Altai mountain system. Our traverses reached the center of
the great continent of Asia, which in all probability has been inhabited by man
longer than any other continent. Across its immense areas of desert and
steppe country, caravans have journeyed from the time commercial inter-
course developed between peoples, and travelers have toiled over these
stretches of sand in the Gobi ever since man began to wonder about the earth
and its possibilities. Here are the oldest trails of the world. Some of them
are primitive transcontinental highways made by the tread of countless bare
and padded feet, and dating back, perhaps, to the time of the great racial
migrations.

Notwithstanding such a history, surprisingly little had been learned
about the content and the meaning of the underground of the Gobi region.
But as students of science continued to gather data of other regions of the
earth, especially those facts bearing on early man and the animal life that
preceded him, the conviction grew that Asia must have seen the evolution
of man, and may have seen as much of many other races of living things.
The race of man may have originated here; there was, however, no basis for
a more authoritative statement, since no one had yet found much evidence in
Asia itself, especially in central Asia.

It is surely not accident that has made the continent of Asia so impor-
tant a factor in the development of man. Both man and beast have been
more directly dependent on the corresponding growth and behavior of the
earth than has been generally assumed. Geologic environment,—the whole
physical setting of the past while this great continent was in the making,—
may have been a more important factor in producing interrelated floras,
faunas, races, and cultures of the present day than is fully appreciated.

Geologic history and the history of life run side by side, and have been
intertwined and interlocked from the beginning; yet they are not equally
dependent one on the other, for life is always subject to the geologic setting,
and to some degree is compelled to adjust itself to the endless changes that
the surface of the earth presents. Too seldom have geology and evolution
been studied together as complementary parts of the same story. Thus
these geological explorations in Asia take on a special significance, for the
unraveling of whatever story there is to be told should give a better under-
standing of the long eras through which life progress has been made.

Some such considerations as these were in the minds of the sponsors and
of the scientific staff of the Third Asiatic Expedition, and served to give
direction and character to their undertaking. The general itinerary was
chosen because the region which it covered appeared to have been touched by
geological exploration less than any other of similar extent in central Asia,
but it was modified in accordance with the problems that arose as the work

GEOLOGICAL OBJECTIVES AND METHODS 5

progressed. The major traverse was projected across the Gobi basin with the
conviction that somewhere in this basin country a series of sediments ought
to have been preserved, which could contribute important facts to a better
understanding of the geologic evolution of Asia, and of the development of
present life forms.

The Expedition hoped to establish a better foundation for the study of
all forms of life in late geologic time. It was, therefore, in large part, an expe-
dition devoted to the unraveling of geologic and palzontologic history as re-
corded in the rocks of the desert regions of central Asia. i

It had been predicted by Professor Henry Fairfield Osborn, many years
before, that central Asia would one day yield important paleontologic contri-
butions and perhaps connecting links in the chain of ancient forms of life, and
that it would prove to have been a great center of origin for forms found in
North America and Europe. It was not the province of the Expedition, how-
ever, to predict or to promise, but rather to investigate and to interpret.

Travelers crossing the desert seem to have found little support for these
more or less speculative predictions, and the general reputation of the Gobi
region was far from encouraging to such an undertaking as the Third Asiatic
Expedition. It was evident, however, from the observations of such men
as von Richthofen (1877) in the region farther south, and Obruchev (1900)
in the Gobi proper, that there were sediments of somewhat uncertain age
which ought to fulfil the requirements; but it took courage to project so
expensive an expedition into a region apparently so desolate and un-
promising as the Gobi. The region, to be sure, has been crossed by many
other explorers, but with less advantage in operation and apparently with
less success in detecting the elements of its complicated structure. In part
this is because of the many natural difficulties that face the would-be inves-
tigator in such surroundings. Slow, tiresome travel; privation and occasional
danger; heat, cold, and ceaseless winds; drifting sands and almost trackless
wastes; scanty food and uncertain water-supply,—all these combine to make
more complicated the task of an investigation whose uncertainties and puzzles
are only one part of the practical problem.

It is quite outside our present purpose, and surely not necessary, to give
a descriptive account of this now well-known Expedition. Its scientific staff
was prepared to make every exertion to prove the quality of the enormous
expanses of desert country in central Asia. The members were resolved to
do as much as was humanly possible, on a hurried reconnaissance, toward
interpreting the physiographic features and the geologic structure, the fossil
content and the prospective paleontology of the region, and to determine, if
possible, the significance of the processes represented in the rocks made in
former times. They set out to discover and to read the story of the fossil

6 GEOLOGY OF MONGOLIA

remains that might lie buried in the strata. Whether any such finds could
be made no one knew, but whatever contribution the country had to offer
must be found. Geographer, paleontologist, geologist—together all bent
their energy to one purpose. Each turned his best effort to unraveling that
part of the story which was written in the language of his own science, from
the present back to the beginning.

The major purpose was to see whether the Gobi region had fossil fields
of late geologic time, to find how promising they might be, and to determine
the structural and stratigraphic history of the plateau and basin country cov-
ered by Mongolia. In any case, whether the major object proved to be attain-
able or not, the staff of the Expedition was prepared to make a line of scien-
tific determination across the northern desert, from China on the one side to
Siberia on the other, so that it could be used as a standard for future reference
and a guide to additional exploratory work. At least it should serve as a
starting point for studies in other parts of the region.

As a step toward this end, a continuous cross-section, representing the
geological changes and structure, was carried from the beginning to the end
of the traverse. A good deal of ground was covered, of course, before much
system could be discovered in the accumulating mass of data. In the course
of time, however, a fair working basis was evolved, and certain localities were
found to be of sufficient importance to deserve special and more extended
study. At those places local, detailed investigations were made on a scale
commensurate with their apparent geologic and palzeontologic importance.
These have become the key studies of the Expedition, and are supported by
maps and large collections, both of rocks and of fossils.

In this manner there was constructed about three thousand miles of
cross-section, part of it with accompanying route maps, and five special
areas were given intensive study as local problems. Areal and structural
reconnaissance maps were made, covering more than seven hundred square
miles. At the same time, the physiographic features and processes of the
desert were observed and studied, so that those features constitute an addi-
tional contribution. The maps of the region were entirely inadequate, and,
therefore, attention was given to their correction, and to the more accurate
location of important physical features or points of special interest.

The major results, in addition to the details of observation, include the
following items:

1. Several great fossil fields of reptiles and mammals have been located.

2. The general geologic structure of the major rock formations of the
Gobi basin region has been determined.

3. The major elements of geologic history of the Mongolian region have
been unraveled, and a fairly complete geologic column has been worked out.

GEOLOGICAL OBJECTIVES AND METHODS 7

4. Extensive collections, both of fossils and of rocks, have been made,
some of which are among the rarest and most valuable ever found in any
region.

5. The principal steps in the physiographic development of central Mon-
golia have been detected in the features of the region.

6. A chain of scientific determinations has been welded between China
on the one side and Siberia on the other.

7. The major problems deserving additional investigation have been
indicated, and steps toward their solution have been suggested.

8. A great field for subsequent research has been opened in central Asia,
which is certain to attract increasing attention in many other related sciences.

g. A series of special key studies has been made, to serve as starting
points for future work.

In preliminary summary, it is, therefore, possible to say that, instead of
being barren and hopeless, the Gobi Desert has proved to be one of the most
productive geologic and paleontologic regions in the world. This volume
records the observations and conclusions covering some of these results.

THE APPROACH TO MONGOLIA

The Expedition outfitted in Peking, and traveled inland by rail as far
as Kalgan before taking to its own means of transportation. From that point
motors were used for the scientific party, its immediate equipment and a few
supplies. A caravan of seventy-five camels had been sent into the interior
about two months in advance, with additional supplies sufficient for the whole
season. After making connections with this caravan at Mount Tuerin, five
hundred miles out, the movements of the two sections were codrdinated in
such a way as to keep the supply caravan within reach of the motor section.

This system of transportation and supply proved to be adequate during
most of the season, and only an occasional side traverse or piece of local work
had to depend on a different method. Rarely were supplies short. The sev-
eral junction points were reached by the whole Expedition, and the move-
ments of both sections were so well timed that no delay was caused by failure
to codrdinate.

Since it is not the purpose of this report to describe the incidents of the
trip, or to cover the several other fields of investigation carried on at the same
time, but instead to emphasize the geological observations, little further at-
tention will be given in this volume to that side of the Expedition’s operations.

The itinerary account will be carried without special reference to the
method of travel, the incidents of camp life in the desert, or other happenings
of the Expedition. These have been given in considerable detail by the

8 GEOLOGY OF MONGOLIA

leader of the Expedition, Roy Chapman Andrews, in semi-popular articles
contributed to Asia Magazine and other publications (1922 to 1925).

Although the scientific work of the Expedition began beyond the outer
walls of Kalgan, a better appreciation of the features first encountered there
can be gathered from a statement of the geology along the route from Peking
to Kalgan.

Peking stands not far from the inner border of the great alluvial plain
of China formed by deposits carried by rivers that have come down from the
mountainous interior of the continent. These alluvial deposits cover a com-
plex erosion floor of older geologic formations (Fig. 1, page 8). The surface
of the deposits is a smooth plain composed of gravel, sand, and silt, rising
almost imperceptibly from the sea to a very abrupt and rugged inner margin
which stands as a mountain wall nearly three thousand feet above the plain
(Plate V, A, page 23). The Nan K’ou Pass pierces this barrier, and through
it winds the old trail—northern China’s principal gateway to the interior—
over which seven centuries ago the Mongol hordes came on a very different
mission. On a modern railroad that parallels the ancient trail to Kalgan, the
Expedition set out to reach the heart of the country of the Mongols, which
has become again a center of world interest, not because of anything the
Mongols have done, but because of the remarkable geologic history of their
vast desert region.

oe
ors

SI

Archean "Proterozoic Granite Cambro- Lower Tertiary
Gneiss Limestone Ordovician Jurassic Cretaceous Lava

Ficurp 1.—Generalized geologic section from Tientsin to the Wan Ch’uan Hsien Pass. The section is
intended to emphasize the step-like approach to the Gobi region from the great plain of northern China. The
major underground geologic structures and the relations of the principal rock formations are indicated. They
fall conveniently into three groups: (a) a simple delta, made of alluvial sands and clays built out into and dis-
placing the ancient Yellow Sea; (b) a basement of complex rocks emerging from beneath the delta at Nan K’ou;
(c) a cover of Mesozoic and Tertiary gravels, sands and clays with associated basaltic lava flows, beginning at
Wan Ch’uan Hsien Pass,

In the Nan K’ou Pass, through which one must go to reach the higher
lands of the interior, the older sedimentary rock formations come to the sur-
face and are plainly exposed in all their complicated structure in the walls
of the gorge. On entering the gorge, simple sedimentary strata are encoun-
tered, dipping southeastward beneath the plain. Then more complex for-
mations appear in the core of the range. They are cut by large intrusions of
granite, which continue to the upper reaches of the Pass. The geology of

GEOLOGICAL OBJECTIVES AND METHODS 9

Nan K’ou Pass is comparatively well known, and it is eminently fitting to
turn to it as a key to help unlock the secrets of the ground that lies beyond.
Many times in the course of the work the features of this pass were recalled,
and the interpretations accepted for them were read into many of the rock
structures of the interior.

On reaching the top of the Pass, however, one is surprised to see another
plain, not very different in appearance from the one at the foot of the Pass,
but this second plain stands at a much greater elevation, for it is not far below
the crest of the mountain range (Fig. 3, page 42). The higher plain looks very
similar to the lower one at Peking, but it lies between mountain ranges, is
far smaller, and has a more variable floor and a much thinner cover of sedi-
mentary strata. We may travel inland on this second level for many miles,
but in due course we come again to mountain barriers that stand across the
way, and if the interior is to be reached we must climb through other passes
to yet higher levels. .

Kalgan lies in a valley at the foot of these passes. It is located on the
alluvial deposits of the Yang Ho, a stream whose variable flow cuts and fills
and shifts enough at times to endanger the very foundations of the city. Occa-
sional floods cause great damage and loss of life, for the inhabitants push their
dwellings and tiny farms to the very edge of the river, on ground which may
have been flooded only a year or two earlier. The trail to the famous Wan
Ch’uan Hsien Pass climbs a tributary of this valley to the Pacific divide, and
by way of it we may reach the edge of the plateau where Mongolia begins.

THE GOBI REGION

The Desert of Gobi occupies a broad, shallow, basin-like depression in the
very extensive plateau of central Asia. From every side it is approached
over a mountainous rim, and the trail stretches away for one thousand miles
and more over desert and semi-desert country (Fig. 2). The monotony is
broken by relief features which are chiefly the result of warping and uplift.
Some of these features, like the Altai ranges, are of mountainous quality, for
each one is a complex uplifted fault-block. Many of the ranges are rugged,
and comparatively difficult to traverse. The less prominent hill-masses mark
areas of gentler warping and smaller vertical displacement. Some of them are
older fault-blocks that have been more completely worn down. Elsewhere
the region stretches out in a succession of plains, marking the location of minor
enclosed basins where down-warped areas have caught the sediments washed
down from the uplifted segments (Fig. 10, page 53).

In spite of these irregularities, the average relief of the Gobi basin 'as a
whole is remarkably slight. The basin measures six hundred miles in width,
and more than one thousand miles in length, yet it is marked by an average

10 GEOLOGY OF MONGOLIA

depression of less than six feet to the mile—an amount not appreciable by ordi-
nary methods of observation. For all essential purposes, then, the region is a
great sweeping upland country, so many hundred miles in extent in every
direction that one feels hopelessly lost in it.

! KALGAN
hart IDE. ee |DABAsU [RIANG PACIFIC DIVIDE y |
50 100 200 300 = 400 500 600 MILES
" P PROFILE A
AD is H bre He BB ————————
PROFILE B
—_—0 oso

FIGURE 2.—Two cross profiles of the Gobi basin. Both profiles are based on hundreds of systemati-
cally distributed aneroid readings. The vertical scale is exaggerated ten times.

Profile A crosses the whole of the eastern Gobi and shows a broad, shallow downwarp between
the Arctic and Pacific divides.

Profile B crosses only the northwestern extension of the Gobi, lying north of the Altais, and shows
much stronger warping and block-faulting than does the eastern section. Index letters are as follows:
AD, Arctic Divide; S N, Sain Noin Khan; G, Gorida; U, Mount Uskuk; TN, Tsagan Nor; BB,
Baga Bogdo.

In very early geologic time, the region had an entirely different physical
habit from the present. Epochs of mountain-folding, of volcanism, and of
great elevation and subsidence, with all their complexities of product and
structure, have characterized the geologic history. In the beginning the region
was mountainous and rugged, with rivers flowing in all directions to the sur-
rounding seas (Fig. 153, page 367). After a very long period of erosion, the
ancient continent was worn down almost to base-level, and the traces of this
old peneplan are what one sees in some of the dissected remnants of the old
rock floor. Since that time mountain-folding has not occurred there; but,
under the influence of some of the inner forces of the earth, the whole region
has been lifted and warped and faulted, so that the streams, which formerly
carried sediments to the sea, turned inland and made deposits in the bottoms of
the inland basins.

Ever since Lower Cretaceous time, when these changes came about, the
region has been continental in habit. Centers of deposition shifted as new
warpings took place, but deposits were nearly always being made somewhere,
while in places deposits were being stripped from areas previously covered.
The complete story of the later sediments of the Gobi region could be com-
piled by interpreting the data of all these basins. Bones of animals that
roamed these plains from Cretaceous time to the present must be buried here
and there in the deposits of the Gobi basins, and the story of the ancient life
of the region, with its changes, could be written if their remains were found.
Many epochs are unrecorded, so far as we know, and the continuity of the
story is broken; but some of the gaps may be filled by future exploration.

GEOLOGICAL OBJECTIVES AND METHODS II

If it were not for the fact that the story is more complex than the part of
it already given, the complete history could not be unraveled. If the pro-
cesses had stopped when the basins were filled with sediment, the history of
the Gobi could not be worked out, because only the surface of the basin could
be studied. However, since deposition began, there have been periods of
deformation so pronounced in certain places that great series of sediments
have been turned on edge and exposed by later erosion (Plate XI, A, page 61).
Here one can measure each bed in detail and examine its content as success-
fully as though he had gone down through the whole series from top to bottom.
In addition to this, there were, in former times, periods of greater rainfall than
now, and streams have cut trenches into deposits already made. There are
gorges and gulches, escarpments and uplands, where strata are exposed, in
some places throughout considerable thickness, even down into the ancient
rock floor itself (Fig. 11, page 54). At such places the meaning of the strata
is legible, and ultimately a geologic column, covering nearly the whole depo-
sitional history, may be constructed by piecing together the data afforded by
the various localities. A trial column, based chiefly upon the work of this
Expedition, has been assembled, and will appear in Chapter XXII.

These are the obvious features of the Gobi region. The geologist soon
learns their significance, so that the character of the country suggests the
nature of the underground structure and something of its geologic possibil-
ities. There is an alternating succession of broad, nearly level, smooth, grav-
elly plains which mark the presence of sediments, separated by more rugged
areas of the uplifted and dissected hard-rock floor. Occasionally a character-
istic dissection of the sediments themselves actually opens to observation the
edges of the strata, and these alone are favorable for fossil collecting (Plate I).
They are the great fossil fields of Mongolia.

Caravan trails reach out bewilderingly across this apparently endless
country, intersected in a confusion of possible courses by the trails of herds
and flocks and wild animals. Across such an open plains country, where
tireless winds sweep the shifting sands, which are the only mantle obscuring
the rock floor beneath, one must find his way and determine the geological
structure, unravel the history of the region, and locate the fossil fields. Each
day’s traverse looks so much like the last in general surface features and land-
marks that the traveler feels hopelessly lost; yet every trail leads somewhere,
for these highways of the desert have served for centuries as the only connect-
ing threads between otherwise isolated centers of culture in central Asia.

A Mongol guide is immensely more useful than any existing map. Save
for his instinct in finding the right trail out of hundreds of possible courses,
or, failing that, except for the use of the compass almost as the mariner uses
it, one would inevitably come to grief in the Desert of Gobi.

12 GEOLOGY OF MONGOLIA
PREHISTORIC ADVENTURE

One of the amusing foibles of a dominant race is its belief that explora-
tion begins when men of its own blood travel into a ‘‘new land’’—disregard-
ing the peoples who have made it their home for untold centuries. So Mon-
golia, which has been a land of varying climate, favorable enough at times to
attract races, and then arid enough to drive them out again, seems to have
been forever under exploration. Before men lived in Mongolia, race after
race of other creatures surged through the land and died in its broad basins,
where we are now deciphering their history.

The tale of early human exploration will make a fascinating chapter
when increasing knowledge enables us to piece it together. As we tell it
now, the story is incomplete. Yet it seems advisable to fit the fragmentary
evidence together as best we can, even though it will be many times corrected
and added to as knowledge increases. We believe that future studies will con-
firm the inference that ever since there were men in Mongolia, their tale has
been one of tribal migrations. As in Europe, clan after clan has entered this
region and driven out or absorbed the earlier inhabitants. As their numbers
grew, streams of nomad wayfarers went out to other lands. Each invasion
brought new blood into Mongolia, and the new conquerors were in turn lured
forth by rumors of better lands or were absorbed by the next immigrant
horde, just as fate, climatic changes, and the laws of inheritance ruled.

History does not begin with the written page. Records are also made by
bones, chipped flints, and ash heaps of vanished settlements, and are as much
a part of human history as the Bible and the Rosetta Stone.

The earliest record is a single tooth obtained from a Chinese apothecary
by Haberer (1903, page 20), and described by Dr. Max Schlosser, who reported
on his suggestive discovery in 1903. It is not wholly certain that the tooth be-
longed to a human being; Dr. Schlosser claims only that it belonged to an an-
thropoid which was structurally nearer to man than any known ape. Clinging
in the roots of this tooth is a bit of red clay. These are the only facts, and
speculation upon them should be made with caution. No one knows where
the tooth was found, or from what geologic horizon it came, but its primitive
character suggests that it belonged to one of the earliest inhabitants of cen-
tral Asia. We can only hope that future discoveries will throw more light
upon this interesting problem.

The second chapter in the history of human adventure in Asia lies quite
outside our region. In 1892, Professor Dubois (1894) discovered in Java
the remains of a primitive human being, or, as some scientists believe, a crea-
ture rather more primitive than man, to which he gave the name Pithecan-
thropus erectus. The age of the beds in which the bones were found is generally

GEOLOGICAL OBJECTIVES AND METHODS 13

considered to be early Pleistocene. No one knows how widely this early
Javanese roamed, but it seems unlikely that the same species lived in both
Java and central Asia.

The next cultural stages in the history of the region are much more defi-
nite and understandable. They are represented by a Paleolithic culture dis-
covered in the Ordos by the distinguished explorers Pére Emil Licent (1925)
and Pére Teilhard de Chardin (1924 0), and one Paleolithic and two Neo-
lithic stages found in the Gobi by our own Expedition. Licent and Teil-
hard found primitive stone implements, not unlike those used by the Nean-
derthal man of Europe, and bones of extinct animals in gravels which were
laid down before the vast deposits of ‘‘loess’’ were formed in northern China
—probably in the Middle or Lower Pleistocene. Of the three stages found
by the Third Asiatic Expedition, the palzoliths are apparently much like
those of the Mousterian culture in Europe, but similar implements need not
imply the same or closely related races or even the same period. These stud-
ies are as yet so incomplete that all we dare to say is that several stone-age
civilizations existed in Mongolia long ago, and it is reasonable to think that
they represent many successive invasions of the land by tribes which brought
much of their culture with them.

Dr. J. G. Andersson (1923 b, c, and 1925) described an ancient Eneo-
lithic culture in China that antedates all written and traditional records of
the Chinese. Excellent pottery, ornaments and implements were made, but
no metals were found in any of the old dwelling-sites. The material includes
vases, plates and ornaments, with patterns so closely related to those of later
Chinese art that he believes that the ancient culture was indeed Chinese.
Dr. Davidson Black has examined the human bones which Andersson col-
lected, and concludes that they belong to a people essentially similar to the
present-day North Chinese (1925 b, page 98). Therefore the Chinese people
probably had occupied China and had developed the elements of their civil-
ization at least several thousand years before the beginning of their written
history. They may have been invaders, too, but not even in their legends
do we find the myth of an original homeland.

In many parts of Mongolia, especially in the north, there are stone mon-
uments of many different types, built by races which were in the land before
the coming of the Mongols. The Encyclopedia Britannica, quoting Radlov,
says that the earliest inhabitants of this northern region were Yeniseians;
remnants of these tribes still linger in the mountain-country of Uriankhai.
They were succeeded in possession of the land by the Urgo-Samoyeds, immi-
grant tribes, who brought a finer art in gold, silver, and bronze than the land
had known before. The Huns drove out these less warlike people about the
third century B.c. The succeeding centuries were periods of vast tribal move-

14 GEOLOGY OF MONGOLIA

ments, and we hear of the Huns invading India on the one hand and Europe
on the other—lured by the hope of better lands than central Asia afforded.
Some of the monuments can be traced to the Turkish tribes which appeared
in the fifth and sixth centuries A.D. They were masters of much of northern
Mongolia and southern Siberia, but they were overcome in turn by the rising
might of the Mongol empire which Jenghiz Khan built up in the thirteenth
century. Gradually the Turkish tribes shifted away from Mongolia, or
merged with the overwhelming Mongol peoples. As the power of the short-
lived Mongol empire declined, the Chinese conquered Mongolia, and made it
one of the ‘‘outer provinces.’’ At about the same time, the Russian peoples
were beginning to turn their eyes eastward to the vast lands beyond the Ural
Mountains.

This sketch of man’s ancient migrations in central Asia is dim and frag-
mentary, yet we think that it contains the first lines of a splendid picture which
future studies may enable someone to paint in full color. We discern two parts
in the composition of this picture: the older part, which deals with man’s
origin and descent, is but faintly sketched; the other part is a picture of migrat-
ing tribes and changing cultures, rather than the history of an evolving race.
The first part reaches back into Pliocene and earlier periods; the second begins
with the Ice Age, and continues to the present. Ancient adventure and the
discovery of central Asia by successive migrating tribes constitute good sub-
ject matter for future scientific study.

HISTORIC TRAVEL AND MODERN EXPLORATION

Exploration implies travel for the purpose of collecting data of one kind
or another. Travel for adventure and to satisfy one’s urge for a larger world
is older than exploration. The list of travelers is legion, but only a few names,
chiefly of those who have made extensive traverses in the interior of Asia,
may be mentioned here. Exploration proper is of very recent development,
and scientific exploration in central Asia dates only from ‘‘day before yester-
day,” with a very short list of names. Among the travelers, Marco Polo is
the dominant figure; among the explorers, von Richthofen is one of the earliest
and most illustrious.

As early as 1253, Saint Louis sent William of Rubrouck to Mongolia to
visit the court of Mangu, the grandson of Jenghiz Khan. William reached
the court in the year in which Marco Polo was born. Fifteen years later,
Marco with his father, Nicolo, and his uncle, Matteo, began the astonishing
pilgrimages that were to fill his life and fire his only too active imagination
(Yule translation, 1921). In the next century, Father Odoric traveled through
Persia, India, and the East Indian islands to China, returning by way of Tibet.

GEOLOGICAL OBJECTIVES AND METHODS 15

These and other travelers brought back to medieval Europe the earliest
accounts of the Far East; but Marco Polo has given us the richest and most
colorful picture of the land and people of Mongolia and China that was ever
prepared before science came to guide exploration.

In the sixteenth century, at about the time when adventurous peoples
from western Europe were beginning to colonize the new-found Americas,
the Russians began to cross the Ural Mountains into Asia. We may picture
two streams of migrating peoples, one pouring toward the west, the other
toward the east: the one to cross the salt waste of the Atlantic to America, the
other to traverse the tundras and swamps of western Siberia—the one to face
the red Indian, the other the bronze Mongol. Each stream went forth to a
pioneer’s life, into a vast unconquered land, and each, despite the emphasis
that written history lays upon wars, went armed rather with the plough than
with the sword. Even the earliest records of Russian occupation of Siberia
tell of farmers who were sent-out with the soldiers: one group to build the
blockhouse, the other to break the soil around it.

The ghastly story of exile and the prison camps of Siberia is one of those
black chapters which no one palliates or defends. Most countries have tried
the experiment of exile colonies in one form or another, and all have found,
sooner or later, that it does not pay. Yet, curiously enough, one of the sto-
ries which concerns us in this book grew out of the prison camps of Siberia.
_ After the battle of Poltava, in 1710, a Swedish officer named Philipp Johann
von Strahlenberg was taken prisoner by the forces of Peter the Great. He
was sent, with many others, to Siberia. During their long exile, Strahlenberg
and his comrades studied peoples, languages, resources, and the geography
and history of Siberia. With older maps to build upon, Strahlenberg made
a new and astonishingly good map of Siberia, Turkestan, Mongolia, northern
China and Tibet. An excellent critique of this map has been published by
Sven Hedin (1917, I, pages 246-252) in his Southern Tibet. Two other pris-
oners of this same war, Schénstrom and Renat, made important geograph-
ical and ethnographical observations in Siberia, and Renat constructed a map
of central Asia which gives the best picture of Dzungaria and the Tarim basin
known up to that time.

Strahlenberg and Schénstrom bought from a Boucharan merchant a man-
uscript which they translated into several European languages. It proved
to be a brilliant account of the Tatar or Mogul (Mongol) people, which was
written by Abul Ghazi, prince of Khiva (translated 1729). The book is un-
finished, for the writing was broken by the prince’s death in 1663. He carries
the history of the world rather rapidly from the creation of Adam to the rise
of Jenghiz Khan, who was Adam’s lineal descendant and Abul Ghazi’s lineal
ancestor. For a man who believed in magic and who dealt in traditional his-

16 GEOLOGY OF MONGOLIA

tory, the work of Abul Ghazi is remarkably literal, and bears evidence on every
page of an honest and scrupulous mind. Although we cannot include him
among the explorers of Mongolia, his Genealogical History of the Tatars has
become one of the source books for the history of the Mongol people.

EARLIER STUDIES OF SPECIAL SIGNIFICANCE

Following the work of Strahlenberg, Schénstrom, and Renat, an epoch
of scientific exploration of Siberia, Turkestan, and Mongolia opened. French,
German, and Russian investigators searched out the resources of the vast
region, studying its plants, animals, soils, rocks, and minerals. Naturally
the work of the eighteenth century was crude and poor, and had to be done over
again by the more fortunate and better trained scientists of the nineteenth
century. A new era in the exploration of this region dawned in 1864, when an
American geologist, Raphael Pumpelly, made the first geologic section across
Mongolia from Kalgan to Urga (Plate III). In his short but excellent account
(Pumpelly, 1866) he tells that Mongolia is an inland basin; that much of it is
a plateau, or series of plateaus, formed of almost level strata of sandstone,
clay, and lava sheets, interrupted by areas of granite and other massive rocks
belonging to the oldrock floor; and that folded rocks, chiefly slates and schists,
form another important structural element. This sane and simple analysis
was the first great step toward the unraveling of the complex structure and
history of Mongolia. |

When Pumpelly crossed Mongolia in 1864, Ferdinand Freiherr von Richt-
hofen (1877) had been two years at work upon his pioneer reconnaissance in
the Far East. He examined the southern part of the great basin, and recog-
nized the Khingan range as a continuation of the great line of flexure and
faulting along which the mountains of Chihli bend down under the soft silts
of the Great Plain. He noted the level strata of sand and clay, and called
these beds the Han Hai Series. The name is Chinese and means the “dry
sea.” It is a beautifully figurative name for these immense and marvelously
level wastes of desert above which the hard-rock hills rise like islands. But
von Richthofen carried the analogy further, for he believed that these depos-
its had been laid down in an ancient sea.

Von Richthofen finished his researches in China in 1873, and was followed
by the new school of Russian explorers. The character of their work changed
from that of travelers to that of more scientific investigators. I. V. Mushke-
tov (1876, 1906) had visited the Tien Shan while von Richthofen was yet
in China, and, like the German pioneer, continued his travels for twelve years
(Plate II). He noted the complexly folded rocks, cut by many kinds of once
molten intrusives—granites, syenites, diorites, and porphyries. He collected

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GEOLOGICAL OBJECTIVES AND METHODS 7,

fossils which told of Devonian, Carboniferous, and Cretaceous seas, and plant
remains which recorded swamp-lands in the Triassic and Jurassic periods.
He made a careful study of the coal in the Kuldsha basin, in Turkestan, and
noted many other mineral resources. Romanovsky (1878) began exploring
Turkestan rather later than did Mushketov; but the two pioneers joined
hands to produce in 1884 a map of Turkestan which has been the foundation
of all our scientific knowledge of that region.

Pievtsov and Bogdanovich (1892) traveled through eastern Turkestan,
Dzungaria, Tibet and western Mongolia in 1889 and 1890. Bogdanovich
(1892) made out the general geology of the region traversed, and laid the
foundations for all later studies.

While the Russians were pushing their investigations outward from
Siberia, the Hungarian count, Béla Széchenyi, undertook an extensive jour-
ney through China, southern Mongolia, and Tibet. He was accompanied
by Gustav von Loczy (1893, 1899), who constructed a geologic cross-section
throughout the entire course of the journey.

The explorations, which began in the west with Turkestan, were con-
tinued eastward by later investigators. It is not possible in a reconnaissance
volume to do justice to the many colleagues who have contributed to this
work through their writings, and the dry acknowledgments convey little of
the appreciation due to the many deeds and thoughts of great summarists
like Suess, and of master pioneers like Obruchev. The maps (Plates II and
III) give some idea of the maze of routes traced across central Asia before us,
but even this picture leaves much untold. We can give space to a very few
outstanding names, and trust that in a later volume we may tell the whole
story.

V. A. Obruchey, greatest of Russian explorers, began his travels in 1892
(1893). His advent marks a new era in the study of these great regions; the
coming of a keener insight into geologic problems, with a better training in
the science than that of any of his predecessors. He began, as was natural,
where von Richthofen, Mushketoy, Roborovsky, Bogdanovich, and others
had left the story. He recognized at least two periods of mountain-building:
an earlier, following the Permian period, when the strata were folded into
ranges like the Appalachians; and a later, in Tertiary time after the earlier
mountains were wholly worn away, when the land was broken along rifts,
and tilted up as block-mountains (Obruchev, 1900, 1912, and 1923). Obru-
chev was the first geologist to discover vertebrate fossils in the Han Hai beds.
His utmost care in collecting the fragments could not prevent their breaking
still further; nevertheless, a tooth of general rhinocerotid type was pieced to-
gether, and was tentatively determined by Edouard Suess (1899) as a Rhi-
noceros or Aceratherium, which he believed to be of Miocene age. Obruchev’s

VOL. IIl—2

18 GEOLOGY OF MONGOLIA

account of this discovery was in the hands of the geologists of the Expedition,
and led them to pay special attention to this region, although, curiously
enough, vertebrate fossils were found at two other horizons before the site of
Obruchev’s discovery was examined. In the northwestern corner of the Ordos,
Obruchev found the skull of a fossil rhinoceros, probably of Pleistocene age,
very near to the place where, more than thirty years afterward, the Jesuit
Fathers Licent (1925) and Teilhard de Chardin (1924 b) made their great dis-
covery of the implements of fossil man.

From the foregoing list of contributions, it is evident that a large number
of exploratory investigations have been made in central Asia. The funda-
mental elements of the work accomplished have been taken as a ready-made
foundation for the new investigation, and the original sources of a particularly
helpful character require some additional comment and acknowledgment.
These are: the contributions of Ferdinand von Richthofen (1877), and of V. A.
Obruchev (1900); Research in China, by Bailey Willis (1907); and Explorations
in Turkestan, by Raphael Pumpelly (1905). Each in his own field has pene-
trated an almost trackless wilderness of scientifically new territory, and has
brought information and order out of difficult, obscure, and comparatively
little-known regions.

The structural and stratigraphic elements of China, as given by Willis,
furnished by all means the best starting point for the geologists of the Third
Asiatic Expedition, because the work had to be projected from China as a
base of operation. Furthermore, his work affords most direct and ready com-
parison, because, after the season’s work, one returns again from the interior
to the standard sections of China. We now know that a remarkable similar-
ity of development is represented in the geology of the interior and that of
the Chinese border. The two regions have approximately the same history.
In past time they comprised a more uniform structural unit than the present
physiographic differences suggest, so that the major structural features are
essentially the same.

It is fortunate, therefore, that the Third Asiatic Expedition found it
convenient to organize in China, as the staff was given special encouragement
and help by the geologists of that country. It was of advantage, also, that
one of the members of the geologic staff of the Expedition, Frederick K. Mor-
ris, had already accumulated considerable field experience in China and had
become familiar with its formational and structural habit. Probably from no
other base and under no other conditions could the work have been under-
taken with so great advantage or so great likelihood of success.

It is certainly not advisable at this stage of the investigation to carry
detailed correlation to such an extreme as to assume the identity of many of
the individual formations in these two widely separated regions,—China

GEOLOGICAL OBJECTIVES AND METHODS 19

proper and the interior desert plateau. Nevertheless it is true that the major
structural units, as well as the principal deformation epochs and changes in
the dominant processes, as marked out by Willis in China, can be recognized
definitely, in the same order and with much the same proportions in the north-
ern region.

The geological statement as given by Willis is, for another reason, par-
ticularly helpful. It not only recognizes formational units, but establishes
them on a broad dynamic basis. Moreover, in describing them Willis man-
ages to allow for expectable variations in the character and physical condition
of the sediments, so that corresponding representatives or equivalents in new
territory might be placed in the geologic column without serious revision.
This is materially aided by a careful determination of igneous and deforma-
tion epochs, so that the whole geologic column is built up with suitable empha-
sis on the major breaks and the dominant forces or processes of each part.
The Expedition came to appreciate the great service of such a contribution
as this of Willis in China. One is the more impressed with its soundness after
working, as this Expedition did, over several thousand miles of adjacent ter-
ritory, where, again and again, this proved to be the only really helpful geo-
logic guide.

In addition to the work of Willis in China, that of Pumpelly (1866, 1905),
Davis (1904, 1905), and Huntington (1905 a, 1907 a), on the west side of the
continent in Turkestan, ranks especially high in its applicability and help-
fulness in a study of problems of the interior. For our region the geologic
column representing the older formations, as given in the Explorations in
Turkestan, is of little service; but the determinations bearing on physiographic
changes, the significance of the processes and effects represented, and the order
of events in later time are of far-reaching application on the continent of Asia.
It would be strange, of course, if this were not so; for the men who participated
are masters in their own fields, and their observations and conclusions could
fail of wider application only in case the geological conditions were actually
different. We now know that there is enough unity in the continent of Asia,
despite its immense size, to enable us to correlate with reasonable success
many important steps in its development, from China on the one side to Turk-
estan on the other.

The emphasis placed by the work in Turkestan on the changes that have
taken place in late geologic time might be applied also in Mongolia, where the
deformations, the processes, and the climatic changes are of similar type and
of corresponding magnitude. The features are more easily read, are more
definite, and are of greater significance in the desert region of the interior than
in China proper. One finds, therefore, in the reports of these western explora-
tions the clearest statement of the history of late geologic time, of the physio-

20 GEOLOGY OF MONGOLIA

graphy, and of the processes that have made present conditions. The prin-
ciples used and the explanations given were found to be a constant help in
our studies of the interior, and we are confident that the summary given in
this volume will be found to establish a reasonably consistent geologic link of
field observation between these two justly famous investigations—that of
Willis and Blackwelder in China, and that of Pumpelly, Davis and Hunting-
ton in Turkestan.

The most suggestive observations of the latter expedition are those which
postulate a fluctuating climate, reaching back to the limits of human occupa-
tion and probably beyond; and those which establish the correlation of uplift
and depression, with the shift of deposition centers during late Tertiary time.

Displacements of ten thousand feet, recognized by Huntington, are
equaled in such movements as that of the Altai uplift of the desert interior;
and the repeated warpings which Huntington refers to in similar connection
are not only duplicated in many parts of Mongolia, but, because of the detailed
determination of horizons in many adjacent basins, are established more
firmly than ever before by the work of the Third Asiatic Expedition. By
means of this work, epochs of deposition, of erosion, and of deformation have
been dated and brought into definite sequence. Davis and Huntington in
Turkestan recognized the significance of the evidence indicating these changes,
and placed them for the first time, in their proper geologic and physiographic
setting. The work of the Third Asiatic Expedition, while it supports the
general conclusions of the expedition to Turkestan, attempts to go further in
the definiteness of its correlations.

The work done in Siberia is more scattered, deals with more limited
problems, and is difficult of access. Nevertheless, important observations
have been made, and some of the conclusions derived from them are readily
fitted to the Mongolian region.

CHAPTER II

BOUNDARIES OF THE GOBI REGION

INTRODUCTION

THE Gobi Desert is merely the arid part of a great inland basin that lies
enclosed between mountainous divides (Fig. 4, page 43). The highland bor-
ders of the basin are for the most part well watered grasslands that change by
almost imperceptible gradations into the central desert. The boundaries of
the desert proper are therefore not only indefinite, but inconstant, because a
few rainy years extend the grassland, while a dry period makes the desert
broader. The boundaries of the inland basin in which the desert of Gobi lies
must be placed along the crests of the mountains that slope into the basin on
the one hand, and outward toward the sea on the other. At first sight it
seems as though the boundary would coincide with the watersheds which part
the inland drainage from the waters that reach the sea, but this is not every-
where true, for the Kerulen River, after draining a part of the Gobi region,
escapes to the sea through the Argun and Amur Rivers; and in the west, the
Gobi is not bounded by marine drainage, but is separated by low, inconspicu-
ous divides from other inland desert basins. In attempting to summarize
the nature and structure of its boundaries, one has to rely largely upon a crit-
ical review of literature concerning the surrounding regions, and the record is
very imperfect.

THE NORTHERN BOUNDARY

The northern boundary is everywhere mountainous, and may be divided
into three general parts. The western part consists of the fault-block range
called Tannu Ola, which connects with the Sailugem Mountains on the west,
and, by several minor fault-blocks, with the Khangai uplift on the east. The
central section includes the Khangai, Gangin Daba, and Kentai ranges, which
may be considered as a series of arched or domed masses of complex rocks.
The eastern section is formed of the Transbaikalian block-mountains which

21

a2 GEOLOGY OF MONGOLIA

overlap one another obliquely along the Gobi basin, the southwestern end of
each range plunging down beneath the Gobi sediments.

Northern boundary—western section

The northwesternmost part of the Gobi basin is a group of mountain-
rimmed depressions called by Pievtsov the ‘‘Valley of the Lakes” (Plate IV).
Its western wall is the Sailugem mountain chain, concerning which we have
very little information. The northern rim of the Valley of the Lakes is formed
by the Tannu Ola range, which is a stretch of the Arctic divide, parting the
headwaters of the Yenisei on the north from the wholly enclosed Ubsa Nor
basin on the south (PlateIV). The abrupt scarp of the Tannu Ola rising above
the broad lake-basin strongly suggests that the range is faulted, forming as
it were a shorter northern counterpart to the longer fault-block range of the
Altai. Suess has assembled (1902, III, pages 111-117) three traverses of the
Tannu Ola, and shows that the western traverse reveals only Devonian rocks
—conglomerates, quartzites, and porphyries. The middle traverse brings in
gneiss and granite, limestone, schists, marls, and sandstone. The sandstone
beds contain plants belonging to the Carboniferous age (Culm). Just east of
this section, on the north side of the range, the same traverse records Devo-
nian strata, followed by Culm and then by the Jurassic Angara series. The
third traverse begins on the north with the Jurassic Angara beds, and passes
to the Devonian on the south side of the range. The rocks are folded and
faulted.

Hausen, in 1917 and 1918, studied the part of the Tannu Ola lying east and
west of the lake Kara Kul (1925). He reports that the rocks are largely igne-
ous masses,—granite, grading into diorite and even gabbro,—which invade
greenstones, limestones, and schists of uncertain age. Devonian sandstones
and limestones with porphyries and lava sheets overlie the older rocks uncon-
formably. The range is made up of several faulted blocks, and some of the
longitudinal valleys, including that of the Kara Kul, are regarded as grabens.
Hausen has thus shown that there are at least two ancient periods of folding,
and that the folded structures of the mountain are broken by faulting of more
recent date.

Northern boundary—central section

The Tannu Ola passes eastward into the general mass of the Khangai
Mountains. The latter name is here used collectively for a mountainous
region, rather than a range, lying between the Valley of the Lakes on the
west and the Tola River on the east (Plate IV and Fig. 4). Suess says (1902,
III, page 118), ‘“Eastward the Khangai merges into the Kentai Shan and the
mountainous regions north of Urga which are continuous with those of Trans-

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PLATE V.

A. THE FIRST MOUNTAIN BARRIER. THE WESTERN HILLS FRONT NEAR PEKING,

B. THE GREAT WALL ABOVE NAN K’'OU PASS.

This view was taken at Ch'ing Lung Chao, and shows the rugged crest of the first mountain barrier back of Peking. In an-
cient times this was the inner defense line for the great plain of China, the outer defense being the Great Wallabove Kalgan, more
than a hundred miles farther toward the interior of Asia. (See Plate XLIII B.)

BOUNDARIES OF THE GOBI REGION 23

baikalia. . . . No tectonic boundary separates the Gobi from the Khangai,
since denuded remnants of a similar structure may be recognized in adjacent
parts of the Gobi itself.’’ Suess recognized horsts and grabens in the Gobi
region, but as both the folding and the later faulting follow in general much
the same structural trend, Suess did not clearly differentiate an age or ages
of dominant folding from another age characterized by dominant faulting.

Tolstikhin (1920) made an extensive study in the region of the Selenga
River in 1919 (Plate IV). We have not a copy of his paper, and must rely
for the present upon Obruchev’s review of it (1921, page 149). Tolstikhin
noted a thick formation of steeply folded crystalline limestones, striking
N. 30° W. to N. 50° W. and extending from the Kosso Gol to the river Telgir
Merin (Plate IV). Northward from the river Murin, dolomitic limestones
striking N.N.E. have been cut and metamorphosed by granites and granite
porphyries. Associated with these limestones are quartzites, sandstones, and
silicious schists, all of which Tolstikhin considers to be pre-Cambrian. He
goes on to say that after a long erosion period, the region came again under
water which covered the lower course of the rivers Murin, Buksui, and Eder
and extended eastward along the Selenga to the mouth of the river Egin Gol.
Gray and black limy clays and sands, with green and red sands and conglom-
erates, were deposited, in which obscure plant remains are found. This
water sheet, he says, retreated toward northeast and east. While the sedi-
ments were being deposited, volcanoes poured out dark basic lavas and their
tuffs. All of these rocks were folded at a later date, and strike W. N. W. to
E. N. E. with dips less steep than those of the pre-Cambrian. The basic erup-
tive rocks were converted into green chloritic schists. Tolstikhin assigns these
sediments and associated igneous rocks to the Jurassic Angara series of Suess.
He describes a later granite-syenite intrusion, which closed with eruptions of
keratophyre.

Obruchev (1921), commenting upon this paper, thinks it more likely that
the later sediments may be not Jurassic, as Tolstikhin supposed, but possibly
may be of Paleozoic age, citing his own discovery of Palzozoic bryozoans and
corals between Urga and Kiakhta in rocks of much the same character as those
observed by Tolstikhin. Obruchev considered it probable that “in northern
Mongolia, as in the rest of Central Asia, except the region of Kashgar, the lat-
est advance of the sea was in Upper Carboniferous or Lower Permian time”’
(1921, page 150). The possibility that these chloritic schists may be of pre-
Palzozoic age should not be overlooked.

The route of the Expedition in 1922 led southwestward from Urga through
a wide belt of the slates and graywackes. As this section is followed south-
ward, igneous rocks appear, cutting the graywacke series, and increasing in
number and size until the granite of the Mongolian bathylith is exposed over

24 GEOLOGY OF MONGOLIA

large areas, while the graywackes lie isolated as roof-pendants and xenoliths
(Figs. 34 and 35, page 95). In the region of Tsetsenwan (Lat. 46° 15’ N.,
Long. 104° 10’ E.), the mountains are disposed in broad, relatively low blocks
whose tops are beveled by an arched peneplane and whose fronts descend ab-
ruptly to basins with almost flat bottoms, some of which are floored by the gran-
ite, while others bear a cover of nearly horizontal sediments (Fig. 37, page
101). In the block-mountains we found in one place gneisses and marbles which
we assigned to the Archean complex; and in other places schists and crystalline
limestones,—striking N. 75°-80° W. and dipping northward,—which we judged
to be Proterozoic, comparable to the Wu T’ai series of China. They are all
unconformably overlaid by the folded conglomerates, sandstones, and surface
volcanic rocks which we tentatively consider to be Lower Jurassic. As we
went northwestward again to Sain Noin, the graywackes reappeared. They
form the entire region for many miles about Sain Noin Khan, save for infolded
synclines of the Jurassic conglomerate. The graywackes are indeed present
on a scale so vast that we called these rocks the Khangai series.

Thus the Khangai mountain region, taken as a whole, includes a great
complex of folded sediments and igneous rocks, ranging from the Archzan to,
possibly, the Lower Jurassic. Their upturned edges are beveled by a pene-
plane which forms the upland of the Khangai, standing as high as ten thou-
sand feet above sea-level. All of these rock types were observed also in the
Gobi desert, where they maintain the same folded structure and essentially
the same strike as in the Khangai. The Gobi and the Khangai are not sep-
arated by a fault line or by an abrupt scarp, and Suess may have had this in
mind when he wrote that there is no tectonic boundary between these two
regions. The Khangai hills become lower, and extend in long tongues out
into the desert; while the lowland of the desert surface extends up the valleys
between the projecting spurs of the mountains. The mountain mass as a
whole rises gradually above the desert and has the aspect of a broad dome in
which faults and fault lines are not so conspicuously in control of the topog-
raphy as they are in the Altai ranges to the southwest or in the Transbaikal
ranges to the northeast. The streams heading on the two opposite sides of
the Khangai watershed form a long dendritic drainage pattern which clearly
suggests that the drainage is consequent upon the Khangai arch. There is
no evidence in the river pattern of a former connection of the Gobi basin with
the Arctic drainage. Along the Khangai front, however, the earth movements
that have made the mountains were essentially a gentle doming or upwarping
of ancient, complexly folded rocks which had been peneplaned prior to the
uplift.

The Kentai Ola continues the Khangai northeast of Urga, and forms the
Arctic divide in this region. J. Morgan Clements (1922) examined part of

BOUNDARIES OF THE GOBI REGION 25

the Kentai hills, and expressed the belief that the graywacke series is of late
pre-Cambrian age. Ussov (1915) reports in general an Archean “ Barchin
Series” and an Algonkian graywacke series which are cut and metamorphosed
by great intrusions of granodiorite. The Expedition in 1922 crossed the
Gangin Daba Mountains, which may be considered the southwestern exten-
sion of the Kentai, and observed in the southern part a belt of schists, which
we regarded as later than the Archean, and probably of much the same age as
the Wu T’ai system of Bailey Willis (1907, II, page 4). This correlation would
not necessarily cast any doubt on Ussov’s identification of Archzan rocks in
the Kentai. Like the Khangai, the Kentai appears to rise gradually out of
the great basin of the Gobi, without any marked tectonic boundary beyond a
broad, gentle upwarp or doming.

Northern boundary—eastern section

Thus far the boundary of the Gobi has been placed along mountain crests
that form the Arctic watershed. All water falling south of this divide runs
southward into the desert, to escape only by evaporation. But the Kerulen
River, heading in the Kentai hills, runs southward into the great basin of the
Gobi, and then flows eastward nearly four hundred miles to the lake Dalai
Nor, from which the waters escape to the ocean along the Argun and Amur
Rivers (Fig. 4). The course of the Argun River follows the fault-graben
along the eastern base of the Argun range to about latitude 50°, where it cuts
across the mountain. Between this and the Khingan range the river follows
some of the fault lines, and cuts across others. It swings around the northern
end of the Khingan and joins with the Shilka River to form the Amur River
(Fig. 4). The genetic origin of the Kerulen-Argun is an unsolved problem.
The course is clearly a complex one, and shows two types of adjustment:
along parts of the course the river is adjusted to the structure of the present
ranges, and along other parts it cuts across these structures. If the moun-
tains arose by faulting at somewhat diverse times, and at diverse rates of inter-
mittent movement, a large river would maintain its course across the minor
ridges, and especially across those whose growth was slow and of more recent
beginning, while larger, older ranges and those whose increments of fault move-
ment were greater, would force the river to take a course adjusted to the
trend of the ranges. The Argun range has in part, and the Great Khingan
has wholly, forced such adjustment to its own lines.

The mountains north of the Kerulen belong to the Transbaikal system
of fault-blocks in which the rivers have a peculiar trellis pattern. Whether
the topography is primarily controlled by block faulting, or whether the rivers
have subsequent courses and have etched out their valleys along the weak
zones of ancient fault lines, is another of the unsolved problems of the region.

26 GEOLOGY OF MONGOLIA

Lvov (1916) says that the dominant topographic feature of the western Amur
region is a group of short parallel-oriented massifs, separated from one another
by narrow or broad, down-sunken depressions.

Obruchev (1899, pages 192-206, and 1926, page 432) has given a descrip-
tion of the broad features of Transbaikalia, in which he emphasizes the great
width of the divides or mountain uplands, and also the great width of the val-
leys. He considers the mountains to be fault-blocks, and his map, which is
redrawn and transliterated in Fig. 135, page 320, shows a long series of faults
defining the present ranges. On the other hand, he does not throw direct
light upon many important problems. He says that several ranges are
“broken through’’ by rivers, but does not explain the origin of the river’s
present course. He stresses the fact that the ranges are bow-shaped, lying
in arcs which are slightly convex toward the south; but he does not tell whether
he believes that this shape is due to pressure or thrust from a given direction.
He shows that the strike of the bedded crystalline rocks diverges in places
quite markedly from that of the ranges. In the west, near Lake Baikal,
the folded structure strikes east-southeast, while the faulted mountains lie
east-northeast to northeast (Fig. 135, page 320). Farther east the prevail-
ing strike is east-northeast, and here it falls into agreement with that of the
mountains. Still farther east the strike of the beds is again east-southeast
to southeast, and locally east-northeast, or northeast or even north, so that
it lies now across, now diagonal to, and now parallel with the direction of the
mountain ranges. Obruchev continues (1899, page 201):

The Mesozoic and Tertiary rocks . . . in most cases strike parallel to the de-
pressions, in which exclusively these rocks are found. It seems that they have been
broken into larger and smaller tablets or plates, which sank steeply in this or that
direction, and, locally, have thereby even formed shallow folds.

The post-Tertiary deposits are not dislocated.
Obruchev says (1899, page 202):

The depositional relationships of the metamorphic schists, the Paleozoic, the
Mesozoic and Tertiary rocks, show that the fault movements repeated themselves
in later periods, and in general remained true to the first established strikes . . .
Secondary horizontal movements accompanied the dominant vertical movements,
as the folding of the sediments shows . . . The vertical movements of the earth’s
crust were accompanied by outbursts of massive rocks, which arose along fault-
clefts. Many of these rocks pressed in as laccoliths into the Archean masses, and
have been laid bare only by later erosion and denudation. Elsewhere they form
many dikes in the older rocks. But the existence and the wide distribution of tuffs,
breccias and conglomerates which are associated with various porphyrites, mela-

BOUNDARIES OF THE GOBI REGION 27

phyres, trachytes, rhyolites, and basalts, and which in other places are interbedded
and alternate with the ordinary clastic sediments, prove that many outbreaks reached
the surface and that long chains of volcanoes were active along the faults at the margins
of the ancient (‘‘Archaische’’) horsts. The products of eruption came to rest partly
on the dry land, and partly in the lakes, which occupied the valleys during many
periods. :

In the east in the district of Nerchinsk, the igneous rocks are as widely distributed
as the crystalline schists and sedimentary strata; and in some localities the horsts
consist only of the igneous rocks. Therefore the arrangement of the rocks in this
district is not so regularly zoned as in the other parts of the region, and the structure
is far more complex.

. . . Besides the dominant fault lines striking east-northeast, which mostly,
even though with interruptions, have a great length, there are in the region other far
shorter fault lines which run north-northwest to north. Such cross-faults, which in
places appear to be connected with horizontal displacements, are seen in the west,
but more especially in the district of Nerchinsk.

Obruchev says that these cross-faults are rudely parallel to the Khingan
range, and therefore infers that they are related to the displacement history of
the Khingan. He calls them the “ Khingan faults” in contrast to the “‘ Baikal
faults’’ which lie parallel to Lake Baikal and form the prevailing mountain
lines of Transbaikalia. He points out that the great Khingan range forms
the eastern tectonic boundary of the Baikal region. At the western boun-
dary is Lake Baikal, which Obruchev considers to be a great faulted depres-
sion, the deepest graben in Transbaikalia. Westward from this graben we
meet with broad areas of Cambrian and Silurian marine deposits, which are
relatively slightly dislocated. Here, then, very different rocks and very
different tectonic relations dominate, so that Lake Baikal closes the western
limit of the Transbaikalian fault lines.

“South and southwest of our region lies northern Mongolia, in which we
find the same ancient metamorphic and massive rocks and the same tectonic
lines as in Transbaikalia.”’

Summary of the Transbaikal border

As in the Khangai-Gobi contact, so also in the contact between the moun-
tains of Transbaikalia and the great basin of the Gobi, there is no difference
in the rock formations or in the lines of ancient folding. These facts point to
the conclusion that the great basin of the Gobi is of more recent origin than any
of the folded ranges. All of the folded mountains, including the Altaids of
Suess, were wholly planed away before central Asia became an inland, un-
drained hollow. The present Gobi basin is relatively young, and was formed
coincidentally with the uplift of the Transbaikal ranges. Both the Trans-

28 GEOLOGY OF MONGOLIA

baikal mountain region and the basin of the Gobi were formed by deforma-
tion of a single great province of peneplaned ancient mountains.

THE EASTERN BOUNDARY

The eastern boundary of the great basin of Mongolia may be taken as
the Ta Hsing An or Great Khingan range, which reaches from the Amur River
at the north to the mountains of the Kalgan and Nan K’ou region in the
south (Fig. 4). The Khingan range strikes about N. 25° E., and so lies athwart
the structure of the Altai, Khangai, and the Transbaikalian mountains. The
following brief account of this range is taken from the summary of Suess
(1902, III, pages 117-122):

According to Gedroits, the northern part of the range has a width of one
hundred eighty to two hundred versts, or about one hundred and twenty to
one hundred and thirty miles. It is formed of igneous rocks and ancient
schists, overlaid by deposits probably of the Palzozoic age, with intrusions of
granite, porphyry, and diabase. Kropotkin (1865) and Manakin (1898) have
recorded in Lat. 49° 30’ N., “‘clay-slate and granite and in places porphyry
and upturned red sandstone’”’ (Suess, 1902, III, pages 151-152). Potanin,
crossing at the headwaters of the Nomin River (Plate IV) noted ‘parallel
chains of granite and porphyry with an almost meridional trend and particu-
larly steep eastern declivity.”” Potanin, coming from the north, encountered
granite, porphyry, and clay-slate down to about 45°N. (Suess, III, page 153).
Prjevalski (1877), traveling northward from Peking to Dolon Nor, reports
gneiss and granulite for one hundred and eighty versts on the western slope
of the Great Khingan (about Lat. 42° 20’ N., Long. 116° 20’ E.).

Northwest and west of Dolon Nor the Mongolian platform rises at once to over
2000 meters; on the summit, the previously rocky character of the landscape dis-
appears and we suddenly enter the monotonous Gobi... The town of Dolon Nor
stands on ancient rocks traversed by quartz veins; then follows to the northwest
quartz-porphyry, and the steps which take us to the summit of the Gobi appear to
be formed of the same rock. (Suess, 1902, III, page 153.)

Upon the ancient complex rocks lie floods of lava, chiefly basalt, which
probably are genetically related to fault movements. Suess mentions ‘recent
voleanic rocks . . . near Nerchinski-Savod, along the upper Argun and on
Lake Kulun (1902, III, page 151)."". Nerchinski-Savod is on the Argun River,
about Lat. 51° 20’ N., Long. 119° 30’ E. Manakin reports an extinct volcano
with a well-preserved crater on the east side of the Khingan, at the bend of the
Nomin River (1898, pages I-79). Potanin notes remains of two other craters
near the town of Mergen. ‘‘Thus the town of Mergen is surrounded by a

BOUNDARIES OF THE GOBI REGION 29

volcanic region of recent date, which measures two hundred versts from west
to east.” . . . and ‘forms an eastward extension of the volcanic rocks of
the Khingan and at the same time a part of the border of the eastern Gobi.”
(Quoted by Suess, 1902, III, page 152.)

Other extensive lava fields lie west of the Khingan. Mushketov (1881)
describes a lava field north-northwest of Dolon Nor, in the hills called Bogdo
Ola (Lat. 43° 35’ N., Long. 115° 30’ E.), where he saw a volcanic cone that still
retained its crater. Very recently Pére Emil Licent and Pére Teilhard de
Chardin visited this general region, and at about Lat. 42° 30’ N., Long. 115° E.,
discovered large areas of basalt and a chain of volcanic cones. The basalt
overlies sediments carrying an Upper Pliocene fauna which would serve to
suggest an approximate date of either late Pliocene or Pleistocene for these
outpourings. (Teilhard, 1924 c, and personal communication, 1924, to W. D.
Matthew.)

Suess believed (1902, III, page 120) that the lavas of the Khingan Moun-
tains probably were correlated with the lava fields of southern Mongolia.
But later studies have shown that the lavas range in age from probably as
old as Oligocene to certainly as young as late Pliocene, so that correlation of
the lavas must be left for future careful investigation.

The eastern side of the Khingan descends abruptly to a broad lowland,
and is considered to be a fault-scarp by Mushketov. Von Richthofen also
took this view, adding: ‘‘. . . it seems that the name Khingan applies to the
steep eastward-facing scarp of a gentle up-swelling of the margin of the plateau
from the west.’’ (1877, page 34.) On page 147 (1877) von Richthofen also
says: ‘‘the gentle up-arching of the Khingan forms the boundary of the north-
ern and eastern transition region [from the Gobi outward to the drained and
watered regions], but is more of a connecting link between them than a separa-
tion of them.”” On page 519 of Volume II, he adds:

If we omit the tongues of the plain that thrust in long valleys into the mountains,

the plain [the great plain of China] is separated from the mountains by a straight line

If we prolong this line north-northeast, it coincides with the eastern descent

of the mountains of Shansi rimming the Bay of Peking. This line, nearly at right

angles to the trend of the Kuen Lun ranges, is of great significance in the configura-

tion of northern China, and it is perhaps not by accident that the Khingan, which

is likewise an up-arched plateau margin and only offers the aspect of mountains when
seen from the east, lies in the prolongation of this very line.

He considers that the uplift of these northeastward-trending mountains took
place at some time later than Lower Jurassic.

Willis, commenting upon von Richthofen’s line of dislocation, the Khin-
gan line, says in part (1907, II, page 106):

30 GEOLOGY OF MONGOLIA

He does not cite any evidence of faulting on the line itself, it being drawn indeed
in the plain of alluvium; and according to our observations on three different sections,
the passage from the plain to the mountains is a zone of warping, not a line of dis-
location. Where, in latitude 49°, the Siberian railroad descends to the Sungari, the
eastern slope of the Khingan is a tilted, dissected, but unbroken peneplane. Where
the Sha-ho, in latitude 39°, has cut its autogenous valley, . . . the effects of modern
warping are obvious. Normal faulting, though present in the Ning-shan basin,
occurred at a remote Tertiary date, and erosion has reversed the relief to which it
gave rise. Again, in latitude 31°, where the Yangtze emerges from its profound
gorges at Ichang, the mountain slope that faces the far-spreading river plain is a
tilted surface of erosion, showing a continuous stratum of Carboniferous limestone,
which toward the base is overlaid by the K’uichow red beds in appropriate strati-
graphic sequence. It is a warped surface, not a fault; and there is no evidence that
it is limited by a fault at the base.

Willis then cites two places at which von Richthofen crossed the Khingan
line—one in latitude 33°, the other in latitude 38°. Von Richthofen found a
simple flexure at the first, but at the more northerly crossing he found that
the descent to the plains was made by a series of step-faults with the down-
throw on the east. Willis points out that the moderate faulting in this case
“is not inconsistent with the warping observed elsewhere,’’ and concludes
that the weight of evidence favors the view that the Khingan line is to be
regarded ‘‘as a zone of monoclinical flexure, not as a fault’’ (1905, page 7).

Summary of the eastern boundary

From the foregoing facts, we may conclude that the history of the Khin-
gan range has been essentially as follows: An ancient mountain range com-
posed of folded schists and igneous intrusives was wholly planed away by ero-
sion before the uplift of the present Khingan. The new range is due to simple
warping or arching of the peneplaned rocks, which are similar to those of the
oldrock floor of the Gobi. The lava fields along the Khingan were formed by
volcanic outpourings which accompanied the growth of the present range
during Tertiary and Pleistocene time.

THE SOUTHERN BOUNDARY

The southern boundary of the Gobi is complex, and locally difficult to
define. In the east it is the edge of a great dissected plateau. The central
section in the Ordos is indefinite. Here the Yellow River makes a great loop
toward the north, east, and south, encircling the Ordos platform: Whether
the Gobi basin should include this block, or whether the boundary should be
placed along the hills north of the Yellow River is an open question. On the

BOUNDARIES OF THE GOBI REGION 31

whole, however, it seems advisable not to separate the two regions, because
in the later history of basin making, they have behaved essentially as a unit.
West of the Ordos, the boundary is to be placed along the Nan Shan and Richt-
hofen ranges. There are thus three sections of the southern boundary—the
eastern section, composed of a plateau edge, largely capped with lava flows;
the central section, composed of hills that swing around the southern end of
the Ordos; and the western section, composed of great mountain chains.

Southern boundary—eastern section

The southern boundary of the Gobi in the region of Kalgan is formed by
the edge of uptilted Tertiary lavas (Figs. 1, page 8, and 5, page 47). The
margin of the lavas presents an imposing scarp, descending abruptly into China
on the south, and sloping very gently away into Mongolia on the north, so
that it forms another “of the great steps by which Central Asia descends to
the sea,’’ as Mushketov so aptly said of the Khingan. The scarp at Kalgan
does not mark a fault, but is simply an erosion scarp. The only faults we
have been able to trace in this region are downthrown toward the north, that
is, toward Mongolia (Fig. 5, page 47). The scarp, therefore, indicates a truly
impressive retreat, through erosion, of lava sheets that once must have cov-
ered a far greater area than at present.

The lavas overlie a thick formation of conglomerate, sand, and sandy
clay in which dinosaur bones have been found. There is apparently a slight
but significant angular unconformity between the sandstones below and the
lavas above. The lavas dip northward at a gentler angle than do the sand-
stones. Andersson (1923 a, page 99) and Barbour (1924,) have reported the
presence of plant beds, believed to indicate Lower Oligocene age, between the
lava sheets.

According to von Richthofen’s map, the lavas continue to the westward
for fifty miles to about Lat. 40° N., Long. 114° E. From this point (Plate IV)
we must again follow Suess’s summary for the general description of the region
(1902, III, pages 255-266). Prjevalski observed limestone hills called
Khara Khada, or Shara Khada, which appear from underneath the lava
cover in about Lat. 41° 30’ N., Long. 113° E., and form a range that strikes
southwestward toward the Yellow River. Two other ranges, the In Shan and
Muni Ola, continue the general line of the Shara Khada along the northern
side of the Yellow River valley. The ranges are not in one line, but overlap
one another (Plate IV). A similar chain of ranges lies farther to the north
and includes the Suma Khada, Sheiten Ola and Narin Ola ranges. They are
all formed of short overlapping ranges, and all are of ancient rocks—dominantly
schists, gneisses, and crystalline limestones, cut by large bodies of granite.
Suess remarks (1902, III, page 257) that the strike of these old disturbed rocks

32 GEOLOGY OF MONGOLIA

does not agree with the present strike of the ranges. Probably the ranges
follow Tertiary or post-Tertiary fault lines.

Southern boundary—central section

The Khara, Narin, Argilintai, and, farther south, the Ala Shan Mountains
bend like a great framework around the corner of the rectangular course of
the Yellow River. The eastern side of this frame is made by the fault-block
mountains of Shansi. Within the mountain-framed bend of the big river lies
the semidesert land of the Ordos. It is a platform of low relief which the
Comte de Lesdain says has sunk somewhat and now lies at about 1500-1600
meters, without high mountains or deep valleys (1908). It is bounded rather
indefinitely on the south by hills from which flow a number of tributaries to
the Yellow River and the Wei Ho.

The Cambrian limestones in the Ordos are still nearly horizontal, indicat-
ing that the region is part of a very stable block of the continent,—a plateau
land very like the Angara element in central Siberia (Fig. 127, page 295).

Suess points out the strong contrast between this ancient plateau struc-
ture and the complexly folded oldrock floor of the Gobi. He adds (1902, III,
page 258): ‘‘The difference between the Gobi and the Ordos could not be
more complete. In the contrast between the ancient folds of the Altaides and
their foreland, the Ordos, we find a parallel to the contrast between the folded
Alps and their foreland.’’ However, Suess has compared only the folded and
the non-folded units of the oldrock floor; the contrast does not extend to the
younger sediments that overlie this floor. Since the time when the latest
folded mountains of Mongolia were peneplaned, Ordosia has been broken up
into a series of elevated and depressed blocks, and the Ordos proper has shared
the warping and inland deposition of the Gobi. In the Gobi, the oldest sedi-
ments resting upon the peneplaned floor of complex rocks are of Lower Creta-
ceous age. The peneplaned oldrock floor of the Ordos is similarly covered by
inland basin sediments, the oldest of which, according to W. H. Wong, are
now believed to be Cretaceous (1924, personal communication). Despite
the warping that has caused the great detour of the Yellow River toward the
north, around the margin of the Ordos platform, it seems fair to reckon the
Ordos as essentially a subprovince of the Gobi.

Southern boundary—western section

The southern boundary of the Ala Shan desert, which lies west of the
Ordos, is formed by the Nan Shan ranges. In describing the relations of the
Nan Shan to the Gobi basin, Suess (1902, III, page 223) draws his data chiefly
from Obruchev’s work (1900, page 581). The Nan Shan is a great and com-
plex mountain range, striking a little south of east, and is cut by a series of

BOUNDARIES OF THE GOBI REGION 33

longitudinal faults. North of it lies a broad lowland bounded by the Lung
Shan, often called Pei Shan, which means North Mountains, in contrast to
the South Mountains or Nan Shan. The intermontane depression which
forms the lowland of Kansu, is watered chiefly by streams from the Nan Shan
and Richthofen ranges. Most of these streams cross the depression at right
angles to its trend, and pass through gaps in the Lung Shan into the desert
beyond; only one runs parallel to the trend, collecting affluents from the
mountains, and then passes through a gap in the Lung Shan to become the
Edsin Gol in the Gobi. Their courses lie upon basin sediments in the low-
lands, and upon the complex rocks of the ancient floor in the mountains. A
critical study of the relations of the rivers, the basin sediments, and the old-
rock floor, would yield a rich harvest of discovery concerning the structure
and age of this part of the Gobi basin.

Obruchev’s cross-sections of the Lung Shan (1900, I, page 553) show
clearly that the rocks have been folded in one revolution, and later have been
faulted by quite another series of earth movements. The folding took place
after the Carboniferous period, as Carboniferous strata are involved in the
folded structures. Three of the four faults are represented in the section as
downthrown on the north side, indicating that the depressed region lay north
of the range, where the Gobi lies. Along the north flank of the Lung Shan,
later sediments of the inland Gobi basin are tilted up, indicating that part, or
all, of the uplifting of the Lung Shan is of later date than the deposition of the
sediments. Similarly in the Nan Shan and Richthofen ranges, the basin
sediments have been displaced, according to Obruchev; therefore, in part at
least, the uplift must be younger than the basin sediments.

Briefly summarizing these facts and the inferences they support, the Nan
Shan and Lung Shan owe their present height to warping and faulting, not
at all to folding. Here, as elsewhere throughout the Gobi region, the ancient
folded mountains were eroded away before the basin sediments were deposited.
The modern uplift is later than some at least of the basin sediments.

THE WESTERN BOUNDARY

On the west, the boundary is still more difficult to define, because we pass
farther into the vast undrained interior of Asia, and must choose a boundary
between basin and basin, not between inland basin and continental slopes
that drain to the sea. Two such enclosed lowlands open from the Gobi,—
the Tarim or Lop Nor on the southwest, and Dzungaria on the west.

Between the Gobi and each of these great basins there is a lowland gate-
way which probably is a peneplaned agd warped divide, not unlike the broad
low saddles that separate the minor basins within the Gobi. Between the

VOL. II-—-3

34 GEOLOGY OF MONGOLIA

Dzungarian gateway and that of Lop Nor, the Tien Shan mountain system
thrusts far eastward, dying out at about the 1ooth meridian. The two gate-
ways can hardly be considered as boundaries of the Gobi. They are, indeed,
merely the connecting links between subdivisions of one great physiographic
unit. It seems logical to include the mountain-circled basins of Dzungaria
and Lop Nor and the Gobi in one central Asiatic basin province.

The Tien Shan is one of the largest mountain systems of Asia, with a total
length of about thirteen hundred miles, and a maximum breadth of nearly two
hundred miles. The range Bogdo Ola, east of Urumchi between latitude 88°
and 92°, is gedlogically the best known part of the Tien Shan. Here Merz-
bacher (1916), Groeber (1914), Machatschek (1918), Friederichsen (1899,
1904, and 1919), and others have conducted careful studies. We briefly
summarize the work of these authorities. The Bogdo Ola rises from the
southern border of the Dzungarian lowland to an altitude of nearly twenty
thousand feet, above which three splendid peaks rise to 21,330, 21,370, and
20,690 feet. From the lowest peak the western part of the chain drops
abruptly in sheer cliffs nearly ten thousand feet, to a depression that sepa-
rates the lower western part of the chain from the high central part (Friede-
richsen, 1919). On the south the range sinks abruptly to a long desert
depression, lying about 4,200 feet above sea level. Beyond this lowland
is a minor range called the Dyjargos, the southern front of which drops
sheer into the Turfan lowland, whose floor is 720 feet below sea level. On
the north, the first and lowest part of the range consists of Jurassic conglo-
merates, sandstones, shales, and marls, cut by younger igneous intrusive
rocks: this series is eroded to a peneplane, and downthrown along a fault
(Friederichsen, 1919). South of the Jurassic belt is a series of more or less
strongly metamorphosed slates, quartzites, graywackes, breccias, hornfels,
and altered eruptive rocks. Groeber (1909, 1914) considers these to range
from Lower Carboniferous to Permian, but fossils are lacking. The high cen-
tral range includes older rocks that rise in a monstrous scarp, three thousand
meters above the northern parts of the range, harboring splendid hanging gla-
ciers. The disturbed Jurassic beds and the folded and metamorphosed ser-
ies which Groeber has called Paleozoic are dislocated along fault lines which
cut and displace the folded structures. Therefore the folding must have been
wholly completed before the growth of the present faulted mountains began.
Such a history is very similar to that of the eastern Altai as determined by
the Third Asiatic Expedition. It also agrees with the masterly summing up
of this range by Obruchev, who concludes (1915, page 321):

But all later observations on the TiengShan system, as well as the Russian Altai,
compel us to doubt seriously the conclusions of Suess as to the simultaneity of the

BOUNDARIES OF THE GOBI REGION 35

folding and faulting movements in central Asia . . . Large mountain-making move-
ments, creating entire ranges of bow-shaped folds with their convexity turned to-
ward the south, and extending from the Russian Altai to the Tien Shan, were com-
pleted at the very close of the Palaeozoic or at the beginning of the Mesozoic. Then
disjunctive movements began creating numerous faults . . . which appeared with
special vigor at the end of the Mesozoic or in the beginning of the Tertiary period.
The lines of rupture . . . diverged . . . from the direction of the Paleozoic folds
much oftener than they coincided with that direction. By these two diagnostic
evidences—diversity of the time of occurrence and lack of agreement in strike—the
movements of folding and of disjunctive dislocation prove to be separated one from
another; and it seems hardly possible, as Suess proposes, to reckon the dislocations
as a supplement to the plication-movements.

This analysis constitutes a definite advance from the view propounded by
Suess, in that Obruchev has recognized a period of mountain making charac-
terized by fault movements, as distinct from the more ancient revolution
which was characterized by folding of the rocks.

The correctness of Obruchev’s general conclusions, as far as he may have
intended them to apply to the Gobi region, is thus confirmed by our more
recent studies, and the fact that our analysis of the region has brought out
many additional details rather enhances than diminishes the value of his
pioneer work. Our own results agree very well with this sequence of events.
We go farther, however, in recognizing the virtual peneplanation of the folded
Jurassic series, and the development of the Han Hai in entirely new depres-
sions, formed by the warping of the post-Jurassic peneplane. We also recog-
nize (1924, b) that the Han Hai deposits began earlier than Neogene, in Lower
Cretaceous time, that they are of diverse origin, and that they are not by any
means the simple filling of large lakes. We believe that warping and fault-
ing took place intermittently through the time occupied by the deposition of
the basin sediments, and that the relief formed by faulting was more than once
peneplaned. Although movements went on intermittently throughout the
Cretaceous period and the Cenozoic era, the chief mountain-making periods.
were probably the Pliocene and the Pleistocene.

The following facts and inferences have been developed in this discussion
of the Gobi borders:

1. Warped borders form the lip of the great basin on the Khangai,.
Kentai, and Khingan fronts.

2. The border on the Kerulen River front is probably a compound fault--
and-warp.

3. On the Tannu Ola and Nan Shan fronts, the wall of the basin is a.
fault-scarp.

4. Along the Ala Shan, Yellow River, and In Shan front the borders are:

36 GEOLOGY OF MONGOLIA

faulted, if we agree with Suess in classing the Ordos as a region quite apart
from the Gobi, but if we include the Ordos as part of the Gobi, the boundary
is the warped southern margin of the Ordos.

5. The basins of Dzungaria and Lop Nor, lying west and southwest of
the Gobi, are virtually continuous with it and are connected with it by low
warped gateways.

6. The structure lines of the Gobi are quite in accord with those of the
surrounding mountains. The oldrock floor of the Gobi is composed of rocks
essentially the same as those of the surrounding lands. In the ancient dis-
turbed rocks and in their structure lines, there is, therefore, as Suess has justly
remarked, no boundary between the Gobi and the surrounding regions. Nev-
ertheless there is traceable a tectonic boundary, formed of later warps and
faults, which marks out the Gobi as an inland basin; and the movements that
have determined this basin character are not older than post-Jurassic. They
seem on the whole to have culminated in the late Tertiary and Pleistocene.

| ROUTE STUDIES OR ITINERARY _

“sy PM Pegi ely ways
ORD 1S OE em

tke

PART II—ROUTE STUDIES OR ITINERARY

INTRODUCTION

CHAPTER |] III—FROM KALGAN TO IREN DABASU

CHAPTER IV—FROM IREN DABASU TO URGA

CHAPTER V—FROM URGA TO TSETSENWAN

CHAPTER VI—FROM TSETSENWAN TO SAIN NOIN KHAN AND THE ARCTIC DIVIDE
CHAPTER VII—FROM THE HOT SPRINGS OF SAIN NOIN TO BAGA BOGDO OF THE ALTAI
CHAPTER VIII—THE RETURN JOURNEY FROM TSAGAN NOR TO ARTSA BOGDO

CHAPTER IX—FROM ARTSA BOGDO TO SAIR USU

CHAPTER X—FROM SAIR USU TO KALGAN

38

INTRODUCTION
THE TASK OF A GEOLOGICAL RECONNAISSANCE EXPEDITION

It is of prime importance for a geological expedition to keep in close inter-
pretative touch with the strata or other rock formations along the route of
travel, for the resulting information forms the basis of such additional explora-
tory investigations as the country seems to warrant. This further study may
resolve itself into acquiring collections of fossils for palaontologic purposes, or
into a search for important fossil fields, with a view to their future exploita-
tion. It may strive to determine the ages of the strata and to read something
of their geologic history, to locate valuable mineral deposits, to solve the
detail of local geologic structure, or to interpret more clearly the meaning of
the surface features of the country.

Important finds in any field, of course, may be made by accident, but no
scientist expects to reach his larger discoveries other than by careful and
systematic work. In any case, he tries to weld his observations into some
form of reconnaissance interpretation. A mineral deposit, a fossil field, or a
surface feature has a good reason for being where it is, and it acquires added
significance because of its surroundings and history. It may not be a geo-
logic achievement to discover such a feature, but it is a geologic problem to
explain it, and to use such factors from the record as may lead to other dis-
coveries. Without such interpretation most finds have little more signifi-
cance than the isolated specimens of a museum, whereas, when properly con-
nected, they not only constitute a definite chapter in local geologic history,
but may lead to still more important finds which would otherwise escape
observation.

Fossil bones had previously been found in central Asia, but they meant
little or nothing to the people who found them—perhaps they were ‘‘ dragon
bones,” good for medicine or magic! An occasional fragment was carried
away from the desert and reached hands of greater competence, but without
corresponding field observations and opportunity to connect with the geologic
story, they have added little to an understanding of this great continental
area.

39

40 GEOLOGY OF MONGOLIA

Such considerations explain in a measure why the Third Asiatic Expedi-
tion has been successful. To the chances of accidental discovery common to
all expeditions was added the result of scientifically directed search based on
a step-by-step field interpretation of the geology; to the large collections of
fossil material were added the determination of the structural setting and of
stratigraphic relations, and the evidence of origin and subsequent changes
belonging to the same deposits. Thus the strata of the desert basins have
been subdivided into formations having a reasonably definite age and history.
Their relations, one to another, have been traced structurally as well as by
their fossil content. The detail of the geologic column has been extended
beyond anything that had previously been attempted in central Asia, the
history has taken on a more definite form, and now the region can be com-
pared more satisfactorily with other regions of the earth.

This is not the first time, of course, that the sedimentary formations of
the Desert of Gobi have been described. Von Richthofen, in the winter of
1871-1872, entered the southern part of the basin, where, it appears from his
description (1882), similar geologic conditions prevail. He called the sedi-
ments the ‘‘Han Hai beds” (1877, I, page 25). Obruchev (1900, I, page 69)
covered a portion of the same route in 1892 and gave the name ‘‘Gobi Series”’
to all these superficial sedimentary deposits. Neither scientist subdivided the
series or determined their age with any considerable accuracy. Neither seems
to have suspected their complexity of make-up, and neither unraveled the
structural or the palzontologic story. One of the accomplishments of the
Third Asiatic Expedition was to subdivide, identify, and correlate the members
that make up the desert basin sediments, to discover their structural relations,
and to collect their fossil content. We now know that they form a complex of
several distinct groups of strata, representing an immensely long period of
geologic time, from the Lower Cretaceous to the present. Nearly every
period is represented in the Gobi, and no fewer than fifteen separate identifi-
able formations of historical and paleontological significance have been added
to the geologic column. The identification of these members, showing that
definite periods of Mesozoic and Tertiary age are represented and that they
contain fossils both characteristic and rare, is the most fundamental strati-
graphic contribution of the Expedition.

In this way the Gobi series of Obruchev has been subdivided and has
taken on a more definite meaning, expanding its history to include a much
greater range of the geologic column than previous investigators had assigned
to it. The limits of some of the formations have been drawn, general terms
have been replaced by others of more specific time value, and the several mem-
bers have been given a very different significance in the geologic history of the
region. For example, we find that three sedimentary formations—the Ondai

INTRODUCTION 41

Sair, the Hsanda Gol, and the Hung Kureh—which are found in the district
immediately north of the Baga Bogdo range of the Altai Mountains, and which
together represent the Gobi series, are of widely different age from one an-
other. In the first, there are dinosaurs of Lower Cretaceous type; in the sec-
ond, mammals of mid-Tertiary type; and in the last, mammals of a time
just preceding the Ice Age. At Iren Dabasu, at Djadokhta, at Oshih, as well
as at many other localities, still different groupings occur, dependent on the
shifting conditions of deposition and denudation that accompanied the suc-
cessive warpings of the Gobi floor.

At most places the ‘‘Gobi Series” is not a series at all, but remnants of
several different series separated by significant breaks in the record. Some
of these gaps correspond to deposits which are still to be found in adjacent
territory. The whole list of distinguishable constituent members together
makes a formidable array, covering a long series of events and a long succes-
sion of life forms, and adds a big chapter to the story of central Mongolia.

Whether such a reconnaissance deserves to be regarded as sufficiently
complete to warrant publication, is a fair question. It is at best a hurried,
preliminary study with the object of determining major features as a guide
for later, more detailed work of many kinds. Its value, of course, may be
measured by its adequacy in meeting the demands for which it was organ-
ized. A geological reconnaissance which has solved the formational struc-
ture, outlined the major historical events, established their sequence, and
assembled the criteria in such a way that they can be used successfully, has
perhaps a right to be considered a unit of investigation. If, in addition, these
criteria have already been used to advantage in the progress of the field work
in a region which bids fair to become a famous source of scientific returns,
there may be good reason to put these reconnaissance results into permanent
form for unrestricted use.

RECORDING OBSERVATIONS

It is not easy to record in usable form the thousands of observations made in
so extended a traverse as that of the Third Asiatic Expedition in its reconnais-
sance of 1922 and 1923 in central Mongolia. Unless they are organized they
become too unwieldy to be of much value beyond that already served in the
progress of the reconnaissance itself. Probably the simplest method is to
avoid printing the record, and to give instead only the conclusions and inter-
pretations reached. It is evident, however, that this alternative is open to
serious objection, particularly because the work has had to be done too rapidly
for uniformly reliable results. It is only fair, therefore, that the major facts
of observation should be given without involving them in much interpretation

42 GEOLOGY OF MONGOLIA

beyond the simplest inferences, wherein there is little likelihood of error. It
is our wish to make these observations serviceable to others, even though they
may lead to a different interpretation, and to this end we have chosen to record
the great bulk of elementary data in an itinerary account.

a

Ficure 3.—Profile and section illustrating methods of recording observations. The figures below the cross
section are the altitudes in feet, as determined by barometer readings. The figures above are miles, representing
the distance traveled from Kalgan. The profile of the section is constructed to scale, both horizontally and
vertically. The bedding of all stratified rocks is drawn in approximately correct position, recording folding,
tilting or horizontal structure. The igneous rocks are indicated by symbols suggesting crystalline structure.
The conventional rock symbols are set forth in Plates VI and VII. In the succeeding sections, beginning with
Figure 5, only a few of the altitude readings are entered, though all were used in constructing the profile; and
the mileages are represented by a graphic scale.

1 460 9 8 7 6 5 454
MILES FROM TSAGAN-NOR
<—— TRAVELLING S40*E

it

We believe that this account will be more interesting and more readily
used if it is in part narrative and descriptive, and in part graphic, represented
by profiles and cross-sections running parallel with the text (Fig. 3, and Figs.
5-97). We are well aware that this is an unusual method—one which, as
far as we know, has never been followed by a reconnaissance expedition. We
are mindful also that to some degree the record thus takes on certain elements
of interpretation, in spite of the wish to avoid it; but only such interpretation
has been used as that attached to field structure, which is represented in this
volume by geologic cross-sections. If presented, therefore, with the explana-
tion that much of the work has been done under rapid movement, requiring
hurried observations, and that this graphic method was found in the field to
be the most practicable way of recording the mass of data gathered, it will
probably serve a useful purpose, and should not lead to any misconception of
its value as compared with those special studies where more careful work could
be done. It is possible by this means to follow the route mile by mile, and to
find the profile, the geologic structure, and the specific rock formation at any
point along the trail without difficulty (Fig. 5, et seq.). Thus the route sec-
tion becomes a base from which additional investigations can be projected.

Wherever we were fortunate enough to make many observations, we are
confident that the suggested determinations and structures are correct. Wher-
ever the observations are particularly scattered or scanty, and where there is

INTRODUCTION 43

obviously greater doubt about the structural interpretation, we have at-
tempted to indicate that difference of reliability in the accompanying descrip-
tion. Yet rather than leave such stretches blank, we have suggested the
underground structure which the evidence best supports, believing that even
this is a step toward a better understanding of this very little known region.
Wherever the field notes are sufficiently complete, we have used them to
construct not only the profile and cross-section of the traverse, but a route
map on which it is possible to indicate more or less of the immediate side coun-
try and guide points.

The scales of the itinerary diagrams are uniform, each covering thirty
miles, although special cross-sections are constructed on a larger scale. In

54°

uf
UF tb
LA
PPENING 1500
SSS

RES Vea “A

Ficure 4.—General location and route map. Mountainous areas are shaded in slanting lines.
Lowlands are white, and the deeper depressions are stippled. Lakes are colored black. The routes of
the Expedition in 1922 and 1923 are drawn in broken lines. Index letters along the routes indicate the
following places: K (northwest of Peking) Kalgan; P K, P’ang Kiang; I M, Irdin Manha; I D, Iren
Dabasu; C J, Camp Jurassic; T W, Tsetsenwan; O S, Ondai Sair; O, Oshih; G S, Gurbun Saikhan; D,
Djadokhta; A O, Ardyn Obo; S M, Shara Murun; B U, Boltai Urtu; U (south of the K of Khangai),
Uliassutai; K (north of the A of Altai), Kobdo. The Ubsa, Kirghiz and Kara basins constitute Pievtsov’s
“Valley of the Lakes.”

order to avoid possible confusion from this cause, we have taken pains to carry
a scale of miles on every sheet, indicating not only the scale of the drawing
but also the distance from the starting point of the traverse.

The running descriptive account which accompanies the continuous pro-
file and cross-section covers the major points of interest in the corresponding
portion of the itinerary (Fig. 4 and Plate IV). Of course, the text and its
corresponding illustration do not in all cases, fall on opposite or on following

44 GEOLOGY OF MONGOLIA

pages but the description always follows the same order as the cross-section
and forms a continuous story of observations. In large part, these observa-
tions constitute the basis of such conclusions as the Expedition has been able
to formulate concerning the geology of Mongolia.

But the data are not wholly confined to the route traverses. Where an
area was found which combined unusual opportunities for all the scientists
of the Expedition, time for longer study was given, and more detailed obser-
vations were made. These studies form the basis of contributions included in
Part III of this Report. Some of the special and local studies covered par-
ticularly critical spots, and some of them have been examined in as great detail
as is ordinarily done in countries much more highly favored with geological
service. Details of stratigraphy, structure, and process were worked out in
certain areas, which led to extensive correction of the geologic conclusions
based on the reconnaissance traverses alone.

The itinerary observations, together with the special and local studies,
have been reorganized, therefore, in the later discussion of the larger geologic
problems. The assembled material, representing the summaries and tenta-
tive conclusions of the geological section of the Expedition, constitute Part
IV of this Report.

,
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PLATE VI.

CONVENTIONAL SYMBOLS
USED FOR THE ROCK FORMATIONS OF
THE SERIES OF STRUCTURE SECTIONS

CHAPTERS Il TO X INCLUSIVE

SIMPLE LATER SEDIMENTS

OF THE INLAND BASINS
CENOZOIC
PLIOCENE
ie od Hung Kureh and Ertemte
PROBABLY MIOCENE

P’ang Kiang

OLIGOCENE
=| Hsanda Gol

aaa Houldjin
Vt «

PALEOCENE
BEES Glehcto
MESOZOIC
CRETACEOUS
Iren Dabasu

Djadokhta

4 Oshih and Ondai Sair

PERIOD UNDETERMINED

DEFORMED SEDIMENTS
OF THE OLDROCK FLOOR

MESOZOIC
PROBABLY JURASSIC

Tsetsenwan
Series

Sandstones
Conglomerates

PALA-OZOIC
CARBONIFEROUS AND PERMIAN

Jisu Honguer
and
Sair Usu

| Sandstone Series

PROTEROZOIC
LIMITS OF THE SERIES UNCERTAIN
RHR | -
INS Limestone

SIRENS
ASHES i
SASS Cherty Limestone

NN Argillite
N i\\ Slate

Quartzite

Khangai

Series

Jasper

Graywacke

PLATE VII.

CONVENTIONAL SYMBOLS
USED FOR THE ROCK FORMATIONS OF
The SERIES OF STRUCTURE SECTIONS
CHAPTERS Tl 10 xX INGLUSIVE
(CONTINUED)
EARLY PROTEROZOIC IGNEOUS ROCKS
THE WU TAI SYSTEM INTRUSIVE PORPHYRIES
BELONGING TO THE OLDROCK FLOOR
Sy Dolomites

“3 Crystalline Limestones : in Granite Porphyries

\ Quartzites
\ Phyllites

Schists

Trachyte Porphyries
Andesite Porphyries

Basalt Porphyries

ARCHA-OZOIC
T’Al SHAN COMPLEX

, : MASSIVE IGNEOUS ROCKS
Crystalline Limestones CHIEFLY BELONGING TO THE
MONGOLIAN BATHYLITH

Injected Schists : “| Granites
Gneisses sonnei! Syenites

Diorites

IGNEOUS ROCKS

VOLCANIC FLOWS mya Cebbros
ASSOCIATED WITH THE LATER SEDIMENTS

Rhyolites
Trachytes

+= Andesites BASIC IGNEOUS ROCKS
PROFOUNDLY ALTERED

Basalts ‘s,0-| Serpentine

CHAPTER III
FROM KALGAN TO IREN DABASU

THE ground on which Kalgan stands is alluvium, or flood-plain deposit, of
the river Yang Ho, which periodically overflows its banks and spreads destruc-
tion along its course; but the rugged hills above Kalgan, at the entrance to
the Wan Ch’uan Hsien Pass, are made up chiefly of igneous rocks (Figs. 1
and 5).

KALGAN TO WAN CH’UAN PASS

For the first few miles the traverse was made over a low spur on the side
of the valley, where the roadway is cut down fifteen or twenty feet into vol-
canic tuffs (Specimen 6, A.M. 9695) and associated lavas (Specimen 7, A.M.
9696), which together form the foundation rock. A small tributary gulch
just above this spur is partially filled with loess, which exhibits the character-
istic habit of this material. Nowhere else on the whole summer’s traverse
was typical loess again encountered. Apparently it is entirely a border accu-
mulation and not at all characteristic of the interior desert region.

The lower pass

The igneous formations belonging to the lower half of the Wan Ch’uan
Hsien Pass are in peculiarly fresh condition except on the lower portions of the
valley sides, where long exposure to weathering has led to the usual decay,
and it is here only that the rocks of this formation show enough weakness to
yield noticeably to the wear of travel. In the course of centuries the wear of
wheels and the tramp of caravans have cut veritable miniature canyons into
the rock. The types of rock include rhyolites, trachytes, trachy-andesites,
porphyritic felsites with corresponding porphyries, and a great variety of
tuffs. Structurally they present great confusion, and leave one in uncertainty
as to their intimate relation. Some of them are clearly lava flows and surface
tuff accumulations in which there is a crude structure that gives locally some
clue to original conditions, but for the most part the structure is obscure.

45

46 GEOLOGY OF MONGOLIA

It is evident that intrusions break through the simpler surface flows, not only
cutting off the original continuations, but otherwise disturbing the earlier
members.

The variety of rock observed is much greater than this list of types would
suggest. A very great range is represented, the like of which had not been
seen elsewhere in our traverse, and was not to be encountered again for sev-
eral hundred miles. The whole complex, however, has a definite character,
due to the prevalence of felsitic and fine-grained porphyritic types of rock, to
the obscurity of their structural relations, and to their habit of breaking into
small angular bits, so that it has been possible to identify the same forma-
tional conditions at many other places in the course of our work in central
Mongolia. The igneous eruptive complex continues halfway through the pass
—indeed, through the whole of the narrow portion of about five miles. Be-
yond it the valley opens out considerably, and although traveling is difficult,
the gorge form disappears.

We found no clue to the age of the igneous rocks exposed in the lower
portion of the Pass, except the fact that they are not metamorphosed and not
intimately deformed, although much broken. The deformational habit,
coupled with the peculiarities of rock type and structure, belongs, as we after-
wards learned, to great igneous eruptive complexes associated with strata
which we judged to be of Jurassic age. At other places, similar porphyries
underlie the Lower Cretaceous sediments, a fact which tends to support the
inference of the Jurassic age of the porphyries.

A very rugged topography is carved on this complex and forms a frown-
ing, jagged barrier rising to one thousand feet and more in the immediate
hills about the city (Fig. 5). It is surmounted by a deeply dissected escarp-
ment, rising two thousand feet higher to the top of the Pass and extending to
the edge of the Mongolian plateau.

Wan Ch’uan basin and the upper pass

Where the road emerges from the narrow gorge into the open portion con-
stituting the upper half of the Pass, the igneous complex is terminated abruptly
against a fault. Higher ground lies beyond, but its slopes are smoother,
because the underlying rocks are of a simpler and less resistant type. They
are sediments that lie nearly flat, dipping only slightly northward toward the
interior of the continent. Curiously enough, they are dragged abruptly
upward at the fault margin, indicating that the ground upon which we have
just entered, despite its greater present surface elevation, belongs in reality
to a down-dropped block rather than to an uplifted one. Above the dis-
sected edge of the escarpment lies the Mongolian plateau.

Stream erosion has carved the usual complexity of a dissected surface, but

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47

48 GEOLOGY OF MONGOLIA

for two or three miles beyond the fault the topography is comparatively
smooth, and the trail rises gently. Beyond this, the climb is more difficult,
and the last three miles are as steep as can be traversed successfully with
motor equipment such as that of the Third Asiatic Expedition. Thus one
mounts, over the edges of a great series of strata, to the top of the Pass.

Two features of this second portion of the Wan Ch’uan Hsien Pass deserve
special note:

(a) The double erosion form noted on the dissected escarpment, and

(b) The occurrence of volcanic outbreaks and intrusions.

Midway in the Pass, the sediments bear traces of an ancient, smoothly
rounded topography, which was developed by long continued erosion during
a preceding epoch (Plate XLI, page 389). In the present epoch these slopes
have been sharply trenched by new, steep-sided, precipitous gullies, which
give a peculiar double form to the hillsides. Whatever the cause, there has
been a rejuvenation, and local erosion is proceeding far more rapidly at the
present time than in the preceding cycle. This is probably due to changed
climatic conditions—apparently to greater aridity. When rainfall is heavy,
a thick mat of vegetation protects the soil so that rainwash tends to produce
a gently rolling surface. At the present time the slopes are poorly protected
by vegetation, and the attack of running water meets with little resistance.
This explanation appears to be more satisfactory than that of change of local
base-level, or greater rainfall, or even that commonest of explanations, defor-
estation by men. Climatic change as an explanation is consistent with many
evidences seen in other regions on this extended traverse. (See Chapter XX1I.)

At many points in the upper half of the sedimentary series in this stretch
of ground there are butte-like hills and unexpectedly rugged spots, which, on
inspection, prove to be due to igneous outbreaks. Some of them are necks
and plugs and irregular, projecting intrusions; others are dikes and sills; still
others are local induration effects produced by escaping gases. All together
they represent igneous activity on a considerable scale. At no place is there
a very extensive modification of the surrounding strata by the invading mate-
rial, but locally the structure is disturbed. The most marked feature is an
abrupt change of rock type, introducing a great difference in erosion effect.
The outbreaks are mainly basalts; some are vesicular, and doubtless volcanic
ashes and tuffs were also furnished, which were distributed with the sediments.
A thick series of basalt flows forms a cap which constitutes the surface forma-
tion inland for many miles.

Here, at the very top, stands the ancient wall that divides China and
Mongolia. Here, also, we could see that we had attained, not the top of a
mountain range, but the outer edge of a great plateau, a portion of which
has been carried away by erosion, while the ground remaining constitutes the

PLATE VIII.

A. VIEW FROM THE HEAD OF THE WAN CHU'AN PASS.

Valley carved in the peneplaned basalts on the inland basin side of the Pacific divide beyond the Wan Ch’uan pass.

B. CULTIVATED FIELDS OF CHINESE FARMERS.

Grain fields of the Chinese settlers developing the agricultural possibilities of the grassland belt of southern Mongolia.

PLATE IX.

A. THE UPLAND AS SEEN FROM THE SECOND CAMP.

Upland of the oldrock hills preserving remnants of the dissected Mongolian peneplane in the Chakhar district of southern
Mongolia.

B. CAMP IN THE GRANITE HILLS OF CHAKHAR,

The photograph, Plate IX A, was taken from the top of this hill.

FROM KALGAN TO IREN DABASU 49

rugged topography and steep ascent of the Wan Ch’uan Hsien Pass from the
valleys below—a distance of twenty miles. In that distance the Expedition
had climbed nearly three thousand feet and had traversed the splendidly
exposed representatives of two great geologic formations,—in the first ten miles
the igneous porphyry complex, and in the last ten miles a great thickness of
sedimentary strata, having the appearance characteristic of formations belong-
ing to the later periods of geologic time.

The sediments of the younger formation have not been much disturbed
or reorganized since the strata were laid down, and present all the evidences of
rapid accumulation by water currents. Sediments of this kind are now usu-
ally considered to be of continental origin. They consist of conglomerates,
sands, and clayey sands—varicolored and rapidly changing in quality and
color both vertically and laterally. Bedding is prominent, but differences of
resistance to erosion in successive beds are not great enough to have much effect
on the topography.

This was our introduction to the simple sediments of the interior coun-
try. Such strata ought to carry fossils. Strata of like appearance, found in
the interior, yielded abundant returns. On the outward journey little atten-
tion could be given to this stretch of ground and a hurried inspection at two
or three points yielded nothing. On the homeward trip, however, fragments
of reptile bones were found close beside the trail. In structure and dynamic
history the Wan Ch’uan formation resembles some of the Cretaceous and early
Tertiary formations seen elsewhere in Mongolia, and the fossil evidence favors
this view, although an exact determination has not yet been made.

THE PACIFIC DIVIDE TO P’ANG KIANG

The edge of the plateau reached by the Wan Ch’uan Hsien Pass, twenty
miles above Kalgan, is a true continental divide. From that point toward
the interior the average elevation gradually decreases, and one enters the great
basin occupied by the Desert of Gobi (Plate VIII, A). While the elevation
at the divide is more than five thousand feet above the sea, at the center of
the basin, three hundred miles away, it is less than three thousand feet. From
the top of the Pass the streams turn inland, and for nearly six hundred miles
the drainage is toward the interior.

For the most part the country is either smoothly rolling or nearly flat
(Plate VIII, B). The surface features are all erosion forms governed by the
underground rock structure. We soon learned that the topography is related
to the deformational history of the region, its faultings and warpings, its igne-
ous outbreaks, and its sedimentary accumulations. The origin of the land
forms is genetically connected with the physiographic history that goes back

VOL. II—4

50 GEOLOGY OF MONGOLIA

to the time when this region first became an inland basin. Traces of a com-
paratively ancient erosion are preserved on some masses of the hard rock,
whereas features which were developed at a much later time appear on sedi-
ments resting upon the oldrock floor. The topography therefore ties up not
only with the underground structure but with the dynamic history, and as
the relations of these forms come to be better understood, they furnish the
first, and in some respects the best, aid in unraveling the geological relations
in rapid reconnaissance.

For the first twenty miles after crossing the divide, the rolling basalt
hills continue, with enough exposures to determine the character of the form-
ations below (Fig. 6). Weathering and decay have formed a soil that has
obscured most of the rock floor, and these processes, together with the stream
wash, have produced an alluvium that covers almost the entire surface. The
surface material is thus, in part, residuary from decay, and, in part, a deposit
from the wash. In many places, therefore, and occasionally over long
stretches, it is impossible to determine the exact quality of the underlying
rock floor, but its general structural habit is, as a rule, indicated by the topo-
graphic features just described. Relying on this evidence, we judge that the
basin of sediments, across whose edge the Expedition climbed at Wan Ch’uan
Hsien Pass, extends inland many miles, capped by basalt, and, in places, also
by sediments covering the basalts. Some of the sediments are doubtless
much younger than those of the Pass, and vary greatly in quality from place
to place. Granite and other crystalline rocks appear abruptly on the inner
margin of the basin. They are doubtless a part of the still more ancient floor
on which all the so-called later sediments rest, and they have been brought
to the present surface either by faulting or by warping.

From this point onward there is a succession of complex features belong-
ing chiefly to the ancient floor (Figs. 6 and 7). The first large basin in which
sedimentation occurred was crossed at or near the 50th mile. A much smaller
basin, only a few miles beyond, has one of its limits at mile 75, and then a series
of schists, quartzites and slates, cut by diorite, porphyry and granite masses,
forms the floor for fifty miles (Figs. 7 and 8). The minor structural relations
of these different formational units are not determinable with so little field
observation, but the major ones are plain. The schists, quartzites, and slates
are comparatively ancient rocks which have been invaded by the granite, and
the diorite porphyries have cut both the granite and the schists. It is possible
that the oldrock hills are the remnants of an ancient fault-block mountain.

This was our first encounter with a type of granite that we were to see
subsequently in a thousand other places, exhibiting a great variety of facies,
but always with similar relations to the major formations of the region. Per-
haps it is the same also as that seen in Nan K’ou Pass, forming the core of the

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51

52 GEOLOGY OF MONGOLIA

mountain barrier above the plains of China. It is this granite that we came
afterward to recognize structurally as part of a great bathylith underlying the
whole Mongolian region, in places penetrating the various oldrock formations
of the roof, forming the surface for long stretches, or appearing as intrusions
of more limited extent and of more variable quality.

At mile 116 a third small basin was reached, where, because of the cover,
‘the nature of the rock is more obscure; but it is certain, judging from the
topography, that the underlying beds should be classed with the basin sedi-
ments. Several miles of granite and graywacke separate this tract from a
larger basin, which begins at about mile 137, and, except for a low inlier of
schists and graywacke, continues for nearly fifty miles, with simple, nearly
flat-lying sediments, capped by, or interbedded with, basalts (Fig. 9).

The granite hills of Chakhar

In the granite hills the first glimpse was obtained of one of the great
peneplanes of Mongolia. The camp here, Camp Granite Hills, lay one hun-
dred and thirty miles from Kalgan and was reached on the second day (Plate
IX, B). It was pitched in a country of rolling hills and broad, open valleys,
but from the summit of any of the surrounding hills we looked over a veri-
table sea of hilltops in every direction (Plate IX, A). From this eminence the
irregularities of relief seemed to fail completely in the distance, as they might
if the relief had been carved by simple erosion from an original plane surface.
In every direction the same features were in evidence. This surface may be
a trace of the ancient erosion floor, originally lifted and warped to make the
great Gobi basin, or it may be the result of subsequent planation, with this
portion still uncovered by sediments. Or perhaps it has been stripped of a
former sedimentary covering and somewhat dissected. Whatever the explana-
tion, it is one of the finest exhibits of planation on crystalline rocks to be seen
anywhere in the region. We named this upland surface the Mongolian Pene-
plane. (See Chapter XIX.)

The P’ang Kiang hollow

Our third camp was established near the telegraph station at P’ang Kiang,
which stands in the midst of the fourth basin of sedimentary strata, on the
border of a hollow which has been cut deeply enough to expose the edges of
the beds (Figs. 10 and 11).

Although the locality promises fossils, diligent search in the time avail-
able failed to reveal any. During the season of 1923, however, a single fossil
was found in this vicinity, proving that the P’ang Kiang beds are of Tertiary
age. The topography of the basin introduces certain physiographic questions
not hitherto presented. There is a post-mature, almost peneplaned upper

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53

54 GEOLOGY OF MONGOLIA

surface on the sediments. It is clearly not the original surface of the depos-
its, but represents a long period of erosion, probably in some late Tertiary
epoch. Later when the significance of the smooth upland came to be more
fully recognized, we referred to it as the ‘‘Gobi erosion plane.” The broad,
undrained hollow, with an almost level floor, carved in this ancient plane, rep-
resents a definitely later stage in the local physiographic history. Similar
relations were later recognized at many other places, so that the erosion fea-
tures of this locality were selected as a type and referred to as the P’ang Kiang
stage of dissection. It is essentially the latest important stage of erosion

Crossbedded Sandstone
Light red soft sandy Clay 1
White Sandstone | fe
Red Sand and Clay

Hard red Sandstone -——
Soft red Sand and Sandstone 15

Ficure 11.—Field sketch drawn southeast of the telegraph station at P’ang Kiang. The station lies ina
large, undrained hollow eroded in the sands and clays, and the sketch shows the nearly horizontal sediments
exposed in the wall of the hollow.

marking the dissection of the almost level surface presented by the sediment-
basins. Even though the rock floor of the hollow is composed of compara-
tively soft sands and gravels, there is very little residuary soil. At most
places there are only a few inches of residuary gravel or a few feet of shifting
sand in the form of mounds.

At P’ang Kiang there is no way of determining how thick the deposits
are. They lie almost flat, for they dip not more than four degrees from the
horizontal, generally less (Fig. 11). ‘The sediments are cross-bedded, and their
structure seems to indicate that they were brought to their present position

FROM KALGAN TO IREN DABASU 55

from the northwest rather than from the south. There are slight discrepan-
cies of dip between the coarser upper beds and the lower fine, clayey beds, but
this is not a marked feature. The whole structure is slightly deformed, or
warped, since the strata of the east side of the valley dip gently to the east,
while those of the west side dip even more gently to the west. It is, of course,
entirely reasonable to believe that the sediments were formerly more exten-
sive, and that they may have covered portions of the oldrock floor which are
now bare. It is possible, also, that many of the last fifty miles of ground,
where only small basins are indicated by the topography, have been stripped
—the warpings that followed original deposition having exposed some of
this ground to an erosion sufficiently deep to remove the whole cover.

The P’ang Kiang basin was the first to throw some light on the geologic
history of the region, and although many other places furnished much more
striking and conclusive evidence, none was more suggestive or helpful to the
geologists of the Expedition.

P*ANG KIANG TO IREN DABASU

A few miles beyond P’ang Kiang we came, without a break in the topog-
taphy, to the ancient floor (Fig. 12). The relations clearly show that at one
time these Tertiary sediments must have formed a complete cover. The
P’ang Kiang stage of erosion has stripped them from a small area, because
here, either by warping or by faulting, the old floor is lifted higher than in
the basin just crossed. It is probable, judging from the apparent dips of the
strata, that the floor is brought up by faulting. This effect is repeated within
ten miles. In both places small patches or strips of the ancient floor form
outcrops at the surface, and are abruptly terminated and succeeded in each
case by smooth gravel-covered ground having the characteristic appearance
of simple, overlying sediments. After the last outcrop, at mile 197, the basin
sediments appear once more and continue unbroken for sixty miles, to the tele-
graph station known as Erlien, or Iren Dabasu—the scene of one of the most
important finds of the Expedition. (See Chapter XI.)

At miles 187, 197, and 217, three quite different types of rock are repre-
sented in outcrops, which are crossed by the trail after leaving P’ang Kiang.
The first is a limestone belonging to old, much deformed units, whose age can-
not be determined from the material itself but whose appearance is similar to
limestones seen in Nan K’ou Pass, and similar also to limestones seen farther
along on the traverse and regarded as of late pre-Cambrian age. The out-
crops examined in the narrow strip exposed on the trail are not fossiliferous.
The rocks dip steeply and are crumpled and sheared. The outcrop adjoins
the sediment-basin along a comparatively straight marginal line, suggesting

56 GEOLOGY OF MONGOLIA

that it is brought up by faulting or by very abrupt flexure, although it is fully
appreciated that original eminences in the old basement might present simi-
lar exposures after stripping. The ten-mile stretch from these limestone
outcrops to mile 197 crosses a broad depression about one hundred and twenty-
five feet deep, and on the next rise of ground, at mile 197, slates are exposed.
Brief examination of these slates indicates that they were ancient sediments
which have undergone considerable deformation. There is no possible way
of determining either their age or their relation to other floor formations of
the district, because the exposures are small, very obscure, and separated from
the nearest outcrops of a different kind of oldrock by several miles of covered
ground. They are similar, however, to many other slates noted on this trav-
erse, most of them much farther along on the trail.

The third exposure of a type of rock different from the usual superficial
sediments is located in the traverse between miles 217 and 229 (Fig. 13). In
this stretch are numerous outcrops of limestone of dolomitic appearance.
The rock has a peculiar porous structure, and there are traces of fossils, the
nature of which was not determinable in the field. Structurally this rock is
much simpler than that of the two outcrops previously described. It does
not show deformation, metamorphic reorganization, or any of the complexi-
ties belonging to the most ancient types. It does not at all resemble the
Palzozoic limestones which were encountered farther to the northwest. It
has the structural appearance of a later time, but the evidence as to its exact
age is not clear. Nothing resembling this formation had been seen previously
along the journey of more than two hundred miles. Later on, we found
patches of similar limestone interbedded in the basin sediments; we believe
that they represent deposits formed in inland lakes and ponds.

The last appearance of the oldrock floor in this stretch from Kalgan to
Iren Dabasu is a very small outcrop of slate, in the depression of Iren Dabasu
at about mile 258 (Fig. 14). At this point a patch, very similar to that noted
at mile 197, has been stripped by the erosion that has made the Iren Dabasu
lowland. Its only significance comes from the evidence that it bears as to the
shallowness of the basin itself, since the floor rocks are actually exposed at
several places and form the northern edge of the sediment-basin. Doubt-
less the basin is deeper between these windows—i.e., between miles 197 and
258—but there is no way of measuring it.

Between mile 197 and Iren Dabasu at mile 260, we passed continuously
over simple, nearly flat-lying sediments. The trail is on the upland to mile
238, where it descends abruptly into an undrained erosion lowland, repre-
senting the P’ang Kiang stage, and continues for seven miles in the hollow.
At mile 245 the upland is reached again. Here a much smoother and much
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57

58 GEOLOGY OF MONGOLIA

Dabasu lowland, which begins at mile 255. These two different erosion forms,
the level upland and the enclosed hollow, are especially well developed at
Irdin Manha (Fig. 13).

Little of the topography, except in very minor ways, is constructional in
origin. The smooth, almost level, gravel-covered surfaces that are encoun-

NW
Camp Iren Dabasu
Houldjin = anni

WLLL I) Mf
“Probakis Granite Basement _ Slates

240 MILES 245 250

FiGcurE 14.—Geologic section from Irdin Manha to Iren Dabasu, crossing a narrow strip of upland
between two undrained hollows. At Iren Dabasu the lowest sediments are Cretaceous, resting directly
upon the folded slates and followed by Eocene and Oligocene in the Houldjin escarpment. (See Chapter
XI.)

tered here and there, stretching away for miles, are strictly erosional forms,
and are typical of the Gobi region. Since the Mongols call these great open
areas of nearly level country ‘‘gobis,’’ we chose the name ‘‘Gobi erosion plane”’
for the surface which we later were able to trace across nearly the whole desert
region.

All the landforms thus far mentioned are destructional. The most per-
sistent constructional form consists of small hummocks or dunes of sand.
They are, perhaps, the “‘tamarisk mounds” described by Huntington (1907,
page 179). In some places they shift more or less, but in most cases they are
small, very closely and irregularly placed, and owe their lodgment to shrubs
and other small plant growths, which knit them together and give them some
slight fixity. It is the shifting surface sands, however, that offer the greatest
difficulty to the traveler and make it quite impossible in certain areas to depart
far from the beaten trail. It is always easier to follow the trail where the dis-
turbance caused by caravan travel enables the winds to keep the trail clear,
so that the loose sands are, to a large degree, swept away. These surface accu-
mulations are, in general, more numerous and troublesome in lowland areas
represented by the P’ang Kiang dissection than on the Gobi erosion plane.
The gravelly residue of the level upland seems to serve as a protection against
the furnishing of sand for drifting, whereas the strata exposed in the walls of
hollows of the P’ang Kiang stage are readily attacked, and sands from them
are spread on the lowland and in places are swept over portions of the adjacent
upland.

FROM KALGAN TO IREN DABASU 59

Nowhere in this whole traverse of 260 miles did we see any large drift-
ing dunes, and nowhere is the constructional deposit more than a thin, super-
ficial covering. No large features are controlled by it, and at no place is the
accumulation, including both the soil mantle and the drifting cover, more than
a film on the surface. Instead of being compelled to travel over great accumu-
lations of loose sand, the Expedition actually traversed rock with only a few
inches or a few feet of obscuring cover. To be sure, the floor itself is com-
posed, in part, of simple and almost unconsolidated sediments, not very unlike
the sands themselves; but they are, nevertheless, definite sedimentary forma-
tions and of varying age from place to place, quite different from one another,
both in quality and in fossil content. The sandy and little consolidated char-
acter is responsible for the common failure of travelers to distinguish between
loose sand and sedimentary rock formation.

Fully half the distance, however, is over a floor of much more complex
rocks, and they also are exposed with little cover. The obscurity due to over-
lying soil is greatest in the first hundred miles, where some of the country has
agricultural capacity—some, indeed, is cultivated by Chinese settlers, who are
in this way encroaching upon Mongol territory (Plate VIII, B, page 48).
The land could be farmed for a considerably greater distance northward, but
by the time P’ang Kiang is reached, conditions have become too desert-like
for agriculture, and the rock formations are too nearly bare, since here the
wind is so successful in blowing away the finer débris that there is little devel-
opment of soil mantle. This, therefore, is the edge of the Gobi Desert proper,
which, as far as we have seen it, is essentially a rock desert.

The first fossils

At P’ang Kiang and again at Irdin Manha and Iren Dabasu (Plate X, A),
the sedimentary formations are especially well exposed in the escarpments,
where the succession of strata can be seen and intimately inspected. On the
first traverse no fossils were found at the first two places, but upon arriving at
the Houldjin escarpment above Iren Dabasu, at mile 255, numerous frag-
ments of fossil mammal bones and teeth were found, while dinosaur bones were
discovered at Iren Dabasu. Later, an extensive fauna was collected at Irdin
Manha, and in the following season a single rodent jaw was brought to light -
at P’ang Kiang. The finds at Iren Dabasu were recognized at once as of very
great importance, and on this account it was decided to spend additional time
on this ground. Here the first local study was made. The results are set
forth in special study No. 1 of Part III, ‘‘The Iren Dabasu Basin,” constitut-
ing Chapter XI of this volume.

CHAPTER IV
FROM IREN DABASU TO URGA

ALTHOUGH sediments are extensively exposed at Iren Dabasu, the slate
floor lies near the surface. One small outcrop is uncovered at the south side
of the basin (Fig. 14), and a comparatively large area is exposed on the north
border of the lake Iren Nor and near the Chinese telegraph station (Plate
XXIII in pocket). The hollow is margined on the south by a scarp of sedi-
ments, and on the north by an abrupt ascent over crystalline rock to a nearly
level erosion plane 200 feet above the lake (Fig. 15 and Plate XXIII in
pocket). The trail over this northern edge’ passes across slates cut by numer-
ous quartz veinlets like those in the bottom of the basin.

An inspection of the veins failed to show mineralization of any conse-
quence. In spite of the abundance of vein quartz—a condition that was noted
at many other places in the course of the summer—there is everywhere a sur-
prisingly small amount of metallic content. Prospectors have dug pits here
and there, and doubtless have made tests of the material for the precious
metals. But we saw nothing that gave any encouragement to such a search,
except the presence of the veins of quartz.

Farther to the east, the Iren Dabasu basin is bordered by intrusive
granites which, with the slates, have been eroded to form the ancient floor
(Plate XXIII in pocket). Within a mile of the north margin, on the way
toward Urga, the slates are cut by basalts or traps (Specimen 35, A. M. 9724),
which continue for about two miles (Fig. 15). The exact structural relations
are not determinable, neither is it certain whether these rocks are intrusive
or are in part surface flows. Some of the material has the aspect of surface
flows, but in any case these rocks are much later than the slates with which
they are associated. There is little evidence of deformation in the basalt,
whereas the slates are considerably metamorphosed.

IREN DABASU TO CAMP JURASSIC

Six miles north of Iren Dabasu, at mile 267, the trail again descends to
a basin-like area, smaller than Iren Dabasu but similar in superficial form
60

PLATE X.

A. IREN DABASU.

Dissected dinosaur beds at Iren Dabasu. The distant skyline is the Houldjin escarpment

B. RESIDUARY GRANITE BOWLDERS.

Bowlders of granite left as residuals on a level erosion surface of almost bare rock, along the Urga trail.

PLATE XI.

A. UPDRAGGED SANDSTONE BEDS AT CAMP JURASSIC,

B. MOUNT TUERIN.

Granite residuals on the lower slopes of Mount Tuerin, Irregular jointing controls weathering of the rock into peculiar
shapes.

FROM IREN DABASU TO URGA 61

(Fig. 15). The floor is doubtless the same slate formation, but it is covered
for several miles with later sediments which have been characteristically
eroded into low, rounded bluffs and undrained hollows. The sediments con-
tinue to mile 272, where basalts are again encountered; then, a little way be-
yond, granites and syenites break through the slates and form the floor
through a somewhat more rugged stretch of country.

At mile 275.8 there is a very striking outcrop of poikilitic syenite which
displays a great many variations of composition and coarseness of grain. It
probably represents, in part, the results of syntexis, and, in part, the end
products of a differentiating magma. The chief local types include horn-
blende syenite, hornblende granites, and quartzose pegmatites. The locality
bears a monument known as Kwei Ming Kwan, near which we noted a most
complex group of variations in the syenitic rock. The dominant type is a
hornblende syenite (Specimen 36, A. M. 9725), but there seems to be no real
difference in age or origin between these and other varieties. Even the granite
(Specimens 36 and 39, A.M. 9725 and 9728), from this same ground probably
has essentially the same significance. At mile 278, there is granite porphyry
(Specimen 40, A.M. 9729), and at mile 280, a very coarse granite (Specimen
41, A.M. 9730), both of which are probably only varietal expressions of the
igneous activity of this peculiar area. In the Kwei Ming Kwan area, also,
there are some older metamorphosed sediments, through which the syenites
and granites have penetrated. The patches of these metamorphics now re-
maining seem to be merely portions of the roof into which the original igneous
mass had advanced. They include a group of strongly veined argillites
which probably have been tuffs (Specimens 37, 38, A. M. 9726 and 9727).
Their peculiar greenish color and their structure suggest intermixture of ash.
At mile 283, an ancient conglomerate is associated with the slates and tuffs.
It carries bowlders of granite and many kinds of porphyry and may be a
basal conglomerate (Specimen 42, A.M. 9731), but a detailed examination
could not be made. There is evidence, at least in this locality, for an important
structural break between two ancient series, and it is possible that this con-
glomerate represents the basal portion of the Nan K’ou series. We believe
that the slates, as well as the limestones seen much farther back along the
trail (miles 187 to 220), belong to the same series.

Argillite outcrops again at mile 286 and is cut by a very coarse hornblende
granite, which is itself cut by dikes, and which continues for two miles or
more. Slates or graywackes reappear at mile 289, and in a short distance
are overlaid by sediments in a basin which extends from mile 289 to mile 302,
a distance of 13 miles (Fig. 16). The slates of the margin are very badly
deformed and stand in all attitudes, cut by granite porphyry and by basalt
sills and dikes in great abundance (Specimen 46, A.M. 9735). The slates

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FROM IREN DABASU TO URGA 63

themselves are variable in quality and extremely thin-bedded, but are well
indurated. They are closely folded, and the strata stand vertically in places;
but on the north side of the basin, where the slates form the surface for seven
miles, the dips are chiefly to the north. The whole formation is undoubtedly
very thick.

The sediments in the basin from mile 289 to mile 302 have a basal con-
glomerate of rather coarse quality on the north margin, at mile 302. At mile
296, there are red shales overlying a pebbly fresh-water limestone and sand-
stone, together with carbonaceous paper shales which contain scattered gyp-
sum crystals. It was appreciated at once that the quality of these strata is
unusual and that nothing quite like them had been seen. A rapid search was
made for fossils, but none was found. Their possibilities in some respects
were not fully understood until three months later, when we studied the
Ondai Sair locality, north of the Altai Mountains. At Ondai Sair, exactly
similar dark paper shales and red and gray sandstones furnished fossil insects,
fishes, and plants, as well as dinosaur bones, by which they were determined
to be of Lower Cretaceous age. The paper shales alone are so characteristic
that there is really little uncertainty in correlating these deposits with the
Lower Cretaceous of Ondai Sair. The sediments in this particular basin,
between miles 289 and 302, are quite different in physical aspect from the
Cretaceous strata which had already been identified at Iren Dabasu. A small
salt pan lies in the bottom of the depression. The conglomerates at the base
may indicate that basal beds of the Lower Cretaceous series are represented
here. It is entirely possible, of course, that the higher red shales are of Ter-
tiary age, but no satisfactory evidence of their age could be found.

This basin deserves additional study. It contains formations of the same
type as those which elsewhere furnished a unique fauna, and probably would
furnish also a valuable contribution if it could be inspected in detail. Un-
doubtedly important stratigraphic data could be gathered in a more extended
investigation of the ground, from mile 289 to mile 302, especially by broaden-
ing the scope of the search to the east and west, within the basin. It is the
only area noted on the Kalgan-Urga trail where the carbonaceous paper shales
are developed in typical form. A week, instead of an hour, should have been
devoted to this locality.

At mile 310, a very coarse granite, of almost binary composition, cut by
pegmatite veins and quartz stringers, breaks through the slates and continues
for two miles (Specimen 47, A.M. 9736). Since the ground slopes gently to
the north, the granite gradually passes beneath a mantle of later sediments
obscured by arkosic sand and many dunes. In a gulch at mile 320, however,
we noted green clays, which are probably Tertiary beds (Fig. 17). The whole
basin is very shallow and is terminated on the north, at mile 323, by slates

64 GEOLOGY OF MONGOLIA

similar to those seen before. From mile 325 the ground slopes rather rapidly
between granite hills, northward toward Ude. An alaskite granite (Specimen
49, A.M. 9738), was gathered beside the trail. Probably a thin sediment, or
at least an arkosic alluvium, occupies the bottom of the valley, in the midst
of which stands a granite hill (Specimen 50, A.M. 9739), which marks the
site of the station of Ude, now entirely deserted. From this point the trail
follows the sandy wash of a stream bed with rugged valley sides of crystalline
rock, probably granite, bowlders of which appear abundantly in the wash.
No motor exists that can plow up this mixture of sand and cobbles without
coming toa stop. And at mile 332, just where the heavy sand stops the car,
the door-flap of a yurt opens, and a long-skirted Buriat official comes down,
inspects the traveler’s permit to enter Outer Mongolia, and invites him to
pay atax. In the hills on either hand sits an invisible garrison, ready to fire
upon a car that is mad enough to try torun. The exaction may not be pleas-
ant, but the strategic choice of a perfect motor trap is admirable.

Above the permit station, we cross a low divide to where, on the farther
slopes, sedimentary strata overlie the granite. The road is smooth and gravel-
ly for five miles, continuing down a gentle slope to a well at mile 341. Although
there was no opportunity to determine the quality of this ground, the sub-
stantial nature of the material led us to believe that it may be older than
Tertiary.. The structure is basin-like, and is terminated abruptly on the north
at mile 343, making a ten-mile stretch of sedimentary strata not very different
from the others passed over in the last 75 miles. All of the sedimentary basins
encountered in that day’s run are smafl and shallow—none more than Io or
12 miles across.

The country at Camp Snow Storm (mile 341) was covered with snow on
the morning of May 8. This entailed some delay, but in the afternoon the
clouds began to break, and a start was made. After a twenty-five mile run,
however, the weather turned bad again, and a long-threatened storm broke
in a furious blizzard. There was no alternative but to keep on as cautiously
as possible, in the hope of driving through. By the time mile 373 was reached,
the clouds had lifted again, and not only had the Expedition passed through
the storm itself but it had come into ground where no snow had fallen. At
this point we noticed a decided difference in the topography, which invited
geological investigation to determine just what change of structure had taken
place (Fig. 18). For some distance the storm, with its blinding snow, had
obscured everything, but it was evident, at least, that a small basin had been
crossed.. Herea well gave opportunity for camp, and the party stopped at mile
374, in one of the most interesting structural areas to be found on the Urga trail.

Immediately after leaving Camp Snow Storm at mile 341, the trail leads
over slates with a strike nearly east and west, and a dip south at an angle of

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VOL. 1—5

66 GEOLOGY OF MONGOLIA

about 60 degrees. There are many interbedded limestone and quartzite
layers, but the most striking feature is the occurrence of great numbers of
granite sills, which penetrate the slates. Weathering and erosion have left
residuary bowlders of granite scattered along the course of the sills, as though
they were the ruins of long, low masonry walls built upon the marvelously
level surface of the desert. No more splendid exhibit of planation across
different types of rock has been seen in the whole of Mongolia than is shown
in this stretch of ground. From mile 343 to mile 353, folded slates which
stand nearly on edge are interbedded with limestones and quartzites. They
are associated with granite sills and all are planed off so perfectly that one can
drive almost anywhere by motor. This kind of ground continues for twenty-
four miles.

It is clear that the formations are folded, but the structure could not be
determined in greater detail, because of the rapidity of travel. Possibly the
limestones furnish evidence of the age of these beds, but no fossils were found
in them. In the northerly half of this stretch of ground the limestone beds
are much more numerous, and there is some variety in the quality of associated
slates and sandstone. Northward from mile 355 there is a valley-like de-
pression in which the character of the material is less easily made out, but the
slates dominate, with some graywacke and a few quartzite beds.

At mile 362 one of the most striking features seen thus far in the traverse
is represented by a volcanic blowout, which forms buttes and minor ridges
bursting up through the slates. The rock is an olivine basalt (Specimen 66,
A.M. 9755). North of this blowout the ground carries more graywacke and
less slate, becomes pebbly, and obscures the relation to the slates and lime-
stones farther south. At mile 367, conglomerates, the like of which we had
not yet seen in our traverse, lie nearly flat beside the trail. Observation of
these beds was interrupted by the snowstorm, and the point at which change
to later sediments takes place was not accurately determined. Their pres-
ence, however, was not long in doubt, because they were found again at Camp
Jurassic, mile 374, on the north edge of the small basin which occupies the
ground from miles 370 to 374.

The belt of slates, from mile 343 to mile 367, deserves more careful in-
spection than could be given. There must be an immense thickness, perhaps
15,000 to 20,000 feet. Their variation in character, the amount of deforma-
tion they have undergone, the strikingly abundant intrusives in the form of
sills, and the numerous limestone interbeds, are prominent features. Prob-
ably more than a hundred granite sills were seen in this stretch of ground,
all of them intersecting the strata in a clean-cut, definite fashion, instead of
impregnating or absorbing or markedly metamorphosing the country rock.
There are many quartz veins and occasional jaspery bands. Doubtless these

FROM IREN DABASU TO URGA 67

latter are related genetically to the intrusives in some way. The slates im-
pressed us as having finer grain and a better slaty structure than most of the
slates subsequently encountered farther north. Indeed, the relation between
these fissile slates and the argillites and graywackes farther north is one of
the questions not satisfactorily settled. They may be equivalent, but, if so,
there is a marked change in lithologic quality.

The structural relations at Camp Jurassic

Because the physiographic features showed that a change in structure
had been reached at mile 374, camp was made there on the evening of May 8,
so as to give an opportunity for more detailed inspection of the ground (Fig. 18).
The strata which occupy the narrow basin extending from mile 370 to mile 374
have been turned on edge by drag against the older formations. The plane
of movement could be readily located (Plate XI, A). In the distance, in line
with this fault, several small volcanic cones were seen. The strata thus turned
on edge are sandstones belonging to the later sediments; their exact age is
undetermined, but, judging from their petrographic character and condition,
they may belong to the Tertiary series.

Special interest centered in the formations standing almost vertically in
the uplifted block against which these later beds are faulted. The older
strata are comparatively hard sandstones in great variety, and are devoid of
fossils except for scanty and obscure plant remains. The agencies of weather-
ing and erosion have carved them into fantastic shapes. The structure and
history of the rocks seem to be consistent with what was expected of the
younger strata belonging to the floor formations; there is no metamorphism,
and the beds are simply folded. In China, rocks which in all essential re-
spects resemble these are regarded as of Jurassic age, and the same correlation
was assumed for this locality. Later, at three other places, strata were found,
similar in composition, structural relation, and history, and apparently con-
sistent with the assumption that they are all of Jurassic or mid-Mesozoic age.

The area is more significant than it at first appears to be. Here one can
see that the basins carrying the later sediments, such as had been crossed re-
peatedly in the traverse from Kalgan, may have originated, in part, by de-
formation of comparatively recent date, for in places the strata themselves
are turned up on edge, indicating that some of the deformation is younger
than the sediments. The making or deepening of basins, therefore, probably
was a long-continued and oft-repeated process.

Within two miles the entire exposure of the steep-standing sandstones
of the floor is crossed, and younger overlying beds appear again. Evidently
there is an uplifted area adjacent to the down-dropped one. The erosion
surface, representing the original floor, has been thrown into broad arches

68 GEOLOGY OF MONGOLIA

and troughs by gentle warping, but there are sharp flexures and faults along
certain marginal lines. It is apparent, also, that volcanic activity found
vent along some of these lines of weakness. An extensive igneous accumula-
tion, both of lava and of tuffaceous material, is associated with the overlying
deposits. There are evidences of volcanism also within the supposed Jurassic
area, but the relations there are obscure.

The supposed Jurassic strata strike N. 60° E., and dip steeply north.
There is a marked angular unconformity between them and the beds coming
in above (Figs. 18 and 19). The area exposing the Jurassic strata is the center
of a breached, dome-like uplift, with annular valleys, dip slopes, hogbacks,
and the other erosion-forms characteristic of a dissected dome of sedimentary
strata (Fig. 122, page 281).

On the whole, this locality was the most illuminating in the matter of
dynamic history and structural relations of any thus far seen in the Mon-
golian region. There is definite evidence of the succession of sets of strata
and of their relation to deformation and igneous activity. Here one can see
the youngest strata that belong to the ancient floor, and this is the first place
where strata suggesting Jurassic age had thus far been seen. It is a clear
case of late faulting, and volcanism seems to be structurally related to that
weakness. It shows that there is a great unconformity between the Jurassic
strata of the floor and the so-called later sediments. It seems also to carry
evidence of an important break in the midst of later sediments, particularly
in the ground forming the north side of this dome. Apparently there are
three important series here, one of Mesozoic age belonging to the old floor;
another, the first series of sandstones above the unconformity, lying on that
floor; and a still younger series, including the topmost beds.

At the time these features were first seen, it was not fully appreciated
that the structural elements of this locality are representative of the most
fundamental structure of the Gobi region. Although not fully proved at the
time, it appears certain from later developments in adjacent regions that the
two sedimentary members at Camp Jurassic, overlying the eroded edges of
the so-called Jurassic strata forming the floor, include a Cretaceous series and,
after a slight unconformity, a series of Tertiary strata of simpler history.
This splendid structural exhibit presents a good opportunity for working out
in much greater detail the sedimentary history of the region, and warrants
the recommendation that this area be given a more extended investigation.
The strata of these different types are well enough exposed to allow a com-
paratively complete column to be constructed, as there must be at least 3,000
feet exposed in the area, a large portion standing almost on edge. Here, an
area that could be extended so as to include the basin to the north deserves
mapping, as well as a structural and paleontologic study.

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70 GEOLOGY OF MONGOLIA
CAMP JURASSIC TO MOUNT TUERIN

The ‘‘Camp Jurassic’’ dome proved to be a very limited one. It is dis-
sected in perfectly typical form, and from the higher eminences one can follow
the hogback border from the south side eastward and northeastward until it
curves back to the west again, completely closing that end of the uplift.

For a distance of little less than a mile, the trail crosses steeply upturned
sandstone beds supposed to be of Jurassic age (Fig. 17). The thickness rep-
resented is at least 3,000 feet, with neither the top nor the bottom exposed.
At mile 375, a new series of sandstone beds appears unconformably on the up-
turned edges of the Jurassic strata, dipping much more gently northward.
They clearly belong to the so-called later sediments of the desert region, al-
though no fossils were found to indicate their age.

This series continues as a foundation rock for four miles to an escarp-
ment and divide at mile 379, where the red-colored strata are made up largely
of basalt bowlders and smaller fragments of igneous material, which, in a
general way, strongly resembles the material noted on the south side of the
dome. It is much more loose in structure than either of the formations oc-
cupying the center of the dome, and, although the structural relations are not
quite clear, the general dips along the trail indicate that there is an uncon-
formity beneath this formation also (Fig. 18).

The upper series of sediments, beginning with the red volcanic débris,
continues for thirteen miles and is succeeded by sandy sediments of consider-
able variety (Fig. 19). Over this stretch of ground the trail is gravel-covered,
and although there is considerable difference of relief the sediments through-
out seem to be of the same simple type. At mile 392, more substantial rocks
form the floor, and we judge that the previously encountered sandstone series
again comes to the surface. This type continues from mile 392 to mile 429,
and constitutes one of the longest stretches of continuous basin structure thus
far seen (Fig. 20). The trail for many miles over the sandstone beds is re-
markably smooth, and furnishes an excellent road for travel. The only diffi-
cult places are where the sandy channels of dry water courses cross the trail.
Over most of the ground there are no outcrops, and the country is a typical
open plains area such as the Mongols call a ‘‘ gobi.”

A significant find was made in the banks of a small stream at mile 394,
where several fragments of reptile bones were recovered. These reptile bones
and the somewhat disturbed and indurated beds suggest Cretaceous age.
These beds we judged to be continuous with the middle member of the Camp
Jurassic dome. It is clear that the ground is all sedimentary, and is a direct
continuation of the sedimentary series exposed to better advantage in the
first few miles of the trail; but from this point to the northern limits of the

FROM IREN DABASU TO URGA 71

basin, at mile 430, no additional exposures of consequence were seen. It is
quite possible, however, that much better conditions for inspection might be
found either to the east or to the west of the trail, and if a point could be
found where the ground is more successfully dissected, this basin would furnish
an exceptionally good field for further investigation and collecting. This is
especially true if our interpretation of the Cretaceous expanse is correct as
represented on the sections, for it is one of the longest stretches of supposed
Cretaceous ground encountered in the summer’s investigation.

At the northern edge of the basin, there is a sharp transition to granite
(Specimen 70, A.M. 9759). There is no indication of the structural causes
of this change, but we have chosen to represent it as a fault. It is possible,
of course, that it is simple warping which brings the floor of the basin grad-
ually to the surface, but the effect is the same. For the next eleven miles
along the trail the rock floor is virtually bare. It is formed of coarse granites
which display considerable variety within the mass. The granites are cut
by many dikes, chiefly syenite, pegmatite, and granite porphyry, while a few
are latite or melaphyre (Specimens 71, 73, A.M. 9760, 9762). There are many
residuary outcrops and bowlders (Plate X, B), around which is strewn a loose
arkosic sand, resulting from disintegration of the granite. The floor is splen-
didly planed, and the profile followed over the sediments for the last twenty
miles continues over the granites without noticeable difference of relief (Plate
XII, A). It is not likely, of course, that the planations of both the crystallines
and the sediments have been accomplished together. It is more probable
that the granite profile represents an older peneplane which has been stripped
and additionally planed during the later stages of erosion that have affected
the adjacent sediments. It is most impressive to find repeatedly such perfect
conformity of profile on adjacent areas of strikingly different quality of rock.

This is one of the largest stretches of granite that we saw in the Gobi
region. The rock is a very coarse-grained, uniform type, not at all gneissic
or gneissoid in structure. It has not been much metamorphosed, but it is
closely associated with gneisses of general granitic composition, which have
had a much more complicated history, and it must cut these gneisses as an
invading intrusive. We were later to discover evidence that a great granite
bathylith underlies central Mongolia, for we found that repeatedly this same
general type of granite cuts through older formations and constitutes the
floor for considerable distances. This granite area doubtless belongs to the
same unit.

From mile 441 to mile 444, the ground is covered and smooth and has
all the outward appearance of being sedimentary. It is apparently a very
small, shallow basin in the granite, in which a thin cover of sediments is
preserved (Fig. 21). From mile 444 to mile 448, the floor is formed of granite

72 GEOLOGY OF MONGOLIA

gneisses representing very much older formations, the precise relations of
which are unknown, but they have the appearance of being the most ancient
of any thus far encountered along the trail (Specimen 74, A.M. 9763). In
most places where gneisses and other ancient crystallines were seen along the
Urga trail, the strike is nearly east and west. In this particular local stretch
the strike is nearly north and south, with a dip to the west.

From mile 448, basin sediments lie on the gneisses for seven miles; but
at mile 455, the floor emerges, either by faulting or warping, and is represented
by a complex series of gneisses, crystalline limestones, schists, and granite
intrusions cut by black dikes. The structure is confused; the rocks are all
somewhat metamorphosed (except the igneous intrusions) and must belong to
some very ancient type (Specimens 78, 79, 80, 81, A.M. 9767, 9768, 9769,
9770). The limestones are strictly interbedded with the gneisses and schists,
but the whole series is cut in a complicated way by granites, probably offshoots
from the bathylith, and by dikes that are of later date. These dikes exhibit
considerable variety, the most important types being syenite and granite
porphyries. The original sedimentary series must have included sandstones,
shales, and limestones which now appear as schists of various sorts. The
origin of the gneisses is less certain, but in part, at least, they are likewise
ancient, injected sediments.

This type of geological structure continues for another ten miles and more,
and presents the most complicated structural unit yet seen in the Gobi region.
It is much too complex a problem to be worked out in satisfactory detail in
so hurried a reconnaissance. The chief items characterizing this belt up to
mile 468 are as follows: a red mica schist, very thick; massive crystalline
limestone, one belt half a mile in width, with numerous smaller belts; several
beds of dense quartzite; and thick graywacke schists. These are intricately
cut by granite and granite porphyries. There is not much intimate injection
or impregnation, in spite of the abundance of igneous material. The igneous
members, with their differentiation products and different stages of develop-
ment, may all belong to a single magmatic source.

The ancient metamorphics, represented by specimens 83, 84, 85, etc.
(A.M. 9772, 9773, 9774, etc.), are of special interest. They have a more com-
plicated structural habit than the Nan K’ou series as recognized in China,
but correspond very well indeed to the descriptions of the Wu T’ai system of
Bailey Willis as determined in China. The Wu T’aisystem includes limestones,
phyllites, green and black schists, and sheared greenstones. The best that
can be said, of course, is that this ancient formation, from mile 455 to mile
469, is similar in all essential respects to the Wu T’ai system. We doubtless
have in the floor of the Gobi representatives of the same age and of the same
general petrographic character as are found in China.

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74 GEOLOGY OF MONGOLIA

At mile 468.5, a sudden, complete change in formations takes place,
probably due to faulting; the floor for a short distance is composed of sand-
stones similar to those seen at Camp Jurassic. It is entirely possible that they
are Jurassic strata. They are cut by granite porphyry and are overlaid by
rhyolite flows and ash-beds of great variety, in which are a few sills (Specimen
89, A.M. 9778). The igneous rocks continue for several miles, dipping gently
northward, and constitute a great series of volcanic flows of both acid and
basic type (Fig. 22). Some of the flows are very pumiceous (Specimen go,
A.M. 9779), and exhibit beautiful minor variations of petrographic quality.
The chief types are interbedded rhyolites and basalts, and each type is repeated
several times. Some of the vesicular basalts (Specimens 91 and 93, A.M.
9780, 9782), are crowded with amygdules of agate and chalcedony, and the
ground is literally strewn with millions upon millions of these pebbles, known
as “Gobi stones,’’ so that the trail looks as if it were studded with jewels.
The series continues for twenty miles, dipping gently northward, but is ter-
minated abruptly along the north side of a range of low hills consisting of
rhyolite breccias and basaltic tuffs. Probably the termination is against a
fault, and the basin has dropped down at this edge against the granite. Here
the strata are structurally more obscure, perhaps because of the deformation.
There is apparently no way of accounting for the change to granite without
such displacement, nor is there any way of telling the age of the formations.
Although lava flows are common elsewhere, at no other place has so remark-
able a series been seen in the Gobi region.

The succession at this place could be worked out in great detail. In
passing over the ground so rapidly it was not possible to detect all the units,
but it is clear that the earlier and lower members are chiefly rhyolites with
rhyolite porphyry sills, rhyolitic ash, and pumiceous rhyolite, whereas the
topmost flows are chiefly basalts with only an occasional rhyolite flow or
rhyolitic sill, The middle members are more mixed than either extreme, and
carry both rhyolites and basalts. The total thickness of these rocks is cer-
tainly over 500 feet, and probably exceeds 1,000 feet.

The granites of Mount Tuerin

At mile 490, the trail enters on a stretch of granite, which continues un-
interruptedly to Mount Tuerin, a distance of twelve miles (Fig. 23). Some
of the ground is covered with débris, but the bedrock outcrops at frequent
intervals, broken only by later injections in the form of dikes, chiefly of several
varieties of granite porphyry (Specimens 97, 98, A.M. 9786, 9787). This is
the granite of Mount Tuerin, which is a residuary remnant of a large boss
and is undoubtedly a part of the great Mongolian bathylith. The rock is a
hornblende granite, and is fairly uniform in the central portion ‘of the mass.

FROM IREN DABASU TO URGA 75

There are many other varieties of granite, however, especially on the north
side of the mountain. The whole area, therefore, is a granite complex, but
there is no indication of great differences of history in its various parts. Its
age relations are unknown, because nowhere along the trail could we see its
structural relations with other rocks of known age. On the south the contact
is formed by a fault, and on the north the granite is overlaid by later sediments.

Mount Tuerin is a conspicuous landmark, visible for a great distance from
every direction as it looms to exaggerated height when seen across the arid
plain (Plate XI, B). It is a splendid example of a residuary remnant of a
very much larger mass, most of which has been worn away to a remarkably
smooth erosion plane. The central portion of the remnant is rugged and
craggy, and difficult to cross with such an expedition as ours.

The west base of the mountain has been chosen as the site for a lamasery,
where about 500 lamas, or priests of Buddha, live. This is by all means the
most pretentious establishment which we saw along the Urga trail. Here,
in 1921, the Chinese made one of their last stands in their retreat across
Mongolia, after being driven out of Urga by the combined Russian and Mon-
golian forces. A year later, at the time of our visit, the ground was still
strewn with relics of that encounter.

In the peculiar mirage-producing atmosphere of the Gobi, Mount Tuerin
presents a more striking appearance at a distance of twenty-five miles than
close at hand. The effect is increased by the crags and pinnacles left by the
weathering of a jointed granite. The master joints fall roughly into a sys-
tem, and the principal weathering and erosion features follow these same lines.
The most marked set have a trend N. 60° E. There are prominent cross-
fracture joints also, which divide the rock masses very unequally. As a
result there are many overhanging and poorly balanced residuary blocks (Plate
XI, B). There are also many small valleys and grassy slopes which add im-
mensely to the beauty and picturesqueness of the place, and add, too, no doubt,
to the interest it holds for the Mongol people. One can readily see the appro-
priateness of locating a great lamasery at the foot of this mountain. So un-
usual is the appearance of Mount Tuerin against the surrounding country
that it seems to the Mongols to be marked by nature as a place deserving
special reverence.

Mount Tuerin was the first rendezvous for the Expedition. It was here
that the motors overtook the caravan and received from it their first supplies,
separating again for the next lap of the journey. Camp was pitched in a
grassy valley of the mountainside, on one of the most attractive sites: of the
whole summer; but the geology, despite the picturesqueness'of the outstanding
features, is too simple to require extended study. The Expedition, therefore,
moved on toward Urga after only a day or two of local inspection. For the

76 GEOLOGY OF MONGOLIA

first four or five miles the same hornblende granite continues. It is a coarse-
grained type with miarolitic structure, slightly pegmatitic in tendency, but
much less so than is usual in a large granite mass. Under weathering, the
rock tends to stain with some black coloring matter, probably oxide of man-
ganese.

MOUNT TUERIN TO BOLKUK GOL

Beyond mile 509, new types of granite were encountered, quite different
in physical appearance from the Mount Tuerin type. The relations of one
to the other were not clearly made out, but probably the new types cut the
mountain mass as later intrusives (Specimens 102, 103, 104, A.M. 9791, 9792,
9793). Farther on the complexity is increased by the presence of still other
intrusives, while some of the preceding varieties are repeated. The granite
complex continues to mile 524, giving thus a section of thirty-four miles
from mile 490, where the unit was first observed. This is the longest single
stretch of granite encountered on the Urga trail.

It is not possible to determine exactly where the granite floor ends and
the new change begins, but at about mile 524, after crossing a small stream,
the country opens appreciably (Figs. 23 and 24). The ground, which has
been comparatively uneven, with a good many outcrops and residuary knobs,
gives place to smooth, level country, and the arkosic character of the soil
gives place to gravel of the sort usually accompanying the later sediments.
The topography is more open, with broad, gentle sweeps and without eminences
of any kind. As there are few outcrops it is not possible to determine much
regarding the character of the underlying material, but it is evident from
every feature that a decided change has taken place. This has come gradually,
probably by the downwarping of the old floor, so that overlying sediments are
preserved, and arkosic disintegration material is mixed with sedimentary resi-
due in a sort of transition zone of surface cover. Farther along, outcrops of
sediments are exposed. This type of country continues for nearly fifty miles
and constitutes the largest continuous basin thus far crossed by the Expedi-
tion. There is no clue to the age of these sediments or to the details of their
structure. They are more completely covered than usual, because the traverse
has now entered the grassland country, where much larger amounts of soil
are held than is the case in the desert.

There is here little similarity to the barrenness seen farther to the south.
At this time of year, early May, we saw only the dried remains of last year’s
vegetation, but over long stretches it must have stood two feet high in the
growing season. The district is a typical grazing country, with here and
there a lake furnishing ample water for stock. Whether the lakes carry water

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78 GEOLOGY OF MONGOLIA

' the year round we could not determine, but they appear to be permanent.
Furthermore, they are not appreciably salt; at least, they are fresh enough to
serve aS a suitable water supply for herds. In every respect the country
resembles the better portions of the great plains of the western United States,
and gives one the impression that some of the land might well repay cultiva-
tion in the same manner. A country in which the native grasses grow so
luxuriantly should mature crops of grain without irrigation. This northerly
border of the Gobi region for 120 miles presents much the same character that
one sees on the south border, where the encroaching Chinese colonists have
already made good use of it. Doubtless there will come a time when this
northerly border will be cultivated also; and when the limits of such encroach-
ment are reached, the true desert region will be materially reduced, for almost
a third of the Gobi country lying between Kalgan and Urga would encourage
a much more intensive use than is now made of it.

The trail is smooth and forms a perfectly safe, first-rate road for motor
travel over the whole of the sedimentary basin, from mile 524 to mile 577
(Figs. 23, 24, and 25). At miles 564-565, three small volcanic vents break
through the sediments, which are well exposed on the flanks of the cones.
The vents are apparently recent, and the openings are filled with scoria,
pumice, pieces of breccia, bread-crust bombs, glassy crusts, and a great variety
of vesicular material in excellent, fresh condition (Specimen 106, A.M. 9795).
Some of the material is red and preserves fragments of sediments and traces
of an original sedimentary structure, whereas other pieces seem to be as
typically glassy and pumiceous lava as could be found anywhere. The ad-
jacent sediments are not appreciably disturbed, and the whole relation seems
to favor the conclusion that not only the red bricklike material, but perhaps
the whole series of volcanic products at this spot is of purely local origin,
made through fusion of the sediments by superheated volcanic gases. Al-
though the cones are almost covered with fragments of this sort, they cannot
have been built of the fragments, since unaltered sediments show on the
flanks. This structure indicates, of course, that the cones are only in small
part the result of accumulation, and are in chief part residuary after erosion
—they stand above the general level because of their more successful resistance.
No other igneous rocks were seen within the same basin, but five miles farther
north a series of lava flows was encountered, lying on the sediments. They
are vesicular trachytes succeeded by basalts and rhyolites (Specimens 107,
108, 109, A.M. 9796, 9797, 9798), and are apparently covered with sediments
which give a very smooth surface for a few miles to the northerly limits of the
basin.

The small volcanic vents seen at mile 565 are among the most striking
expressions of volcanism seen in the Gobi region. It is not possible to date

Sie
Later Sediments

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Injected Schists and Gneisses

FiGurE 25.—The margin of the northernmost large sediment basin traversed by the Expedition

Daba Mountains.

, and the beginning of the upward slope into the Gangin

The thickness of the lava flows between mile 570 and mile 577 is exaggerated.

79

Later Sediments

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FicuRE 26.—Gradual rise on the southern flank of the Gangin Daba Mountains, which here form the northern lip of the great basin of Mongolia.

80 GEOLOGY OF MONGOLIA

the outbreaks because the age of the sediments is not determined. One of
the most surprising things about the small cones is the great range in quality
of the material found on them. The maximum diameter of the top of the
first of the hills is not more than fifty feet. The second one is larger, but
apparently not over a hundred feet. There are no other representatives of
volcanic activity until the flows five miles to the north are reached. Perhaps,
however, the vents belong to the same epoch as the flows, and mark the
escape of gases from the same under-sources which gave rise to the flows.

At mile 577.5, the sedimentary basin is abruptly terminated, probably
by a fault, and ancient granite gneisses (Specimen 110, A.M. 9799) form the
floor for ninety miles. They are cut by dikes of granite and trachyte porphyry
(Specimens 111 and 112, A.M. 9800, 9801). The ground is for the most part
thinly covered with soil, but outcrops are frequent enough to determine the
quality of the floor. The composition of the rock is that of granite; the
structure is strongly gneissic, thoroughly crystalline and considerably in-
jected. The strike is across the course of the traverse; typical measurement
gives strike N. 55° E., and dip 80° S. There is considerable variation, how-
ever, in both strike and dip. The rock is a true gneiss. It shows shearing
effect and has the appearance of being the most thoroughly metamorphosed
and perhaps the most ancient formation in the region. This type of country
continues from mile 577.5 to mile 598, a distance of twenty miles (Fig. 26).
In a general way the character corresponds closely to that of the T’ai Shan
complex of China as described by Willis.

At mile 598, basin structure is encountered again, with basalt flows and
sediments not very unlike those seen a few miles south, but on a much smaller
scale. Outcrops are few and the structure is obscure, although the dip of
the flows first encountered is to the north. The basin continues to mile 604,
a distance of over six miles, where the old floor comes to the surface again,
apparently by warping. The formation here, for several miles, is a blackish-
green schist with a strongly developed structure, a strike N. 45° E., and a
dip to the northwest at a steep angle (Specimen 116, A.M. 9805). This is
undoubtedly one of the ancient series also, but its relation to the gneisses,
on the south side of the basin, is undetermined.

The same formation continues for two miles, then is covered for several
more. At mile 610, the granite gneisses with interbedded schists appear as
the floor again, very similar in general quality to those seen twenty-five miles
to the south. They are succeeded in a few miles by greenish and black
schists, which continue to mile 622 (Fig. 27). The schists are spotted and
streaked and very micaceous, and even, in places, very carbonaceous (Speci-
men 118, A.M. 9807). There are many whitish, lenticular bunches that look
like pebbles which have been sheared out and turned into sericite and quartz.

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82 GEOLOGY OF MONGOLIA

The range of composition indicates rather definitely a sedimentary origin.
The formation is cut by numerous quartz veins, and occasionally by dikes
or sills. The average strike is not far from east and west—N. 80° W. is a
common measurement. In general this formation has a simpler appearance
than the gneisses and is probably younger. The green schists probably be-
long to the Wu T’ai system of Bailey Willis (1907, II, page 4). This forma-
tion continues to the Arctic divide at mile 624.

The Arctic divide

An inconspicuous ridge marks the dividing line between the water sheds
of the desert and those of the Arctic. From this point northward and west-
ward for many miles the traverse lies within the Arctic slopes; and with this
superficial change there is a complete change also in the geology. An entirely
new series of rock formations constitutes the floor, and no later sediments are
preserved. At mile 623 beds of graywacke and quartzite (Specimen 1109,
A.M. 9808) form the adjacent hills. The rocks have a very definite, bedded
structure, dipping gently to the north, and are very hard and resistant to
weathering. Associated with them are interbedded layers of jasper (Speci-
men 120, A.M. 9809) and quartzite, but the dominant type is a graywacke.
Farther along, interbedded slates are abundant, but are inconspicuous in
the portion of the formation represented in the first few miles. The beds
first dip to the north, then in the course of a few miles the dip reverses toward
the south, apparently repeating the formational succession. The hills are
grass-covered, and the country becomes mountainous though not very rugged.

The structural relations between the graywacke formation and the green-
black schists seen farther to the south are not determined, though it is entirely
possible that they could readily be worked out in the vicinity of mile 622.
In our rapid reconnaissance, however, we had time to note only that the
quality of the floor and of the topography changes markedly in a comparatively
short distance.

The graywacke series continues without interruption and without marked
change of quality, except an occasional intrusion of igneous rock, to mile 636
along the main trail, and is known to continue on the same trail to Urga.
At mile 636, however, the direction of the traverse was changed, and, led by
a Mongol guide, the Expedition turned westward through valleys parallel to
the main mountain range, the Gangin Daba. Then by side trails over the
alluvial slopes of the ridges, the course continued thirty-two miles, to the
Bolkuk Gol, a tributary of the Tola River (Fig. 28). This change of direc-
tion carried the party diagonally across folded graywackes and graywacke-
slates to the new camp. Much of the journey lay over the trails of herds,
and part of it over alluvial slopes without a single mark of travel, yet in the

FROM IREN DABASU TO URGA 83

whole distance of thirty miles, after departing from the main caravan trail,
not a single stretch of ground too difficult for the motors was encountered.

Small streams appear here and there in this territory, and large flocks
and herds were seen everywhere. It is an ideal grazing district, but there is
absolutely no cultivation of the soil. Alluvial slopes extend down from the
higher ridges into the valley, but they are not heavily bowlder-strewn and are
nowhere deeply dissected. Rock ledges are exposed extensively in the hills
and locally along the valley floor on the banks of a stream.

Although mountainous country with more abundant precipitation has
been reached, there is no evidence whatever of general glaciation. The soils

May 18,1922

FicureE 29.—The northern valley wall of the Bolkuk Gol. The bowl-shaped head of a small lateral valley
resembles a cirque. It is possible that the shape of such valley heads is due to local glaciation. The floor is
solid rock, and at least the upper part of the sloping approach to the bowl is over bare rock.

and deposits of all kinds are essentially alluvial. But some valley heads in
the side of a mountain present the form of a cirque, and it is believed that these
are evidences of former very small, local glaciers of alpine type (Fig. 29). At
best they could have been of little consequence, although it is difficult to see
how the forms could be developed without the action of ice. Only three or
four such bowl-shaped valley heads were seen along the Bolkuk Gol, but
several hundred miles farther west, in the Sain Noin country, much more
typical cirques were seen and accepted, without question, as of glacial origin.

OBSERVATIONS IN THE VICINITY OF CAMP BOLKUK GOL

Bolkuk Gol, fifteen miles southwest of Urga, became the second ren-
dezvous. Here, near a local shrine, the whole Expedition assembled for the
first time, and established headquarters for the various parties, while Mr.

84 GEOLOGY OF MONGOLIA

Andrews was completing negotiations in Urga for the further progress of the
Expedition.

On the traverse across the Gobi we had become accustomed to a great
variety of weather. The first day had ended in rain. It was generally cold,
but while we were at Iren Dabasu, several days were uncomfortably hot.
On the way to Mount Tuerin, snowstorms were encountered, and a short part
of the traverse was made in the snow. As the party traveled northward
the cold increased, and, with the high winds that prevailed, some of the work
was done under extreme discomfort. On the whole, however, the air was dry,
clear, and bracing. .

The extreme cold of the season came at Camp Bolkuk Gol, where the
temperature readings, through the four days at this camp, ranged from 50°
Fahrenheit on May 15 to 17° on May 19. Readings just a little below freez-
ing were more common. Such temperatures as this, with the high winds,
made the task of keeping warm a difficult one for those of us who had work
to do on notebooks or maps; nevertheless, the surrounding country was scouted
persistently, since this camp was to be our closest approach to the city of
Urga.

Although the Bolkuk Gol valley is well grassed and is largely alluvial,
there are many outcrops on the higher reaches of the valley side, as well as
along the course of the creek. Everywhere the rock is either graywacke,
argillite, or quartzite. These different varieties belong to the same large,
important formation, characterized by dominance of graywacke over large
areas and by argillites in others, or by interbeddings of these two types. In
many outcrops of the graywacke the rock is almost structureless; but wherever
the argillites occur in them the bedding features are comparatively regular
and strongly marked. In general the strike is N. 60° E., with variable dip,
representing a succession of folds. In many places the dip is vertical or over-
turned. This general structural habit prevails over the whole territory crossed
from the main Urga trail to Bolkuk Gol, and afterwards was found to con-
tinue for a very long distance to the west. Toward the north, also, as far
as our explorations were carried, the same formations and formational habit
prevail, extending through the Tola River valley and the Gangin Daba range.

Igneous rocks and igneous influences are insignificant in this whole ter-
ritory adjacent to the Bolkuk Gol, although there is an occasional dike, and
perhaps there are metamorphic effects that are too obscure to trace. Farther
to the west, however, these igneous associations are greatly increased. The
deformation represented by the graywacke-argillite series is that of mountain
folding, of a regularity and persistence that is very striking indeed. The
topography is markedly different from that seen farther to the south, where
the country is characterized by the alternation of basin-filled sediments and

FROM IREN DABASU TO URGA 85

almost peneplaned oldrock divides. Here, on the contrary, the country is
mountainous, with subdued ridges and open valleys. The slopes are gentle,
especially as they approach the valley bottoms, and become much steepened
in the higher valley sides. All the slopes carry much finely broken material,
weathered to soil so that it is practicable to drive with motors even without
the aid of a trail. These slopes are remarkably free from gullies and outwash
courses, although there are occasional small side streams across which one has
to pick his way. Little material is being added or taken away.

Course of the Bolkuk Gol

In view of the fact that the rock structure of the region strikes N. 60°
E. or thereabout, and is of approximately the same quality on both sides of
the valley of the Bolkuk Gol, it is surprising to find the course of the stream
so poorly adjusted to it. The course lies diagonally across the structure.
It may follow a fault weakness, or it may, on the other hand, be a superim-
posed stream, preserving a course inherited from an original drainage that
was developed on some overlying formation which has been completely re-
moved. Later experience in other districts on the margin of the Gobi strength-
ens the inference that the superimposed stream explanation is probably the
better one. It is not an uncommon thing to find stripped areas where former
overlying sediments have been completely removed, and yet in such physio-
graphic relation that this step in its history is perfectly clear. It is only a
step, therefore, to such conditions as are presented at Bolkuk Gol, where
mature dissection has been reached in the underlying floor subsequent to the
stripping. The first stages, therefore, must have been reached much earlier
in the history of the Gobi region, and perhaps the stripping, in this case, cor-
responds to one of the periods of marked erosion, such as that following the
Cretaceous, or in one of the earlier epochs of the Tertiary.

The stream is a misfit in a valley too large for it, and there is a crude
development of planation that should correspond to one of the planation
epochs of larger development in the Gobi.

Glacial evidence in the Gangin Daba

The dominant erosion form is that of simple weathering and stream
work, but in a few places the heads of tributary valleys present a form which
attracts special attention because of its similarity to that of a cirque. No
perfectly formed cirques were seen in this vicinity, although several were
found in the Sain Noin region much farther to the west (Plate XIV, B, page
130). At Bolkuk Gol, however, there are the unusually steep heads of three
or four side tributaries. They are crudely bowl-shaped, with a distinct change

86 GEOLOGY OF MONGOLIA

of grade below the first portion of the deepened course (Figs. 29 and 30).
In one of them the excavation is made directly across the geological structure,
on rocks so slightly different in quality that there seemed to be no good struc-
tural reason for the form, unless, indeed, it had been made by ice. The floor
is virtually bare rock, and this, we believe, adds to the evidence favoring a
glacial gouging action. The bowl-shaped valley heads have not the freshness
of the cirques which were developed in the latest stage of the Ice Age. Their
form is subdued by weathering and stream erosion, and if they are of glacial
origin, they must have been made during one of the earlier glacial advances.

It is very clear that even in the mountains around Urga there is no evi-
dence of extensive glaciation. In the entire region there are no glacial de-

FicurE 30.—Field sketch in the Gangin Daba Mountains near Camp Bolkuk Gol. The streams head in
deep, bowl-shaped cavities which resemble cirques and may be due to a brief glaciation. (Compare with
Plate XIV, B, page 130.)

posits and no suggestion of the presence of an ice-sheet. During the glacial
period this may well have been a very cold, desolate region, but evidently
there was so little precipitation that an ice cover could not form, and, at best,
only here and there were small alpine glaciers developed.

This kind of history is indicated still more clearly in the district of Sain
Noin, where there is an entire lack of evidence of general glaciation, but where
glacial cirques are prominent, with characteristic form and product. Through-
out the whole Pleistocene, therefore, it appears that this region was exposed
to destructive attack from the ordinary surface agents. Erosion and deposi-
tion continued, in so far as they could continue with changed climatic con-
ditions, in the whole Gobi area, while regions much farther west and favored
with much heavier precipitation, were covered with glacial ice. No exception
to this rule was found in all our traverse in the Gobi. Nowhere, even in the
highest mountain ranges, was there evidence of glaciation, except the former
existence of small alpine glaciers.

FROM IREN DABASU TO URGA 87

Ground ice

A curious instance of ice beneath the soil was found first in the vicinity
of Camp Bolkuk Gol. In one portion of the valley bottom, attention was
attracted to a system of nearly straight, knife-sharp cracks cutting the turf.
The valley bottom was well grassed and constantly pastured by herds of cattle
and sheep. On examining the exposed edge of this ground, along the trench
made by the creek, we discovered that the turf was underlaid by a stratum of
ice a few feet thick. The overlying soil attains a thickness of on®to two feet.
The chief interest lay in the question whether the ice is a comparatively per-
manent member of the soil system, or whether it is formed each year by some
method of accumulation beneath the soil. The smoothness of the soil-surface
would argue for comparative permanence. If as much ice as this were to form
each year and be dissipated each summer, the repetition of lifting and drop-
ping the soil layer with its many cracks should result in considerable deforma-
tion of surface. Some deformity was observed at the edges of the system
and at places where thawing was more active, but on the whole there was little
irregularity traceable to this cause. We are inclined to think, therefore, that
the ice layer does not wholly thaw away in the short summer season, and that
such thawing as does take place proceeds with enough regularity to pre-
vent irregular slumping. Apparently the ground water, fed from the ad-
joining hills, enters the valley bottom and circulates in the gravel beneath
the turf at about this level, and develops a layer of ice as freezing pro-
ceeds in the winter season. Coming as a sheet of water, rather than as a
spring, it develops a thicker and thicker layer beneath the tough mat of
turf.

A similar process was noted at Sain Noin, at a valley-side spring. In
that case a mound several feet high, the core of which was solid ice, was
formed by lifting the firm turf layer. At the time this particular formation
was observed, the first warmth of spring had thawed the ice far enough to
have destroyed the peak of the mound. The turf had been cracked far
apart, and thawing had proceeded in the open portion, where a pool of water
was held in by the surrounding ice and turf. Whether the ice accumulation
at the spring is wholly destroyed each year and a new deposit subsequently
developed with each winter season, we were not able to determine; but here
again the simplicity of the form as a whole suggests the comparative perma-
nence of it, as though the ice were not each year wholly destroyed. Probably
the more fully covered portion is preserved beneath the soil.

Opportunity was not presented for other observations upon this under-
soil ice, but in many places similar physical surroundings were duplicated.
Underground ice seems to be a common feature in this far north country of
small surface precipitation.

88 GEOLOGY OF MONGOLIA

Local culture

Here, at Bolkuk Gol, for the first time, conditions were found favorable
for the support of large numbers of people. The country, although moun-
tainous or very hilly, is well grassed, even far up on the mountain sides.
Flocks and herds can be kept in large numbers, and villages are comparatively
numerous. Urga, the capital of Mongolia, lies in the district and is said to have
20,000 inhabitants. On one of the adjacent mountain ridges bordering the
Tola River #8 located the lamasery, which is the seat of the Great Lama, former-
ly the actual ruler of the country and still the head of its whole religious system.

In spite of the fact that the camp stood within fifteen or twenty miles
of the city of Urga, it was thought unwise to take the caravan or other equip-
ment into the city, because of a feeling of uncertainty in government circles
about the nature of the Expedition and the advisability of allowing it to con-
tinue upon its exploratory work. It was thought that further delay might
be occasioned by any move of this kind. Mr. Andrews, therefore, and Mr.
Shackelford, with Mr. Larsen, the official interpreter, conducted negotiations
within the city and ultimately secured the desired permit to continue. As
soon as this was.done, the whole Expedition moved westward, on the traverse
which led to Sain Noin and to the Altai mountains.

Side traverse towards Urga

During the stop at Bolkuk Gol, a side run was made toward the city of
Urga, for the purpose of connecting the geological section with that point
(Fig. 31). The same formations continue, with graywacke as the dominant
type of rock. In the cross section representing the side traverse, for the first
time a clear-cut, anticlinal fold in the graywacke series was detected. The
fold lies within the valley of the Tola River, which apparently is developed in
large part on such an anticlinal structure. The strata on the south side dip
toward the south, and those on the north dip northward. Whether the anti-
cline extends across the whole valley we could not tell, because we did not
cross the valley, but the minor structure is clear-cut and simple, and the rocks
on the south side of the valley correspond to the structure on a large scale.

The geologic section of this side traverse, added to the distance covered
after leaving the Urga trail to Bolkuk Gol, shows nearly fifty miles on a single
rock formation; but the course taken is roundabout and therefore gives an
exaggerated picture of the importance of the simply folded graywacke series.
If the traverse had been continued directly to Urga from mile 636, it could
have been completed in an additional twenty miles, and the structure would
have been correspondingly simpler. It is evident, however, that the same
type of country prevails still farther to the north, and later experience showed
that the series of formations encountered here, forming the basis of these

FROM IREN DABASU TO URGA 89

mountain ranges, characterizes the country and gives it its mountainous as-
pect for several hundred miles to the west. It is evident, from the immense
area covered and the many miles that one may travel across the truncated

675 680

FicurE 31.—Side traverse from Camp Bolkuk Gol to Urga through open folds of graywacke,
argillite, and slate.
edges of the strata, that this is one of the great formations of Mongolia. Its
enormous extent, its constancy, and the complete failure of fossil content,
make it not only an important structural element but a great geological prob-
lem as well.

Mineral resources

It is well known, of course, that there are mineral deposits within the
borders of Mongolia, not far from Urga, but the conditions where these occur
must be materially different from those prevailing in the districts farther
south. There are probably few regions in the world with a mineral supply
as scanty as that of the greater portion of the Gobi desert. Mineral develop-
ment is a practical impossibility. It is appreciated, of course, that this com-
ment does not apply to the marginal country where the geologic conditions
may be very different, but it does apply to the region as far north as the
Tola River, in the vicinity of Urga. In this traverse of more than 650 miles,
not a single mineral specimen of economic interest was discovered. Quartz
veins were seen by the thousand, and rock ledges of all kinds were examined,
but no promise of economic returns was found in them. A few beds of coal
of poor quality were seen later in Jurassic strata farther to the west, and coal
may possibly exist in the Jurassic formations crossed by the traverse to Urga,
but it was not exposed, and none, thus far seen anywhere in the Gobi, is of
sufficiently high quality to attract attention. Besides these coal beds there
is nothing of economic interest, and at present they have absolutely no value.
We believe that there are good reasons for the mineral habit here encountered,
and for the fact that in some of the marginal country much better conditions
are reported. This will be discussed more fully, however, at a later time, in
connection with the final summaries, which, it is hoped, may grow out of this
series of studies.

CHAPTER V
FROM URGA TO TSETSENWAN

On May 19, in an early morning temperature of 17° Fahrenheit, we be-
gan the westerly traverse from Camp Bolkuk Gol. Our way lay through
valleys tributary to the Tola River, and within ten miles led into the main
caravan trail along the Tola River from Urga to Sain Noin (Fig. 32). The
route at first followed an exceedingly roundabout course, and was covered
more deeply than usual with soil and river alluvium, presenting great diffi-
culties in the task of keeping a true geologic cross section. We have, however,
attempted to do this, by eliminating as much as possible of the duplication,
and by reducing the whole traverse to simple structural form.

TRAVERSE ALONG THE TOLA RIVER

When the course has reached the Tola River it becomes more direct, and
continues so for many miles, though outcrops are less frequent. The average
course cuts diagonally across the general formational trend, at a low angle.
There is nowhere any doubt about the character of the underlying formation.
It is the graywacke-argillite series, with simple folded structure and post-
mature erosion. The recorded dips indicate that the first day’s journey crossed
two large folds in this diagonal manner.

At numerous places the terraced form of the valley appeared prominently.
The Tola is one of the great rivers of northern Mongolia, with a course westerly
and southwesterly from Urga for about fifty miles, before it makes an abrupt
turn to the north, joining the Orkhon River and following the Selenga drainage
to Lake Baikal. For the greater part of its course the Tola seems quite in-
dependent of the folded structure of the country, although in minor respects
some adjustment is evident. The valley ranges from three to five miles in
width, and has a wide, flat floor on which the river meanders broadly. The
flood plain is bordered by wide rock-terraces bearing remnants of older al-
luvium.

For fifty miles our route lay along the Tola River, so far down in the val-

90

FROM URGA TO TSETSENWAN gI

ley that the traverse profile is unusually monotonous and the geologic structure
largely a matter of inference (Figs. 32 and 33). Outcrops are rare along the
trail, and it was thought inadvisable to take the time to check by means of
side excursions, since there was no doubt whatever of the general structural
relations. But as the traverse proceeded, igneous activity became more and
more evident. An occasional dike of porphyry appeared at first, and as we
progressed these occurrences became more numerous and more prominent.
At the Tola River camp, mile 732, sixty-four miles southwest of Bolkuk Gol,
we saw in the distance many of these large dikes in the mountain-sides across
the river (Fig. 34). They can be traced in parallel alignment, brought out
by differences of relief, since most of them are more resistant to weathering
than the graywackes and slates through which they cut. They constitute a
striking minor physiographic effect. Evidently they are the result of out-
break from some extensive magmatic body which lay beneath. Further ex-
plorations along this traverse were soon to reveal the character of this igneous
mass, but up to this point its nature had not been determined.

At mile 748, the trail turned southward, leaving the valley of the Tola
River, and began to climb into higher country, at the same time turning more
directly across the rock structure of the region. The same kinds of graywackes
and slates continued for another day’s journey, but igneous intrusions increased
in number and size. The commonest rock type is a form of granite or granite
porphyry, and an occasional mass of this kind stands in isolated mountain-
like relief, adding materially to the picturesqueness of the country. It is
clear that the igneous rock has forced its way up through the graywacke-slate
series and is a much later formation, yet there is, in most cases, very little
metamorphic effect. This is especially true where the granites break through
almost vertically.

In a few places, however, where there is reason to believe that the contact
is more gently sloping, we noted a variety of metamorphic effects. In some
places the formation becomes phyllitic and schistose, and much more crystal-
line than the original rock. These changes are so striking in the vicinity of
the first camp south of the Tola River, located at mile 769 and called Five
Antelope Camp, that specimens of the modified rocks, taken out of their known
setting, would present few evidences of their true relationship (Fig. 35). One
would be inclined to classify them with older and more uniformly metamor-
phosed formations; but the party was fortunate enough to find all gradations
of quality in the neighborhood of the camp, from the simple graywacke-slate
varieties to highly metamorphic phyllites and schists,—the difference appear-
ing to depend on their proximity to or distance from the igneous contact.

In this locality, granite and granite-porphyry outbreaks constitute at
least a quarter of the total section, and as a consequence a much greater pro-

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3 Ajueau ayxiays spjo4 a 199 4N410G

g2

FROM URGA TO TSETSENWAN 93

portion of the associated rocks are modified, and attain greater complexity of
composition and minor structure than elsewhere. These intrusives are all
doubtless representatives of an underlying magmatic body,—a mass that we
later designated as the great Mongolian bathylith (Berkey and Morris, 1924a).

THE VICINITY OF FIVE ANTELOPE CAMP

A day’s stop at this camp enabled us to make a more thorough inspection
of the rock floor, which had attracted attention at the close of the preceding
day, and which proved to be more complex than any within the last hundred
and fifty miles. There is considerable schistosity, with a great abundance
and variety of igneous material. Not only is the igneous content abundant,
but it has its own complexities, represented in great numbers of dikes and
sills, stringers and lenses of almost every range of composition between alaskite
and pegmatitic quartz on the one hand, to the composition of a dolerite on the
other.

The foundation rock is a sediment, very siliceous and of rather obscure
structure. It is highly indurated and tends to become phyllitic. The general
strike is nearly east and west,—N. 80° E. being one of the definite readings,—
and the intrusions take on about the same structural trend; some of them
show streaked structure, in contrast to the majority which do not show any
similar deformation effect. They cut each other in a complex way, and some
of them include xenoliths exhibiting a schistose structure older than the in-
trusion. Such phenomena seemed to indicate beyond question that the for-
mation was considerably metamorphosed before these igneous members were
injected into it, but it still was not clear whether this particular igneous activity
was responsible for the greater metamorphism of the formation found at this
camp.

The first working hypothesis assumed that there are two very different
formations,—one the graywacke-slate series of the country traversed from
Urga to this camp, and the other represented by this more metamorphic type
of rock. Such field examination as could be made, however, failed to show
any distinct break in the sedimentary series, and it was finally concluded that
the whole assemblage of beds, with its many differences of condition and qual-
ity, really belongs to the graywacke-slate series, and that the strip of ground
between the granite gateway at mile 760 and Five Antelope Camp at mile
769 represents a deeply eroded anticline in this formation. The center of
the anticlinal arch exposed the lower beds of the series, which are somewhat
different from the upper members over which most of the traverse had been
made. They have also been modified extensively by the encroachment of a
bathylithic granite mass, which in some places sent into the former roof

94 GEOLOGY OF MONGOLIA

larger intrusive masses which now stand as granite mountains at numerous
points in the vicinity. Some of these are essentially stocks or bosses, many
of which stand in very bold relief above the average sweep of the country.

The sedimentary formation, interpreted in this way, includes not only
the graywackes and slates or argillites but also a few beds of jasper and
quartzite, as well as local developments of phyllite and of schist. Some of
them are very massive and exercise a distinct local control in the deformation
of the region, causing considerable minor crumpling and drag effect in the
associated, less competent members.

The conclusion that no new formation had been encountered but that the
difference in appearance and quality of the rocks is a contact effect, due to
the influence of an underlying bathylith, served as a successful working hy-
pothesis for the next steps of the traverse, and the experience of the remainder
of the season’s investigation tended to support this explanation in all respects.

From the field observations thus far made, it is reasonable to conclude
that the great graywacke-slate series in this northerly region along the Tola
River is made up of three members: (1) banded slates or argillites, which
probably form the top; (2) interbedded graywackes and argillites, forming
the middle; (3) quartzites, jaspers, and phyllites, forming the bottom. It
is chiefly with the lower member of the series that we have concerned ourselves
in the vicinity of Five Antelope Camp.

It is evident, of course, that the series must be of immense thickness—
many thousands of feet—but there has been no way of determining its measure.
That it is one of the great formations of the region is perfectly clear, and its
general structural position is also determinable; but its exact age is in doubt,
as no fossils whatever were recovered from it.

The origin of so immense a series of sediments, especially with the dom-
inance of the graywacke type of sandstone and wholly barren slates, is equally
obscure, and when one takes into account the great areal extent over which
rocks of this type are now known to be distributed, one is confronted with a
major problem of the geologic column, deserving special study on its own
account.

FROM THE TOLA RIVER TO TSETSENWAN

After leaving the Tola River, the course of the traverse lay to the south
and southwest across a region of broad, rolling topography, above the general
level of which stand numerous rugged mountain masses of no very large
extent (Fig. 35). These all prove to be igneous intrusions of granite-porphyry,
granite or syenite. Their sharply defined, cliff-like margins rise abruptly
above the general erosion level, which seems to control the rest of the topog-

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aures ay} Ul peziesoues ‘Ef pue zf sain3y jo sayoemAess papjoy Apdums ayy Jo woryenuTyUOD) “dureg Jeary BOL 4 uorjoas o1s0joan—'be aUNoIy

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Kayjea sata
sadser buraeay Yynog pausny

95

96 GEOLOGY OF MONGOLIA

raphy. This habit prevails as far as one can see in every direction, and seems
to characterize a certain zone of transition between areas in which the granite
bathylith forms large portions of the surface, and others in which the over-
lying formations prevail.

One of these granite mountains, standing now as an erosion remnant,
lies along the line of traverse a short distance southwest of Five Antelope
Camp. In its vicinity all of the sedimentary formations are much modified,
a condition which is due to the proximity of igneous masses. In the gray-
wacke-slate series along this traverse, near the granite mountain, we encount-
ered an extensive development of comparatively pure quartzite. It is, however,
only a facies of the graywacke formation.

Our course led us to the southwest toward Tsetsenwan, past a lake basin
from mile 780 to mile 790, small and shallow like hundreds of others scattered
along the northerly border of the Gobi proper. It contains two small lakes,
both salt; in one of which water was standing, while the other was almost
dry. Accumulation of sediment is continuing in this basin, and on the margin
at one or two points we saw a little of the exposed material; but it was not
possible to determine either how much of an accumulation there was at this
point or what its age might be.

Beyond this basin the graywacke-slate series continues with granite in-
trusions and many porphyry dikes, repeating the general structural relations
of the country (Fig. 37). A considerable change, however, takes place at
miles 795-796, where much greater igneous activity is in evidence. There
are large black sills at mile 795, cutting through the graywacke, and at mile
796 a great exhibit of basalt and andesite flows and breccias. This is a type
of rock entirely different from any seen within 200 miles, and it is clearly of
much later origin than anything encountered since leaving the vicinity of
Urga. Subsequent inspection revealed that there is a large crush zone, 300
feet wide, with no appreciable change of topography; the general erosion plane
is unbroken. Here is evidently a great zone of weakness belonging to a large
fault movement, and it is reasonable to assume that the lavas have found
their way up along the weaknesses of the old fault zone. Structural relations
such as this have been noted in many other places, particularly along the edges
of faulted Jurassic strata; but this is the first time in the traverses of the
Expedition that basalts and tuffs were noted wholly associated with very
ancient formations. In all essential respects the products look the same as
those associated with Jurassic strata. It is possible, however, that they are
still younger, because similar outbreaks were noted in other places associated
with fault blocks of Cretaceous and Tertiary age.

The igneous formations extend toward the south beyond the range of
our inspection. Doubtless in that direction a more complete structural re-

FROM URGA TO TSETSENWAN 97

lation could be worked out. A traverse made from Tsetsenwan some days
later, extending forty miles to the south, actually encountered rocks of this
type associated with deformed Jurassic strata, and perhaps still younger
formations. The two areas of igneous rocks are probably continuous, belong
to the same structural unit, and have the same explanation.

The exposure at mile 796 is a remarkable breccia, almost like a conglom-
erate in form, three or four hundred feet wide in the best exposure, and vary-
ing in quality in successive layers or zones. In spite of this appearance, a
thorough inspection indicates that it is essentially a fault breccia and belongs,
undoubtedly, to the crush-zone already described. It is very deceptive be-
cause of its variable appearance, which strikingly resembles bedding.

Beyond the complexities introduced by the fault zone and the volcanics
at mile 796, the traverse lay for a short distance across the graywacke series,
and then over granite cut by many dikes, to the edge of a block of ground
standing somewhat above the average level and presenting a rather abrupt
escarpment ten to thirty feet in height, which proved to be the edge of another
area of graywacke and slate. Further examination showed that it is again
bordered by granite on the opposite side, two or three miles farther west.
Moreover, when the northerly end of the block was reached by a detour on
the trail in that direction, we found that there, too, it is bordered by granite.
The block is clearly a great roof-pendant, wholly surrounded and supported
by granite. It is not necessarily the first one encountered, but it is the first
large, clearly outlined block whose structural relations we had been able to
prove definitely. It isa typical roof-pendant, a remnant of downward extend-
ing portions of the original cap now almost completely removed by erosion.

Granite, cut by thousands of dikes, continues as far as the temple of
Tsetsenwan, which stands on an alluvial fan at the foot of a mountain block.
A run of several miles across the fan, to the mouth of a gulch at the edge of
the block, brought us to the camp at Tsetsenwan, mile 815 (Fig. 36, A and B).

THE VICINITY OF TSETSENWAN

At Tsetsenwan a stop of several days was made, connection with the
caravan was established, and supplies were obtained for the next lap of the
journey, which would take us to Sain Noin. This stop afforded an oppor-
tunity for examination of the features of special interest in the vicinity, and
for side traverses both to the north and to the south.

The district about Tsetsenwan proved to be especially favorable for the
geologists. The key to several puzzling geologic problems was found at this
place; in fact, some of the most fundamental structural relations, which have
a bearing upon the geologic problems of the entire Gobi region, were exhibited

VOL. I—7

98 GEOLOGY OF MONGOLIA

FiGuRE 36A.—The fault block at Tsetsenwan. The front is abrupt, and a gently arched peneplane bevels
the upland surface of the block.

so plainly here that they could not be mistaken. Among these problems are
the following:

1. The bathylithic nature of the granites of Mongolia.

2. The contact metamorphic origin of many of the variations exhibited
by the graywacke-slate series.

3. The extremely great erosion interval between the Jurassic strata and
the graywacke-slate-granite floor.

4. The great thickness of Jurassic strata, and their simple folded structure.

5. The appearance of intrusive porphyries and outbreaking volcanics on
a large scale, either during or immediately following the Jurassic period.

6. The probable unity of the varied igneous formations of the Gobi re-
gion, and their genetic connection with the great bathylith.

7. Block-faulting as a type of deformation in later geologic time.

A side traverse north of Tsetsenwan

North of Tsetsenwan a stretch of several miles is thinly covered with
disintegration débris and soil, so that the character of the rock floor is a little
more obscure than usual. The outcrops indicate that the chief underlying
rock is a coarse-grained granite, which is cut by thousands of dikes of great
petrographic variety and of equally variable size (Fig. 38). The dikes cut
one another at all angles, and traverse the granite in winding, snaky courses,
giving the landscape the aspect of some ruined city of giants, where the dikes
are the wreck of a thousand crooked walls. Our field name for the district
was the “country of the serpent-form dikes” (Fig. 39).

The serpent-form dikes

The dikes have a decided tendency to stand out in relief on the surface,—
enough relief, at least, to make the courses of the larger ones readily traceable

FROM URGA TO TSETSENWAN 99

FicureE 36B.—Continuation of figure 36A, the fault block at Tsetsenwan.

as one views such an area,—whereas the granite foundation through which
they cut has been more deeply etched under weathering influences. This
was noted repeatedly in areas of similar structural relation, where the granite
surface, as now exposed, corresponds closely to the upper portion of the bathy-
lithic mass, and where the roof has been removed.

Some of the larger dikes, particularly those of granite or granite-porphyry
composition, take a comparatively direct course, as if following weaknesses
of larger regional significance (Fig. 39); but the others are surprisingly irreg-
ular, and must have broken through the granite along weaknesses distinctly
different in origin from those connected with regional deformation. Later
observations of similar occurrences in other places,—none of them, however,
as well developed as those at Tsetsenwan,—finally led us to the conclusion
that, in spite of the great compositional variety, which ranges from alaskite
porphyry to melaphyre, and in which undoubtedly hundreds of facies of a
general porphyry type can be observed, the dikes are all the product of the
differentiating granite mass itself. There is, of course, no way of proving
this; but it is unbelievable that there are as many new igneous encroachments
from beneath as there are types of rock. It is much more reasonable to sup-
pose that the great bathylithic magma underwent elaborate differentiation
over an immensely long period after its upper and marginal portion had
cooled sufficiently to form the granite now found on the margins and beneath
the old roof. Eruptive activity must have been many times renewed, and
there must have been abundant opportunity subsequently for the sending
out of molten material from the still unsolidified deeper portions. As the
outer margins cooled still more and developed shrinkage cracks, and as de-
formation developed and originated other weaknesses, the still molten por-
tions beneath were enabled to penetrate through the cap of solidified granite
and into the roof. Under such conditions, multitudes of intruding tongues

100 GEOLOGY OF MONGOLIA

and stringers would develop, and an abundance of penetrating solution-differ-
entiates would finally solidify in this marginal and roof portion, where they
now appear as dikes.

If this were to happen repeatedly through long periods of time, at suc-
cessive stages of local differentiation as diverse compositional concentrations
were reached, there should be dikes of exceedingly great variety, as great
in range of composition, indeed, as it is possible for the differentiation of
a great bathylithic magma to produce in the whole course of its history.
It is reasonable to expect that at certain stages the composition would
not vary greatly from that of the granite of the roof itself, though the tex-
ture might be quite different. At the other extreme, it is reasonable to
expect that very basic constituents should prevail and that a composition
approximating that of a diorite, a dolerite or a basalt-porphyry should be
produced.

With this explanation, the whole system of confused dikes takes on a
comparatively simple genetic meaning. Even the great variety of composi-
tion is understandable, for it reduces to a single process. The dikes are all
relatives—they are all the products of the same process—and they all come
from the same general source; but they are of different ages, and represent
very diverse stages and degrees of modification in the history of the original
magma. It is our belief that they have no other significance, and that this
interpretation is supported by the fact that the dikes clearly cut one another
in a most complex, interlocking system, and present distinct proof, in them-
selves, that some of them had been formed and were solid crystalline bodies
long before others were intruded.

If this explanation be true, the dikes present a most remarkable exhibition
of the great range of differentiation products formed by a large bathylith.
How many of them ever extended up into the overlying roof of graywacke-
slate, one cannot tell; but the roof-pendants which still remain carry great
numbers of them. This condition is admirably illustrated in the large roof-
pendant east of Tsetsenwan which has already been mentioned in the itinerary
description. The intrusives habitually take on a certain regularity imposed
by the structural control of the bedded host. It is our observation that they
are not by any means as numerous in the roof formation as in the granite
beneath; and probably many of the dikes, especially the smaller ones, were
virtually confined to the granite itself, penetrating its shrinkage cracks but
not reaching far into its cover. At any rate, in immediately adjacent ground,
where conditions should have been almost precisely similar, as, for example,
in the ground north and east of Tsetsenwan, there is a much greater number
of noticeable dikes in the exposed granite areas than in the ground still covered
with the graywacke and slate.

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101

102 GEOLOGY OF MONGOLIA

Contact effects of the granite margin

The granite country, cut by dikes, continues for some few miles, beyond
which the floor is again the steeply upturned graywacke series. From the
first high prominence one can look out to the north and northwest over an
endless sea of similar hills that are probably of the same formation. Turning
to the north, a distinct line of demarcation was detected, dividing this rugged
graywacke country from an area of more rolling ground. The latter is charac-
terized by granite cut by the serpent-form dikes (Fig. 39). On the west side
of the line the country exhibits a rugged character, with a mountainous aspect
in the distance. It also has a certain regularity in the relief forms produced,
because of the structural trend of the graywacke formation. At first we as-

‘=

FicureE 39.—Margin of a roof-pendant north of Tsetsenwan. (See figure 38.) The hills are of folded
graywacke. The granite, less resistant than the graywacke, has weathered out to form a lowland, shown
in the right-hand half of the sketch. The dikes withstand weathering better than the granite, and are etched
out in relief so that they stand up like stone walls. The section below is drawn through the large obo-crowned
hill in the background of the sketch, and shows the folded graywacke undercut by the granite, which is traversed
in turn by a host of dikes of many kinds,

sumed that there must be a fault line between these two distinctly different
types of country, and that considerable displacement might be represented in
bringing side by side two areas so strikingly unlike.

A traverse was therefore made across this contact line, for the purpose
of determining its structural significance. We had already noted that the
graywacke-slate series in this mountain is exceedingly variable, showing con-
siderably greater tendency to crystalline condition than usual, and that there
are occasional small dikes or sills injected into the strata. When passing down
over the mountain side to the contact zone at the base of the hill, we observed

FROM URGA TO TSETSENWAN 103

that the graywacke becomes still more complexly modified, and in places is
saturated with and almost replaced by substances of foreign origin. One of
these substances, most prominent in the changed material, proved to be tour-
maline. This is, therefore, an igneous contact, and has no necessary relation
to faulting. On crossing from the graywacke to the granite there is no de-
formation effect or displacement to be seen. The granite simply emerges
from beneath the graywacke, which is extremely modified at the margin, the
intensity of the modification diminishing noticeably with distance from the
contact.

This exhibit proved beyond question that the variety of qualities in the
graywacke seen at many other places, where question of its meaning was
raised, is an entirely normal product of the contact with the granite bathylith.
Close inspection was made of this ground, which is heavily mineralized, to
determine whether it carries metallic content. Practically nothing of this
sort was seen, even though extensive replacement and large introduction of
other substances are evident. The mineralization is essentially of non-metallic
type. Marked effects are produced, but metals are lacking. This may not
be universal with respect to the Mongolian bathylith. It may be characteristic
only of the central portion, while quite other conditions and products may
characterize the marginal zone of the great mass. It is our judgment, from
the structural relations encountered, that we did not reach such a margin on
any other part of the summer’s traverse; but we are quite prepared to believe
that an entirely different mineralization might be found outside of the region
covered by our own work.

Jurassic structural relations

The northerly traverse from Tsetsenwan had been undertaken in response
to a report from local Mongols that ‘‘dragon bones’’ were to be found in that
vicinity. We found none, and it is not at all likely that the strata of the
locality could by any possibility furnish such material. The trip proved in-
teresting, nevertheless, for our attention was arrested by two hills which are
distinctly different in appearance from their neighbors, and which show an
outline suggesting a much simpler structure than that of the granite or of the ©
graywacke series (Fig. 38, and Fig. 126, page 293).

In order to examine these two hills, a second trip was made, which yielded
very suggestive results. The formations involved in the two hills proved to
be simple, unmetamorphosed sedimentary strata lying nearly flat, or at most,
only slightly folded. The rock is conglomeratic sandstone, with plain bed-
ding. It is almost devoid of fossils, but we discovered a few plant remains.
Some of the stems of these plants are several inches across. They not only
make impressions on the sandstone, but occasionally leave cavities in it, where

104 GEOLOGY OF MONGOLIA

the woody tissue has been removed by percolating waters. The fossils are
not perfect enough to enable one to make specific determination of the age
of the sandstone, but the fact that only plant remains are found in it led us
to conclude tentatively that the formation is of essentially the same age as
the coal-bearing sandstones of northern China, which have been assigned to
the Jurassic. North of Tsetsenwan these strata rest unconformably on the
eroded granite and its many dikes, whereas the graywacke series is invaded
by the same granite (Fig. 126, page 293). It is clear that the graywacke
series was folded and extensively invaded by granite coming from the develop-
ment of a great bathylith beneath several hundred thousand square miles of
territory. Then followed a long interval of erosion, during which the gray-
wacke roof was worn away before the Jurassic conglomerates were laid down.
Since the graywacke is invaded by the granite, while the conglomerates rest
unconformably on the eroded surface of the granite, it follows that there
must be a great difference in age between the graywackes and the con-
glomerates.

The Jurassic strata in turn have been tilted, faulted, and locally folded,
and very deeply eroded. Only a few hundred feet of the conglomerate re-
main in this outlying formation, but the sediments are similar in all respects
to the conglomeratic sandstones forming the basal members of the series at
many other places. Much better opportunity was found a few miles south
of Tsetsenwan to determine the great thickness of the whole formation, but
in the northerly outliers were found the first clear evidences of the great
erosion break that lies beneath them. Elsewhere this formation is folded in
much the same manner as the graywacke series, and except for some such
exhibit as this, the importance of the structural break between them would
not have been so apparent.

It is difficult to reconstruct the topography of the country when the
Jurassic strata were laid down. The most suggestive thing about it is the
fact that on this old floor there was deposited an enormously thick series of
conglomerates and quartz sandstones. Wherever the floor is visible, it looks
sufficiently uniform to have been a peneplane, but it is clear that the thick
beds of pebbles and sand were washed down upon it from fairly rugged hills.
There is no evidence whatever of marine influence. Throughout the Gobi
region the Jurassic strata lack marine fossils, and, so far as seen, contain only
afew plant remains. It isa striking feature of the geologic history of Mongolia
that, from the time of the deposition of these continental conglomerates to
the present, there is no evidence whatsoever of marine invasion. We con-
clude that the region has been continental since the great conglomerate-sand-
stone series first began to form, at some time in the Triassic or early in the
Jurassic period.

FROM URGA TO TSETSENWAN 105

A side traverse to the south of Tsetsenwan

On May 29, a side traverse for a distance of forty-five miles was made
to the south of Camp Tsetsenwan, in order to extend geologic observations
across the structure of the country (Fig. 40).

The geologic formations crossed in the course of this southerly traverse
included an exceptionally good display of the strata that lie on the old eroded
granite floor. The first two miles of the trail led across the fan at the foot
of the Tsetsenwan mountain block, and in that distance no trace of the rock
floor could be seen, although there is little doubt that it is the same granite
which emerges from beneath the edge of the fan a little farther on. From
mile 5 to mile 8 the trail lay across the eroded edges of a portion of the gray-
wacke-slate series which represents a roof-pendant wholly surrounded by
granite. This block of graywacke has no topographic expression, except that
outcropping ledges are a little more frequent along that portion of the trail
than on the granite. In some places it is very greatly modified by contact
influences, undoubtedly originating from the granite bathylith. Beyond this
the trail lay for three miles over bare granite with the usual dikes: but from
this point onward the graywacke and slate roof again forms the country rock
continuously to mile 20, where the first conglomerates were found, lying un-
conformably on the deeply eroded graywacke series.

The first patch of conglomerate, less than a mile in extent, is only an out-
lying erosion remnant, occupying the bottom of a shallow syncline. Beyond
it the broad anticlinal arch is completely denuded, so that the graywacke
series peeps through and forms the surface again. From mile 21.5, however,
for a distance of twenty miles, sedimentary formations of Jurassic and later
age are crossed continuously. For the first ten miles, the strata, consisting
of conglomerates and sandstones of exceedingly great thickness, clearly belong
to a single formation. The basal members are heavily conglomeratic and
obscurely bedded, while the higher members, consisting chiefly of sandstone,
are clearly stratified. The dip varies from only a few degrees to as much as
60 degrees at different places within this ten-mile section. For the first five
miles, the dip gradually increases from a scant two or three degrees where the
basal members are first encountered, to 30 and 35 and finally to 60 degrees;
over the first three miles the rocks are chiefly conglomerate, and over the last
two, dominantly sandstone. Beyond that point the sandstones continue
with varying dips, for five miles.

In general, the dip is to the south throughout this whole distance, and as
far as could be determined there is no repetition of the individual beds except
at one point, near mile 30, where there is a possible reversal of dip for a short
distance. If it be true that there is no repetition of beds of any consequence,
the section represents an enormous thickness of strata. There is a total

106 GEOLOGY OF MONGOLIA

distance of ten miles before any change of formation takes place, and the
average dip must be close to 30 degrees throughout the distance. On this
basis, there must be no less than twenty or twenty-five thousand feet of strata,
all conglomerates and sandstones. For long stretches the trail passes over
bare, thin-bedded sandstones standing on edge in so simple and so well-exposed
a condition that no important change could have been overlooked. The
inspection, of course, could not be exhaustive, but the fact that no fossils
were found in such extensive exposures of strata, largely quite open to observa-
tion, shows that the formation is unusually barren. There is no way of telling
the age. These strata are referred to the Jurassic period on the same grounds
as those which previously have been relied on for conglomerates and sand-
stones of this type. They are undoubtedly the same formation, and were at
one time doubtless continuous with the remnants found ten miles north of
Tsetsenwan, where plant remains were found in some abundance in basal
conglomerates of the same sort. It is possible, of course, that no portion of
this series is of Jurassic age, but the number of other possibilities is not great.
There is everywhere an unconformity above this conglomerate-sandstone
series, and the lowest formation above the unconformity, as we afterwards
learned, belongs to Lower Cretaceous time. It is clear, therefore, that these
strata must have been formed long enough before the beginning of Lower
Cretaceous time to allow for deformation and profound erosion. It is prob-
able that, in part, they reach back into the Triassic period, but Palzozoic
possibilities are eliminated by the discovery that the Permian was a time of
marine deposition, and strata of that age carry abundant marine fossils.
Profound deformation, followed by extensive erosion, then preceded the de-
position of this formation.

The conglomerate-sandstone series, therefore, which represents the con-
tinental type of deposit, must have developed in the time intervening between
two profound breaks in the column, one at the end of the Palzozoic era, when
changes occurred which transformed the region into a continental area, and
the other, just preceding Lower Cretaceous time, when the last mountain-
folding took place and the last great unconformity was developed. Perhaps
the series is in part Triassic and in part Jurassic, but it has the same significance
in either case. For simplicity, it is convenient to refer to this formational
series as of Jurassic age.

At mile 31.5 to mile 32, the trail crosses a complex of eruptive porphyry
(Fig. 41). The rock is dense, brittle, much fractured, variable, with little
regularity of structure. It is not clear whether the rock should be regarded
as part of the Jurassic formation just described, or part of the volcanic series
to follow, or, yet again, a part of the older floor unrelated to either. At the
time of making the traverse these porphyries were considered as being in all

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107

108 GEOLOGY OF MONGOLIA

probability a part of the Jurassic series, representing flows and intrusive sills
of that time, which have shared the deformation affecting the whole system,
and so have taken on some of the complexities of structure, especially a
peculiarly fractured condition. This explanation seemed reasonable in view
of the fact that the porphyries, whose structure could not be clearly made out,
are succeeded immediately on the south by a great series of volcanic flows,
breccias and tuffs, with associated sills and reworked ashes, which dip south-
ward at first, in much the same manner as do the Jurassic sandstones. They
are much less consolidated, however, and, in the absence of any evidence as
to their age, it is quite impossible to determine whether this volcanic series
really belongs to the Jurassic formation or to an entirely different and later
one. Later experience in other localities led us to consider that they may be
of Cretaceous age, or even younger.

The volcanic series forms a simple syncline for a distance of six miles,
and represents again an enormous thickness of material. The beds begin
with a dip of 30 degrees to the south, but this is much flattened toward the
bottom of the syncline and is completely reversed on the other limb. The
total exposed thickness is probably five thousand feet.

On the south, at mile 38, the country is comparatively smooth, where the
series passes beneath a cover of uncertain character. The chips of rock seen
on the surface do not resemble the volcanics seen farther north, and probably
in this shallow basin from miles 38 to 41, there is a thin cover of later sediments
not belonging to either series just described.

After crossing three miles of this covered ground, an abrupt change was
encountered. Sheared porphyritic granite, granite porphyry and gneiss, with
a prominent internal sheared structure, form the floor, all well exposed in a
trench marking the outlet of the basin. This abrupt change undoubtedly
marks a fault. The ancient floor is brought to the surface again, and the
broad sedimentary block crossed in the last twenty miles is at that point
sharply terminated. If the whole block to the north belongs to the Jurassic
formation, there may be an enormous displacement along this fault line.
The whole of the sandstone-conglomerate series is cut out, either here or
farther back along the traverse at mile 32, where the complex porphyries
were first encountered. In either case, there must be great displacement
along the fault at mile 41, or at mile 32, where the porphyries have broken
through, or, perhaps, at both places.

The chief reason for considering that the volcanic series, from miles 33
to 38, belongs either to the Jurassic or to the Cretaceous, is the fact that the
series is gently folded. The strata form a simple syncline quite consistent
with the gently arched structure of the sandstones and conglomerates of the
northerly edge of the block. Elsewhere in the Gobi region, folding on such

FROM URGA TO TSETSENWAN , 109

a scale has not been seen in formations later than the Cretaceous. At Camp
Jurassic on the Urga trail, an anticlinal fold affects the formations, which
overlie the Jurassic strata unconformably, forming an arch several miles across.
Perhaps this is a similar structure,—it is certainly not very often found in
strata of Tertiary age.

With some authority, therefore, for the latter interpretation, and with
the support of observations made elsewhere in the Gobi region, we are inclined
now to regard this sedimentary block as being made up of four different units:

1. A simple conglomerate-sandstone formation, of Jurassic age.

2. An eruptive outbreak of porphyries, probably along a great fault line
at the southerly edge of the Jurassic block.

3. A group of sediments, with volcanic flows and ashes of probable
Cretaceous age, gently warped rather than strongly folded.

4. A very thin local accumulation of later basin sediments.

Beyond the fault at mile 41, the traverse crossed an exceedingly complex
series of granites, granite gneisses and shear schists of strongly metamorphosed
character. They are doubtless representatives of the most ancient formations
and are comparable to the T’ai Shan series in China (Willis, 1907, II, page 1).
The amount of internal deformation is striking. All are crushed and sheared
to greater degree than was found in any other district. Perhaps this struc-
ture, however, can be over-emphasized in its bearing on the question of age;
some part of it may be connected with the faulting of the Jurassic block.
But the important feature is the occurrence of a great granite gneiss series
with schists of largely dynamic origin, quite different in all respects from
anything seen since leaving the Urga trail, nearly 200 miles back on the
traverse.

CHAPTER VI

FROM TSETSENWAN TO SAIN NOIN AND THE ARCTIC DIVIDE

WESTWARD FROM TSETSENWAN

For the first half mile westward from Tsetsenwan beyond Camp Tsetsen-
wan, the trail lies across the alluvial fans fringing the north edge of the granite
mountain block. Then it turns and rises abruptly to the block itself, and for
many miles crosses a more rugged country belonging to this structural unit.
The principal formation is granite, of coarse to medium texture and of essen-
tially the same composition and general appearance as has been noted at many
other places. It is doubtless a portion of the exposed bathylith. The granite
is cut by multitudes of dikes of the serpent-form type, although they are not
so numerous here as they are north of the lamasery at Tsetsenwan. There
are also numerous small xenoliths, representing remnants and roof-pendants
of the older formations which once served as a cover for the granite mass.
These are largely hornblendic and siliceous in composition, and have in some
cases a markedly schistose structure. They range widely in quality, and may
represent several quite separate formations of very different age.

In general structural habit, the rocks resemble those around Tsetsenwan,
which have been described in Chapter V. In all essential respects this is a
block of the same kind of country, but it stands higher, doubtless due to
block-faulting. The uplifted block probably has been eroded to a greater
depth, and this may account, in part, for the smaller xenoliths and for their
greatly modified condition, as well as for a smaller number of dikes.

This kind of country continues for nearly twenty miles, but at mile 831
a new type of very red granite (Specimen 196, A.M. 9885), is exposed for more
than a mile (Figs. 42 and 43). It is a later intrusive, and belongs to the same
history as the dikes which cut through the granite floor. It reminds one of
the variety noticed in the Mount Tuerin district, where a number of strikingly
different granites occur together. These simpler, later granites are not much
cut up by dikes and carry few inclusions or xenoliths, although they are in
all probability genetically connected with the great granite bathylith itself.

110

PLATE XII.

A. PENEPLANED GRANITE FLOOR ALONG THE URGA TRAIL,

B. TYPICAL SURFACE FEATURES OF THE EXPOSED GRANITE BATHYLITH WEST OF TSETSENWAN.

PLATE, XIII.

A. CANYON BROOK,

An aggraded trench in an ancient dissected fault block of complex crystalline rocks at Canyon Brook. The skyline is the profile
of an older peneplane.

B. RAINY GULCH.

Rock terrace beveling complexly folded graywackes above the camp at Rainy Gulch on the Tarmil River.

FROM TSETSENWAN TO SAIN NOIN III

They must be later outbreaks, representing renewal of activity and surface-
ward invasion.

At mile 833 a complete change in the character of the rock floor takes
place. The structure is very confused indeed. There is considerable crushing
and shearing, and some of the rock is strongly silicified. Much of it resembles
the quartzites of the graywacke-slate series, but for some distance not a
single good characteristic specimen was secured. Farther on, beyond mile
840, it is clear that the rocks are chiefly sheared granites which have become
gneisses. In this respect they resemble the still more ancient rocks which
we have correlated with the T’ai Shan system (Willis, 1907, II, page 1).

From such inspection as could be made, it appears probable that a por-
tion of the graywacke-slate series is represented for the first few miles, and
that the rest of the block is made up of more ancient gneiss (Specimen 197,
A.M. 9886), although there is no apparent unconformity. It is certain, at
least, that the larger portion of this ground is occupied by shear-gneisses and
schists, made largely from igneous rocks, and that they belong to a much
more ancient time than do the graywackes and slates. They may, indeed,
be comparable to the gneisses seen forty miles south of Tsetsenwan, and it
is not improbable that the two exposures are structurally continuous with one
another. No other rocks seen in Mongolia up to this point have shown so
much intimate internal deformation. Clearly some of these rocks have been
made from simple granites and porphyries, and some of the schists from
porphyries also, but there are other members whose origin is quite uncertain.
Because of the very extensive brecciation, the ledges in many places break
down into small chips on weathering. This further tends to obscure the
character of the rock, and makes identification more difficult.

Nowhere have we seen the bathylithic granite as much deformed as are
the granites of this ground. We are inclined, therefore, to believe that they
represent earlier invasions. At any rate, in the run from mile 840 to mile
843 deformation is extremely prominent. It is possible that these are the
very oldest formations to be found in the Gobi, and should be classified with
the T’ai Shan complex (Specimens 198, 199, 200, 201, A.M. 9887, 9888, 9889,
9890).

At mile 843, granite of the regular bathylithic type is encountered again
(Specimen 203, A.M. 9892). The crest of the range has been crossed, and the
trail descends the westerly slopes on the exposed granite, which, farther along,
is overlaid by a thin layer of sediment concealing the granite for three or four
miles. At mile 848, however, the granite is exposed again, with the usual
accompaniment of dikes and xenolithic masses, and this continues for several
miles. The trail now lies through a mountain valley in which the wash
almost completely buries the rock floor under a coarse pebbly cover, which,

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II2

FROM TSETSENWAN TO SAIN NOIN 113

in some places, makes traveling extremely difficult. At mile 852, the party
turned from the trail into a side canyon, and pitched camp by a mountain
brook, which, for want of a local name, we called Canyon Brook (Plate XIII, A).

THE VICINITY OF CAMP CANYON BROOK

A day’s stop at this place, where a beautiful stream of pure water comes
down the small canyon, gave opportunity for much closer inspection of the
rock formations of the locality than had been possible on the previous day.
All rocks near the camp are exceedingly crushed and broken. Both igneous
and metamorphic rocks are involved in the general deformation, although
some of the porphyries have resisted better than other members. Further
observations led us to conclude that this spot lies within a crush zone of
moderate proportions and that such effects are not characteristic of the whole
region.

We examined the canyon for a mile or two upstream, finding a very great
range of different kinds of rock (Fig. 44). There are schists cut by red granite
porphyry dikes and sills and a complicated lot of associated injections. One
of the most prominent members of the series is a broad belt of quartz-mica

FicurE 44.—Geologic section of side traverse northward from Camp Canyon Brook.

schist, with which is associated a formation of limestone and limestone schists.
Blue and white crystalline limestones occur together; and the structure is
complicated by bunches of deformed dikes and other intrusives. All of the
members are folded and in some places intimately crumpled. With the lime-
stones there are interbeds of schists, phyllites and quartzites, some of which
are graphitic. The general strike of the formation is N. 75-80° W., and in
the vicinity of the limestones the dip is 50—60° to the north.

The series must be comparatively ancient, much older than the gray-
wackes and slates that form the major surface rock from the vicinity of Urga
westward. How much older it is, there is no means of determining. Its
structures are more complicated in all respects, and the series is much more

VOL. 11—8

114 GEOLOGY OF MONGOLIA

variable in composition and much more metamorphosed. The basis is a sedi-
mentary series of the usual succession of original sandstones, shales and lime-
stones, which have been metamorphosed and extensively injected and cut by
igneous masses. Subsequently they were all subjected to extensive deforma-
tion, including mountain-folding and crushing. Apparently, crushing is the
latest important effect, and this is probably connected with block-faulting.

The series must be very extensive since we found no repetition of beds.
Because of the dominance of sedimentary formations, and the fact that some
of the limestones are only slightly metamorphosed, we believe that the series
is older than the graywacke-slate series, and probably corresponds to the Wu
T’ai system of China. We saw virtually no limestones in the graywacke
series proper, but in this series limestones and quartz-mica schists are common,
whereas typical gneisses are rare or wanting. At Canyon Brook, much more
of the terrane is igneous and the rocks are more highly metamorphosed than
in the graywacke series.

The canyon opens into a broad lowland, chiefly underlaid by granite of
supposed bathylithic relations, and cut by many dikes. In a side trip of
about eight miles across the lowland and to the north, we saw two hills which
deserve special description. The first is a roof-pendant of older rock, such as
has been described as occurring in the canyon. It stands above the valley
simply because it is more resistant to weathering than is the average granite
of the floor.

The other hill, which is perhaps a third of a mile long and 500 feet wide,
is igneous, and represents a volcanic outbreak through the granite floor. It
is an olivine-aphanite (Specimen 206), with a strong platy structure, which
gives it, at a distance, almost the appearance of a sedimentary rock. This
peculiarity of structure seems to have attracted the attention of former in-
habitants of the region, who seized the opportunity which these blocks pre-
sented for carving inscriptions. On several of them are remarkable drawings,
quite unlike the work of the Mongol people, both in subject and in method of
making the incisions (Fig. 45).

It is evident from the physical condition of this rock that it belongs to a
period entirely different from those with which it is now associated in the
district. It is practically fresh, and quite unaffected by the shearing and
recrystallization which is so prominent in all of the older formations. It is
another of the occurrences of later igneous rocks which were encountered
at many places, sometimes associated with formations whose age can be de-
termined. This one is quite without such relations and indicates only that
igneous activity of later age extended into regions quite beyond the reach of
the typical sediment-basins and far into the foothills of the mountains.

In this traverse across the lowland, one soon discovers that the alluvial

FROM TSETSENWAN TO SAIN NOIN II5

deposit, which at the mouth of the canyon seems to be important, is a com-
paratively shallow accumulation and not nearly as extensive as it at first

ae

FicurE 45.—The rock drawings near Canyon Brook were found at the dolerite hill shown on figure 46.
They had been incised with a sharp stone tool upon natural slabs of dense black dolorite. The horns of the
mother, and the short, lifted tail of the single figure, suggest the antelope.

appears to be. The granite floor is well exposed at many places, and small
irregularities of the floor appear as completely bare outcrops (Fig. 44).

The character of the alluvium attracted some attention. It is composed
of an unusual proportion of large pebbles, some approaching bowlder size,
which have been spread far out into the valley. In addition, the deposit is
terraced, and the whole setting raises a question of the competence of the
present small stream to accomplish such extensive work. There may be
times of the year, of course, much more favorable to transportation than early
summer, but under the best circumstances it does not appear likely that the
present climatic conditions would present sufficient water-supply for so enor-
mous an accomplishment. Perhaps there have been times more conducive
to the transporting of such heavy material. This inference is somewhat sus-
tained by the terraced condition of the deposits.

The dominance of worn bowlders and pebbles led at first to the suspicion
that ice had played an important part in transporting the material; but this
hypothesis was later abandoned as unsupported by any evidence of glacial
work on the adjacent ground (Plate XIII, A).

FROM CAMP CANYON BROOK TO THE ONGIN GOL

The party left this camp on Thursday, June 1, 1922, with a recorded
mileage of eight hundred and fifty-four miles from Kalgan, and made a run
of forty-six miles during the day to the crossing of the Ongin Gol (Fig. 48).

116 GEOLOGY OF MONGOLIA

The route lay across the open lowland for a distance of eight miles, over the
alluvial deposits already referred to, with an occasional hillock of bare rock
standing above the general level. These in some cases are roof-pendants of
the older meta-sediments, with their typical crushed and sheared igneous
members, but most of them are surrounded by granite which appears to form
the major portion of the floor just as was found in the side traverse of the
previous day. There are granites associated intimately with the older rocks
whose relations seem to indicate that they also date from a more ancient
time than the bathylith. They are crushed and sheared with the meta-sedi-
ments, whereas the bathylithic granite, forming the major floor, is compara-
tively free from such deformation (Fig. 47).

A number of xenoliths and roof-pendants of these older rocks are en-
countered within the valley and on its westerly margin. The trail then passes
into the upland that divides this valley from others tributary to the Ongin
Gol, the highest point in the first range being 700 feet or more above the
valley of Canyon Brook. On this traverse the whole series of modifications,
which mark the transition from a roof of ancient rock to the invading, under-
lying bathylith, is passed in review. At first, on the margin of the valley,
the old meta-sedimentary series is dominant; but outbreaks or injections of
granite are encountered repeatedly, and finally they become close-set, Jit-
par-lit injections, making a complex of oldrock types and intrusive granite,
so commingled that it would be impossible to determine which type predom-
inates. In many places, therefore, the rock is an intimate mixture, a compound
rock, not capable of simple classification. It is a variable injection-gneiss.
In some places the injected portions are porphyritic; in many others, they
maintain the flow structure of the invading material; and in others there is
a partial preservation of the original structure of the older host rock. Effects
of this kind are exhibited in great variety. As there is little soil cover, the
rock is almost bare, and so there is no mistaking the facts of structure. No-
where in the whole Gobi region have we encountered a better exhibit of the
intimate interrelations and complexities of product that may be made by the
invasion of older metamorphic rocks by younger igneous magmas.

It is in this region, also, that several other striking features were first
seen, such as fine white porphyry dikes, wholly lacking in ferro-magnesian
content, and looking almost like pure quartzite. Here, too, the porphyritic
varieties of granite were first definitely proved to be simply a facies of the
major bathylithic mass.

Rock-bound hollows

One of the most striking physical features belonging to the locality is
the occurrence of small, rock-bound hollows entirely within the granite. Not

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II

118 GEOLOGY OF MONGOLIA

only are they surrounded by granite, but undoubtedly they are floored with
the same rock. Some of the shallower small hollows are dry, but the deeper
ones, in June at least, are partly filled with water. In the shallower hollows
the floor rock is badly broken up, the ground being strewn with blocks of
granite in hopeless confusion.

At first we were inclined to ascribe these hollows to the gouging action
of ice, but the failure to substantiate the occurrence of ice of any such capacity,
or to find any deposits that might have belonged to such an agent, led us to
abandon that hypothesis entirely. There is no evidence of local deformation
with which they could be connected. There is no outlet for water, and under
these conditions only ice or the wind could transport the material. There
is no chance for solution to accomplish much in such a rock, so that sink-
holes are out of the question (Fig. 46).

It is very noticeable that the coarser-grained bathylithic granite has a
distinct tendency to disintegration. As we had already noted at many places,
great blocks are tumbled about in utter confusion; and apparently the process
of breaking down to finer and finer particles goes on uninterruptedly, reducing
coarse fragments to smaller ones, even to sizes that the wind can carry. Evi-
dently certain spots yield to extreme disintegration more readily than others,
and these places have been etched out deeper and deeper. Weathering fur-
nishes the first step, and the transporting power of the wind completes the
process by sweeping the ground virtually clean.

Climatic changes, however, must have affected the process to some
extent. Disintegration and removal could not have been accomplished under
water, and evidently the hollows now containing small lakes could not have
been made under present conditions. The hollows, however, might have been
made in a more arid time, and they may be subject to repetition of the process
if the climate should change again. We judge, therefore, that they have
been formed by deflation and that they have significance in the problem of
climatic change. It is certain that some superficial process is responsible for
them, and the only agencies capable of making these hollows seem to be the
wind and the weather. The hollows and their meaning are discussed in later
chapters of this report.

Continuation of the traverse

The bathylithic granite continues for several miles to the top of the
range, but after passing a short distance beyond the summit, the metamorphic
rocks belonging to the roof are encountered again. In the next ten miles
we crossed a series of ancient metamorphosed sediments more complicated
and interesting than those in any other stretch of equal length in our itinerary;
but the work was done at a terrific pace, and under these circumstances the

FROM TSETSENWAN TO SAIN NOIN 119

full significance of the rocks could not be determined. It is most unfortunate
that there was not more time for this geological work, for the section exhibited
here is regarded by us as one of particular significance, and would well repay
more detailed study.

For the first five miles the rocks are intimately intruded, giving a great
series of injected and granitized gneisses. The major basis, however, is a
series of schists and phyllites, and the final product is a complex of these
ancient meta-sediments plus the injected material.

At mile 874, we saw the first conglomerate of importance, much sheared
and metamorphosed, and evidently belonging to a very ancient formation.
The whole succession to this point probably belongs to the same series of
formations that were described at Canyon Camp, twenty miles back. The
significance of the conglomerate, however, was not fully determined in the
necessarily hurried reconnaissance. Whether it is a true basal conglomerate
of a system, or simply an intraformational conglomerate, we do not pretend
to say. It is certain, at least, that the succeeding formations of the next four
miles are in a simpler physical condition. They are chiefly slates and phyl-
lites, and there are many interbedded conglomerates. A slaty condition is
especially prominent as the higher beds are reached. It is possible, therefore,
that, in the beds crossed from mile 877 to mile 879, an overlying formation,
younger in age than the outcrops farther back on the trail, is represented.
On this evidence, we are inclined to represent two different series of formations.
Another interpretation requires that originally a highly variable group of
sediments existed in this locality, with several conglomerates separated by
finer-grained beds; that later on, the finer-grained members were metamor-
phosed into phyllites and schists; and that, where certain of the schists were
reached by the invading magma of a large igneous body, they were trans-
formed into injection-gneisses.

At mile 879, a still more pronounced change takes place. Fine-grained
phyllites, which look almost like slates, predominate. They give the im-
pression of being a much simpler formation than the conglomerate-bearing
series covered in the last ten miles; but the exact structural relation between
the two could not be determined. It was noted, however, that the dips of
this formation are gentler than those of the other formations; and we judged
that there is an angular unconformity between them. At mile 881, granite
breaks through this series also, and forms the surface for a short distance;
but half a mile farther along, the whole floor is covered by Jurassic strata
which come in on top, in the form of a shallow, synclinal remnant extend-
ing for about six miles (Figs. 48 and 49). West of the Jurassic remnant, we
encountered argillites and graywackes of structure so broken that the dips
are obscure, instead of the plainly bedded phyllites seen ten miles farther back;

120 GEOLOGY OF MONGOLIA

and from this point onward to the Ongin Gol the country rock is continuously
the graywacke series.

It appears, therefore, that a decided change of formation has again taken
place, either beneath the synclinal covering of Jurassic strata or back at the
eastern edge of the phyllite area, where the unconformity is drawn in the cross-
section (Fig. 49). We judge that the phyllites at the eastern margin of the
Jurassic syncline belong to the graywacke series and that these beds are
simply the lower members of it. We are satisfied from petrographic and
structural field evidence that the other formations still farther east belong
to older series. This is the only place in the itinerary of the summer where
these two series are so well exposed and where there is so good an opportunity
for determining the exact relations. It would undoubtedly be possible to
work out the structure and determine relations in detail; but the whole ground
was covered by the Expedition in half an hour, and some of these critical
structures escaped direct observation.

For the next fifteen miles the ground descends very gradually to the
valley of the Ongin Gol. The lower ground is covered with soil and
the rock floor is seldom seen. There is, however, no real doubt of the
continuance of the graywacke formation for the entire distance to the Ongin
Gol.

At mile 895, there is an outbreak of dolorite, in the form of a dike, sill
or plug. It has resisted erosion much better than the surrounding graywacke,
and makes a prominent landmark standing perhaps 200 feet above the average
level (Fig. 48). This is only one of a chain of four or five hills stretching away
in an almost straight line toward the northwest, as though they all might be-
long to a single body whose course is marked out by these remnants. The
material is an exceedingly tough and durable aphanite of doleritic composi-
tion, not very different in general character from that seen in the valley ad-
jacent to Camp Canyon Brook.

The Ongin Gol

From the Khangai Mountain range, which forms the Arctic divide farther
to the north, the Ongin Gol flows southward toward the desert. At mile
goo, the river occupies a valley several miles wide, within which a still lower
trench about a third of a mile wide is cut. Within this lower portion, the river
maintains a braided form due, as usual, to the amount of sediment carried
to this part of its course. The stream occupies only a small portion of the
total width of the inner valley, though it is evident from the form of this
bottom-land that the river has occupied different portions of it at different
times. The upper, wider valley is apparently not heavily covered with river
deposits, although rock is not exposed at many places. The inner valley

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is cut down into these loose deposits, with a drop of fifteen or twenty feet,
and the stream channels cut a little below this.

The Ongin valley, therefore, is a terraced valley, indicating some change
in the transporting competence of the river within moderately recent time,
and at present it is in essentially graded condition. Subsequently a more
pronounced terracing was noticed farther south, where the stream enters
upon younger sedimentary strata which are more easily eroded. In that
part of its course continuous and prominent terraces of fifty or sixty feet are
cut out of the rock floor.

The river had to be forded, and, because of the shifting sand of the bot-
tom, it presented a treacherous ground for motor travel. The geologists’
car was hopelessly swamped in one of these distributaries and twisted off an
axle, a misfortune which led to considerable difficulty in making the repair,
and necessitated some little delay.

FROM THE ONGIN GOL TO SAIN NOIN

Camp was pitched at the Ongin crossing on Thursday, June 1, 1922, and
repairs consumed the whole morning of the following day, but by eleven
o’clock the Expedition was moving again toward Sain Noin.

On the west side of the Ongin Gol valley rock outcrops are more numerous
than had been found on the east side. A short distance south of the camp
a large area of coarse-grained, massive granite of bathylithic type is exposed,
with large residuary bowlders and remnants, similar in all important respects
to the granite areas seen many miles farther back along the trail (Specimen
229, A.M.9918). It is the first important occurrence of granite that we had
seen for twenty miles. The area exposed here extends to the south beyond
the limits of the inspection, but to the north the granite area is sharply de-
limited and other rocks come in. A large dike of felsite makes a prominent
outcrop about a mile and a half from camp (Specimen 230, A.M. 9919), and
with it is associated granite (Fig. 48). Only a little farther on, however, at
mile 904, slates and argillites are encountered again, similar in general type
to those seen farther back on the east side of the Ongin.

The course of the trail lay S. 55° W. over a smooth road, easy for motor
travel. No side inspections were made, but the hills at a little distance to
the north have a form and appearance quite unlike those of granite, and are
probably made up wholly of slates and graywackes, belonging to the gray-
wacke formation. Much of the ground along the trail is covered, so that one
catches glimpses of bedrock less frequently than was usual on the traverse
out from Urga. The ground rises gradually and the country becomes less
barren. Indeed there are many stretches which, by contrast with areas crossed

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124 GEOLOGY OF MONGOLIA

farther south, look quite fertile. Flocks find good grazing, and it is evident
that the region is one of greater prosperity than most of those encountered
in the 900 miles of our traverse to this point.

The course S. 55° W. was maintained for eleven miles, to mile 911, and
then was changed abruptly to N. 50° W. toward the official capital of the
province of Sain Noin (Figs. 50 and 51). The trail lay for several miles over
the same open country with the soil-covered slate formation, and then con-
tinued through a broad valley bordered by hills on either side. Here and there
an isolated hill rises within the valley itself, and one of these, at mile 917,
stands one hundred feet above the smooth slope of the valley. The hill is
made of Jurassic conglomerates and sandstones, dipping gently northward.
The beds form part of a synclinal fold belonging to a down-dropped fault
block. The trail had crossed the bordering fault about a mile south of the
hill. Northward, the block of Jurassic sandstones underlies the valley for
more than ten miles, and in large part controls its width. The hills on either
side, not more than three or four miles away, are made of an older and entirely
different kind of rock enclosing the down-dropped block. In the midst of
this stretch of Jurassic strata, the trail passes near the hot springs and the
Holy Mountain of Sain Noin. About a week later a special study was made
of this small area, and the structural relations were worked out in enough de-
tail to determine the principal features. (See Chapter XII, page 213, ‘‘Ari-
shan, the Sacred Mountain of Sain Noin.’’)

At miles 928-929, the graywacke-slate series is encountered again, after
passing over more than ten miles of Jurassic floor. The rock is badly cracked,
jointed and confused; the structure is somewhat obscure, and in many places
the bedding cannot be made out at all. Later it was found to be quite vari-
able, and the whole formation, which is cut by dikes of porphyry, shows very
pronounced internal deformation. The strike at this point is roughly N.
45° E., with a dip to the north. Only a few miles of the graywacke are crossed
by the trail. As the ground rises and a small divide is approached, the trail
enters upon another block of Jurassic sandstones and conglomerates, which
continues for about four miles. At mile 936, after passing over another fault,
the graywacke is encountered again and this formation continues to the new
camp at Rainy Gulch, a name given to it in memory of our day’s experience
(Plate XIII, B).

The vicinity of Sain Noin

The day’s traverse from Ongin Gol to the vicinity of Sain Noin was a
strikingly unusual one. It was begun in threatening weather, and rain
seemed imminent during the whole day. Near the end of the journey, after
reaching the small divide, the storm broke full upon the Expedition and we

FROM TSETSENWAN TO SAIN NOIN 125

were treated to the most terrific rain experienced in the whole summer. The
water came down in torrents, and we could begin to appreciate the effect of
such a rain on the barren slopes of the country bordering the desert. With the
passing of the storm the trail became treacherous, and a chance for camp
was welcomed at a small gulch in the edge of the next valley, that of the
Tarmil Gol, which flows northward into the Ongin Gol. On the main stream,
only two or three miles away, are the lamasery of Sain Noin and the seat of
local government (Plate XIV, B). The valley bottoms are well grassed, flocks
and herds are numerous, and more people live in them than we had seen
hitherto in any part of the journey. For Mongolia, the region is a prosperous
one.

We had now reached a higher elevation than at any previous time in
our itinerary, the aneroids indicating elevations of more than six thousand
feet. In addition to this, we were within sight of rather majestic mountain
ranges lying twenty or thirty miles farther to the north. Peaks with rem-
nants of snow could be seen from the camp. These mountains form the true
divide between the drainage flowing toward the Arctic and that flowing south-
ward into the desert. On account of the promising geologic conditions pre-
sented by this divide country, it was decided to go beyond the present camp
at least another day’s journey and to camp in one of the adjacent ranges.
This was done after two or three days’ work in the vicinity of Rainy Gulch.

While other members of the Expedition studied the people, their customs,
and their living equipment, the geologists investigated the complex rock
structure of the valley. Patches of Jurassic sediments overlie the graywacke-
argillite formation, and in some places there is a simple sedimentary contact
between the two, whereas in others there are faults. The graywacke series
is much folded, jointed and crushed. For a long distance it forms the hills
and mountains immediately northeast and east, which rise to elevations of
over one thousand feet within a mile or two, and appear from a distance to
be all of the same type. Subsequent examination of some of the higher ranges
showed that they, also, are made up of the graywackes. It is the dominant
formation of the Khangai range, and its structural habit and petrographic
quality control the minor erosional forms of these ranges.

The Jurassic strata of the vicinity of Rainy Gulch are quite conglomeratic
at the base, and the cobbles are almost all of graywacke, like the rock of the
immediate floor. This is true especially of the beds at the very base, but higher
up in the series one finds in addition pebbles of many crystalline rocks. They
vary in size up to a foot or more in diameter, and are angular or subangular,
rather than rounded. Evidently the immediate floor rock contributed to
this material without much transportation. The conglomerates are not very
strongly bedded, but the structure is readily determined, and the dip changes

126 GEOLOGY OF MONGOLIA

markedly from one point to another, showing considerable folding. There is
no fossil content whatever.

The structural conditions of the valley are indicated reasonably well by
figure 54. It is marked by a graywacke foundation, carrying Jurassic strata,
folded, faulted and eroded, with a valley cut out at one edge of the Jurassic
block. Subsequent volcanic eruptions poured out basalt, which flowed down
the valley, choking it somewhat and oblitering the topographic detail of the
valley bottom (Fig. 52). Since that time, erosion has continued and the
stream is again channeling its way through the volcanic material. This

3 MILES Ww
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FicurE 52.—Sketch map of the route in the vicinity of the great lamasery at Sain Noin. The map shows:
(1) the flat flood plains of the rivers; (2) a sloping rock bench above the flood plains, separated from them by a
low but abrupt scarp; (3) the higher hills rising above the bench. These three elements are especially well
shown near “Camp” and north of the word, “‘Ongin.” Note the sudden narrowing of the Ongin Gol valley
near the words, ‘Vesicular basalt,’ where the river has cut its way through a lava flow which dammed the
valley. The narrow ridges just north of the direction-vane are of infaulted Jurassic conglomerates, which
stand vertically. From the top of a hill east of Camp Rainy Gulch a sketch was made of the country toward
the northwest, shown in figure 149, pages 344-345. (See also Plate XIV.)

basalt out-flow is clearly a very late geological event. The vesicular pumi-
ceous material is virtually fresh.

Jurassic strata, more or less deformed, are found at several places in
these valleys at Sain Noin. The occurrences all appear to be remnants of
fault blocks, and everywhere they are bordered by the graywacke-slate series.
This is true everywhere except where the younger basalts cover the ancient

FROM TSETSENWAN TO SAIN NOIN 127

complex, as can be seen just above the lamasery of Sain Noin. One of the
impressive things is the comparatively small size of these remnants. Doubt-
less Jurassic strata of similar character covered most of this region at one
time, and only these few patches are preserved after long-continued erosion.

The most characteristic of these, and the one most fully studied, is that
at the Holy Mountain of Arishan, where a special areal study was made.
This is essentially a graben with a jagged series of bordering faults and a
large though uncertain amount of displacement.

Later volcanics were found at three places: (1) a few fragments of vesicu-
lar lava at the hot springs near the Holy Mountain, (2) a series of tuffs and
lavas near camp at Rainy Gulch, and (3) a basalt lava flow of considerable
extent, which fills the valley bottom above the lamasery at Sain Noin. All
of these are certainly very late geological products; but they are not associated
with strata from which their position in the geologic column can be fully
determined. It is very likely that the volcanic outbreaks followed the same
weaknesses and belong to the same structural adjustments that mark the
graben and block faulting, and we believe that the hot springs are connected
with the same history.

All of the lavas are comparatively basic; some are exceedingly vesicular
and amygdaloidal, and tend to be porphyritic. Some of the material is frag-
mental, while some pieces are strictly bombs. One bread-crust bomb was
found on the crest of a minor divide, 2,000 feet above the valley carrying the
basalts; but, of course, its relations are unknown, and it may have been car-
ried there by man. The lavas are not very extensive or very thick, but at
one time they choked the valley bottom, displaced the stream, and modified
some of the minor drainage; but the major features were preserved throughout
that history, and re-excavation is in progress at the present time.

FROM RAINY GULCH TO THE ARCTIC DIVIDE

The traverse toward the Arctic divide from Camp Rainy Gulch follows
the Tarmil River to the lamasery of Sain Noin and crosses the main stream,
the Ongin Gol. Thence we climbed to a camp near the top of a minor divide,
at an elevation of between seven thousand and eight thousand feet (Figs.
53 and 54). The underlying ancient floor, over most of the distance of six-
teen miles, is doubtless the graywacke-slate series, but the first two miles
from camp lay across basalt flows that were poured out in the bottom of an
older valley. Our trail then crossed a small dividing spur over folded Jurassic
sandstones and conglomerates, and across a fault to the graywacke-slates
again, into the Ongin Gol valley where basalt flows are exposed. We judged
from local conditions that a fault may lie beneath and parallel to this larger

128. GEOLOGY OF MONGOLIA

valley and that the vent for the lava flows was probably guided by this weak-
ness in the valley bottom. After crossing the river, there was no change in
formation to the top of the divide. The rock is graywacke-slate, of more
obscure bedding structure, and in a more broken and confused condition than
usual. The formation at the divide has a strike of N. 60° E. and dips north
at about 30°. As far in the distance as one can see, the hills have a similar
appearance, and probably the same formations prevail (Plate XIV, A). In
all likelihood the whole range is composed dominantly of the graywacke-
slate formation (Specimens 245, 246, A.M. 9934, 9935). It is folded and ex-
tremely jointed. There is probably repetition of beds, but this detail could
not be worked out. The rocks in general dip toward the north, at a steep
but variable angle. There are occasional dikes, but igneous rocks are not
very abundant. The formation varies much in quality and is considerably
modified, so that in some places it is not easy to distinguish the regular country
rock from the medium basic dikes that cut through it.

Camp was pitched in the second grove of forest trees, ten miles north
of the lamasery at Sain Noin, just below a very sharp minor divide separating
two portions of the Ongin Gol drainage system. It is not the Arctic divide
proper. The trail rises at the pass to an elevation above eight thousand feet,
and the crest is exceedingly sharp. From it one may look off northward across-
country to the real Arctic divide, which lies at the head of the next valley;
but bogs rendered it impossible for us to make the traverse across this inter-
vening ground.

We had now reached an entirely different set of physical conditions; we
were quite out of the desert and entering the forest country. Indeed, camp
was pitched on the very picket line between the forests of the north and the
deserts of the south. With the swing of the climatic pendulum to more or
to less favorable conditions, this belt must become a fighting zone for occu-
pancy. In certain years conditions must be a little more favorable for the
growth of trees and for the lodging and germination of their seeds. If such
conditions were to prevail for a considerable length of time, small groves of
seedlings would be started on the protected northerly slopes a little farther
out toward the desert than usual. With a change of conditions and a return
to more arid seasons, growth would be checked and germinating seeds would
not survive, so that there would be no addition to the colony formerly estab-
lished. It might be, though, that the seedlings already rooted could maintain
themselves, and remain as an outpost of forest encroachment, to last as long
as they could keep their footing.

A return of favorable conditions might permit reseeding of the same ground
and enlargement of the earlier-established colony, so that occasionally one
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VOL. 11—9

130 GEOLOGY OF MONGOLIA

sizes. The grove in which we camped, near the divide, is made up of trees
of two markedly different stages of development,—a very young growth of
seedlings and saplings, and a much older growth of mature trees, more scatter-
ing, and apparently the survivors of much hardship. The larch, or tamarack,
is the only kind of tree that has been able to persist. Apparently no other
is able to maintain itself under these severe conditions. We have found no
better explanation for this unusual grouping of sizes than the one just sug-
gested. It has to do with cycles of climatic change.

In many other ways this is a peculiar zone of conflict between quite differ-
ent interests, conditions and processes. From this point northward the deer
is native, and may be found in abundance, with many other denizens of the
forest, while southward this fauna is entirely wanting.

Grasses and flowers grow richly, whereas only a few miles to the south the
ground becomes barren and such vegetation as may be seen at this spot is
unknown. It is, therefore, not only a divide separating the waters of the
desert from those of Siberia, but a dividing line where fauna and flora change
abruptly; and here one becomes interested in problems which are strange to
other parts of the region. It is a critical conflict zone of special biologic
interest.

Quite different traveling conditions also prevail. The trails are well
marked but treacherous. Miry places are numerous. Wet spots with stand-
ing or flowing water are frequent. A stretch of country which appears in
the distance to be not only clear but easy, proves difficult and sometimes
impassable.

A week was spent in this camp. From it hunting expeditions were made,
and deer were taken in the very grove in which the camp was pitched. From
that point northward every northern slope is wooded, whereas the southern
slopes of ridges and hills are bare of trees. This condition exists as far as
the eye can see; but beyond the next divide, well within the zone of forest
conditions, even this feature disappears and the region becomes one of con-
tinuous heavy forests.

A step across the divide

From this point several splendid cirques, representing glaciation of an
alpine type, could be seen at no great distance. It was estimated that they
were not more than ten miles away, and on breaking camp it was decided
that the geological party should make an attempt to reach these exhibits of
glacial action by crossing the divide, pushing up an adjacent valley and down
to Sain Noin by a different route. The topographic relations looked simple
enough, but an attempt to carry out the program almost led to disaster, for
the small tributary valley on the opposite side of the divide was found to be

PLATE XIV.

A. THE ARCTIC DIVIDE IN THE KHANGAI MOUNTAINS.

The upland surface is a peneplane above which rise monadnocks. The southern outposts of the Siberian forests occupy the north-
ern slopes.

B. SAIN NOIN KHAN AND THE KHANGAI RANGE.

Cirques in the Khangai range north of the great lamasery at Sain Noin Khan. Rock terraces are well developed along this por-
tion of the Ongin valley.

PLATE XV.

ro Ha 7

ee

MONGOL LAMA AND HIS SMALL DAUGHTER AT SAIN NOIN.

FROM TSETSENWAN TO SAIN NOIN 131

quite impassable. After pulling the car out of miry ground several times,
it was found necessary to return by the same route. The return journey
proved to be one of the most difficult of all the feats attempted with the
motors in Mongolia. For the last few hundred feet it was too steep to climb
‘even in low gear, with no load. Everything was unloaded, and the motor
driven at high speed and caught with the clutch so that with the help of hard
pushing a few feet of progress could be made. This was repeated often
enough to reach the summit, which was gained at sundown, with the whole
party exhausted. After a brief rest, each piece of equipment was carried
laboriously to the top and packed again, but it was then too late to gain camp
that night. The main camp had been moved in the meantime to the vicinity
of the Holy Mountain, about twenty miles away; so our party slept in the open,
along the banks of the Ongin Gol, near the lamasery of Sain Noin. Thus
we lost our last opportunity to visit the cirques of the Khangai Mountains.

CHAPTER VII
FROM THE HOT SPRINGS OF SAIN NOIN TO MOUNT USKUK

On breaking camp at Arishan after the special study of the Hot Springs
of Sain Noin was completed, the Expedition retraced its steps for a few miles,
passing southward down the same valley through which the country had
been entered two weeks before. Then the course was changed so as to bear
farther to the west in the general direction of Gorida on the old Uliassutai
trail (Figs. 55 and 56). For this cross-country journey local trails were used
which made fairly good going, but ultimately, at the divide, very steep slopes
were traversed which taxed the transport to its limits.

ARISHAN TO GORIDA

The first ten miles led over the southerly extension of the Jurassic fault
block represented on the map at the Holy Mountain (Chapter XII, page 213);
but at mile 971 the southern boundary of this block was crossed at the fault
line (Fig. 56). The strata exhibit synclinal structure, dipping toward the
south for the first four miles, and then toward the north for the next six miles,
with comparatively sharp up-drag against the fault. From this point to the
top of the divide, for the next thirteen miles, a difficult trail was followed
across the folded graywacke-argillite series of rocks. All the strata are steeply
upturned, the dips commonly reaching 65 degrees, inclining first to the north,
and then, after passing the arch of the fold, to the south. In this stretch of
country much argillite was encountered, some of it dull gray and dense, and
some more highly colored, green and red predominating (Specimens Nos. 255,
256, A.M. 9944, 9945). The whole divide, standing nearly one thousand
feet above the valley by which it is approached, is graywacke. Occasional
dikes cut the formation, but they are neither very numerous nor very large.

After crossing the divide the trail follows a general course of about S. 60° W.
down a broad valley, with considerable alluvial fill. On this slope the gray-
wacke is left behind, and the bathylithic granite, which throughout this coun-
try lies beneath all other formations, appears at the surface. One finds,

132

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FROM SAIN NOIN TO MOUNT USKUK 135

therefore, that the graywacke is more and more modified by contact meta-
morphism with more numerous intrusions, until finally the trail passes over
simple granite, cut by dikes. Much of this ground is difficult to read, how-
ever, as it is more heavily covered with wash than most of the country trav-
ersed in Mongolia. Only here and there could we judge of the quality of the
underground without going to the sides of the valley. Subsequent observa-
tions tended to support the structure as drawn in the accompanying profile.
A continuance of this course, S. 60° W., carried the Expedition off the granite
and across fifteen miles more of graywacke (Figs. 57 and 58). The granite
bathylith must lie close beneath the graywacke roof, as its projecting intru-
sive masses break through here and there. Gradually, however, the bathy-
lith passes so deeply beneath that it is not seen at all for the last ten miles.
In this journey down the valley from the divide the course lay toward a
most strikingly rugged-looking mountain range (Fig. 59). A very irregular
skyline is formed by sharp, jagged peaks, which in the distance appear to stand
out in bold relief to great height, but diminish as the mountains are approached.
They are essentially granite mountains, and their jagged outlines are due to
the disintegration of the crystalline rock guided by prominent systems of
joints. There is no regularity of form; but there are precipitous portions and
a sharpness of outline that has not been seen elsewhere on such an impressive
scale. This is the bathylithic granite again coming to the surface from be-
neath the graywacke, and we judged that a new fault block had been reached.

The Gorida basin

In the depression lying between the graywacke range just traversed and
the granite range to the south, there is a small basin with horizontal sediments
belonging to a comparatively late period of sedimentation, although as far as
inspected they are barren. At the present time they are being redissected by
streams from the adjacent mountains.

West of the small basin an entirely different series of formations is ex-
posed. Ancient gneisses and schists, much older than the graywacke series,
are undercut and injected by granite. It is this series that the granite range
carries on its flanks. How they are related to the graywackes is not determin-
able in this region, because the contact zone is covered by the basin sediments
lying in the valley between. We have chosen to represent them as shoved up
by faulting, but it would be equally possible for them to have been warped
up instead. The sharpness of the break, together with the narrowness of the
valley, has caused us to prefer the former explanation; and the finding of
crush zone material at one point is considered additional supporting evidence
for this interpretation.

The intimate penetration of the ancient metamorphic rocks by the gran-

136 GEOLOGY OF MONGOLIA

ite invasion is shown very well in the vicinity. The mixture is so intimate
that it is very difficult to determine how much of the rock is of one type and
how much of the other, or what the proportion of intermixture is. Associ-
ated with them are schists which strike N. 30° E. and dip steeply to the south-
east, forming many low parallel ridges. They are chiefly micaceous and of
very fine foliated structure, but there are also amphibolite members. They
are shear-schists, and some of them appear to be derived from igneous rocks.
They undoubtedly have had amore complicated history than have the rocks
of the graywacke series.

The lower part of the valley is covered still more heavily with later sed-
iments, which are not very extensive. They have every appearance of being
very late deposits, as they are not indurated or deformed. How recent they
are, it is impossible to say. The simple surface form which they represent
appears to continue indefinitely toward the south, but in the course of less
than eight miles of travel down stream, they completely disappear, and the
granite floor emerges from beneath them.

The old Uliassutai trail

At this point also the old Uliassutai trail crosses the valley, and here we
pitched camp for a few days. For the first time in a month the Expedition
was in touch with the main transportation arteries of the Gobi region, and
we had the good fortune to see some of the great caravans that carry trade
from China to the most distant portions of Mongolia. It is not a rare sight,
of course, but, for all that, a most impressive one each time it is witnessed,
to see these silent caravans swinging along an old trail. They carry merchan-
dise of surprising value, and represent the chief means of intercourse which
this interior country has with the outside world. We saw a caravan of two
hundred camels, bearing tea, silk and cotton cloth, glide off into the fading
glow of sunset, in train almost a mile long. Sixteen unarmed men were the
only guards and guides for nearly two thousand miles through the Gobi to
Uliassutai and Kobdo. One has no more thrilling experience in a desert
than is furnished by this phantom-like procession with its tinkling bells, the
soft, almost noiseless shuffling of padded feet, and the strangely majestic
forms that loom up in the gathering dusk as the caravan starts on its nightly
journey. One can never forget the strange sight, and one conceives a last-
ing respect for the strong, patient, ungainly beast of burden that lends such
dignity to commercial adventure and such picturesqueness to these silent,
endless stretches of hopeless sand and jagged rock.

In exploring near the camp it was soon discovered that the recent sedi-
ments observed along the valley to this point disappear within a short dis-
tance; and although the same surface levels are maintained and the relief

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138 GEOLOGY OF MONGOLIA

is regular enough to suggest the same simple understructure, actually within
a mile of the camp, ledges of granite and gneiss outcrop and the stream carves
its trench across these ancient rocks. The valley is an oasis-like spot, with a
multitude of springs, whose streams divide and subdivide and swing across
the flat bottom-lands in a most delightful way. Good pasturage for caravans
and abundant water make it a convenient place to stop for a day’s rest; but

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FIGURE 59.—The ragged mountains of Gorida. The rock is granite cut by numerous dikes, and weathers
into fantastic, bare, and rugged ‘‘monument” shapes.

it does not offer much promise of geological discovery. There is nothing
unusual in the region, except the small display of later sediments that we
have just been over, and these seem to be barren. It was decided, therefore,
to continue southward toward the Baga Bogdo mountain mass, which could
be faintly seen in the distance, seventy-five or one hundred miles away.

OLD ULIASSUTAI TRAIL TO MT. USKUK

From the jagged granite crags near the last camp we looked across a
great basin extending far to the south toward the Altai Mountains, and saw
in the distance two very prominent mountain ranges. The nearer one, called
Uskuk, is clearly of moderate elevation though striking enough in its relief
as viewed across the intervening distance of thirty or forty miles. Far beyond
Uskuk, towering to what seemed an enormous height, loomed a magnificent
range. The snow which capped the crest descended through a network of
valleys in a delicate, lacy pattern, ending in a fringe of dangling sabre-points.
At this great distance, through the matchlessly clear air, the snow reflected its
daintiest blue where the light fell, deepening a little in the shadows. The
base of the mountain was almost invisible, so that the infinitely delicate snow-
tracery seemed to grow from the air, reaching upward into gleaming peaks,
less of the earth than of the skies. Some touch of primitive poetry in the
Mongols seems to respond to the thrill of this ethereal beauty, for they have

FROM SAIN NOIN TO MOUNT USKUK 139

named many of their greatest mountains after their God. The range before
us was the Baga Bogdo, the Lesser Buddha, while, far to the west of it, a mere
outline in the sky, stood the Greater Buddha, Ikhe Bogdo of the Altais.

We were prepared to believe from previous observations during the sum-
mer’s work that, in this great basin which stretched out between the Uliassutai
trail and the Uskuk range, and again between that mountain and the Baga
Bogdo, we should find larger deposits of the later sediments, and that these
might give better returns than had been found elsewhere. Certainly the
promise in every geological respect seemed better toward the south than in
any other direction, so our course was taken straight toward Mt. Uskuk,
which we later learned was the chief peak of this range.

The first step was a detour to reach the platform, or erosion plane, bev-
eling the crystalline rocks already noted as forming the surface along the
Uliassutai trail. From this point southward for many miles there are no
covering sediments; the rock is almost bare, but is cut by erosion to a fairly
uniform level which must represent some kind of planation (Fig. 60). What
particular explanation it may have is quite another question, but no such
uniformity of level, across such a variety of formations, could be developed
except by base-leveling of some kind. (See Chapter XIX.)

Granites cut by multitudes of dikes and pegmatite veins, and carrying
more or less completely digested xenoliths of older rocks, were crossed for at
least twelve miles. They undoubtedly represent the granite bathylith, which
varies somewhat from the average type, to be sure, for in this locality the
granite has the most striking development of pegmatitic facies thus far noted.
It is possible that here the magma was capable of more mineralizing effect
on the original enclosing rock than is produced elsewhere, but no metallic
mineral exhibits were seen.

But about ten miles south of the Uliassutai trail, there is a striking change;
the granite is replaced abruptly, at mile 1,027, by older gneisses, schists, and
interbedded limestones dipping steeply southward (Fig. 61). Whether the
change which comes so abruptly is due to a fault, we do not know, but this
seems probable as we did not see a transition or other form of contact. It
is equally possible, of course, that the granite bathylith has invaded higher
levels in the country just crossed and that the traverse now enters upon an
undestroyed portion of the roof. One of the most striking constituents of
this ground is crystalline limestone, which appears in thin layers, associated
intimately with gneisses and schists. Most of it is bluish to white in color,
thoroughly crystalline, and still preserves some bedding structure. There is
a great variety of other rocks, but they all fall within the general groups of
gneisses and schists, and they are all heavily injected by igneous material.
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FROM SAIN NOIN TO MOUNT USKUK 141

tion of the series. Their composition ranges from granite to diorite. Much
of the ground is covered with disintegration débris of finer material, the char-
acter of which is not readily determined in the field. Perhaps they are phyl-
lites or slates; or a younger infolded and much metamorphosed formation,
possibly representing the graywacke-slate series.

The gently sloping erosion plane continues over granites, gneisses, schists
and limestones, for twenty miles from the Uliassutai trail, and descends into
a shallow basin-like depression ten miles or more across. Later sediments
form the floor, and the older basement-rocks are completely covered for the
rest of the distance to Mount Uskuk. For the most part these sediments are
not well enough exposed to secure typical samples or to determine their con-
tent. The surface is not the result of deposition, but is strictly an erosion
plane, and, instead of being a down-warped segment partly filled with depo-
sitional débris, it is an erosion valley cut into a body of sediments which form-
erly had greater extent and more nearly filled it.

The Saltpan of Guchu Burt

This erosion form is clearly seen near a small saltpan in the very bottom
of the valley, where the sediments are exposed in a low scarp, proving that
the hollow is not a place of present deposition, but one where for a long time
erosion has been the dominant process.

The saltpan is not more than an eighth of a mile long, and was the first
we had seen since leaving Iren Dabasu, a thousand miles back along the
route. Salt is actively precipitating at the present time, and we had the
good fortune to see Mongols gathering it by the bucketful and loading it on
camels (Plate XVII, B). The little dry lake has no outlet, but there must
have been a time not long ago, when this was simply a portion of a valley
through which a stream ran, and there must have been an outlet through
which most of the material, now missing, was carried. But the present bot-
tom is a series of undrained hollows, and there is no semblance of a stream
course in its minor features. It is evident that something else has modified
the form, and the only agency that could have done it is the wind. Here in
the basin immediately north of Mount Uskuk, as in many other places, the
wind is eroding and modifying in a minor way the sculpture work of former
streams. There is thus a superposition of topographies by different successive
agencies.

ONDAI SAIR

The traverse was continued across the east foot of Mount Uskuk, over a
divide into the next basin beyond. This is extremely rough ground, especially

142 GEOLOGY OF MONGOLIA

on the margin of Mount Uskuk, which stands more than fifteen hundred feet
above the salt basin. Only a few miles beyond the peak, the Expedition went
into camp near a well at Ondai Sair, which lies in a reéntrant of the Uskuk
block on the slopes that lead down toward the basin at the foot of Baga Bogdo.
This was Camp Ondai Sair, destined to be for a few weeks an important cen-
ter of Expeditionary geological activities. A stream course called the Hsanda
Gol rises on the slopes of Mount Uskuk and flows southward into the pied-
mont lowland, where it is lost. Some twelve miles down the Hsanda Gol, is
Loh, another spot where extensive explorations were made. Beyond, in the
bottom of the basin near the foot of Baga Bogdo, lies Tsagan Nor, a small,
shallow salt lake where the central camp was located during the progress of
work in this region.

In this manner we entered the Tsagan Nor basin. Only a casual exam-
ination was needed to prove that this was a promising geologic and palzonto-
logic field, and it was decided to spend some weeks in an attempt to unravel
some of the stratigraphic intricacies of the later sediments. This is the dis-
trict known as the Mount Uskuk-Tsagan Nor area, where a special local
study and a reconnaissance map of nearly seven hundred square miles were
made, and the stratigraphic and structural relations were studied in greater
detail than at any other place in the Gobi region. Descriptive detail as well
as summaries of the results of this work are included in Chapter XIII, in Part
III of this volume.

PLATE XVI.

A MONGOL BOY AND HIS FATHER INSPECTING THE CAMP AT SAIN NOIN

PLATE XVII.

A. THE BLOCK STAGE OF DISINTEGRATION IN THE GRANITE AT ARISHAN,

B. GUCHU BURT, A SALT PAN IN THE BASIN NORTH OF USKUK MOUNTAIN,

CHAPTER VIII

THE RETURN JOURNEY FROM TSAGAN NOR TO ARTSA BOGDO

CHOOSING A ROUTE

THE month of July and the first part of August, 1922, were spent in the
Tsagan Nor basin, with the headquarters camp pitched on the north side of
the lake. By that time it was deemed wise to begin the return journey. The
distance to be covered on the way back to Kalgan was estimated to be approx-
imately one thousand miles, and since we expected to travel more or less inde-
pendently of the main caravan trails, across a country quite unknown to the
Expedition and on which very little information could be gathered from the
natives, we anticipated that it would take the rest of the good-weather season.
If traveling proved to be good it would be possible to stop at several places
on the way and make the usual local field studies. On the other hand, if the
region proved to be impassable the whole Expedition might be compelled to
turn back and seek a different way out. The uncertainties of the trail, there-
fore, gave some concern, because of the approaching limit of supplies, par-
ticularly of gasoline for the motors.

Accordingly, on Sunday morning, August 13, 1922, the whole camp
was packed up and the return journey begun. The plan was to travel along
the Kweihwating-Kobdo trail, which follows the bottom of the Tsagan Nor
basin, skirting the east end of Baga Bogdo, and passing not far from the north-
erly edge of the Artsa Bogdo range, fifty miles farther to the east. This was
known to be a commonly used caravan trail, but its character as a roadway
for motors was wholly unknown. Many difficulties were encountered, none
of which, fortunately, proved to be insurmountable, and the whole Expedition
moved along it without much real hardship, although there were frequent stops
for reconnoitering and the finding of passageways across treacherous places or
around impassable ground.

On this return journey the same methods were employed by the geologi-
cal staff as were used on the outbound traverses. A running profile and a
geologic cross-section were kept, and a route map was constructed as the

143

144 GEOLOGY OF MONGOLIA

Expedition proceeded. Wherever possible a more detailed study was made.
Over part of this return journey, ground was covered very rapidly, and in
those places observations were necessarily scattered, leaving much to be
desired in the construction of an adequate cross-section. In the belief, how-
ever, that such facts as were determinable are more useful in this form than
in any other, the running section was continued and has been reproduced in
as much detail as the conditions of travel warranted.

Beginning with the summer camp at Tsagan Nor as a zero point, the mile-
age is counted continuously along the course of the journey, and wherever side
trips have been made they are included as extra traverses.

TSAGAN NOR TO THE VOLCANIC CLIFFS

At first the trail runs for miles over the simple, nearly flat-lying, sandy and
clayey sediments of the Tsagan Nor basin, of Miocene to Pliocene age (Fig.
62). All of the strata for the first fifteen miles are apparently Miocene; at
least, they are closely associated with beds of known Miocene age, as indi-
cated by the summer’s fossil collections. Most of the irregularity of bedding
and most of the deformation of the beds are exhibited in a north and south
section more prominently than in the east and west section along our traverse.
The course of the trail for the first fifteen miles was about S. 70° E. and in this
section, therefore, the beds have to be represented as lying nearly flat. They
appear to rise gradually in the scale until mile 15 is reached, where the strata
are notably upwarped. At this point we pass from Miocene strata below to
the overlying Pliocene beds, which are prominently exposed in the adjacent
Hung Kureh foothills bordering the northern side of Baga Bogdo.

The strata for the most part contain no fossils, and although beds con-
siderably higher are known to be Pliocene and beds below are known to be of
Miocene age, it is quite uncertain just where the boundary lies between them.
The strata present a regular succession of apparently conformable beds. If
there be a break of any kind it was not evident along the trail, or at any place
in the district. Doubtless there are gentle overlaps, not only between form-
ations of different age, but within those of a single period or epoch; and some
overlaps doubtless occur in this district, where the developing strata must
have been affected by the continued Baga Bogdo uplift. The only result
noticeable along the trail, however, is a slight warping of the beds so that
one traverses the beveled edges of strata that dip very gently. This is espe-
cially prominent from mile 13 to mile 18, along the Kobdo trail.

We drive, therefore, not across a plain of deposition, but across shallow
basins and very low ridges, over gravel-covered flats and across sand-covered
stretches where the surface forms, developed by erosion, suggest the inter-

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VOL, II—I0

146 GEOLOGY OF MONGOLIA

pretation just given of the underground structure. Some of these forms are
cuesta-like, facing north and south, but so gentle that their significance is
readily overlooked. In general, there is a pitch to the eastward, so that as
one progresses along the trail one rises in the stratigraphic scale and comes
in touch with higher and higher beds.

Desert hollows

At many places along this part of the journey, small, undrained, basin-
shaped hollows are seen. They are not related directly to structural deform-
ation, but are the result of erosion. Very often the edges of sandstone and
clay-sand beds are fairly well exposed in the sides of the hollow. It is unusual,
of course, to find such a depression surrounded by very high enclosing walls,
but it is not unusual at all to find a hollow that is noticeably lower than any
possible outlet. In no case were we able to detect deformation that would
account for the form; and to our best belief the only defensible theory of origin
is that of deflation. Apparently the wind has gouged out these places and
has carried away the fine sands and the still finer silts of the original beds.

There seems to be no escape from the conclusion that this is a compara-
tively common type of desert erosion. It apparently starts at some point
of easy attack on one of the formations, perhaps because of peculiar textural
quality, and when it has once begun, the process seems to be capable of con-
tinuing and in some cases accomplishes a surprising amount of excavation.
The bare surfaces of exposed ledges indicate clearly that the process is con-
tinuing now, and that these hollows are being enlarged by removal rather
than being filled by deposition. Moisture is found at the bottom of some
of the hollows, suggesting that downward erosion by the wind is limited by
reaching the water-table. At any rate, there must be some very regular
dependence or control which limits the depth of excavation; and it appears
that when this is once attained, enlargement proceeds laterally, almost as
lateral planation does with a river (see Chapter XIX).

On the margins of these hollows a good deal of drifting sand was encoun-
tered, making it difficult to follow the trail through the depressions. Upland
traveling is, on the whole, reasonably safe, but difficult going and treacherous
ground are not at all uncommon in hollows of this kind, especially in the
broader ones.

For the next fifteen miles, to mile 30, it is judged that the strata are prob-
ably Pliocene—at least they are a continuation of the uppermost beds of the
ground crossed by the trail. At first the dips are easterly; then they begin
to change, until between miles 25 and 30 they are reversed, and the bedding
dips westerly or backward along the course. Transverse to this direction, on
a line running from north to south, the beds also show warping; and the geo-

PLATE XVIII.

A. TSAGAN NOR.

The sand dune belt of Tsagan Nor, with Hung Kureh in the middle distance and Baga Bogdo in the background. (See Plates
IV and XXX for location.)

B. EASTERN END CF THE LAVA-CAPPED MESA AT OSHIH.

(Compare Fig. 118.)

FROM TSAGAN NOR TO ARTSA BOGDO 147

logic structure along that line is indicated in generalized form at mile 25 (Fig.
62).

Some of the waters that drain from the northern slope of Baga Bogdo
form permanent or intermittent streams with occasional pond-like enlarge-
ments. Along this part of the route we crossed the dry valleys of many such
distributaries, but between miles 18 and 23 we forded several small running
streams with much difficulty, building narrow dams of turf-sods across them
for the wheels of the motor cars. The water-table lies close to the surface,
just as was the case for miles this side of Tsagan Nor, and the cars were in
almost constant danger of miring.

There is a long, narrow, scimitar-shaped area of dune-sands lying between
the trail and the margin of Baga Bogdo, several miles to the south (Plate
XVIII, A). The dunes are most extensively developed just south of Tsagan
Nor, where they form a belt probably fifty miles in length, and where individ-
ual dunes are 50 to 300 feet high. It was quite impossible for the Expedition-
ary outfit to cross the dunes. Indeed, the whole belt bordering the south-
ern trail for many miles is so heavily clogged with drifting sands that it is
quite impassable for motor cars. But the caravan trail farthest from the
mountain is comparatively good, and even where it traverses ground hope-
lessly covered with drifting sands, the trail itself can be followed with compar-
ative ease and safety. On either hand the heaving waste of dunes stands
back from the trail as the waves of the Red Sea hung back along the flanks of
the Israelites’ march out of Egypt. Along the trail itself, the feet of count-
less caravans have stirred up the sand and trampled out the plants that grow
on the dunes, so that the fine sand has been blown away by the wind. Only
the heavier particles remain, forming a firm pavement. This peculiar condi-
tion was found at many other places on the return journey, and for much longer
distances than had been encountered before. Sometimes for many miles at
a stretch the only possible progress was made by keeping cautiously in the
major trail. Even then the traveling was difficult indeed, and sometimes
looked quite hopeless. However, the worst of this kind of ground lies to the
east of the Tsagan Nor basin and was yet to be encountered by the Expedi-
tion.

In this district a surface-water supply was more common than elsewhere
in central Mongolia. Drainage from the near-by mountains finds its way out
into the basin, and the water does not always sink completely into the sands.
Here and there streams actually flow for miles across the plain, and consti-
tute one of the real difficulties of the trail. Uljitundur Gol, a stream at mile
30, is typical of these distributaries coming from the mountain side. It would
doubtless be possible to use this water for irrigation, but as far as observed by
the Expedition, no such use is being made of it in any part of the region.

148 GEOLOGY OF MONGOLIA

At mile 28 the course of a large valley could be distinguished plainly,
leading away from the mountain toward the northeast. It must have been
made by stream erosion where now no competent stream exists. This is
another of the numerous evidences of changed climatic conditions. With the
present rainfall it would be impossible to carve a valley of such magnitude.
Therefore, the presence of these forms, demanding different conditions for
their sculpturing, lends strong argument for a different climate in former
times. There must have been more water, and there must have been greater
streams—for valleys were carved out which are now slowly filling or are kept
cleaned out by the work of the wind, which helps to remove the new deposits.

Because of the westward dip of the strata, it is judged that the beds com-
ing to the surface between mile 30 and mile 33 are probably Miocene (Fig. 63).
A little distance away, at the side of the trail, volcanic rocks of the typical
chip-fragment type, apparently like those that have elsewhere been found
closely associated with the Jurassic strata, project through the surrounding,
simpler, Tertiary beds. The trail passes 6ver the very edge of a small knob
which extends some distance north of the trail and appears to be surrounded
by sediments. It is a ridge of the oldrock floor, laid bare by the removal of
the sediments which once covered it.

No fossils were seen in any of this ground. General petrographic and
structural quality are the only criteria present for use in correlation. Perhaps
the strata are still older than Tertiary, but they are simple in their relations
and evidently are unconformably placed on the volcanics.

The first volcanic outcrop was found at miles 34-35, and then the trail
passes for several miles over sediments again, but farther on it winds through
a low ridge of rugged hills, made up wholly of igneous rock associated with
fragmental beds of red and yellow sands. The igneous rocks are chiefly lava
flows, including rhyolites, felsites, and basalts. In addition, there are pyro-
clastics, but these are not so common as are the normal flows. Passing through
this ridge and down into a small stream valley at mile 45, the day’s traverse
was ended, and the Camp of the Volcanic Cliffs was pitched on the banks of a
stream of water which was usable, though somewhat charged with mineral
matter. Up to this ridge the country had been smooth and open, chiefly of
the simple sedimentary basin type. Here we entered a section of ruggedly
dissected older rocks. The volcanics may be of Jurassic age, but of that there
is no certainty.

VOLCANIC CLIFFS TO ARTSA BOGDO

The traverse was continued on the following day over the trail leading
rather abruptly out of this valley to an elevation 400 feet higher. The vol-

FROM TSAGAN NOR TO ARTSA BOGDO 149

canics continue for a few miles and then give way at miles 47-48 to conglom-
erates, which have the general appearance of the Jurassic strata. After climb-
ing out of the valley to the plain again at mile 49, another change in the geol-
ogy was observed. The conglomerates gave way to the much older and more
complicated formations of the Shara Khata hills. Limestones standing on
edge, associated with schists and quartzites, form the floor for a good many
miles. Evidently this is a part of the pre-Cambrian, such as had been en-
countered at many other places where the most ancient floor is exposed.

In this territory the traveling is good, the trail is well marked and sound,
and there is little drifting sand. At either side of the trail craggy remnants of
these ancient rocks stand up 100 or 300 feet in height, and the trail passes
through the gaps and saddles between them. ‘There is a cover of disinte-
gration soil, so.that outcrops are not very numerous, but the floor can be
seen at intervals frequent enough to determine beyond question that the
whole of it is of this complex series, of which crystalline limestones form the
most characteristic member (Specimen 459, A.M. 10148). The floor rocks
are all folded and faulted, and extensively modified by metamorphism of
various sorts. The strike of the beds varies considerably, and the dip is still
more variable. At mile 51 the strike is southeast; at mile 56 the strike is east
and west and the dip is north; at mile 56.5 the strike is S. 80° E. and the dip
is south; at mile 57 the strike is north and south, with the beds standing almost
vertically.

The complex ancient series continues without interruption to mile 60 or
mile 61 (Fig. 64). In the vicinity of miles 61-62, volcanic rocks of felsitic and
rhyolitic type, similar to those that we have usually credited to Jurassic out-
breaks, are encountered on the edge of an open plain which extends thence
eastward for many miles. Certainly a younger series comes in at this place
rather abruptly, just as it did on the western side of the Shara Khata hills,
and includes types of rock similar to some of those seen at the opposite side in
the vicinity of Camp Volcanic Cliffs. The open plain, which begins at mile
63, is covered; nevertheless it is certainly made up of simple sedimentary strata
lying nearly flat and probably corresponding to some of the strata seen many
miles farther west in the Tsagan Nor basin. Their age could not be deter-
mined. They are thickly covered with sand, and the trail is gravelly. At
mile 68, however, a flow of basalt is intercalated with the strata.

At mile 69, the course, which to this point for 69 miles has been easterly,
was broken, and the Expedition turned directly southward, toward the foot
of Artsa Bogdo. The new course led over simple strata and in a short dis-
tance began to climb up over the superposed alluvial fan carried out from the
edge of the mountain. From this point to the camp, which was pitched at
mile 80, ten miles south of the trail, the ground rises more than 1,200 feet.

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150

FROM TSAGAN NOR TO ARTSA BOGDO I5I

Camp was pitched here on a sloping shelf of ground, at the very edge of
the steep front of Artsa Bogdo. With good water near by, the location is
admirable for a camp. It is within easy reach of the rugged Artsa Bogdo
mountain group, and within striking distance of the sedimentary basins of
the plain. This camp, therefore, became another expeditionary center, and
side traverses, occupying the next two weeks, were made by the several divi-
sions of the Expedition.

NORTH MARGIN OF ARTSA BOGDO

A day was spent in examining the margin of the Artsa Bogdo uplift. On
the way to camp we had noted a large mesa south of the road, near the foot of
the range. Its cap of black basalt, and the white and red sediments showing
here and there on its scarred flanks, make a rare color composition. In: the
lowland between the mesa and the road, extensive red-beds are exposed. Our
first side traverse was to the west, to study the mesa and the adjacent red-
beds. For many miles alluvial fans cover the underground structure. At a
distance of 13 miles west of camp, however, ancient limestone ledges form
outcrops very similar to those along the trail farther north. From this point
on, we encountered limestone carved into rounded knolls which fall into a gen-
eral uniform level, constituting a platform above which the craggy moun-
tains stand abruptly. This is either the original erosion floor on which the
sediments of later age were laid down, or a still younger planation product.
Whatever the origin, it is now entirely bare for many miles.

Beyond the outcrop of ancient, folded limestone, at a distance of 15 to
16 miles, a much younger series of beds was found, dipping gently westward
and consisting chiefly of clays and sands. Interbedded with them are thin
limestone layers, which carry abundant plant remains of a cone- or tube-shaped
form, which have been coated with lime carbonate and then partly replaced
by colloidal silica. The structural relations suggest that the limestone is
essentially an ancient, calcareous tufa formed about plant-stalks and later
affected by silica-bearing waters. In the field we judged these beds to be
of Cretaceous age, and this inference seems to be supported by such addi-
tional facts as are now available.

Simpler strata, belonging to the beds that continue to the mesa, come in
next, and with them are associated amygdaloidal basalts which on weathering
furnish great quantities of agate and chalcedony pellets.

At the white-cliffed mesa, later sediments, capped with lava, rest upon
the gently sloping erosion plane. It is a large mesa, standing well out in the
valley, with a series of contiguous outliers. It owes its preservation to a
basalt cap which protects it from erosion. Beneath is a series of sandy, clayey
and nodular, limy beds. Lava blocks have slumped down over the ‘slopes so

152 GEOLOGY OF MONGOLIA

that few exposures can be seen. It is clear, however, that the basalt, which
is very vesicular, belongs on top. No fossils were found, although diligent
search was made in the hour available. The age, therefore, is wholly un-
known; but the general aspect is like that of some of the Tertiary beds, espe-
cially those of the Hsanda Gol in the Tsagan Nor basin, where unconsolidated
sediments are associated with flat-lying lava beds.

Picture writings

On some of the basalt blocks on the white-cliffed mesa, we noted a strik-
ing display of prehistoric pictures. One group of these, found on a large, flat
fragment, carries the following: two men, one with bow and arrow, several
animals, one of which was probably intended to represent a horse, another a
reindeer, another possibly a moose, while still another is perhaps a fox (Fig.
159, page 394). None of these animals, except the fox and the horse, is to
be seen within several hundred miles of this region at the present time, yet
it must be that the artist was familiar with them. Either he belonged to a
tribe which came from a region where these animals were native, or else the
region itself was occupied in his time by such animals. It is entirely reason-
able to believe that the latter is the explanation, and that the artist pictured
animals living in his vicinity. This interpretation would mean that the
people who made these drawings were not of the Mongol race, and that the
climate was other than it is now.

Upon completion of the inspection at the white-cliffed mesa, we turned
northwestward across the depression, toward the Kobdo trail, which lay a
few miles farther to the north. In making this traverse, beds of considerable
thickness and of simple structural condition were crossed. At one point a
small development of later conglomerate and sandstone caps a ridge, indicat-
ing that, subsequent to deep erosion of the underlying beds, sediments were
deposited above the break. These have again been attacked by erosion and
almost completely destroyed. The conglomerate carries cobbles of the same
basalt as that seen on the mesa, and it must, therefore, be younger than the mesa-
cap basalt. Possibly it is Pleistocene. While we made no important finds in
any of these strata, it is a district which might well repay additional inspection.

While the Expedition camped at Artsa Bogdo, Mr. Granger made a side
trip to the stratified red-beds of Oshih (Ashile), which could be seen in the
distance to the north. The geological division, after making a reconnaissance
of the Artsa Bogdo block and its margin, took supplies for a ten-day journey,
crossed the mountains and the adjacent basin beyond, and finally reached
the Gurbun Saikhan, the easternmost of the great uplifted blocks of the Altai
mountain system. The observations of these side traverses, as well as those
of the vicinity, are included in Chapter XV.

CHAPTER IX
FROM ARTSA BOGDO TO SAIR USU

On Friday, September first, the return journey was resumed. The camp
of the Expedition at that time was located on the Kweihwating trail, out on
the plain ten miles north of Artsa Bogdo. Since the character of the coun-
try for the progress of such an expedition was undetermined, and it was not
possible to get satisfactory information about it, it was decided to travel
eastward on the Kweihwating trail to the vicinity of Ulan Nor, and then try
to find a way northward to the Uliassutai-Sair Usu trail. This, we had reason
to believe, would furnish safe going and probably could be followed into
Kalgan. If the plan proved feasible, the trip could be made in reasonably
short time and without overtaxing our remaining supplies, but if for any
reason it should be necessary to turn back and retrace our steps, they would
be exhausted before the journey could be completed. We had to start with
an overload of supplies, because it would not be possible to replenish any of
the stock for several hundred miles. On this account the trucks were put toa
severer test than usual, and repairs were frequently necessary. These were
accomplished, however, with eminent success, through the great skill of Mr.
S. Bayard Colgate, Chief .of Motor Transport, who succeeded in keeping
the whole Expedition together to the very end.

Every stop, for repairs or for other purposes, was seized upon by the geo-
logical party to make closer investigation of the geological features of this
portion of the itinerary, but despite every effort the return journey was accom-
plished much too rapidly for efficient work. The remaining 800 miles of the
route to Kalgan, which crosses some of the rarest geological formations encoun-
tered in the entire Gobi region, must be described for the most part in general
terms, accompanied by sketches of structure which are based on a minimum
of actual examination. While the simpler portions are reasonably correct, the
more difficult ones are necessarily sketchy and suggestive, rather than depend-
able representations. There is some hesitation, therefore, in presenting a part
of this itinerary, and the decision to include it with the other portions is based
on the belief that this is the most effective way to present such observations

153

154 GEOLOGY OF MONGOLIA

as were made, and to record the interpretative judgment of the Expedition
on the geology of this traverse. This is all the more desirable because of
the fact that the traverse is very distant from the route followed on entering
the Gobi, and because certain new stratigraphic elements come into the geo-
logic column, and because their relation to the major features, both of a physio-
graphic and of a stratigraphic and structural nature, are in this way deter-
mined and located. It is felt, therefore, that the minor features and detail
may well be sacrificed, or left to supplementary determination at some later
time.
ARTSA BOGDO TO DJADOKHTA

For convenience, the return traverse begins again at zero miles (Fig. 65).
For the first five miles the sediments of the lowland continue. We consider
them to be Cretaceous sediments related to those of Oshih, ten miles north,
where an important fossil field had been located. Beyond mile 5, the ancient
crystalline rocks of the floor form the surface, having been stripped by the
same erosion that dissected the overlying sediments. The relief features,
therefore, are not more prominent in this portion than on the sedimentary
plain farther back. Ancient gneisses and schists form the floor at first, then are
overlaid by blue and white banded limestones which dip toward the north,
and which follow a general strike north of east. The floor formations are
evidently folded into a series of anticlines and synclines, which are truncated
by this plane and newly exposed. In the course of a few miles, alternations
of limestones and gneisses or schists were crossed, but the major portion of
this formation is crystalline limestone throughout the whole of the distance
from miles 7 to 16. Some of the ground, of course, is covered and outcrops
are not very prominent, but there is no difficulty in determining the general
underground structure. The presence of brecciated zones, some of which are
rehealed, clearly indicates that the rocks have been faulted, but no displace-
ments are recorded in the present topography. All of the deformation recorded
in the floor rocks has preceded the development of the present surface. The
age of the limestones, of course, could not be determined. They represent
some ancient pre-Cambrian type,—probably some portion of the Wu T’ai
system, which immediately preceded the graywacke series.

At mile 16, new sedimentary strata begin. Probably they do not repre-
resent a new basin, but a portion of an ancient filling or cover of this whole
area, not yet destroyed by later erosion. The former sedimentary cover has
been stripped from a ten-mile stretch, but for the next ten miles sediments
are still preserved, although they probably form a comparatively shallow
cover. The country is more rolling than one would expect, and the relief is
quite as marked as is that of the crystalline floor just traversed. No fossils

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155

156 GEOLOGY OF MONGOLIA

were found here, but in general appearance the rocks are similar to the Cre-
taceous beds farther to the west. Though there is loose gravel and sand along
the dry stream channels, most of the trail passes over bare bedrock, whether
the rock be the nearly level sediments or complex folded crystallines. The
general underground structure could be determined beyond question, but in
a traverse covered so rapidly it is not possible to fix the age or work out minor
detail.

At mile 26, the limestones of the ancient floor are at the surface again,
and for a short distance the sediments have been stripped off. The lime-
stones form a small arch here, with strata lying nearly flat, but dipping first
to the south and then to the north, passing beneath sediments again at mile 29.
This patch is small,—not more than a mile in extent,—and then, at mile 30,
granites come to the surface with the usual accompanying dikes and inclusions
of country rock, exhibiting all the features of the bathylithic granite described
in many other places.

The granite at mile 31 is very coarse-grained, massive in structure and
without internal deformation (Fig. 66). It cuts through rocks belonging to
the graywacke series and includes xenoliths of this formation. This content
increases somewhat regularly, and becomes much more pronounced as the
traverse continues toward the southeast along the trail.

At mile 33, the rock is chiefly gneiss and schist with dikes (Specimens 549,
550, 551, A. M. 10238, 10239, 10240). The variety included in the stretch
from miles 30 to 35, presents some difficulty in formational classification. That
it is a part of the roof material invaded by granite is clear enough, but how
much of it belongs to the graywacke series and how much to older formations
is not so clear. Apparently the graywacke content is not large, and the
stretch as a whole probably represents older formations.

At mile 35, we reached the edge of a new basin, beginning with flows of
basalt which dip gently toward the southeast. Above the basalt lie reddish
sediments forming a smooth plain which with slight undulations extends for
many miles. The age of the sediments at this place could not be determined,
but their relation to similar sediments, found a little farther on at Djadokhta,
is such as to lead one to believe that they are essentially the same—probably
of Cretaceous age, and a lower part of the Djadokhta series. These sediments
continue from mile 36 to mile 48, where the basalt flows come to the surface
again, this time dipping gently westward into the basin. At mile 45, we
crossed a broad valley called Dubshih, in which we encountered standing
water and some bad ground. Basalt flows and other igneous materials form
the surface then for five miles, and, although the minor structure is not worked
out in detail, they apparently form a gentle anticlinal arch and beyond mile
53 dip under again toward the southeast.

y XIX,

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PLATE XX.

A. THE GREAT TEMPLE AND LAMASERY, ONGIN GOL IN SUMU,

The view is taken in the Ongin valley, with low hills of the oldrock margin of the sediment basin in the background.

B. A NEST OF DINOSAUR EGGS IN THE RED CRETACEOUS SANDSTONE AT DJADOKHTA.

FROM ARTSA BOGDO TO SAIR USU 157

In this stretch, near mile 53, we crossed some very treacherous ground,
for the water-table is near the surface, and the ground is miry and very diffi-
cult to get through with the kind of equipment used by the Expedition. At
mile 54, rhyolite flows and andesites, with bombs and ash beds, lie above the
basalts, and are in turn overlaid by reddish sediments, chiefly sandstones,
which continue to the red cliff badlands of Djadokhta (Fig. 67, Plates I and
MAX).

The Flaming Cliffs of Djadokhta

The country is smooth and the trail is firm and easy to follow, but since
we had reached a point somewhat to the east of Ulan Nor and this was as far
east on this trail as we desired to go, we sought an opportunity to turn north-
ward across country. Every effort was made to find such a trail, and inquiry
was made of the few natives who were seen. At a point near mile 65, the
leaders halted, and while Mr. Andrews sought information at a nearby yurt,
Mr. Shackelford inspected the red-beds exposed in the adjacent escarpment, a
few hundred yards north of the trail. His first specimen was a nearly perfect
skull of the rare dinosaur later named Protoceratops andrewst.

The importance of the locality was at once appreciated; so, in spite of the
wish to push forward, camp was pitched a short distance beyond, at mile 65.5,
and the rest of the day was given to search of the cliffs and talus slopes of
the locality. In the two hours left for this purpose a large number of frag-
ments were recovered, forms which were recognized as new. We appreciated
that it would not be advisable to spend the time necessary to investigate this
ground thoroughly, so it was marked as one of the fields to be exploited more
carefully in the following season. This additional work was done in the sea-
son of 1923, and the locality yielded some seventy more or less complete skulls
and a dozen skeletons as well as the dinosaur eggs. The first year a small
egg was found but not reported—because the full significance was not appre-
ciated at that time. The second season, however, when more than twenty
others were unearthed, some of which were in nests with associations that
proved them to be dinosaur eggs, the importance of the discovery was com-
prehended, and the find received the attention that it deserved (Plate XX, B).

The rock is a red sandstone of very uniform grain and comparatively
simple structure. The grains are exquisitely graded, so that when the stone
is weathered, it yields a sand that will run like the sand in an hour glass. There
is virtually no admixed clay, and separate beds of clay are few, occurring
chiefly as channel fillings. Many of the sandstone beds contain myriads of
small limy concretions, some of them being traceable for as much as a mile.
In places the sandstones appear massive and structureless, forming sheer
vertical walls as much as forty feet high, in which neither bedding nor color-

158 GEOLOGY OF MONGOLIA

banding is seen; yet these massive layers pass laterally into well-bedded depos-
its. Cross-bedding, apparently of eolian type, is developed on a large scale
at certain horizons. We believe that the formation is in large part wind-
blown, and that this history is the major factor in accomplishing the perfect
preservation of the delicate fossils which the Expedition recovered from it.
The beds are very faintly disturbed, and show slight arches and saddles, but
dip very gently toward the south. To the south of the road, therefore, one
might hope to find higher formations. In 1923, a series of clays and clayey
sands, bearing a fauna of rare and primitive mammals, was discovered over-
lying the Djadokhta dinosaur-sands. It was called the Gashato formation,
and is believed to be of Paleocene age.

Looking off toward the south we could see that we had reached a point
opposite the Gurbun Saikhan, the last of the uplifts of the Altai system. The
three sections of this compound group of mountains could readily be made out;
the Baron Saikhan on the west, the Dunde Saikhan in the middle, and the
Dzun Saikhan to the east. They are essentially ranges of high mountains
separated by broad vales, so that they appear as three major units. They
form, therefore, a mountain uplift of greater extent and complexity than is
usual in the Gobi region, but they have the same fault-block features as the
other members of the Altai system,—they are apparently a group of three
fault blocks behaving as a larger complex unit.

DJADOKHTA TO ONGIN GOL IN SUMU

The following day the course was turned northward and the traverse
was projected across the lowland below the red cliffs (Fig. 67). The lowland,
which is an ancient erosion valley, has a steep southerly margin formed by the
cliffs of Djadokhta, but on the north side the slopes are more gentle, and it
would be easy to climb out of the valley except for the drifting sand, which is
more troublesome at several places on this northward traverse than at any
other place reached by the Expedition. ‘The difficulty is due in large part to
the fact that there is practically no trail, and traveling is always hard in drift-
ing sand, unless there is a good trail to follow.

Later sediments continue almost horizontally, probably dipping slightly
to the south—and this partially accounts for the difference in marginal slope
in the two sides of the valley. The same general type of sedimentary strata
continues for twenty miles. One can see variations in the country, especially
to the east, where hills of hard rock are plainly visible; but no other rocks than
sediments are encountered on the traverse itself to mile 86, where a sharp flex-
ure in the strata is noticed and there are outbreaks of basalt of irregular form
on both sides of the trail (Figs. 67 and 68). Doubtless the volcanic outbreak
has come up along a weak zone caused by faulting.

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FROM ARTSA BOGDO TO SAIR USU 161

Basalts are prominent in the floor from this point for many miles along
the trail (Fig. 69). They appear to be interbedded with sediments, and in
many places there is a thin wash of silt and sand partially covering them.
They are doubtless of the same age as the sediments, probably Cretaceous.

Simple sediments come in again at mile 98 and continue to mile 159 in
one of the smoothest, longest-continued level stretches seen anywhere in the
Gobi region (Figs. 69, 70, and 71). Throughout the whole distance the differ-
ence of relief amounts to a total of less than 550 feet, and for stretches of ten
miles together the total variation is not more than one hundred feet, with no
local irregularity of more than ten feet. The area is in appearance as level
and flat as a floor, but the whole plain rises gently northward at the rate of
about ten feet per mile. In the first forty-five miles the greatest local irreg-
ularity on the line of the traverse amounted to forty feet. For the first twenty
miles considerable trouble with drifting sand was encountered, and in the
failure of a definite trail there was delay in finding the best course. The route
finally selected lay along the east side of the valley of the Ongin Gol on the
very edge of the trench made by the river.

This trench is carved out of the plain to a depth of fifty feet or more.
There are almost no side tributaries, so that the margin is smooth and almost
unbroken, and traveling on the upland at the edge of the valley is better than
elsewhere. After getting somewhat out of the worst belt of drifting sand,
the run was made with great speed over one of the smoothest stretches of basin
country seen in Mongolia. This upland is a portion of the Gobi erosion plane.
Except for the patches of drifting sand, it is covered with gravel—the typical
covering of surfaces of this origin. It is essentially an erosion surface under-
going some dissection, and, despite the fact that the Ongin Gol carries sedi-
ments from the Khangai Mountains, it is in this region primarily an agent of
erosion, and its valley represents the deepest and broadest dissection of the
plain (Plate XX, A).

Ongin Gol in Sumu

For many miles the strata are not sufficiently well exposed to determine
their precise character, but that they are later sediments of some kind is per-
fectly evident. Within the edge of the Ongin Gol valley, the Expedition went
into camp at mile 147, near the temple and lamasery of ‘‘Ongin Gol in Sumu”’
(Plate XX, A, and Fig. 70). On inquiry there, it was found that the Sair Usu
trail lay only a few miles beyond, and that the major uncertainties of the
return traverse were already safely past. ,

This lamasery seems to be one of the better Mongol establishments.
There are hundreds of lamas and novitiates, who literally swarmed around
the Expedition while their representatives made official calls. Old men,

VOL.. II—II

162 GEOLOGY OF MONGOLIA

gray-haired, dignified and thoughtful-looking, sat on the ground in groups,
doubtless considering the significance of so unexpected a visit. It was an
interesting day, and might have been still more so if we could have gained the
confidence of these elder priests of Buddha, and entered into the secrets of
their thinking. Year after year, their whole lives through, they serve Buddha,
some of them doubtless with a singleness of heart quite in keeping with the .
great master whose teachings they follow. Here and there an older man
impressed us with his apparent superiority and look of intellectual acumen.

It would have been especially interesting to talk freely with such a man.
One would like to know his view of life, what it is he hopes to accomplish,
why he lives this kind of life, quite apart from the opportunities and the activ-
ities of his own people—an existence apparently wholly unproductive, unrespon-
sive, unimpressive, unpromising, devoid of almost every element of modern
interest and service. He seems a relic of ancient times—times when learning
and culture centered in and were preserved by just such priestly communi-
ties as this. From our latter-day viewpoint, their life seems that of a day that
is past, a clinging to a familiar yesterday, for fear, perhaps, of looking squarely
at to-day; yet one should recall that just such apparently unpromising sur-
roundings have furnished some of the great contributions to the religious move-
ments of the world. Most great religions have come from the wilderness or
from the desert. _Who knows but that there may come again, from quite as
hopeless-looking ground, concepts and teachings to shake the foundations of
the world? Is the magic of the desert lost? Has God abandoned the quiet
of the open wastes to make his revelations in the market place or the preten-
tious habitations of the arrogant and the scheming babels of the earth? Who
knows? And if we do not know, how is it that we so freely condemn these
quiet fate-defying priests of Buddha who spend their days in contemplation of
the mysteries of life and believe that they are doing the will of God?

ONGIN GOL TO SAIR USU

The lamasery on the Ongin Gol is situated on a terrace within the valley,
about thirty feet below the general level of the Gobi erosion plane. The ter-
race at this point is several hundred yards wide, and is level and dry. It
must represent a stage in the Ongin Gol erosion developed during an epoch of
climatic conditions somewhat different from those of the present. On leaving
this camp the course of the Expedition lay northeastward to the nearest point
on the Sair Usu-Uliassutai trail near Tugurik (Fig. 72). For ten miles the
same sedimentary strata previously described continue, but the topography
is a little more varied than in the plain farther to the south.

At about mile 160, igneous rocks appear at the surface in the form of

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low hills and ridges with a still more rugged me Le (Fig. 71). The char-
acter of the porphyries corresponds precisely with that of porphyries usually
found associated with Jurassic strata, and no other strata appear for several
miles. From mile 164 to mile 167, the floor is covered with alluvial or sedi-
mentary material in a shallow basin, so that the floor conditions can not be
seen clearly. Somewhere beneath this shallow basin, however, the floor rocks
change, because typical Jurassic sediments with southerly dip appear just
beyond, and continue thence for five or six miles. The dips vary in steep-
ness, but do not change from a southerly direction throughout the distance.
From mile 174 to mile 176, porphyries appear again, apparently breaking
through the Jurassic sediments, but the conglomerates and sandstones con-
tinue from mile 176 to the Sair Usu trail at mile 193. This is one of the fin-
est exhibits of Jurassic sandstones and conglomerates seen anywhere in the
Gobi region. The deformation is of simple folded type, as indicated by the
changes in dip, and includes two anticlines and a dividing syncline in the last
fifteen miles of the traverse.

At the Sair Usu trail the course was changed abruptly toward the south-
east, which was the general course through Sair Usu toward Kalgan (Fig. 72).
From the Altai Mountains up to this point the traverse lay across a great
basin filled with later sediments, with the margin along the Sair Usu trail
formed by folded Jurassic strata. At this very spot, where the course was
changed, the still more ancient floor comes to the surface where a gentle anti-
cline of Jurassic strata is cut away by erosion. Here there is no question of
the gently folded character of the Jurassic strata. The rocks outcropping
from beneath the Jurassic are a complex of granite and syenite,—probably
some representative of the granite bathylith,—and these massive rocks cut
one another in a confused way. This exposed strip is only two miles across,
for, at mile 196, the trail reaches Jurassic strata again, dipping this time
gently southeastward for several miles, and then sweeping up again to form
the other limb of a shallow syncline. Patches of later sediments lie on the
truncated Jurassic strata at mile 207 to mile 210, where porphyry breaks
through to the surface; beyond this point the Jurassic strata outcrop again
for a short distance.

On this journey along the Sair Usu trail, small, shallow basins with alluvial
deposits of recent origin are common. It is quite impossible in most cases
to determine what is beneath them, but judging from the surroundings they are
all very shallow and insignificant, belonging essentially to the present epoch of
alluvial wash (Fig. 73). Mile 215 marks the beginning of a long strip of coun-
try with the most complex structure seen along the Sair Usu trail (Fig. 74).
In the first thirty miles it is essentially a portion of the granite bathylith, cut
by many igneous masses of syenite and porphyries, and by exceedingly numer-

FROM ARTSA BOGDO TO SAIR USU 165

ous small dikes of great variety. The separate field units are too numerous to
put on a section, but the principal ones and the characteristic structure are rep-
resented in the stretch from mile 215 to mile 245. Certain portions of it, such
as from mile 220 to mile 235, are comparatively simple because of the domi-

Los in Sumu

Later Sediments

7 ARO
sUsua rail
Ficure 73.—Margin of a sediment-basin at Losin Sumu. The Sair Usu trail follows an inner low-
land between oldrock hills on the north and the beginning of a broad basin of sediments on the south. The
trail is marked by many narrow parallel camel paths.

nance of the massive bathylithic granite, whereas other less extensive portions,
such as that from mile 240 to mile'245, are much more cut by a variety of in-
trusives (Fig. 75). From mile 248 to mile 253, Jurassic conglomerates and
sandstones form the floor again, dipping to the southeast.

The interval between mile 214 and mile 248 appears to represent a stripped
or eroded portion of the old floor which probably at one time was covered with
Jurassic strata. From mile 253 to mile 262, the country is formed of a com-
plex of porphyries which either have cut through or peep through the Jurassic
conglomerates and sandstones, leaving scattered patches of Jurassic strata, such
as the strip from mile 256 to mile 258. One of the most complex portions lies
between mile 259 and mile 262 in the vicinity of one of the wells of the Sair Usu
trail. Here the Expedition pitched the camp which we named “the Camp of the
Porphyry Hills” (Specimens 580, 581, A.M. 10269, 10270). The distance of
112 miles from the Ongin Gol to this point was covered in a day,—one of the
longest runs in the summer’s work. Nevertheless, the geologic route sec-
tion was kept fairly true. It is a terrific task to unravel more than a hundred
miles of such structure in a working day. The geologist finds himself com-
pletely exhausted, and even dreams of underground structure, of strata and
fossils, bathyliths, roof-pendants and ancient metamorphic revolutions, until
the sun rises again on another day of geologic puzzles. One must try it to
appreciate in full the effort that must be made to keep the story straight.

Beyond mile 262, essentially the same type of geological structure con-
tinues for a short distance; but in a few miles massive syenites and granites
prevail over the porphyries, and, from mile 267, ancient crystalline lime-

166 GEOLOGY OF MONGOLIA

stones (Specimen 584, A.M. 10273), cut by granite, form the country rock
for several miles. The rock is undoubtedly one of the very ancient formations,
and apparently forms the roof of the bathylith, portions of which have en-
croached on the overlying rock and penetrated it far enough so that present-
day erosion enables one to cross alternately patches of granite and strips of
limestone. The limestones have various attitudes, but stand rather steeply
and for the most part dip to the south or southeast.

Several shallow basins, with alluvium and sediments of perhaps Tertiary
age, were crossed, but none of them is more than a few miles in extent. At
mile 275, a patch of sandstone and conglomerate suggests the presence of a
wedge of Jurassic strata, but an abrupt change is made to the older formations
with limestones, granites and schists as the dominant country rocks (Fig. 76).
A patch of flat-lying sandstone at mile 279 suggests that Jurassic strata for-
merly lay over this whole country, and that since being stripped off, the addi-
tional dissection has not been very great. The crystalline limestones and
interbedded schists are probably the Wu T’ai (Willis, 1907, II, page 4); they
are folded and crumpled, showing all the complexity usually present in forma-
tions older than the graywacke series. The best exhibit of this series of forma-
tions is found from mile 280 to mile 285 of the itinerary. Smooth sediments
then cover the floor for a few miles, but it reappears at mile 292, and thence
for thirty or forty miles to Sair Usu there is a succession of changes from lime-
stones to schists cut by granites and in places by later porphyries (Fig. 77).
The structure runs simply enough, but with a great showing of granite for the
first twenty miles, to mile 311, where the course of the itinerary changed from
east to south (Fig. 77).

This change of direction was occasioned by our having departed somewhat
from the main Sair Usu trail, along one leading too far north. The change to
a southerly course was made at the intersection of this byway with the reg-
ular Urga-Sair Usu trail, and from that point a run of seventeen miles brought
us to Sair Usu, a well-known caravan station on the edge of the desert. The
same formations continue, represented by crystalline limestones, schists of
various sorts, granites and porphyries. The general dip at first is southerly,
but this changes at about mile 319 to a northerly dip which continues for sev-
eral miles, and finally to a southerly dip again at mile 324, where the older
floor passes beneath a cover of overlying sediments of uncertain age. The
general quality of these sediments is about that of the Cretaceous beds of
other places, and we surmise that this may be another Cretaceous area; but
no other data in support of it were found, except a considerable warping of
the beds, which seems to be more characteristic of the Cretaceous sediments
than of later ones.

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168

FROM ARTSA BOGDO TO SAIR USU 169

taking on supplies, and from this point on to Kalgan the run was made with-
out further additions. The geologists took advantage of the opportunity to
examine the strata of the basin and the adjacent hills. It was then found
that the bare hillsides seen at a distance east of the trail are weathered ancient
rock. The basin sediments, beginning somewhere in the vicinity of mile
324, yielded after diligent search traces of fossils judged to be plant remains.
These beds are considerably deformed, and are made up largely of pebbly
sandstones bound with lime (Specimen 602, A.M. 10291). The proportions
of sand, lime and clay vary greatly, so that the sediments grade from sand-
stones to limestones and shales. The shales are of many colors, and some of
the dark carbonaceous beds are almost paper shales. This quality, together
with their structural relations, the occurrence of limy layers, and the finding
of plant remains, leads us to believe that the strata belong to a Cretaceous
-basin, the extent of which is as yet quite unknown, but which continues in the
vicinity of Sair Usu along the trail for more than five miles.

CHAPTER X
FROM SAIR USU TO KALGAN

THE trip of 535 miles from Sair Usu to Kalgan, which was begun Sep-
tember 7, occupied twelve days, including brief stops for geologic and palzon-
tologic inspection of promising localities. The journey was made over the regu-
lar caravan trail, which is intersected by very numerous local trails and minor
caravan routes, so that the most direct course is not readily followed without
local guides. This region is more prosperous-looking than that in which the
Expedition had worked for the last two months. One meets travelers fre-
quently, and sees occasional villages, lamaseries, temples, and local shrines.

PALZOZOIC STRATA

The traverse from Sair Usu crosses later sediments between mile 326 and
mile 327, and in general topographic features there is no very striking break
for a short distance farther; but hills appear on the south side of the trail,
and the older floor of schists and limestones comes to the surface again at
about mile 330 (Fig. 78). Apparently this is just a stripped portion of the
old floor, because sediments appear again from mile 333 to mile 336, succeeded
by another stretch of schists.

At mile 337, however, an entirely different formational type was found.
At a cliff of fossiliferous limestone just south of the trail, hasty inspection
revealed a fauna indicating Paleozoic age. The sandstone beds and con-
glomerates beneath the limestones lie unconformably on the ancient schists.
The column at this point, therefore, is represented by: conglomerates at the
base, approximately 150 feet thick; sandstones, 200 feet; cherty limestones,
richly fossiliferous, 500 feet; gray limestones, 200 feet; very dark brown lime-
stone and sandstone, 200 feet; yellow dolomitic limestone, 100 feet; fossilif-
erous, pink, granular limestone, 500 feet; dark brown limestone, 200 feet.
This section is suggested by the outcrops that stretch from mile 337 to mile
338.5. The structure is that of an anticlinal fold, breached at the crest so
that the schists are exposed along the stripped arch.

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FROM SAIR USU TO KALGAN 173

This was the first time in the whole summer’s itinerary that determinable
Paleozoic strata were discovered in place. Only one other piece of Paleozoic
rock, and that a loose fragment, had been found up to this time. The fossil
content of these beds near Sair Usu has proved on later comparison to be of
Dinantian (Mississippian) age. Fossils are abundant, and undoubtedly an
extensive collection of later Palaeozoic forms could be obtained from this local-
ity and a much more extensive series of beds could be made out if the strata
were followed along the strike, either to the east or to the west. We propose to
call these beds the Sair Usu formation.

CONTINUATION OF TRAVERSE TO ARDYN OBO

Along the course of the trail, however, the strata are cut off abruptly
at mile 335, either by a fault or by an igneous outbreak, or by both; and here
a basin of later sediments begins, which continues for at least thirty miles.
Perhaps this is one edge of a down-dropped fault block, or a very sharp warp
of the ancient floor, causing the retention of the sediments. It is not pos-
sible anywhere along the trail to determine either the age or the thickness
of this sedimentary deposit. It is probably comparatively thin, for from mile
345 to mile 347 the floor is stripped by present-day erosion, exposing slaty
and shaly rock with limestone layers and conglomerates standing on edge.
Just what these rocks represent in the series of formations could not be deter-
mined. They are not exactly like any seen elsewhere, but may be some facies
of the Paleozoic strata associated with the limestones seen farther back along
the trail at mile 337.

Except for this window exposing the ancient floor, the later sediments
continue with rolling topography to mile 374, where porphyries of trachytic
composition come from beneath and form the floor for some distance. From
mile 374 to mile 389, a complex of porphyries forms the country rock (Fig.
79). The porphyries are brecciated and brittle, cracking into small chips.
They are prevailingly fine-grained, of very great variety, and belong to many
different individual field units—too many to illustrate on the scale used for the
cross-section. Some of the units show flowage lines, but for the most part
there is little structure to be made out, except that these porphyries cut one
another in a most intricate way, making an exceedingly confused structural
complex.

From mile 389 granite is the dominant rock for several miles (Fig. 80).
Its relation to the porphyry complex is not clear, but in general lithologic
appearance the porphyries seem to correspond in some measure to the dikes
that almost everywhere else are prominent in granite areas. A simpler form-
ation of bathylithic granite, cut by dikes, continues to mile 397, where a
change to complex porphyries again takes places and continues to mile 4or.

174 GEOLOGY OF MONGOLIA

From mile 401.5 to mile 407, conglomerates and sandstones, standing on edge
but dipping steeply to the north, are crossed by the trail. This makes a
stretch of five and a half miles directly over the edges of strata whose struc-
tural relations are not fully determined. Probably they are all of Jurassic
age, and this is another of the down-dropped blocks so often seen carrying
strata of this type.

Volcanic rocks cut in again at mile 407, and continue with a porphyry
complex and volcanic flows of obscure structural relations for several miles,
until the edge of a basin is entered at mile 413, and from that point later
sediments form the terrane for forty miles. The underground structure is
simple, but the profile is more varied than in most sedimentary basins. Ero-_
sion has exposed the strata at numerous places, so that one can see their
character, and at one point, mile 433, the ancient floor comes to the surface,
apparently indicating that the basin is considerably deformed (Fig. 81).
Judging from the physical appearance of the strata, a part of these sediments
should be of Tertiary age, and a portion underlying, with somewhat more
indurated habit and somewhat greater deformation, containing occasional
limestone layers, is probably of Cretaceous age. Certain of the thin-bedded
limestone layers carry traces of fossils (Specimens 636, 637, A.M. 10325,
10326) of a type similar to specimens found north of Artsa Bogdo and subse-
quently shown to be plant remains. On the north margin of Artsa Bogdo
we had found beds of fresh-water limestone, carrying obscure plant remains
intercalated in the basin sediments (see page 151).

Sediments of Ardyn Obo

At mile 453, the ancient schists, gneisses, and porphyries of the floor
appear at the surface again and continue to mile 458 (Fig. 82). This ground
is very smooth and almost as level as the surface of the sediments themselves.
Sediments come in again at mile 458, and continue, with only one break at
mile 473, to Ardyn Obo, where an escarpment of these sediments exposes
more than 200 feet of strata (Fig. 83). Here tossils of Lower Oligocene age
were found (Fig. 86, and Plate XXI).

The bluff of Ardyn Obo can be seen south of the road, extending east
and west for many miles. At mile 487, the direction of the scarp changes
sharply and lies almost north and south (Fig. 84). Upon the cliff-top at this
abrupt corner, which, high over the vast lowland, looks like a cape on a sea-
coast, the Mongols have built a large obo which can be seen for twenty miles
(Fig. 85). An obo is a pile of stones beside the road, in which every stone
represents a prayer to Buddha. Some are mere heaps, while others are built
like towers, with minor piles around them. Pitiful sacrifices of the few things
the desert can spare are added by pious wayfarers—torn strips of garment,

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176 GEOLOGY OF MONGOLIA

nose-thongs of camels, small Chinese coins, and sticks from broken yurts or
carts. The obo serves not only to mark a sacred spot, but is of serious import-
ance to the traveler as a cairn to indicate the trail. A Mongol guide will tell

ARDYN OBO

Vast lowland

Broad by pas

Latitude: 43°24 Je"
Longitude: 108° 21 26°

Contour interval 50 feoi Elevations determined by barometer

Ficure 84.—Map of the Ardyn Obo locality.

the way in terms of obos, no less than of temples, wells, and topography.
The impressive pile at mile 487 is the Ardyn Obo, the ‘‘prayer pile of jewels,”
and gives the district its name. The name was also adopted for the strati-
graphic unit. The word ‘‘ardyn,’”’ meaning ‘‘jewels,’’ proved to be the same
word which we had spelled ‘“‘irdin’’ farther east. It refers to the glittering,

Ficure 85.—Field sketch, made at Ardyn Obo from the lowland, showing the abrupt cliff-like scarp and the
very slight back-cutting of the young gullies.

highly polished pebbles of quartz and chalcedony which are found in the
upper sandstones of the formation.

Three days were spent at Camp Ardyn Obo (Plate XXI), to enable us to
make collections and additional studies. The strata are well exposed, so that

FROM SAIR USU TO KALGAN 177

a detailed column of the beds forming the escarpment could be measured
(Fig. 86). They consist essentially of two members, sandy clays below, and
cross-bedded sands and gravels above. All were probably water-borne sedi-
ments, and the channeled and cross-bedded upper member testifies to vigorous

YELLOW SANOS AND GRAVELS

FOSSIL

BEARING BED
MIDOLE MEMBER

GRAY AND YELLOW SANDS

LOWER MEMBER
CHIEFLY REO SANDY CLAY

FicurE 86.—Geologic section at Ardyn Obo.

stream work. Consistently with this interpretation, no associated skeletons,
such as were common at Djadokhta, were discovered here—only scattered
bones, some of which were much worn, as though they had been rolled about
by streams. Caches, or pockets, of bones were found in the bottoms of the
old stream-channels, the cross-sections of which are clearly seen in the bluff
(Plate XXI, B). Although the fauna includes an aquatic rhinoceros, a large
titanothere and several carnivores—enough of a fauna to determine the approx-
imate age as Lower Oligocene—the locality is not prolific in fossil forms, and
the great majority of the beds seem to be entirely barren.

At this point, also, there is an exceptionally good exhibit of characteris-
tic erosion forms, and some of these have been made use of in our studies of
erosion and physiographic history (Chapter XIX). The bluff itself is very
simple, steep and straight, and extends unbroken except for minor short
gulches, as far as the eye can reach. It is topped by a gravel-covered, smooth
surface which represents the Gobi erosion plane.

ARDYN OBO TO SHARA MURUN

On leaving Ardyn Obo the trail continues over sedimentary strata of the
same sort to mile 493, where, only three miles from camp, granite and granite

VOL, II-—I2

178 GEOLOGY OF MONGOLIA

gneisses cut by dikes come to the su face again. This change makes no
impression on the topography, for erosion has simply stripped certain of the
irregularities of the old floor, and these portions merge into the general level
without any noticeable change of feature.

Again, at mile 495, sediments are reached in a shallow basin extending for
two miles. They are unfossiliferous sandstones, which, from the character of
the rock, were judged to be probably of Cretaceous age. From mile 497 on-
ward, the granites of the floor form the surface again, and erosion has left
upstanding, jointed remnants that look like the ruins of giant masonry, con-
veniently referred to as ‘‘monument granite.’’ The same type of erosion and
relief was noted at several other places along this traverse. At mile 500, a
gabbro-diorite cuts into the granite, and then there follows for two miles a
great igneous complex of many rock varieties, which in turn is succeeded by a
series of limestones and schists, dipping steeply to the north. Strips of this
kind of rock are repeated several times in the next five miles, separated by
occurrences of granite. The granite is probably of the same bathylithic
type that has been noticed over so large a portion of the Mongolian territory,
and the occasional strips of metamorphic rock are probably only portions of
the roof not wholly destroyed in the peneplanation that preceded the depo-
sition of the later sediments.

An abrupt change takes place at mile 507, where sandstones come in,
which we believe to be of Jurassic type; but within three miles they are cov-
ered by alluvial material and perhaps shallow basin sediments, which entirely
obscure the older structure (Fig. 87). The basin is limited at mile 512, and
again for many miles the trail crosses the ancient floor, consisting of lime-
stones, quartzite schists, granite gneisses, shear-schists, and greenstones.
They form a great schist-gneiss complex, including a large amount of green-
stone sheared into schistose condition, with much smaller amounts of dolomitic
limestones and other metamorphic rocks, including gneisses. Nowhere have
we found so great a series of this type, and nowhere in the whole summer’s
work has so much greenstone been seen. The series probably belongs to the
Wu T’ai system—perhaps very old Wu T’ai.

Farther along, at mile 519, red quartzites and slates are intercalated with
the greenstone schists, but greenstone predominates to mile 522, a distance
of three miles. Quartz veins and lenses are abundant, but they appear to
be barren. Toward the south the greenstones pass into brown, siliceous meta-
dolomites, sericite schists and porphyritic greenstones (Specimen 673, A.M.
10362), conglomerates, and tuffs or tuff-schists.

From mile 524 the trail passes over much ground covered with pebbles
that might be derived from Jurassic conglomerates, but no outcrops were seen
and it was not possible to determine in this stretch what the structure is.

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FROM SAIR USU TO KALGAN 181

The next outcrop of definite character is a quartz-porphyry at mile 526, and
the ground is covered again for several miles, making it impossible to deter-
mine its exact character. Ridges seen in the distance at the side of the trail,
indicate formations with a prominent structure, dipping to the north. Frag-
ments, chiefly of mixed schists, are washed in from the adjacent country
and lie about on the ground. Among the fragments are pieces of limestone,
some of which are fossiliferous, and it is judged from this that Palzozoic
strata are exposed in the higher ground on the east side of the trail. The cov-
ered ground extends to mile 531, and then granite, cut by dikes of various
porphyries, is exposed again in smooth topography and is the chief type for
several miles. At mile 538, the trail crosses green schists cut by granite;
the schists probably represent roof-pendants extending down into the bathy-
lithic granite. This kind of country continues for several miles, with areas of
granite, short stretches of covered ground, and the usual dikes and remnants
of the ancient roof.

At mile 546 an entirely new series begins, represented by schists, quartz-
ites, phyllites, and slates, developed on an enormous scale (Fig. 88). At first,
reddish slates, and phyllites with considerable vein quartz predominate; the
strike is N.80°E., and the dip is northerly. Camp was pitched here at mile
549, on the Bilgoho flats. The same formations continue, becoming gradually
more granular and dipping to the south. Farther along, the quality changes
to typical graywacke and gray sandstones, while reversals of dip indicate that
the series is folded. This graywacke quality predominates in the portion of
the series crossed from mile 554 to mile 560. There are marked differences of
color, and the quality is much more variable than has been seen elsewhere in
the so-called graywacke series. Phyllitic slates are abundant, and in some
places the rock is sheared almost to a schist (Plate XXII, A). It is possible,
of course, that there is more than one series represented in this stretch of
ground, which continues to about mile 571, but in the time given to it no defi-
nite formational separation could be made, and it is assumed that this whole
succession from mile 547 to mile 571 is equivalent to the graywacke series as
typically exposed on the Tola River far to the north.

Paleozoic strata of Jisu Honguer

Beyond mile 571, the strata change abruptly—probably along a fault
(Fig. 89). The graywackes, seamed with multitudes of white quartz veins,
give place to a series of simpler sandstones, fine fissile shales and gray lime-
stones, striking a little north of east (Specimens 691, 692, 693, A.M. 10380,
10381, 10382). The new series contains none of the quartz veins seen in the
graywackes. The shales and sandstones are barren, but the limestones are
richly fossiliferous. A.W. Grabau later identified the Permian Lyttonia fauna

182 GEOLOGY OF MONGOLIA

in the fossils collected here, thus proving that the beds are not correlated with
those of Sair Usu. The district is called Jisu Honguer by the Mongols, and
the name Jisu Honguer formation was therefore chosen for these Permian
beds. In the second season the geologist stopped for several days at this
place, mapping and recording the rocks and collecting their fossils. The beds
are very strongly folded; the folds are overturned toward the south and are
broken by two series of faults—an older set of fold-thrusts parallel to the
strike, and a younger set of cross-faults. The formation occupies a narrow
strip bordered by fault-contacts between the granite hills on the south and the
graywacke series on the north. At about mile 576, the entire series passes
under sediments which, fifteen miles farther east, carry a rich fauna of later
Eocene vertebrates (Fig. 89).

Tertiary sediments of Shara Murun

There is an escarpment at mile 589, where beds of characteristic Tertiary
habit are exposed (Plate XXII, B). The gullies of the badland area descend
northward into a hollow lowland of large extent, exposing beds of red, green-
ish, and purple clays amazingly rich in vertebrate fossils. Above the clays
lie white and gray sandstones which carry fewer fossils. In our swift recon-
naissance no adequate study of these beds could be made, but the place was
noted and reserved for careful search in the season of 1923. We have called
them the Shara Murun formation, and Osborn and Matthew consider them to
be of Eocene age—rather younger than the Irdin Manha beds.

At this point the trail climbs to the top of the escarpment and crosses a
stretch of level Gobi erosion plane from mile 595 to mile 606 (Figs. 89 and go).
This ground is underlaid in its entire extent by Tertiary strata such as were
seen at mile 593, and they are found again at mile 606, on the opposite side of
this broad mesa, where a very prominent bluff exposes a few hundred feet of
white and reddish sands and clays.

The Shara Murun, a broad valley 300 feet deep, had to be crossed from
mile 606 to mile 615. The course of the traverse across the Gobi erosion plane
had been approximately S.60°E., but across this valley the direction was
changed to S.80°E., and on climbing the escarpment beyond mile 615 not
much difference was made in the course, except to turn a trifle more to the
south. The country is underlaid by sediments of later type but of undeter-
mined age, as no sufficiently good exposures were seen to warrant closer in-
vestigation. The upland, across which the traverse lies, is undoubtedly again
the Gobi erosion plane, but the ground, as it reaches away indefinitely in every
direction, is more rolling than in many other places. There is an older topog-
raphy still preserved on this upland, with shallow valley courses apparently

PLATE XXI.

A. ARDYN OBO.

The upland surface is the level Gobi erosion plane. The heap of stones on the upland is the Ardyn Obo, or “prayer pile of jew-
els,"" which gives the locality its name and for many miles is the traveler's chief guiding landmark.

B. STRUCTURAL DETAIL OF THE SCARP AT ARDYN OBO.

The crossbedded sands and gravels exhibit channeling. Traces of contemporary plant roots have been preserved as tubular
concretions which are best seen at the right. An important find of Oligocene rhinoceros skulls was made in one of these ancient
channel fillings, where Mr. Granger and Mr. Andrews are seen at work.

PLATE XXII.

nascent ot imi!

A. BILGOHO,

The bare rock floor near Bilgoho bevels the upturned edges of folded slates and phyllites. Deeply worn grooves made by caravans
are seen in the foreground.

B. BADLANDS OF THE EOCENE SHARA MURUN FORMATION AT ULA USU,

FROM SAIR USU TO. KALGAN 183

unrelated to the more pronounced valleys, such as the Shara Murun which
is now dissecting the country.

SHARA MURUN TO KALGAN

This rolling upland continues for many miles. The camp of September
15 was pitched beside the trail on this upland country, at mile 632, and the
journey the following day continued over the same kind of ground to mile
657, twenty-five miles farther along (Fig. 91). At some point within this dis-
tance the quality of the ground changes, but because of the soil cover and the
increasing failure of outcrops, the change was not noticed until mile 657 was
reached, where granite, quartzite, limestone schists, and other associated
schists again form the floor.

Not many observations were made, but occasional opportunities for
identification indicate a complex floor of ancient rock, probably Wu T’ai, with
schists, limestones, quartzites, and granites as the dominant elements (Fig.
92). For the next twenty miles or more no great change is registered. Long
stretches of ground are covered with disintegration débris, but some indication
of rock character can be made out from the fragments that lie scattered about,
and there is every indication that the underlying structure is complex. In
the last few miles approaching the next camp, granites dominate, weathered
into ‘“‘monument granite” form.

The day’s travel was accomplished under extraordinarily trying condi-
tions. The autumnal storms had set in; the journey began in rain which
changed to snow, with a tremendously strong, cold wind, and ended with
everyone chilled through, for the party had not yet changed from midsummer
clothing. Less than two weeks had elapsed since almost tropical tempera-
tures had prevailed. As a result of the day’s exposure, those who were least
protected were so thoroughly chilled that there was real danger of more seri-
ous consequences, and the experience served asa broad hint to use all possible
speed in reaching Kalgan.

From Camp Granite Monument, at mile 697, the trail crosses granite with
patches of quartzites and schists as xenoliths and pendants, for the next twenty
miles or more (Fig. 93). The structure appears to be simple; granites greatly
predominate, with numerous roof-pendants, the usual porphyry dikes, and the
normal variations in quality.

Chinese settlements

Within a day’s traverse, the desert country gradually greened into
the typical Mongolian grassland, and at about mile 725 the first cultivated
fields of the encroaching Chinese came into view (Fig. 94). From this point

184 GEOLOGY OF MONGOLIA

onward the occupants of the country are Chinese, and, except for travelers
along the trail, one sees no Mongols. The outermost patches under cultiva-
tion are scattered, and clearly represent a pioneering experience; but within
a very short distance there appear extensive, well-cultivated fields of grain
with ripening barley, oats, flax, and millet. It is noted also that the soils
look rich and heavily cover the rock floor. Outcrops now are rare, and it is
more difficult to determine the structural character of the country in a recon-
naissance as rapid as the return journey required. Only occasional observa-
tions could be made from this point onward. Wherever granites are present,
outcrops could usually be seen at the side of the trail. Occasionally, as the
trail crossed them in the next thirty miles, limestones, quartzites, gneisses,
and schists could be identified. It is very evident, therefore, that the floor
is made up of ancient rocks rather than of later sediments. Wherever the
ledges are to be seen, they indicate closely folded structure and crystalline
condition. This type of country undoubtedly prevails without material
change, but also without possibility of very accurate correlation, to about
mile 760 (Fig. 95). The trail at that point enters a prosperous agricultural
plain dotted with Chinese villages. Its smoothness suggests a sedimentary
basement, but the understructure is obscure.

At mile 773, the Expedition went into camp at the side of a small pond
in the midst of a farming community, where fields of grain and general agri-
cultural prosperity were the predominant features. In the distance one could
see flocks of wild fowl, and as the whole aspect was that of a grain-farming
region, not very unlike portions of our own Northwest, we named it Camp
North Dakota.

In a single day the Expedition had returned from the barrenness of semi-
desert to comfortable agricultural surroundings. For the first time in five
months one could see cultivated plants being grown for food. The prosper-
ity of this immediate country naturally suggests the possibility that a much
greater portion of the Mongolian border might be adapted to the same pur-
pose. This is undoubtedly true, and advantage is being taken of it by the
Chinese, who are encroaching on this territory at the rate of two or three
miles a year. How much farther this kind of cultivation can be extended
is a serious question. The limits of farming without special adaptation to
arid conditions appear to be not far beyond the present zone of occupation.
Within two days’ journey in our mode of travel the bounds would surely be
reached, and cultivation beyond would have to be confined to local areas of
special favor, where irrigation could be practised.

Certain it is that the Mongols themselves are not making this use of their
own land, although there is evidence that irrigation was practised far inland
in former times. Whether they ever will change their mode of life sufficiently

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FROM SAIR USU TO KALGAN 187

to enter upon agricultural undertakings is extremely doubtful. It is certain,
of course, in that case, that other peoples sooner or later will occupy the arable
lands, and it is probable that the time is not far distant when the Chinese,
on whom the pressure for an outlet is greatest, will claim most of them.

It is most amazing that these arable lands have lain untilled, on the very
borders of a land where the struggle for existence has been so severe for gen-
erations. With an example of great agricultural competence before them,
even at their very doors, the Mongol owners of these tracts seem never once
to have tested this quality; and, on the other hand, the Chinese, even though
visited by famine time and time again, and doubtless fully appreciating the
advantages of additional agricultural facilities, seem to have avoided this
encroachment. It is an almost unbelievable situation, for which there must
be some special explanation. Perhaps the Mongols have been too fierce, and
the unprotected outposts of unwarlike settlers could not maintain themselves
against their raids. The region lies outside all the ancient walls, and many,
many miles beyond the Great Wall of China, the existence of which throws
some light on the same problem. A people with a desire for isolation so strong
that they were willing to build such a vast, protective barrier, perhaps should
not be expected to venture far into the barbarians’ territory beyond; and it
is equally reasonable to believe that a people outside, against whom such bul-
warks must be built, were vicious enough to prevent any unwelcome encroach-
ment. Something has happened in more recent times to modify materially
this age-long stalemate of nations confronting each other. Perhaps the Mon-
gols, weakened as a people by the loss of their best tribes and of their best lands
north of the desert, and weakened as a state by the disintegration of their
ancient empire, have become less watchful and jealous of their heritage.
Perhaps, on the other hand, the Chinese have become more venturesome,
emboldened by the pressure of their teeming millions and by the pact with
Russia which gave them Inner Mongolia as a sphere of influence. It is pos-
sible that the world is now witnessing one of those transitional periods that
have marked the rise and fall, the expansion and the elimination of competing
peoples.

The return to Kalgan

Monday, September 18, the return journey was continued across this
cultivated plain. Occasional outcrops of rock between mile 775 and mile
776 indicate that the ancient floor of complex rocks lies close beneath the
trail, but for the longer stretches the simplicity of topography suggests that
the underlying strata are sediments of later age.

The country is well watered; streams and small lakes are numerous, and
roads are treacherous and heavy. Limestone fragments were found along

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FROM SAIR USU TO KALGAN 189

the trail at mile 798 (Specimen 717, A.M. 10406), and basalts are exposed in
prominent ridges from mile 800 to mile 804 (Fig. 96). This is probably the
same series of basalt sheets or flows that is encountered in the edge of the escarp-
ment at Wan Ch’uan Hsien Pass above Kalgan. From mile 805, the country
clearly consists of sediments for some distance, and basalts are encountered
again from mile 816 to the Great Wall (Fig. 97).

At mile 824, after following an easterly course for many miles—indeed
for a whole day—we reached the main road and telegraph line leading to Kal-
gan, and the Expedition turned southward on the same route over which
it had entered the Gobi region almost five months earlier. The night of the
eighteenth was spent at the Chinese inn at Miao T’an, and the following day

Hamha
French Mission“ Basalt Lava Flows

820

FiGuRE 97.—End of the traverse of 1922. This section connects with the outward
bound traverse at Miao T’an. (See figure 6.)

the return traverse was completed to Kalgan without further incident than
the finding of extremely bad roads, much worse, indeed, than they had been
in the spring when the journey was begun.

In the whole distance of more than three thousand miles, nowhere had
there arisen conditions so difficult for travel as we now encountered on the
Kalgan-Urga trail. More damage was done to the transportation equipment
on this day’s traverse, together with the first day out over the same ground,
than was done over all of the remaining three thousand miles. The road for
the last three days had been through cultivated country, where the transport
of produce to market is one of the occupations of the people, and where long
trains of carts pass daily from the upland country to the markets of Kalgan.
In consequence of the heavy traffic and of the fact that no road is ever re-
paired, the trail is one vast stretch of interlacing ruts, with almost bottomless
mud in the flat country, and with stones and bumps in the rugged country of
the pass.

Finally, however, the journey was completed to the Anderson-Meyer
compound in Kalgan without the loss of any of the transportation equipment.
This was on September 19, five calendar months to a day from the time the

190 GEOLOGY OF MONGOLIA

party left Peking. In that interval the Third Asiatic Expedition had crossed
the Gobi desert region of Mongolia twice, had traversed several hundred miles
of northern border country, had covered more than three thousand miles of
itinerary, had crossed the Altai Mountains, had made geologic, palzontologic,
and zodlogic observations in considerable detail along the whole route, and
had found several important fossil localities from which enough rare material
was recovered to make the scientific reputation of the Expedition secure.
On this reconnaissance a sufficient number of promising fields had been dis-
covered and enough additional scientific work had been laid out to occupy
years of further investigation.

PART III
SPECIAL LOCALITY STUDIES

191

PART III—SPECIAL AND LOCALITY STUDIES

INTRODUCTION

CHAPTER XI—IREN DABASU AND IRDIN MANHA

CHAPTER XII—ARISHAN, THE SACRED MOUNTAIN OF SAIN NOIN
CHAPTER XIII—THE TSAGAN NOR BASIN

CHAPTER XIV—THE GURBUN SAIKHAN RANGES

CHAPTER XV—ARTSA BOGDO AND OSHIH

CHAPTER XVI—PROBLEMS AND AREAS DESERVING SPECIAL STUDY

192

INTRODUCTION

ALL the traverses of the Expedition, even those made under the most
deliberate and favorable conditions, compelled rapid work and gave no oppor-
tunity whatever for detailed investigation. It was possible, however, to
keep the run of the general geologic structure, as given in Part II of this
volume, besides paying special attention to the selection of localities where
it would be profitable to make a more extended examination. At three of the
shorter stops, side traverses were made which proved to be especially helpful
in gaining an understanding of the geology of the region. The side traverses
are included in the itinerary account, Chapters III to X.

Wherever more time was available, the geologists made an areal study.
In the course of the summer, several such places proved to be of exceptional
interest, and these reconnaissance investigations were expanded as much as
time permitted. The local studies constitute the chief basis for the state-
ment of the systematic geology compiled by the Expedition. They are the
key studies by means of which the fundamental structural and stratigraphic
features were established.

Where the geologists made areal and structural studies of the rock form-
ations, Mr. Granger conducted paleontological investigations and collected
fossils. The two lines of research are correlated so as to tell a single story of
the land and its inhabitants in successive periods. The detailed account of
the palzontological researches is to appear in later volumes of this series.

In certain of the localities where time was available reconnaissance maps
were made, showing topography and geology; they are the first of their kind
for this part of central Asia. It is hoped that the local studies may furnish
starting points for additional investigation and may be useful for comparisons,
in the same manner as guide localities and standard sections are used in other
countries.

Method of mapping

The maps of special areas were made with a Gurley mountain alidade
and Johnson movement plane-table. A base line was measured in most
cases with stadia, and this once accomplished, the stadia rod was but little

VOL. I—13 193

194 GEOLOGY OF MONGOLIA

used, for virtually the entire map was worked out by means of triangulation
and vertical angle-readings. The Mongols have a most laudable custom of
building stone-piles called obos, each stone of which is said to represent a

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FréurE 98.—Location map of the eastern Altai region. This sketch map shows the route of the recon-
naissance, and the localities referred to in this report. Reconnaissance made since this map was drawn has
shown that all the lakes except Orok Nor are intermittent.

prayer to Buddha. All important hilltops and many minor ones are crowned
by obos, which enable the grateful topographer to locate points with a precision
which in Christian countries would be impossible at this stage of the mapping
program.

INTRODUCTION 195

In special cases, where thickness of strata as well as topography was being
recorded, the stadia rod again came into play, especially in the case of com-
plexly folded rocks, where the precise location of all outcrops was necessary.
In certain cases, where great speed of work was required, and where the nature
of the ground permitted, the base line was run by means of a motor car.
Several times in the course of such a run the car was stopped and the plane-
table set up, so that the base line included a number of triangulation points in
its length. Maps were surveyed on the scales of one inch to one mile and two
inches to one mile, with contour intervals ranging from ten feet in the flat
country about Iren Dabasu to one hundred feet in the mountains at the foot
of the Altai. On the whole, a fifty-foot interval was found most practical for
reconnaissance mapping.

The areas covered in this way by special maps total more than seven hun-
dred square miles, and include the Iren Dabasu hollow, the Holy Mountain
of Sain Noin, and the Uskuk-Tsagan Nor basin. Other studies in the list
were made under conditions unfavorable to the making of a map, but such
studies have yielded results which have helped materially toward an under-
standing of geologic structure, correlation, and origin. Five of these locality
studies are covered in this report, as follows:

1. Iren Dabasu and Irdin Manha.

2. The Sacred Mountain of Sain Noin.

3. Uskuk Mountain and the Tsagan Nor Basin.

4. The Gurbun Saikhan.

5. Artsa Bogdo and Oshih.

Another chapter is added for the purpose of enumerating and discussing
briefly other areas deserving additional exploration, and problems worthy of
special study.

CHAPTER XI

IREN DABASU AND IRDIN MANHA
[Geologic Map, Plate XXIII, in pocket at end]

INTRODUCTION

AFTER traveling many miles over almost perfectly level upland, the
Expedition came, on the evening of April 24, 1922, to the edge of an escarp-
ment, 255 miles out from Kalgan, from which one may look out across a
basin-like depression. This is Iren Dabasu where the first important palzeon-
tological discoveries and the first studies of structural detail were made (Figs.
99 and 100).

In the lowest part of the hollow the ground water stands at the surface,
and in rainy seasons there is a shallow lake which at other times dries up,
leaving a crust of salt. The lake bed is soft and miry, but the surrounding
lowland is firm ground. A little to one side of the trail are two wells from
which caravans obtain the only water available for many miles. They mark,
therefore, one of the established camping spots along the Urga trail. Here
also is the telegraph station Erlien.

Four miles away, near the north margin of the depression, stands a small
Buddhist temple, known as Boro Sunit (Plate XXIII). Aside from the oper-
ator of the telegraph station, a Chinese inn-keeper, and one or two Mongol
families, there are no other inhabitants, for, though the place is less than
two hundred miles distant from prosperous cultivated fields, here it is hope-
less desert. ‘There is a steady descent of more than two thousand feet from
the edge of the Mongolian plateau at the head of the Wan Ch’uan Pass to
this point, which lies in the lowest portion of the larger Gobi basin. The
station is known as Erlien by the Chinese, and the saltpan is called Iren
Dabasu by the Mongols. It is this latter name which has been adopted as
a suitable designation for the whole local area and for one of the geological
formations. ;

Our first discovery of fossils was made at the very lip of the hollow, where

196

IREN DABASU AND IRDIN MANHA 197

the geologists found teeth and bones of rhinocerotids in the yellow gravel cap-
ping the upland. But the most significant find was made the morning follow-
ing our arrival at this camp. Fossils picked up by the chief geologist in the
immediate vicinity of the camp proved to be dinosaur bones. With a little
further search, the locality was found to be rich in these remains, and it was
then decided to devote a week to more detailed study of the ground. Conse-
quently the forces were divided, Mr. Andrews, with a part of the staff, pro-
ceeding toward Urga, while Messrs. Berkey, Granger, and Morris remained
to carry on this work. Additional study proved that the locality was of
even greater importance than had at first been supposed, and the stay length-
ened beyond the original intention to about ten days, until the supplies were
exhausted and we had to move or starve.

In the meantime, special attention was given to every phase of inspec-
tion and examination that could be carried out, and extensive collections were
made. A map of the locality was prepared, the geologic column and struc-
tural details were worked out, and everyone turned a hand to collecting fos-
sils from as wide a territory as was practicable. It was found advisable
even to turn back along a part of the trail, to study the dissected bluffs noted
twenty miles back, at mile 235, where sedimentary strata are exposed in even
greater prominence than at Iren Dabasu. This is the Irdin Manha escarp-
ment, where subsequently a very rare fossil fauna was discovered. Study of
the Iren Dabasu locality, therefore, properly includes Irdin Manha. The
strata exposed at the two places are intimately related; they belong to the
same structural basin, which has had a long history. The problems of both
spots are essentially the same, hence there is a distinct advantage in discuss-
ing the two localities together.

The geological complexity of the area was not at first apparent, and it
was not until the fossil contents of the different strata were compared that the
nature of the structural difficulties was appreciated. Structural relations in
general are uncertain where so much of the ground is covered with a shifting
mantle of sand, and where marked breaks in the succession of fossil forms cor-
respond to very insignificant-looking unconformities in the stratigraphic suc-
cession. It was all the more important, therefore, to discover a key locality
thus early in the expeditionary work, for similar conditions and obscurity of
structural relations prevail in many other districts, and it is not likely that
they could have been interpreted as rapidly, had it not been for the fortunate
advantage furnished by the Iren Dabasu investigation. The geologists, there-
fore, look on this small depression of Iren Dabasu as one of the key localities
of the Gobi region,—not only because it yielded the first dinosaur bones found
in Central Asia, and because the Expedition’s first announcement of scientific
discovery was written at this camp (Granger and Berkey, 1922), but also

198 GEOLOGY OF MONGOLIA

because the geological studies made here became the criteria by which
our reconnaissance was more rapidly conducted.

GENERAL ASPECT AND LARGER RELATIONS OF THE BASIN

In its broader lines, Iren Dabasu and Irdin Manha form a part of one of
the largest sedimentary basins we have seen in Mongolia. The sediments

FicureE 99.—Sketch map of the region between Iren Dabasu, Shara Murun and P’ang Kiang. Known
areas of different major types of ancient floor rocks are shown by appropriate symbols, while the overlying
later sediments along the several traverses are left blank, with the exception of a single limestone area just south
of Irdin Manha, which is of fresh water origin and belongs to the later sediments.

are reached at mile 197 on the road from Kalgan, and there is no interruption
of them until the slates appear at Iren Dabasu, mile 260. Along the Urga
trail, therefore, from southeast to northwest the basin measures sixty-three
miles in width. Toward the east, the basin has been followed for ten miles
from the Irdin Manha bluff, without a glimpse, even in the distance, of a
hard-rock wall terminating the sediments. Toward the west, the basin

IREN DABASU AND IRDIN MANHA 199

appears to be quite continuous with that of Shara Murun, nearly one hundred
miles away. The width of the basin from north to south along the trail
between Iren Dabasu and Boltai Urtu is at least seventy-five miles. The
measurement would have been at least twenty miles greater if the trail had
lain a dozen miles farther west. The total length of the basin in an east-west
direction is not known; but the Shara Murun wing extends westward to Jisu
Honguer, an approximate distance of ninety-seven miles west of Irdin Manha.
If we allow only twenty miles for the continuation of the basin eastward from

SHARA MURUN

PANG KIANG

FicurE 100.—Block diagram of the area between Iren Dabasu, Shara Murun, and P’ang Kiang. This
shows the major physiographic features and the general underground structure of a part of southern Mongolia,
approximately 100 by 125 miles in extent. The front edge runs northwestward and follows the Kalgan-Urga
trail through the P’ang Kiang and Iren Dabasu basins. The rear diagonal cut shows the section between
Boltai Urtu and Shara Murun, along the Kalgan-Uliassutai trail. The cut from front to rear illustrates one
interpretation of the relations between the Irdin Manha and the Shara Murun formations, indicating that the
Irdin Manha represents a lower horizon.

Irdin Manha, it still is more than one hundred miles long and averages more
than fifty miles wide, with an area of nearly six thousand square miles (Fig.
99).

The area of our special study lies near the northern border of this large
basin. It is bounded on the north by the hills of slate forming that part of
the basin rim; on the south, our study did not extend beyond the scarp of sed-
iments at Irdin Manha. Eastward and westward, the area examined by us
had no definite boundaries.

There are three areas of oldrock in the vicinity—the slate hills on the
north, a low ridge of granite on the east, and an exposure of graywackes and
quartzites, thirteen miles south of Iren Dabasu. Remnants of the sediments
are found clinging to the front of the slate hills, showing clearly that the
sedimentary cover once extended entirely across the hollow in which the

200 GEOLOGY OF MONGOLIA

salt lake now lies. The floor of the lake lowland is part slate and part sedi-
ment. The strata of sand and clay form a low, irregular, much dissected
scarp some distance south of the lake, and pass southward into a terrace about
two miles broad. From the terrace the Houldjin bluff rises abruptly. It is
a smooth-faced slope, dissected by short gullies, and at its top the level upland
surface, covered by a thin mantle of gravel and drifting sand, extends south-
ward for ten miles. Here the smooth upland is again broken by an immense

FicurE 101.—The scarp at Irdin Manha. The field sketch shows the very level erosion upland, the short
gullies of the cliff front, and a portion of a broad lowland belonging to the P’ang Kiang stage of dissection.

hollow, extending westward beyond the reach of vision. On the east and
south, the hollow is bordered by a well-dissected bluff which we called the
Irdin Manha escarpment (Figs. 101 and 102). Just at the angle of the scarp,
where its north-south face swings around toward the west, there is a well
called Jia Jing Shando, which Obruchev noted, more than thirty years ago,
as a good camping place for those who would study the cliffs and their fossils.

As the names which we have used differ in important respects from those
used by Obruchev, when he crossed this region in 1892, it will be well to pause
at this point and compare the records of the two traverses.

Coming from the north, Obruchev passed Iren Dabasu, which he spelled
Iren Dabassun Nor (Fig. 102). He ascended the scarp which our guides
called Houldjin. He had no name for the bluff itself, but for the upland he
records the name Kharangoin Tala (1900, I, pages 86-89). At Irdin Manha, he
discovered fossils in a small butte near the well, Jia Jing Shando. He collected
a fragment of a jaw which Suess (1899) later identified as Rhinoceros or Acera-
therium and which he considered to be of Miocene age. Obruchev called the
scarp Khuldyin-gobi—undoubtedly the same word as our Houldjin, which,
we are told, applied to the scarp farther north. He called the plateau up-
land, south of the scarp, Irdin Mankha.

Because of the doubtful application of so many Mongol names, and
because the names, as we applied them, have been used to define standard
stratigraphic units, whereas Obruchev used them only as geographical terms
to define localities, we have decided to retain the usage established in the

IREN DABASU AND IRDIN MANHA 201

papers already published. Professor Obruchev has courteously accepted this
arrangement, saying:

“Your observations concerning the place of discovery of the Rhinoceros which
I found in 1892 are wholly correct. I found it in the scarp of the plateau, Irdin
Mankha, which my guides had called Khuldyin-gobi. My guides were Mongols who

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t t t i

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“ undrained lowland undrained | lowland

iRDIN MANHA FORMATION “ss BEDS...

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ARSHANT DABASU'2 te ee a v PROBABLY “GRANITE Vuevvvveyed \
‘Ont S le) eee 20 25 MILES

FicurE 102.—Generalized geologic section between Iren Dabasu and Irdin Manha. The section shows
at least three periods of basin making and three corresponding unconformities in this repeatedly warped basin.
The unconformities lie (1) at the base of the Cretaceous Iren Dabasu beds, upon the uneven floor of crystalline
rocks; (2) at the base of the Eocene Arshanto formation, upon the slightly disturbed and eroded Cretaceous
beds; (3) at the base of the Oligocene Houldjin gravels, upon the eroded Eocene. Each unconformity implies a
long period during which deformation was so gentle and erosion so slight as not to wholly remove the preceding
thin deposits of sediments.

had traveled this road many times with tea caravans, so that they knew the road
very well; but, of course, they could err very easily in the matter of exact names of
localities. From your profile and my description of the journey, one must conclude
that the rhinoceros tooth was laid in the Irdin Mankha formation and not in the
Houldjin beds which are exposed much nearer to the lake, Iren Dabasu, and which
I have briefly described on page 86 (1900, I), without having discovered any fossils
in them.”’

Origin of the hollow at Iren Dabasu

The Iren Dabasu lowland is only eight miles wide along the Urga trail,
but is more extensive east and west. The western scarp lies three miles
beyond the telegraph station, while in an easterly direction the hollow extends
for at least twenty miles. The lowland is not a basin structure at all, but is
purely an erosion form, representing the P’ang Kiang stage of dissection
referred to in Part II of this report, and discussed more fully in Chapter
XIX. The depression is basin-like in topographic form, without outlet, and
without very direct evidence of the process by which it has been made. The
elongated depression has many of the features of a stream valley, and it is

202 GEOLOGY OF MONGOLIA

possible that a river started the excavation; but it is certain that the undrained
hollow which contains the lake must have been scoured out by the wind.
Here is a record of changing climatic conditions. Only when water was
absent could the wind have excavated the lake bed. The presence of the
lake tells of a return to rainier conditions than those prevailing when the wind
was etching out the hollow, while the drying of the lake records a minor arid
cycle.

There are many small stream courses, dry most of the year, leading from
the bluffs toward the lake, and proving that both erosion and transportation
are accomplished by running water. The fact that the stream courses do not
lead out of the basin, and yet the basin does not fill, presents a problem that
requires explanation. Even though there must be extensive movement of
detritus, there is virtually no sediment accumulating in the bottom of the
hollow itself. It is incredible that stream erosion could be a large factor in
sculpturing the margins without corresponding sedimentary accumulation in
the bottom, unless some other agent removes the deposits as fast as they can
form. Only the wind could doit. The wind must be responsible for carrying
away large quantities of loosened and transported material, so that accumula-
tions are kept at a minimum.

The general effect is a depression of basin-like form, the margins of
which, on the south side and around the west end, are formed by an escarp-
ment of sedimentary strata, whereas the northern margin is formed by the
stripped floor of complex ancient rock.

GENERAL ROCK STRUCTURE

In a very broad way, only two strikingly different classes of rocks are
present—(a) metamorphic, of which slate is the commonest, representing the
ancient floor, and (b) later sediments, mostly sandstones and silts, which con-
stitute the nearly flat strata overlying this floor.

The rocks of the ancient floor are, we judge, of pre-Cambrian age. It is
possible that they belong to the Nan K’ou series of China, but this cannot be
determined in the Iren Dabasu locality. These slates carry numerous vein-
quartz stringers and many dikes of igneous rock, some of which are trap or
diabase, whereas others are granite. None of these intrusive units is of suffi-
cient importance in the local study to warrant further description.

The sedimentary strata, which are of much more immediate interest, can
be divided into three groups marked by differences in petrographic quality
and in fossil content. The evidence indicates that the whole triple series is
of inland continental origin. Part of the sediment was deposited in shallow
lakes; much of it was laid down by streams with vigorous currents, and prob-
ably also a part is wind-blown dust and sand.

IREN DABASU AND IRDIN MANHA 203

The Iren Dabasu formation

The first and oldest of the basin formations, to which the name of the
locality was given, occupies the southern half of the hollow, where bare sand-
stone ledges form many outcrops (Fig. 103). With them are associated strata
of fine clay, which are less resistant than the sandstones. The sandstones,—
particularly the coarser varieties,—are cross-bedded and do not lie perfectly
flat. The dip is always small, and its variation from place to place indicates
slight deformation since the beds were laid down. These strata are much
more indurated than are the overlying Tertiary beds, and some of the exposed
layers are substantial enough to withstand the attack of weather fairly well,—
so that they form projecting ledges, flat mesa-tops, and the caps of upstand-

ond

Houldjin formation
Ole

Disconformity —> gocene

Eocene Beds
Arshanto formation?

Talus slop
Tren Dabasu formation -« ee
Cretaceous :

nconformity*

FicurE 103.—The stratigraphic section at Iren Dabasu.

ing pillars. For the most part the clays are reddish or buff or slightly yellow-
ish in color, and are comparatively thin-bedded. They vary from pebbly
internal structure to extremely fine, almost silt-like material. There are a
few lenticular, limy layers of very limited extent, one of which was found
to carry fossil fresh-water bivalve shells. The principal constituent is quartz
sand, and in composition there is no striking or unusual peculiarity. But cer-
tain beds carry such fine material that they might be called silts or even mud
rocks, rather than sandstones. The finer silts also are simpler in general struc-
ture than are the coarser layers.

The only important differences, in addition to these petrographic varia-
tions, are presented by the fossil content. Certain beds carry multitudes of
fossil fragments, and have yielded incomplete skeletons of several large dino-
saurs. The formation proves to be of Cretaceous age. Only a remnant of
the original deposit is left, and it is quite impossible to determine the original
thickness. In 1923, a thicker deposit of Cretaceous beds was found several
miles east of the lake (Plate XXIII). At least one hundred feet of sediment
are exposed there, but only the uppermost thirty feet have as yet proved

204 GEOLOGY OF MONGOLIA

fossil-bearing. Sandstones are rather less abundant in the eastern exposures
than in those west of the lake, but they are identical in character,—rather
coarse, light gray sandstones consisting of angular grains of quartz with minor
amounts of chalcedony, quartzite, feldspar, granite and epidote rock. The
standstones are cross-bedded and bear only fragmentary fossils. The better
fossils, especially the bones which were found associated in their skeletal posi-
tions, were found almost wholly in the beds of gray-green clay. One bed of
red sandy clay, the highest fossil-bearing horizon yet found in the Iren Dabasu
formation, contained numberless fragments of smooth, curving plates, which,
it was thought in the field, might be the shells of dinosaur eggs. The discov-
ery of perfect eggs at Djadokhta had not been made at that time, and these
fragments were collected as doubtful material. An examination of thin sec-
tions under the microscope, however, has demonstrated that they are dinosaur
egg-shells. The shells vary in size, and doubtless belong to several different
types of dinosaur. The larger eggs are much bigger than those found at Dja-
dokhta. Hence there are at least two places in central Asia where dinosaur
eggs are found.
The list of fossils for the Iren Dabasu formation is as follows:

Reptilia
Dinosauria
Predentata
Undetermined iguanodonts of various sizes and in great abundance
Theropoda
Numerous remains of struthiomimid and deinodont types.
Chelonia
Fragments of turtle bones.

Variegated Tertiary beds

The next overlying beds were at first believed to belong to the Iren Dabasu
formation, but they are now known to belong to a different age and to overlie
that formation unconformably. Obviously, therefore, the same name can
not apply to these beds.

Everywhere the loose cover of wash conceals the transition from the
sandstone ledges of the central part of the basin to the beds of an entirely
different quality exposed on the marginal escarpment (Fig. 103). As nearly
as can be determined, they lie flat and are less indurated than the Cretaceous
strata. Some of the beds are better designated as silts or clays; most of them
have a finely laminated parting, and would be classed as clay shales. They
include a great range of color, varying from brick red to green and from yel-
low to blue. Some individual beds are variegated red, green, and drab.

IREN DABASU AND IRDIN MANHA 205

In places the beds are concretionary, and there are a very few limy lay-
ers carrying septarian nodules. The most constant differences between these
beds and the sandstone beds below are their less consolidated state, finer grain,
higher color, less deformed condition, and their comparative barrenness. No
fossils were found in them during the first season, but in 1923 a molar of a
small lophiodont was found, together with great numbers of ostracods. The
tooth, so far as it has been studied, is like those of the lophiodonts found in
the Arshanto formation which will be described later. The stratigraphic posi-
tion of the barren beds of Iren Dabasu agrees very well with that of the Ar-
shanto, which is undoubtedly of Eocene age. At the base they pass beneath
a cover that conceals their relation to the beds below; but it is now certain
that between them and the dinosaur-bearing sandstones below there is an
erosion unconformity which constitutes a floor for these much later sediments.
A corresponding relation was proven later on with similar groups of strata in
the Uskuk region.

The Houldjin formation

Upon the second series of beds described above lie about fifteen feet of
yellow, pebbly gravel, which forms the top of the escarpment. The layer
carries pebbles of quartzite, vein quartz, chalcedony, slate, and graywacke,
and in both internal structure and fossil content is totally unlike any of the
others thus far described. Even the fossils are but fragments and are dis-
tributed as if carried into place by currents. Many are well worn and
rounded, and are more abundant in the coarse-grained, rusty basal part of
the formation than in the higher levels. The deposit is of a rusty to buff
color, but is not well cemented. Such partial cementing as there is readily
breaks down under weathering, and the loose sands and gravels slump over
the escarpment, where small streams distribute the débris over the lowland.
Fossil fragments are found, therefore, scattered over the slopes across the
lower formations, but are readily traced back to this layer as a source.
The first bones found by the Expedition were obtained from this horizon,
and here, too, the calcaneum of the giant hornless rhinoceros, Baluchitherium,
and many other fragments were collected, giving the first hint of the pres-
ence of this huge mammal in central Asia. The following fossils have been
collected from the Houldjin formation:

Mammalia
Carnivora
Undetermined fragments
Perissodactyla
Baluchitherium sp.

206 GEOLOGY OF MONGOLIA

Mammalia— Continued
Perissoda tyla—Continued
Cadurcotherium sp.
Cenopus or Preaceratherium sp.
Artiodactyla
Entelodon dirus, new species
Reptilia
Chelonia
Fragments of a large tortoise.

Apparently the Houldjin is a mere remnant of a once much more extensive
formation, most of which has been completely removed by the processes that
have made the Gobi plane. In spite of the fact that the formation cannot be
traced on the upland, it is exposed again sixteen miles south of Iren Dabasu
station, where the upland has been somewhat dissected, and where exactly
the same relation of formations is found in the edge of a small depression.
The drab clay is overlaid by a red clay, upon which rests a cap of rusty grav-
elly sand, which is undoubtedly the Houldjin. At that point the formation is
about thirty to forty feet thick.

The sedimentary strata lying on the crystalline rock floor in the Iren
Dabasu basin may be grouped into three formational units—(a) an older
formation, chiefly of sandstones bearing dinosaur remains and, therefore, of
Mesozoic age, (b) a clay formation, lying unconformably above, and clearly
much younger, and (c) a topmost member, of sand and gravel, carrying a
mammalian fauna, separated from the formation immediately below by an-
other hiatus. The three distinct portions of the sedimentary column above
the crystalline floor are represented in the accompanying columnar section in
their approximate proportions (Fig. 103, page 203).

The Arshanto and Irdin Manha formations

Twenty miles south, at Irdin Manha, the Houldjin does not appear, and
the escarpment is formed of Eocene strata. The beds can be divided into two
groups,— one of red clays at the base, and the other of gray sandy clays, sands,
and gravels above. There is a marked contrast between the two groups, and,
while the name Irdin Manha formation was given to the entire sequence of
red and gray sediments, it seemed well to reserve the possibility of separating
the groups. Accordingly the name Arshanto was assigned tentatively to the
lower red beds, while the name Irdin Manha was reserved for the upper gray
formation. For clearness of reference these names will be used in describing
the sediments, even if ultimately the Arshanto proves to be merely the lower
Irdin Manha.

IREN DABASU AND IRDIN MANHA 207

The Arshanto beds are prevailingly red clays and fine silts, which, on
weathering, crumble into small hard chips. Much of the deposit is structure-
less, almost like loess; however, it lacks the vertical cleavage and the concre-
tions so commonly noted in loess. In some places, a faint color-banding is
seen in the red beds, but no shaly structure or marked bedding. Color-banding
is held by Matthew (1915, pages 395-398) to be not inconsistent with wind
deposits, and our own observations tend to support his opinion. In still other
places, chocolate brown beds and, more rarely, thin gray layers appear among
the red, and here at least there is definite bedding. About a mile east of Irdin
Manha escarpment lies another broad, undrained hollow (Fig. 99, page 198),
part of which is called Arshanto, from which the formation is named. Here
some lens-shaped beds of gray and red sandstone were found in the red clays.
A limited collection of small lophiodonts was made, which has not yet been
fully studied; but the preliminary examination by Matthew and Granger shows
that they are quite different from the lophiodonts of the Irdin Manha beds.
The base of the red beds was not seen; the only bottom known for the forma-
tion is at Iren Dabasu, where it rests upon the Cretaceous beds. Whether
there are Cretaceous beds beneath this part of the basin is not definitely
known, but it seems probable that the Iren Dabasu formation extends farther
south than is revealed in the outcrops around the lake.

The barren clays above the dinosaur beds at Iren Dabasu are distinctly
shaly and well bedded; there is no doubt that they, at least, were laid down
in water. They probably represent a different facies of deposition in Ar-
shanto time, a local deposit in a shallow lake.

The Irdin Manha formation at the type locality consists of the prevailing
gray beds of sandy clay, sand, and gravel. Some strata are mottled red and
green; some few are even red; but there is a knife-sharp contact between the
Arshanto and the overlying beds. The Irdin Manha is characterized by num-
berless channel-cuts and fillings, so that no individual bed can be traced very
far. The quality of the beds also varies greatly, ranging from a sandy clay
through sand and gravel. Some of the sand is exquisitely fine and uniform,
and may well be a wind-borne deposit. Many of the beds of sand contain
clay-galls, and some of the clays carry lumps of agglutinated sand grains.
Most of the coarser sands are cross-bedded, with longer foreset beds than are
seen in the Cretaceous sandstones at Iren Dabasu. The prevailing dip of
the foreset beds is northward, suggesting accumulations from the south.

Two peculiarities of the Irdin Manha beds are especially notable. One
is the presence of innumerable white marly or limy lumps, like concretions.
In some cases at least, they show evidence of wear, as though they shad been
rolled along by water after they had become hardened. Possibly they repre-
sent limy concretions or fragments of lime-cemented sediments which later

208 GEOLOGY OF MONGOLIA

were broken up and washed out by the streams that laid the gravels and sands
in which these lime-cemented lumps are found. The other peculiarity of the
Irdin Manha formation is the lustrous polish of the pebbles found in it. Jas-
per, chalcedony, agate, quartz, and quartzite are brilliantly polished, while
feldspar fragments are dull, save for the brightness of cleavage faces. The
handsome lustrous stones have given the place its name, for Irdin Manha
means “Valley of Gems.”

Most of the fossils are rather fragmentary. There were no complete
skeletons and no associated chains of vertebre, though occasionally a manus or
pes was found. For the most part, pieces of jaw, isolated teeth, or ends of
limb bones make up the collection. Assiduous search revealed several fine
skulls. The generally broken and scattered condition of the bones suggests
that most of them were deposited by running water,—an inference which
agrees well with the channel structures which are so common in the deposits.

The following fossils have been identified:

Mammalia
Carnivora
Paracynohyenodon morrisi Matthew and Granger (1924a), new species of
Hyznodontide
Andrewsarchus mongoliensis Osborn (1924e)
Mesonychids—genus and species undetermined
Rodentia
Genus and species undetermined
Insectivora
? Pantolestes, or Pantolestidz
Perissodactyla
Desmatotherium mongoliense Osborn (19230)
Lophiodonts, undescribed (Shara Murun and Irdin Manha)
Protitanotherium sp., undescribed
Titanotheres (dolicocephalic) undescribed (Shara Murun and Irdin Manha)
Artiodactyla
Undetermined genus, Anthracotheriide
Amblypoda
Eudinoceras mongoliensis Osborn (1924d)

In the season of 1923, the Expedition camped for two weeks at Iren
Dabasu, and for three weeks at Irdin Manha, near the well Jia Jing Shando.
The survey was extended in several directions, and our general knowledge of
the region broadened. One observation that seems to be of especial interest
is the very irregular distribution of fossils in the Irdin Manha formation.
The camp lay in the midst of the richest deposit of fossils seen in this vicinity.

IREN DABASU AND IRDIN MANHA 209

Northward, the bluffs were prospected all the way to the granite ridge and
along its entire front, without disclosing any but scattered and fragmentary
bones. Southward, the bluffs were followed for ten miles, but were for the
most part barren. A traverse southward from Iren Dabasu reached the Irdin
Manha escarpment at a point about ten miles west of the type locality, and
here a considerable deposit of bones was found, but all were fragmentary and
water-worn. At another place, some miles south of Holti temple (Fig. 99),
President Osborn found a tooth of an amblypod, Eudinoceras mongoliensis.
No locality in this part of the basin proved as rich as the first prospect, which,
though we did not know it at the time, was the place where Obruchey, in 1892,
had taken the first fossil recorded from Mongolia.

Shara Murun

Leaving Irdin Manha on June 12, 1923, the Expedition journeyed south-
ward across seventy-five miles of unbroken basin country, finding Irdin Manha
fossils at one place. The road leads through an area of granite and quartzite
at Holostai temple (Fig. 99), but a splendid escarpment of sediment was vis-
ible some miles to the west, unmistakably resting upon the granite surface
(Fig. 104). At Boltai Urtu the course turned westward and at once encoun-

Gobi Upland on Piateau of Tertiary Sediments

Granite

PES Sagas
eed
_- Kwethwating

FiGurE 104.—The scarp at Holostai. The eroded surface of the granite is overlaid at the western end of
the sketch by Tertiary sediments.

tered sediments, across which it continued for sixty-five miles to the Shara
Murun district, where, in 1922, Mr. Granger had noted a rich deposit of fos-
sils. Although so far away, Shara Murun is inseparable from Irdin Manha,
and it carries a related, though distinctly later fauna.

A high bluff extends nearly east and west, and at the camp site is divided
into two slopes, separated by a terrace nearly a mile wide (Plate XXII, B, page
183). The well near camp is called Ula Usu, which means ‘‘ Mountain Water”’
or perhaps ‘‘Water on the Hill.” Meanwhile a paper had been published
(Berkey and Granger, 1923), using another name, Shara Murun, for this
place, and the beds will therefore be called the Shara Murun formation, al-
though the river Shara Murun actually lies sixteen miles east of the Ula Usu
camp.

The upper part of the formation consists chiefly of white and light gray
sandstones which are gravelly in places. Locally brownish and gray clayey

VOL. II—I4

210 GEOLOGY OF MONGOLIA

beds appear in the upper member. The lower member is almost wholly sandy
clay, richly variegated in color. Reds predominate, but there are purple,
brown, and green layers, and some are mottled with many colors. The beds
are lens-shaped, and some can be traced along the outcrop for as much as a
mile. In some places, large crystals of gypsum are abundant, locally forming
fibrous masses two feet long, and in one place beds of gypsum a foot thick were
seen. No fossils were found where gypsum was plentiful.

Below the many-colored clays lies a hard red clay, which we named the
Tukhum. It is nearly barren, but a few fossils were obtained which do not
match those of the typical Shara Murun. Considering this difference in phys-
ical condition and faunal content, it seems probable that there is an older
Eocene formation below the Shara Murun, in much the same stratigraphic
position as the Arshanto occupies at Irdin Manha, but it is not yet possible
to correlate the red Tukhum clays at Shara Murun definitely with the Ar-
shanto.

The fossil field is extraordinarily rich, yielding not only numberless bones,
jaws, and teeth, but several associated skeletons and many excellent skulls.
From palzontologic evidence, it appears that the horizon is but a little higher
than that of Irdin Manha.

AGE AND CORRELATION

At the time of the discovery of fossils in the Iren Dabasu, there was prac-
tically no basis for comparison between the strata found there and in other por-
tions of the Gobi. Strata of similar appearance, but not yielding fossils, had
been seen at several places on the journey, particularly at P’ang Kiang and at
Irdin Manha, and it was conjectured that the beds of those localities are closely
related to the sedimentary rocks at Iren Dabasu. When it was found, however,
that certain beds of the Iren Dabasu district, including those at Irdin Manha,
produced characteristic fossil faunas, it was possible in a general way to fix
the age of those portions of the series. Thus it was evident that the dinosaur
beds, found near the base of the geological column at Iren Dabasu, are of
Cretaceous age. Nothing equivalent to them was known in central Asia.
At that time it was supposed that the overlying beds of red and drab and varie-
gated shales and sandstones belonged with them, and the whole combination
was referred to at first as the Iren Dabasu formation. Subsequent work indi-
cated much more clearly an unconformity between the dinosaur-bearing beds
and the overlying, simpler, and barren strata. These were subsequently
proven to be of Tertiary age, but in the Iren Dabasu basin, because of the lack
of fossils, the exact horizon remained somewhat doubtful. East of Irdin
Manha, a productive horizon in the barren-looking series yielded a primitive

IREN DABASU AND IRDIN MANHA 211

lophiodont fauna, indicating Eocene age. An effort was made to trace the
connection between Irdin Manha and Iren Dabasu across the upland, but
without much success. It is evident that the same general series continues;
but whether the barren beds at Iren Dabasu lie above or below the lophiodont
horizon is not yet clear. They are probably closely related beds, and are
surely of Eocene age.

It is evident, therefore, that the Iren Dabasu formation, as revised, does
not include quite as much as indicated in the original description (Granger and
Berkey, 1922), and properly applies only to the Cretaceous sandstone beds
below the unconformity. It is evident, also, that the overlying variegated
barren beds of Iren Dabasu really belong to the same series as those found at
Irdin Manha, and are very probably a part of the Arshanto formation.

At Iren Dabasu there is a disconformity separating the Arshanto from the
Houldjin formation above, but the Houldjin beds do not appear at Irdin
Manha (Fig. 102). The Houldjin formation carries a strikingly different
fauna, indicating Lower Oligocene age. No higher fossil-bearing formations
have developed at either place. Only a fraction of the original Houldjin form-
ation remains beneath the Gobi erosion plane. The gravels and shifting sands
on the smooth surface of the upland are recent. When these relations are
plotted, it appears that there is no way of accounting for the peculiar distri-
bution of the Houldjin formation, unless the region was warped subsequent
to the production of the pre-Oligocene erosion surface. These relations are
graphically indicated in the accompanying geologic cross-section (Fig. 102).

The total thickness of the sediments at Iren Dabasu is not more than
two hundred feet, yet there are four plainly marked erosion and planation
surfaces, as follows:

1. The floor of ancient rock on which all the sediments are laid down.
This is the pre-Cretaceous peneplane and unconformity.

2. An undulating surface truncating and limiting the Cretaceous dino-
saur-bearing beds. This is the pre-Tertiary unconformity.

3. An erosion surface separating the Arshanto formation from the
Houldjin beds above. This is the pre-Oligocene erosion surface.

4. The present level upland, which also is clearly a product of erosion,
we have called the Gobi erosion plane. On it rest only the drifting sands.

At Irdin Manha the erosion breaks are not visible, except the Gobi plane.
The old floor is completely covered, as are the Cretaceous strata, while the
pre-Oligocene plane must be warped to a position so high that it is actually
above the present Irdin Manha surface, and has been completely destroyed
by the beveling work of the Gobi planation.

Even the Gobi erosion plane is warped, as indicated by the elevation
readings across the upland. Apparently this region has been for ages affected

212 GEOLOGY OF MONGOLIA

by gentle warping, with periods of deformation separated by periods of com-
parative stability. This kind of history has continued since the beginning of
Cretaceous time, and the striking feature of it is the small total algebraic
sum of plus or minus readjustment that has taken place. The alternating
periods of deposition on the one hand and of distributive erosion on the other,
each inaugurated and closed by slight deforming movements, are all com-
prised in this basin within present vertical limits of only two or three hundred
feet of deposits.

It would be an error, of course, to assume that the present sedimentary
column includes all the vertical movements which have affected the region, or
that it contains all the sediments that were ever deposited there. Probably
greater thicknesses have been removed from each of the formations than now
remain, and if the total column could be reproduced and its erosion then
accounted for to make the present structure, a much greater total vertical
range would be covered.

The beds of the successive formations of later age often lie so nearly
parallel that the actual physical break, caused by one of these erosion epochs,
can rarely be seen. In the Iren Dabasu basin physical breaks were so obscure
that two of them were not detected until a detailed study was made. When
this was done, however, it was clear that the lowermost beds have been more.
deformed than those above, and in a few places slight angular unconformity
can be measured. The deformation, however, has been too gentle to develop
very pronounced structure, and in some cases the finding of a change in the
fauna is the first suggestion of the presence of a physical break in the strati-
graphic succession.

The evidence, as a whole, suggests a region of very gentle shifting of levels,
the erosion work of one stage tending to destroy and remove from certain up-
warped areas the deposits of the preceding stage. Then, after additional warp-
ing, the whole process is renewed, often with the directions reversed, and part
of the deposits of the preceding epoch are removed from the newly uplifted
area to the newly formed depressions. Thus, from one epoch to another
these changes have taken place, with the result that there is an enormously
complex interlocking and interweaving of formations, no single one extending
continuously, or free from partial destructive erosion, for very great distances.
The construction, therefore, of a complete geologic column for the Gobi will
probably require correlation of all the scattered remnants of warped and eroded
basin sediments in Mongolia. It may not be possible to formulate a com-
plete column.

AIXX ALVId

j
|
i
j

PLATE XXIV.

THE SACRED MOUNTAIN OF ARISHAN.
(Painted by C. Lester Morgan after field sketch in crayons by Frederick K. Morris.)

CHAPTER XII

ARISHAN, THE SACRED MOUNTAIN OF SAIN NOIN
[Geologic Map of Arishan, Plate XXVI, in pocket at end]

INTRODUCTION

AFTER we had traveled more than a thousand miles and had gained a
little confidence in our understanding of the geology of the region, we came
one day, in the province of Sain Noin, to the vicinity of a strikingly majestic,
black-browed, sharp-edged mountain, whose high-hanging, dark talus forms
a frowning crown that seems to give it additional distinction (Plate XXIV
and Fig. 105). Blocks of fallen rock in some places reach still farther down
the mountainside and form a festooned fringe on its upper slopes, in sharp
contrast with its own smooth base and with its simpler neighbors.

That it is an unusual mountain could be seen at once, but that it holds
mystery and magic was still to be learned. Instinctively we felt that it must
have a history somewhat different from anything we had yet seen. Later
we were to find reasons for this and to learn why the locality is regarded with
great veneration. At that moment we could not know that, for centuries
past, generation after generation of men had respected this mountain, and
that we were now in the presence of one of the natural wonders of their world,
the Sacred Mountain of Sain Noin.

At the foot of the mountain stands a shrine, and a small group of yurts
occupied by priests, among them a high priest, or Da Lama, who assumes
charge of the place. He is a member of the official family descended from the
Khans of Sain Noin, a relative of the ruling prince of a fourth of Mongolia.
Apparently he considered himself directly responsible for the sacred place,
representing in his own person both church and state, and carrying in his
blood the pride of centuries of authority and privilege traced back to the
golden age of the Mongol Empire.

Only a few feet from the lama’s yurt runs a sparkling brook, whose waters,
hot as the hand can bear, issue from the ground at the base of the shrine.
These are the far-famed healing waters of Sain Noin, credited for ages with

213

214 GEOLOGY OF MONGOLIA

miraculous properties and bearing their own evidence of direct connection
with supernatural powers. Else why do these waters issue forth hot, when
the earth all around is frozen? Why, also, should they pour out in such
abundance in a region where water is scarce at best, and where all other
waters disappear into the ground? Even the mountain above stands frown-

Sacred

ae / Mountain

| Arishan ~~ Hii CAz «en Elevations determined by barometer.
Hot Spr: fj & ht Contour interval 100 Feet.
6200}

Scale of Miles

= 1 ° 1 2 3

FicureE 105.—Location map of the route between the Tarmil River and the Sacred Mountain of Arishan. :

ing and threatening in a mutely protective way. Here is mystery. Here is
result without adequate cause. Here the course of nature is reversed. What
better sign could there be that this spot is to be revered more than other
places? It must be that a god has touched the waters, or a benevolent demon
dwells beneath, or the King of the World, whose realm lies in the earth below,
has permitted this evidence of his power to mark the spot. Here is where
the natural and the supernatural merge. It is a holy place!

Doubtless this mountain has been the object of pilgrimage through ages,
and it must have looked down on many savage scenes as the sacred rites of
primitive tribes were performed. So we liked to let our fancy spin the story
of the Holy Mountain and clothe it with the superstitions of that early time,
some of which still linger. Some of them, indeed, may come down from the
long ago when primitive man stood for the first time awestruck before this
evidence of hidden powers. ;

When we asked the Da Lama for permission to inspect the spring, take
a sample of its waters, make a map of the district, visit the mountain, examine
the formations of the locality, and study its geological history, he readily
consented, but warned us, in a gentle though confident way, that nothing
should be killed on this sacred ground (Plate XXV, A). The holy spot lost
none of its charm for us because it happened to fit our purpose to study the
mystery itself, for it is one of the type geological localities of northern Mon-
golia.

ARISHAN, THE SACRED MOUNTAIN OF SAIN NOIN 215

We found a great fracture in the earth’s crust at the base of the mountain,
and the strata have been displaced along it so that one side has dropped down
several thousand feet. Taking advantage of this weakness, volcanic lavas
have attempted to break through the fault zone, but their energy died out
before much of an outbreak was accomplished. They came so near the
surface, however, that even to-day one can find a few fragments of volcanic
rock lying about on the ground, and probably there is much more below.
Now, the rain that falls on the adjacent hills and sinks into the ground finds
its way into the fractured zone, following along the hillside and under the
valley till it encounters the lava below, which must still be hot. Heated by
the lava, the waters rise of their own accord, issuing as hot springs.

Thus it has come about here, as so often elsewhere, that obscure and hid-
den geologic processes came to be interpreted in terms of mystery, and that
natural forces, operating in full conformity with physical laws have been
clothed with supernatural powers and have been ascribed to gods or demons
not subject to law of any kind. These powers must be worshiped or consulted;
at the very least, they must not be offended. And we heeded the serious-
minded request of our priestly host not to kill anything here because ‘this
is holy ground.”

But we felt the strange spirit of the place—the sacred springs and the
Sacred Mountain—and soon came to realize that here we were face to face
with one of the primal elements of man’s dawning wonder, respect, and super-
stition which together somehow seemed to involve the very rudiments of
religion. Our physical explanation of the mystery and the Buddhist’s worship
of it were brought into clear contrast by the parting request of the lama
prince. When asked what he would like to have us do for him or send him
on our return to America, he said simply: ‘I wish you would send me an
analysis of the water.’”” To him it meant: ‘Tell me what makes this differ-
ent from other waters—what has the King of the World done here?’’

Race after race has dwelt here in the forgotten past, and each people
has ascribed the hidden power to its own deity. Tribe after tribe has looked
upon this place as one demanding reverence, recalling duties, warning the
indolent of shortcomings, kindling aspiration in finer minds, and telling all
of the presence of influences which they could not understand. For ages,
god has succeeded god, each to inherit the magic springs from his vanishing
predecessor. Buddha is only the latest of many masters. Each one has
poured forth blessings at his pleasure, and called upon his people for sacrifice
and for prayer, in accord with the custom of the day. It is a crude beginning
of religion, but perhaps there is no other way to begin. One could stand be-
fore magnificent church edifices which have no greater charm and perhaps
no wider influence than has this lone mountain of Sain Noin. One could

216 GEOLOGY OF MONGOLIA

listen to long sermons containing no more food for serious thought than the
simple request of the Buddhist priest to be mindful of his holy ground.

FEATURES OF THE LOCALITY

Only a small area was included in our special study of this locality.
The map covers about twenty-five square miles. It extends from one side
of the valley to the other, just to the hill margins, and far enough on the south
side to include the Sacred Mountain. The area covers a typical section of
the local geology. The central portion is a smooth erosion surface, with gently
rolling ground in which a stream with minor tributaries flows toward the
southeast. At either side there is more pronounced hill country. On the
whole, however, the topography is not especially striking. In certain places,
the minor relief features indicate the structural attitude of gently dipping
sedimentary beds, but elsewhere the topography is not particularly suggestive.
The Sacred Mountain itself stands in marked relief, largely because of its
cap of metamorphosed graywacke. The area has, however, a very com-
plicated geological structure, with many faults and considerable variety of
ancient crystalline rock, and with great thickness of sedimentary strata.
Although the faults are of considerable magnitude they are wholly without
topographic expression.

GEOLOGICAL FORMATIONS

Three important geological formations are represented, all in typical
character (Plate XXVI). The oldest is the graywacke series; associated with
it is a portion of the bathylithic granite, which is much younger. The most
recent group consists of the Jurassic conglomerates and sandstones which
occupy the central portion of the valley. The graywacke and granite form
the valley sides. The only additional structural material is found in the
dikes, which cut the granite and graywacke series.

Graywacke series

On the east side of the valley the graywacke series constitutes the border-
ing region for many miles. It is the chief formation of the Khangai moun-
tain range, and we have called it the Khangai formation. In the Arishan
locality its bedding is especially obscure, and it is neither as good a graywacke
nor as good a slate as we find in some of the other localities farther to the east
or to the south. It is grayish or greenish, dark-looking, clastic rock of the
sandstone type, of very mixed composition. There has been considerable
deformation, with development of cleavages and fractures that help to con-

ARISHAN, THE SACRED MOUNTAIN OF SAIN NOIN 217

fuse the original structural habit of the rock. Enough of the bedding can
be seen almost anywhere, however, to show that the formation stands
steeply inclined, and that these hills are only the eroded remnants of a great
series of mountain folds.

On the west side of the area, in the vicinity of the Sacred Mountain,
there is a markedly different relation. The top of the mountain is capped
with graywacke, as are also several other adjacent hills; but beneath the cap,
and forming the base of the mountain with the adjacent ground, is granite.
Erosion has cut down through the bathylithic roof represented by the rem-
nants of graywacke formation, and has exposed and cut into the underlying
granite. The granite magma had previously eaten its way up into the basal
and down-folded portions of the graywacke series, destroying all the founda-
tions on which that series had formerly rested. Thus there is here a splendid
exhibit of the very roots of a roof-pendant (Plate XXV, B).

The capping graywacke has been cut and broken and somewhat meta-
morphosed. The granite, too, has been modified near the contact, for it has
absorbed some of the cap-rock and has included fragments of it within its
own substance. No place in the Gobi region shows these relations more
clearly or to greater satisfaction for structural or historical study than the
Sacred Mountain. The contrast between the graywacke and the granite is
striking, and it is this change in quality of material, with almost black, meta-
morphosed graywacke forming the top of the mountain and furnishing the
streaks of dark talus on its upper slopes, that gives the mountain its frowning
appearance. The base is simple granite, weathering out to granular condi-
tion and pulverizing to fine sand that can be blown away. Even the granite
is more readily destroyed than the graywacke which caps the hills.

The granite

The base of the Sacred Mountain and the adjacent hills, together with
the ground sloping toward the hot springs, are all of a biotite granite. It is
a massive type, of comparatively coarse composition, disintegrating in this
climate somewhat more readily than the other rocks with which it is associated.
The granite, therefore, is worn down in some places as low as the simple sand-
stones of Jurassic age which underlie the rest of the valley. There is other-
wise nothing peculiar or unusual about it. The mountain stands so high
wholly because it has been protected by the cap of relatively resistant meta-
morphic graywacke.

Near the hot springs the granite has broken into a multitude of massive
blocks (Plate XVII A, page 143). They lie tumbled about in utmost confusion,
and give at first the impression that they are the ruins of some great structure
built by man. The same habit can be seen in many other places, always in

218 GEOLOGY OF MONGOLIA

low or wet spots, and we judge that the granite breaks up, by jointing, into .
large slabs which are dislodged by frost; and the rgck in the low, wet places
is more prominently affected in this way than the average rock, simply because
the joints are more commonly filled with water, the freezing of which supplies
the lifting energy. The blocks are then exposed to additional atmospheric
attack, and at last disintegrate into arkosic sand which the wind can blow
away. This is one of the steps, we judge, in the development of the peculiar
depression-topography noted in most of the granite areas thus far seen in
the Gobi region. Under the most favorable circumstances depressions of con-
siderable depth are etched out.

The granite covers the whole western portion of the map, except for the
remnants of the older graywacke which are part of the roof of the bathylith.
Toward the east, however, the granite is limited at a definite line which proves
to be a fault boundary. The formations on one side of the fault line are the
granite and graywacke series, and on the other, the Jurassic sandstones and
conglomerates.

The granite is intersected by many large dikes that trail off across-
country, in wavy courses somewhat resembling the serpent-form dikes described
in greater detail in the account of the Tsetsenwan country (Chapter V).
These are of exactly the same type, only less pronounced in their serpent-
form habit. They run somewhat more evenly, and are not quite so numerous
as they are in the Tsetsenwan district. They vary in quality from granite
porphyry to medium basic rocks, such as andesites; but their whole range of
composition might easily come within the possibilities of differentiation in
the granite bathylith itself, the same explanation applying to these as to the
myriad dikes of Tsetsenwan (page 98). Some of the dikes break through
the capping graywacke, and in such places tend to follow the structural lines
of the rock.

Another feature, somewhat different in its meaning, is the presence of
almost completely digested xenoliths of graywacke within the granite mass.
In some places little is left to remind one of the graywacke substance, but
there is a certain superimposed structural habit in the granite and it has a
slightly different composition from the average, suggesting absorption of
graywacke substance by the granite. In some places there are contact meta-
morphic minerals, such as tourmaline, which indicate local concentration of
special constituents.

Thus the granite varies both in composition and in structure, especially
as one approaches the cap of graywacke or the included xenoliths. In ex-
treme cases the granite and the invaded rock may be so commingled that a
given specimen consists of nearly equal parts of each kind of rock.

Besides the variation in quality due to absorption of the overlying rock,

PLATE XXV.

A. GUARDIANS OF THE SACRED MOUNTAIN,

The lama prince stands at the door of his yurt with attendant priests. The yurts and tents at one side belong to pilgrim visitors,
and those in the middle background stand near the shrine and over the principal springs.

B. XENOLITH OF GRAYWACKE CAPPING A GRANITE HILL.

The black capping rock is graywacke metamorphosed by the bathylithic granite which surrounds it.

ARISHAN, THE SACRED MOUNTAIN OF SAIN NOIN 219

there is variety due to differentiation of the magma represented especially
by the dikes (Specimen 251, A.M. 9940).

The Jurassic formation

In the middle of the valley the formation is wholly sandstone and con-
glomerate, which we have tentatively assigned to the Jurassic. The sediments
are well-bedded and form a very thick series, terminated abruptly on the west-
ern margin along the fault line. Along this margin the beds are tilted so that
one may walk across their upturned edges. Elsewhere in the valley they lie
in various attitudes, for the most part much more nearly horizontal. Erosion
of the tilted beds of varying hardness has produced many small hogbacks and
cuestas. The rocks for the most part are thin-bedded and contain a sur-
prisingly high proportion of quartz-sand. Sandstones prevail over all other
types, and are associated with conglomeratic or pebbly beds, but shales are
not developed to a great extent. It is not possible to tell the thickness of
the formation, but several thousand feet of beds are actually exposed by the
tilting of the strata. No fossils were found. Some very black, coaly-looking
material was seen near the springs; this is regarded as equivalent to some of
the coal layers found elsewhere in the Jurassic series (Specimens 252, 253,

A.M. 9941, 9942).

Special products

The only unusual type of material found at Arishan consists of fragments
of vesicular basalt. None was found strictly in place; all were scattered
promiscuously in the vicinity of the hot springs. They are not especially
attractive fragments, such as might have appealed to men who could carry
them there as peculiar or fascinating pieces of rock, neither are they a part
of the shrine. The one suggestive feature of their distribution is that they
were found only along the precise line of the fault. The evidence favors the
conclusion that they are residuary blocks from a formation that is in place.
We believe that they represent a portion of a plug or outlet of volcanic mate-
rial that at one time came up along the fault zone to at least the level of the
present surface, and that they mark an incipient volcanic vent, which appar-
ently did not fully develop. This, however, is not essentially different from
the volcanic history of the area along the Ongin Gol at Sain Noin, where
recent lavas have poured out and choked the stream (page 126). It is our
opinion that the two volcanic outbursts are comparable in age and origin.
The one at Arishan, however, did not succeed in furnishing a lava flow, as
did that of Sain Noin, but followed the zone of weakness along the fault
line almost to the surface. We are inclined, therefore, to assume a volcanic
basalt plug in the fault at this point, and it is our belief that the hot springs

220 GEOLOGY OF MONGOLIA

owe their existence to this volcanic history and favorable structural relation
(Specimen 254, A.M. 9943).

STRUCTURAL FEATURES

The Sacred Mountain area covers a portion of a down-dropped block—a
typical structural feature of the Gobi region. The fault block extends north
and south for at least ten miles and for a shorter distance east and west.
Its margins are not simple lines, but, particularly in the vicinity of the hot
springs, are formed of intersecting faults, enclosing angular blocks.

One cannot tell how much displacement there has been along these faults
to bring the sandstones and conglomerates down against granite and gray-
wacke. The strata are turned on edge for a long distance on the west side,
and, from this alone, we would judge that the displacement was a large one,
easily more than one thousand feet, but there is no other measure of it. The
block, therefore, has the structural relation of a graben. There are faults on
all sides, except, possibly, the north; and the dips are inward, showing drag
effects along the north, east, and south margins.

There seems to be no way to determine with much precision the age of
the faulting. It is a typical example of block-faulting, which is one of the
characteristic modes of deformation in Mongolia, but the surprising thing is
its limited size. The whole area of the block is not much more than twenty
square miles. It is, therefore, a good example of the very moderate propor-
tions of areas affected by vertical movement. In areas covered by the later
sediments, the vertical movements commonly observed are simple warpings.
But probably in the oldrock floor beneath, the faulted graben and horst are
not rare.

Portions of at least two other down-faulted blocks were crossed in the
Sain Noin district, within a traverse of twenty miles, and there must be liter-
ally scores of others in the general region. Some of the up-faulted blocks are
great masses, such as form the individual Altai ranges, but a much greater
number are small and much less conspicuous.

THE SPRING WATERS AND THEIR ORIGIN

The waters of the hot springs at Arishan reach a temperature of 127°F.
as they issue from the ground. Although the ground is covered with a thin
mantle of disintegrated rock and soil, there is no doubt of the position of the
fault, on the exact line of which, as projected through this ground, the spring
rises. For a short space the ground along the zone of the springs is richly
carpeted with grass, but beyond that the barrenness of the desert reasserts
itself.

ARISHAN, THE SACRED MOUNTAIN OF SAIN NOIN 221

The water itself is clear, tasteless, tinged with a slight bluish cast, and is
not highly charged with mineral matter. One can detect the presence of
slight amounts of H.S, as the water issues from the ground, and bubbles of
some other gas emerge—probably CO., but this could not be determined.
No actual difference from pure water was detected by the gravitation method,
both instruments reading 1,000 points. There is no mineral deposit, but the
water feels smooth, almost soapy, and furnishes a delightful bath. There is
scant opportunity for that, however; two or three small tents pitched over
shallow excavations in the ground, where flat stones are laid on the edge of
the excavation, furnish the sole equipment.

The same question of source is raised in this case, as with all hot springs.
Are they meteoric or magmatic? Has the water entered the ground else-
where as meteoric water and come up here, or is it pouring out from cooling
or reacting volcanic sources beneath? In this case it is more reasonable,
we believe, to hold that the water is of essentially local origin, particularly
because of its purity. There is some question, of course, because of its con-
tent of gas, and it is probable that volcanic emanations of some sort join with
it, even though its chief source is meteoric. We think that the waters are
essentially meteoric, or local surface collections, which penetrate the ground
and find their way into the fault zone. They can circulate more readily along
the crushed rock of the fault zone than elsewhere, and tend to advance south-
eastward in the direction of a lower outlet. In their course, they come into
contact with the still hot volcanic plug which represents the abortive attempt
at eruption. We see no other adequate source of heat.

It is possible, of course, as far as the manner of issuing is concerned, for
the springs to be of magmatic origin, but in that case one would surely expect
a water of greater mineral content. The escaping gases favor a contribution
from igneous sources, but even this is not a sound argument for magmatic
source of the whole supply. Even meteoric water, which has circulated to
considerable depth and reissues at the surface at a higher temperature, might
give off bubbles because of the changed pressure conditions. But the presence
of H,S is more suspicious, especially as there appears to be a total lack of
sulphides in all the rock formations of the locality. The H,S may be derived
from volcanic emanations.

In this connection, we recall that the basalt flow obstructing the Ongin
River above the lamasery at Sain Noin, is so recent that the glazes of the
glassy crusts are still preserved, and the stream has not yet accomplished
dissection of it. The flow seems to have come up along a similar fault zone,
at the edge of another down-faulted block. Probably the movement of the
fault-block, as well as the outburst of lava, is dependent upon subterranean
igneous activity.

222 GEOLOGY OF MONGOLIA

At the surface, the commonest and plainest evidence is a lava flow, but
the unseen escape of gases and the still more rare occurrence of hot springs
may have the same meaning. Block-faulting and igneous activity are so
commonly associated in the Gobi region as to indicate some vital genetic
interrelation.

CHAPTER XIII

MOUNT USKUK AND THE TSAGAN NOR BASIN
[Geologic Maps, Plates XXVII, XXVIII, XXIX and XXX, in pocket at end]

On the traverse southward from Sain Noin to the Altai Mountains, the
edge of a basin was reached in the vicinity of Mount Uskuk (Fig. 98, page
194). It was at once recognized as one of great promise for every phase of
work in which the Expedition was engaged. The later sediments are splen-
didly exposed; many of the beds are fossil-bearing; and the total thickness
accumulated in this basin is probably greater than in any other seen by us
in the Gobi region. The major unconformity within the later sediments is
very evident, and a series of deformation events, with their consequent struc-
tural features, is unusually well recorded. Surface features, also, characteristic
of different physical conditions of the ground and different deforming and
modifying processes, are clearly displayed here. The geologic story is carried
with fewer breaks, through a greater range of time, and to later epochs, than
anywhere else in Mongolia. This, therefore, proved to be a remarkably good
locality for a special study; and the fact that all the other interests of the
Expedition could be cared for there made it practicable to spend a longer
time in the Uskuk-Tsagan Nor area than was devoted to all the other special
localities together.

PRINCIPAL STRUCTURAL FEATURES

The major structural units are marked by prominent differences in physio-
graphic features. The area is terminated on the south by the face of the
great mountain, Baga Bogdo, one of the fault-block units of the Altai system
(Plate XVIII, A, page 146, and Fig. 106). It is an upthrust mass of old pre-
Cambrian metamorphic crystalline rock, undercut by bathylithic granite.
Thirty miles to the north is another compound block of older rocks, thrust
up sufficiently high to present rugged, mountainous topography (Plate XVII,
B, page 143). This is the Mount Uskuk uplift. It is essentially a double
fault-block, each half composed of pre-Cambrian crystallines of complex

223

224 GEOLOGY OF MONGOLIA

PIAS END

FicureE 106A.—Field sketch of the Baga Bogdo front. The skyline, which is about fifty miles long, repre-
sents the remarkably smooth Mongolian peneplane, beveling granites, schists, and graywackes. (Compare
with figure 118, page 269.)

structure and very great petrographic variety, modified by intimate igneous
injection of dominantly granitic material (Fig. 107). Within them, also are
areas of conglomerates and sandstones of supposed Jurassic age, with accom-
panying brittle porphyries. The whole series, from ancient rocks to Jurassic
beds, is closely folded, extensively deformed in every way, and much affected
by deep seated volcanism. The mountainous units are bounded in all cases
by faults and sharp flexures, and are surrounded by later sediments, which
lie unconformably on an ancient floor of the same kinds of rock as are exposed
in the mountain units (Plates XXVII and XXVIII in pocket).

Baga Bogdo, forming the southern border, and the Uskuk block near the
northern border of the area, are the two great relief features. The lowland
between them is covered with sediments and is structurally a basin, although
it is not everywhere of basin form. An arch has been lifted across the original
basin, so that now there are two basins—a small one just north of Uskuk
at the extreme northerly margin of the map, with small saltpans in it; and a
very large, open one lying between Uskuk and Baga Bogdo, with the lake,
Tsagan Nor, occupying its lowest depression. Separating the two parts of
the original basin, is the uplifted compound block of Mount Uskuk and the
arch of sediments, with remnants of erosion in the form of lava-capped mesas
that stand up as great landmarks not much less conspicuous than Uskuk
Mountain itself (Figs. 108,109). Indeed, Uskuk Mountain is capped with the
same lava that caps the mesas (Fig. 110). Undoubtedly the lavas of the
entire area once formed a single unit, of which only scattering remnants are
now left to mark the position of a former continuous cover.

Principal interest in the basins attaches to the subdivisions, content,
character, and correlation of the different sedimentary formations, which
cover nine-tenths of the whole area. At Ondai Sair, on the easterly margin
of Uskuk, it is plainly seen that there are two distinctly different series of
strata, the lower one of which is more deformed than the upper, with an

MOUNT USKUK AND THE TSAGAN NOR BASIN 225

MIM ny,
PO A ONS
ae S iW vans VSR aS
Ss Oe ae am ORTON

=—

?

ee LEED a -
i TS SNe aes Se A a oe ae STE

Ficure 106B.—Continuation of field sketch. The slope at the foot of the mountain is a series of alluvial
fans, the oldest of which have been disturbed and eroded. Remnants of them appear as ridges partially buried
in the younger fans, which are themselves trenched and partially refilled.

erosion break marking an unconformity between them. The beds beneath
the unconformity carry an entirely different fauna from those above; the
lower fauna is of Mesozoic, the upper of Tertiary age. A similar faunal
difference was noted in the Iren Dabasu basin described in Chapter XI, but
there the unconformity is not so plainly defined as at Ondai Sair, where it can
be traced to actual contact. In both series of strata,—those below and those
above the unconformity,—fossil faunas of definite scientific interest were re-
covered, and it is to these that one must turn for help in correlation. They
indicate beds of Lower Cretaceous age at Ondai Sair and of Oligocene age at
Loh, twelve miles farther to the south. At Loh, also, the paleontologists
report the presence of a Miocene fauna, and twenty miles farther south, near
Baga Bogdo, beds of Pliocene age were discovered.

THE ROCK FORMATIONS

The formations of the Uskuk locality may be grouped under two great
divisions: first, the rocks of the ancient floor, represented by the upthrust fault
blocks; and second, the division of later sediments which have been laid down
on the more depressed portions of the ancient floor, and which have been
preserved from subsequent erosion (Fig. 107). Chief interest attaches to the
younger group, because of the advantages of extensive exposures, determina-
tive fossil content, and relations that aid materially in unraveling their structure
and in building up a dependable geologic column.

ROCKS OF THE ANCIENT FLOOR

The ancient rocks of the floor, although well exposed and exhibiting an
unusually fine petrographic range, are not better developed here than else-
where, and alone would not be as readily interpreted as those found in two or

VOL. II—I5

226 GEOLOGY OF MONGOLIA

three other localities. They include the following units: (1) Ancient Crystal-
line Metamorphic Rocks; (2) The Graywacke Series: (3) The Bathylithic
Granite; (4) Jurassic Strata; and (5) Post-Jurassic Intrusives.

These require brief additional descriptions.

The crystalline metamorphics

The major formations of the Uskuk block are crystalline metamorphics
or injection products of them, folded sediments of less modified habit, and
intrusives cutting the sedimentary members (Plates XXVII and XXVIII).
The general effect is to produce ridges, extending in general east and west,
with an average strike of about N.60°W. Virtually all the rocks with either
schistose or bedded structure stand almost on edge. On the south side of the
block the average direction of dip is to the north, and on the north side the
average dip is to the south, as if the whole complicated series constituted a
synclinal structure. The synclinal form is not simple, however, and per-
haps the fold is not complete, since the same beds were not identified on the
two limbs. On the whole, the metamorphics are simpler in the southern half
than in the northern, and perhaps the difference in dip is due to relations of
regional unconformity rather than to simple folding.

The series of formational members, in a traverse from south to north, is
as follows: (1) intrusive porphyries; (2) Jurassic sandstones; (3) granite por-
phyry; (4) bedded rocks changed to schists; (5) crystalline limestones; (6)
spotted gneisses; (7) mica schists; (8) sheared porphyry gneisses; (9) gran-
ite porphyry gneisses; (10) crystalline limestones; (11) gneisses; (12) crystal-
line limestones; (13) gneisses and schists with injections; (14) large crystal-
line limestone belt; (15) granite gneisses; (16) crystalline limestones and
schists; (17) gneisses; (18) schists with many injections; (19) crystalline
limestones and schists.

In this list, which is a generalized lithologic cross section of the southern
Uskuk block, minor injection products and all intrusives except the major
ones are neglected. One of the striking features is the abundance of lime-
stones and schists. There are also extensive bodies of porphyries sheared into
gneisses. Some of them are unusually fine examples of their kind, and mark
a period of profound regional deformation. They are believed to represent
one of the oldest series of rocks in the Gobi region.

The dominance of metamorphic rocks; the prominence of limestones and
schists and shear gneisses; the abundance of injection phenomena of all kinds,
from minute impregnation effects to large intrusions; the prominence of an
east-west trend; and the commonness of faulting are the most characteristic
structural features of the block. In spite of the confusing list of rock types,
it is possible to group them into two fairly well-defined series. A portion of

MOUNT USKUK AND THE TSAGAN NOR BASIN 227

the column is made up chiefly of gneisses and schists; with minor silicated
limestone members, all much modified from their original lithologic habit.
We regard them as forming the more ancient portion of the complex. In
contrast to these rocks it is possible to distinguish a series characterized by
simple physical conditions. The members of the second series are crystalline
also, but they have retained evidences of their original character more suc-
cessfully than those of the other series. Limestones are present in larger
proportion; schists and phyllitic rocks are more abundant; and quartzites are
associated with them. They are strictly a series of ancient sediments but are
probably much younger than the other series. There is, however, no other
means of determining either their relative age or their correlation with the
rocks of other regions. The simpler series may correspond to the Wu T’ai
system as used in China, and the more complicated series may belong to the
still older T’ai Shan complex. The thickness of all these rocks could not be
determined, but each series must be represented by some thousands of
feet.

The graywacke series

On the margins of the Uskuk block, and particularly in a second block
of hill country some twelve miles east of Uskuk, there are extensive areas of
slates and graywackes. The structural trend of the rocks carries them be-
neath the sediments of the Tsagan Nor basin; so it is certain that a portion of
the floor of the basin is made up of rocks of this series. They are chiefly
slates and graywackes with prominent bedding, much deformed by folding and
some faulting, but not thoroughly recrystallized. The slates are fine-grained
and slightly phyllitic, and the graywackes are granular, ranging from fine,
almost slaty condition to conglomerate. Bedding is prominent, the trend of
_ the folds is nearly east and west, and the series is of immense thickness. It
is undoubtedly the same series as that noted along the Tola River and de-
scribed in much greater detail on the traverse from Urga to Sain Noin (Chap-
ters V and VI).

No determinable organic remains were found in the rocks, despite a very
critical inspection for possible fossil content. The search yielded only some
obscure tubular structures which may be borings or burrows, and some traces
of unidentified minute forms which resembled alge, but whose organic nature
is inferred rather than proven. The lack of fossils in so great a formation of
such simple structural condition is, we think, an argument in favor of its great
age. We tentatively regard the whole series as pre-Cambrian, and as essen-
tially the Mongolian equivalent of the Nan K’ou series of von Richthofen or
the Sinian system of Grabau. This is one of the unsettled questions of the
graywacke problem to which we hope to give more extended study.

228 GEOLOGY OF MONGOLIA

Bathylithic granite

Large granitic intrusions with great variety of petrographic expression,
are to be seen in the Uskuk block. It is not certain that all are of the same
source, and probably they are of very different ages. Some of them, how-
ever, are of the type that has been designated on this Expedition, ‘‘the great
Mongolian bathylith.”” The best, and by all means the most characteristic,
exhibit is that forming the northern face of the Baga Bogdo mountain block.
Here the oldrock floor has been thrust up to an elevation so great that the
whole mountain front is made up of granite. The ancient crystallines and
other formations which originally must have formed the roof of the granite
mass, have been carried off by erosion, and the débris is now distributed as
detritus with the strata that formed at its foot in the Tsagan Nor basin.

Jurassic strata

In our opinion there are in the Uskuk region no representatives of Palzeo-
zoic age. Between the graywacke series, which we consider to be late Pro-
terozoic, and the sandstones and conglomerates with coal beds, which are
probably of Jurassic age, the whole geologic column is a blank.

In the south margin of the Uskuk block, a thick series of sandstones and
conglomerates stands on edge; with them there are thin, badly crushed coal
beds of very poor quality (Plate XXVIII). The rocks are almost wholly
non-fossiliferous, but there is no mistaking their general significance. They
are simple, unmetamorphosed sediments, which are wholly of continental
type. Most of the pebbles are quartz and quartzite; they are only moder-
ately well bound together, and while certain beds are very substantial and
hard, on the whole they are not strongly resistant to weathering. The con-
glomerate beds stand out better than the simpler sandstones. The series is
closely folded, considerably faulted, and crushed. The total thickness could
not be determined, as the whole series is cut off by porphyry eruptives,
which form one margin of the area of Jurassic strata.

Post-Jurassic intrusives

In this area, as in many others, a very complex series of volcanic intru-
sive porphyries is associated with the Jurassic strata; and these together con-
stitute the south margin of the Uskuk uplifted block. Most of the porphyries
are of the usual brittle, fine-grained, or finely porphyritic type, varying from
trachytic to andesitic composition, and, in-rarer cases, to much coarser and more
massive-looking granite porphyries. The granite porphyries form a larger
part of the assemblage than was found elsewhere, but they are associated here
with the Jurassic units. They are all somewhat deformed and are judged to

MOUNT USKUK AND THE TSAGAN NOR BASIN 229

be the youngest representatives of the floor on which the later sediments were
laid down.

ROCKS OF THE SEDIMENTARY COVER

All the formations of critical importance in the Tsagan Nor basin belong
to the so-called later sediments, which have accumulated unconformably on
the erosion surface of the deformed, metamorphic strata and igneous masses of
Jurassic and pre-Palzozoic age. The old, deformed masses will be referred
to in the following discussion simply as the formations of the oldrock floor.

The strata lying above this floor fall into two definite series, separated
by an unconformity (Fig. 107, page 229). The series beneath the unconform-
ity, and occupying the interval between it and the oldrock floor, carries a

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Shitiuta Dzun Hsir Tsagan Nor
Ondai Sair £599.
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FicurE 107.—Generalized geologic section across the Tsagan Nor basin. The figure shows that, in general,
the sediments of the basin dip toward the Altai front, so that younger beds are encountered as one goes south-
ward toward the Altai range. The formations of the oldrock floor are drawn approximately as they are found
in the southern block of Mount Uskuk, just west of the section. Next above the oldrock floor lie the Ondai
Sair beds, of Lower Cretaceous age, which were deformed and deeply eroded before the early Tertiary strata
were laid down. These include a thick series of barren beds, at the base, and the Hsanda Gol beds, of known
Oligocene age. The overlying Loh beds have been tentatively assigned to the Miocene. At Hung Kureh,
the strata bear a late Pliocene fauna, and there is good reason for representing a physical break both above and
below them. The rubbles of Gochu, regarded as early Pleistocene, were deformed and deeply eroded during
Pleistocene time, and are partly covered by later alluvial fans.

fauna that indicates Lower Cretaceous age. Whether Upper Cretaceous beds
are present under cover, or were at one time deposited in this region, is not
known, but it is clear that there was considerable erosion of the lower series,
and formerly there may easily have been a much greater thickness, represent-
ing younger beds that have been destroyed.

Above the unconformity there is a much more extensive series of sedi-
ments, which are all judged to be of Tertiary age. The lowest beds of this
series are non-fossiliferous, and their exact position in the scale is, to this
extent, doubtful; but they form a succession in which the higher strata, begin-
ning with the Oligocene and closing with the Pliocene, are fossil-bearing. The
series can be followed from bottom to top in a succession of exposures, contin-
uous except for a long stretch of covered ground between the fossil-bearing

230 GEOLOGY OF MONGOLIA

beds of Pliocene age near Baga Bogdo and the beds of Miocene age at Loh. In
that stretch, of course, the exact structural relations are lost. As far as can
be seen, the strata appear to be conformable, and the succession might be
essentially continuous, except for the overlapping which accompanied the re-
peated deformation of the basin. But the faunas which have been collected
seem to indicate that there are breaks in the series, and that the sequence
is not continuous, even though the gaps are physically inconspicuous in the
field. If there is any break of greater physical consequence than elsewhere,
it is that beneath the Pliocene beds, where the structural relations are not clear.

The uppermost series is limited at the top by a plane of erosion, parts
of which are unusually smooth so that it bevels even the upturned edges of
beds previously deformed (Fig. 111). No sediments of consequence have
been deposited since that time, except along the immediate base of Baga Bogdo
and, because of the fact that Pliocene strata are affected by this erosion, we
infer that the erosion epoch belonged to Pleistocene time. Deformation fol-
lowed, however, so that the planation surface is domed and warped, and the
succeeding epoch is marked by a rejuvenation of stream work, which has
accomplished extensive dissection. The only types of deposition noted above
the erosion surfaces are: (a) very local stream wash, which is carried part
way down the new trenches and is now being deposited along the lower reaches
of the valleys (Plate XX XI, A); (b) alluvial fans on the margins of the up-
lifted mountain blocks, both Uskuk and Baga Bogdo; (c) wind-blown mater-
ial, especially the sand dunes of the Tsagan Nor lowland (Plate XVIII, A,
page 146, and Plate XXX in pocket).

The petrographic character of the strata forming the two greater series
is, in each case, typical of the age represented, for similar characteristics have
been noted in other districts. The older series, of Cretaceous age, is much
the more hardened and substantial in exposed ledges, simpler in its coloring
and more uniform in its bedding habit. The upper series is much the more
variable; some of the beds are very highly colored; changes take place within
a short distance, in thin layers, and some portions are strikingly variegated
in color; the sediments are not hardened, and readily break down into sands,
clays, and loams, so that there are no jutting ledges, and exposures are pre-
served only on actively eroding surfaces. The erosion history of the district,
however, has favored the exposure of both series on a remarkably large scale
in the Hsanda Gol valley, so that it is possible to examine an extensive suc-
cession of the beds in intimate detail.

Cretaceous strata

The Cretaceous strata are chiefly gray, yellowish, or rusty sandstones,
made up of quartz sand of uniform grain, extensively cross-bedded but not

PLATE XXXI.

A. THE HSANDA GOL.

Lower Cretaceous and Tertiary sediments are exposed along this valley. The Gobi planation surface forms the skyline.

B. SOUTH USKUK BLOCk.

Along the southern margin of the south Uskuk block, tilted Tertiary lavas form a ridge, marking the flexure and fault that form
the boundary of the Mount Uskuk uplift.

PLATE XXXII.

North UskKuK Block

A. USKUK BLOCK FROM DZUN HSIR,

View westward from Dzun Hsir across the dissected Gobi surface, the adjacent red badlands, and the Hsanda Gol, into the south
Uskuk block with its enclosed sedimentary remnants.

B. SEDIMENTARY REMNANT AT USKUK,

In the eastern end of the south Uskuk block, one of the sedimentary remnants is shown, surrounded by hills of the crystalline

rocks.

MOUNT USKUK AND THE TSAGAN NOR BASIN 231

strikingly pebbly or coarse. The sandstone beds alternate with thinner beds
of drab shales and thin, black, perfectly fissile paper-shales. Together they
form a very characteristic lithologic habit to which it is convenient to refer as
the ‘‘sandstone and paper-shale series.’’

There are three places within the district where these strata are exposed:
one in the angle between the two Uskuk blocks; another along the Hsanda
Gol at Ondai Sair; and the third two or three miles farther to the southeast,
where tilted strata of the same type, chiefly sandstones, are exposed. For
convenience, the first two, which are connected in the field, are referred to
jointly as the Ondai Sair locality. By far the best exhibit is obtained on the
east side of the Hsanda Gol, where the strata can be traced eastward almost
continuously from the edge of the fault-block to the unconformity at the top
of the series. The succession from top to bottom is as follows:

Unconformity Feet
1. Gray and purplish sandstones ‘ ; ; ; . : $4.6
2. Sands with greenish shaly layers . ; : : : ‘ “ph 4
3. Cross-bedded sands with harder red layers. , : . mer
4. Rusty layer with white sandstone : ; : ‘ in 5
5. White and gray sands with many shaly Streaks i ; ‘ 428
6. Paper-shales 3 : i : P ; : . ‘ sen 4
7. Concretionary gray sand. 2 : ; : : : Seat)
8. Dark paper-shales : ? ; , ; Ars
9. Indurated sandstone, dip spatneast 16 dees

10. Sandstone with many dark paper-shale layers
11. Paper-shales

_
iS)

. Hard sandstone layer .

2
7
3)
I
13. Yellowish and rusty, cross- epedded Hebbly, Ealdstotie 3 ay
14. Dark paper-shales. ; d : : . bint 6
15. Strongly bedded and idiaeted Seeanone 2
16. Yellowish and white sandstone layers 17
17. Paper-shales 2
18. Cross-bedded white Sandstone 3
19. Coarse, nodular sandstone layer I
20. Thick series of paper-shales, and thin, an, ona nated i many
thin, sandy layers. ; : : 2 5 : Be
21. Strongly indurated sandstone tye : : ‘ : : 5 ate
22. Dark paper-shales with interbedded sands. . : : 820
23. Strongly indurated sandstone : ; : ; ; : hii |
24. Paper-shales with sandy layers : :
25. Indurated sandstone, dipping Saher 10 deere i 2 : ore <I
26. Dark paper-shales : ; : : , : d : itt
27. Strong sandstone layer ; : : : ; : when

232 GEOLOGY OF MONGOLIA

Feet

28. A series of drab and gray paper-shales with associated sandy layers. 21

29. Dark-colored paper-shales

30. Rusty sandstone bed

31. Dark-colored paper-shales

32. White, fine sandstone .

33. Yellow sandstone with thin, drab pee

34. Drab-colored shales and paper-shales :

35. White sandstone with occasional shaly layers .

36. Drab shale .

37. White sandstone .

38. Dark paper-shale

39. White cross-bedded Ba eieeenie :

40. Sandstone, with more uniform structure

41. Gray shales ‘

42. Dark shales with thin Papet-ehale end white aude 4

43. White sandstone . ;

44. Drab and yellowish shale eee

45. Yellow and white sandstone .

46. Paper-shale

47. Yellow sandstone

48. Covered ground, adoubtedly anderieeel cas, by santisbeties: to the
river level, estimated : : ‘ f ‘ ; ¢ . 50+

ol iS)
Ha ananmnouowunoaununns bb -&

im
1 ae)

on

The base of the section was not reached, and the top of the section has
been cut off by erosion. The striking point about the series is the alternation
of simple sandstone beds with sharply defined and finely developed fissile
shales of the paper-shale type.

A summary of this detailed column gives a total exposed thickness of
390 feet. The difference in elevation between the exposed base and the top
is 215 feet. In the total there are twenty-nine definite sandstone beds, and
nineteen thin shale or paper-shale layers. The sandstones total approxi-
mately 230 feet, and the shales or mud rocks, 160 feet. The typical paper-
shales amount to nearly 100 feet. This section offers one of the best exhibits
in variety of Cretaceous strata found in the Gobi region. We found a
larger number of paper-shale beds at Ondai Sair than in all other districts
put together. Beds of Cretaceous age elsewhere are dominantly sandstones,
such as those at Oshih and Djadokhta; but the paper-shale type is found again
at Oshih (Chapter XV, page 271), and on the Urga trail north of Iren Dabasu
(Chapter IV, page 63).

Fossil content.—-Although the formation is not highly fossiliferous through-
out, there are many beds which carry fossils. The paper-shales are the most
prolific, but not all of them contain fossils. Some of the forms are new, but

MOUNT USKUK AND THE TSAGAN NOR BASIN 233

a fauna which is in part identical with it has been reported from eastern
Siberia. The fossils have been reviewed by Professor Cockerell (1924). The
complete list thus far identified is as follows:

Reptilia )
Protiguanodon mongoliense Osborn (1923d, 19242).
Large sauropod undetermined.
Pisces
Lycoptera middendorffii Johannes Miller. Exceedingly abundant in the
paper-shales. The specimens agree with L. middendorffi in the propor-
tions of the fins, rather than with L. sinensis A. S. Woodward, from the
lower Jurassic (?) of the Province of Shantung, China. These fishes are
discussed more fully in a later contribution (Cockerell, 1925); they form a
new family, Lycopteride, apparently ancestral to the Cyprinide, and have
scales scarcely differing from those of the European minnow.

Crustacea
Estheria middendorffi T. R. Jones, 1862. Exceedingly abundant in the paper-
shales (Pl. II, fig. 2).

Insecta
Ephemeropsis trisetalis Eichwald (1864)
Ephemeropsis melanurus Cockerell (1924)
Cymatophlebia (?) mongolica Cockerell (1924)
Trichopterella torta Cockerell (1924)
Indusia reisi Cockerell (1924)
Chironomopsis gobiensis Cockerell (1924)
Coleoptera, spp. incert.

Plants
Baiera furcata (Lindley and Hutton)
Phyllocladites (?) morrisi Cockerell (1924)
Czekanowskia sp.
Equisetacez (?) fragments.

Tertiary strata

Above the unconformity representing an old erosion surface on the de-
formed Cretaceous strata, there is a series of younger beds several thousand
feet thick. It is not possible to measure the total thickness accurately, but
certain portions of the series may be measured in detail. These portions in-
clude a great succession of beds near the bottom, a small section near the
middle, and a somewhat larger section toward the top. All are undoubtedly
of Tertiary age, but the horizons marked by definite faunal content fall within
the Pliocene at the top of the series, and within the Miocene and Oligocene
near the middle. It is assumed that the lower beds, three thousand feet of

234 GEOLOGY OF MONGOLIA

which lie beneath these determined horizons, may reach down into the Eocene,
but definite determinations of such horizons have not been made in this area.
Fossil content.—The fossil content of the upper sections is listed below:

1. Oligocene strata along the Hsanda Gol at Loh, which we have called the Hsanda
Gol formation.
Mammalia
Carnivora
Hyenodon pervagus Matthew and Granger (19242)
Didymoconus colgatei Matthew and Granger (1924a)
Didymoconus berkeyi Matthew and Granger (1924a)
Amphicticeps shackelfordi Matthew and Granger (19242)
Bunealurus ulysses Matthew and Granger (1924a)
Bunelurus parvulus Matthew and Granger (19242)
Paleoprionodon gracilis Matthew and Granger (1924a)
?Cynodictis elegans Matthew and Granger (19242)
?Cynodon (Pachycynodon) teilhardi Matthew and Granger (1924a)
?Viverravus constans Matthew and Granger (1924a)
Glires
Tsaganomys altaicus Matthew and Granger (1923c)
Cyclomylus lohensis Matthew and Granger (1923c)
Cricetops dormitor Matthew and Granger (1923d)
Selenomys mimicus Matthew and Granger (1923d)
Tataromys plicidens Matthew and Granger (1923d)
Tataromys sigmodon Matthew and Granger (1923¢)
Karakoromys decessus Matthew and Granger (1923d)
?Prosciurus lohiculus Matthew and Granger (1923d)
Eumys asiaticus Matthew and Granger (1923d)
Desmatolagus gobiensis Matthew and Granger (1923d)
Desmatolagus robustus Matthew and Granger (1923d)
Insectivora
Tupaiodon morrisi Matthew and Granger (1924)
?Tupaiodon minutus Matthew and Granger (19240)
Paleoscaptor acridens Matthew and Granger (1924)
Paleoscaptor rectus Matthew and Granger (19240)
Perissodactyla
Baluchitherium grangeri Osborn (1923d)
?Epiaceratherium sp. Matthew and Granger (1924)
Artiodactyla
Eumeryx culminis Matthew and Granger (1924))
Batrachia
Pelobatidz
Macropelobatis osborni Noble (1924)

MOUNT USKUK AND THE TSAGAN NOR BASIN 235

2. Miocene strata at Loh
Mammalia
Rhinocerotide
Baluchitherium mongoliense Osborn (1924g)
Mastodontide
Serridentinus mongoliensis Osborn (19242)

3. Pliocene strata at Hung Kureh, at the foot of Baga Bogdo
Mammalia
Artiodactyla
Large cervid
Gazella sp.
Camelus (?)
Perissodactyla
Hipparion sp.
Rhinocerid
Proboscidea
Mastodont
Rodentia
Castor sp.
Aves
Struthiolithus
Fragments of a smaller bird

Recently Mr. Granger has come to believe that the ostrich and perhaps
the mastodon are of Pleistocene age, and that they come from beds that over-
lie the typical Hung Kureh formation.

The whole series of beds is undoubtedly of continental type, including
stream wash, lake sediments and wind-blown material. The quality ranges
from loose, poorly bound conglomerate or pebbly sands, to simple, massive
sands and marls and loamy clays. The colors vary from white to brilliant
reds and dull greens, and many beds are variegated. Changes in quality
are sharply marked, bedding is a prominent feature, and most of the thicker
beds show marked change of quality. All are poorly indurated and weather
down readily, so that the slopes are covered with loosened débris which con-
ceals the beds beneath. Only in exceptional positions along the stream courses
and in the badland patches, are the beds well exposed. Wherever erosion is
comparatively rapid, as it is in the softer beds exposed on the east side of the
Hsanda Gol near Black Butte, the strata are well exposed and are carved into
badland topography. Midway in the series are basaltic lavas and tuff beds,
which formerly covered a large portion of this area and doubtless were con-
nected with the more extensive beds of lava still covering the upland country

236 GEOLOGY OF MONGOLIA

farther to the west. Above the lavas, similar variable sands and loamy clays
of brilliant colors continue. Higher in the series, especially in the Pliocene
beds of Hung Kureh, the colors are softened—many of the beds are almost
pure white or slightly yellowish—the bedding becomes less regular, and
finally pebble beds are interlayered, doubtless representing alluvial fan ex-
tensions from the Baga Bogdo uplift. The upper pebble beds may mark the
limit of the Pliocene.

The whole series can be regarded as a large unit with three rather distinct
physical divisions. The first division begins at the Cretaceous unconformity
and includes the lower portion of the series along the Hsanda Gol, up to the
lava beds. The second division begins with the beds above the lavas, and
includes the strata exposed through the district of Loh to the valley of Tsagan
Nor, beyond which beds of a different quality appear. The third division
includes the lighter colored beds of Hung Kureh. Roughly, this division
marks the most prominent physical breaks and in a general way corresponds
to the faunal changes. The beds of the lower division are essentially barren,
though Oliogocene fossils are found just below the lava sheet; the middle
division carries a lower Middle Oliogocene and a Lower Miocene fauna; and
the upper division is of late Pliocene age. There is, however, no noticeable
physical break corresponding exactly with the Oliogocene-Miocene hiatus.

Clearer understanding of the character of the strata constituting this
Tertiary series can be secured by referring to the detail at points where sec-
tions are best exposed. The best exposure for the lower division is that in
the vicinity of the Black Butte, called Dzun Hsir by the Mongols, where a
total thickness of more than two thousand five hundred feet of sediments is
exposed along a stream course which cuts across the upturned edges of these
beds. Here the whole series is flexed as it merges into the fault bordering
the south side of the Uskuk block, and is planed off by Pleistocene erosion.

At this point the unconformity at the base is not exposed, but the con-
glomeratic beds, which are recognized everywhere as forming the lowest
layers, are exposed, and undoubtedly the basal member is at no great depth
beneath them. The succession from bottom to top in the Dzun Hsir section
is as follows:

1. Yellowish conglomerate, with reddish sand streaks . ‘ ‘ + al ct
2. Red sandy bed. 5 5 : ; 4 ; ; ah, Siaae
3. Heavy yellowish Seva Shae : : ; ‘ ; ; Seog tete}
4. Green sandy layer. : : ‘ 4 5 2 ; F I
5. Heavy yellowish paieiaicnee é : : : : : SPs
6. Red sandy layer . : ; é : : f : 3 I
7. Heavy yellowish Sotigidadinte : : : : ; ; amet |
8. Red sandy bed. : : : é : : é : : 4

MOUNT USKUK AND THE TSAGAN NOR BASIN

. Heavy conglomerate

. Succession of red beds

. Heavy conglomerate

. Red and buff layers

. Coarse white sand

. Deep red bed

. White conglomeratic rock

. Red checker stratum

. Coarse conglomerate

. Variegated pebbly layer

. Conglomerate bed with red and es ies

. Red and white and drab alternating beds :
. Pebbly, white and red, variegated beds, with drab inves ;
. Red beds, with many alternating white layers .

. Conglomeratic white and red beds .

. Succession of red and white alternating beds

. Dominantly white beds, with reddish layers

. Series of red and white alternating beds .

. Red and white variegated beds

. Alternating red and white beds

. White pebbly conglomerate, with at eat Sitoe lesiee
. White, gravelly sand. ‘ :

. Buff and orange and light red one

. Brown-colored stratum .

. White, coarse sandstone

. Succession of drab and white sand and Sher in 5- res Pees
. White and reddish sands and gravel beds

. Hardened white sandstone stratum

. Variegated beds

. Drab-colored and white inves

. Buff sandstone layer

. White sandy layer

. Deep red stratum

. Tuff or ash bed

. Bottom flow of basalt

. Top flow of basalt

64
218

In this long succession of beds, which, even in this statement, is generalized

so that some of the details represented by the section itself are merged into
groups, it is possible to make three—or possibly four—major divisions, charac-
terized by dominance of certain lithologic qualities.

(1). The bottom group is represented by a dominance of yellowish con-

glomerates and conglomeratic sands.

The beds lying immediately above are

238 GEOLOGY OF MONGOLIA

also prevailingly conglomeratic, but have a larger intermixture of fine-grained
layers and a greater proportion of reddish colors. These two groups, which
together represent a total thickness of about 400 feet, form the conglomeratic
base of the column.

(2). Above the conglomerates there is a complex series of alternating beds
of sand, loamy silts, and clayey or marly material—with a generally variegated
aspect, ranging through every shade of color from white to black, from deep
red to pink, and from brown to yellow, with the reds predominating in the
general color scheme. In places where there is badland erosion, the dominance
of reds is so strong that it creates a striking scenic effect, and when seen from
a distance, in the slanting rays of the sun, these dissected areas become what
we called them—‘‘the flaming badlands of the Hsanda Gol” (Plate XXXII,
A). The type of sediment with dominantly high colors continues from the
conglomerate beds of the bottom group to the tuffs and basalts that form the
top, and measures a total thickness of more than 2,000 feet.

(3). The last group is made up of tuffs and basalts, with a total thickness
of nearly 300 feet.

The total thickness, measured along the course of the stream where the
section was made, amounts to 2,761 feet after correction for dips. This can
not be taken as quite accurate, however, for there is a slight additional cor-
rection to be made because the section does not lie exactly normal to the strike
of the beds. This would reduce the total thickness as exposed to perhaps
2,600 feet, but the bottom is incomplete, and there are minor variations in
dip, introducing additional errors. The best that can be said, therefore, is
that the section includes more than 2,600 feet of sediments including 300 feet
of volcanic material.

The dips vary from almost flat to vertical; the lowest beds, which are
exposed in the northern end of the section, are the least disturbed, and the
successive layers increase in dip until the central portion of the variegated beds
is reached, where many of the strata are vertical. From this point on, the
dips gradually diminish, until the capping basalt is reached, where there is a
dip of 48 degrees.

Former extent of later sediments

Later sediments of Cretaceous and Tertiary age occupy the basin country
around Mount Uskuk, and thence southward to Baga Bogdo. Nowhere in
this basin portion does the oldrock floor come to the surface, and it is quite
certain that in the deeper portion it must lie beneath several thousand feet of
later sediments (Fig. 107). Certain features of the mountain blocks of north
and south Uskuk, as well as of Baga Bogdo itself, lead one to believe that at
one time sediments of similar type may have covered even these areas, and

MOUNT USKUK AND THE TSAGAN NOR BASIN 239

that their present relief, as well as their ruggedness, is due to uplift, stripping,
and further erosion. The features supporting this view are:

FicurE 108.—Domed and dissected planation surface of the Mount Uskuk area. This is a clearly
marked example of recent doming of a small area. Mid-Tertiary sediments have been strongly flexed and
faulted. In early Pleistocene time erosion developed an almost perfect plane which, still more recently, has
been arched and redissected. Both the sediments and the oldrock floor exposed in the background record the
deformation.

(a) The faulted margins of the blocks, (b) The presence of remnants of
sediments within the blocks, (c) Remnants of a similar basalt cap on Mount
Uskuk and on adjacent sedimentary strata, (d) The evidence of planation.

Evidence of the faulted margins —Wherever the facts could be determined,
there is faulting along the margin of the blocks, and it is this faulting that
disturbs the bedding of the later sediments adjacent to these margins. Every-
where remnants of these strata are dragged up so that the nature of the dis-
placement, as well as some approximation to its age, can be determined. On
the south side of Mount Uskuk block, for example, there is a sharp flexure
against which the sediments stand on edge, and great thicknesses are cut out
(Plate XXIX, Plate XXXI, B, and Fig. 107). The total displacement along
this flexure must measure more than 2,000 feet. The strata involved, although
not very narrowly determined, are at least of mid-Tertiary age. On that side,
therefore, the deformation which has given some, if not all, of the present
relief to the Uskuk block is as late as mid-Tertiary. Along every margin where
the detail can be seen, such displacements are noticeable; but it is not every-
where that the age of deformation can be traced to so late a stage. At the
foot of Baga Bogdo, strata of late Pliocene age are disturbed, indicating that
part of the uplift is post-Pliocene.

Evidence of the sedimentary remnants.—At several places within the Uskuk
blocks there are remnants of later sedimentary strata on the unevenly eroded
crystalline floor, still preserved from the erosion that has increased the present
relief (Plates XXVIII and XXXII). Some of the remnants lie in rather

240 GEOLOGY OF MONGOLIA

elevated positions, the most westerly one attaining an elevation quite com-
parable to portions of the adjacent blocks themselves. It is very evident,
with such relations, that they are no more than remnants, left from the strip-
ping of a very much more extensively covered area; it is entirely probable,
indeed, that the sediments at one time covered every bit of the crystalline
areas now exposed, even to the tops of the highest mountains. What the
original relief of the floor beneath was like is not so easily determined, but it
is clear, from the pocket-like distribution of some of the remnants of sediments,
that it was uneven in the Uskuk area.

The upland portions of the Uskuk blocks fall fairly well into peneplane
form, which seems to have prevailed much more extensively in the beginning
of basin history. The sediment remnants, however, lie below the upland

Dzun Shiliuta
Baron Shiliuta

Lava Cap.

Erosion Plane

FicurE 109.—Field sketch of Baron Shiliuta. The lava-capped mesa of Tertiary sediments stands east of
Mount Uskuk. The gullies are all tributary to the Hsanda Gol. The dissection is subsequent to the planation
epoch marked by the Gobi erosion surface. (Compare with figure 108.)

25,1922.

levels, in pocket-like or trough-like depressions (Plate XXXII). The sim-

plest explanation, perhaps, is that the peneplaned Uskuk area was uplifted
and dissected in some earlier time, possibly in the Upper Cretaceous; and
then it was covered with sediments again, so that the inequalities of that new
erosion surface were filled. Subsequently, the area was once more differ-
entially uplifted and has been stripped, with the preservation of only a few
patches of the sediments in what remains of the deeper valleys belonging to
the earlier dissection.

In support of this hypothesis, we may cite a rather striking example of
retained sediments in the south Uskuk block, where it is very clear that they
occupy a valley (Plate XXVIII). In later redissection a stream course was
laid out in much the same form, passing down this valley, with an outlet at
one time near the southeast corner of the block. At the present time the
drainage from this section is all toward the northeast, and the valley with
its sediments is abruptly terminated against a wall of crystalline floor rock,
which now rears across its course. It is reasonably clear that the abrupt
termination of the valley against the wall is caused by faulting, but the evi-
dence is not clear as to when the faulting took place. It must have taken

MOUNT USKUK AND THE TSAGAN NOR BASIN 241

place, either during the downwarping which subjected it to sedimentation,
or, more likely, during the later upwarping which preceded the present strip-
ping. The valley is an ancient one, and belongs to a previous erosion epoch
quite as prominent as the one now in progress. The present form is due to
the reexcavation of the ancient valley down to the point where the fault was
discovered. There the course was blocked, and the stream has found an
outlet across the north side of the valley and through the ancient crystallines.
It is evident, therefore, that the original peneplaned block was deeply dissected
before the sediments were laid down, and that the new streams of the present
day are redissecting both the ancient block and the pocketed remnants of
its cover. Perhaps the earlier dissection corresponds to the Cretaceous uncon-
formity, but of this there can be no conclusive proof until the age of the
sediments can be established.

Evidence of basalt remnants.—The highest mountain in the Uskuk blocks
is Mount Uskuk itself, which stands as a prominent landmark, capped by
more than a hundred feet of basalt (Plate XXVII and Fig. 108). None of
the other nearby peaks or upland portions of crystalline rocks is thus capped
with lava, but farther to the west, within easy range of vision, we saw extensive
continuations of lava fields, which are doubtless of the same age, and probably
once belonged to the same unit. However, with the exception of Mount
Uskuk itself and the three mesas, all portions of the uplifted areas are stripped
of lava. It seems to us entirely allowable to postulate their former connection,
thus assuming that in former times these lava remnants were continuous,
covering not only the three mesas and Mount Uskuk, but many scores of
square miles beside. Since that time the Mount Uskuk block has been re-
excavated, and the southern block has been completely stripped. At the
time of the basalt flows, the area, including most of the Uskuk block, must have
been covered with sediments, because the same basalts in the vicinity are
known to be both underlaid and overlaid by Tertiary sediments.

Basalt flows of the same type form small remnants in the basin far to
the northeast and in the saltpan basin immediately north of Mount Uskuk,
but here they are at a level hundreds of feet lower. If these basalts are re-
lated, there must have been a great amount of warping and faulting since
they were poured out, and this inference is supported by all the structural
evidence. On the south side of the Uskuk blocks, the basalts stand on edge
along a sharp flexure or fault, and thus complete the picture of a great area
of sediments capped by basalt, which has been deformed by the upthrust of
the twin blocks of Uskuk (Plates XXVII and XXVIII). The subsequent
period of erosion has been long enough to transform the Uskuk blocks into
rugged mountain areas, almost completely stripped of their former cover.

VOL. II—16

242 GEOLOGY OF MONGOLIA

This has happened since the mid-Tertiary period, for the lava flows lie within
the series of sediments whose age is definitely indicated.

Evidence of planation.—The tracing of a possible ancient peneplane, as
represented by the elevated remnants and upland portions of the Uskuk
blocks, is of itself little evidence of former cover. It is evidence, however,
of a former regional habit. It could scarcely happen that such striking uni-

Uskuk Peak

«<—Lava Ca
<a. P Basalt P.

f

“ Cretaceous and Tertiary Sediments

cap on Mount Uskuk; (2) planation of the crystalline rocks; (3) the abrupt front of the block; (4) the planed
and dissected Cretaceous and Tertiary sediments in the foreground.

formity of levels could be developed except by regional planation. This
habit, also, is consistent with the rest of the geologic history. General plana-
tion surely followed Jurassic folding, and preceded the earliest deposition of
Lower Cretaceous sediments. Wherever the floor can be seen without sub-
sequent modification, its simple form is its most striking feature. Not all
portions, of course, were covered with sediments at any one time, because
there must have been basins and arches in the first deformation stages; but
it is probable that all parts were covered at some time, and it is possible,
indeed, that many of them were covered and stripped, dissected and re-
covered, re-stripped and re-dissected, to reach their present topographic com-
plexity.

The ancient peneplane is more pronounced on Baga Bogdo than on
Uskuk, but it can be traced on both (Plates XXXII,‘A, XXXIII, A, and
Figs. 106, pages 224-225, and 118, page 269). The broad, gentle, arching sweep
of the Baga Bogdo uplift. as one approaches the Altai range, is a most mag-
nificent spectacle. The arch of the skyline from peak to peak, as traced from
end to end of the block, at a distance of forty miles, is so uniform that it could
scarcely be drawn more truly than the present form of the mountain range
itself. It seems impossible for it to be anything but the trace of an ancient
peneplane that has been uplifted, arched, and dissected. Yet it was prob-
ably at one time covered with sediments, just as were the other areas. A full

MOUNT USKUK AND THE TSAGAN NOR BASIN 243

statement of the Baga Bogdo evidence belongs to another story that must
be worked out by additional field investigation.

During subsequent time, while other areas have been lifted and depressed
and lifted again, Baga Bogdo has risen highest of all. It is a magnificent
landmark, showing a displacement of many thousands of feet, for the peak
stands 7,000 feet above the level of the sedimentary plain below. If one
adds to this the thickness of sediments which lie on top of the crystalline
floor in the Tsagan Nor basin, the two, measured together, must amount to
not less than 10,000 feet, much of which must have been attained in the de-
formations of the region during very late Tertiary time. Baga Bogdo of
the Altais appears, therefore, to be a comparatively young mountain, and the
stratigraphic and structural evidence of its margins fully supports that in-
ference,

EROSIONAL HISTORY OF THE SOUTH BLOCK OF USKUK

Viewed from every direction, the south Uskuk block is a rugged-looking,
much dissected area, rising abruptly from the surrounding country. The
margin is sheer on the north side, and is equally abrupt, although not quite
so steep and high, on the south side; but the easterly margin is much more
broken and more deeply dissected. The difference in marginal feature is due
largely to the fact that the structural trend of the formations which make up
this block is about N.60°W., thus presenting, both to the north and to the
south, the broad sides of the structural units, whereas the ends of the struc-
tures come to the margin on the east side. Readjustments of drainage have
resulted in extending many of the stream courses east and west, and it is this
adjustment to structure that gives the more ragged appearance of the easterly
margin.

From the higher central portions of the block, still another feature is
presented which is not visible from the plain. We approach these central
areas through deep notches cut by streams, and toil up precipitous valley
sides that are extremely difficult to traverse. In striking contrast to this
steep, rugged topography is the rolling surface of the upland. There is an
immediate change to broad, open valleys from the narrow gorges which lead
up to them. Evidently the upland topography reaches back to an entirely
different history. It must belong to an earlier epoch, and the deep notches
leading up to it must represent rejuvenation, caused by uplift or reuplift of
the block after erosion had established broader and more mature valleys of
an earlier time. This double history is very like that which we read later
in the Artsa Bogdo range of the Altai system, where a mature topography
characterizes the uplands, whereas all approaches have an extremely rugged,
youthful form. (Compare Chapter XV, page 266.)

244 GEOLOGY OF MONGOLIA

The valleys leading out to the south and east are long and deeply cut,
whereas those leading out to the northern margin are very short indeed, and
extremely steep. The block as a whole must have been tilted to the south
and southeast, enabling the drainage to establish itself more successfully on
that side and to capture nearly all of the watershed of the block, which thus
has a very unsymmetrical drainage pattern.

The north block, in which Mount Uskuk itself is located, has a similar
topographic habit and history; but it is not quite so striking in its asymmet-
rical development. The relatively smooth, rolling upland topography is very
well developed, and the only approaches to it are through the rugged, gorge-
like entrances (Fig. 108). Some of the upland valleys afford good grasslands,
where the natives pitch their yurt villages and live with their flocks.

DEFORMATION OF THE MOUNT USKUK REGION

The Uskuk area, including the Tsagan Nor basin, exhibits somewhat
definite and detailed evidences of its deformational history. Not only are
the deformations of pre-Jurassic time preserved in the Uskuk blocks, in the
unnamed block to the east and in Baga Bogdo, but the continued deformations
of post-Jurassic time are so related to strata of definitely determined age
that their time-distribution and significance in historical geology are unusually
well established. In addition to the warpings which must represent the earli-
est happenings after the post-Jurassic peneplanation, there have been repeated
warpings and faultings almost to the present time. The most significant of
these are the following:

(a) Faulting and tilting of the Lower Cretaceous strata initiated condi-
tions leading to the erosion break and unconformity above them.

(b) Relative downwarping must have followed to make the region one
of subsequent deposition during early Tertiary time.

(c) Warping and uplift raised the Uskuk blocks to an altitude of such
prominence that erosion was initiated again, and accomplished the entire
stripping of these areas, with the development of their present mountainous
topography. This was accompanied by sharp flexures and faulting on virtu-
ally all sides, notably on the north and the south, where great faults extend
in a general easterly and westerly direction for many miles, and where strata
stand on edge or are entirely cut out by the movement (Plates XXXI, B,
and XX XIII, A).

(d) The faulting of Baga Bogdo has resulted in lifting a portion of the
peneplaned and sediment-covered floor to its present altitude of certainly no
less than 10,000 feet, part of which has been accomplished since Pliocene time.

(e) Even the latest Gobi planation surface, which bevels all strata of

MOUNT USKUK AND THE TSAGAN NOR BASIN 245

the region, including the Pliocene beds of Hung Kureh, has been warped
(Plate XXX). There is evidence that this warping continues to the present
time and accounts for some of the basin-like depressions of the region (Figs.
107 and III).

Deformation by warping and faulting, therefore, has been in evidence at
intervals throughout the time of later deposition, but no folding has been
produced. The movements are vertical, resulting in uplift and depression,
varying in expression from warping to sharp flexures and marked faulting.
Whatever the causes are, they are inherent in the substructure of the earth
immediately beneath. They are up-thrustings and down-sinkings of com-
paratively simple mechanical expression. They may well be the resultant

EOri eet on brown while brown

Ficure 111.—Gobi planation surface beveling tilted strata. Field sketch of planed and dissected Tertiary
sediments about thirty miles north of Baga Bogdo. In the badland area formed by the re-dissection of this
surface, the upturned beds have developed varying topographic expressions.

of changing isostatic conditions connected with subcrustal volcanism. The
most common accompaniment of these disturbances has been volcanic erup-
tion. Outflows of basalt mark certain stages, and basaltic rocks break out
through certain lines of weakness. Probably the movements themselves are
initiated by some form of volcanism in the interior of the earth, which in
some places causes failure of adequate support and in others causes strikingly
contrasted uplift. Certainly they are accompanied by surface volcanism.
We believe that the movements of the up-thrust blocks of the Gobi are local-
ized, and are quite independent of larger regional influences. They are
independent also of the shifting of superficial sedimentary load.

Crowding, with folding and complicated igneous injection and intrusion,
accompanied all the deformations up to the close of Jurassic time; but since
then all deformations have been simpler, non-regional, essentially local, and
strikingly epeirogenic; and the accompaniments have been volcanic activity
with the outpouring of extensive surface flows. The whole exhibit suggests
that volcanism in one form or another is itself responsible for both types of

246 GEOLOGY OF MONGOLIA

deformation, and that the nature of the deformation characterizing the differ-
ent times, varied with some fundamental change in volcanic activity.

THE SALTPAN OF GUCHU BURT

A long and comparatively narrow basin borders Mount Uskuk on the
north (Plates XVII, B, page 143, and XXVII). It is bounded on the south
by the Uskuk uplifted block with the adjacent upwarped sediments on one
side, and on the north by the margin of the crystalline floor, only a few miles
distant. It has the appearance of a valley extending indefinitely eastward.
It doubtless was once a valley occupied under favorable climatic conditions
by a stream.

The general topographic features are favorable to such an interpretation,
but it will not account for the chain of minor hollows in the bottom of the
lowland. There is no outlet for the depressions, the principal one of which
is occupied by a saltpan. From every side the wash is toward the hollows,
yet there are practically no modern deposits in them. On the very borders
of the saltpan itself one can see exposed strata quite open to present-day
weathering, and, although the wash from all sides is toward this ground,
there is virtually no cover on it. On the south valley side, toward Uskuk,
where the wash comes out from the uplifted fault block, there are extensive
alluvial fans sloping down and distributing coarse fragments.

Under these circumstances it is most surprising that the valley bottom
is not covered. Even more striking is the fact that the floor is too uneven
in depth for any stream to occupy it. There would be many lakes if there
were water enough, for the bottom is eroded to uneven depth. Only two origins
present themselves: one, that the valley bottom has been warped so that
there are basins and ridges along it, and the other, that it has been deepened
in places by wind erosion. Possibly both causes have been at work, but
apparently the erratic work of wind has been a more prominent process than
that of warping. It fully accounts for the exposure of strata in the very
bottom, and for the unevenness of depth and form and the very small size
of these depressions. The saltpan of Uskuk, therefore, appears to occupy a
portion of an ancient valley differentially deepened by wind work, and now
lying in a wind-swept hollow which has reached the water-table.

TSAGAN NOR

Tsagan Nor is a lake with an area of several square miles, not too salt
to be used for stock, for the Tatsin Gol, an intermittent stream coming in
from the west, delivers some water to it. The lake water is only a few feet
deep, and the depression it occupies lies in a broad, open lowland, which was

MOUNT USKUK AND THE TSAGAN NOR BASIN 247

produced in part by downwarping, and in part by erosion (Plate XXX and
Plate XXXIII, B). The major outlines could have been made most consist-
ently by stream erosion in a former more humid time, but the minor depression
occupied by the lake could not have been made either by warping or by water
erosion. Apparently the only agent competent to the purpose is the wind,
and even this could not have accomplished the work under present conditions.
This phase of the local problem will be discussed more fully under the chapter
on Climatic Changes (Chapter XXI), where other evidence is assembled.
Perhaps it is sufficient at this point to state the problem and to describe the
physical features which seem to bear on it.

There are several abandoned strand lines or old beaches above the present
water level (Plate XXX and Plate XLII, B, page 396). The only possible
way these could have been made is by waves, during the time when lake water
stood at these respective levels. The lake, therefore, has stood at various
higher levels long enough to accomplish this work; hence, there must have
been periods of greater humidity than the present.

The depression occupied by the present lake could not have been made
by any known operation of water and wind while water lay in it; but it could
have been made by deflation if the water were dried up. It is clearly an erosion
product of some kind, and the only process which seems to meet all the de-
mands is erosion by wind. Hence, there must have been a former time still
more arid than at present.

The saltpan of Uskuk and Lake Tsagan Nor, therefore, are diagnostic
features on which a fluctuating climate has registered some of its recent
changes. We see no escape from the conclusion that there have been periods
more arid and more humid than the present. There may have been many
changes of this kind, but as yet we see no way to work them out in greater
detail.

CHAPTER XIV

THE GURBUN SAIKHAN RANGES

THE JOURNEY

On August 19, 1922, the two geologists of the Expedition packed a small
camping outfit on camels and with saddle horses started over Artsa Bogdo,
with the intention of crossing the next desert basin to the Gurbun Saikhan
ranges, which could be seen fifty miles or more away. A local guide undertook
to lead the party, which set off with the two geologists, a Chinese cook, and
a native Mongol interpreter. Nine days were consumed on this side trip,—
nine of the most difficult and trying days of the whole summer, for, in the Gobi
region, July and August cover the period of maximum heat. Many very hot
days had been passed in July near Tsagan Nor; but at no time in the summer
of 1922 was the weather so nearly unbearable as those days in mid-August,
when we crossed the desert between Artsa Bogdo and the Gurbun Saikhan.

The first day we crossed the mountains, inspected the south margin of
Artsa Bogdo, and established camp. The next day the local guide was sent
forward with the camels, while the geologists paused to examine some fossil-
bearing sediments. But the guide took the wrong trail, turning eastward
along the mountain instead of southward across the desert. The day was
almost gone by the time the little caravan was found, and camp had to be
pitched again on the southern margin of the Artsa Bogdo range, many miles
farther to the east. This mishap was serious, for it upset the original plan
of the traverse. It also engendered a well-grounded distrust of our guide
and caravaneer. Yet it yielded one bit of good fortune, for the following morn-
ing we discovered some chalcedony artifacts of Paleolithic type. The site
lies in a little cove-like patch of meadow between two ridges where water is
obtainable. On account of the structural relations, we judge that water is
furnished even in the most arid times, and the lavas of the vicinity yield a
never-ending supply of chalcedony and jasper. These conditions may have
encouraged primitive man to dwell there, for it is a spot highly favored with
supplies essential to human life. Nowhere else during the summer was a
locality discovered so promising for further investigation of primitive cultures.

248

PLATE XXXIII.

A. FLAMING BADLANDS OF USKUK.

In the badlands along the south margin of the Uskuk block, nearly 3,000 feet of tilted Tertiary strata are exposed.

B. SAND MOUNDS AT TSAGAN NOR,

A sand flat with scattered growth of tamarisks and other shrubs in the Tsagan Nor basin.

PLATE XXXIV.

A. THE INTERMONT SEDIMENTARY BASIN BETWEEN THE ARTSA BOGDO AND THE GURBUN SAIKHAN RANGES.

Artsa Bodgo is faintly outlined in the background about thirty miles away. The view is taken near the only well in the
basin, with the geological party in the foreground.

B. TYPICAL MORNING SCENE AT A DESERT WELL,

THE GURBUN SAIKHAN RANGES 249

With an additional guide and better directions, the traverse began more
auspiciously on the following morning, so that we finally reached the trail
that we should have found the previous day. On this day’s journey, however,
we experienced some of the most discouraging travel that fell to our lot in the
Gobi. Loose, drifting sand, so loose and so difficult to cross that the horses
became almost exhausted, prevailed everywhere, and the flaming heat made
us long for the little cloud-shadows which brought a moment of relief. Only
the camels held steadily to their course. Late in the afternoon the party was
glad to go into camp beside the only well in the district, about midway be-
tween the two mountain groups (Plate XXXIV, A).

A desert well

The ground is remarkably barren and parched. There is a very large
tract covered by drifting sand, which is more prominent toward Artsa Bogdo
on the west and north than on the south and east nearer Gurbun Saikhan.
In spite of its desolate appearance, there is a sparse growth of desert vegeta-
tion upon which the Mongols manage somehow to graze surprisingly large
flocks and herds. After a day’s ride across such ground, it is difficult to be-
lieve that water could be reached anywhere. Nevertheless, here in the middle
of this desert plain is a well with abundant water. Sheep, horses, and camels
come literally in thousands to the well, and here we saw an exhibit of the
water situation and its management (Plate XXXIV).

In this plain, as in many others, the water-table lies at no great depth,
and water could doubtless be obtained at many other spots. Here it rises
to within two feet of the surface, and the supply seems capable of responding
to any demand. Apparently this well has served the desert folk from time
immemorial. The Mongols’ primitive method of handling water requires that
it be found abundantly very near the surface. A pole, four or five feet long,
with a baby goat skin tied to one end, serves as sweep and bucket. A wooden
watering trough completes the apparatus for drawing and distributing water
(Plate XXXIV, B). A few flat stones are placed around the mouth of the
opening to protect it, and sometimes there is a cover to prevent an animal
from falling in. By means of the goat skin, the water is slushed into the
trough, and the animals stand there and drink. The water that splashes
over helps to enlarge the mud-puddle, which overflows and returns to the
well with whatever else may fall there. So there is an endless round from the
well to the surface and back again. The only real barnyard mess of the region
is immediately around the mouth of the well. It is a discouraging supply
for human use, but it is the only supply, and one drinks this water or nothing.

On the whole, the water of the plains is good—surprisingly good consider-
ing the surroundings. Itisseldom salt or brackish; never bitter, but commonly

250 GEOLOGY OF MONGOLIA

contaminated; it is cool and wet and therefore comparatively satisfying. It
would be easy to improve these conditions, of course, but they are in every
respect the conditions that have prevailed here, as in all primitive countries,
ever since wells were made.

At such a spot one has ample opportunity to study the people and their
equipment. They all come to the desert well. Watching the scene as the
herds and flocks straggle in from the skyline of the desert and from yurts that
lie far beyond the range of vision, one could almost believe that the world
has not moved in 3,000 years. In the early morning, women come, bringing
camels laden with casks, to draw their day’s supply of water; in the evening
the herdsmen drive in their flocks and herds. One sees the usages of past
ages repeated. It is the same picture that one might have seen long ago,
even in early biblical times, when Abraham and Lot divided between them
the pasturage and water privileges,—one taking to the open plain and the
other to the hill country beyond. Not a thing has changed from that time
to this, except—the rest of the world.

In three days of hard travel it appeared that we were only halfway to
our destination, and on inquiry from the natives, who came to the well for
water, little encouragement for proceeding farther was offered us. The Mon-
gols would not admit that they knew where the next water could be obtained,
and our informants hinted that it was quite uncertain whether we could find
any wells. Neither our guide nor the people who came to the well would admit
ever having been to the Gurbun Saikhan Mountains, which lay in plain sight.
In a country of nomads, such an attitude seemed incredible. We afterward
came to believe that this unusually uncommunicative stand was taken be-
cause we were then approaching the boundary line between two native states
and because another day’s journey in that direction would carry us across
the border. These men probably had reason to believe that our course
would not be favored by the local authorities, and therefore they took this
means of discouraging the venture.

Nevertheless, we crossed the basin, and found excellent camping condi-
tions, with good running water, in one of the canyons opening out at the north
side of the Gurbun Saikhan range. No sooner had we reached this ground,
however, than we were told through our interpreter that the locality was
haunted. Mysterious things were said to happen in this canyon. It was
said that the grass would poison the animals and that some of them would
surely die if kept there overnight. This greatly disturbed our native helpers,
but we could not see any good reason for such consequences. An inspection
of the surroundings satisfied us that the tradition was unfounded as far as
natural conditions were concerned, and, since the party was completely tired
out and the animals had to be relieved of further travel, we decided to camp

THE GURBUN SAIKHAN RANGES 251

at this place in spite of its bad reputation. That night proved to be the most
depressing of our many trying times. Every one was exhausted, the natives’
were frightened, and both of us were ill. The trip had been undertaken not
only at the height of the summer season, but at a time also when vitality had
run low from long-continued exertion and illness. We had long ago lost
confidence in our original guide, and now found that we could not secure
very reliable information along the way. When we turned in that night,
the world ahead looked to us as dark as the haunted canyon behind our camp.
Although this seemed quite enough worry for one day, additional trouble
was brewing for us, for we had no more than finished breakfast the next morn-
ing when four natives appeared and sat down solemnly in front of our tent.
Then we were apprised of the fact that these men constituted the local govern-
mental authority and that we were under arrest. Our chief crime seemed to
lie in having crossed the state line without proper credentials or permission.
There are four great states or kingdoms in Mongolia, and unwittingly we had
crossed the line between two of them,—that between Sain Noin and Tushetu.
We told our captors frankly what we were trying to do, and, ignoring
the arrest, asked whether it would be practicable for us to go farther up this
canyon or whether it would be easier traveling to the crest of the mountains
by some other trail. At first they would offer no advice. We were given to
understand that traveling was bad in both adjacent canyons, and the local
chief advised against attempting to go farther, as it would be extremely diffi-
cult to proceed along seldom-used trails. He assured us, in fact, that he could
not very well permit us to travel to the point indicated, because the boundaries
of his jurisdiction did not extend so far, and that he would be blamed for allow-
ing us to pass through. At first he wanted to take us to the Yamen, or police
station of the district, a two or three days’ journey away. We told him then
that we could not afford the time for such a journey, even if it should gain
special permission for us, and that our purpose was only to go one day farther.
We were especially anxious to reach the divide in the hope of determining
from that vantage point the nature of the country and the larger geological
features of the Gurbun Saikhan. In response to his objection that we were
likely to be stopped by the local authorities beyond and that he might be
accused of negligence, we assured him that we would not advertise the fact
of having been interviewed in his territory, and that, in any case, we would
take every care not to compromise him. With this assurance he good-
naturedly gave his consent for us to proceed up the canyon toward the divide,
and seemed quite satisfied with the explanations we gave of our purpose, with
our evident desire to avoid complications, and with our promise to return.
The canyon, with its tributary valleys toward the crest of the first range
of the Gurbun Saikhan, is one of the finest seen in our Mongolian experiences.

252 GEOLOGY OF MONGOLIA

Flowing water, cold and clear and pure, comes to the surface at many places,
then disappears for long stretches in the gravels and sands of the canyon
floor, only to reappear beyond. These outwellings of water became less fre-
quent as we advanced, and when camp was pitched on the divide that night,
there was no water to be had. For the whole day the trail had lain through
grassy patches, instead of over the barren ground that had made the trip
so desolate between the two mountain ranges; and, as the higher ground was
reached, the country became more and more open, grades became gentler, and
all the slopes were grass-covered.

From the divide a landscape opened to our view, so attractive that we
could well understand the reason for the name, Gurbun Saikhan, which means
“the three fair ones’’ or ‘‘the three good places.”” It refers to the three
major ranges rising out of the plain here, which together form rather a complex-
looking group of mountains. They are of such height and extent that moisture
is precipitated much more abundantly there than in the plain, and as a con-
sequence they are covered with richer vegetation. Seen from a distance,
they gleam with a velvet softness in the sunlight, and on the hillsides are
numerous flocks. The region looks prosperous, and its people seem to live
under especially favorable circumstances compared with the people of the
plains. It would be difficult to imagine a greater contrast than that between
our camp at the well in the desert, two nights before, and this one on top of
the first range of the Gurbun Saikhan.

The return journey

A storm arose during the night, and on the following morning it was still
raining. The breaking of camp was delayed for some time in the hope that
the weather might change. The whole setting, including the weather, pre-
sented a vivid contrast to the experiences of the last two weeks. The beauty
of the scenery, the splendid grazing, and the cool air, all led us to think of
the place as a park rather than a desert, and as it came in the midst of most
disheartening desert experiences, we called the camp Paradise Park. Few
places in any grazing country could present a more attractive appearance,
and as we looked off across the depression beyond our camp to the flocks on
the distant hillsides, we could have believed ourselves in a much more pro-
gressive and prosperous country. The whole mountain area is in truth an
oasis, a resting-place, surrounded on every side by desert waste.

The gorge and valley, through which we had come to this spot, are known
to the natives as Ulan Huataka Ama. Ulan means red, and refers, doubtless,
to the red Han Hai beds at the mouth of the canyon. Huataka means water
or well, and refers to the many sources of water along the course of the valley.
Ama means valley. Therefore, this is ‘the watery valley of the red rocks.”

THE GURBUN SAIKHAN RANGES 253

On our journey back to camp at Artsa Bogdo by way of the well in the
midst of the desert, a new road was attempted, and the second day spent in
reaching Artsa Bogdo on this traverse was one of the most trying of all our
experiences. There were miles upon miles of trail covered with drifting sand,
making progress extremely difficult and slow; and then, after reaching the
outer edge of the southern slope of Artsa Bogdo, there were additional miles
of traverse over alluvial fan and pebble-covered lava slopes, across which the
trail could not be followed, and where it was quite impossible, even with local
guides, to determine whether we were taking the best course. We simply
stumbled along toward a distant landmark in the hope that the close of the
day would put us in better surroundings. The ground between the well and
Artsa Bogdo is the most desolate part of the region; but as soon as the higher
slopes of Artsa Bogdo are reached, grazing is better again and once more one
sees flocks in the small valleys and an occasional spring of water.

It took four days to make the return journey. The trip proved to be so
heavy a tax on the endurance of men who were sick when they started out,
that when camp was reached, the whole party had to go to bed for general
recuperation.

GEOLOGICAL OBSERVATIONS AT ARTSA BOGDO

Because of the mistake made by the guide, opportunity was given for
a more extended inspection of the southern margin of the Artsa Bogdo up-
lift than was originally planned. On that side the range descends gradually,
while on the north side there is an abrupt escarpment, with much dissection,

SE

Gurbun Saikhan~,
Well

Sediments of Unknown Age

20

FicurE 112.—Generalized geologic section from Artsa Bogdo across the intermont sedimentary basin to
the Gurbun Saikhan. :

to be sure, but still plainly a fault-scarp. The core of the range is made up
of complicated geological formations; to the south overlapping lavas and tuffs
cover these rocks as they descend beneath the plain. The lavas reach far
up into the mountains, where the formations are all igneous. Farther out
toward the plain, reddish sandstone sediments lie on top, and in some of them
a few fragments of reptile bone were recovered. An extensive area of vesicular
and amygdaloidal basalts borders the south side of the range (Fig. 112). On

254 GEOLOGY OF MONGOLIA

its inner edge this ground merges into the more rugged topography of the
mountains, while on its outer margin it slopes gently out into the sediments
of the southerly basin. The intermediate ground is a rock desert, covered
with broken lava and countless fragments of agate, chalcedony, and jasper.
The many-colored pebbles make a brilliant display as they sparkle in the burn-
ing sun. They are the so-called ‘‘Gobi stones’’ of the desert, some of which
are polished by the Chinese for use as jewels.

This is the character of the country for many miles; but still farther to
the east, near the end of the range, the ancient rocks beneath the lavas are
exposed again and exhibit a feature not elsewhere seen on such a scale. In
the distance the ground looked gleaming white,—far whiter than the ordinary
sands. We found that the country rock is an ancient mica schist, not very
different from that seen in many other places, representing the complicated
pre-Cambrian series; but here quartz veinlets and bunches of vein quartz
are so abundant that the ground is literally covered with residuary fragments
of snow-white quartz. This seems to be a very local development, but it is
of the same origin as the veins and pegmatite dikes seen elsewhere. All
were doubtless derived from a granitic invasion from beneath. Special in-
terest was aroused by this extraordinary display of vein material, and care
was taken to look for metallic mineralization; but, as far as observed, the
veins are barren. There is so little metallic content that weathering does
not even stain the quartz. It would be difficult to find, in any region, so bar-
ren an exhibit of vein material developed on an equally large scale.

The south basin

The basin country between Artsa Bogdo and the Gurbun Saikhan is a
sort of connecting lowland between a great open plain extending indefinitely
eastward toward and beyond Djadokhta, and another series of basins lying
to the southwest (Fig. 98, page 194). This connecting basin is underlaid by
sediments. Most of them are fairly substantial sandstones which are not well
exposed. They are covered almost everywhere by a mantle of some kind,
commonly drifting sand. In some places, especially adjacent to the mountain
ranges, the cover is stream wash. The ground surface, however, is not one
of simple accumulation, for the deposits of an earlier epoch, which make up
the strata, have been slightly deformed and lifted from their original position,
and erosion has carried away the uppermost members. ‘The larger features,
therefore, are erosional; only the most superficial and minor forms are de-
positional. The deposits of recent time have tended to fill up some of the
inequalities of earlier erosion, but even this has not destroyed the broad,
gently rolling topography.

Only fragments of fossil bones were found, and these are not decisive as

THE GURBUN SAIKHAN RANGES 255

to exact age, but Mr. Granger considered them to be Lower Cretaceous.
The petrographic and structural habit also gives some suggestion of correlation.
When our traverse was made, nothing was known of the nature of the sedi-
ments lying farther to the north, in the Oshih basin. Later, they were
examined, and are now known to be of Cretaceous age. Their similarity to
the beds seen on the Gurbun Saikhan traverse leads us to believe that these
also are Cretaceous rather than later. As far as we could see in the distance,
no striking changes in the topography of the basin were noticeable; this type
of country prevails over a large area. Perhaps, farther to the west, as well
as farther to the east, still younger sediments have been deposited on the
Cretaceous strata; but in the narrow lowland between the two mountain
ranges, only the older representatives of basin-sediment type were discovered.

APPROACH TO THE GURBUN SAIKHAN

The last miles of this journey took us over alluvial fan deposits built in
a gentle slope leading up to the mountains. In crossing the fans, special
interest was aroused by the variety of pebbles distributed there. They are
prevailingly ancient crystalline types, together with igneous rocks. One peb-
ble, however, is of red limestone crowded with fossils, and is quite unlike any-
thing we had hitherto seen in Mongolia. It is certainly of Paleozoic age and
similar in structural quality and general aspect to the Devonian of America.
Professor A. W. Grabau has since examined this pebble, and says that the
fossil fragments, which include crinoids and corals, together with smaller
fragments of gastropods and brachiopods, suggest either late Devonian or
Dinantian age. We take this pebble as evidence of the presence of mid-
Paleozoic strata somewhere in the Gurbun Saikhan uplift. Subsequent in-
spection of the mountains themselves failed to discover any of this rock in
place; but since the stream course which furnished the lone specimen traversed
a different part of the mountain block from that reached by us, it is entirely
possible that Palzozoic strata are still exposed in some parts of the range.

This piece of red limestone first attracted attention because of its peculiar
appearance, and was picked up while crossing a dry distributary course.
Although half an hour was spent in an attempt to find additional material,
no other fragment appeared. One of the striking things about the whole
summer’s work thus far had been the failure to discover definitely identifiable
Paleozoic strata. The graywacke-slate series on the Siberian border has been
considered by certain of the Russian geologists to be of Paleozoic age, but it
seems to us improbable that the graywacke-slate series we have seen can bear
this interpretation. We were aware that Paleozoic strata had been reported
from the Altai Mountains, much farther to the west, but neither in the Baga

256 GEOLOGY OF MONGOLIA

Bogdo region nor in the Artsa Bogdo had any such rocks been found by us.
Here, however, in the Gurbun Saikhan, more than one hundred and fifty
miles east of Baga Bogdo, in the easternmost of the separate uplifts that
continue the Altai system, the first scrap of evidence was found pointing to
the presence of definite Paleozoic strata. We were later to see considerable
bodies of strata of Palzozoic age, still farther to the east in the vicinity of
Sair Usu, and it is probable also that there are Paleozoic strata involved in
the complex floor of the basin region, in a belt that extends much farther
toward the east.

These remnants of Paleeozoic strata, however, belong strictly to the ancient
floor, and their folded condition has nothing whatever to do with the uplifts
that make the present-day Altai Mountains. Each of the three mountain
areas, Baga Bogdo, Artsa Bogdo, and the Gurbun Saikhan, is strictly a fault
block in its present relations, and the fact that each contains folded strata
is incidental and no more significant than the additional fact that it carries
still more ancient strata and igneous intrusions of many kinds. In this con-
nection, it is suggestive that the piece of Palzozoic rock is not metamorphosed
in the least, nor is it deformed internally, whereas the Khangai rocks—in-
cluding the graywacke series—are, in the same vicinity, considerably modified
in this way. It seems to us that the evidence is fairly conclusive that rocks
of two very different ages are here represented: on the one hand, ancient
strata older than the chief igneous injections and metamorphism; on the other
hand, representatives of Palzozoic age, not nearly so much affected by meta-
morphic changes and not more deformed than is consistent with simple fold-
ing. Although so sweeping a conclusion is surely not allowable on the ev-
idence of a single loose fragment, it nevertheless holds good after the gathering
of considerable additional evidence from the Palzozoic strata of the Sair Usu
region. The lack of all fossil content in the graywacke series, together with
the more complex condition of the rocks, throws a strong balance in favor of
the greater age of the graywacke series as compared with the known Palzo-
zoics, which are richly fossiliferous and are in comparatively unaltered con-
dition.

THE FORMATIONS OF THE GURBUN SAIKHAN

The north margin of the Gurbun Saikhan is formed by a fault (Fig. 113).
The block is uplifted, and the sedimentary formations that border it on the
north are dragged up until they stand virtually on end for several hundred
feet. The material is largely a reddish conglomerate, containing pebbles,
and interlayered with pebbly sandstone. Some of these beds are well ce-
mented and carry much angular material. The dips are steepest at the very

THE GURBUN SAIKHAN RANGES 257

edge of the crystalline rock next to the fault, and flatten gradually in passing
northward away from the mountain, until, at a distance of a quarter of a
mile from the margin, the dips are gentle and even slightly reversed, inclining
southward. Beyond that, they change abruptly again and dip to the north.
The strike of the series is about N. 60° W.

They are probably the same beds which were seen by Koslov (1907) in
his traverse across the Gobi, and which he considered part of the Han Hai

N
pepe Argillites, Grawwackes, Sheared

ediments

Silicified
Limestone ‘as ce aS

oe waches, Lire (or es

I NG
Ce ‘
ES

2k Le
Syenite Sheared_
Diorite Gneiss
reese

FicurE 113.—Generalized geologic section of the Gurbun Saikhan.

beds. The thickness observable here must be at least five hundred feet, but
no attempt was made to measure the beds accurately. Their age could not
be determined, but their petrographic and structural similarity to the Creta-
ceous strata found in the Djadokhta district farther north, suggests the same
general age. A persistent attempt was made to find fossils in the upturned
beds, but not a single one was discovered. The general appearance and
condition of the rocks are very similar to the rocks seen on the Hsanda Gol
and in the Oshih basin. In the absence of definite proof of their age, however,
there is no way of classifying them more specifically than to put them with
the strata laid down on the oldrock floor of the region, the earliest of which
are now known to be of Cretaceous age.

Ancient rocks

A traverse was made from the north margin of the Baron Saikhan to
the top of the first divide, a day’s journey. The rock formations which we
examined are listed in their geographical order from north to south (Fig. 113):

Limestones

A meta-limestone (Specimens 496, 497, A.M. 10,185, 10,186), is the
principal formation of the north margin. Its appearance is deceptive, be-
cause of extensive silication and considerable injection by igneous material.
On this account its primary character is obscure, and it is easily overlooked.

VOL. 1I—17

258 GEOLOGY OF MONGOLIA

Serpentine
Next beyond the limestone there is a considerable body of serpentine
(Specimen 495). There are many irregular, projecting masses of it, cutting
up into the overlying limestone. These are encountered, here and there, over
a distance of two or three miles along the canyon. The original rock was
doubtless a pyroxenite, but other basic types occur with it and are also some-
_what serpentinized. The behavior suggests the presence of an igneous mass
beneath, from which solutions have been given off that have penetrated the
overlying formations. These solutions have furnished the material for trans-
forming and metamorphosing the adjacent rock.

Graywacke-slate series

Farther up the canyon, the great graywacke-slate series was encountered,
and this continues as the chief formation throughout most of the distance
to the summit. The formation is folded so that the strata stand always in
deformed position, and it is much more modified in this block of ground than
in most of the other places along the Expedition’s traverse. There is, how-
ever, no reason whatever to regard it as a different series. The graywacke is
the chief formation seen in the northerly Gurbun Saikhan range. It is cut
by many intrusive masses, among which are granites and also more basic
types.

The same general types continue almost to the top of the divide, where
the rocks are concealed beneath the soils of a beautiful open country which
forms the crest of the Baron Saikhan block. Not only is the country rolling
rather than sharply dissected, but, because of the elevation, it is well grassed,
and flocks are to be seen everywhere. This change, of course, is gradual;
we had noticed in the journey up the canyon that cattle and sheep and horses
were more numerous as the high ground was approached. Here and there
a yurt village nestles in a nook in the valley side, and we seized the oppor-
tunity to replenish our supplies by purchasing a sheep from one of the herds-
men.

Topographic character of the Gurbun Saikhan

The open character of the upland country is most striking. Its com-
paratively mature topography belongs to a previous cycle, which has not yet
been destroyed by rejuvenation. Farther out toward the margin, a new dis-
section has developed a youthful character, leaving only here and there much
smaller remnants of the former topography. In the more distant upland
portion of the range, however, the encroachment of the new dissection is
almost entirely wanting, and the open, broad valleys, with rounded hills
‘between, are the dominant features. The smooth surface is disturbed by

THE GURBUN SAIKHAN RANGES 259

deformation which includes both block-faulting and warping, not very differ-
ent from the deformations noted at very many places in the sedimentary basins.
In all probability, the sea of mountain peaks which comes into view southward
and eastward from the first divide, and which falls into a comparatively even
skyline in almost every direction, marks the trace of one of the ancient pene-
planes, perhaps the Khanghai peneplane, a prominent feature of the Arctic
divide, or the Mongolian peneplane (see Chapter XIX). The peneplane has
been dissected, and the upland valleys and high rolling upland country were
developed in a second attempt to reduce the region by planation. This was
far from complete but must represent at least very long-conitinued erosion.

Lower than the upland peneplane, there are shoulders within the present
valleys, many of which are even now capped with recent conglomerate, well
enough cemented to stand as remnants in spite of the renewed dissection.
These together seem to represent a still later level to which erosion had re-
duced some portions of the ground. Perhaps this last corresponds to the Gobi
planation stage which is prominently represented on the soft sediments of
the plains.

CHAPTER XV

ARTSA BOGDO AND OSHIH

THE ARTSA BOGDO MOUNTAIN BLOCK

ArTSA Bocpo is a mountain unit quite separate from any other. It is
not as high as Baga Bogdo farther to the west, but toward the east no others
rise so high for hundreds of miles. Like the Baga Bogdo, its longer axis lies
east and west, and this makes it appear to be almost in line with the Baga
Bogdo mass as though they formed a single continuous chain. As a matter of
fact, however, Artsa Bogdo is separated from Baga Bogdo by low ground and
is somewhat offset besides, standing a little farther to the south (Fig. 98, page
194). The Gurbun Saikhan, another mountain group farther to the east, is
still more completely separated and stands with more ‘distinct offset to the
south. This is the characteristic structural habit vand arrangement of all
the easterly representatives of the Altai system. Still farther eastward the
mountainous character is not so conspicuous and finally dies out, although
some of the structural characteristics remain.

The representatives of the Altai system in the mid-Gobi owe their uplift
and relief to block-faulting. A row of narrow blocks, whose longer axes lie
east and west, has been elevated and tilted. The whole system dies out
toward the east, each block being lower than its western neighbor. It is an
incidental matter that the rocks involved have exceedingly complicated struc-
tural features and exhibit close folding nearly parallel to the elongation of the
block.

The Artsa Bogdo differs from the Baga Bogdo unit in that the dominant
rock formations are crystalline metamorphic representatives of the ancient
series, whereas Baga Bogdo exposes an immense area of granite, flanked by
metamorphic series. This difference, we judge, is in large part the result of
a different amount of elevation and erosion. Most of the stratified and meta-
morphic cover of Baga Bogdo has been stripped from the underlying bathy-
lithic granite, whereas from Artsa Bogdo the stripping is not so complete and
the usual cover of ancient metamorphosed strata is preserved throughout most
of the range (Fig. 64, page 150).

260

ARTSA BOGDO AND OSHIH 261

The marginal shelf

All along the north margin of Artsa Bogdo a pronounced erosion shelf
or pediment exists (Fig. 114). Here rocks of all kinds are beveled and reduced
to a comparatively smooth surface. In some places the complicated ancient
igneous rocks are involved; at others, later strata, which have been turned
up on edge in the uplifting of the mountain. It must be, therefore, that
this represents a definite attempt at base-leveling. It is, at least, the trace
of some form of planation. Above this general platform rise the mountains
of Artsa Bogdo.

Out in the plains to the north there is a similar relation. Small isolated
hills stand from 100 to 300 feet above the general level. They are made up
of complicated folded strata, largely limestones and schists, all rising from a

oe
g wed
a aev
eer
vow

-Updragged Sedimen MOM ou Peres vneunee

G-
v
“
v
”
v
.
¥
.
.
v
v
%

7
v
v
v
v
-
¥
.
v
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v
»

FicurE 114.—Generalized structure section of the piedmont shelf at Artsa Bogdo.

crude platform level, in the manner of monadnocks. This type of relief,
which characterizes the ground traversed by the Kobdo-Kweihwating trail,
appears to correspond closely to that of the shelf along the margin of Artsa
Bogdo. The origin and correlation of these different planation products is
reserved as an unsolved problem for the next field season of the Expedition.
There are several alternatives. The planation surface may possibly be a
part of the old pre-Cretaceous floor, stripped of its former cover of sediments.
Or it may be the pre-Cretaceous floor, stripped and partially replaned by post-
Cretaceous erosion which beveled the disturbed Cretaceous strata before Ter-
tiary beds were laid down. Or, it may be in part a still more modern erosion
surface. At least, it is certain that since the later sediments were laid down
and deformed, new erosion has succeeded in developing platforms and ter-
races, above which are many remnants, and below which there is considerable
dissection.

History of the shelf —Attention has been called to the deformed condi-
tion of the later sedimentary beds, which have been steeply tilted and then
beveled by erosion along the shelf following the north margin of the Artsa
Bogdo. There are represented three epochs of deposition and three stages
of erosion, with structural relations which deserve further description.

A series of conglomerates and sandstones, with interbedded basalts, is

262 GEOLOGY OF MONGOLIA

turned up on edge, the deformation being due to the uplift of the Artsa Bogdo
block. In some portions of the series, basalts are more abundant, but they,
too, are tilted at steep angles. Subsequent to their deformation they were
planed by erosion; on the new surface lies a much more recent conglomerate,
remnants of which are still preserved. The conglomerate fills the inequal-
ities of the old surface and is preserved now chiefly because, in a more recent
erosion epoch, those portions which occupied the hollows were well protected.
Finally, recent alluvial fan deposits lie on this second erosion surface, and these
are slightly dissected. In some places the original slope is so steep that ero-
sion still continues; whereas in other places there is considerable alluvial de-
posit. This is largely due to warping, but in occasional instances perhaps to
changes in supply of detritus.

The alluvial fans and the protected remnants of older conglomerates are
the most recent deposits seen along the Artsa Bogdo. One group is still
accumulating at the present time, while the other represents an earlier stage
in the development of alluvial fan deposits. Beneath the whole series lies
the floor of older crystallines, representatives of which may be seen along the
inner margin of this pediment or shelf.

Rock formations of the mountain

There was no opportunity for a study of the whole Artsa Bogdo area,
but the range was crossed several times and examinations were made at
widely separated points (Fig. 64, page 150). The most characteristic rock
formations of the area are as follows:

Ancient meta-crystallines and intrusives—There is a great series of very
ancient rocks with considerable petrographic variety, the chief members being:
banded blue limestone, finely crumpled schist, bluish ribbon-limestone. black
quartzite, buff dolomite, granite, injection gneiss.

Folded sediments and associated eruptives—A group of infolded sediments
occupies some of the higher points of the mountain block. . They are conglom-
erates and sandstones, resembling the type regarded by us as Jurassic, but
possibly older. With them are associated porphyry intrusives. How closely
these intrusives are related to the sediments we do not know, but they have
the same fractured habit and obscure structural relations which have char-
acterized the series of porphyries commonly found associated with the Juras-
sic strata.

Later igneous rocks.—The two groups of rock formations noted above con-
stitute the chief foundation of Artsa Bogdo; but in the south side of the block,
both at the east and at the west end, immense tracts of basaltic flows and
bedded tuffs lie unconformably on the others and are distinctly younger than
any thus far described (Fig. 115).

ARTSA BOGDO AND OSHIH 263

The make-up of the mountainous area known as Artsa Bogdo is, there-
fore, a triple complex, the lowest members of which belong to pre-Cambrian;
the youngest represent Cretaceous and Tertiary time, and an intermediate
group is tentatively considered to be of Mesozoic age, probably Jurassic. All

Uy TT HOTT ry

ining
|

He

FicurE 115.—Generalized structure section of the eastern end of Artsa Bogdo.

the sedimentaries, except those of the youngest series, are folded and other-
wise much deformed. The crest of the range is made up, in part, of a syn-
clinal remnant of Jurassic conglomerates and sandstones, the structure of
which is indicated in the diagrammatic sketch reproduced in figure 64, page
150. On each side of this syncline, and beneath it, lie the still older meta-
morphic rocks of much more complicated internal structure.

Crystalline limestones are especially abundant in the ancient series. The
blue limestones are strongly bedded and banded and, at first glance, give the
impression that they cannot be as ancient as are some of the other portions
of the older series. They do not carry fossils, but are completely crystalline
and vary greatly in habit and quality. Some portions are heavily affected
by contact metamorphism. It is hardly believable that so prominent a series
of limestones would carry no fossils if it were of Paleozoicage. The fact that
the limestones are barren and that they are associated with the schists leads
us to conclude that they are pre-Cambrian and belong to one of the oldest
series.

Part of the range is occupied by a very great thickness of coarsely crystal-
line dolomitic limestone which has all the characteristics of a very old forma-
tion. These beds may be simply a different member of the same series to
which the blue limestones belong, but they have a more ancient aspect. Be-
tween them is a crumpled sericitic schist, which undoubtedly has been made
from a shale of some kind. Its minor wrinkled structure resembles the cor-
rugated surface of a washboard so strikingly that in the field we called it the
“washboard schist’’ (Plate XX XIX, B, page 303). In addition, there are
quartzite beds of considerable thickness, some few of which are iron-bearing
and are very black, reminding one strongly of the iron-bearing formations of
the Lake Superior district in North America. None of the beds, however, as
far as observed, carries iron ore of any consequence—the content is much too
poor for any possible use.

264 GEOLOGY OF MONGOLIA

The buff dolomite referred to is a very thick formation, and is the most
disturbed of all of these old meta-crystalline members. It has been exten-
sively modified by the invasion of granite which forms the core of the range.
Because of the structural relations and its petrographic condition, we think
that this dolomite belongs at the base of the series represented here.

Granites cut the old meta-crystallines, coming through the uppermost
members in bosses, Jit-par-lit injections and irregular forms. On the southern
third of the range, granite is the chief rock and is believed to represent the great
Mongolian bathylith. It is cut by many dark-colored dikes which are them-
selves of variable quality.

Many of these rocks show the effect of contact influences, but the lime-
stones and dolomites are most strongly affected. Some of these are so much

hina ite SE ME Se nn

FicurE 116.—Analytical profile of the upland of Artsa Bogdo. The area records three stages of dissection.

modified by silicification and silication that one cannot tell, from a hand speci-
men alone, what they originally were. This is especially true of the silicated
rocks. Even greater confusion results from weathering, when the rock be-
comes a yellowish, crystalline mass. The granites can be followed almost
down to the level of the plain on the south side, where they are overlapped
by basalts and tuffs, and finally, as the next basin southward is entered, by
sediments, probably of Cretaceous age.

Toward the east end of the Artsa Bogdo range there is a remarkable
development of quartz veins in the schist of the older floor (compare Chapter
XIV). They are so abundant that from a distance their milky, glittering dis-
integration material whitens the landscape, like a sheet of snow. Inspection
revealed nothing peculiar in the composition of the ledges, except the abun-
dance of vein quartz. It is unaccompanied by any important mineralization,
as far as observed, and this lack allows the rock to disintegrate and weather
almost perfectly white, instead of staining yellow or brown in the usual form
of gossan.

Tectonic features
The structure of all of the rocks, except the eastern lavas, indicates pre-

vious folding. Plainly there have been at least two widely separated periods
of folding,—one affecting the ancient rocks that are now metamorphosed, and

ARTSA BOGDO AND OSHIH 265

one affecting strata of supposed Jurassic age, which were deposited on the
eroded edges of the ancient series. Although the younger conglomerates and
sandstones must have been laid down essentially flat, they also now stand on
edge and form a beautiful syncline, a remnant of which constitutes a portion of
the highest ground in the range. This is the latest stage of the folding, which
in all cases had an east-west axial trend. One is at first inclined to think of
the folding as belonging to the uplift which has made the mountains, but this
is not the case. Any other block of the ancient floor, lifted above the sur-
rounding region to sufficient height, would present similar features, —that
is, ancient crystalline metamorphic rocks with nearly east-west strike and a
structural relation indicating ancient folding. It is certain that the rocks of
such an uplift as Artsa Bogdo do not owe any of their folding to present moun-
tain uplift, which is of an entirely different origin.

The key to the present mountain structure may be seen along the north
margin of the Artsa Bogdo block. Isolated, craggy hills rise abruptly above a
very gently sloping shelf, over which one can drive with a motor. At the
mountain front, however, there is an abrupt change—so abrupt and steep
that one cannot proceed except on foot. That a block has been lifted above
the marginal plain by faulting is shown by the structure of the adjacent strata,
beneath the shelf at the foot of the escarpment. At a point where a new
stream channel has cut a trench into the shelf, later sediments are turned on
end for a distance of several hundred feet. The age of these upturned beds
is not determined, but they are comparatively unconsolidated and have all
the petrographic and structural habit of the Cretaceous and Tertiary beds
seen at many other places in the basin regions. It is clear that, since they
were formed, a portion of the block which formerly lay below their level, has
been thrust up along a great fault, and the broken edges of the overlying
strata have been turned up by the drag. Once they must have extended
across even the uplifted block. It is necessary to conclude, therefore, that the
uplift has taken place in comparatively late geologic time—certainly some
time in the Tertiary—and the features of erosion in the upland portion of the
mountains indicate that there have been at least two such upward movements,
and perhaps others. This estimate is consistent, also, with the extreme rug-
gedness of the northern margin. Uplift has been by stages separated from
each other by more stable conditions, and the latest increment of this move-
ment has been so recent that even the marginal rejuvenation has not caught
up with it.

Traces of former sedimentary cover

With such a dynamic history, one would expect to find evidence that
ancient folded rocks of the Artsa Bogdo mountains were once covered com-

266 GEOLOGY OF MONGOLIA

upland valleys.

pletely by younger formations. This is precisely what one sees as he looks
along the range toward the east, for there is no mistaking the structural char-
acter of the distant parts of the uplift. Instead of a rugged outline, formed
by the ancient rocks standing on edge, the topography is decidedly simple,
and the structural lines lie nearly horizontal, but bend gently down toward
the south (Fig. 115). These lines are formed by a series of lava flows, tuff
beds, and sediments that still cover a portion of the uplifted block. We do
not know their age, but they lie unconformably on the oldrock floor and descend
to the south beneath strata of Cretaceous age. Such lavas are abundantly
associated with strata which elsewhere have been proved to be of Cretaceous
age. On these bits of evidence we judge them to be Cretaceous also. This
seems to have been one of the great centers of volcanism of that time. Few
localities in Mongolia furnish so impressive a series of basalts. The whole
southerly slope of Artsa Bogdo is smothered by the lava cover. The basalts
are stripped and deeply eroded on the higher flanks of the range; they are
simply stripped and but slightly dissected on the lower slopes, and along the
basin margin they are still covered by the sandstones which once must have
had much greater extent.

Topographic features

Within the higher parts of the range there are smooth, open, park-like
tracts between the rugged dividing ridges. They look like ancient stream
valleys, belonging to a former cycle of erosion, which was so long-continued
that mature valleys with broad subdued slopes were developed even in the
higher mountains. The modern streams have cut many deep, sharply V-

ARTSA BOGDO AND OSHIH 267

FicureE 117B.—Continuation of figure 117A. Showing the rugged canyons dissecting the abrupt front, and
the broad basin, north of the range, in the background.

shaped trenches across and along the older valleys even to 1,000 feet deep
(Fig. 116). The redissection has been on so gigantic a scale that the gentler
slopes of the upland parks have been almost completely destroyed over large
areas. Thus there are two types of valley in the Artsa Bogdo—youthful,
sharply incised trenches, superimposed on broad, mature valley floors, which
were themselves the product of erosion of a still more ancient surface, per-
haps a portion of the original upland peneplane. The new dissection is prob-
ably due to an uplift, after a period of stability long enough to permit open,
gently sloped, mature valleys to develop.

Stream pattern.—The stream courses of the newer dissection do not fol-
low either the structure of the rocks of the trend of the older upland valleys;
they cut quite independently across all of these features (Fig. 117). Evidently
they were laid out on an entirely independent plan. The drainage pattern
is superimposed. The major streams probably took their courses on the simple
upwarped sediments which formerly covered the block, and, now that the
cover has been stripped off of the core in the central portion, these streams
still maintain much of their original pattern, with partial adjustment to struc-
ture in the tributaries.

Glaciation.—The heads of certain valleys along the northern front of the
range are bowl-shaped, and resemble cirques. They are developed on hard
rock, apparently quite independent of minor structural control. They are
very small for true cirques, but both the form and the distribution of débris
led us to believe that the only satisfactory explanation for the feature was the
accumulation of ice. Only two or three bowl-shaped valley-heads were noted,
and they are not at the highest points of the range. It is our opinion,

268 GEOLOGY OF MONGOLIA

therefore, that glaciation has been a negligible factor in the Artsa Bogdo.
The evidence favors the former presence of a few small ice bodies of the alpine
type, but the features are not prominent, and this kind of erosion never became
important here.

In discussing the topographic features, it has already been noted that the
upland portions of the Artsa Bogdo are characterized by many park-like ex-
panses, with comparatively smooth, grassy floors, separated by more rocky
divides. These we attribute to ordinary erosion work carried nearly to old
age, at a much earlier stage in the history of the block.

THE OSHIH HOLLOW

While the geological party was engaged in making the side traverse to
Gurbun Saikhan, Mr. Granger explored the paleontological possibilities of
the depression to the north of Artsa Bogdo, in a district known as Oshih
(Ashile), and on our return we spent a day with him in this locality. The fol-
lowing year, additional time was spent there. The traverse from the main
camp to Oshih covered about forty miles, the first ten of which led down over
the alluvial fans and piedmont slope, stretching out from Artsa Bogdo into
the basin to the north; then the traverse continued northward across the
basalts and sediments lying on the ancient floor. Most of these are quite
unfossiliferous, and in much of the territory the strata are not well exposed
and the structure is somewhat obscure.

Features of the locality

The depression known as Oshih is not a structural basin, but is essentially
a desert hollow about eight miles wide from north to south, and at least fifteen
miles long from east to west (Fig. 118). The erosion features, therefore, are
particularly impressive. The high colors of the strata and the badland dis-
section stand in striking contrast to the simple features of the surrounding
rolling plains. A red mesa about three miles long, capped with black lava,
stands in the middle of the hollow. It is a striking landmark (Plate XVIII,
B, page 146, and Fig. 118). At the eastern end of the mesa, one enters a great
amphitheatre-like depression, extending about three miles to the top of the
hill Oshih Nuru, which forms the eastern end of the Oshih hollow. The sides
of the amphitheatre are fretted into a splendid badland area (Plates XXXV,
B, and XXXVI). From the north wall of the hollow, a great bluff about ten
miles long, called Urulji Nuru, one can look northward over a perfectly simple,
almost level topography representing the Gobi erosion plane; but in the
depression everything is minutely dissected down almost to the very bottom.
The myriad gullies, joining into fair-sized canyons which open into smooth-

PLATE XXXYV.

er
om-ce-e

annonbal

A. A FAULT ZONE IN THE OSHIH BASIN,

B. THE BADLANDS AT OSHIH NURU.

View northeastward toward the badlands and faulted lava flow at Oshih Nuru.

PLATE XXXVI.

A. THE TILTED LOWER CRETACEOUS STRATA AT THE FOOT OF OSHIH NURU,

B. CANNONBALL CANYON,

Lenses of gray, cross-bedded sandstone between red, clayey sands in the Cannonball member of the Oshih formation (Psittacosaurus
beds).

ARTSA BOGDO AND OSHIH 269

floored valleys, indicate that the dominant erosive agent in the hollow is
running water (Fig. 118). Several inliers of oldrock stand as considerable
hills at the western end of the hollow,—two of them, called Baron Khara and
Dunde Khara, being made up of folded porphyries, chiefly trachytes of the

Camp arog Khara Dunde Khara Baga Bogdo Urulji Naru
Dsun cs ial

PEAR NO rrr gaa PTT Cer Pore
— = 2 (=

‘kelch from western head of Oshih Mountain looking mestward. Shows:
Distant profile of Baga Bogdo~a crossprofile from N to S.~Feneplane +
Tnliers in Oshith Basin ~ Dsun Khara = limestone + syenite, monadnocks.

‘Donde Khara and Baron Khara = porphyries.
Sediment -shoulder of Urulj Nuru overtopping the oldrock éniiers. This sedimentary
mass continues westward toward Ondat Sair

Wo Java-capped mesas, one at Camp + one at foot of Baron Khara.

Lava sheet and higher sediments tn Oshih Nuru.

FicurE 118.—The mesa and stripped inliers of the Oshih basin. Baga Bogdo, seen at a distance of
about fifty miles, exhibits a typical cross profile of an eastern Altai range. The high scarp at the right, called
Urulji Nuru, represents later sediments that once extended over and entirely covered the hard-rock ridges of
the middle distance. Two areas of badlands, one capped by a lava sheet, lie at the foot of these ridges. The
same lava covers the broad mesa in the center of the sketch, and it reappears, upfaulted, in Oshih Nuru, in the
extreme foreground. The sketch emphasizes three of the dominant processes of the later Mesozoic and Tertiary
history: (a) accumulation of sediments, (b) later deformation, (c) local stripping.

type which we have elsewhere seen associated with Jurassic strata. The
westernmost inlier, Dzun Khara, is of ancient crystalline limestones, invaded
by syenite and granite. There are extensive badlands at the foot of these
oldrock inliers, but time was not available to search them adequately for fos-
sils.

The Oshih strata are not folded, but are somewhat tilted, and are cut by
numerous faults. The dips are steepest near the faults, where, in many cases,
the strata are dragged into a vertical position (Plate XXXV, A). All orig-
inal fault-scarps have been eroded away, and even the redissection of the
region has brought about but little topographic expression of the faulting
(Fig.119). Thestrike of the faults curves around ina peculiar manner. They
are all ‘‘normal’’ faults, dipping toward the downthrow side, which, in most
cases, is toward the west. The throw is believed to be moderate; where it
could be measured, it ranged from as little as three feet to sixty feet, but in
many instances the throw is much greater. Along several faults, recognizable
groups of strata have moved into juxtaposition, and by this means we know

270 GEOLOGY OF MONGOLIA

that the movement must be equal at least to the thickness of the units cut out
by the faulting. The full thickness is not exposed for any one of the recogniz-
able members of the Oshih formation in the eastern badlands, but even the

"FIELD SKETCH From
WW Ap? > Book Vill. 1923 p.si Sho
from Book VIII 1923. p.5i | fT Wud pA pp es= ay Fault striking N 70° E

looking North. Shows AN EDEY b) bed No.4 downthrown
ay Fault, a Paikling NB8°E. by Bed No VN 09 Now S neo No.4 ve ekg
upthrown. cy Bed No4, downthrown Nf \ fs 04 & Orag elava cap
MT cw
FiGuRB 119.—Two fault zones in the Oshih basin. The sketches record the result of deformation in post-
Cretaceous time, as expressed in the modern erosion features.

visible parts are enough to indicate that movements of several hundred feet
have taken place.

Most of the curving faults strike toward the north, in a direction approxi-
mately perpendicular to the Altai front. It is quite impossible to determine
the date of the dislocations, beyond the fact that they were certainly later
than the deposition and cementation of the sediments, and earlier than the
carving of the very level upland surface which bevels the disturbed sediments
round about the Oshih hollow. These limits would place the date of faulting
some time between Lower Cretaceous and Pleistocene. However, a sugges-
tion is offered by evidence gathered in the region around Uskuk mountain,
farther west. The Ondai Sair beds carry a fauna which the paleontologists
correlate very nearly, if not exactly, with the Oshih fauna. The Ondai Sair
formation was faulted, and the fault-scarps were leveled by erosion before the
early Tertiary beds of the Hsanda Gol formation were laid down. There, at
least part of the faulting took place before Lower Oligocene time. By impli-
cation it may be argued that the faults of the Oshih region are of much the
same age.

The geologic column

The following groups of sediments are recognizable:
1. A group of greenish-gray sandstones alternates with red, hard, siliceous
clays and red, concretionary sandstones. This is the ‘“‘cannonball’’ group, in
which spherical sandstone concretions are found. It is surely more than two

ARTSA BOGDO AND OSHIH 271

hundred feet thick, lies nearer than any other unit to the base of the Oshih
formation, and is brought to the surface only in the upthrow blocks along
the larger faults. The greenish sandstones, cross-bedded and of variable thick-
ness, are best interpreted as channel fillings (Plates XXXV, A, and XXXVI,
B). Reptile bones, including a very perfect skeleton of the small parrot-
beaked dinosaur, Psittacosaurus, were found in the red sandstones.

2. A group of thin-bedded red sandstones alternates with thick beds of
hard, red, sandy clay. The sandstones are less commonly greenish gray, but
the clays form the principal ingredient. In one section, one hundred and
eighty-five feet could be seen; but as the bottom is not exposed, the thick-
ness is surely greater than this figure. No fossils were found in this group.

3. A group of pink and buff clays. The clays are color-banded, and
locally contain thin partings of sandstone. They lie immediately under the
lava bed, and form the body of the central mesa. The thickness is unknown,
because the base is concealed by the red clay-and-sand group, which is up-
faulted at the eastern end of the red mesa (Plate XVIII, B, page 146, and
Fig. 119).

4. A gravel group of rather uncertain position. It is found downfaulted
against the cannonball group, as is the younger unit, and it seems to bear
the same relation to the red clay-and-sand group. It forms a high hill south-
east of camp (Fig. 118), where a thickness of at least two hundred feet is vis-
ible. The bottom is not exposed, and the upper part has been eroded away.
It is probable that gravel members are a matter of locality rather than of
horizon, for a thick gravel deposit underlies the lava cap in part of the hill
Oshih Nuru (Plate XXXVI, A,), while fine pink clays underlie the same lava
at the red mesa.

5. A group of brown clays overlying the lava flow. It is very well ex-
posed in the eastern end of Oshih Nuru, and brown clays that may be its
equivalent are seen in the north wall of the hollow at Urulji Nuru. The gigan-
tic sauropod A siatosaurus was found in this layer.

6. A group of gray, thin-bedded deposits, shales, sands, and limestones.
The latter are thin sheets of calcium carbonate—some beds composed of ver-
tically directed columnar crystals, others of cone-in-cone structure. The
shales include thick deposits of typical carbonaceous paper-shales in which
our brief search failed to reveal any fossils except some macerated plant frag-
ments. Selenite crystals were present, suggesting that possibly the shallow
lakes, in which these beds were laid down, were bitter waters that did not sup-
port the rich fauna which at Ondai Sair is preserved in paper-shales just like
these in all respects, except for the absence of gypsum.

7. At Oshih Nuru, only gravels, which may be of recent age, cap the gray
beds. But on the north flank of the Shah Ola, in a high hill of conglomerate

272 GEOLOGY OF MONGOLIA

some eight miles east of camp (Fig. 98, page 194) the paper-shales are suc-
ceeded by fine red sandstones of well-assorted quartz grains. They closely
resemble the Djadokhta sand; but twenty miles farther east, near the temple,
Sairim Gashato, these sands were found to contain bones which Mr. Granger
thinks represent the Oshih fauna. In a concretionary stratum of the Sairim
sand, some thick-shelled pelecypods, very like those collected at Iren Dabasu,
were found.

The measurable thickness of these units does not indicate the true total
thickness of the beds in the Oshih basin. Neither the top nor the bottom of
the local series of beds is in sight. The simple elevation difference between the
bottom of the basin and the south basin wall is 860 feet, neglecting the dip,
which, in this part of the basin, is slightly southward, so that it would have a
tendency to increase the total thickness. At least 900 or 1,000 feet might be
given as directly measurable in this way. When the north valley side alone
is estimated for thickness, it measures, in simple elevation, difference from the
north wall to the bottom of the valley, 560 feet. The valley is two miles wide,
measured from the red mesa, and the average dip is something approaching
10°. This figure would allow an additional thickness, within the range of
two miles, of 740 feet; so that the total thickness of exposed strata on that basis
would be about 2,300 feet. It was impossible to check these figures closely;
but from them alone it is reasonable to place a thickness of approximately
2,000 feet for all the strata exposed in the Oshih basin. Half of this is directly
measurable, and the rest of it is estimated on dip and distance. (Specimens
Nos. 534, 535, 536, 537 and 538, A.M. 10223, 10224, 10225, 10226, 10227.)

On the south wall of the hollow, opposite the western end of the red
mesa, there are many stacks or pillars of chalcedonic silica, which look, at
first glance, like a grove of petrified trees. They are all in one small area near
the foot of a lava-capped cliff, and have weathered out of fine clay-like sedi-
ments which may prove to be volcanic ash, and which dip gently toward the
southeast, passing underneath the basalt cover. The stacks seem to repre-
sent a replacement of ash by colloidal silica, along vertical channels which
served as conduits for the escape of uprising hot waters, at the time of vol-
canic activity. It is possible that they may be silica fillings of geyser necks,
but they appear to lack the concentric structure one would expect in this case
(Plate XXXVII).

The overlying lavas at the south rim of the hollow are several hundred
feet thick, and dip south and southeastward toward the Altai front. They
contain numberless amygdules of chalcedony, agate, and quartz, which on
weathering out, literally cover the ground. They are called ‘‘Gobi stones’”’
by travelers.

The Oshih sediments and their associated lavas dip generally southward

PLATE XXXVII.

RESIDUARY STACK OF JASPERY SILICA BELONGING TO THE ASH BEDS OF OSHIH.

=
° =
7 s
‘
7 -
3 - : -
ra
‘
a é¢ ~
4
‘
*
’

= a iy
o«
a alae
=-

a : y
= e ~~ 7
= —— Sa. ae, eS =

ARTSA BOGDO AND OSHIH 273

toward Artsa Bogdo, just as the Cretaceous and Tertiary sediments at the foot
of Baga Bogdo dip southward toward the Altai front. If the Oshih be the
same formation as the succession of lavas and red sediments seen by us south
of the Artsa Bogdo, it may very well be that the site of the present range
formerly was covered by the Oshih formation.

VOL. 11—18

CHAPTER XVI

PROBLEMS AND AREAS DESERVING SPECIAL STUDY

INTRODUCTION

OnE of the specific objects of the Expedition was to determine the nature
of the geologic problems of Mongolia and to locate areas where additional inves-
tigations would be likely to produce results. The first season was strictly a
reconnaissance venture, and it could best meet the original requirements if
its findings proved suggestive and supplied reliable guides to new fields of
investigation.

When the Expedition started, there was complete uncertainty regarding
the prospect of locating fields of palzeontologic importance; but, fortunately,
several productive localities were found, and it was possible at the close of the
season to indicate with precision the most important of these fields and to
suggest the nature of the returns that might reasonably be expected from them.
The results of the second season fully corroborated these forecasts.

In similar manner, it is now possible to indicate the nature of other geo-
logical problems suggested by the studies which were made in the interpre-
tation of the geologic structure and the geologic history of the region. In
some cases, these problems are more difficult to carry forward than are the
locality studies in proven fossil fields; but their nature can be stated quite as
definitely, and it is possible also to indicate the district in which important
data pointing toward their solution can be obtained.

A great number of minor problems, also, have presented themselves in
the course of these field studies. Many of them, of course, are only partly
solved. It is beyond the purpose of this chapter to state them. They are
characteristic.of every such investigation, and have to do with the working
out of geologic structure and history in greater detail and with greater cer-
tainty.

In the course of the reconnaissance season, progress was made on several
special locality studies, as opportunity offered, and the results are included
in the preceding chapters. In each case, we have been rewarded for the

274

PROBLEMS AND AREAS DESERVING SPECIAL STUDY 275

effort put on them, either because of important faunas discovered or because
of suggestive structural relations determined. The special districts embrace
the study of the Iren Dabasu basin, the hot springs area of Sain Noin, the
Uskuk Mountain area. In the second season, 1923, certain problems, sug-
gested by the first reconnaissance, were pursued with excellent results. They
include the special stratigraphic and palzontologic studies of the Djadokhta
district, the Shara Murun locality, Ardyn Obo, Oshih (Ashile), and a restudy
of the Iren Dabasu-Irdin Manha district.

Those suggested for additional consideration fall under the following
heads (Fig. 120):

SEDIMENTARY BASINS AND POTENTIAL FOSSIL FIELDS

The area of the great pass

In the Wan Ch’uan Hsien Pass, above Kalgan, the trail crosses later sed-
iments for several miles as it approaches the top of the escarpment on the
edge of Mongolia, where strata, at least 5,000 feet in total thickness, are

Z TUERI
aly, aan
ie
eos!

"tle,

PRETO &
ese

im.
Ge

FicurE 120.—Outline map showing the location of the places deserving additional study.

exposed by rejuvenated erosion (Fig. 5, page 47, and Plate XLI, A, page 389).
Most of the beds are unfossiliferous, but a few have been found to carry

276 GEOLOGY OF MONGOLIA

fragments of fossil bone, and others, obscure plant remains. They are
clearly a great series of sediments of the same structural relations as the later
sediments of the desert basins proper, and they appear in much greater thick-
ness than is represented in any other locality of our experience—unless it be
in the Tsagan Nor basin. It is entirely likely that more than one series of
strata is represented. Their exact age is not known, although we infer
from the fragmentary material in hand that the greater part of them are of
Cretaceous age. The area of the great pass is, therefore, one of the most prom-
ising localities for special study. It is not difficult to reach, and it promises
important evidence in the chain of geological history.

Following up the conclusions reached at the close of the first season,
1922, Prof. George B. Barbour of Peking was advised to make a further in-
vestigation of the area. Several brief notes have appeared, and additional
results are promised in due time (Barbour, 1924).

P’ang Kiang

P’ang Kiang lies on the main caravan trail toward Urga and is the first
great basin north of the pass where strata of later age are well exposed (Figs.
10, page 53; 99, page 198, and 100, page 199). No very great thicknesses
appear, but there are long stretches of erosion-scarps and valley sides, where
strata can be examined in detail. The beds are chiefly unfossiliferous, although
the one fossil found indicates Tertiary age. It might prove to be a fossil field
of some consequence, but in any case it is a field where additional stratigraphic
evidence is available, and it lies within easy reach of Kalgan as a center of
operations.

The paper-shales of mile 299

A basin of moderate size is crossed by the Urga trail about 290 to 300
miles from Kalgan (Fig. 16, page 62). The area was crossed rapidly, with
only two halts in the basin, one of them at mile 295, where reddish sediments
of coarse grain and somewhat limy character were seen, but no fossils were
found. A few miles farther on, at mile 299, after crossing the Gobi upland,
the trail passes down over an erosion-scarp into a depression in which are
several outlying remnants of shales. An inspection of two of these showed
the presence of a very fissile paper-shale of black carbonaceous appearance
and, as far as observed, without fossil content. Only a few minutes were
devoted to the inspection, and the failure to discover fossils should not dis-
courage further study of the locality. This was the first encounter with the
paper-shale type, and its importance was not appreciated. Later studies,
however, several hundred miles farther west, revealed a very important fauna

PROBLEMS AND AREAS DESERVING SPECIAL STUDY 277

of Lower Cretaceous age in exactly similar paper-shales at Ondai Sair (Chapter
XIII). Apparently the area at mile 289 carries the same series, and if the
beds were inspected carefully, following up the outcrops both to the east and
to the west of the trail, it is almost certain that an important link in the his-
tory of the region could be determined. It is one of the few places in the
Gobi region where paper-shales have been seen.

The flaming cliffs of Uskuk

On the west side of the Uskuk area, which was made the subject of a special
areal study on the reconnaissance, there is a remarkable exposure of variegat-
ed and highly colored sedimentary beds, called in the field ‘‘the area of the flam-
ing cliffs.” They are undoubtedly a continuation of the sedimentary series
underlying the Tsagan Nor basin (Fig. 120). We believe that they represent
the lower part of the Tertiary column, and probably are to be correlated with
the beds exposed in the badlands of the Hsanda Gol, already described (page
233). At the new place the beds are turned up on edge against the south
Uskuk block (Fig. 121), so that minute inspection could be made, and these

125 450

same conditions appear to extend far to the west. How well they are exposed
farther along in that direction, we do not know, but the structural conditions
indicate their continuation. They are probably exposed at other places along
the same fault line that marks the south side of the Uskuk block. The beds
actually examined at the flaming cliffs proved to be poor in fossil content, but
there is reason to believe that productive spots could be found.

The strata are, of course, closely related to the strata described in the
Uskuk mountain area; but it may be that the lower members of the series not
so well exposed in the places already studied, could be seen in greater detail
on the westward extension. It is apparently as encouraging ground as the
Hsanda Gol locality itself, and promises results, both as a fossil field and as a
basis for a better understanding of the stratigraphy of the later sediments.

Red sandstones and sands, lying on the complex rocks of Mt. Uskuk
Reddish clayey sands with red and gray layers
Dark drab and black beds with central zone of red
Variegated thin-bedded red and white strata
Red and buff thin-bedded clays and sands
White and drab beds and thin red layers
White and gray sands and gravel (
Reddish sands Red clays with concretions 65 \

Black lignitic beds, alternating with yellow gravel
Yellow beds and gray sandstones
and sandstones Red Cle6 aN

25 320 305
v v av .

278 GEOLOGY OF MONGOLIA

Oshih (Ashile) basin

Out in the plains country, twenty-five miles north of the Artsa Bogdo
mountain block, a great series of Lower Cretaceous sediments is exposed in
fine erosion remnants and badland country. (See Chapter XV, and Plates
XVIII, B, page 146; XXXV, XXXVI, XXXVII, and Figs. 118, 119, pages
269 and 270.) The locality is known as Oshih. Several days of exploratory
inspection during the first season revealed dinosaur-bearing strata, and one
excellent skeleton was found by Mr. Granger. The large area of splendidly
exposed beds offered encouragement for further study.

Only ten days’ inspection could be given to the locality in the second sea-
son, so that, despite the work of two summers, the vast badlands of Oshih
remain a legacy for future investigators. In 1923, other areas of badlands
were discovered, east and west of the original camp. A few fossils were
found in the new eastern extension of Oshih, some miles north of the temple
Sairim Gashato. Several areas of paper-shales like those of Ondai Sair were
found, but no fossils were seen in them.

Undoubtedly there are exposures of like nature at other places in the
large basin extending eastward from this point; from the mountain-tops it
is possible to discern badland areas in the distance. Perhaps the region to
the north would also be a promising area for additional inspection, but, in
all probability, the strata in that direction are of later age than those at
Oshih.

Djadokhta

A locality, known to the natives as Djadokhta, is reached on the Kobdo-
Kweihwating trail, almost due north of the great mountain mass of the Gurbun
Saikhan (Plates I, XIX, page 156; XX, B, page 157, and Figs. 67, page 159; 152,
page 356). The place was discovered and the nature of its fauna determined
on the homeward traverse of the reconnaissance expedition in September,
1922. Only two or three hours were actually spent in inspection of the local-
ity, but in that time an unusual number of dinosaur specimens was recovered,
together with one dinosaur egg. These finds showed that not only were the
remains abundant, but, more important still, they were of a rare and unfamiliar
type. The urge to hurry the return journey prevented more detailed exam-
ination, but enough was done in a few hours to prove that here is one of the
great fossil fields of Mongolia; and on the return of the Expedition to Peking,
when the following season’s work was tentatively laid out, Djadokhta was
listed as one of the principal places for detailed examination. The results of
the following season, 1923, fully corroborated the opinion, for the locality
yielded an extraordinary harvest of discovery. The fossil remains, even those
of delicate structure, are unusually well preserved, showing that peculiarly

PROBLEMS AND AREAS DESERVING SPECIAL STUDY 279

favorable conditions must have prevailed at the time of deposition and bur-
ial. Twenty-six dinosaur eggs were recovered in the course of a month of
exploration in 1923, with no less than thirteen nearly complete skeletons and
more than seventy skulls. In addition, a few,skulls of minute, rare mammals
were obtained.

The area, of course, is not exhausted; but sufficient work has been done
to show the range of possibilities. In a formation so favorable to the preser-
vation of the frailest material, there is encouragement to further work.

Overlying the Djadokhta sands is a group of clays and gravels called the
Gashato formation, which yielded a very small collection of rare and primitive
mammals belonging to the Paleocene. The extent of this formation is enor-
mous, and as yet only a few days’ time has been spent upon it.

North and northeast of Djadokhta there is an immense basin, well-exposed
in places, on which almost no work has been done.

Ardyn Obo

At Ardyn Obo, on the trail between Sair Usu and Kalgan, a great erosion
escarpment, which stretches westerly as far as the eye can reach, exposes
more than two hundred feet of Tertiary strata (Plate XXI, page 182; Figs.
83 to 86, page 175, and 137, 142, page 324). An inspection of these beds was
made during the first season, at the point where the trail touches the escarp-
ment, and several skulls of Rhinocerotidze were found. Fragments of other
forms were found also, but the beds, as a whole, were not very prolific. Appar-
ently the district is one well worth additional exploration in the prospect of
finding beds of more varied content. It is favorably located on a main trail,
but probably has the disadvantage of a narrow range in age.

Shara Murun

At Shara Murun on the Sair Usu trail, one hundred and five miles southeast
of Ardyn Obo, where a halt of only half an hour on the return journey revealed
fossil bones and teeth, a locality of special promise was discovered (Plate
XXII, B, page 183; and Figs. 89, 90, page 180; 99, page 198; 100, page 199; 143,
page 339; and 146, page 340). An exposure of later sediments, carrying fossil
remains, extends for several miles. The examination at that time, although
insufficient to determine the possible range, was enough to show clearly that
the locality deserves much more attention. Further examination was made
the second season, and the connections of this basin with the Irdin Manha to
the east were reasonably well traced, yet the extent of the basin is so great
that it seems wisest to class it among the less explored regions as a hopeful
project for further study.

280 GEOLOGY OF MONGOLIA

Ulan Nor

Ulan Nor appears to be an area of deposition at the present time. The
Expedition on the return journey passed about fifteen miles east of the lake,
and crossed several miles of deposits where anastomosing streamlets are
now stripping the basalts lying to the east and distributing the sands. It
is a difficult country to traverse—dangerous for heavy motor cars—but it is
one of the few places in the Gobi region where the Expedition encountered
active deposition. On this account it ought to repay critical examination.
Here one might make out the nature of the deposits made under present cli-
matic conditions. Somewhere in the region, sediments must still be accumu-
lating, and perhaps this section would throw additional light on Pleistocene and
Recent history. It may hold the key to a better understanding of the later
sediments of the Gobi region, and it is well worth the critical examination
recommended. It does not necessarily promise important fossil returns; but
if it produces any returns at all, they are likely to be quite different from those
already secured and would aid materially in unraveling the problem of the
later geologic history of Mongolia (Fig. 98, page 194).

The badlands of the Ongin Gol

The Ongin Gol has trenched the Gobi upland for a hundred miles, and
in places the strata of the plain are exposed (Figs. 69, 70, page 160). On the
whole, however, there is little encouragement for examination of the Ongin
trench itself. Twenty or more miles to the west of the river, a short distance
south of the latitude of the temple Ongin Gol In Sumu, one can see extensive
badlands (Fig. 98, page 194). Brilliantly red strata are exposed; and un-
doubtedly there is a favorable opportunity for inspection of additional sedi-
mentary strata belonging to the great series of basins lying between the foot-
hills of the Khangai Mountains on the north, and the Altai uplifts on the
south. It is probably a difficult locality to reach, but it is accessible from the
vicinity of Ongin Gol In Sumu with a camel train, even if not by motor.

STRUCTURAL AND HISTORICAL PROBLEMS

In the following list a different type of problem is presented. In cer-
tain of the localities mentioned, the chief feature is one of structural relation,
while in others the major point is one of age, correlation, and historical mean-
ing.

The Camp Jurassic area

At approximately three hundred and seventy-four miles north of Kalgan,
the main Urga trail crosses one of the outcrops of Jurassic strata. For a few

PROBLEMS AND AREAS DESERVING SPECIAL STUDY 281

hours before darkness set in, we inspected the locality and found that the place
exhibits extraordinarily well some of the critical structural relations of the
Gobi region. Here later sediments are tilted on edge by a sharp flexure which
brings the oldrock, in this case represented by Jurassic strata, within reach of
surface erosion (Plate XI, A, page 61, and Figs. 18, 19, page 65). A small,
unsymmetrical, structural dome is breached so that three different series of
strata are exposed. We judged, from the short inspection given, that Tertiary,
Cretaceous, and Jurassic strata are all involved in this structural unit, which
is one of the smaller and sharper uplifts of the Gobi basin (Fig. 122). With it
are associated basaltic eruptives that may have special significance, and the

Domed Cretaceous Strata Cuesta of Tertiary Strata

see
ene
* sla
meee

Annular Vatiey

FIGURE 122.—The northern slope of the dissected dome at Camp Jurassic showing cuestas and annular
valleys.

typical erosion features of a dome are beautifully developed. The intensive
study of an area, five or six miles from north to south and perhaps ten miles
from east to west, would make one of the finest contributions to structural
geology afforded by the Gobi region. It promises fossil remains in all the
formations, although it is not recommended as a fossil field. Even in the
short time devoted to it, this place proved to be especially suggestive and
helpful in working out the structural and stratigraphic history.

The crystalline upland east of the Ongin Gol

Between Canyon Brook and the Ongin Gol, along the trail toward Sain
Noin, there is an upland block of complex structure (Fig. 47, page 117, and
Fig. 49, page 121). In it are ancient crystalline metamorphics made from
sediments of various types, cut and injected by dikes and stringers derived

282 GEOLOGY OF MONGOLIA

from the granite bathylith. The rocks belong to an era promising returns
along structural, stratigraphic, and historical lines, as well as in physiographic
forms. It is a region of depressions in the granite, which are deep enough to
hold water and are without adequate explanation under present conditions.
The ancient sediments in the block exhibit a succession of conglomerates,
limestones, and slates, showing different degrees of metamorphic complexity,
and apparently representing several entirely different formational series of
ancient types.

The ground was covered too rapidly to enable us to make the required
determinations of structural relation and succession. This is especially un-
fortunate because we rushed through one of the few localities where we might
have been able to distinguish the different ancient series. An area of complex
gneisses ends abruptly against a zone of conglomerates, which is succeeded by
less complex schists, by slates, phyllites, and more conglomerates, and then
by graywackes overlaid by Jurassic strata. How important the breaks are
between the different series and what significance the conglomerates have,
we do not know, but in this locality these questions could be determined.

Transition country of Gangin Daba

Approaching the mountain country south of Urga, the trail crosses a
broad succession of gneisses and schists of various kinds, of high structural
complexity and apparently of great age. They continue with little change—
except that they apparently become somewhat more simple in petrographic
character—to the vicinity of the mountains which separate the desert from
the Tola River valley (see Chapter IV). Between miles 619 and 623 a change
takes place (Fig. 27, page 81). Instead of schists, phyllites, and gneisses, the
country rock becomes wholly graywackes, slates, and quartzites, with a few
jasper beds. The graywacke-slate series is certainly simpler in petrographic
quality, and therefore is regarded as younger, although the relation between
it and the formation next to it could not be ascertained. This critical strati-
graphic and historical point could be determined here but the Expedition
rushed past without stopping. The whole stretch of more than twenty miles
deserves detailed structural interpretation, and it would”yield definite returns
toward the unraveling of the structural features of the pre-Cambrian.

Sair Usu

Nine miles southeast of Sair Usu, Paleozoic strata, chiefly limestones, are
found (see Chapter X, and Fig. 78, page 171). They are deformed by fold-
ing and faulting but are so little modified in other respects that the fossils are
not destroyed. Our meagre collection from the richly fossiliferous beds indi-
cates that the age is Dinantian. The formation, which appears to continue

PROBLEMS AND AREAS DESERVING SPECIAL STUDY 283

indefinitely westward, would yield unusually good returns toward a better
understanding of Paleozoic stratigraphy. This is one of the few places in cen-
tral Mongolia where fossiliferous Palzozoic strata occur under conditions
favorable for detailed inspection, and where their relation to other formations
could be determined. It deserves careful and detailed examination.

Los in Sumu

At Los in Sumu, about seventy-four miles west of Sair Usu on the Ulias-
sutai trail, an exceptionally good opportunity is afforded to study the edge of
a sedimentary basin where it adjoins the margin of hard rock. On the north
are rugged hills of the graywacke-slate series, which descend in spurs to a
smooth floor. One may suppose that sediments have been stripped from
this floor, for the level-lying gravels, rubbles, and sands form a low scarp
about a mile south of the oldrock hills (Fig. 74, page 167). It would be inter-
esting to find an answer to such questions as the following: What is the age of
the sediments? Are they beveled by an erosion plane, or is their present sur-
face one of deposition? How was the floor formed on which they are laid
down? By what agencies was the inner lowland formed which now separates
the infacing scarp of sediments from the hills of ancient rock? What has
become of the sediment removed from the site of this inner lowland?

Jisu Honguer

A second area of Palzeozoic strata was discovered at Jisu Honguer, after
crossing many miles of upturned slates and graywackes of uncertain age
(Chapter X, and Figs. 89, page 180, 127, page 295). A stretch of fifty miles
deserves careful structural determination, and the area in the immediate vicin-
ity of Jisu Honguer would repay special study in Paleozoic stratigraphy. Per-
mian fossils were recovered during the first season. In the second season, a
map of part of the area was prepared, and a larger collection of fossils was
made. The Permian beds extend far toward the southwest from Jisu Hon-
guer, beyond the limits of the map. The Permian beds are overlaid by con-
glomerates of uncertain age, which we supposed to be Jurassic.

GENERAL PROBLEMS

There are, of course, many general problems not to be solved by study of
a special locality, but which become more and more understandable as observ-
ations accumulate over a great region. Some of these are:

The study of both recent and more remote changes of climate.

The major steps in physiographic history.

The problem of peneplanation and the correlation of many planation rem-

284 GEOLOGY OF MONGOLIA

nants and partial planation effects, such as those on the sediments, and simi-
lar effects on the crystalline rocks.

The means or method by which the very perfect Gobi planation was
accomplished.

Origin of the thousands of hollows that have no outlet.

The question of whether the wind has been as great a factor in erosion as
water.

Evidence which the Gobi region offers on the problem of isostasy.

The part played by volcanism in deformation. Is it cause or effect; or
are volcanism and deformation only incidently associated?

The reason why the region has changed so strikingly in its deformation
habit since Jurassic time.

The age and history of the graywacke-slate series.

What evidence is afforded as to the life history of a bathylith?

What was the Paleozoic history of the Gobi region?

What was happening in the Gobi region in Pleistocene time?

The date of man’s appearance and his reaction to the conditions of the
region.

One of the most unproductive epochs in the first reconnaissance was the
Pleistocene. Most of the Gobi seems to have been subject to erosion in that
period. The life of that time had little opportunity to register itself by bur-
ial in accumulating sediments. But if the Gobi was characterized by erosion,
there must be other regions, perhaps beyond the margins of our preliminary
search, where sediments were being laid down. If one could discover where
these are still preserved, better fortune ought to reward search for evidence of
early man. Better conditions, we think, may prevail both farther south and
farther east, and perhaps also along the disturbed margin of the Altai Moun-
tains.

This opinion seems to be strengthened by the recently reported finds of
the two Jesuits, Pére Emil Licent and Pére Teilhard de Chardin, who have
discovered implements and dwelling sites of primitive man in Pleistocene beds
of the Ordos, south of the Gobi. Reference has been made to these discover-
ies, and also to the primitive stone implements found by the Expedition, in
Chapter I of this volume.

PART IV
SUMMARIES AND DISCUSSIONS

285

PART IV—SUMMARIES AND DISCUSSIONS

INTRODUCTION
CHAPTER XVII—STRUCTURAL ELEMENTS OF THE OLDROCK FLOOR

CHAPTER XVIII—SURFACE FEATURES AND THEIR ORIGIN:
PART 1—BASINS AND MOUNTAINS

CHAPTER XIX—SURFACE FEATURES AND THEIR ORIGIN (CONTINUED):
PART 2—PLATEAUS, PLAINS AND FLATLANDS

CHAPTER XX—STRATIGRAPHY OF THE LATER SEDIMENTS
CHAPTER XXI—SUCCESSIVE CLIMATES OF MONGOLIA
CHAPTER XXII—SUMMARY OF GEOLOGIC HISTORY

286

INTRODUCTION

THE chapters included in this portion of the volume are devoted to sum-
maries, discussions, interpretations, and generalizations based on the obser-
vations and data gathered from the various fields and traverses whose more
detailed descriptions have been given in the preceding chapters. In the fol-
lowing chapters, we attempt to correlate, explain and unify some of the results
of the exploratory work of the Expedition in so far as this seems to be possible
at the present stage of the investigation. This step is undertaken despite
the fact that active work still continues with every prospect that modifica-
tions of the present understanding will grow out of subsequent work.

The purpose is not to state a final conclusion, but rather to outline those
allowable inferences and generalizations which seem to be best supported, in
the belief that in this form they not only are more useful in prosecuting our
own future work, but will be of greater service to others.

With this purpose in mind, and with this explanation, we feel justified in
presenting certain of these problems in a more definite form than would other-
wise be warranted. They are the problems that have appealed to us and on
which observations have been made which should lead at least in the di-
rection of their solution. Whatever the answer may be, this evidence and these
arguments will be taken into account, and we trust that additional contribu-
tions of our own in this direction may some day be added.

In studying the data on which these statements are made, we have
worked with as much discrimination as we possess; and in reasoning from the
assembled and assorted facts, we have tried to avoid all unfounded specula-
tion. Asis usual in geology, there are fields in which facts are scarce and data
too limited for fully establishing any final conclusion; but where such is the
case, we have tried to indicate it by a clear statement of the limitations. The
purpose, therefore, has been maintained, even in these portions, in a form that
we trust will serve as a record and as a suggestive discussion.

287

CHAPTER XVII
STRUCTURAL ELEMENTS OF THE OLDROCK FLOOR

DEscRIPTIONS have been given in several articles already issued (Berkey
and Morris, 1924 b and 1924 c), as well as in preceding chapters of this vol-
ume, of the structural make-up of the rock foundations of the Gobi region.
Portions of the published accounts fit our present purpose so well that it is
considered appropriate to reproduce essentially the same discussion.

TWO MAJOR DIVISIONS

The most important single structural feature is the unconformity between
the folded strata of comparatively ancient formations, which together make
up a complex oldrock floor, and the nearly flat-lying sediments of Cretaceous
and younger age, which lie above this floor.

Large areas in the Gobi region are covered with younger sediments that
lie nearly flat (Fig. 10, page 53). The strata themselves are simple and, wher-
ever they are disturbed, the deformation is of comparatively simple type
also,—either gentle warping, or, somewhat more rarely, sharp flexure and
normal faulting (Fig. 111, page 245).

In all other areas, much more complex rock formations are exposed,
representing a more ancient floor which is doubtless continuous beneath all of
the sediments. Wherever the old floor rocks are encountered, the type of
deformation and the degree of internal modification exhibited by them are very
different from those of the simpler overlying strata (Fig. 123). Everywhere
the floor rocks are folded; in many places they are cut by igneous intrusives
and notably metamorphosed. These features are, of course, more pro-
nounced in the older members.

Wherever the rocks of these two very different types of formations—the
sedimentary cover and the floor—are seen in contact, or where their structural
relations can be determined, a great unconformity is found between them.
The hiatus is so extensive that mountain-folding and erosion of thousands of
feet of material were accomplished before the first basin sediments were laid

288

STRUCTURAL ELEMENTS OF THE OLDROCK FLOOR 289

down. Furthermore, it appears that during this interval an entire change
in the diastrophic habit of north central Asia came about. Mountain-folding
characterized the deformations that took place before that time, whereas
warping and block-faulting, without mountain-folding, characterized subse-
quent epochs.

Late Mesozoic and Tertiary continental sediments carrying a remark-
able new fauna constitute the formations developed above the unconformity.
The rocks below, representing together all the ages preceding the Lower Cre-

Rhyolite
dikes

basin sediments Sediments

q granite

=
*O
a:
oa
o
c
oO
a
yt
"
i
Mt

4 =A
MILES

SSS \
Saar wits

FIGURE ACM cxistin margin of a gobi sediment basin. This figure shows the upwarped, complex
oldrock floor partly stripped but carrying small remnants of the former sedimentary cover lying on a nearly
smooth erosion surface. This section represents a strip along the Sair Usu trail, about 400 miles northwest of
Kalgan.

taceous, form the floor immediately beneath the younger sediments, and con-
stitute the present surface in other parts of the region. ‘The overlying sedi-
ments are at best but a thin veneer with many interruptions, and the domi-
nant structural foundation for the whole of Mongolia is the ancient rock
series below the great Mesozoic unconformity.

Traces of the peneplane developed at that time still form major elements
of the topography, and surprisingly large tracts of this old floor are to-day
entirely bare. Many of the bare areas have been covered at one time or an-
other by later sediments, only to be denuded subsequently to some of the
minor warpings of Tertiary time. In the more elevated areas, of course,
agents of erosion, working toward a new level, have dissected this old surface,
leaving only upland remnants of the former peneplane, whereas in well-pro-
tected areas, or those recently stripped of their cover, little change has been
effected in all the intervening time.

SUBDIVISION OF THE OLDROCK FLOOR

From the great variety of rocks noted as belonging to the ancient floor,
and the very great differences of physical condition represented by them, it
is evident that this floor is of compound make-up. It has been possible to
distinguish several sharply defined series of sedimentary strata, other more
obscure metamorphosed formations, and yet other definite igneous units. In

VOL. II—I9

290 GEOLOGY OF MONGOLIA

some cases there are prominent structural breaks between them, or the forma-
tions have structural relations characteristic of important differences either in
age or in origin. Some are strictly igneous types of large extent and evident
structural importance; some, on the other hand, are profoundly metamorphosed
and have taken on all the complexities which commonly characterize the crys-
talline gneisses and schists of very ancient time; still others are only moder-
ately affected by such modifying processes, amd*consequently are regarded as
of much later age, corresponding in some degree to their greater simplicity.
At least six great groups are thus distinguished, some of which are capa-
ble of additional subdivision (Figs. 124, 130, and 161, page 415). This is

Sa Pang Ktang

. al’ Han a
Mountainous areas SY wut,
Jurassic porphyr =
and con lormehake

Late Paleozoic

Siiqe Y
gion OP 9 pt
MA

(Sinian of Grabau)

Early, Proterozoic

rchaeozoic
100 so ie) 100 200 Miles__. 300 400

FIGURE 124.—Sketch map of Mongolia, showing the general geology of the rock floor along the route
traversed by the Expedition.

true particularly of the lowest, most ancient one, where for present purposes
all the strongly metamorphosed units are grouped together. These major
groups, in descending order, are as follows:

6. Mesozoic porphyry intrusives and extrusives, cutting all formations
up to and including the sedimentary series involved in the last folding of the
region previous to the development of the great Mesozoic unconformity.

STRUCTURAL ELEMENTS OF THE OLDROCK FLOOR 291

5. A great series of folded conglomerates and sandstones of continental
type, considered to be chiefly of Jurassic age.

4. Strongly folded, fossiliferous Palzozoic strata of marine origin.

3. An extensive underlying and invading mass of granite, described as
the great Mongolian bathylith.

2. Avery thick and widely extended series of folded, unfossiliferous gray-
wackes and slates, older than the granite bathylith and only moderately meta-
morphosed. We have called this the Khangai series, and tentatively consider
it to be of late pre-Cambrian age.

1. Still more ancient, underlying complex groups of quartzites, slates,
phyllites, schists, gneisses, crystalline limestones, and other associated meta-
morphic rocks. This complex is undoubtedly made up of more than one
series. The upper members are judged to belong to the division distinguished
in China as the Wu T’ai system, and the oldest members are regarded as local
representatives of the T’ai Shan complex.

Mesozoic intrusives

A great variety of porphyries, forming dikes and irregular intrusive bod-
ies of much larger extent, have been seen at many places. In some cases, at
least, they cut the latest sedimentary series preceding the great unconformity.
This clearly establishes the fact that they are the youngest of the formations
of the ancient floor. The strata above the unconformity are regarded as of
earliest Lower Cretaceous age, whereas those immediately below are judged,
on rather obscure grounds, to be Jurassic. These intrusives, therefore, must
also be of Jurassic age.

Representatives of this group are widely distributed, and in many places
exhibit a formidable complexity of relations. So many different units are

RY

Eee granite

SE é. :
405 4 3 2 1 400 9 8 7

piucped conglomerates
Ro ie CaS
Ne

aL of ee i

FicurE 125.—Typical structure section of the oldrock floor. The section is based on ten miles of route
traverse, seventy to eighty miles southeast of Sair Usu, on the Uliassutai trail. Folded Jurassic conglomerates,
cut by many dikes, are seen at the left. A complex of eruptive porphyries occupies the center. The por-
phyries are invaded and undercut by the granite of the great Mongolian bathylith, which fills the right-hand
end of the section. The ancient peneplane is remarkably smooth in this district, despite the complexity and
variation in quality of the rock floor.

represented, and they cut one another in so irregular a way, that in certain
areas they form a veritable igneous complex (Fig. 125).

292 GEOLOGY OF MONGOLIA

Field investigations since the original article was published indicate that
there are yet other ancient porphyry complexes, and that large areas are occu-
pied by them. The porphyry problem, therefore, is a more difficult one than
was at first thought. A discussion of it in the light of the newer observations,
however, must await a later volume.

The commonest type is an acid porphyry, ranging in minor character
from a simple quartz porphyry to granophyre and granite porphyry of com-
paratively massive habit. Intermediate composition is common also, espe-
cially trachytic or syenitic porphyries, and occasionally there are more basic
types; so that the compositional and structural range is very wide. The most
constant characters are fine grain, dense texture, and only moderately porphy-
ritic habit. These rocks are brittle and exhibit a very broken condition, due
apparently to deformation. This physical condition, together with the great
irregularity of form and occurrence as part of a confused complex, is not so
strikingly exhibited by any other series of rocks.

Wherever such an igneous complex intrudes the Jurassic strata, the orig-
inal sediments are entirely displaced and none of the original structural trend
is preserved. Areas represented by such rocks, observed at several points,
cover many square miles. The best examples are those seen at Tsetsenwan,
at Sain Noin, in the Mount Uskuk district, in the Artsa Bogdo range, on the
Sair Usu trail east of the Ongin Gol, and on the trail southeast of Sair Usu.
It is worth noting that igneous activity of a somewhat similar sort is promi-
nent in China also, in association with exactly the same sorts of sedimentary
formations. The type is constant enough in character, no matter how widely
the porphyry areas are separated, to warrant the belief that some very wide-
spread general source for these intrusions must have existed, operating under
regional rather than local control. Weare inclined to the belief that the active
history of the great granite bathylith, in spite of its much greater age, is in
some way connected with the genesis of the Mesozoic intrusions. There are
certain differences of habit in the intrusives that appear in one age after an-
other, that make the whole lot look like a genetic succession, as if they all, from
beginning to end, represented only different stages in the active history of a
single great, slowly differentiating, and repeatedly rejuvenated bathylithic
mass. Perhaps these peculiar porphyries are only the normal product of par-
ticular stages from the master source (Fig. 161, page 415).

Jurassic sediments

The youngest of the sedimentary groups forming the old floor is a great
series of conglomerates and sandstones of continental type, simply folded or
in some places block-faulted, and quite free from important metamorphism
(Figs. 125, 126). A large proportion of the material is coarse-grained, and

STRUCTURAL ELEMENTS OF THE OLDROCK FLOOR 293

considerable thicknesses are strictly conglomerates. Other great thicknesses
are simple, well-bedded sandstones. Locally, interbedded finer sandstones
are abundant, but as far as noted there are no large developments of shale or
limestones. Nowhere is there any evidence of marine conditions. The entire
series consists of stream deposits. The only fossils seen are plant remains,
chiefly stems, which are poorly preserved. In certain portions of the series,
however, there are thin beds of coal of low grade. Even in the coaly layers,
the original fossil forms are nearly destroyed by deformation, so that the fossil
content has proved thus far to be quite inadequate to determine the age of
beds.

The material of the sediments is largely quartzose, or at least very silice-
ous, and the forms of the fragments indicate much wear. The three striking
features are the enormous thickness of the series, its wide distribution, and
the abundance of quartz pebbles and grains.

The rocks were found at several widely separated localities, the principal
ones being at Camp Jurassic, fifty miles north of Ude (pages 67 and 280);
at Tsetsenwan, one hundred and twenty-five miles west of Urga (page 103);
at Sain Noin, three hundred miles west of Urga (pages 124 and 219); in the
Mount Uskuk region, forty miles north of the Altai Mountains (page 228);
in the Artsa Bogdo range (page 150); on the trail midway between Sair Usu
and Ardyn Obo; and at a few other spots where the evidence was insufficient
‘to determine the extent and local importance. ‘There is evidence that strata
of this series formerly extended over a much greater area than that covered by
the Expedition.

Almost everywhere the strata stand on edge, or are strongly folded (Fig.
125); in some localities they are mashed and faulted, while a few synclinal
remnants show little disturbance (Fig. 126). The total thickness in the dis-

WwW —
: Contact_metamorphic effects Erosion Unconformity cy

The Graywacke Series

Jurassic Conglomerates
Graywacke roof pendant

ree Barre a Deen
a NONMTONGe caatas

Underlying granite bathylith cut by great numbers of dikes

FicurE 126.—Geologic section north of Tsetsenwan. A synclinal remnant of Jurassic conglomerates and
sandstones rests uncomformably upon the graywackes and upon the bathylithic granite with its myriad dikes.

trict where this point could be best determined is no less than 25,000 feet, and
at several other places great thickness is indicated, although estimates were
not made (Figs. 40, 41, page 107). Doubtless the same series of strata forms

294 GEOLOGY OF MONGOLIA

the floor beneath the covering of simple sediments at many places, but it
appears that erosion has cut so deeply into the geologic structure of that
time that only the lowest portions of the synclinal folds and the bases of fault
blocks are preserved.

The whole series is pre-Cretaceous and precedes the general planation.
On the other hand, it lies above another unconformity, the exact position of
which in the geologic scale is undetermined except that it belongs above the
latest Palaeozoic sediments. The series, therefore, is apparently mid-Mesozoic
and in all essential respects is analogous to and in many important features
similar to the Lower Jurassic formations of China proper. The fossil evi-
dence for age determination is inadequate, but no fossils other than plant
remains are found either in the Mongolian conglomerate or in the Jurassic of
China, and there are enough points of similarity in type of sediment, charac-
ter of content, and deformation history to warrant tentative assignment to
the same age. On this basis we are referring to the series as wholly of Jurassic
age, although there is no good evidence against the presence of representatives
of the Triassic also. In any case, the series must be regarded as a unit in
which the only breaks of consequence are those marked by the igneous intru-
sions described under the preceding heading.

These intrusions occur at so many places where they are associated with
the Jurassic sediments directly, that one is impressed with the necessity of
accounting in some way for the close association (Fig. 125). It may well be
that the deformation that accomplished the foldings and faultings of Jurassic
time was occasioned by igneous activities in the depths beneath, one expression
of which is marked by the intrusives. Down-faulted blocks, therefore, or
very deep down-foldings may mark the places of weakness which guided the
outbreak, and thus the sedimentary remnants and the associated intrusives
are now preserved together. Other higher portions were more successfully
removed by erosion.

Palzozoic strata

It is a most striking fact that all the sediments thus far found in central
Mongolia, from the present back to the break at the close of Paleozoic time,
are of continental type. But beneath the unconformity, at the base of the
so-called Jurassic sediments, there is a great series of strata of marine origin,
carrying abundant and characteristic fossils (Fig. 127). They include basal
sandstones of only moderate development, with much greater thicknesses of
limestones and shales preserved in down-folded remnants. Several thousand
feet in thickness have been seen, but the total or maximum thickness is un-
known.

Strata of this age have thus far been found in only two areas, both south-

STRUCTURAL ELEMENTS OF THE OLDROCK FLOOR 295

east of Sair Usu. The Tertiary faulting, which raised the modern Altai
ranges, did not extend into this portion of the desert, so that the folded Palzo-

Honguer A Jurassic ? Oshigo
Ola Tertiary Conglomerates | Ola
Sediments }

Ficure 127.—Geologic section of an area of marine Permian beds at Jisu Honguer on the Uliassutai trail,
about 330 miles northwest of Kalgan.

zoic strata have not been uplifted and are still simply part of the peneplaned
floor (Figs. 78, page 171, and 89, page 180).

The larger proportion of the strata is judged by Dr. A. W. Grabau, from
fossil collections made during the first season of the Expedition’s work, to be
of Permian age, but there is a continuation downward into a still earlier pe-
riod, at least into the Pennsylvanian. Curiously enough, no representatives
of these strata were identified in the Altai Mountains themselves, in either the
Artsa Bogdo or the Baga Bogdo district. A single pebble carrying fossils of
Palzozoic age was found loose on the northerly flanks of the Gurbun Saikhan.
It therefore seems probable that representatives of Paleozoic age are to be
found somewhere to the south in that region. Probably marine Paleozoic
strata were formerly extensively distributed in this central Asiatic region, but
the early Mesozoic or mid-Mesozoic epochs of diastrophism and erosion
wrought such havoc that now only a few remnants are preserved.

The rocks are all closely folded, but the fossil content is fairly well pre-
served. The strike is nearly east and west, conforming in this respect to the
average structural trend of the other elements of the ancient floor. Nowhere
have we seen the exact relations between this series and the Jurassic above or
the graywackes below. But the relative position in the scale can be inferred,
and the general nature of the relation is reasonably well determined by differ-
ences of structural habit and physical condition. An important break is
clearly indicated between the Palzozoic sediments and the Jurassic series,
since the former are marine, whereas the Jurassic beds are strictly continental.

It is especially disappointing that the Palzozoic beds have not been seen
in direct sedimentary contact with the graywacke-slate series beneath. This
leaves some uncertainty about structural relation and relative age. It is
clear that the graywackes are older, and, in view of the fact that they are
unfossiliferous, more metamorphosed, and of an entirely different petrographic

296 GEOLOGY OF MONGOLIA

habit, we are inclined to believe that the graywackes are much older and are
probably separated from the Palzozoic strata by an unconformity as pro-
nounced as either of those above. There is abundant evidence that the great
Mongolian bathylith, described as the next unit under the following heading,
is later than the graywacke series and is very much older than the Jurassic
sediments, which in some places lie on an erosion floor of granite (Fig. 126).
But it is not entirely clear, from any relation yet observed, whether the Palzo-
zoic sediments are younger or older than the maximum invasion stage of the
bathylith. All the early and mid-Palzozoic strata are missing, so that there
are no representatives yet found from Cambrian to Mississippian time. Ap-
parently the Paleozoic era is the most defective one, as indicated by the few
sedimentary remnants still preserved. The Paleozoic rocks mark a transient
marine history between two very long epochs of continental control.

The great Mongolian bathylith

Between the sedimentary series just described and the older ones to
follow, there developed in central Asia a granite bathylith, exposures of which
can be seen at many places over a very large territory (Figs. 125, 126, and 128).
The formations existing at that time, including the Archzan crystalline rocks
and the graywacke-slate series, are invaded by the granite, and at many
places where subsequent erosion has been deep enough, remnants of these
earlier formations are preserved as roof-pendants. The granite appears as
large areas of massive rock, and also as smaller intrusive masses, even dikes,
which cut all the formations up to and including the graywackes. It is not
so clear what its relations are to the Paleozoic series, but in one place the Pale-
ozoic strata appear to rest upon granite with the normal sedimentary contact
(Fig. 127). As the Permian beds near the contact are not metamorphosed,
and as no granite dikes are seen cutting them, it seems fair to infer that
the Permian sediments are younger than the granite.

It is possible, of course, that even the later intrusives, such as those which
cut the Jurassic series, are products of the same great bathylithic magma; but,
if so, they belong to a much later stage in its own development than that rep-
resented by the large areas of true granite (Chapter XXII, page 408). The
stage of maximum invasion, in which the massive granite was formed, is prob-
ably pre-Pennsylvanian, and is certainly later than the Khangai graywacke.
It is entirely possible that every igneous unit in the region, no matter what
its age, is genetically connected with this immense bathylith. Its early
developmental stages may have been responsible for the injection phenomena
of the ancient gneisses; its maximum encroachment was attained in Palzo-
zoic time, and its minor rejuvenations during old age may be recorded in the
outbreaks of later periods (Fig. 161, page 415).

STRUCTURAL ELEMENTS OF THE OLDROCK FLOOR 297

The granites show considerable variety of composition and minor habit,
but the dominant type is a biotite granite of medium coarse texture and
massive structure. It has produced an extraordinary variety of end-product
effects and considerable contact metamorphism. Its relations, distribution,
and special features are made the subject of a separate paper already pub-
lished (Berkey and Morris, 1924, a).

The Khangai graywacke series

An extensive series of graywacke sandstones and interbedded shales or
slates is widely distributed in Mongolia. It forms the major composition of
the mountains of the Arctic divide, and constitutes the country rock of the
Urga-Tola River-Tsetsenwan region, as well as the Khangai mountain range
through the province of Sain Noin toward Uliassutai (pages 82, 90, and
122). The rocks are well exhibited in the Gurbun Saikhan (page 258) and in
the small mountain tract twenty miles east of the Mount Uskuk block (page
227), as well as in the Ude region along the Urga trail (page 61). Repre-
sentatives of the same formation are found in many other areas.

In the Tola River region and at Tsetsenwan and westward, graywackes
predominate, whereas southward and eastward we find a greater proportion
of interbedded shales. Neither the top nor the bottom of the series has been
determined, but it is certain that it is of very great thickness,—probably at
least 20,000 feet. .

As far as observed, the strata are unfossiliferous throughout. Diligent
search was made for possible fossil content, and, in the hills east of Mount
Uskuk, slates were found with obscure markings that are believed to repre-

7

S
SW Folded Graywacke-slate series NE

MIO: YYW fp ip Gee
DLO cue

Y
We
FicurE 128.—Typical geologic section of the Khangai series. The section lies one hundred miles south-

EY) fi
C27 Ls
west of Urga. Simply folded graywacke strata are undercut by the great Mongolian bathylith and are locally

Y gr y
metamorphosed by contact influence.

6

G Te tf 4 OY j ee
Cee

Great Mongolian Granite Bathylith

sent imprints of some simple organic form, probably alge. A few thin, limy
lenses contain tubular structures which may be borings made by some worm-
like animal. A striking thing, of course, is the unfossiliferous nature of these
rocks in spite of their simple sedimentation structure, in strong contrast to
the richly fossiliferous habit of the Upper Paleozoic strata. It is not believed
possible that the graywackes and slates, with their splendid bedding structure

298 GEOLOGY OF MONGOLIA

and abundance of original shales, could be of Paleozoic age without bearing
better evidence in the form of fossil content.

We are well aware that the graywackes farther north and east are regarded
as of Paleozoic age by Russian geologists, who report fossils from certain local-
ities. Inthe Gobi region covered by our own traverses, however, we have not
found any determinable fossil content in the graywackes, and these rocks are
more modified than are the recognized Paleozoic strata. The Carboniferous
strata begin near Sair Usu, above a basal conglomerate, instead of making a
transition to the graywacke series. At Jisu Honguer, both the graywacke
series and Paleozoic strata are present, but again there is a structural break
between,—this time a fault instead of a transition.

As we interpret the ground at Jisu Honguer, the Paleozoic strata belong
to a down-faulted and crowded block which brings the two formations side by
side. In physical habit, as well as in surface form, they differ greatly. It
is our purpose to make a special study of this problem at a later time. Under
these circumstances, we think that the evidence still favors more ancient cor-
relation for the graywacke series, and we regard all of the Paleozoic rocks as
down-folded and down-faulted remnants of superimposed strata of a later
age.
! Dr. J. Morgan Clements (1922), working north and northeast of Urga in
the Kentai Mountains, found graywackes which he considered pre-Cambrian.
This is doubtless our Khangai series. Ussov (1915) has recorded his opinion
that the graywackes of the Kentai are Algonkian. In view of the difficulties
in the way of correlating this formation and the widely divergent views regard-
ing it, we are inclined to let it stand as probably pre-Cambrian.

Siliceous limestones of dark gray to blue color are found associated with
slates in some localities; they also are quite without fossils so far as we have
seen. Probably the limestones represent incursions of a shallow sea, and the
very fact that they lack fossils so completely suggests that they are pre-
Cambrian. The graywackes are probably non-marine and of the same age,
and, especially in the Khangai Mountains and the region about Urga, where
limestones are quite lacking, it is believed that the great series is essentially
continental.

Everywhere the series is folded. In places where it is made up largely of
original shales, there is much internal deformation, so that typical slates have
been formed, but wherever the much more massive graywackes predominate,
simple folding is more common, with very little internal deformation or meta-
structure.

The series was invaded by the granite bathylith subsequent to its
folding, and in many places over extensive areas the graywackes and slates
now form the only roof. Great numbers of dikes cut through the series, and

STRUCTURAL ELEMENTS OF THE OLDROCK FLOOR 299

the surface of the bathylith itself is very uneven, so that after prolonged ero-
sion, patches of granite are exposed, alternating with patches of graywacke
(Fig. 128). In places where the granite lies close beneath, considerable con-
tact metamorphic effects have been produced on certain qualities of the gray-
wacke-slate series. In some places a crystalline condition and moderately
schistose structure are thus exhibited, whereas normally the rock is plainly
granular and not schistose.

THE ANCIENT CRYSTALLINE COMPLEX

Clearly older than the graywacke series, as indicated not only by their
structural relations but also by the much greater metamorphic modification,
is a great group of formations which doubtless includes several different series,
but which together may be conveniently referred to as the ancient crystalline
complex (Fig. 129). The simplest of the rocks of this class are slates, phyl-
lites, schists, limestones, and conglomerates that are clearly derived from

NW
4 310
MILES

Granite

H}

, Sry We My h fy fi 5 ZeThld iba ely ok
c\ wy 14 PNM YAY, i Cee if | * ¥ d aie soe HL
A complex of schists, injec stalline limestones cut by granite
FIGURE 129.—Typical geologic section of the most complex pre-Cambrian rocks. ‘This section lies nine

to nineteen miles northwest of Sair Usu, on the Uliassutai trail, The mountain structures have been pene-
planed, and bear two very shallow basins of later sediments.

some ancient sedimentary series, of much more variable habit and somewhat
different origin from that of the overlying graywacke series. They stand
everywhere on edge; they are repeatedly exposed in many places as widely
separated as are the observations of the Expedition. There are few places,
however, where they cover extensive tracts, and fewer still where many dif-
ferent members of the series are exposed together.

The Wu T’ai system

Because of the fact that in certain places conglomerates are found in the
midst of these series, and on account also of an observed difference in the
degree of metamorphic complexity of certain members, we think the evidence
favors the existence of at least two great systems of pre-Cambrian rocks,
older than the rocks of the Khangai graywacke series.

If we are right in regarding the graywackes as pre-Cambrian, they must
be late pre-Cambrian, and as such they should correspond approximately to

300 GEOLOGY OF MONGOLIA

the Nan K’ou series of von Richthofen (1877), and Hu t’o system of Bailey
Willis (1907, II, page 7), in China, or the Sinian system of Grabau (1922).
Our next older series in Mongolia may include the greenish chloritic schists
in the Artsa Bogdo range of the Altai Mountains (Chapter XV, page 263).
There is also a vast series of greenstones and chloritic phyllites in the moun-
tains east of Ardyn Obo, which may be of igneous origin, and perhaps represent
ash beds and surface flows that have undergone thorough reorganization
(Chapter X, page 178). More data bearing on this problem may be expected
when these rocks are studied with the microscope. Thin beds of limestone
were found in the greenstone area. Mica schists and mica phyllites were
seen on the Kalgan-Urga trail north of P’ang Kiang, and crystalline limestones
are associated with them (Chapter III, page 55). The phyllite-schist-lime-
stone-greenstone group outcrops at many places, yet is nowhere as thoroughly
pierced and saturated by invading igneous material as is the group of rocks
next to be described. In his ‘‘Research in China”’ (1907, II, page 4) Bailey
Willis classified a series of phyllites, limestones, quartzites, schists, and green-
stones under the Wu T’ai system, which he assigns to the early Proterozoic.
We see no better classification for these very similar rocks in the Gobi re-
gion (Figs. 130 and 161, page 415).

The T’ai Shan complex

Still more complexly modified rocks are found in the Gobi region. They
are largely gneisses, associated schists, and crystalline limestones. The
gneisses range from granitic to dioritic in general composition, but they are
not simple granites and diorites. They represent a complex in which the
original rock probably was a schist. This original rock has been invaded by
igneous material which has penetrated and saturated the original, following
in the main the structural lines of the schist. The magma has replaced as
well as penetrated the host rock, so that now the igneous matter is a streaked
gneiss because it has inherited the structure of the schist which it has largely
destroyed and replaced. Where the igneous streaks are less predominant,
the rocks may still be classed as schists, and among these the commonest type
is a coarse-grained muscovite-biotite schist, streaked with lenses and thin
sheets of granite or pegmatite. The limestones are white to blue or gray
crystalline marble.

Such rocks were especially noted in the block mountain south of Tsetsen-
wan (page III); in another block mountain forty miles southwest of Tsetsen-
wan (page 114), and north of Kalgan on the road to the pass, as well as at
many other places.

Because of the great complexity of structural habit, these rocks are
regarded as still older than those we have referred to the Wu T’ai system, and,

LOWER PORTION OF THE
GENERALIZED GEOLOGIC COLUMN

FOR CENTRAL MONGOLIA
COVERING THE STRUCTURAL UNITS OF THE BASIN FLOOR

PLEISTOCENE
THROUGH
CRETACEOUS
Go eee) tN ON FOUR LM hey

ALL ROCKS BELOW THIS LINE ARE FOLDED

A SERIES OF SIMPLE SEDIMENTS
LYING NEARLY HORIZONTALLY

CENO-
ZOIC

A GREAT SERIES OF CONGLOMERATES, SAND-=
STONES AND SHALES,WITH ASSOCIATED
LAVA FLOWS, TUFFS AND ASHES;CARRYING
OBSCURE PLANT REMAINS AND LOCALLY,
COAL; THE WHOLE ABOUT 20,000 FEET THICK,
APPARENTLY CORRESPONDS TO LOWER JU-
RASSIC OF NORTHERN CHINA

SERIES

JURASSIC

ME a2 OE

EARLY MESOZOIC
TSETSENWAN

UINC ONE OL ROMY

NOT HITHERTO SUBDIVIDED

Ss easrnes
O|N THE JISU HONGUER SERIES | SANOST
Ni @ LIMESTONES
Sy
QE} to
ay 3 Bs z5 CONGLOMERATES
ZIE| of THE SAIR USU SERIES

<|/ Cy
Oj] gu DOLOMITES

UNCONFORMITY COVERING EARLY PALAZOZOIC TIME

GREAT BATHYLITHIC INVASION GRANITE

NANK’OU SYSTEM THE THE THE TOLA RIVER}

we) SINIAN KHANGAI GRAYWACKES
= 4 ayes PROPOSED SYSTEM SERIES AND SLATES
Sr AS USED BY} INMONGOLIA [WITHOUT FOSSILS

GRABAU

WILLIS IN CHINA

ASERIES SCHISTS

PROTEROZOIC

> , IT

a THE WU T’Al SYSTEM INMONGOLIA | [IMESTONES
o ; COMPARABLE DOLOMITES
<< | AS USED BY WILLIS IN CHINA TO THE UARTZITES
uJ REENSTONES

WU TAI

CRYSTALLINE
LIMESTONES,
SCHISTS,

A SERIES
INMONGOLIA
COMPARABLE

RECOGNIZED IN PART
BY EARLIER EXPLORERS

ARCHAOZOIC

THE TAI SHAN COMPLEX

ARCHA-AN

AND COMPLEX
AS USED BY WILLIS IN CHINA Ta otiiy | INJECTION
GNEISSES

FIGURE 130.—Summary table of the formations of the oldrock floor.
301

302 GEOLOGY OF MONGOLIA

again following the usage established by Willis in China, we regard these
oldest of the formations yet seen in the Gobi as Archzozoic and equivalent
to the T’ai Shan of China.

Undoubtedly there are great breaks between the principal members of
the pre-Cambrian series. It is believed that the conglomerates seen on the
Urga trail in the midst of these formations (page 80 and Fig. 27), and the
conglomerates seen between Tsetsenwan and Sain Noin, two hundred miles
west of Urga (Fig. 45, page 115), mark some of the important breaks, but even
without these there is sufficient evidence in the differences of the rocks them-
selves to warrant such subdivision as has been made and such age differences
as are indicated in the tabulation (Fig. 130). Doubtless very much greater
detail of formational make-up is actually exhibited than has been determined
as yet, but the major elements of the ancient rock floor and the major charac-
ters of the individual series are reasonably well expressed as a working basis
by the accompanying table (Fig. 130).

PLATE XXXVIII.

A. BEVELED SURFACE OF UPTURNED PHYLLITES AT GORIDA.

The photograph is taken looking down on a nearly level surface,

B. SHATTERED GRAYWACKE,

Upturned and broken graywacke strata cut by a shattered quartz-porphyry dike, near Jisu Honguer.

PLATE XXXIX.

A. DEFORMED JASPERITE IN THE GRAYWACKE SERIES.

The photograph was taken near Five Antelope Camp, south of the Tola River.

B. THE PHYLLITES OF ARTSA BOGDO,

The fine, micaceous schists of Artsa Bogdo have a remarkably regular crumpling across the schistosity and bedding structures.

CHAPTER XVIII

" SURFACE FEATURES AND THEIR ORIGIN

BASINS AND MOUNTAINS

THE BASIN OF MONGOLIA

AN inland basin of more than a million square miles lies enclosed between
the Great Khingan range on the east; the Transbaikal, Kentai, Khangai, and
Tannu Ola, on the north; the eastern ends of the Sayan, Sailugem, and Altai,
on the west, and the Nan Shan system on the south (Fig. 131). The essen-
tial basin shape is well shown in Fig. 4, page 43. The minor basins, moun-
tain ranges, isolated peaks and hills, and hollows containing lakes, playas or
saltpans, which rise above or sink below the general sloping surface, do
not disturb its basin-like character. Across these many and various surface
features the Desert of Gobi is flung like a pall; it has no definite boundaries,
but covers mountain, plain, and hollow.

Deserts in general

In a broad sense, a desert is any part of the land where plant and animal
life are under notable disadvantage, so that only a sparse population of rather
specially adapted forms can live. We may recognize three types of desert:

1. Deserts due to scanty rainfall.

2. High altitude deserts in lofty mountains.

3. High latitude or snow-deserts.

The Gobi desert, as the arid part of a great inland basin, belongs to the first
class.

The shaping processes

The inland basin of Mongolia, like all other land surfaces, has been
molded and carved by geologic processes of two types: the constructional pro-
cesses which tend to build up or make a country, and the destructional, which
tend to destroy rocks and lower the land surface. Under the term construc-
tional are included:

303

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speosfrey =——— i,
-. IUPA 184d m~~ GZ6l JO e4nod| — -—
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Aiepunog (eucen —:— 2Z6l jo oyned gy i.
POR a BE

SO Se eee

304

SURFACE FEATURES AND THEIR ORIGIN 305

1. Tectonic movements: warping, faulting, thrusting, folding, uplift.

2. Volcanism: the outpouring of lava, the ejection of ash, tuff, and bombs,
and the building of volcanoes.

3. Sedimentation: a process which lies on the borderland between ‘the
constructional and the destructional.

In general, all sediments except volcanic ash and tuff result from the
destruction of rocks by weathering and erosion. But those sediments which
fill basins in the interior of a continent may be considered constructional,
as they tend to build up the surface by means of new strata of rock
which are as permanently a part of the continent as any other rock it
possesses.

Accordingly, the constructional landforms in Mongolia will be the major
and minor warped basins and their contained sediments which form the plains
and plateaus; the mountain ranges and the volcanoes and remnants of vol-
canoes.

Destructional processes include:

1. Weathering: mechanical and chemical.
2. Erosion: the work of ice, running water, waves, and wind.

In the Gobi region, the mechanical processes of weathering are rather
more active than the chemical. Temperature changes are abrupt and great,
and frost action is vigorous. Quartzites, flints, jasper, chalcedony, siliceous
argillites and other very elastic and brittle materials tend to chip, flake, and
split when rapidly heated and chilled, so that the outcrops are commonly
seamed with a network of fine cracks. Rocks which are composed of several
different kinds of minerals tend to crack apart, yielding nearly unaltered crystal
fragments. Thus granite, consisting chiefly of quartz and feldspar, breaks
down into a coarse sand of quartz, feldspar, and granite chips, covering large
areas of the rock floor. The surface of the granite bedrock is so disintegrated
that it is difficult to obtain a fresh specimen. In addition to the vigorous
mechanical weathering, there are also some chemical changes, for the ferro-
magnesian minerals are altered and rusted, and the feldspars are somewhat
kaolinized. Basalts and other dark, igneous rocks split into blocks; unlike
the granites, they do not break down into a sandy residue of fresh minerals,
but alter chemically into clayey soils. The sudden changes of temperature
have less effect upon the limestones than upon the more siliceous rocks, but
their surfaces are commonly etched by the solvent action of rain water into
a fretted pattern of shallow pits.

The chief erosional processes in a desert are the work of running water
and of wind, and almost all destructional land-forms are due to their activity,
although it is often very hard to tell which plays the dominant rdle. Glacial
ice and the waves of lakes are both minor erosive agencies in the Gobi region.

VOL. II——20

306 GEOLOGY OF MONGOLIA

Each of the factors will be discussed in connection with the land-forms which
it helped to shape.

MAJOR SUBDIVISIONS OF THE MONGOLIAN BASIN

In Chapter II, we present evidence of warping and faulting in the border-
ing ranges of the Gobi, indicating that the vast, shallow depression enclosed
by these ranges is essentially of tectonic origin. The earth movements were
not uniform throughout the region, so that the inland basin of the Gobi may
be divided into an eastern and southern warped province and a western faulted
province. The warped province is a broadly open country in which exten-
sive plains, underlaid by Cretaceous and Tertiary sediments, are separated by
areas of low rolling hills of ancient complex rocks. The present topography
is chiefly the result of warping and differential erosion. In the western prov-
ince, block-faulting of the Altai type controls the major relief features.

Because of the unequal warping, each province is divided into a number
of broad depressions which we have called “‘talas,”’ from a Mongol word mean-
ing an open, intermontane country. In the east and south, the talas are
large, rudely oval, shallow basins; while in the Altai province, they are nar-
rower and deeper, walled in by high mountains, and their surfaces are divers-
ified by minor mountain ranges. Within each tala, there are many lowland
areas filled with sediments, for which we adopted the term “‘gobi,’’ as the
Mongols told us that “gobi” is the name for the flat desert floors which we
recognized as deposits of Tertiary and Cretaceous sediments. While the
tala is a topographic element, the gobi is a lens-shaped mass of sediments,
underlaid by the eroded and downwarped oldrock floor.

The talas of the eastern and southern province

The eastern and southern province of the Gobi falls into three great warped
basins or talas, which we have called the Dalai, the Iren, and Gashuin talas,
each from a lake lying in the deepest part of the depression. The Dalai tala
is the one least known to science, and in the present volume it cannot be
treated authoritatively, as the Expedition has never entered it. The north-
ern part of its broad floor is occupied by a chain of lakes, some of which drain
through the Argun River to the Amur. South of the lake region lies a rougher
country which is apparently a continuation of the oldrock hills of Chakhar.
The most recent explorations have been conducted by the Jesuit Fathers,
Teilhard de Chardin and Emil Licent (1924 c), who, after traversing a consider-
able area of the oldrock hills, discovered abundant Pliocene fossils in a
series of sands, clays, and marls, which are overlaid by lava flows. They also re-
port a ‘‘chain of Quarternary volcanoes extending more than one hundred kilo-
meters.” (Teilhard, personal communication, 1924.)

SURFACE FEATURES AND THEIR ORIGIN 307

The divide between the Dalai and the Iren talas is a broad swell of the
oldrock floor, dissected into many low ranges which, according to the maps,
strike nearly east and west (Fig. 131). The main axis of the arch lies rudely
parallel to the Khingan range, and may have been formed at the same time
and by the same processes of warping which uplifted the Khingan.

The Iren tala was traversed by the Expedition in 1922, and part of it was
restudied in 1923 (page 198). It is a broad, open country, unmistakably
higher everywhere than in the center about Iren Dabasu, for which our barom-
eter readings recorded an altitude of 2,930 feet. From this general low cen-
ter, the tala surface rises gradually northward to about 6,000 feet at the
Arctic divide, and southward to about 5,000 feet at the Pacific divide. Prob-
ably at least half of the area of the tala, judging from what we have seen, is
occupied by gobi-basin sediments resting upon a floor of older complex rocks.
It was here we first recognized that a single tala may carry many gobis of
widely diverse size and age. Thus, in the seven gobis crossed between Kal-
gan and Urga, sediments were found ranging from Lower Cretaceous to at
least Pliocene age. The sediments are beveled by a remarkably smooth
surface which we named the Gobi erosion plane (Chapter XIX, page 332).
Undrained hollows are excavated below this level surface, and many of them
contain small salt lakes. The Iren tala lacks an integrated drainage more
completely than does any other tala as all streams are short and drain radially
into the hollows. The gobis are separated by low ranges of maturely dissected
hills composed of ancient complex rocks, whose upland is crowned by a well-
developed erosion surface which we named the Mongolian peneplane (Fig.
147, page 342).

Obruchev (1901, II, page 434) crossed the desert from the Yellow River
to the Gurbun Saikhan range, along the very boundary between the Iren and
the Gashuin talas. He reported outcrops of the oldrock floor throughout
almost the entire route, but noted one area of very smooth desert which we
take to be a gobi.

The Gashuin tala forms the southwesternmost part of the warped prov-
ince and extends, indeed, west of some of the typical faulted talas. It is
pierced by faulted ranges on the west, and bounded by faulted ranges on the
north, thus marking the transition from the warped to the faulted province.
Gashuin Nor is the largest of a group of lakes lying in the lowest part of the
depression and receiving from the Nan Shan and Lung Shan ranges a wither-
ing and dividing stream called the Edsin Gol. Very little is known of the
geology. Chernov (1908) visited the region but gives little more than a
geographical description of it. He says, however, that there is good evidence
that the lakes and rivers were once larger than they now are, indicating a
recent change in climate toward greater aridity.

308 GEOLOGY OF MONGOLIA

The western faulted talas

The talas of the western region are much smaller than the warped talas
of the eastern and southern province. They are bounded by faults on at
least one side, and, according to Suess (1902, III, pages 132, 135), some parts
are reported to be true grabens, enclosed between faults on both north and
south. In another paper (1924 b) we have called them the Tsagan, Shargin,
Kisin, Kara, Kirghiz, and Ubsa talas. Only one of these, the Tsagan tala,
has been visited as yet by the Expedition. Klementz (quoted by Suess, 1902,
III, pages 122-125) has described the basins of the ‘‘Valley of the Lakes,”
which we have called the Kara, Kisin, and Ubsa talas, as a succession of steps,
like the treads of a great stairway, descending northward from the Altai front
to the Ubsa tala, which lies only 2,660 feet above sea level at the foot of the
Tannu Ola. He says that the depressions are separated by faults or faulted
ranges.

The Tsagan tala lies between the Khangai arch on the north, and a series
of overlapping ranges of the Altai system on the south (Fig. 131). Like the
Iren, the Tsagan tala illustrates strikingly the fact that a number of gobis,
carrying sediments of very diverse age, can be contained within a single tala.
At the foot of Baga Bogdo lies a gobi about thirty miles broad, between the
Altai front and the Uskuk block (Fig. 98, page 194). The sediment-fill in-
cludes Lower Cretaceous, Oligocene, Miocene, Pliocene and Pleistocene beds
with a total thickness of about 10,000 feet. North of the Uskuk block is
another beveled and redissected gobi which is far less warped and has a lower
relief than the gobi to the south. At one point we found reptile bones, indi-
cating that at least some of the beds are Cretaceous. North of the chain of
saltpans that occupy the deepest part of its concave surface, the gobi ends
against the rock shelf that rises steadily into the great swell of the Khangai
Mountains. As we go eastward to Artsa Bogdo and again plot a section north-
ward, the mountain front descends abruptly to a gobi consisting of sediments
and lava flows, all more or less disturbed by faulting and tilting and exposed
in the eroded walls of several large hollows (Fig. 118, page 269, and Plates
XXXV and XXXVI, page 268). The only fossils found indicate a Lower
Cretaceous age. The sediment-basin extends far to the northward beyond
our reconnaissance.

East and somewhat south of the Artsa Bogdo, lies the Gurbun Saikhan
tange. The basin north of it is more gently warped than either the Artsa
Bogdo or the Baga Bogdo piedmont basin. Diversified by desert hollows and
oldrock inliers, and cut through by the river valley of the Ongin Gol, the smooth
surface of this gobi extends over one hundred miles to the northward, ending,
like the others, at the foot of the Khangai arch. At three widely separated
places where the geologists reached the mountain, the basin sediments were

SURFACE FEATURES AND THEIR ORIGIN 309

seen steeply upturned against the front of the range. At least three distinct
formations have been recognized in this gobi: (1) the thick Lower Cretaceous
Ochungchelo formation, probably of the same age as the Oshih, is exposed at
the foot of the middle range of the Gurbun Saikhan; forty miles to the north-
west, another thick formation, the Dubshih, believed to be also of Lower
Cretaceous age, underlies the Djadokhta sands. It is as yet impossible to
say whether the Dubshih and the Ochungchelo are of the same age; (2) the
Djadokhta formation carries a Cretaceous fauna quite different from that of
the Oshih; and (3) the Gashato formation bears a very primitive mammalian
fauna and has been tentatively called Paleocene.

These brief descriptions omit much detail, but with their aid one may
form a mental picture of a faulted tala,—a structural basin with a very com-
plex history of warping, faulting, sedimentation, erosion, and occasional
volcanism.

MOUNTAINS

Bounding the Gobi, and partly included within it (Fig. 131), are some six
groups of mountains that represent but two general classes: (a) tectonic moun-
tains; (b) volcanic mountains, which are of very minor importance in Mon-
golia.

To the tectonic class belong: (1) the Khingan range, which lies nearly
athwart the Altai trend, striking about 26° east of north; it is probably up-
faulted along the eastern front, while its western face slopes gently into the
Gobi basin; (2) the Baikal-Yablonoi group, which consists of relatively short,
broad, fault-block mountains, controlling a special type of drainage pattern
in which the rivers seem to be adjusted to fault lines and grabens; Obruchev
(personal communication, 1925) writes that they are peneplaned blocks, and
that the valleys are tectonic and bear fills of Tertiary sediments which are
being eroded by the present rivers; (3) the Khangai-Kentai group, which con-
sists of broad, gently upwarped masses of peneplaned mountains, now deeply
dissected; faults are present, but, in the main, they do not control the drain-
age, which is essentially of dendritic pattern; these three types are considered
in more detail under the heading ‘Boundaries of the Gobi’? (Chapter II);
(4) the Altai group of long, relatively narrow, fault-block ranges, which
trend about east-west in gently curving, parallel lines, enclosing strips of desert
lowland between them; (5) the older ranges in the warped province, consist-
ing of low, maturely dissected hills.

Fault-block mountains of the Altai type

The Altai belongs to a great group of ranges between the Khangai and
the Tien Shan. All, including the Tien Shan, are essentially long, narrow

310 GEOLOGY OF MONGOLIA

fault-blocks extending nearly east and west and enclosing broad, intermontane
basins (Fig. 131). They form the most important mountain system of cen-
tral Asia, and their structural features and history are characteristic of the
deformations which have taken place in this region since the beginning of
Lower Cretaceous time. We saw four ranges of the eastern Altai,—Ikhe
Bogdo, Baga Bogdo, Artsa Bogdo, and Gurbun Saikhan, named in order
from west to east. In the same order, they diminish in size, Ikhe Bogdo being
the highest of the four. A touch of primitive poetry in the Mongols has led
them to name many of their finest mountains after their gods: thus, Ikhe
Bogdo means the Greater Buddha; Baga Bogdo, the Lesser Buddha; Artsa
Bogdo, the Buddha of the Trailing Junipers; and Gurbun Saikhan, the Three
Fair Ones.

In general form, the eastern ranges are typical examples of the long,
narrow Altai fault-blocks. They overlap one another as shown on the map

NEC A\NZ\. DOWN THROWN BLOC
3 “ Z

a

“N AIN AYTIANS KS

FicureE 132.—Generalized block diagram of a faulted basin. This diagram represents a rectangular block
sliced out of the earth. The front of the block shows the cut edges of the rock structures. The center of the
block is represented as the down-sinking area where sediments have continued to accumulate as deformation
progressed. The uplifted portion, stripped of its original cover, ultimately forms a long, straight, mountain
front of old crystalline rocks more or less deeply dissected by continued erosion. Volcanic eruptions have taken
place along the zone weakened by faulting. This feature of the diagram represents conditions that existed
in Cretaceous and Tertiary time rather than the present, for the volcanic cones have been destroyed by later
erosion; but lava flows and ash-beds are still preserved, interbedded with the sediments. The latest develop-
ment is an overspread of alluvial fans along the mountain front.

(Fig. 98, page 194.) The northern front of each rises steeply to a crest which
lies but a short distance behind the base line. From the crest we saw that
the whole top of the range is a sea of nearly accordant peaks, which we judged
to bea peneplane. In most of the ranges this upland surface tilts gently down-
ward toward the south. In Artsa Bogdo and Baga Bogdo, and probably in
Ikhe Bogdo, the southern face is notably lower than the northern, and drops
steeply to a broad lowland. In each lowland, but especially in the northern
basin, sediments have accumulated, associated with lava flows (Fig. 132).
Ikhe Bogdo.—Concerning Ikhe Bogdo, Suess’s summary of Klementz’s
reports (Suess, 1902, III, page 134) indicates that the range is narrow, that

SURFACE FEATURES AND THEIR ORIGIN 311

it is made of granite and some schists, and that the northern slope is ex-
tremely steep and difficult. In 1922 we saw from a distance that the range has
a remarkably smooth upland surface, or peneplane, and that the heads of the
major valleys are cirques, recording the easternmost glaciation in the Altai
system. At the foot of the range lies a broad piedmont basin. We visited
the range in 1925 and observed so many special features that a separate
description will be required. This will appear in a later publication.

Baga Bogdo.—The range is about fifty miles long from east to west, and
not more than twelve miles wide from north to south. Its position is indicated
on the sketch map (Fig. 98, page 194). From Mount Uskuk, forty miles
north of Baga Bogdo, the surface of the piedmont basin slopes downward
into the depression of the lake Tsagan Nor, and then gently rises 2,000 feet
to the foot of Baga Bogdo. From this slope, the mountain juts up more than
4,000 feet, attaining an altitude of nearly 12,000 feet above the sea, and a
height of about 7,000 feet above Tsagan Nor. The summit peneplane slopes
gradually from the east and west ends of the range, up to the culminating
twin peaks near the center (Fig. 106, page 224). The stream valleys that
dissect the mountain front at this point are so short and straight that, stand-
ing in the mouth of Tiger Canyon, one can clearly see the two peaks, 4,000 feet
above, yet less than three miles away. As we climb the valleys, they open
through terrace after terrace. The lower benches are narrow and relatively
modern, and correspond to several stages in the history of the alluvial fans.
But the higher shoulders, 2,500 feet and more above the inner gorges, are
remnants of much wider and more mature valleys which have been almost
wholly destroyed by later dissection. They must mark earlier stages in the
growth of the range, when the mountain, much lower than it now is, had
paused long enough in its uplift to develop a network of broad valleys.

The range is made entirely of ancient, complex rocks of diverse kinds: and
ages, but it is a significant fact that the folding and the granite intrusions were
accomplished long before the uplift of the present fault-block range began
(Plate XXX in pocket)

The alluvial fans are of extraordinary interest, as they contain part of the
record of mountain growth and part of the record of the climatic changes of
the region. There are at least four series of fans. The oldest is the Pliocene
Hung Kureh formation which outcrops less than five miles north of the moun-
tain, and consists chiefly of thin-bedded clays and fine sands. The modern
streams bear coarse gravels far beyond the northern scarp of the dissected
Hung Kureh. It is, therefore, probable that the mountain was lower when
the Hung Kureh beds were deposited. If the range had been as high as it now
is, the accumulation of fine pond clays so near the mountain would not have
been possible, as the average slope of the present piedmont surface is more

312 GEOLOGY OF MONGOLIA

than 200 feet per mile. The Hung Kureh beds are tilted and even somewhat
folded, indicating that mountain-movements have crowded them since their
deposition. Upon the Hung Kureh lie the rubbles of the Gochu fan, which
have been tilted so that their bedding planes dip as much as 20° southward
toward the Altai front, quite reversing the original dip (Fig. 107, page 229).
They bear a record of deposition followed by tectonic movement, followed in
turn by prolonged erosion before the third fans were laid. The third, or
high fans, overlie and partly bury the eroded remnants of the Gochu fans,
but even they have been so dissected that some of the cones have been almost
wholly cut away, leaving remnants that now stand 300 feet and more above
the modern fans. The remnants connect with a rock terrace in the mountain
valleys, below which the modern streams have cut deep trenches. The dis-
sected fans record a long period of accumulation, followed by a reversal of
conditions and the removal of vast amounts of material. The fourth and
most recent series of fans has been built in the gaps torn through the older
fans, and is chiefly of later date than the cutting of the inner trenches of the
mountain valleys. The modern fans also record changing conditions: first,
the piling of gravels along the mountain front in the canyon mouths and in
the broad gaps torn in the older fans; then a dissection of these fans to a depth
of 150 feet. At the mouth of Tiger Canyon, terraces are cut in the modern fan
(Plate XL, B, page 388), which indicate balanced conditions, when the stream
widened its bed without greatly deepening its channel.

The record demands very careful reading because effects of mountain
growth may sometimes be confused with the results of climatic change. Dur-
ing dry cycles, when the protecting vegetation of the mountain slopes withers
and the streams disappear shortly beyond the canyon mouths, the soil and
loose rock wash down and overload the streams, which pile the burden along
the mountain front, building up the alluvial cones. But during rainier epochs,
living roots form a thick mat which blankets the mountain slopes, holding the
soil in place so that the rivers are not overburdened, while their actual supply
of water isincreased. With increased water and diminished load, the streams
begin to excavate their channels and to dissect the alluvial fans which they had
piled up. Uplift, with the steepening of all gradients within the mountain,
tends to rejuvenate the streams and push the alluvial fans farther out into the
piedmont lowland. A long pause in the uplift permits the streams to lower
their gradients and so to increase the fans which previously they had piled up
against the mountain front. Uplift, therefore, works a result similar to that
of a moist climate, while the effect of a period of quiescence in mountain growth
resembles that of a change to a dryer climate. Where the fans are not only
dissected but tilted, which is true of the Hung Kureh and the Gochu fans,
the fact of disturbance in geologically recent time is plain. In the case of

SURFACE FEATURES AND THEIR ORIGIN 313

the two later series of fans, where there is no change of dip and no direct evi-
dence of renewed faulting, a change in climate seems to be the most rational
explanation (page 390).

Artsa Bogdo.—Artsa Bogdo is notably lower than Baga Bogdo; the high-
est peak thus far reached stands about 8,000 feet above sea level. The north-
ern front juts up steeply from the smooth piedmont slope to an upland
surface which rises gradually and as gradually descends toward the south.
Notable monadnocks stand above the upland peneplane. The southern face
of the range is less steep than the northern front. Within the mountain block
most of the streams have a dendritic arrangement, although a few subsequent
streams have developed along belts of weak rock. One well-marked terrace
borders the valleys and broadens upstream until it becomes the valley floor,
and the younger, inner gorge disappears. In the high valleys, which are broad,
richly grassed meadows, herds of big-horn sheep and ibex graze side by side
with the sheep, goats, and yaks of Mongol drovers. The slopes are beautifully
decorated with the spreading rosette growth of the artsa or juniper, which
gives the range its name. There are thus four topographic elements within
the range: (1) the steep-sided inner gorges of the lower valleys; (2) the ter-
races and high meadows in the upper valleys; (3) the summit peneplane; and
(4) the monadnock peaks rising above the peneplane (Fig. 116, page 264).
To these might be added two exterior surface elements,—the front slopes of
the range, and the smoothly eroded rock shelf or pediment, at the foot of the
mountain (Fig. 114, page 261).

Like Baga Bogdo, the range is composed of an uplifted block of complex
rocks—schistose conglomerates, schists, limestones—with large and small
* intrusive masses, chiefly granites and diorites. Infolded or infaulted in the
oldest rocks are slates, sandstones, and conglomerates which we tentatively
assigned to the Jurassic. At the eastern end, the crystallines disappear be-
neath a great field of lava, probably Jurassic or Lower Cretaceous (page 262).

Only a portion of the history of the piedmont basin at the Artsa Bogdo
range is known (Fig. 114, page 261). Along the northern front there is a
_ narrow but distinct pediment, which bevels the same hard rocks that com-
pose the mountain. The slope at the foot of the mountain front is an erosion
plane, and, as at Baga Bogdo, the modern alluvial fans are relatively small.
The rock waste issuing from the mountain is carried through the beveled basin
sediments in stream courses which are trenched well below the erosion plane.
Each stream valley bears a terrace, coated with a heavy layer of partially
cemented coarse conglomerates. The piedmont slope is locally varied by
mesas, capped with black basalt. The tilted conglomerates seen at the foot
of Baga Bogdo are absent.

Twenty miles north of the range, a thick deposit of Lower Cretaceous

314 GEOLOGY OF MONGOLIA

sediments is exposed in the scarps about the Oshih hollow. Between the
Oshih and the Artsa Bogdo the rocks are for the most part covered. Close to
the northern face of the range, however, unfossiliferous sediments are exposed
and steeply upturned, indicating that they once extended farther. Along the
southern flank of the mountain lie red and gray sediments with reptile bones
of the Oshih fauna. We consider it probable, therefore, that the present site
of the Artsa Bogdo range was covered by Oshih sediments in the Lower Cre-
taceous, and has been uplifted and cleared of sediments since that time. So,
the peneplane on the top of Artsa Bogdo might well be the ancient floor on
which these sediments were laid down; in this case, it would be a stripped or
resurrected peneplane. The more conservative view is that the upland level
is part of the later Mongolian peneplane, for which we have tentatively pro-
posed an early or middle Tertiary age (1924 d).

A brief summary of the history of Artsa Bogdo, so far as it is known, is
as follows:

1. A complex mountainous country was peneplaned during or before
Lower Cretaceous time.

2. The Oshih sediments were deposited upon the old floor in Lower Cre-
taceous time and covered certain low monadnock ridges of the old hard rocks.
Volcanic activity accompanied the deposition of the sediments, and possibly
there were outpourings of lava after the sedimentation was completed.

3. The region was disturbed by faulting and tilting, which were renewed
at intervals. Some of the resting periods were so long that the relief features,
due to faulting, were planed away. The upland peneplane of Artsa Bogdo,
the broad terrace along the valleys in the mountain, and the planed rock
shelf at the foot of the range, are records of the larger intervals of quiescence
and erosion.

Gurbun Saikhan.—The Gurbun Saikhan is not a unit, but a broad moun-
tainous area including a number of nearly parallel ranges, separated by long,
relatively narrow depressions. The Mongols have given names to the three
chief blocks—the Baron Saikhan, the Dzun Saikhan, and the Dunde Saikhan,
meaning the Western, Middle, and Eastern Good One. ‘To these should be
added a smaller but structurally similar ridge along the northern front of
Baron Saikhan, and a long block extending southeast of the Dunde Saikhan.

Each is a long, narrow, steeply rising uplift, essentially like the other
eastern Altai ranges, and each is composed of old complex rocks. In the
Baron Saikhan, we observed silicified and partly replaced limestone, large
intrusive, stock-like masses of serpentine and diorite, and steeply folded argil-
lites, slates, and graywackes (page 256). In the front range, we noted a
younger series of argillites and graywackes, and an older series of phyllites,
chloritic schists, and sheared, schistose conglomerates. The Dunde Saikhan

SURFACE FEATURES AND THEIR ORIGIN 315

consists wholly of one closely folded series, including graywackes, slates,
siliceous argillites, and green, dense, silicified ash, the age of which has not
been satisfactorily determined.

The front of each range rises sheer from the piedmont slope, without any
foothills. There is an upland peneplane, which is tilted and bent at various
angles in the several ranges. Above it stand higher monadnock peaks. The
southern slope of Dunde Saikhan is much steeper and higher than the northern
face, reversing the rule for this part of the Altai system. Within each range,
the valleys are terraced, and the cutting of a younger, narrow, winding gorge
has destroyed the terraces near the mountain front; but deeper within each
range, the upper parts of the valleys open into gently sloping, beautiful mead-
ows, affording rich pasturage and unfailing water.

Along the foot of the Dunde Saikhan, two series of alluvial fans are dis-
played. The older fans are sixty to a hundred feet higher than their modern
successors, and have been much dissected. In places, they have buried the
mountainous topography of the front range, and the entrenched river valleys
are superposed across the hard-rock ridge and have cut gorges through it.
The detritus that forms the younger fans is led in channels through the rem-
nant of the dissected high fans and is spread out on the long, smooth piedmont
slope, thinning gradually until the underlying basin sediments of Cretaceous
and Tertiary age again form the surface.

The history of the Gurbun Saikhan must be read in part from the ranges
themselves, and in part from the record of the surrounding sediments. Tilted
and faulted Lower Cretaceous sediments have been seen along the northern
and southern fronts of the ranges. At the foot of Dunde Saikhan, they must
be nearly 2,000 feet thick, and consist chiefly of evenly bedded clays and fine-
grained sands. At the Baron Saikhan, red sands and conglomerates without
fossils are upturned to a dip of 78° upon the front of the mountain (Fig. 113,
page 257). Along the low front range a dip of 56° was recorded. The sedi-
ments were formerly more extensive and may well have covered the site of
the present ranges. The Djadokhta sands, consisting of almost pure silica
(page 356), could not have been derived from the Gurbun Saikhan oldrocks
by simple decay and transportation, but must have been derived from an
older, sandy sediment that had already been considerably assorted. This
inference suggests that the oldrocks of the Gurbun Saikhan were not exposed
during the accumulation of the Djadokhta sediments (Fig. 133). The first
sediment in which the fragments of the oldrocks appear prominently is a gravel
of angular pebbles, overlying the Djadokhta (Fig. 133, C). The gravel forms
the base of the Gashato beds, which consist of variable sandy clays, sands, and
gravels, which may have been derived directly from the weathering of the
Gurbun Saikhan.

Site of the Gurbun-
Saikhan range Sagdy sediments, pooal ly Sairim or Oshih

were neturee
rr @ Pe

; 77 YY LG, on .
Bal KY Cj: PEPE Y SUSE Sans aves ROWE
AAW ite

AL Eronnn OF WAM, etna ) N

FIGURE 133.—Four stages of warping and sedimentation at the Gurbun Saikhan:

A shows a cover of Lower Cretaceous sediments resting on a complex oldrock floor.

B shows initial upwarp of the Gurbun Saikhan range and corresponding deposition of the Djadokhta
sands,

C shows renewed upwarp and deposition of angular pebbles from the newly exposed Gurbun Saikhan
rocks,

D shows the deposition of the Paleocene Gashato beds.

Subsequent to this last stage there has been renewed deformation and extensive erosion, and alluvial fans
fzom the mountains are now encroaching upon the remnants of all three formations,

316

SURFACE FEATURES AND THEIR ORIGIN 317

The history of the Gurbun Saikhan may be summarized as follows:

1. A complex mountainous country was peneplaned prior to Lower Cre-
taceous time.

2. Basin-making began in Lower Cretaceous time, and the site of the
Gurbun Saikhan was covered with sediments (Fig. 133, A).

3. There followed a period of quiescence, indicated by the break between
the Lower Cretaceous and the Djadokhta formations.

4. Warping began again, and material derived from the older basin sedi-
ments was carried into the new basin to form the Djadokhta beds (Fig. 133, B).

5. Slight warping and disturbance took place.

6. A period of quiescence and erosion continued through most of Upper
Cretaceous time. During this interval, a new erosion plane was carved
upon the Djadokhta beds.

7. Probably in Paleocene time, warping and uplift were renewed, and
the Gurbun Saikhan oldrocks were laid bare; the Gashato formation began
with a wash of gravels derived from the mountains; clays, sands, and gravel
followed, and at least one lava flow tells of volcanic activity (Fig. 133, C, D).

8. Uplift of the mountain continued with faulting. The latest faulting
may be as recent as we judged it to be at Baga Bogdo, i.e., early Pleistocene.

9. The range has been vigorously redissected by the streams of at least
two cycles—an older stage marked by a terrace in the river valleys, and a
younger, indicated by deepening and gullying.

The mountains of the eastern province

No ranges of the Altai type were seen in the Iren tala and none is reported
from the Dalai tala. As a typical example of the mountains of the warped
province, we may describe the hills of Chakhar along the Kalgan-Urga trail
(Figs. 7 and 8, page 51). The range extends for about fifty miles, east and
west, and is about twenty miles wide from north to south. The highest alti-
tude is somewhat less than 5,500 feet above sea level, and the upland stands
not much more than 1,000 feet above the general desert surface. The splen-
didly developed Mongolian peneplane (Fig. 147, page 342) crowns the sum-
mits, and has been very maturely dissected. In some of the valleys which
are as much as six miles wide, there are shallow accumulations of sediment
which bear Pliocene and Pleistocene fossils (Andersson, 1923. a). There is no
doubt that the range stands as a monadnock mass above the lower level of
the desert, but the age of the monadnock remains a problem for future study.
There are two possible explanations of its origin: it may be a peneplaned rem-
nant of a mass that was upwarped or faulted during early Tertiary time (in
this case the weathering away of the mountains furnished the sediments now
retained in the surrounding gobis) ; or it may be a disinterred monadnock which

318 GEOLOGY OF MONGOLIA

was formerly buried under the sediments which lie north and south of it. We
are inclined to favor the first interpretation.

There are many such masses as the Chakhar Mountains, especially in the
eastern talas. In general, their history seems to be very simple. They are
probably fault-blocks or broadly arched domes, which, since the time of their
uplift, have been so thoroughly eroded as to destroy all the diagnostic signs by
which we might recognize their precise mode of origin. An example of a dis-
interred monadnock is found in Oshigo Ola, which is a low granite ridge near
Iren Dabasu (Fig. 134). It rises very gradually, less than 300 feet above the
smooth, planed sediments that surround it. The oldest of these sediments

Granite Ridge Gobi Erosion
Scarp of Tertiary Sediments Plane

Ze

EXM. May 13,1983.

FicurE 134.—Field sketch of Oshigo Ola, from the south. The low ridge of granite is being exhumed from
the sediments which still partially bury it, and which have protected it from erosion since Cretaceous time.

are the Cretaceous Iren Dabasu beds, which outcrop some three miles north
of the ridge. They are overlaid by Eocene strata, which show an undisturbed
sedimentary contact with the granite ridge, for the beds are horizontal, and,
as we follow them toward the ridge, there is a distinct increase in the amount
of granite fragments in the sediment. Therefore, the ridge surely existed in
Eocene time. The Cretaceous beds are not faulted, are only very faintly
warped, and contain abundant fragments of granite waste. It is probable,
therefore, that Oshigo Ola is not a post-Cretaceous fault-block but was a
knob upon the oldrock floor when the Iren Dabasu beds were being laid down.

‘It is still partly buried in sediments, and its preservation throughout Upper
Cretaceous and Tertiary time can be explained only by supposing that it was
buried during most of this interval.

Many other hills of hard rock rise through the gobis, and their history, if
studied, would probably be very like that of Oshigo Ola. Two such exhumed
monadnocks are shown in the sketch of the Oshih basin (Fig. 118, page 269).
It is not impossible that some of the major ranges, such as parts of the Altai,
may have chapters in their history that tell of burial and reexcavation, though
in their case there is always a more extensive history of faulting and warping.

Volcanic mountains

Although in the Dalai tala, volcanoes with fresh cones are reported by
Teilhard de Chardin (1924, c), no true volcanic cones were found in the terri-

SURFACE FEATURES AND THEIR ORIGIN 319

tory covered by our Expedition. However, about one hundred and ten miles
southeast of Urga, we passed two tiny, cone-shaped hillocks, not over sixty
feet high (Fig. 25, page 79). They are covered by a rubble of spongy scorias,
which, under the microscope, are found to be chiefly fused clay. For this
reason, they should be considered as outlets of hot volcanic gases, a very spe-
cial type of volcano. They probably are of Quaternary age.

In all other areas traversed by our Expedition, the volcanic cones that
must have existed in the past have been worn away. Lava flows, which
have been buried in sediments for several geologic periods, and upstanding
dikes and necks were found at many places. For example, isolated black
peaks were seen in the distance, from the vicinity of Camp Jurassic, west of
the Urga trail, but were not visited. Their real character, therefore, is not
known; but the fact that volcanic eruptive material is abundant, and that
these hills lie along the projection of one of the major fault lines, seems to favor
the conclusion that they are remnants of small volcanic outbursts (page 68;
Fig. 18, p. 65). North of Djadokhta there is a sharp volcanic neck called
Khurul Obo—the Bronze] Peak—surrounded by many dikes and flows (Figs.
67, 68, page 159). Together they represent a volcanic center extending over
fifty square miles. The original cone of Khurul Obo has been eroded away.

No laccolithic mountains have been reported by any observers in Mon-
golia.

THE STRUCTURE LINES OF MONGOLIA

By way of summary of the general relations of mountains and basins, it
is well to review briefly the structure lines of the region; that is, the compass
directions in which the following structural elements of the rocks lie: the
cleavage-planes of gneisses, schists, and slates; the axial planes of folds; the
strike of tilted rocks; the strike of fault-planes; the axial trend of mountain
ranges and basins.

In Mongolia these structures can be considered in four classes: (1) Those
of the oldrock floor of Mongolia; (2) those of the Tertiary fault-blocks; (3)
those of the warped and faulted talas; (4) those of the gobis.

1. The trends of oldrock lines belong to two groups (Plate IV, page 22):

(a) A group with a general northeasterly direction, includes:

The Khingan trend, a little east of north, for the eastern ranges (von
Richthofen, 1882, II, pages 520, 734).

The northeastward Baikal trend for Transbaikalia (Obruchev, 1899, and
1914) (Fig. 135).

The Sayan trends in the region west of Lake Baikal; they swing in a great
S-shaped curve from northeast-southwest to northwest-southeast and back
again (Suess, 1902, III, pages 90, 91, 102, 107).

320 GEOLOGY OF MONGOLIA

(b) A group with a general easterly direction, includes:

The Altai trend, curving from southeastward to nearly east and west, for
the region between the Tannu Ola and the Tien Shan (Suess, 1902, III, pages
FIT, 226, 132):

The Kuen Lun trend, swinging from 20° north of east to 15° south of east
for the Richthofen, Nan Shan, and Tsingling Shan ranges.

The structure lines of the ancient gneisses and schists vary extremely.
East-west lines predominate. In the region traversed by the Expedition, we

TECTONIC MAP
of southern
Tranobaikalia
compiled by

V. A. Obruchev A

,

—~_- Faultlines without topo
jason

seiR Distribution of massive rocks

FiGuRE 135.—Tectonic map of Transbaikalia (after Obruchev). The faults which mark the present
Tanges are drawn in heavy broken lines; areas of massive igneous rocks are shaded with short dashes. Faults
of two trends are shown, the northeasterly Baikal trend and the northwesterly Khingan trend.

noted nearly north-south strikes at a few places, although in the main they
follow the Altai trend, varying between N. 60° E. and N. 60° W. The strike
of the latest pre-Cambrian graywackes varies also, but in most places it aver-
ages N. 70° W. Near Urga, the strike swings toward northeast as much as
N. 40° E., approximating the trend of Lake Baikal. The Paleozoic rocks
strike about N. 80° W., though N. 65° E. was observed near Sair Usu. The
Jurassic folds are the least regular that we have seen, but even these vary,
for the most part, within twenty degrees of the east-west line. Much more
detailed study is needed before this system of folding can be reconstructed.
2. The faults of Tertiary and post-Tertiary ranges trend, in many places,
at an acute angle to the direction of folding in the rocks which compose the

SURFACE FEATURES AND THEIR ORIGIN 321

mountains, and there are oblique faults which divide the Altai into separate,
overlapping ranges. But, taken collectively, all the Altai blocks form a series
of somewhat arc-shaped chains, lying, as Obruchev has pointed out (1912,
1915), with their convexity toward the south. Granting all divergence from
the structure lines of the older folding, there remains, through at least five
mountain-making revolutions, a fair agreement between the old and the new
trends. The repetition of trend in mountain-making revolutions which are

\

SIBERIAN
LOWLAND

‘ike,
Ay;
Anh, SS

oe

fe

ASIA 0

STRUCTURE LINES AND BASINS
COMPILED FROM THE WORK OF
SUESS , WILLIS, HOBBS, STAMP, GREGORY
UMPELLY , HUNTINGTON , KONSHIN , BURRARD, HAYDEN
GRABAU, DE LAUNAY, LEUCHS, OBRUCHEV, GERASIMOV

FicurE 136.—General structure lines of Asia. The solid lines represent the trend of folded and faulted
mountain ranges. The stippled areas indicate the great interior basins. The shaded areas represent the
recognized positive elements.

separated by vast time intervals is also well shown in the Khingan range,
where the ancient pre-Cambrian gneisses and the Tertiary arch lie almost
parallel to one another, though this direction is nearly at right angles to that
of the Altai ranges.

3. The talas of the faulted province are distinctly elongate east and west.
The Iren tala is nearly round, and the Dalai tala’s longer axis lies parallel to

VOL. II—2I

322 GEOLOGY OF MONGOLIA

the Khingan. Evidently the basins are controlled by the same regional ten-
dencies of deformation that have upwarped and upfaulted the mountain
blocks and inter-tala divides.

4. The gobis are less definitely outlined, but in those of more readable
history and more suggestive form, it is clear that the east-west axis is the longer.
In some cases at least, there is sharp flexing or minor faulting along the north
or south margins.

The axes of the Cenozoic mountains and lowlands (Fig. 136) agree very
nearly with the strike of the folded Palzozoic sediments and with the struc-
ture lines of the ancient schists and gneisses. The general agreement is far
more striking than the local divergences. The folding, thrusting, and fault-
ing that have made mountains, have followed much the same directions in
successive revolutions since early pre-Cambrian times, indicating an orderly
constancy of control in the earth movements that, throughout the immensely
long sequence of sedimentations, foldings, and igneous intrusions, have welded
together the continental elements that are now Asia.

CHAPTER XIX

SURFACE FEATURES AND THEIR ORIGIN (Continued)

PLATEAUS, PLAINS, AND FLATLANDS

INTRODUCTION—THE LEVEL LANDS OF THE DESERT

BETWEEN the mountains around and within the great basin of Mongolia,
we journey across wonderfully smooth and level areas that have been called
“‘plains’”’ by travelers in the past. An examination soon shows that in one
place the smooth surface is underlaid by granite (Plate X, B, page 60), in
another place by folded slates (Plate XXII, A, page 183), in a third by tilted
sands and clays (Fig. 111, page 245), and in others by horizontally lying strata
of many kinds (Figs. 137 and 138). These flat surfaces are the result of
erosion, but the granites and closely folded slates belong to ancient mountain
structures which have been worn down, and only the horizontally lying strata
come within the definition of the true plain or plateau. A plain is a region
of horizontal strata, smooth, or dissected into low relief (Johnson, 1916,
page 444). The plain and plateau differ only in that the plateau is carved
into high relief. In Mongolia, the horizontally lying strata—either plains
or plateaus—are found in the gobis, and therefore all the typical sediment-
basins or gobis are true plains or plateaus, resting upon a foundation of com-
plex mountain structures. They would be classed as plains throughout the
open country of the large warped talas, where their relief is low—rarely exceed-
ing 300 feet (Plate X, A, page 60). Deep dissection in the piedmont basins
along the Altai has developed such rugged country in the sediments that pos-
sibly the term ‘‘plateau’’ would be appropriate there (Plates XXXV, XXXVI,
page 268). The only term we used in the field for the horizontal or but slightly
tilted strata was ‘‘plains’’—because the areas of rugged relief are relatively
small pockets of dissected badland country along the few stream valleys or
around the walls of undrained hollows.

The erosion surfaces which bevel the gobis and the hard rocks cannot
be called peneplanes, for a peneplane is a vast erosion surface developed by
streams whose base-level is the sea (Davis, 1909, and Johnson, 1916). A true

323

324 GEOLOGY OF MONGOLIA

peneplane, therefore, cannot be carved in an inland desert basin. Pending
the general adoption of a special name for the desert erosion surfaces, we shall

Old rock fills PangKiang lowland

FiGuRE 137.—The Gobi upland at Ardyn Obo. The field sketch shows: (1) the remarkably smooth
erosion plane of the upland, developed on nearly horizontal strata; (2) the extremely short gullies dissecting the
scarp; (3) a portion of an extensive lowland, representing the P’ang Kiang stage of dissection.

call them simply erosion planes, and have grouped them, because of their
similarity and wide prevalence in the Gobi region, under the generic name
“‘the Gobi erosion plane.”’

AGENCIES WHICH CARVE THE EROSION PLANE

The only agencies which are capable of carving the erosion planes of the
desert are wind and running water, assisted by weathering. It is impossible

FicurE 138.—Field sketch of the scarp of eroded Eocene sediments west of Irdin Manha. The smooth
erosion plane of the upland bevels the white and gray strata. During Eocene time a small stream channel
in the white sands was filled with gray clay. A remnant of the clay is still preserved beneath the thin cover
of modern desert gravel.

that waves or glacial ice had any part in the work. The wind acts not only
as a carrier for loose waste, but as an actual sculptor. However, the relative
importance of wind work as compared with that of running water is difficult

SURFACE FEATURES AND THEIR ORIGIN 325

to evaluate. There are two general schools of thought on this subject. One
group of authorities (Gilbert, 1880; McGee, 1897; Paige, 1912; Bryan, 1922,
1923; Jutson, 1914, Lawson, 1915) holds that running water is a potent, even
a dominant, factor in the sculpturing of desert surfaces, while other author-
ities (Passarge, 1904, 1909; Bornhardt, 1900; Walther, 1912; Davis, 1909, for
the later stages of the desert cycle; Hobbs, 1917, 1918; Keyes, 1903, 1908a,
1916) attribute the chief r6dle to the work of wind. We shall review briefly
the results which in Mongolia we attributed to each of these agencies—wind
and running water—before discussing the origin of the erosion planes.

Erosion by wind

Despite the stinging sandblast of the big storms, which makes one’s
skin smart, there is less evidence of actual abrasion by wind than might be
expected, and erosion forms are almost exclusively the result of weathering
and running water. In the hard-rock regions, we saw monument-like piles
of granite, many with overhanging ledges (Plates X, B, page 60, and XI, B,
page 61). If they had been carved by sandblast, the undercut bases of the
columns, where the attacking sand grains are largest and most numerous,
would soon lose their husk of weathered and loosened crystals, and the sand-
blast would bite into the fresh granite, while higher up, where fewer and smaller
grains strike, the rock should stand out as an overhang, its surface deeply
weathered but not wind-carved. This interpretation is not sustained by the
facts. In every case, the undercut pedestal is so deeply weathered and dis-
integrated that the specimens we collected with our hammers crumbled to
bits. In the same regions, some of the ‘‘monuments”’ have a pyramidal
form, and the lower part, most exposed to wind abrasion, is unworn. Thus,
both the projecting ledges and the spiring tops are due chiefly to simple
weathering processes, acting upon unequally resistant portions of the rock.
Where the granite is coarsest, where the crystals are less firmly interlocked,
or when the joint cracks are closely spaced, the rock tends to disintegrate
more rapidly. No doubt, the wind is one of the destroying agencies, but it
plays by no means the chief réle, since the slow process of disintegration can
outstrip it.

In the soft sandstones of the gobis, undercut cliffs were seen which showed
evidence of true windblast. The surface is fluted and channeled parallel to
the direction of the wind (Plate XIX, page 156), and every little concretion
or pebble stands out as a resistant button, followed by a trailing ridge of
protected sand on its leeward side. The surfaces parallel to the prevailing
direction of the strongest winds are those most effectively carved. Bluffs
that stand transversely to the prevailing winds are commonly banked by
drifting sand, and so are protected from erosion.

326 GEOLOGY OF MONGOLIA

The wind undoubtedly plays an important part in excavating the un-
drained hollows which form one of the most conspicuous features of the desert
landscape; but as their development involves a number of complexly inter-
acting agencies, it seems best to treat the origin of desert hollows as a special
subject (page 336).

The destructive work of wind is very easily hindered. A blanket of
drifting sand protects level surfaces and even steep declivities from active
erosion. Where level surfaces are exposed to the strongest blasts, the wind
carries away the finer particles of weathered rock, and the heavier and firmer
fragments remain as a ‘‘desert armor’”’ which reduces wind-erosion to almost
infinite slowness. If there is enough moisture to support either a lake or an
effective mat of vegetation, wind scour is checked. The wind is, therefore, a
weak agent of erosion, abrading rocks no faster than they break down under
ordinary atmospheric weathering. But as a transporting agency, moving the
finer and lighter materials about and depositing them, it plays a vitally im-
portant part, and is the only power that can carry sediment out of a wholly
enclosed basin (page 336).

In taking up the work of running water in a desert region, the questions
which first arise for consideration are: How much water does the desert
receive, and how is it distributed?

Rainfall

Rain, in this northernmost of the world’s desert basins, is most frequent
and heaviest along the Arctic and Pacific divides, and lightest over the open
lowlands. The bordering ranges, receiving their supply from the great stretches
of Siberia on the one hand, and the Pacific Ocean on the other, have a richer
water supply than even such high interior mountains.as the Altai. Lacking
all figures, and basing our estimate upon the general aspect of the vegetation,
we hazard a guess that the rainfall varies from about thirty inches along the
Arctic divide and about twenty inches on the Pacific divide, to eight or ten
inches in the arid centers.

The Mongols tell us that the midsummer rains, occurring in late July
and August, are the heaviest of the year. As a rule, the rain falls in light
showers, occasionally in long wetting drizzles, and still more rarely in short
squalls that beat through the tents and cover the flat desert surface with a
thin sheet of water. We have seen as much as one third of an inch fall in
a single storm; but we never saw such violent ‘‘cloudbursts’’ as Walther de-
scribes for the tropical deserts: storms which, at long intervals, break over
quite restricted areas, carving and removing large amounts of coarse material
in a very short time. For example, at Wadi-Abu Shuasha, ‘‘blocks of the
pavement ran for many meters through the streets of Helwan, gliding on the

SURFACE FEATURES AND THEIR ORIGIN 327

small pebbles as if on rollers’”’ (1912, page 34). The evanescent streams in
the Gobi must be vigorous when they run at all, judging from the great ripple-
marks and scour channels and from the cobbles and coarse gravel which they
move; but the moraine-like masses of washed-out débris, described by Walther,
are absent. It seems probable that in tropical deserts, especially those near
the sea, sudden and extremely violent storms are to be expected, but that
rains are lighter in the cooler deserts of higher latitudes, even though there
be a greater annual rainfall. There is a light winter snowfall, but not enough
to prevent the herds from grazing on the dried remnants of the summer vege-
tation. This condition is so nearly uniform that hardly any of the Mongols
store winter food for their herds.

Drainage

In a well-watered region, large rivers can maintain their courses during
deformation of the country, provided the rate of deformation is slower than
the rate at which the river can cut down its bed. In a desert, where few
streams are constant and fewer streams are large, the rivers, as a rule, must
adjust themselves to any change of slope that may arise through earth move-
ments (Davis, 1909, page 299). Nearly all the streams of Mongolia are
consequent upon the slopes of the warped and faulted basins and ranges.
They therefore flow inward, radially, toward the centers of the depressions.
This centripetal drainage pattern is repeated on an ever diminishing scale
from the great basin of Mongolia and its subdivisions, the talas, down to the
small erosion hollows excavated in the desert surface, each of which has its
own little system of contributory gullies. The larger rivers are not connected
to.form an integrated drainage system; instead, each terminates in a lake.
The smaller drainage systems are still less integrated, most of the streams dying
out before they reach the center of the desert hollows. There are two general
types of stream in the Gobi region: (a) Streams rising beyond the desert
border in humid areas, and flowing thence into or through the Gobi Desert;
(b) streams that originate in and belong wholly to the desert. The first type
may be called immigrant streams, and the other, native streams.

Immigrant Streams.—Most of the larger rivers of the Gobi are immigrants:
either the traversing rivers that pass through the desert, or the terminating
rivers that die in the desert. Only two reach the ocean, the Hwang Ho and
the Kerulen. The Hwang Ho, in passing from Lanchowfu to its junction with
the Wei Ho, makes a great bend around the Ordos Desert, and virtually into
the Gobi. On the northern border, the Kerulen rises in the well-watered
Kentai Ola and runs through the desert to the Dalai Nor which drains into
the Amur River. Most of the larger streams of the Gobi rise in the rainy

328 GEOLOGY OF MONGOLIA

mountains, and terminate in the lakes of the desert basins. They have many
tributaries near their headwaters, but few or none in the arid lowlands, where
they flow through swampy valleys, walled in by low, steep bluffs. If they
receive water in the desert, it must come from local rains, springs, and seep-
ages. As they run, they lose water through evaporation, and locally through
seepage, until the loss just balances the supply. Here they end, whether the
terminal basin is reached or not. Thus, the Ongin River ceases to flow as
a surface stream long before it reaches its terminal lake, Ulan Nor; and the
Tatsin River for many years has not as a living stream touched Tsagan Nor,
which is its geographical terminus (Fig. 98, page 194). While the immigrant
rivers from the Arctic divide run to within twenty miles of the Altai front,
most of the native streams rising in the Altai die out at its very foot.

Native Streams.—In contrast to the long streams that head in the border-
ing mountains, the desert is carved by many short river systems which depend
upon local rain, melting snows, and ground-water issuing as seepages and
springs. Many of the mountain-fed streams disappear into the gravels of
the alluvial fans. Where mountains immediately adjoin gobis which are
filled with porous sediments, the sinking streams and melting snows of the
mountains feed the sponge of sediments, especially when the strata in the
basin are upturned along the mountain front. The ground-water table in
the mountains is higher than in the gobis, so that water also passes underground
through crevices from the mountains to the sediments. The mass of ground-
water moves slowly downward toward the depressed centers of the talas and
gobis. As it moves, some of it is drawn upward by capillarity and evaporates
from the sun-baked surface. Whenever the water-table meets the surface
of the earth, seepages and springs break forth, which may feed existing rivers,
or may start new drainage systems.

Rainwash will, at favorable places, incise the desert surface and start
a new set of stream heads, which may, or may not, receive support from
ground-water. Running together, great numbers of such rills form a river
which cuts a valley in the erosion plane of the desert. But the thirsty air
takes its tribute of vapor, and water is lost by sinking into the ground, until
the new river finds its load too great to carry, and deposits part of it. The
stream now ceases to cut a valley and spreads over the surface, laying out a
broad thin apron of loose sediments which is attacked by the wind as soon as
it is dry.

Farther down the slope of the erosion plane, the rain-wash and ground-
water may again start a new system of streams—a system which converges
into a main trunk and dies out, like the first, in a flat outwash apron. Streams
such as these may succeed one another until the lowest one falls into a lake

SURFACE FEATURES AND THEIR ORIGIN 329

basin where a final balance is struck between water supply and evaporation.
This type of stream does important work in carving desert hollows, and is
discussed further in that connection on page 338. One of the best examples
of this type of stream is the Hsanda Gol, of which we made a reconnaissance
map (Plates XXVII, XXVIII, and XXIX). It collects innumerable tribu-
taries about its head, which lies between Uskuk Mountain on the west and the
three big buttes on the east. It carves a trench 150 feet deep and locally
more than half a mile wide. After running southward twenty miles and de-
scending more than 1,500 feet, it emerges into a broad desert hollow. The
walls on either side of the valley turn east and west away from the river,
which there distributes a broad, flat, alluvial fan over several square miles
of country, and comes to anend. The amount of sediment it has actually
laid out is astonishingly small when compared with the immense excavation
of its valley and the valleys of its tributaries. Going southward, within
three miles after leaving the valley of Hsanda Gol, the river deposits thin
out and come to an end, and we find ourselves again traveling upon bedrock.
We suppose that, when the river is in flood, it spreads out so thin a sheet of
fine waste that much of it can be carried away by the wind after the river has
withered.

Erosive work of running water

One of the many surprising paradoxes of the Gobi Desert is that the most
important eroded surfaces are not, as one might expect, the work of wind,
but the result of erosion by running water.

In the well-watered mountains, such as the Khangai, the rivers merit no
special discussion, as they behave like mountain streams in any other part of
the world. But in proportion as the mountains become drier, the valleys
become more closely spaced—another of the paradoxes of the desert; for with
a thinner web of roots to hold both soil and water, the rainwash cuts shallow,
closely spaced gullies instead of the fewer and larger valleys of the Khangai
(compare figure 106, page 224, with figure 149, page 344). When rain falls
or snow melts in the dry mountains, the water finds a steep slope to give it
power, and plenty of loose material to carry; and in its short-lived vigor, it
rushes blocks, bowlders, and sand down stream. The pebbles and sand are
more easily moved than the larger blocks, and eventually the floors of the lower
valleys, where the gradient is less steep, become armored with bowlders, while
the finer materials are carried down into the alluvial fans. Not all the streams
acquire their armor by the simple method of transporting it; some inherit
bowlders which descend from terraces through which the stream has cut, and
some receive them from landslides or from cataract tributaries. Weathering,
soil-creep, rainwash, and tributary streams continue to wear the walls of the

330 GEOLOGY OF MONGOLIA

valley, while the main river does at least a little lateral cutting. The valley
thus widens slowly in its lower reaches, without much deepening, provided
the supply of water remains nearly constant; but in wet climatic cycles, the
increase in rainfall revives the river, which, leaving a remnant of the bowlder-
strewn floor as a terrace, sinks a new narrow trench in the bedrock, at the
same time dissecting its own alluvial fan along the mountain front.

THE PIEDMONT SLOPE

The gently sloping surface that descends from the mountain is an erosion
plane, diversified by alluvial fans draped along the front of the range (Fig.
139). The slope is concave, steepening toward the mountain base, until it
attains a gradient of five to seven and even ten degrees, which exceeds by
three to four degrees the maximum inclination of the alluvial fans.

In August, 1922, we were camped at the foot of Artsa Bogdo, on the rock
shelf which stands forty feet above the streams issuing from the mountain,

=e

eR Le
2 8——Caravan Trail

FiGuRE 139.—The piedmont slopes of Los Ola, froma photograph. The piedmont slope is a composite of
eroded rock bench and alluvial fans.
and has a slope of eight degrees. During a rainstorm which lasted for about
two hours, the water swept in a sheet under our tents, though after the heaviest
squall it settled into shallow runways. The rock surface of the erosion plane
is marked by an infinite network of shallow rill courses, and is thinly and un-
evenly strewn with small chips of native rock, which no doubt are carried by
vigorous little streams such as we saw form and fade during the storm. They
are the tools with which the running water abrades the bédrock, which is so
deeply weathered and disintegrated that we found it impossible to secure
fresh samples, except in the walls of the deeper gullies; for mechanical and
chemical weathering are especially active along the mountain foot, where
rocks are alternately saturated and dried at frequent intervals, and where
the surface run-off and the underground escape of water are slower than in
the steep mountains. The wind does not contribute to the carving of the
erosion surface, except in a very minor degree; the chief réle must be ascribed
to the myriad short-lived streamlets acting upon the weathered rock of the
piedmont. These transient rills run together into little drainage systems,
which are not wholly integrated, but may either fall into some larger channel,
or may die out leaving a thin, flat cover of angular fragments over the slope.

SURFACE FEATURES AND THEIR ORIGIN 331

The larger channels of the mountain streams have cut shallow but steep-
sided trenches across the slanting piedmont surface, whether this be composed
of alluvial fans or bedrock or both. Their intrenchment is due at least partly
to a revival of the streams during the latest epoch of wet climatic conditions.
The valleys flatten out at varying distances from the mountain front, and the
streams vanish into the general waste-covered slope. Still farther down, new
systems of valleys appear, where ground-water codperates with rainwash
along the thinning edge of the alluvial fans. The piedmont bench is thus a
complex surface, or rather several surfaces of diverse age and origin, including
alluvial fans formed by deposition, rock benches cut by streams, and the ever
widening valley bottoms of the streams which dissect the fans and the benches.

In order that a nearly smooth surface may be carved across rocks of
diverse quality and structure, there must be some means of limiting the tend-
ency to cut downward; otherwise, all streams would continually deepen their
valleys, and those which rest upon soft rocks would dig in faster than those
which flow over hard rocks. The influences which tend to limit the depth
to which the piedmont streams may cut are the relations between the supply
of water, the load of sediment, and the gradient of the river-bed. When the
climate is dry, the supply of available débris is always large in comparison
with the running water, so that the floor of the lower part of the valley, near
the mountain front, becomes thinly armored with waste which the stream can
just move when in full flood. The rate of downward cutting is now slowed
to a minimum, and will remain so as long as the rainfall, and especially the
number of effective storms per year, stays fairly constant, maintaining the
equilibrium between current and load; a river in this balanced condition is
said to be ‘‘graded,”’ or “‘at grade.” (Davis, 1909, page 390.) With an arid
climate, a stream will become graded with slopes far steeper than in a rainy
climate. The small rills on the divides between the main streams, issuing
from the ravines in the mountain, are working with a steeper gradient on bare,
much weathered rock, and carry a light load. They cannot wear down quite
to the level of the graded master streams, which serve as a controlling base-
level, but they can keep the divides worn down to a slope not many degrees
steeper than that of the graded streams.

The whole piedmont slope becomes lowered very gradually as the graded
streams cut downward, and becomes lengthened very slowly as the mountain
front is cut away. Here and there, masses of upstanding rock are left tower-
ing above the nearly smooth surface. Just why they should be spared in
the general planation, it is often difficult to see. Some are preserved because
they are made of or are capped by hard rock, such as lava or conglomerate;
but others, such as folded limestones or uptilted porphyries, are apparently
of the same stuff as the beveled floor surrounding them. Where there is a

332 GEOLOGY OF MONGOLIA

piedmont slope, it merges into the smooth erosion surface which bevels the
broad gobi basins, and which we shall call the Gobi erosion plane. ‘This does
not mean that a single erosion plane extends continuously throughout the
minor basins of Mongolia, for a very similar surface is developed in separated
basins and at different elevations above sea level.

THE GOBI EROSION PLANE

The popular concept of the typical desert basin is a sediment-filled hol-
low, in which material washed down from the surrounding mountains has been
collecting continuously from the time of the formation of the hollow to the
present. If this were the case, the topmost sediments would be recent, and
the surface of the basin would be, in the main, an original surface of deposi-
tion, sloping at the angles of repose characteristic of alluvial fans, flood plains,
deltas, and lake deposits. On the contrary, in Mongolia, most of the sedi-
ments are Cretaceous and Tertiary, and the surface proves to be one of erosion
rather than of deposition. East of the southern margin of Uskuk Mountain,
the surface bevels steeply upturned sediments (Fig. 111, page 245), while
along the general line of the Hsanda Gol, the nearly flat upland rests succes-
sively upon beds ranging from Lower Cretaceous to Miocene age; near Shara
Murun, the same smooth and almost flat upland bevels Oligocene and Eocene
formations; between Irdin Manha and the Houldjin bluff, a similar level
upland passes from Eocene to Oligocene beds (Fig. 102, page 201). Even
upon horizontal strata, where the evidence is less openly readable, the fact
that the surface rests successively upon formations of diverse age can only
mean that it bevels across their structure. It is impossible that the original
surface of a Cretaceous deposit could be preserved so perfectly that it would
be now in the same stage of dissection as the original surface of an Eocene or
Oligocene deposit, nor could they both be in the same stage of dissection as
an original surface of Oligocene or Miocene age. The striking similarity in
the smooth uplands, developed upon rocks of diverse age, resistance, and
structure, throughout a vast area, must mean that these surfaces are erosional,
are due to the same agencies, and are everywhere of nearly the same age.

Superposed on the erosion surface are Pleistocene fans and gravels, and
post-Pleistocene sands forming sheets and dunes. Here and there the upland
is carved by streams, and in many places it is indented by large and small
hollows.

There are thus four elements in the surface of the desert basins—the
first three belonging to the erosion plane: (1) Slopes and irregularities of
the original erosion surface; (2) deformation of the surface by tilting or warp-
ing; (3) erosion of the surface; (4) deposition of material upon the surface.

SURFACE FEATURES AND THEIR ORIGIN 333

In a region where warping has taken place at intervals throughout the entire
history of basin-making, it may be very difficult to tell an original slope from
a deformation.

Original slopes of the erosion plane

The plane is remarkably smooth; in some places the expanse is so broad
that the horizon appears as unbroken and as level as the sea-circle around a
ship. The gradients of most of the Gobi uplands cannot be stated accurately,
as the Expedition traversed most of them along a single line. Some of our
records of extraordinary flatness, for example across the Irdin Manha basin
(page 57), are probably due to our having traveled almost exactly at a right
angle to the direction of the slope.

The Gobi upland, east of the Ongin Gol valley, between Khurul Obo
and the Sair Usu trail, is one of the smoothest stretches we have seen (Figs.
69 and 70, page 160). It rises northward, at about ten feet per mile—a
gradient that in a wet climate would give ample energy to running water; a
gradient many times that of the Mississippi below St. Louis. It seems pos-
sible that, in a climatic cycle somewhat more humid than the present, sheet
flood erosion and the wash of a net of small local streams might do much toward
the leveling of an erosion surface upon soft Cretaceous or Tertiary sediments.

Deformation of the Gobi surface

Great caution is required in judging whether a given surface has been
tilted by earth movement or carved at a slant. In general, slopes which are
notably steeper than those of the graded streams are judged to have been
tilted. Arched or domed surfaces are undoubtedly deformed.

As one of the best examples of deformation, we may cite the domed
surface east of the Uskuk block (Fig. 108, page 239), an account of which is
given in Chapter XIII. Farther south, near the Altai front, there is evidence
of faulting since the broad upland valleys were carved (Plate XXX, in pocket).
In a later publication, we shall offer a more complete discussion of the pied-
mont of Baga Bogdo.

In some cases, slight deformation of the Gobi surface seems to have de-
termined the courses of rivers whose valleys now dissect the erosion plane.
For example, the course of the Hsanda Gol is down the shallow notch between
the Uskuk block and the Shiliuta dome just east of Uskuk.

Deposits on the Gobi upland

Two characteristic deposits overlie the Gobi upland. Gravel covers by
far the larger part of the upland surface. Its only rival is wind-blown sand,
which, though far less extensive, covers broad areas. The gravels on the

334 GEOLOGY OF MONGOLIA

erosion plane are of two general kinds: those which come from the underlying
materials, and those which are carried from some other place.

As the surface of the desert becomes lowered by weathering and removal
of the finer grains, the most durable materials remain on the surface, gradually
accumulating until they form a cover (Fig. 140). We have observed the fol-
lowing common types: (1) Dense dike-rocks and veins of quartz, epidote

Original Surface

Pavement of Residual Gravel _

FIGuRE 140.—Diagram illustrating the origin of residuary gravels. The left half of the figure shows a bed
of friable sediments containing pebbles. The right half shows the fine material blown away, concentrating the
pebbles in a surface gravel.

and colloidal silica, weathering out of granites; (2) quartz veins, weathering
out of schists, phyllites, and graywackes; (3) veins and masses of jasper or
chalcedony, weathering out of ash or sediment which they have replaced;
(4) beds of silicified ash, breaking down into hard strong fragments; (5)
amygdules of silica, weathering out of basalts; (6) flints and partly silicified
concretions, weathering out of limestones; (7) pebbles, released from con-
glomerates through the dissolving of the interstitial cement; (8) concretions
of sandstone and lime. Neither of these types of concretions lasts long.

Each type of pebble is especially abundant near its source, but all peb-
bles tend to be distributed over the surrounding country wherever a downward
slope favors the wash of shallow temporary streams. The gravel cover may
be almost wholly a residue of the decaying rock beneath, or it may be wholly
washed in by streams, or it may represent a mixture of residual and trans-
ported pebbles.

Where the surface truncates the lava flows of Cretaceous or Tertiary age,
the gravels consist largely of chalcedony and agate amygdules, weathered
out of the basalt. The pebbles have traveled somewhat, as foreign pebbles
are mingled with the residuary agates, while the latter are found far outside
of the lava areas. At Oshih, for example, the gravel overlying the basalt
consists almost wholly of chalcedony amygdules weathered out of the lava,
while the exposed sediments are covered by pebbles which could have been
concentrated from the pebbly strata (Fig. 140). At Ardyn Obo and at Irdin
Manha the upland gravels are largely residual, as the pebbles match those of

SURFACE FEATURES AND THEIR ORIGIN 335

the underlying beds. Even in these places there has been some transporta-
tion, for locally we find the gravel concentrates in shallow channels, where the
contact between the gravel and the underlying sediment is sharp.

On the upland at Djadokhta, the dark gravels are one to five feet thick,
and are notably cross-bedded. They contrast sharply with the fine uniform
red sands of the Djadokhta formation. The latter does not contain such peb-
bles, and therefore the prolonged erosion of the sand could not have yielded the
gravels as a concentrated residue. The fact that the pebbles were not derived
from the underlying sediment would not, of course, preclude the possibility that
some younger formation, such as the Gashato, which once rested upon the Dja-
dokhta, might have yielded the pebbles, and might have been wholly destroyed.

At Shara Murun, the erosion plane descends westward across Oligocene
beds of two diverse aspects, and one Eocene formation. It is mantled with
gravel, which varies in thickness from three to twelve feet, and is everywhere
coarsely cross-bedded. The gravel is clearly later than Oligocene, and rests
upon the smoothly eroded surface of the early Tertiary beds. It evidently
was washed into place by streams—very probably the same streams that carved
the surface upon which it rests. In general, the residuary gravels are thinner
than the beds of transported pebbles. Very commonly, a cement of limy
matter deposited by evaporating ground-water binds the upland gravel loosely
together, or in rare instances, cements it into a massive conglomerate.

Where the Gobi upland is trenched by stream valleys or by undrained
hollows, the gravel cover is redistributed over the slopes and upon the new
lowland. In some cases, it covers the floor as effectively as it formerly covered
the upland. In others, it may be commingled with, or buried by finer sedi-
ments washed from the surrounding scarps. Even in the latter case, the
selective removal of finer dust and sand by the wind tends to bring the pebbles
to light again, and to reform the gravel armor.

The only other cover commonly seen upon the upland is sand, which is
blown out of the valleys and hollows, and is spread over the upland in the form
of a thin sheet, or of hummocks, held by grass and bushes; more rarely the
sand may be heaped up as veritable dunes. Wind-blown deposits are dis-
cussed further on page 350.

Redissection of the Gobi erosion plane

At the present time the Gobi erosion plane is not being formed; on the
contrary, it is being eroded and destroyed—in part by streams and in part by
the work of the wind. Undoubtedly there has been a change from conditions
which favored the making of such a plane to conditions favoring its destruc-
tion.

We have considered the probability that, in carving the smooth erosion

336 GEOLOGY OF MONGOLIA .

plane of the Gobi, a considerable part was played by graded streams. An
increase in water or a deformation which resulted in steepening the slope
would cause all rivers to incise their channels and sink new and narrow
trenches into the underlying rocks. A lessening of either water or slope would
cause the streams to die at shorter distances from their sources, dropping their
load, and so building up the surface until it attained a slant that again estab-
lished a graded condition of the rivers. In the case of a diminution in the
activity of running water, the wind would accomplish more efficient erosion,
partly because the protecting tissue of plants becomes thinner, and partly
because the power of the wind has remained constant while that of its great
rival, water, has diminished. Wherever the ground is especially favorable
to attack by wind, undrained hollows will be excavated.

Summary of the Gobi erosion plane

The Gobi erosion plane thus appears as a series of wide stretches of won-
derfully smooth country, sloping very gently except where it has been recently
deformed. It bevels horizontal and tilted strata with almost equal smooth-
ness, and in places laps over the complex rocks of the ancient floor. It is of
comparatively recent origin, as it overlies all the sedimentary formations,
including the latest Pliocene or earliest Pleistocene, yet it is being destroyed,
not formed, in the present epoch—a fact which indicates that it was formed
under somewhat different climatic conditions from those of the present time.
Its destruction is being brought about partly by the work of streams, especially
near the rainy mountains, and partly by the excavation of undrained hollows,
especially in the arid central basins. We are not yet sure what may be the
details of the origin of such erosion planes; more light is to be expected from
the study of regions where these planes are now being formed.

DESERT HOLLOWS

On the night of April 23, 1922, we camped near the telegraph station of
P’ang Kiang, on the floor of the first large, clearly defined desert hollow which
we had crossed in our journey (Fig. 141). Subsequently we were to see hun-
dreds of such hollows, some larger and many smaller than the one at P’ang
Kiang. They are all formed in the same way, and are of later origin than the
Gobi erosion plane in which they are cut. We gave the name “‘P’ang Kiang”’
to the stage of erosion which they represent.

The desert hollows could not possibly be excavated without the work of
wind; no other agency could lift material out of an enclosed lowland. Al-
most every day while we were camped in such a hollow, we saw the “‘ whirling
pillars’’ of dust racing across its dry floor. During violent windstorms the

SURFACE FEATURES AND THEIR ORIGIN 337

air was dark with flying dust and sand and the sun was dim and red. Dust
veils hung in the air for several days after the biggest windstorms. Much of
the stirred-up sand and dust settles upon the surrounding country, but no

Gobi Erosion Plane Pang Kiang Lowland

FIGURE 141.—The western scarp of the lowland at P’ang Kiang.

doubt some of the finer material is exported to great distances, even beyond
the rim of the Gobi.

We have tried to determine the conditions which favor the commence-
ment of excavation, and may list them briefly as follows: .

(1) The gravel cover of the Gobi upland is thin or absent;

(2) The underlying sediments are fine-grained, such as sand or sandy
clay, and are not well cemented;

(3) The ground-water level does not lie near the surface;

(4) Vegetation is sparse or has been killed by drought;

(5) A small pond dries, leaving enough salt to prevent vegetation from
covering the exposed bed, but this condition rarely obtains on the Gobi up-
land, though it is not uncommon on the floors of the hollows;

(6) A stream has cut a shallow trench in the upland, piercing the gravel
cover and exposing the finer sediments, which during the dry seasons or
epochs lie bare to the attack of the wind.

Any of these conditions, or any combination of them, may determine the
place where the wind can begin to excavate a hollow. How deep the wind
can dig is controlled, in turn, by a number of limiting factors, among which
are the following:

(1) The floor of the hollow may come near enough to the level of ground-
water to encourage a vigorous growth of plants, which checks further erosion.

(2) A layer of gravel may be developed, partly by blowing away the finer
particles from the beds of sand and clay, and concentrating the pebbles which

VOL. II—22

338 GEOLOGY. OF MONGOLIA

they contain, and partly by washing down gravel from the upland surface.
Where the lowland gravel is thick enough, it eflectively checks the ravages
of the wind.

(3) The floor may be cut down until it rests upon a layer so well cemented,
or composed of conglomerate so coarse, that the rate of wind-erosion is slowed
to a minimum.

(4) Rainy seasons or a rainy epoch may raise the erowndrwmater level
sufficiently to support a shallow lake in the-hollow. -

(5) Salts may be brought to the dry surface oe eepitianer, and may
cement the-ground so firmly, that it is virtually a rock, on which the wind
works very slowly. The drying lakes may leave crusts of salt, which protect |
the surfacé, and the salt absorbs enough moisture from the air, or from the
ground, to enable it to recrystallize and recement itself when broken by the
tread of animals. Nevertheless, ane the lake plain 1 is thoroughly dried, salt.
is exported along with dust.

_ Any of these conditions,’ or any poentinitiren of them, may | tend to aise
the depth of the hollow, which, throughout Mongolia, is shallow—rarely
more than 400 feet.

The sides of the hollow are dissected by rainwash and by rills from the
upland. During epochs of active erosion, the sloping bluffs are fretted into

FIGuRE 142.—The abrupt heading of the short gullies that dissect the scarp at Ardyn Obo,

typical badlands, and the loose sediment is washed down to the floor of the
hollow. As soon as the thin flat apron of sediment is dry, it is exposed to the
tireless winds, which carry much of it out of the hollow. The retreat of the
bluffs and the lateral enlargement of the hollow is thus mainly the work of
running water. Because most of the streams that run into the hollow are
temporary rills that flow only while the rain falls, vigorous cutting will not
be done, except on the steep slopes. Gullies are rarely seen on the floor of
the hollow, and still more rarely on the upland. The gullies along the bluffs
are very short, heading a little way behind the edge of the scarp and dying out
a short distance beyond its foot. (Figs. 137, 138, 141 and 142.) ‘There is

SURFACE FEATURES AND THEIR ORIGIN 339

thus a narrow zone of active stream-dissection along the scarp—a zone which
retreats with the scarp as the latter is cut back.

Where there is enough rain to encourage a thick growth of herbs and small
bushes, yet not enough to support a drainage system of permanent streams,

FIGURE 143.—Re-dissection of a smooth scarp at Shara Murun. The drawing illustrates the transition
from the smooth slopes, at the left, to the redissected badland condition, with reversed scarps, seen in the
foreground. :

the mat of vegetation may check the dissection of the scarp fora time. The

profile of the bluff weathers into smooth, gently rounded slopes (Fig. 143).
Locally, gravel from the upland washes down and mantles the lower slopes,

~ gravel
white sand —:
red sandy clay

FIELD SKETCH
of scarp at Shara Murun,
Showing gravel- covered
cuestas Book X,1923.p.47.

FIGURE 144.—Early stage in the development of a reversed scarp. The smooth back slope of the reversed
scarp in the foreground, preserves a remnant of the former condition in which the whole scarp had this form.

though it cannot cling to the steeper part of the declivity. It forms a pro-
tecting cover even in the absence of a well-rooted mat of plants.

With the approach of a dry epoch, the plant cover dies on the exposed
slopes, just where the steepness gives the rainwash its maximum speed and

340 GEOLOGY OF MONGOLIA

erosive power. Short gullies are carved (Figs. 144, 145, and 146), transform-
ing the green, rounded slope into a torn badland of exposed strata. The cliff
is cut back, enlarging the hollow, while the rills die out on the floor, spreading

FIELD SKETCH
at Shara Murun,look-
ing East. Shows revers-
ed scarps facing toward

the main scarp bie

FicurE 145.—Late stage in the development of a reversed scarp.

their burden of sands and clay. The significance of the periodic renewals of
dissection is considered further in Chapter XXI. :

Undrained hollows were found in granite regions also. They are most
common in coarse-grained granites, and seem to coincide, in part at least,

FiGuRE 146.—Reversed scarps at Shara Murun. The scarps resemble cuestas but are composed of hori-
zontal strata. The cross-section, below the sketch, extends a half mile farther out into the lowland than the
sketch above, and shows an additional older reversed scarp. The area of the section covered by the sketch is
included within the limits of the bracket.

SURFACE FEATURES AND THEIR ORIGIN 341

with the pattern of stream courses (Fig. 46, page 117). There is surprisingly
little waste in the hollows—only a thin cover of arkosic sand and, locally, a
rubble of disintegrating granite blocks. (See Plate XVII, A, page 143.)
The coarse granite tends to break up under the action of frost and changes of
temperature. It weathers chemically also, for the rock bears rusty stains,
and the feldspars are somewhat kaolinized. As the rock disintegrates into
grains which the wind can carry, these grains are removed, and the hollow
deepens until it approaches the depth at which ground-water lies. The sides
of the growing hollow are carved by short evanescent streams, just as in the
sediment-basins. During dry seasons or cycles, the wind will continue to
blow away the finer dust and to deepen the excavation. Should the ground
water level rise during a season or cycle of heavier rainfall, a pond will form
in the bottom of the hollow, and the exportation of material will be checked,
although rills which course down the sides of the bowl may continue to enlarge
it laterally. (See also Chapter XXI.)

THE OLDER PENEPLANES

The oldrock surfaces of Mongolia bear clear record of several periods of
planation older than the Gobi upland. We observed at least two such plana-
tion products, whose relations to one another are not quite clear. Doubtless
there are still others, less conspicuous, whose traces may serve to confuse
the problem (Fig. 110, page 242). Lacking reliable maps or surveys, and travel-
ing as we did along a single route, it was impossible to study adequately these
erosion surfaces, but, at least, the major features of the problem deserve a
brief statement.

The pre-Cretaceous peneplane

The oldest basin sediments, of Lower Cretaceous Age, rest upon a nearly
smooth erosion surface, which truncates the edges of the complex rocks of the
floor. In some localities it can be shown that the peneplane exposed at the
mountain tops, was once buried and is now being laid bare by the stripping
of the sediments which formerly covered it. This is the case at Uskuk Moun-
tain (Fig. 110, page 242), where early Tertiary sediments, and possibly
Cretaceous beds as well, once covered what is now the top of the mountain
block.

It is a possible interpretation, therefore, that the Mongolian peneplane,
which bevels the mountain tops, is the surface upon which the earliest basin
sediments were laid down, and this was, indeed, the theory with which we
at first worked in the field. But as we obtained increasing evidence of the
widely different periods of basin-warping, it was judged that the entire history

342 GEOLOGY OF MONGOLIA

was more complex than it seemed at first. The following considerations are
some of the elements of the problem.

Buried unconformities and disconformities are numerous and imply long
gaps in the sedimentary record. Thus, in the Altai region, at least two of
the gaps in the sedimentary record represent planations—one at the base of
the Cretaceous, and the other at the base of the Tertiary, when the faulted
and tilted Mesozoic rocks were reduced to base-level (see Chapter XIII).

The Mongolian peneplane

A clearly defined ancient erosion surface bevels all the’mountainous areas
of Mongolia. Remnants of an older unreduced upland rise above it as monad-

FiGcuRE 147.—The Mongolian upland in th The maturely dissected Mongolian
peneplane bevels hills of granite and schist in southern Mongolia. The distant landscape appears mono-
tonously smooth in every direction.

nocks, and the valleys carved out below the peneplane represent a dissection
so mature that by far the major part of the old flat surface has been destroyed.
Provisionally, this upland surface has been called the Mongolian peneplane
because of its widespread distribution in Mongolia.

Mongolian peneplane

S Monadnocks N

Inner Valley
Rock terrace jlola River

FIGURE 148.—The Tola River terraces. The figure shows: (1) the Mongolian peneplane with surviving
monadnocks, (2) the broad rock terrace of the Tola River, (3) the modern river trench within which the Tola
River meanders on an aggraded floor.

The peneplane was first observed in the granite mountains of southern
Mongolia, at an altitude of 5,300 feet (Fig. 147). But as we climbed toward
the Arctic divide, south of Urga, an old surface was again seen at about 6,000
feet, beveling the schists and the younger graywackes. At the Tola River,
near Urga, the following topographic elements were observed: (1) Monad-

SURFACE FEATURES AND THEIR ORIGIN 343

nocks rising above the Mongolian peneplane; (2) the peneplane; (3) a broad
rock shelf, or terrace, within the valleys of the Tola and its tributaries; (4)
an inner valley, sunk gorge-like below the rock terrace, yet having a mature
floor upon which the river meanders. These relations are expressed in figure
148.

The Khangai peneplane

When we climbed into the Khangai Mountains north of the great lamasery
of Sain Noin Khan, we saw a smooth upland, dissected by streams and, in
the highest parts, by former glaciers. Above the peneplane rise notable monad-
nocks. The physiographic unconformity between the valleys and the ancient
upland surface is clearly seen (Fig. 149). We were not wholly sure of the
identity of the upland with the Mongolian peneplane, and so called it pro-
visionally the Khangai peneplane. It stands at about 10,000 feet where we
observed it, and so should be older than the Mongolian peneplane, unless the
two erosion surfaces can be shown to be identical. There is a shoulder of
rock terrace bordering the valleys.

Descending from the Khangai Mountains and going southward, we fol-
lowed the peneplane, as carefully as possible, to test its relations with the
Mongolian peneplane. The valleys broaden southward, and the rock benches
within the valleys tend to coalesce so that remnant hills or outliers of the
sx<hangai Mountains are cut off from the long spurs and lie isolated, surrounded
by the erosion lowland. As we look southward, from such border fringes of
the Khangai hills, it seems that a new, lower beveling continues southward
over the tops of the ranges there. Lacking adequate maps or surveys, we
found it very difficult to be wholly sure of such a correlation; but if our observa-
tion was not at fault, it would support the inference that there are two pene-
plane surfaces beveling the hard-rock structures of Mongolia: an older Khangai
peneplane, and a younger and lower Mongolian peneplane (Fig. 150, A).

Relations of the Khangai and Mongolian peneplanes

If we assume that the Khangai peneplane is the older (Fig. 150, A), the
significant steps in the physiographic history may be summarized as follows:

(1) There was a complex mountainous oldland; (2) the oldland was re-
duced to a base-level or peneplane, above which stood low residual elevations
or monadnocks; this is the Khangai peneplane; (3) the region was uplifted;
(4) it was then subjected to erosion so prolonged that the Khangai peneplane
was wholly destroyed over a broad area, and a new base-level was achieved,
south of the Khangai region. This is the Mongolian peneplane, above which
the Khangai itself is a monadnock unit.

If, on the contrary, we assume that the two peneplanes are identical

344 GEOLOGY OF MONGOLIA

2 aie at as EEC mgt i as a ae —
sears ————
—< Z ~~ . a

(Fig. 150, B), the significant steps may be summarized as follows: (1) There
was a complex mountainous oldland; (2) the mountainous country was worn
down to a peneplane which we have called the Mongolian peneplane; (3) the
peneplane was warped and locally faulted, especially in the Altai and Trans-
baikal regions, and was deeply dissected. In the Khangai Mountains the
uplift was a very broad, gently sloped anticlinal warp, which was much higher
than some of the other upwarped areas farther south.

The crux of the problem lies in the question of the age of the Mongolian
peneplane. The oldest basin sediments are of late Mesozoic, probably Lower
Cretaceous age. They rest upon a peneplane which was carved upon the old-
rocks after the last mountain-folding. If the last folding took place, as we
believe, in Middle Jurassic time, the mountains must have been reduced
virtually to base-level by the beginning of the Lower Cretaceous (Coman-
chean). It seems very improbable that two major peneplanes, the Khangai
and the Mongolian, could have been developed in this interval. If then the
Mongolian peneplane is the surface on which the oldest Gobi sediments rest
(Fig. 151, A), we should consider this an argument in favor of regarding the
Khangai and Mongolian peneplanes as the same warped surface. But if the
Mongolian be a much younger surface than the pre-Cretaceous peneplane, as
suggested in figure 151, B, the question of the identity of the Khangai and
Mongolian peneplanes would be reopened. Even in that case, the following
considerations are opposed to the idea of there being two separate stages: (1)
The Mongolian peneplane is as elaborately dissected as is the Khangai upland;
if it were so much more recent it should be less dissected; (2) it can be shown
that so much warping and faulting have taken place in Mongolia that an
ancient upland peneplane might be expected to lie at very different levels in
different parts of the country; (3) it seems to us that it is not logical to expect
a great upland peneplane like the Khangai to survive the removal of the
enormous quantities of hard rock, together with the long period of very slow
decay and removal that must come in the later stages of post-mature dissec-
tion, involved in the carving of a new peneplane several thousand feet lower
than the Khangai, over a large part of Mongolia.

SURFACE FEATURES AND THEIR ORIGIN 345

FicurE 149B.—Continuation of figure 149A. Showing the cirques which indicate an incipient glaciation.

Relations of the Mongolian peneplane and the Gobi erosion plane

The Mongolian peneplane always lies higher than the Gobi erosion plane
that bevels the basin sediments, and there is no doubt that the Mongolian
base-level is much the older of the two. The questions to be solved are the
following: Is the surface that underlies the sediments in the gobis the down-

Arcti
S divide N

an

(

(A) The Khangai upland as a separate and older surface than the Mongolian peneplane.

Archie

S divide

Wome Cee

mu
B

Bae) See eles sre Piedmont b
(B) The two planation surfaces as a single warped peneplane.

FicurE 150.—The two diagrams show alternative explanations of the relations between the different
planation surfaces developed on the ancient complex rocks.

warped Mongolian peneplane (Fig. 151, A)? If not, at what date was the
Mongolian peneplane completed?

The relatively thin fills of sediments, deposited at great intervals of time,
seem to indicate that during each of these long periods in which no deposit
was made, erosion might well have base-leveled the very moderately uplifted
land from which the preceding sedimentary fill had been washed. It seems
that in a region where epochs of slight warping and of quiescence have followed

346 GEOLOGY OF MONGOLIA

one another all through the late Mesozoic and Tertiary, a peneplane like the
Mongolian might be made pari passu with the deposition of sediment. Hence,
the Mongolian peneplane may not be of pre-Cretaceous age, but may have been
completed at a much later time, say in the Middle Tertiary, and may have
been finished at different times in different parts of the country.

The absence of Miocene sediments over most of the region studied by the
Expedition,.and over all or nearly all of northern China as well, rather suggests

tae

ei Sobi erosion plane
i 2.

2. “=e.

wm ge = = mee fe oe J ene ewe ee

(B) The Mongolian peneplane is represented as an additional and later product of erosion.

= — a

er cman

—

2
a Ee ee

Gobi erosion plane

ee ee ee ee
<= woe ceraen ssbrs Sacha nS Ws ihd AASNER OUERDGN a

ORR en Ne DRaC ROD Sao gern

FicurE 151.—The two diagrams illustrate alternative explanations of the ancient erosion surfaces de-
veloped on the oldrock floor.

the Miocene as a period of widespread erosion, in which the closing stages of
peneplanation were completed. The great post-Oligocene disconformity, which
is very striking everywhere except at Hsanda Gol, where the break between
beds of Lower Oligocene and those of Lower Miocene age is inconspicuous,
may correspond to the Mongolian peneplane developed on the hard rocks.

It seems possible that the peneplane may prove to be the surface upon
which the oldest Lower Cretaceous sediments were laid down (Fig. 151, A);
but if so, it could not be the surface upon which were deposited such sediments
as the Ardyn Obo formation (Oligocene) and the P’ang Kiang formation
(Miocene or later). It seems more likely that the pre-Cretaceous peneplane
has been destroyed everywhere, except (1) where it is still covered by basin
sediments (Fig. 151, B), or (2) where it has been reéxposed in comparatively
recent time by the removal of its sediments. In this case, the Mongolian
peneplane would be much younger than the pre-Cretaceous, and might have
been completed in Tertiary time, perhaps in the Miocene.

The conditions under which the Gobi erosion plane was formed and the

SURFACE FEATURES AND THEIR ORIGIN 347

date of its completion are questions of some difficulty. Since late Pliocene
beds are beveled by this surface at Hung Kureh, the plane must have been
formed during the Pleistocene. Its recency is also attested by its extra-
ordinarily smooth surface, undissected save for the hollows that represent
the P’ang Kiang stage of erosion. Part of the difficulty lies in the question
of how such widespread planation could have been accomplished in a region as
uplifted as the interior of Asia must have been during the Pleistocene, which
was undoubtedly a period of mountain growth.

Relations between the Gobi erosion plane and the lowlands of the P’ang Kiang stage

As there is no doubt that the P’ang Kiang hollows are younger than the
Gobi erosion plane, the only problem is their mode of origin. We believe that
the hollows are the result of combined action of wind and running water as
explained on page 336.

COMPARISON OF PENEPLANES WITH THOSE OF SURROUNDING REGIONS

Mongolia is bordered by peneplaned mountain countries. Obruchev has
described a very perfect upland peneplane beveling the mountains of western
Dzungaria (1923, page 259). Ina letter to us in 1925, he states that a similar
peneplane is to be observed on all the mountains of Transbaikalia. Davis
(1904, and 1905, pages 21-119) and Huntington (1905, pages 157-216) have
described a well defined upland peneplane in Turkestan. To the south, in
China, we have the classic work of Willis and Blackwelder, which establishes
the presence of several erosion stages in China, the most ancient of which is
an upland peneplane (Willis, 1907). We offer for brevity’s sake the stages
recognized by Bailey Willis as revised in part by Andersson:

1. The Pei T’ai stage (Early Tertiary). ‘‘We take this broad flat form
to represent a stage of erosion to advanced old age, the nearest approximation
to a peneplain which we have found in the course of our journey’’ (Willis,
1907, page 237).

2. ‘“T’ang Hsien stage (Pliocene). Deposition of gravels and clays with
the Hipparion fauna. Land forms of advanced maturity.

3. “Fen Ho stage (Early Pleistocene). Earth movements and subse-
quent revival of vertical erosion.

4. “Ma Lan stage (Middle Pleistocene). Cold arid climate. Deposition
of valley gravels and eolian loess with Elephas sp.” (Andersson, quoted by
Yih, 1920, pages 65-77.)

5. “Pan Chiao stage (Late Pleistocene). Climate semi-arid, abundant
summer rains. Dissection of the valley gravels and primary loess. Forma-
tion of redeposited gravels and loess, with Bos sp., Ovis sp. and Cervus sp.”
(Andersson, quoted by Yih, 1920.)

348 GEOLOGY OF MONGOLIA

It is as yet too early to seek an actual correlation in physiographic stages;
but those of China may tentatively be compared with the stages recognized in
Mongolia, as follows, placing the most ancient stage at the bottom of the
column:

China Mongolia
8s Moders. Aissection...: iaslesiivad-adtnsk cob oe-phide< Modern dissection
7. Pan Chiao dissection. «.:.’. imeuiew myeterew dase P’ang Kiang dissection
6. Ma Lan, loess stage
5. San Men pluvial epoch tion att link ang stotence Gobi erosion plane and
(valley gravels) Pleistocene fans
Ap em FIG As putt os <6 Sn'ecoe sap iainnls eee RS Warping and uplift
3. T’ang Hsien, partial peneplanation............. Rock terraces
POE 1 Al ONOTMANIE. 0s icicle elec estes aie ore shnko ge BAe Mongolian peneplane
Khangai peneplane?
1. Monadnochkpcusesicivsts sis nope soe MARR A GETS Monadnocks

LAKES

Walther (1912, page 39) has classified desert lakes under three groups,
as follows:

(1) Rain-fed lakes; (2) spring lakes; (3) terminal lakes, into which one
or more rivers empty.

This classification applies rather to the water supply of the lakes than to
their mode of origin. A little difference in rainfall for a few seasons would
change a lake from one class to another. For example: the lake Ulan Nor,
fed by the river Ongin Gol, is a terminal lake; but if a few dry seasons withered
the Ongin Gol, the Ulan Nor would depend upon the rainfall and ground-
water in the tectonic basin in which its shallow erosion hollow is carved. The
Tsagan Nor, at the foot of Baga Bogdo, occupies part of a shallow erosion
hollow in the floor of the tectonic basin (Plate XXX, in pocket). Its chief
affluent stream, the Tatsin Gol, disappears at least ten miles west of the lake.
The published maps show the Tsagan Nor as the terminal lake of the river
Arguin Gol; but the Arguin Gol turns southeastward into another lake called
Erin Nor. The Tsagan Nor is fed by vigorous springs in its base, but if
several successive dry seasons should lower the level of the ground-water
under the Tsagan Nor, this supply would be cut off, and the lake would
depend chiefly upon rainfall.

The lakes of Mongolia are of widely diverse geologic origin, and many
of them have had a complex and interesting history. The following types
are known to be present, though the list is not exhaustive:

1. Lakes which occupy true fault-depressions, such as the Kosso Gol.

2. Lakes which occupy basins built by the heaping up of loose materials

SURFACE FEATURES AND THEIR ORIGIN 349

so as to enclose a low area. Many ponds are found in hollows among sand
dunes; and shallow lakes, or ponds, form along the advancing fronts of alluvial
fans.

3. Lakes which occupy basins scoured out by some eroding agency.

(a) We have not seen lake basins due to the plucking and scouring
of glaciers, but they should be present in the high Altai Mountains,
west of the region explored by our Expedition.

(b) Plunge pools at the foot of waterfalls, and potholes excavated
by swift torrents, are found in the high mountains.

(c) Crescent-shaped pools in the bends of drying rivers, and oxbow
lakes in abandoned courses, are found along the larger streams.

(d) Small pools at springheads are not uncommon.

(e) The commonest of all lake basins in Mongolia is the desert
hollow—a shallow basin due largely to wind work. This case receives
special attention under the heading ‘‘ Origin of desert hollows,” page 336.
4. Lakes which occupy basins of a complex history, due partly to earth

movements, partly to deposition of sediment, and partly to erosion. As an
example of this type, we shall attempt to present the history of Tsagan Nor,
one of the piedmont lakes of the Altai Mountains:

(a) There was a complex mountainous old land.

(b) It was peneplaned in pre-Cretaceous time.

(c) It was warped and somewhat eroded. In the warped basin and
in erosion hollows in its surface, beds of Lower Cretaceous age were de-
posited. This step initiated a complex history of warping and deposition
(described in Chapter XIII) which has lasted virtually to the present
day, and which includes deposition of Oligocene, Miocene, Pliocene and
Pleistocene beds, all somewhat deformed.

(d) A smooth erosion plane was carved, beveling the tilted and
faulted sediments, in the course of which process a vast amount of mate-
rial was removed. Sediments many hundreds of feet thick have certainly
been eroded and exported from the region. In this sense an erosion basin
of vast extent was formed; but whether it was an inland basin or had
drainage connection with the sea at the time is an unsolved problem.
The closing stages of the planation took place in the late Pliocene or
post-Pliocene time.

(e) The erosion plane was warped, forming a tectonic depression
which was further emphasized by the faulting that elevated the Altai
front.

(f) The beveled sediments were scooped out by the wind, and a
new lowland was made, which is the basin of the lake. Necessarily,
since a deflation hollow cannot be excavated when standing water is

350 GEOLOGY OF MONGOLIA

present, the region must have been drier when the hollow was dug than

it is now.

(g) In the deflation hollow a lake was formed which surely was
much larger than the present Tsagan Nor. It extended eastward, prob-
ably to the Hung Kureh bluffs, and westward through what is now a
great swamp.

(h) This larger lake has gradually diminished. We noted seven old
beaches, the highest of which was twenty-nine feet above the water of
the lake in 1922.

A careful analysis shows, therefore, that there are many stages in the
development of the lakes of the Gobi region. The history here given is rather
typical of the region; but each lake should be considered as an individual
problem and many lake histories must be worked out in detail before a more
sweeping generalization becomes possible.

The most common lakes seen in Mongolia are shallow playas in deflation
hollows, which are dry most of the year. The short-lived streams wash silt
into the lakes, where the material spreads out over the floor. When the lake
is dry, this silt is bared for wind erosion, and almost every day clouds and
whirling pillars of dust are seen rising from the dried playa beds. There is
thus a direct correlation between the work of lakes and that of winds.
Most lakes, including the more nearly permanent ones, are at least partially
surrounded by sand dunes.

HUMMOCKS AND DUNES

The floors of the hollows and of the few river valleys that traverse the
Gobi vary in condition from swamps to deserts. Asa rule, the swampy parts
develop a very uneven lumpy surface which is exceedingly difficult to traverse
with motor cars. The hummocks are due to various causes. Grass and
iris grow in thick clumps, so persistently that they form solid protuberances
one to two feet high. Many kinds of low bushes or shrubs net the soil with
their thick roots, and form a small hummock at the site of each plant. Rills
tend to wash out the more loosely held soil around them, and the wind adds .
dust and sand to the hillock. Where drifting sand is available, lumps of these
kinds may grow into sand-hummocks, as described on page 351. We gave the
generic name ‘‘playatuft”’ to the plant-mounds of the playa-bottoms.

Mounds of another kind are made by a wrinkling of richly grassed turfy
bottoms. In this case, the irregular surface is due to movements under the
mat of turf, caused by freezing of ground-water (see Chapter IV, page 87),
or by movement of saturated mud.

The wind-borne dust and sand form deposits of several types, depending
upon the abundance and quality of the sand, the direction, strength, and con-

SURFACE FEATURES AND THEIR ORIGIN 351

stancy of the wind, the topography of the region, the rainfall, and the vege-
tation.

Where the supply of sand is abundant, true dunes arise on the lee-side
of the source, and march in the direction of the wind. At the foot of Baga
Bogdo, a long scimiter-shaped area of big dunes curves eastward for more
than thirty miles from the dissected sediments and dry stream beds, which
furnish the sand (Plate XXX, in pocket). The shape of the dunes varies
with the wind. Most of the time, the westerly wind is so strong and constant
that the dunes approach the shape of typical barchans. But during violent
storms from the east, the shapes become sinuous and irregular, though the
crestlines still lie rudely north and south, transverse to the direction of the
prevailing winds.

It was our good fortune to cross the dune belt six times in 1922, twice
during high winds which permitted observation of the movement of the sand.
A thin sheet of sand-filled air leaped from the crest of the dune, like spray
from the crest of a big sea wave. The sand spray dropped upon the leeward
side of the dune, down which it ran in long rills that looked like a syrup spill-
ing down the face of the dune. New rills started from the crest, or near it,
every few seconds. We looked for eddies on the leeward side, and saw some,
but they were small, and were not important factors in shaping the advancing
front of the dune. The chief factor was the simple delivery of sand from the
back slope to the front.

With less abundant supply, the sand forms thin sheets over the level
surfaces, especially on the upland above the valley, or hollow, from which
the sand comes. It climbs the windward slopes of hills, blanketing them
against effective wind-erosion. |

The growing vegetation entraps the drifting sand, growing as the sand
accumulates, until low hillocks are built up. They are probably similar to
the ‘‘tamarisk mounds’”’ described by Huntington (1907 a, page 179), and
should be clearly distinguished from true sand dunes built by the wind, even
though dunes may be invaded by plants and fettered by their roots. In the
case of these sand hummocks, on.the contrary, the plants were on the ground
before the sands. Layer by layer the sand is entrapped and held. As the
hillock grows, other plants join the colony. Thus a hillock may be started
by grass, or peas, and when it has attained a fair round convexity, the ‘‘dune
plum”’ may gain a roothold, and may replace the original plants. Or the
hillock may be mothered by dune plums from the beginning. In suitable
places, the sage, or more rarely, the saxaul or the tamarisk, may start the
hillock, which commonly grows about three to five feet high, though ten feet
has been noted. Internally, these hummocks are made of thin, nearly hori-
zontal beds, faintly arched; the convexity of the beds is less than that of the hill.

CHAPTER XX
STRATIGRAPHY OF THE LATER SEDIMENTS

ONLY the formations from which fossils have been collected are of service
in the present discussion. There are many basins of sediments which have
not yet yielded fossils, but we shall not consider them here.

The record of the beds of gravel, sand, and clay which have been laid
down on the floor of the Gobi basin in the past has an important bearing upon
many other large problems. For example, an attempt to read the climates
that prevailed during past ages in Mongolia requires consideration of the
kinds of sediments deposited during each period and the arrangement of their
strata. Another question, which the study of the sediments helps to decide,
is whether Mongolia has always been a great inland basin or whether its rivers
ran to the sea until comparatively recent times. The sedimentary record
helps us to answer yet another large problem: does the weight of sediments
tend to press down the earth’s crust, and does that sinking of the crust tend
to push up mountain ridges along the margin of the sediment basins?

THE LATE MESOZOIC ERA

The complexly folded oldrock floor of Mongolia includes early Mesozoic
strata which we consider to be Jurassic (Chapter XVII). The earliest sedi-
ments which lie unconformably upon the oldrock floor bear fossils of Lower
Cretaceous age. The great break between the earlier and the later Mesozoic
formations marks a change in the physical history of central Asia (Chapter
XXII); and the deposition of the late Mesozoic sediments marks the initia-
tion of basin history, with which also begins the record of vertebrate pale-
ontology in central Asia.

CRETACEOUS DEPOSITS

Five formations of Cretaceous age have been distinguished and given
locality names. They are the Ondai Sair, Oshih, Iren Dabasu, Wan Ch’uan,
352

STRATIGRAPHY OF THE LATER SEDIMENTS . 353

and Djadokhta. It is not yet possible to determine their detailed structural
or their exact time relations. In all probability no two are physically con-
tinuous throughout, and probably no two are of precisely the same age, but
it is likely that some of the horizons overlap one another. We do not assume,
therefore, in using these formational terms, that they are each of equal in-
dependent significance in the geologic column. They are each distinct enough
physically and geographically and even palzontologically to warrant retention
of the field terms, and they have been arranged in the sequence suggested by
the paleontologists who have studied their respective faunas.

The determinations of faunas have been made by Cockerell, Granger,
Gregory, Matthew, Mook, Osborn, and Simpson. The suggested ages and
correlations of the strata are chiefly the work of Matthew and Osborn, who
consider the correlations to be tentative, and subject to revision as more fossils
are collected and studied.

The Ondai Sair formation

The Ondai Sair type locality is the reéntrant of sediment on the east side of
the Mount Uskuk block, about seventy-five miles west of Oshih, and about forty-
five miles north of the Altai range, Baga Bogdo (Fig. 110, page 242, and Plates
XXVII, XXVIII, in pocket). The formation includes gray sandstones and dark
paper-shales, with a few beds of red sandy clay. Like the Oshih, the Ondai
Sair formation is disturbed by faulting and tilting. The upper limit is an erosion
plane on which rests the basal conglomerate of an early Tertiary formation.
The whole thickness must exceed five hundred feet, as the base is not exposed
where the section was measured. The details of the section and the list of
the fauna have been included in the description of Cretaceous strata, in
Chapter XIII, page 230.

The correlation of these beds will be discussed after the Oshih formation
has been described.

The Oshih (Ashile) formation

The type Oshih section is in Oshih hollow, about twenty-five miles north
of Artsa Bogdo of the Altai mountain system. The Oshih formation includes
the oldest of the later sediments lying upon the complex oldrock floor of Mon-
golia. The base of the formation is covered, the top has been eroded away,
and the remnant which is exposed in Oshih hollow is much disturbed by faults,
so that nowhere can a complete section be seen. The sequence has been un-
raveled by studying the faults and determining which is the older upthrown
group and which the younger downthrown group along each fault. Combin-
ing these groups, we have in the eastern and southern badlands the following
sequence:

VOL. 11—23

354 GEOLOGY OF MONGOLIA

1. The ‘‘cannonball”’ group of sandstones and sandy clays, the “ Psit-
tacosaurus beds.”

2. A group of red clays and thin sands.

. A group of pink and buff clays.

. The lava flow.

. A gravel member of rather uncertain relations.

. A group of brown clays, the ‘‘ Asiatosaurus beds.”

. A group of gray, thin-bedded shales and sands, with fresh-water lime-
stones.

8. A fine red siliceous sand, the Sairim formation, found on the upland
northeast of the hollow.

The fauna thus far collected includes fresh-water bivalve shells and three
dinosaurs—a primitive predentate, Psittacosaurus, a large sauropod, and a
theropod.

The paleontologists agree that the Protiguanodon found at Ondai Sair,
and the Psittacosaurus of Oshih are very closely related, if not identical genera.
The similarity in lithologic habit and the presence of the peculiar paper-shales
at both places further suggest that the two formations are of approximately
the same age. They lie in the piedmont basins of two adjacent mountain
ranges, Baga Bogdo and Artsa Bogdo, and are probably in physical con-
tinuity with one another. Cockerell (1924) considers the Ondai Sair to be
of essentially the same age as the’ fish-bearing shales of Transbaikalia. Grabau
correlates the Ondai Sair with the fish-bearing shales of Kansu, Shensi, and
Shantung, and with the Kweichow formation of Honan (Grabau, 1923 a, 8,
and c).

The faunas, including those of the paper-shales of Siberia and North
China, are all of an inland type, and are very difficult to assign to a definite
place in the geologic column. Reis (1910), after discussing evidences suggest-
ing correlations which range from Jurassic to Lower Tertiary, doubtfully
assigned the Siberian faunas to the Upper Jurassic. Cockerell, after critically
considering Reis’s Siberian biota and the Ondai Sair fauna, concludes ‘‘it
presumably belongs to a period near the beginning of the Lower Cretaceous,
and its classification as Jurassic or Cretaceous may be merely a matter of
arbitrary definition’’ (1924, page 135). Grabau’s revision of the entire sub-
ject, in view of the discoveries of similar or related faunas in China, will be
awaited with great interest.

The structural relations of the Oshih and Ondai Sair formations must be
taken into account in any attempt at correlation. The formations rest directly
upon the oldrock floor, and mark the very beginning of ‘‘basin-history,”’ as
we have explained in Chapter XVII, page 289. The youngest folded rocks
in the oldrock floor, beneath the great unconformity, are the conglomerates

Nu Ww

~sI

a

STRATIGRAPHY OF THE LATER SEDIMENTS 355

and sandstones which we have considered to be Jurassic. The great structural
break between the two series of beds renders it unlikely that both could be
Jurassic, and rather supports the suggestion that the unconformity should
be used to mark the separation of two periods.

The Iren Dabasu formation

The Iren Dabasu formation is exposed in a low, dissected platform, along
the southern side of the saltpan, Iren Dabasu Nor, or Shaya Nor (Plate
XXIII, in pocket). Slate hills, cut by granite, limit the basin on the north
and form the oldrock floor which slopes gently southward under the Creta-
ceous sediments. The floor is not smooth, as low ridges of granite and of
quartzite rise through the sediments. The Iren Dabasu beds are favorably
exposed in two places, which we called, respectively, the eastern and the
western areas.

In both areas, the strata consist of red and gray sandy clays and coarse
gray sandstones. In general, the clays occupy the lower part of the section
and are well bedded, with marked color zones. The dinosaur bones found
in them are less scattered than in the sandstones; many partial skeletons were
unearthed with the bones in articulated skeletal position. These facts suggest
quiet deposition, either on broad flood plains or, more probably, in shallow
lakes. The details of the sections are given in Chapter XI, page 203. The
fauna includes iguanodonts of several sizes, which are very abundant, as well
as numerous remains of “‘ostrich dinosaurs” and deinodonts. Fragments of
dinosaur eggs are so plentiful at one place as to suggest our field name, “‘dino-
saur-rookery.”’ One sandstone bed carries abundant shells of a fresh-water
mollusk, not yet studied. The correlation of the dinosaur beds at Iren
Dabasu must await a detailed study of the fauna. W. D. Matthew considers
the fauna to be rather primitive, and holds that it is best to place the Iren
Dabasu tentatively in the Lower Cretaceous (personal communication).

The Wan Ch’uan formation

Another pre-Tertiary basin lies in the passes north of Kalgan (Fig. 5,
page 47). Its sediments are exposed in the very edge of the Mongolian
basin and are over five thousand feet thick. The lower three thousand feet
of rock consist of coarse pebbles, chiefly of trachyte and granite porphyries.
The conglomerates grade upward into finer sands and clays, in which scattered
dinosaur bones have been found. The strata are tilted toward the north
and dip as much as thirty degrees, especially near the faults which cut the
basin at several places. The average dip is about ten degrees. The cross-
bedding dips northward, indicating currents flowing from the south. The
Wan Ch’uan beds are overlaid by Tertiary basalts and associated sediments

356 GEOLOGY OF MONGOLIA

which form the top of the Mongolian plateau at this point. The basin is of
great extent and splendidly exposed, and a thorough examination of it should
yield a rich harvest of discovery.

UPPER (?) CRETACEOUS DEPOSITS

The Djadokhta formation

As one comes eastward from Oshih, the first recognizable beds of the
Djadokhta are encountered about forty miles from the last identified outcrops
of the Oshih. The formation is about five hundred feet thick. The sand
grains are moderately well rounded, exquisitely fine, and of uniform size,
averaging 0.Imm.and smaller. While the great majority are of quartz, some
are of chalcedony and jasper, and a few are of feldspar, chiefly microcline.
There is a very loose binder of calcite which fails to fill all the interstices.
The vertical cliffs, shown in Plate I, are massive and structureless, and are
composed wholly of fine red sand. We believe that wind-borne sand was
entrapped by a sparse vegetation, which grew upward as the sand accumulated
and so built up a thick, uniform deposit resembling loess. Some strata show
cross-bedding of zolian type, in which the sloping beds dip as much as twenty-
four degrees and range from fifteen to fifty feet in length (Plate I and Fig. 152).
They indicate the presence of large sand dunes in Djadokhta time. On the

u

Cross-bedded Stra
23.

EK™.19
FicurE 152.—®olian cross-bedding at Djadokhta.

other hand, the concretionary beds are horizontal, and some of the larger ones
can be traced for as much as three miles (Plate I). Where the limy or marly
matter exceeds the sand, the rock becomes almost a limestone. This fact
favors the inference that at least the broader marly beds were deposited in
shallow ponds, rather than the view that they are limy concretions, developed
at a former ground-water level.

STRATIGRAPHY OF THE LATER SEDIMENTS 357

There are very few beds of deep red clay which outcrop in the form
of lenses. We examined the clays for plant remains, but found none.
Large pebbles of dense hard materials, chiefly jasper, quartzite, and vein
quartz, are found lying isolated, here and there, in the beds of fine sand.
They are all faceted pebbles with very well-rounded interfacet edges, and
are polished to a waxy luster. The fine grain, the uniformity of texture
in virtually all the sand beds, the great predominance of quartz grains, the
paucity of feldspar, the very few beds of clay,—all these facts support the
inference that the Djadokhta beds were derived from the erosion of an older
sandy sediment, possibly the Oshih, which once may have covered the site
of the Gurbun Saikhan range. The scattered pebbles, too, were probably
derived from some older pebbly sediment rather than from the crystalline
rocks of the Gurbun Saikhan range. But the pebbles were probably faceted
and polished in Djadokhta time, when wind-blown sand was abundant.

The fauna thus far recorded include the following:

Mammalia
Djadochtatherium matthewi Simpson (1925)
Several other rare primitive mammals, as yet undescribed.

Reptilia
Dinosauria

Predentata
Protoceratops andrewsi Granger and Gregory (1923)
A larger predentate, either ceratopsian or iguanodont

Armored dinosaur, undescribed

Theropoda
Velociraptor mongoliensis Osborn (1924c)
Saurornithoides mongoliensis Osborn (1924c)
Oviraptor philoceratops Osborn (1924c)

Crocodilia
Shamosuchus djadochtaensis Mook (1924)

Lacertilia
Two undescribed forms

COMMENTS ON CRETACEOUS SEDIMENTS

All the formations thus far examined bear evidence of having had a greater
areal extent in the past; in the Altai region the evidence suggests that certain
deposits once covered the site of the mountain ranges. All the deposits are
somewhat disturbed and warped. Some, like the Oshih, Ondai Sair, and Wan
Ch’uan, are faulted and tilted. The Oshih and Wan Ch’uan formations
indicate that the country had a rugged relief in the region where they were

358 GEOLOGY OF MONGOLIA

deposited, and possibly a relief whose height was renewed from time to time
by earth movements during the deposition. Most of the formations, how-
ever, indicate moderate or low relief. None of them grades into the formation
which lies directly above or beneath it. The faunas, too, are sharply distinct
from those of the overlying or underlying sediments. These facts indicate
either that there are time intervals unrecorded by sediments, or else that
many basins yet remain to be discovered. Locally, at least, the first inter-
pretation is the correct one,—if other Cretaceous horizons are to be found,
they will not be found in sequence with the known formations. For example,
the Djadokhta is directly succeeded by Paleocene beds; the Iren Dabasu by
Middle or Upper Eocene; the Ondai Sair by Oligocene, and the Wan Ch’uan
by basalts and interbedded clays which Andersson considers to be Oligocene.
The discontinuity of sediments and of faunas is a very striking fact. The
faunas all consist of new types, and bear marked evidences of isolation, sug-
gesting that central Asia was shut off from Europe and America during a
great part of Cretaceous time.

THE CENOZOIC ERA

The Cenozoic deposits include at least ten formations from which fossils
were recovered in 1922 and 1923. They differ from the Mesozoic deposits in
their complete lack of dark carbonaceous beds and in their generally less
indurated and less disturbed condition.

PALEOCENE PERIOD

Only one formation, the Gashato, was found which could be assigned
tentatively to the Paleocene period.

The Gashato formation

The earliest post-Cretaceous deposit of which we have record is the
Gashato formation, which directly overlies the Djadokhta beds, at the foot
of the Gurbun Saikhan range. It begins at the base with four to six feet of
coarse gravel composed of angular pebbles of the ancient rocks of Gurbun
Saikhan. A group of sandy clays, mostly of deep red and chocolate brown
colors, about one hundred feet thick, follows the gravel; then come fifty feet
of sands and gravels and a small lava flow twenty feet thick, above which are
reddish sands and clays. The basin extends at least fifty miles to the east-
ward from the type locality at Gashato, which lies seven miles southeast of
Djadokhta (Fig. 98, page 194). The total thickness of the beds must exceed
the measured sections. The fauna determined by Matthew and Granger
(19252) is as follows:

STRATIGRAPHY OF THE LATER SEDIMENTS 359

Mammalia
Notoungulata
Paleostylops iturus Matthew and Granger
Multituberculata
Prionessus lucifer Matthew and Granger
Glires
Baénomys ambiguus Matthew and Granger
Menotyphla?
Eurymylus laticeps Matthew and Granger
Condylarthra?
Phenacolophus fallax Matthew and Granger
Creodonta
Hyracolestes ermineus Matthew and Granger
Uncertain, creodont or carnivorous marsupial?
Sarcodon pygmeus Matthew and Granger
Reptilia
Chelonia
Fragments of an emydid turtle.

EOCENE PERJOD

The Arshanto formation

The Arshanto formation consists chiefly of fine, almost structureless, hard
red clay which, on weathering, breaks up into chips and ‘‘crumbs.”’ It is
possibly a wind-borne deposit, similar to loess. In the Houldjin bluffs its
horizon is represented by red clays and olive green, fine, fissile shales, which
are undoubtedly lake deposits.

The only fossil thus far described is a small lophiodont, Schlosseria
magister (Matthew and Granger, 1926). None of the typical Irdin Manha
fossils has been obtained from the Arshanto. The contact between the two
formations, where it has been studied, is clearly an erosion plane. Probably
the break between the two is much less significant than most of the other gaps
in the sedimentary record in Mongolia, yet it seems advisable to separate the
Arshanto from the Irdin Manha because of its notably different physical
and faunal aspect, and because of the clear-cut boundary between the two
formations.

The Tukhum formation

A hard red clay, physically resembling the Arshanto, underlies the Shara
Murun beds, at the type locality of the latter formation, more than one
hundred miles southwest of Irdin Manha. It seemed best to call these red

360 GEOLOGY OF MONGOLIA

beds by a new name, as they differ notably from the Shara Murum. Accord-
ingly, they were called the Tukhum beds, from the broad lowland in which
they are exposed.

Matthew and Granger (1926) have described a small hyracodont, Teil-
hardia pretiosa, from the red clay. According to the opinion of these authori-
ties, Tetlhardia may be ancestral to Ardynia of the Oligocene Ardyn Obo
formation. They add that possibly the Tukhum formation may be as young
as the Irdin Manha.

The Irdin Manha formation

The unit which succeeds the Arshanto consists of brown, red, and gray
soft clays, with associated beds of gray sand and gravel. In general, the
coarser beds are above the clays. The sand and gravel fill shallow channels
inthe clay. There are a few thin and local beds of exquisitely fine and powder-
like sand. There are broad lens-shaped beds of sand with streaks of gravel
and clay, as well as cross-bedded gravels and coarse sands with foreset beds
three and four feet long. All these facts point to varying conditions of de-
position, such as might be expected on a broad flood plain or in a large, low
alluvial fan. The fauna has not been fully studied as yet; it includes eight
genera of cursorial carnivores, one of which was bigger than the Kodiak
bear; at least nine perissodactyls, all of which were ranging, browsing
forms, chiefly titanotheres and lophiodonts; and at least four artiodactyls.
Turtle bones are abundant, and a few fish vertebrae were found. The fauna
is neither a strictly forest-dwelling group, nor a true desert fauna, but rather
belongs to open plains which bear patches of trees along the stream courses.
Several of the types have wandered in from America, while others represent
types which later migrated to America and Europe. W. D. Matthew con-
siders the Irdin Manha to be comparable to the Bridger of North America.

Mammalia
Carnivora

Mesonychide
Andrewsarchus mongoliensis Osborn (1924e)
? Gen. indet., resembling Harpagolestes.
? Synoplotherium sp. gen. indet.
Hapalodectes serus Matthew and Granger (1925¢)
? Hapalodectes auctus Matthew and Granger (1925¢)

Hyznodontide
Paracynohyenodon morrisi Matthew and Granger (1924a)
Hyenodon irdinensis Matthew and Granger (1925¢)
Propterodon irdinensis Matthew and Granger (1925e)

STRATIGRAPHY OF THE LATER SEDIMENTS

Mammalia—Continued
Carnivora—Continued
Miacidee
Miacis invictus Matthew and Granger (1925e)
Rodentia
? Paramys
Undetermined fragments
Perissodactyla
Helaletide?
Teleolophus medius Matthew and Granger (1925/)
Desmatotherium mongoliense Osborn (19230)
Desmatotherium fissum Matthew and Granger (1925f)
Hyracodontide
Cenolophus proficiens Matthew and Granger (1925)
Lophiodontide
Lophialetes expeditus Matthew and Granger (1925/)
Lophialetes minutus Matthew and Granger (1925/f)
Titanotheriidee
Protitanotherium grangerit Osborn (1925)
Telmatherium berkeyi Osborn (1925)
Dolichorhinus olseni Osborn (1925)
Manteoceras ? irdinensis Osborn (1925)
Metarhinus ? mongoliensis Osborn (1925)
Chalicotheriide
(doubtful specimen)
Amblypoda ,
Eudinoceras mongoliense Osborn (1924d)
Artiodactyla
Helohyidee
Gobiohyus pressidens Matthew and Granger (1925¢)
Gobiohyus orientalis Matthew and Granger (1925e)
Gobiohyus robustus Matthew and Granger (1925e)
Achenodont, indet.
Tragulina
cf. Archeomeryx, gen. indet.
Insectivora
? Pantolestes sp.
Aves
Undetermined fragments of tarso-metatarsus,
Reptilia
Chelonia
Trionychid turtles
Pisces
Undetermined vertebre

361

362 GEOLOGY OF MONGOLIA

The Shara Murun formation

The beds at Shara Murun lie in the gobi which contains the Irdin Manha,
but are considered to be of somewhat later Eocene age than the Irdin Manha
(Figs. 100, page 199, and 156).

The section is at least three hundred feet thick and includes a group of
soft clays of many colors,—brown, deep red, purple, greenish-gray, and mot-
tled. They rest upon the hard red clays of the Tukhum formation, and are
overlaid by light gray sands and gravels. Bones are abundant and well
preserved in the clays, and nearly complete skeletons were unearthed. Gyp-
sum crystals are present at several places along the bluffs, and at these places
no fossils have been found. Only scattered bones were seen in the sandstones.
The faunal list, so far as it is worked out, follows:

Mammalia
Carnivora
? Mesonychidz
Olsenia mira Matthew and Granger (1925d)
Hyznodontide
Pterodon hyenoides Matthew and Granger (1925d)
Rodentia :
? Desmatolagus
Perissodactyla
Helaletidee
Deperetella cristata Matthew and Granger (1925d)
Hyracodontide
Cenolophus promissus Matthew and Granger (1925d)
Cenolophus obliquus Matthew and Granger (1925d)
Caenolophus progressus Matthew and Granger (1925d)
Cenolophus ? minimus Matthew and Granger (1925d)
Amynodontidz
Amynodon (or new genus)
Titanotheridze
Protitanotherium mongoliense Osborn (1925)
Protitanotherium andrewsi Osborn (1925)
Dolichorhinus kaiseni Osborn (1925)
Chalicotheriide ~
(doubtful specimen)
Artiodactyla
Hypertragulidz
Archeomeryx optatus Matthew and Granger (1925d)
Reptilia
Chelonia
Tortoise, undescribed.
Trionychid turtles, undescribed.

STRATIGRAPHY OF THE LATER SEDIMENTS 363

OLIGOCENE PERIOD
The Ardyn Obo formation

At Ardyn Obo, about ninety miles northwest of Shara Murun (Fig. 83,
page 175), the Expedition stopped in 1922 and 1923 to examine a scarp of
well-exposed sediments. It lies at a corner of the cliff, which extends south-
ward along one front and southwestward along the other (Fig. 84, page 176).
The cliff itself is nearly three hundred feet high, and the slopes below it add
another two hundred feet. The sediment rests directly upon the granite and
gneiss of the oldrock floor.

The lowest beds exposed in the floor of the lowland are red clays, followed
by light gray clays in which a titanothere skull was found. Red clays appear
at the base of the bluff, and are succeeded by gray clays, passing upward into
one hundred and fifty feet of cross-bedded sand and gravel, the foreset beds
of which dip toward the northeast and north. In the upper gravels, polished
pebbles are found, though they are not as lustrous as those of Irdin Manha.
A partial list of the fauna follows:

Mammalia
Carnivora
Hyznodontide
Hyenodon eminus Matthew and Granger (19250)
Oxyenide
Ardynictis furunculus Matthew and Granger (19250)
? Canidee
? Cynodictis
Rodentia
Ardynomys olsent Matthew and Granger (19250)
Ardynomys chit Matthew and Granger (19250)
Desmatolagus robustus Matthew and Granger (19250)
Perissodactyla
Helaletidee?
Colodon inceptus Matthew and Granger (1925c)
Paracolodon curtus Matthew and Granger (1925c)
Hyracodontide
Ardynia precox Matthew and Granger (1923) and 1925c)
Cadurcotherium ardynense Osborn (1924f)
Titanotheriide
Brontops gobiensis Osborn (1925)
Menodus mongoliensis Osborn (1925)
Chalicotheriidz
Schizotherium avitum Matthew and Granger (1923b and 1925c)
Amblypoda
Gen. indet.

364 GEOLOGY OF MONGOLIA

Mammalia—Continued
Artiodactyla
Anthracotheriidz
Gen. indet.
Hypertragulide
Lophiomeryx angare Matthew and Granger (1925c)
Lophiomeryx gobie Matthew and Granger (1925c)
Miomeryx altaicus Matthew and Granger (1925c)
Reptilia
Chelonia
Testudo insolitus Matthew and Granger (19230)
Emydid, gen. indet.
Trionychid, gen. indet.

The Houldjin formation

The top of the Houldjin bluff, south of Iren Dabasu, is capped by a soft,
friable, yellow gravel, the lower parts of which are loosely cemented in places.
The pebbles are chiefly vein quartz, with quartzite, chalcedony, graywacke,
argillite, and other hard, durable fragments. Many pebbles are well rounded,
especially those of quartz, but others are angular. At the type section, the
gravel is only fifteen feet thick, but southward it increases to at least forty
feet. Fossils are abundant, but all are fragmentary. Many bones are so
well worn that the edges of fractures are rounded. The following have been
collected (Matthew and Granger, 19232).

Mammalia
Carnivora
Undetermined fragments.
Perissodactyla
Baluchitherium sp.
Cadurcotherium sp.
Cenopus or
Preaceratherium sp.
Artiodactyla
Entelodon dirus Matthew and Granger
Reptilia
Chelonia
Fragments of a large tortoise.

The Hsanda Gol formation

In the great gobi at the northern foot of Baga Bogdo, there are exposed
at least 3,500 feet of early Tertiary sediments, overlying the Lower Cretaceous

STRATIGRAPHY OF THE LATER SEDIMENTS 365

Ondai Sair formation. There is a basal conglomerate of well-rounded peb-
bles, grading upward through finer gravels and sands to a series of clays. A
basalt flow divides the formation into an upper and a lower member. At the
Tatal Gol (Plate X XIX, in pocket), the lava flow dies out, and here no clear
distinction could be drawn between the upper and the lower beds.

All of the Hsanda Gol fossils at the type section came from the red and
brown clays above the lava flow. At the Tatal Gol, however, Mr. Granger
reports finding fossils below the horizon corresponding to that of the basalt.
The list of fossils thus far studied is given in Chapter XIII, page 234.

MIOCENE PERIOD

The Loh formation

Directly overlying the Hsanda Gol clays, about five miles south of Uskuk
Mountain, there is a group of olive green clays less than one hundred feet
thick. No clearly defined physical break can be seen between the two forma-
tions, but the upper olive clays yielded fossils which Dr. W. D. Matthew
correlates with the Lower Miocene of Europe. The name of this place is
Loh, which, the Mongols told us, means dragon, and refers to the presence
of the “dragon bones”’ in the clays. Accordingly, the clays were called the
Loh formation (Plate XXVIII, in Docket A list of fossils will be found in
Chapter XIII.

The strata dip southward at a very slight angle, which should bring
them underneath the white sands and light gray clays that outcrop ten miles
farther south (Plate XXIX, in pocket, and Fig. 107, page 229). No fossils
were found in the southern beds, but they must occupy a position intermediate
between the typical Loh horizon and the Pliocene beds of Hung Kureh, which
outcrop still farther south.

PLIOCENE PERIOD

The Hung Kureh formation

Southeast of Tsagan Nor, at the foot of Baga Bogdo (Plate XXX, in
pocket, and Fig. 107, page 229), there is a bold white scarp, facing west and
north. Near the base of the scarp, the lowest beds exposed are yellow sands
which contain fossils. Above them are fine white sands and light gray clays,
forming the face of the bluff, which is about two hundred feet high. The
clays and sands are abruptly succeeded by a rubble of coarse pebbles which
are only slightly rounded. The contact between the clays and sands and the
overlying rubble is sharp, and tentatively we regard it as marking the upper
limit of the typical Hung Kureh. It undoubtedly indicates a change in

366 GEOLOGY OF MONGOLIA

physical conditions in the region. A list of fossils will be found in Chapter
XIII.

In the valley of the Da Ying Gol, south of Hung Kureh (Plate XXX, in
pocket), we studied a succession of gray and light brown clays and sands which
yielded small gastropods and plant fragments. The relation between them
and the typical Hung Kureh beds could not be definitely determined because
of covered ground and the fact that both formations are disturbed by tilting,
faulting, and even a moderate folding. Therefore, the southern beds were
named the Da Ying Gol beds. They can be traced southward to the ‘‘ West
Knolls” (Plate XXX, in pocket), within three miles of the Altai front, and
retain their character of fine clays, brown, red, and gray colors predominating.
Small black nodules of oxide of manganese and abundant gypsum crystals
are found in the clay at the West Knolls.

It is probable that the Da Ying Gol and the Hung Kureh are the same
formation, and for the present the name Hung Kureh will cover both groups.
It is impossible to state the thickness of the Hung Kureh, as the base is not
exposed, but it exceeds three hundred feet without including the Da Ying
Gol. If the latter be added, it will exceed 1,000 feet.

SUMMARY OF THE CRETACEOUS AND TERTIARY DEPOSITS

Among the interesting facts which the Cretaceous and Tertiary deposits
bring to light are the following:

All the deposits are relatively thin. The fact that the actual thickness
in most cases exceeds the measured thickness of a visible section is somewhat
compensated by the evidence of structural geology, which implies that most
of the gobis of the Iren tala are shallow. Those of the Tsagan tala, along the
foot of the Altai, are deeper, but the evidence collected between Mount
Uskuk and Baga Bogdo implies that the combined thickness of all formations
there does not much exceed 10,000 feet; and these formations range from
Lower Cretaceous to late Pliocene.

The large gaps in the record are very noticeable (Berkey and Morris,
1924b). No period of geologic time is represented by any approach to a
complete section. For example, the three best represented periods are the
Cretaceous, Eocene, and Oligocene. Of the Cretaceous, we have three hori-
zons: (1) The Oshih-Ondai Sair, which, according to Grabau, is about
equivalent to the Wealden. (2) The Iren Dabasu and Djadokhta beds,
which, though clearly later than the Oshih, are very difficult to correlate
with American or European horizons. They carry primitive faunas, and can-
not be later than early Upper Cretaceous. (3) The greater part of Cretaceous
time is unrecorded in the known Cretaceous sections of Mongolia. The

STRATIGRAPHY OF THE LATER SEDIMENTS 367

Eocene is represented by the Paleocene Gashato, the Middle Eocene Arshanto,
and Upper Eocene Irdin Manha and Shara Murun formations, which surely
leave out most of the Eocene period. The three Oligocene horizons,—
Ardyn Obo (Lower Oligocene), Hsanda Gol (Middle Oligocene), and Houldjin,
(Middle Oligocene)—cover more or less of the lower and middle parts of the
period.

The gaps in the record of other periods are still greater, as only one hori-
zon has been found for the Miocene and one for the Pliocene. The only
Miocene beds yet seen in Mongolia are those of Loh. Similarly, in China,
Andersson cites no positively determined Miocene formations, but refers a
few beds to the ‘‘Lower Pliocene or Upper Miocene” (Andersson, 19232,
Table II). This almost complete absence of the Miocene over vast areas of
basin lands constitutes one of the major problems of the region. No marine
Miocene beds are known in northern Asia; even within the Arctic circle, the
Miocene lignites reported by Baron von Toll (1900, quoted by Suess, 1902,

Z = ania

ANCIENT SOON

AZZ. YGBRAAG
(C) Bape of sedimentary beds of later AL. after the region had been MZ o domes and
basins.

FIGURE 153.—Successive stages in the development of a gobi basin.

III, page 29), are continental deposits. Hence the north Asia continent was
large at that time.

It follows, therefore, either that more extensive inland sediments of
Miocene age should be discovered by future exploration, or that northern
Asia was undergoing general erosion during that period.

The suggestion made by the patchy distribution of all the post-Jurassic
basin sediments is that they are all inland deposits. Despite the intermittent
warping which accompanied the sedimentation, the continent, as a whole,

368 GEOLOGY OF MONGOLIA

was stable and was undergoing removal rather than deposition of sediments
during the late Mesozoic and the entire Cenozoic eras.

For the Pleistocene, we have the Olan and Diske beds, which Andersson
has named (1923a, pages 45-47), and the tilted rubbles of Gochu ridge that
overlie the Hung Kureh. Except along the foot of the Altai, however, we
know of no other deposits of this period in the great basin of the Gobi. In
this respect, the Pleistocene record is comparable to that of the Miocene.
We consider that probably the Pleistocene in Mongolia was dominantly a
period of erosion.

The origin of a basin, whether as large as Mongolia or as small as a gobi,
may be summarized in its simplest terms by figure 153. Section A shows a
rugged, mountainous region, and may represent Mongolia as it appeared
after each of the great mountain-making revolutions, including the post-
Jurassic disturbance. Section B shows the mountains worn down to base-
level by long-continued erosion. In Section C, the land has warped, and
sediments are being washed down from the uplifted parts into the down-
warped areas.

Probably the sediment basins are not of simple origin, but are the resultant
of a number of interacting factors:

1. The erosive work of water and wind, which carves out lowlands wherein
sediments may be laid down.

2. The agencies which move and deposit the sediments.

3. The supply of sediments available.

4. The slow and intermittent warping of the earth’s crust (Figs. 154 and
155). If we suppose that in general the basins form where all the conditions
are most favorable (Fig. 155, A-B), the amount of warping that need be as-
sumed would be reduced to a minimum.

Among the most interesting inferences is the shifting of the locus of de-
position from place to place (Fig. 156). The Lower Cretaceous Oshih and
the Ondai Sair gobi basins are almost certainly connected. Deposition may
have begun in the Oshih earlier than in the Ondai Sair, since the Oshih beds
are much thicker, and of notably coarser grain than the Ondai Sair beds, and
since the paper-shales and large sauropods are found only in the upper beds
of the Oshih, whereas they appear in the lower beds of the Ondai Sair. After
deposition of the Oshih and Ondai Sair, sedimentation in this region ceased;
or, if beds were deposited, they have been removed by erosion. But at
Djadokhta, forty miles east of Oshih, a new gobi received at least five hundred
feet of fine red sands, probably derived from the destruction of older sediments.
At Iren Dabasu, three hundred and eighty miles farther east, the Cretaceous
beds rest directly upon pre-Cambrian slates. The locus of deposition shifted,
therefore, during Cretaceous time.

STRATIGRAPHY OF THE LATER SEDIMENTS 369

At the base of the Altai, referring again to figure 156, the Gashato
beds, which are the lowest Eocene strata we have seen, rest on the Djadokhta
red sands (Fig. 133, page 316). The gap between the Djadokhta and the
Gashato represents an interval of Cretaceous and Paleocene time that has
left no sedimentary record, so far as we now know. No younger formation

iil il

iii ii

FIGuRE 155.—The same region after warping.

Two diagrams to illustrate the relation of warping to topography in the formation of basins, showing how
basins may be the result of original relief and deformation combined. Let both higher relief and upward warp-
ing be denoted by the plus sign, while lowland and downwarping are denoted by the minus sign. Then, in A-B
both relief and movement are negative, so that a basinisformed. In C-D both influences are positive, while in
E-F and G-H positive and negative relief and movement are combined, making either basin or highland,
according as the relief factor is greater or less than the warping.

was seen in this region, but both to the west and to the east there are later
sediments. The Irdin Manha and the Shara Murun formations represent the
highest Eocene yet found in Mongolia. Thus there was a shifting of the locus
of deposition during the Eocene, from the Altai region in earlier Eocene
toward the Iren tala in later Eocene time.

The oldest Oligocene beds yet seen in Mongolia are, apparently, those
of Ardyn Obo, which rest directly upon the crystalline oldrock floor. The
Oligocene of the Houldjin bench at Iren Dabasu should be rather younger,
since it contains Baluchitherium bones. The Baluchitherium beds of Hsanda
Gol are considered Middle Oligocene by W. D. Matthew (19246). Whether
or not these formations prove to be of the same age, the evidence of shifting
of the locus of deposition is convincing. In at least three widely separated
regions, warping recommenced in Lower Oligocene time, so that beds of this
age were laid down in one locality upon eroded Lower Cretaceous (Coman-
chean) beds, in the second upon the bare crystalline rocks, and in the third
upon the Eocene Arshanto beds (Fig. 156).

The only Miocene beds yet seen in Mongolia are those of Loh, which rest
upon the Hsanda Gol without any notable appearance of a break in sedimenta-
tion. Pliocene beds are known from three widely separated localities: the

VOL. I—24

370 GEOLOGY OF MONGOLIA

Hung Kureh, at the foot of Baga Bogdo; the Ertemte, described by Andersson,
from a place on the Kalgan-Urga trail about ninety miles northwest of Kalgan,
and an unnamed but large gobi, discovered by Teilhard and Licent, near the
Tsilin Gol in the southern part of the Dalai tala.

It seems to follow, independently of the inferences just offered, that the
weight of the sediment could have had no causal relation to the depression

g
PLEISTOCENE

Woog

PLIOCENE

| LOH =
OLIGOCENE

4000°

EOCENE

PALEOCENE | / ai seep _ai

EOCENE

LOWER beiei : mie
CRETACEOUS :
: ?

TYPE OF BASIN—> —FAULTED BASINS WARPED BASINS FAULT
MILES ——> 10 al 80 * 70 t 300 { 120 170 20 4 70 4 80 t 70 ‘
DIRECTION-> s |SE| SE] E SE SE NE SE | SE | SE | SE

ONDAI HUNG DJA- ARDYN IRDIN TABUL CHINA
SAIR KUREH | DOKHTA OBO MANHA

stains | HSANDA —OSHIH SHARA —_IREN PANG WAN
GOL MURUN DABASU KIANG — CHUAN

FicurE 156.—Generalized columnar diagram of the later sediments. Warped basins have been dis-
tinguished from faulted basins, the latter carrying by far the thicker deposits. The different basins are arranged
in the diagram from west to east. The distances indicated are measured between working stations and camps,
and do not measure the limits of the basins. The section for northern China is compiled from J. G. Andersson's
“Essays on the Cenozoic of Northern China.”

The reconnaissance of 1925 has added a station 40 miles west of Ondai Sair, with 120 feet of early Pleisto-
cene, a post-Pleistocene section at Djadokhta, and a thin Oligocene section at Shara Murun.

of the earth’s crust. The fact that areas as large as ten thousand square
miles in extent are floored by thin formations, as in the case of the Irdin
Manha and Shara Murun basin, seems to indicate that these beds were laid
down upon a relatively flat surface that did not tend to sink under the load
(Fig. 100, page 199). The crust evidently supported these and other forma-
tions while they were peneplaned. The planations are taken to imply great

_ ee

STRATIGRAPHY OF THE LATER SEDIMENTS 371

The stratigraphy of the later sediments is summarized in the accompany-
ing table.

TABLE OF THE LATER SEDIMENTS

AGE ForMATIONS THICKNESS CHARACTER OF SEDIMENTS |TyYPICAL FossILs
Playa deposits I-50 Fine clays and silts
Alluvial fans 2000+ Cobbles, gravels, sand, silt
Sheet-wash I-20 Chiefly clay-sand mixtures
Recent Drift-sand I-300 Assorted sand, quartz, feld-
Ce Spar, mica, hornblende
Loess ; I-10 Fine silty loam
Older playa and wind de- 10-200 Clays, silts and sands
posits
“High fans” of the Altai 1000+ Coarse gravels
front
Pleistocene Gochu formation of de- 1000 Cobbles and gravels

formed fans along the

Altai front

Hung Kureh sands and 2000+ Finely bedded pond-de- |Hipparion
Pliocene clays posits, windblown sands,

and alluvial fan gravels

Loh 1000? Olive green and brown |Serridentinus
: sandy clays
Miocene P’ang Kiang? 500-E Wee and gray sands and
silt
Hsanda Gol 3000 Red sandy clays ay
Glicocene Houldjin 15-150 Gravels and yellow sands i Baluchitherium
B Ardyn Obo 500 Gray and red silt; white |Menodus
and yellow sands
Shara Murun 500 Variegated clays and |Protitanotherium
white sands
Biocede Tukhum 100+ Hard red sandy clays Teilhardia
Irdin Manha 100 bila sands and gray |Eudinoceras
clays
Arshanto 100+ Red sandy clays Schlosseria
Gashato 300 Red and brown sandy silts |Pal@ostylops
Paleocene and thin gravels
SLIGHT ANGULAR UNCONFORMITY
Cretaceous Djadokhta 500 Fine uniform red sands Protoceratops
Iren Dabasu 180 Red and gray clays and |[guanodontia
; thin white sandstones
Lower Oshih (Ashile) 2000+ Red clays and sands and |Prodeinodon
Cretaceous : black paper-shales
Ondai Sair 500 White and gray sands and |Protiguanodon

black paper-shales

GREAT UNCONFORMITY
All rocks below this line are folded.

The Oldrock Floor of the Gobi Region.

Folded and partly metamorphosed sediments, ranging from early pre-Cambrian to Jurassic age, and
cut by large and small igneous bodies. (See Figure 130, page 301.)

372 GEOLOGY OF MONGOLIA

stability and rigidity of the bedrock floor; that is, the crust neither sank
under the load sufficiently to deform the sediments and encourage thicker
deposits, nor rose under the negative load, when the region was eroded to a
peneplane. We believe isostatic balance can be overset only by very large
positive and negative loads, and that the stripping and loading observed in
Mongolia have been of an order of magnitude too small to overcome the inertia
of the earth’s crust, even where fault lines exist. In the dynamics of the
region, the sediments played a passive part. Basin-warping and mountain-
building were complementary parts of a great orogenic movement or succes-
sion of movements, and were controlled, we believe, by deep-seated causes,
such as the intrusive movements of large magmas.

CHAPTER XXI

SUCCESSIVE CLIMATES OF MONGOLIA

INTRODUCTION

IN our opinion, the case for fluctuating climate in recent time and the far-
reaching effect of these changes on the migrating movements both of men
and of other animals, are supported by much geologic evidence. It is entirely
likely that a peculiarly delicate climatic balance, where even a slight shifting
of factors could make decided differences in living conditions, together with a
long continental history, have had very far-reaching consequences on the
racial history of many forms of animal life. Favorable and unfavorable con-
ditions, by turns inviting conquest or occupation and relentlessly driving out
again; nourishing for a time and then ruthlessly applying the chastening
hardships of want, trying endurance to the utmost,—these constitute a rough
kind of geologic providence whose hand has been felt by every living creature
whose life was spent on the open plain. On account of the importance of the
whole problem and the broad application that it appears to have, we have
undertaken to assemble the evidences in hand which seem to us to bear on
climates of the distant past as well as on climatic changes of more recent time.

PAST CLIMATES OF MONGOLIA

We shall not enter into speculations on the climates which may have pre-
vailed in the eras represented by highly metamorphosed gneisses, schists, and
crystalline limestones. We shall omit consideration of the Archean and
Huronian eras, and begin with the great graywacke series. We are aware,
also, that interpretations of past climates are, at best, tenuous inferences, and
that they should be taken cautiously.

LATE PRE-CAMBRIAN OR SINIAN PERIOD

The graywacke series of the Gangin Daba and Khangai Mountains con-
sists essentially of well-bedded, dark sandstones and gray slates, with some
373

374 GEOLOGY OF MONGOLIA

conglomerates and a few beds of jasper, but virtually no limestone. The
sandstones are composed of bits of slate, phyllite, quartz, and feldspar, vol-
canic ash, and several types of porphyry. Most of the fragments are fresh
and unweathered.

So far as these facts enable us to postulate climatic conditions, we should
infer a cool or temperate rather than a tropical climate. Blackwelder (1917)
has called attention to the prevalence of dark sediments of the graywacke type
in Alaska, and places this same interpretation upon it. Blackwelder reasons
that, in a warm and moist climate, the chemical processes of weathering would
predominate, and would reduce the country rocks to clay soils or to laterites,
which, when washed into their final resting place, would form clay shales. He
argues that in a cold climate, on the other hand, the mechanical work of frost
and temperature changes and abrasion by wind, water, and ice would pre-
dominate among the weathering processes. These agencies tend to break
up the country rock into small grains which are but little altered in their
chemical composition. In a granite region, arkosic sands made of feldspar,
quartz, and mica flakes are formed, while the wash from a region of slates
would bring down myriads of particles of scarcely altered slate, which, when
mingled with quartz grains and cemented together, make a graywacke. The
more finely ground particles, together with some true clays made by chemical
weathering, form dark-colored shales. Doubtless a part of the coloration is
due to carbonaceous matter derived from plant growth.

The climate of Mongolia could not have been dry, for the sediments are
well assorted into distinct, fine clay slates and sandy graywackes, which
range from single beds a foot thick to large formations of slate or graywacke
many hundreds of feet in thickness. Such widespread masses of sediment
could not have been assorted and distributed by any agency other than water.
If there were abundant rains when the graywackes and slates were forming,
it seems not unlikely that the climate was also cold, for in a warm and moist
climate, the minerals bearing iron would be altered to form oxides and car-
bonates, and the complex silicates would break down to form clay, so that the
end product would have been red and brown clays rather than dark gray-
wackes and slates.

PALZOZOIC ERA

Of the marine Palzozoic formations, only the Permian beds have been
studied in any detail. They consist of limestones, sandstones, quartzites,
shales, and conglomerates. The gray limestones are composed chiefly of brach-
iopods and bryozoans, lime-sand, and mud. There are but few corals, and
none of the reef-building types. The fauna, therefore, suggests a temperate

SUCCESSIVE CLIMATES OF MONGOLIA 375

rather than a tropical climate, though mud and admixture of fresh water may
well account for the scarcity of corals.

The clays are thin-bedded and greenish, and may owe their color to chlor-
ite derived from the older phyllites, graywackes, and greenstones underlying
the Permian beds. The coarse sandstones include bits of quartz, graywacke,
slate, and argillite. Fragments of feldspar and porphyries are present also,
suggesting the disintegration of granite and dikes. At least the sandy part
of the sediments was derived from several kinds of older rocks which were re-
duced to small angular fragments without being thoroughly altered by proc-
esses of chemical weathering. Either aridity or cold could retard the chem-
ical processes of weathering so that the lands might yield a sediment com-
posed of fairly fresh rock fragments; while a warm and humid country would
be apt to send down to the sea maturely weathered rock waste, such as clay.
The evidence of the rocks is not conclusive, but it suggests either a cool or
an arid climate for the land from which the sediments were derived, and so
is not at variance with the faunal evidence.

MESOZOIC ERA
Lower Jurassic Period

The Lower Jurassic rocks of Mongolia consist of conglomerates, sand-
stones, and shales, with associated lava flows, tuffs, and ash-beds.

The pebbles of the conglomerates are well-rounded and range in size
up to boulders a foot thick. They are of many kinds of rocks,—gneisses,
granite porphyries, sandstones, quartzites, graywackes, argillites, jasper,
and silicified ash, with scattered limestone pebbles. The rounded form sug-
gests mature weathering with enough abrasion to keep the pebble fresh. The
presence of pebbles of many different rock types suggests that many forma-
tions have contributed to the streams that made the deposit.

The Jurassic pebbles give evidence of having had a complex history.
Some are fragments of older conglomerates which were themselves composed
of rounded pebbles before the Jurassic period. How many times they have
been loosened from a bedrock, worn, weathered, rolled, and again deposited
and cemented into a new generation of bedrock it is impossible to say. The
important thing is not the number of rock generations that have employed
the pebbles, but the fact that they have had a long history.

It seems to us that these considerations lead toward several important
inferences:

1. In Lower Jurassic time, relief of the land surfaces must have been
great enough to give the streams rather steep gradients so as to move the vast
quantities of pebbles which now form the Jurassic conglomerates. Probably
the Jurassic deposits were laid out in a series of separate intermontane basins.

376 GEOLOGY OF MONGOLIA

2. Weathering and erosion of exposed rock surfaces were continued long

enough to change all new rock fragments into well-rounded pebbles with
‘which older pebbles were commingled.

3. Water must have been abundant enough not only to move the pebbles
but to wash out the finer sediments and to assort the rivers’ load.

Coal is found associated with this series in one place at least in the Gobi
region, and remains of woody plants are common, even in the conglomerates.
In China and in Siberia, coal in commercial quantities is associated with
Lower Jurassic beds. The coals probably represent broad swamps in the old
land surface of Jurassic time. Probably the belt of heaviest rainfall lay around
the margins of the Gobi region—in Siberia, Manchuria, and China—while
the climate in Mongolia was less moist and more variable, as suggested by the
abundance of coarse clastics and the paucity of clays and coals. The rocks
and obscure plant remains in Mongolia fall in with this interpretation, and
indicate a warm climate, with only a moderate rainfall.

Cretaceous period

Oshih and Ondai Sair time.—The thick variable deposits at Oshih (page
270) probably represent a large alluvial fan built during Lower Cretaceous
time at the foot of an ancient mountain range. The beds of sand and pebbles
represent the zone of active deposition—a zone which shifts from one part of
the fan to another with the changing courses of the distributary streams. The
fine clays and paper-shales represent the zone of shallow lakes, which is shifted
from place to place around the foot of the developing fan. The variations in
quality of the Oshih sediments, therefore, probably imply the normal changes
in the distribution of coarse and fine material and in the distribution of water
supply during the growth of the fan. The variations do not necessarily indi-
cate changes of climate during the deposition of the Oshih sediments.

The dark paper-shales are clearly lake deposits. Those at Oshih and at
mile 298 on the Urga trail (page 63) apparently lack fossils and contain gyp-
sum, suggesting that the water was bitter, which would imply the undrained
lakes of a semiarid climate.

The abundance of large insects with aquatic larve in the Ondai Sair
formation suggests that the climate was at least moderately warm. The
fauna and flora are so nearly uniform from China to Siberia that we
must suppose very similar conditions to have prevailed throughout the
region.

Iren Dabasu time.—The red and gray, variable, locally cross-bedded strata
of the Iren Dabasu formation (page 203) probably were deposited upon flood
plains and in shallow lakes. This inference is supported by the composition
of the fauna, which includes a bivalve mollusk, and abundant trachodont din-

SUCCESSIVE CLIMATES OF MONGOLIA 377

osaurs, together with crocodilians, turtles, and fishes. On the other hand,
the carnivorous dinosaurs and “‘ostrich’’ dinosaur are not distinctively water-
loving forms.

The small isolated group of lake beds and the fauna suggest an environ-
ment, but do not prove a climate. The angular-grained sands containing
fresh feldspar, the rapid alternation of clay and sand, the absence of lignite,
coal, or dark-colored beds, the lack of plant remains except for a few bits of
silicified wood—all suggest a semiarid rather than a humid climate.

Djadokhta time.—The Djadokhta formation is found about forty miles east
of the Oshih. It is almost wholly composed of a red sand, so uniformly fine
that it will run like sand in an hourglass. Its grains are fairly rounded and
polished. ‘Virtually no clay is admixed with the sand, and there are very few
beds of clay in the formation. These facts suggest that the sand has been
assorted by wind.

Some of the beds are apparently structureless (Plate I) and probably rep-
resent settled dust or loess. The zones of small, white, nodular concretions,
described in Chapter XX, probably were formed by precipitation in pond bot-
toms, though it is possible that some of them represent the work of ground-
water at the time the sediments were forming. Many strata are cross-bedded,
some with foreset beds as much as forty feet long, dipping as steeply as twenty-
four degrees (Plate I, and Fig. 152, page 356), others with curved and dis-
sected beds of the type which we commonly ascribe to wind work. All the
cross-bedded sands are probably part of dunes. No plant remains and no
dark or gray-colored beds have been seen.

The fauna consists entirely of genera which are new to science. Turtle
and tortoise bones, which are common in other fossil fields of Mongolia, are
rare, and the only other water-loving form is a small crocodilian. All the din-
osaurs are small, and all are light-limbed, fleetly-running types, except the
Protoceratops and an undescribed armored reptile.

Associated bones, even to complete skeletons, are more abundant in
the Djadokhta beds than in any others, except perhaps the Shara Murun
formation. The ribs of the dinosaurs are expanded, not crushed—a feature
which, as Mr. Granger pointed out, suggests that the carcasses were sun-
dried and then buried in drift sand. The dinosaur eggs are not broken in
fragments but are found whole; they have been somewhat crushed, and in
many cases the larger end of the egg is slightly telescoped over the smaller
end. Many of the eggs are in their original position in the nest. Washing
by water probably would not produce these results, but would displace the
eggs and break them up. On the other hand, slowly accumulating wind-blown
sand would gradually bury the eggs, crack them and fill them with fine sand.
Once filled, they could not be crushed further.

378 GEOLOGY OF MONGOLIA

All the evidence,—in the rocks, in the fauna, and in the mode of preser-
vation of the fossils,—seems to point to a semiarid climate. There probably
were shallow lakes, withering rivers, belts of sand dunes, and sparse vegeta-
tion, including trees along some of the water courses. Nevertheless, this inter-
esting formation and its fauna do not prove that this climate extended over
the entire country. The Oshih and Ondai Sair beds denote conditions found
over a very large area and therefore give some clue to the climate of the region.
But the Djadokhta beds may rather indicate a special and local condition in
a region that as a whole was more moist. Until more deposits are found and
correlated with it, we cannot say with confidence that these conditions were
widespread.

On the whole, the evidence suggests that Mongolia in Cretaceous time
was semiarid yet somewhat rainier than the Tertiary, as will be brought out
more clearly in the pages that follow.

CENOZOIC ERA

. There is abundant evidence that during the Cretaceous period Asia was

already a vast continent with large inland basins and that this condition con-
tinued into the Cenozoic era. It has been considered that the great central
area in Asia may have played an important part in the evolution and distri-
bution of mammalian faunas, including man. The Tertiary climates of Asia,
therefore, are of especial interest.

In the Himalayan Sea, south of the great Cretaceous continent of Asia,
there were stirrings of uprising land masses even in the Eocene, although lime-
stones with nummulites were still forming in northwestern India. Some time
afterward, that seaway was drained, and the Himalaya ranges were folded and
uplifted, welding the southern lands of India and Arabia to the northern con-
tinent. Probably this greatest of mountain-making revolutions occupied the
whole of Tertiary time. It is very probable that the uprising of the ranges
was slow and that it took place in a series of minor movements. All the suc-
cessive phases of this complex history of uplift, erosion, and renewed uplift
must have had far-reaching effects upon the climate of the lands around, but
especially in the regions north of the Himalayas.

The same era witnessed the building of the arc-shaped ranges that fringe
the eastern coast of Asia, as well as the uprising fault-block mountains of the
interior. Beside the mountain-building, a general continental uplift must have
been in progress, and broad areas were warping into high inland basins. There
is evidence which suggests that, though there have been many changes of cli-
mate, the range of variation has been small. No evidence of very heavy rain-
fall has been found in the entire record of the Tertiary, and the climate seems
to have varied between semiarid and desert conditions during the entire era.

SUCCESSIVE CLIMATES OF MONGOLIA 379

Eocene period

Gashato time-—The Gashato beds have been described on page 358.
The clays, sands, and gravels are poorly assorted; the coarser grains are com-
mingled with the finer, and all beds, even the gravels, carry some clay. These
facts suggest rapid accumulation by streams that were not sufficiently per-
manent to assort their load. The abundance of gypsum in some of the clay
beds suggests the concentration of salts in enclosed waters. There is no coal
or peat, or any plant remains other than a few fragments of silicified wood,
though the gray and brown beds probably carry a little admixed carbon.
Probably there were no large swamps, and the vegetation was sparse. The
fauna includes turtles and a few aberrant mammals, most of which were
adapted for herbivorous feeding, though some were insect-eaters. The fauna
gives little clew to the climate, but it could well have been adapted to a tree-
less prairie or a semidesert. These meager data indicate a climate that was
not markedly wet, though water played the dominating réle in the deposition
of the sediments.

Arshanto time.—Part of the Arshanto beds are structureless red clay
which resembles loess except that it lacks the characteristic vertical cleavage.
True loess is formed by the settling of dust from the air, or by the action of
rain, which washes the dust out of the air. There must be enough vegetation
to entrap and hold the dust, so that the winds that brought it cannot carry it
away again. The well-bedded, greenish fissile shales, which outcrop in the Iren
Dabasu hollow are undoubtedly the filling of a shallow lake. The lens of
greenish sandstone in which skeletons of small lophiodonts were found is a
stream channel filling. The Arshanto suggests an open grass country with
semiarid climate, rather than a desert.

Irdin Manha time.—The Irdin Manha beds (page 207) consist almost
wholly of the deposits laid by streams. The inference that the streams were
evanescent, but ran vigorously when there was a supply of water, is sup-
ported by the following facts: (1) The sediments are not well assorted, but
consist of commingled pebbles, sand, and fine dust; (2) many strata are cross-
bedded; (3) many of them are channel fillings, and cut one another in a com-
plex way. The presence of highly polished pebbles in great number sug-
gests effective windblast.

The fauna (page 208) includes ranging animals adapted to the open plains;
but tortoise and turtle bones are abundant, as well as a few fish vertebra.
There were, therefore, some aquatic forms as well. Fragments of silicified
wood are not uncommon, but still there are no dark-colored beds, and no
lignite or coal.

It is probable that the Irdin Manha formation represents a rather
dry flood plain or alluvial fan, with trees or patches of wood along the

380 GEOLOGY OF MONGOLIA

courses of intermittent, shifting streams. Such a deposit suggests a semiarid
climate.

Shara Murun time.—At Shara Murun the chief sediment is varicolored
clay, capped by light gray sandstones. We judged the clays to be deposits
made in shallow lakes, as they are well bedded and contain abundant gypsum.
No fossils were found in the gypsum-bearing beds, but where the gypsum is
absent, great quantities of bones and several nearly perfect skeletons were
found in small areas or pockets. Probably there were fresh-water springs
or water holes that made these spots at once a gathering ground and a ceme-
tery for the inhabitants of the country in Eocene time. The fauna includes
ranging types resembling those of Irdin Manha in their adaptations. All these
facts suggest a semiarid climate with inland drainage and drying lakes in which
gypsum was precipitated.

Oligocene period

Ardyn Obo time.—The Ardyn Obo formation (page 174) includes green
and red sandy clays, in the lower part of the section, while the upper part con-
sists almost wholly of sand and gravel. The cross-bedding of all the sands and
gravels dips at low angles of fifteen degrees or less. Vast numbers of channel
fillings, intricately crossing and cutting one another, contain a mixture of large
and small pebbles and grains. No associated skeletons, but only scattered
bones and skulls were found. Many of the fossils were discovered in the
bottoms of the channels, suggesting that they were washed down by the
streams of Lower Oligocene time.

The fauna includes a slender-jawed carnivore with much the same general
build as a fox; two rhinocerotids, one of which was probably semiaquatic; at
least three artiodactyls, and fairly abundant tortoises. The physical and
faunal evidence suggests flood plains or alluvial fans with water at least as
abundant as in the Eocene. We again note the absence of forest-living forms,
and of plant remains or carbonaceous beds, although some of the strata have
a grayish color. We judge that the climate was semiarid.

Houldjin time.—The Houldjin gravels (page 205) are much like the upper
gravels at Ardyn Obo—yellow gravels and sand, well washed. They contain
teeth and worn fragments of the bones of ranging animals, but no associated
skeletons have been found. Probably, like the Ardyn Obo, the Houldjin beds
were laid out on a broad flood plain or fan under semiarid conditions.

Hsanda Gol time.—The Hsanda Gol formation (page 233) begins with a
heavy conglomerate, locally three hundred feet thick. It passes upward into
red sands and clays, which are thick-bedded, lens-shaped, uniform deposits,
and no bed or unit can be traced for more than a half mile. Such deposits
might be formed along the foot of a large alluvial fan, where there were broad,

<<

SUCCESSIVE CLIMATES OF MONGOLIA 381

shallow, playa basins that filled when water was abundant. It is possible
that some of the thick, structureless beds are dust deposits rather than lake
fillings.

The fauna includes a group of ranging, browsing animals, one of which is
the giant Baluchitherium, which was found also in the Houldjin gravels; a
number of insectivores, most of which resemble the shrews and moles, but one
of which may have been arboreal; and a great host of burrowing rodents of
which eleven species have been named. This assembly, according to W. D.
Matthew (1924 b), has the aspect of a desert fauna, or at least a fauna adapted
to an open plains country. Most of the beds bear no plant remains, though
a few fragments of silicified wood are found, and the chocolate brown and
gray beds contain a little carbon.

The Ardyn Obo, Houldjin, and Hsanda Gol formations were probably
deposited chiefly by running water. In all three, the sediments and faunas
offer evidence that suggests vigorous: temporary streams and vanishing lakes,
rather than permanently well-watered conditions. We may picture, under a
temperate, semiarid climate, a land of open plains with long, low aprons of
alluvium stretching down from low ranges of oldrock hills; dry stream courses
with patches of trees along their banks, and shallow, often drying lakes in the
bottoms of broad depressions.

Miocene period

The only Miocene beds thus far found in Mongolia or in China are the
gray, brown, and olive-colored clays at Loh (page 365). If Mongolia were an
inland basin in Miocene time, it is hard to see why deposits of that age were
not formed, and we may hope to find them in future explorations. If, on the
other hand, the entire region was being eroded, it is possible that the exporta-
tion of material exceeded the deposition, and that almost no Miocene deposits
were formed. In any case, the lack of adequate stratigraphic data makes Mio-
cene history one of the large unsolved problems of Central Asia.

Pliocene period

Hung Kureh time-—The Hung Kureh beds (page 365) include fine white
and yellow or buff sands, and light gray well-bedded clays. The sands have
yielded the bones of deer, Mastodon, Hipparion, and a large ostrich, while the
bedded clays contain small gastropods and plant remains. The deer might
be a forest animal, but the horse and ostrich were both adapted to open plains.
The clays are clearly pond deposits, but at least part of the sands are prob-
ably wind-blown. It is difficult to estimate the climate, but it must have been
more humid than the earlier Tertiary climates.

382 GEOLOGY OF MONGOLIA

Ertemte time.—Dr. J. G. Andersson (1923, a, page 40) has described a
deposit of clay and sand at Ertemte, near Hallong Ossu in southern Mongolia
(Fig. 7, page 51), which he believes to be an old lake bed. He obtained
fresh-water mollusks, some small fish bones, and many rodents including a
beaver, as well as teeth of deer and rhinoceros, and an egg of the extinct os-
trich, Struthiolithus. On the whole, this is a water-loving, forest-dwelling
fauna, although the ostrich and rhinoceros are typical creatures of the open
plains. It needs, as we have seen in Mongolia, only a small patch of trees to
contain an isolated woodland fauna.

In an unnamed basin, south of the Tsilin River, in southeastern Mon-
golia, the two distinguished French scientists, Pére Teilhard de Chardin and
Pére Emil Licent (1924, c), found a Pliocene fauna with remains of Giraffide,
a rhinocerotid, a horse, a mastodon, a hare, several mustelids, and an ostrich.
This is a typical plains fauna, and is found in ‘‘red beds.” Above the red
beds are white sands and marls, with turtles, mastodon, a horse, a badger, a
beaver, smaller rodents, mustelids, and large deer. Pére Teilhard correlated
the higher beds with those found by Andersson at Ertemte (personal communi-
cation). The beaver and deer again suggest a forest environment, although
this need not be implied for the whole country, especially as most of the
other animals were rovers of the plain. The deposits themselves are simply
described as ‘‘terre rouge’’ and ‘‘marnes et sables blancs,” but at least no car-
bonaceous beds or plant remains were noted.

It seems not improbable that the Pliocene was rather moister than the
present, and that lakes existed and patches of trees maintained themselves
in places that are now desert.

Pleistocene period

The Pleistocene had the most variable climate of any post-Cretaceous
period, including epochs warmer than the present and epochs in which the
higher mountains bore large alpine glaciers, while portions of the Siberian
coastal slopes were covered by broad ice caps. The Gobi region was not
glaciated, but its geologic processes were affected by the climatic revolutions
of that time.

In the excellent little book on the Evolution of Climate, C. E. P. Brooks
(1922) reviews the evidence on the glaciation of Asia. Only two glacial
epochs seem to be demonstrated for northern Asia. The earlier glaciation
was the more severe, and ice sheets formed in the extreme north (Fig. 157).
The later glaciation was confined to the mountains. Brooks (1922, page 82)
believes that the first advance is ‘‘contemporaneous with the first glaciation
(Gtinz) of Europe,” while the second coincides with the third, or Riss, of
Europe. For the Himalaya and Tien Shan, three to five ice advances have

SUCCESSIVE CLIMATES OF MONGOLIA 383

been recorded by Oldham (cited by Brooks, page 82) and Huntington (1905, a,
page 186). Epochs of glacial advance seem to have been marked by moister

FIGURE 157.—Glacial map of Eurasia. The shaded areas show the maximum known extent of glacial ice
during the Pleistocene. All those south of the Verkhoyansk Mountains were of alpine type. The striking
feature is the small extent of glaciation on the continent of Asia. A light line shows the probable maximum
extent of the Caspian Sea. A dotted line represents the present 49-fathom submarine contour showing the
possible land connections of that time during epoch of uplift.

climates, with increased stream erosion and increase in the size and number
of lakes.

We observed in the Gangin Daba Mountains, at altitudes between 6,000
and 7,000 feet, that many valleys start in bowl-shaped heads (Fig. 158 and
Figs. 29 and 30, pages 83, 86). They are weathered and rounded, and no mo-
raines are connected with them. In one of the bowls the stream cuts a
slight trench through the rock at the entrance, which suggests that the floor
of the bowl was somewhat excavated by ice that once occupied it. If they
are cirques, they represent only an incipient glaciation. Streams are dissect-
ing them and changing their original form. Their worn and weathered con-
dition suggests that they belong to an early epoch of glaciation.

384 GEOLOGY OF MONGOLIA

At our camp in the Khangai Mountains, some ten miles south of the
Arctic divide, and at an altitude of about 8,000 feet, the valleys are flat-floored,
steep-sided, and head in broad hollows that may have been cirques. No

FIELD SKETCH

at Bolguk Gol 22 miles south-
west of Urga,showing a bowl,
that may bea cirque.

Book IT 7922. p.58

FicureE 158.—A bowl-shaped valley head. Field sketch in the steeply upturned graywackes of the Gangin-
Daba Mountains.

moraines were seen, though large beds of washed cobbles were found on the
flat floors. If, as we suppose, the valleys were glaciated in an early epoch,
the effects of ice action have been largely cleared away.

Looking northward from our camp, we saw at the Arctic divide undoubted
cirques with sheer, precipitous walls (Plate XIV, page 130, and Fig. 149, pages
344-345). We estimate that the mountains in which they were carved stood
about 10,000 feet above sea level. They are young cirques, separated by broad
areas of smooth, unglaciated upland. Although we had no opportunity to
visit and examine these glacial bowls, we infer that they were made in the
last glacial advance. It seems probable that alpine glaciers formed in this
region in two distinct epochs, separated by a long interglacial period. If the
inferences drawn from the obscure cirque forms are correct, the earlier glacia-
tion was much the more severe and affected valleys two thousand to three
thousand feet lower than did the glaciers of the later epoch.

In the Altai, we saw no undoubted cirques except the distant view of
those in Ikhe Bogdo. The deep narrow valleys of the ranges visited by us
have the V-shaped cross-section characteristic of stream work. Except for
the record of the cirques, the Ice Age in Mongolia is the most difficult period
to read, because of the lack of recognizable deposits. In northern and central
Mongolia no undoubted Pleistocene fossils were found in either 1922 or 1923,

SUCCESSIVE CLIMATES OF MONGOLIA 385

and the only evidence that suggested Pleistocene age is the abundance of
coarse rubble in the ridges between Hung Kureh and the Baga Bogdo front
(Plate XXX, in pocket). In 1925, Pleistocene deposits were found at Hung
Kureh at the north base of Baga Bogdo in the Tsagan Nor basin, and at Lake
Kholobolchi, forty miles west of Tsagan Nor. They will be discussed in a
later volume. Andersson reports, in southern Mongolia, deposits of fine sand
in which bones of an elephant and a rhinoceros were found (1923, a, page 45).

That peculiarly characteristic Pleistocene deposit called loess seems to
be lacking in Mongolia. In our explorations we saw little or none, and we
find no reports of it in the literature except from the extreme southern mar-
gin of the Gobi. The loess of China, south of the Gobi, may well have been
blown from Mongolia and from the basin of Lop Nor, especially if the moun-
tain barriers that now separate these lands from China were lower during
part of Pleistocene time than they are now. Deposition of loess probably
implies at least a moderate rainfall in the area of accumulation, for the rain
washes down the dust that the wind has carried, and there must be vegetation
to hold it and prevent the wind from taking it up again. But the basin from
which the dust is carried must be one in which fresh exposures of fine sedi-
ment are offered to the wind. Drying lakes, the outwash of withering rivers,
or fresh deflation hollows in fine sediments offer favorable supplies of dust and
sand to the winds. These considerations suggest that during part of thé
Pleistocene, while the loess was forming in China, Mongolia was being actively
eroded, and that large amounts of material were being exported from Mongolia
by winds. Whether the rivers had a better connection with the ocean than
they have at present is not known, but certainly one of the chief problems of
the physiography of Mongolia is the enormous amount of material that has
been removed from the country in geologically recent times.

The map (Fig. 157), assembles the known evidence for maximum glacia-
tion of Asia; the contrast with the vast ice sheets of Europe and North Amer-
ica is salient. Apparently Asia as a whole was markedly drier than Europe
or North America during the entire Pleistocene; for, whereas these other con-
tinents had at least four major glaciations, Asia had but two, corresponding
rudely with the first and third glacial epochs of Europe. Even granting this,
however, Asia was probably lower, warmer, and rainier during much of the
Pleistocene than at present; and that period witnessed the growth of moun-
tains and several adjustments of continental level, besides great changes in
climate.

RECENT CHANGES OF CLIMATE

The retreating glaciers between the Pleistocene and the present, through-
out most of Asia, were not large enough to have such profound effects upon

VOL. II——25

386 GEOLOGY OF MONGOLIA

the climate as they had in Europe and North America. For this interval
we find many evidences of climatic changes: in the building and partial destruc-
tion of several successive alluvial fans along the Altai front; in the ancient
beaches at Tsagan Nor; in the redissection of the gently sloping walls of hol-
lows (Fig. 143, page 339); in the renewed dissection of smooth surfaces which
formerly supported a richer carpet of vegetation than at present; in renewed
dissection of the Gobi erosion plane; in the carving of broad valleys by streams
which have vanished, and whose channels are now filled with wind-blown sand;
in the failure of rivers to reach their terminal lakes; in the shrinking of the
size of meander-curves of rivers, as shown by the large abandoned meander-
scarps; in the drying of salt lakes to form salt pans; in old drawings cut upon
rocks by a vanished race, picturing animals that lived in woodlands, though
the region is now bare of trees. It is not yet possible to correlate and date
these changes, but they imply many alternations of climate, with a steady swing
toward a drier climate in the present day. The nature of the evidence is
discussed at greater length under the following individual heads.

EVIDENCE BEARING ON CLIMATIC CHANGE

Dominant denudation

None of the streams of the central Gobi flows out of it to any other region.
Numerous streams from the north flow from the Khangai Mountain country,
across hard and soft rocks alike, into basins within the Gobi. Their sedi-
ments are deposited either along their present courses or in lake basins at the
end of the system. In spite of this fact, which would lead one to expect ex-
tensive recent accumulations, there is surprisingly little new deposit to be seen.
The land forms of the present surface are almost wholly the result of erosion.
Most of the erosion is plainly the effect of running water, though there are
abundant evidences of wind-erosion in rocks of many kinds. It is clear,
therefore, that there is greater total erosion than total deposition, and that the
present is a destructional epoch, in striking contrast to certain earlier periods.
The only explanation which appears to meet all the conditions is a change of
climate toward greater aridity. Probably this is only one of many alterna-
tions of climate, which extend back throughout the history of the continent,
and which may account for some of the gaps and changes in the sedi-
mentary record.

Double dissection

In the open ground of the Wan Ch’uan Hsien Pass, above the porphyry
gorge (Fig. 5, page 47), a remarkable double erosion-form is presented by the
marginal slopes of the valley. There are clearly two stages of erosion,—one

SUCCESSIVE CLIMATES OF MONGOLIA 387

comparatively ancient, the other new and active. Smoothly rounded, gently
sloping forms of rolling country are cut sharply by vigorous new gullies,
making a most complicated and intricate picture (Plate XLI, A). There
is no doubt that the older form represents a control of some sort, strikingly
different from that of the present epoch. Formerly there was a tendency to
develop simple, smooth outlines, as though the land were protected from vig-
orous destructive erosion. More recently, these protective controls have
failed, and at the present time a vigorous dissection is in progress and has
been operating long enough to partly destroy the older topography.

It is commonly suggested that deforestation is responsible for removing
the original protection, but we believe that artificial deforestation has been
an incident rather than a large factor. At the present time, seedlings do not
catch and grow on this ground; the only trees are along the stream in the bot-
tom of the valley. If artificial deforestation had been the cause of the change,
it would not prevent the growth of new seedlings on the unoccupied ground.
The fact that they do not catch suggests that the real controlling conditions
are climatic, and that the present cycle is one favoring vigorous dissection
because of its greater aridity and the consequent failure to support a protec-
tive cover of grass. Therefore, the double dissection pattern of Wan Ch’uan
Hsien Pass is direct evidence of a change in climate toward increased aridity.

The choked gorge of Wan Ch’uan Hsien Pass

Below the redissected sedimentary ground which has just been described,
the stream runs through a short, deep, narrow notch with almost vertical
walls, which has been cut across a range of tilted porphyries (Plate XLI, B).
If the river were in full eroding operation at the present time, the rock gorge
would be virtually bare, with a floor of solid rock. But if the stream has been
reduced in volume, and is overloaded with sediments washed from the sharp,
new gullies just described, it would pile along its valley floor the burden that
proved too great to carry, and so would choke the narrow gorge with gravels.
This is exactly the present condition. The pass is clogged with gravel, and
the stream is not actively cutting on the rock bottom (Plate XLI, B). There
must have been a time when the stream was vigorous enough to cut out the
notch that is now partially filled, and that time represents an epoch of greater
humidity. Wan Ch’uan Hsien Pass, therefore, in its choked condition, is an
evidence of change of climate toward more arid conditions than formerly
prevailed.

Desert trees

Trees of any kind are rare in the desert region of Mongolia, but isolated
trees, and even rows of large elms, are seen at a few places along the margins of

388 GEOLOGY OF MONGOLIA

dry stream courses. The mere fact that mature trees, gnarled and battered,
exist there unaccompanied by saplings, points to a time more favorable than
the present for the germination of seeds and the success of new tree growth.

The forest margin

It was the good fortune of the Expedition to spend several days at Sain
Noin, on the very margin of the northern forests, where groves of trees occupy
the northern slopes of the ridges.

Here along the picket line between the forest and the desert or non-
forested region, the evidence of fluctuating climate, changing from favorable
conditions to those unfavorable for the lodgment and growth of trees, is quite
plain. In a grove covering a stretch of half a mile, the trees are prevailingly
of two sizes. There is a growth of old trees, averaging two hundred and fifty
to three hundred years in age, which forms the chief stand and covers the max-
imum area of ground. In addition, and in contrast to the old trees, there is
a rather dense growth of young saplings, with only a few members of inter-
mediate sizes. There is no regular gradational series of younger trees of vari-
ous ages. It is very striking that there are just two dominant sizes,—a large
size represented by the old trees, and a small size represented by the younger
ones.

In the absence of artificial means of control, such as deforestation, which
is not practised by the Mongol people, there appears to be only one good
explanation. Evidently in an epoch, two hundred and fifty to three hundred
years ago, the climatic conditions were such that the forest was able to advance
by the catching of seeds on this hillside, until a grove was started thatis now
represented by the old trees. Subsequently, for a long time, conditions were
not so favorable, and, although a few trees were started in the more protected
central portions of the grove, the outer margins were wholly unoccupied by
any new growth until comparatively recent time, when conditions again
became favorable for young trees to take root in great numbers. Some of
these are seventy-five years old; much the greater number are younger,—
twenty-five to fifty years old. Apparently the present is again not quite so
favorable for seedlings.

Only one small grove had lodged beyond this picket line—a group of a
dozen or more trees on the next hill to the south. That grove marked the
extreme encroachment of the forest in the most favorable epoch of recent
time. There are no representatives of the second or latest favorable epoch;
in other words, there are no saplings, but instead only old, wind-battered
trees, isolated as if abandoned to their fate by the retreat of the forces that
set them out, and not reached by the last rally in their behalf fifty years ago.

It appears, therefore, that the earlier epoch, centering about the time

PLATE XL.

A. AN OUTPOST FOREST AT THE ARCTIC DIVIDE.

The larger trees represent a successful early invasion of the Siberian forest into the edge of the Mongolian basin. The smaller
growth represents a later invasion, after an interval of unfavorable climatic conditions.

B. TERRACES IN THE LATEST ALLUVIAL FAN OF TIGER CANYON AT THE FOOT OF BAGA BOGDO,

PLATE XLI.

A. TWO STAGES OF DISSECTION AT WAN CH'UAN.

The gullied valley side at Wan Ch'uan, showing an older smooth surface which is being actively redissected. The oversupply of
waste thus furnished is filling the valley bottom.

B. THE CHOKED GORGE AT WAN CH'UAN PASS.

The rock gorge is much deeper than the present profile which represents an excessive filling of the valley with waste.

SUCCESSIVE CLIMATES OF MONGOLIA 389

three hundred years ago, was a more favorable climatic epoch than the last
one, centering about the time fifty years ago. In any case, the evidence
seems to support the hypothesis that the climate of Mongolia fluctuates
between changes from more arid to less arid conditions, in cycles of some
kind, with intervals measured in only a few hundred years.

These observations were checked up by counting the rings of trees that
were available in the vicinity of our camp. The forests are wholly of larch
(tamarack), which apparently is one of the most hardy trees under these try-
ing conditions, and thus serves as an advance guard for the forests of the north
in their contests with the forces of the interior deserts.

Tree growth at Tiger Canyon

Under a different head, attention has been called to the fact that the
original maximum development of the modern alluvial fan at the mouth of
Tiger Canyon has been trenched by stream action, with the development of
two or three pronounced terraces (page 312). The trench at the apex of the
fan is at least a hundred feet deep. The water, issuing from the canyon, sinks
completely out of sight in the first few hundred feet after reaching the allu-
vial fan material; but far out along the distributary channels, which bear evi-
dence of carrying water at certain seasons, there are poplar trees of a species
resembling cottonwood. They are fairly numerous for the first part of the
stretch, but gradually become scattered and more widely spaced, until at the
distance of a mile, there remain only a few rugged, gnarled sentinels. The
point of special interest is that nearly all of this tree growth is mature; among
the more distant groups there are no young trees or saplings. Almost all the
trees represent a former condition which must have been much more favor-
able than the present to the establishment of young tree growth.

This single generation of trees must indicate a marked change of condi-
tions. A lessening of moisture is the only change that seems most likely,
as the rock floor has not altered, and there is no possibility of the soil having
behaved very differently one time from another. It is, however, entirely
possible that there were times of greater rainfall, so that abundant water
could be carried to greater distance beyond the mouth of the canyon, making
the distributary courses favorable to the establishment of young trees. The
lack of saplings indicates that the present is a period of comparative aridity,
perhaps not by any means the driest interval experienced by the region, but
still sufficiently arid to produce conditions less favorable than those at the
time when the groves of trees were first established. Only the most vigorous
of the old stock have maintained themselves to the present time, and evidence
of their coming annihilation is apparent in the fallen trunks that lie half bur-
ied in the shifting gravels.

390 GEOLOGY OF MONGOLIA

The trees that grow along the distributaries from Tiger Canyon, there-
fore, seem to have the same significance as is gathered from features of very
different sort in other places. At several other localities with slightly differ-
ent physical surroundings, we noted the same phenomenon of mature trees
which have all the appearance of being the last remnants of more vigorous col-
onies. Some of these are far out on the open plain along dry stream courses,
and occasionally one sees two or three lone trees, clinging to the side of a hill
or at the outlet of a ravine. The evidence presented by desert trees from
many localities favors a climatic change, in comparatively recent times, great
enough to register its effects in numerous ways.

Ancient stream courses

In many places in the Gobi region there are typical valley forms in which
there is no water. It is quite beyond reason to believe that these could be
made without the work of running water, and yet, under present climatic
conditions, it is impossible that any such forms could be produced. It seems
to us that the facts point to the conclusion that in comparatively recent time
more humid climatic conditions prevailed, with rainfall sufficient to develop
streams of running water in valleys that are now dry. At that time, the
larger erosion effects were produced,—effects that are much greater than could
result under the present régime. In the same category should be placed the
evidence offered by ‘‘misfit’’ streams; for the meanders of all the larger rivers
are of much smaller curvature than the concave scarps left by the meanders of
a former epoch, and therefore indicate a shrinking of the rivers in compara-
tively recent time.

Terraced alluvial fans

A magnificent series of modern alluvial fans borders Baga Bogdo on the
north side. The streams that have dissected this recently uplifted mountain
block have deposited the large fans that reach out for several miles beyond
its base and form a striking physiographic feature. They are developed at
certain places on such a scale as almost to bury the bordering hills of a previ-
ous cycle of erosion; but the feature of immediate importance in this connec-
tion is their own dissected condition.

At the mouth of Tiger Canyon, for example, the last alluvial fan, in its
maximum development, reached into the mouth of the canyon more than a
hundred feet above the present level of the stream. This fan has been trenched
by the same stream which originally made it, and has not only developed a
miniature canyon in the fan itself, but has marked its stages by several
terraces. Whatever the causes may be, the later tendency was to erode the
fan already built and to redistribute the material.

SUCCESSIVE CLIMATES OF MONGOLIA 391

This same general effect is apparent in each of the fans throughout the
length of the Baga Bogdo block, and is apparently quite independent of the
rock composition of the block from which the material has been derived. Fur-
thermore, there is in sight no evidence of renewed uplift or of dynamic dis-
turbance of any corresponding sort since the time of maximum development
of the fan itself. ;

We judge, therefore, that the chief cause of the shift of processes, from
deposition to marked erosion, is climatic change. The present is an epoch of
greater or of less aridity. It is a fair question whether greater aridity with
less water in the stream would tend to build up or to destroy the deposits
already made. Our judgment, in this case, is that less water would build up
and more water would erode the fan. The terraces apparently indicate a more
balanced condition than is represented by either the building or the dissecting
processes.

Quite aside from the problem of whether dissection would be aided by
more arid or by more humid conditions, it is evident that, in either case, cli-
matic change seems to be supported by this feature. There have been times
which facilitated the accumulation of such deposits, and times subsequent to
these when the fans were destroyed; these two very different kinds of epochs
are distinguishable from a third, in which the balance of opposing processes
is much more nearly reached. Such evidence as there is in the alluvial fans
points to recurrence of climatic change.

Minor scarps in erosion hollows

» Intimately associated with climatic changes are the ‘“‘false cuestas,”’
described in Chapter XIX in connection with the origin of desert hollows.
The ‘‘cuestas” are clearly remnants of the once smooth and gentle slope from
the edge of the Gobi upland to the floor of the hollow (Fig. 143, page 339).
At one time there must have been sufficient protection of vegetation on the
smooth slopes to prevent active gullying. When the change to greater arid-
ity weakened or destroyed the protecting vegetation, the rainwash dissected
the steeper parts of the slopes, but was too scanty to destroy the gentler slopes
below, where decrease of gradient lessened the erosive power of the water.
These gullied bluffs and their fringes of minor scarps are found especially
along projecting headlands of the walls bordering the larger hollows (Fig. 146,
page 340). Therefore, they are where the soil dries most readily, and where a
lessening of the rainfall would first affect the protecting vegetation.

Lakes

In many of the enclosed hollows, for which wind erosion is the only ten-
able explanation, there is at the present time standing water, which forms

392 GEOLOGY OF MONGOLIA

small lakes, saltpans, or swamps. At most places, the waters are doubtless
shallow, but in others there is considerable water. This condition exists both
in the sedimentary and in the granite areas.

The standing water in these holes prevents all wind attack, and it is evi-
dent that the bottoms could never have been scooped out under the present
conditions. The only logical conclusion is that they must have been made
under an earlier climatic condition of greater aridity, with a consequent lower
water-table or less precipitation, when the floors of the hollows were dry.

It is possible that in a still earlier cycle the climate was moister than at
present; in that case, there should still be traces of former strand lines and
greater depth, or evidence of the former presence of water where none is now.
In the hollows occupied by larger lakes, such as Tsagan Nor (Chapter XIII),
where the conditions were more critical and more nicely balanced, a history of
climatic change is legible.

Ancient lake beaches of Tsagan Nor

Tsagan Nor, just north of the Baga Bogdo range, is the largest desert
lake yet visited by the Expedition. The floor is so gently sloping that one
can readily wade out into the water several hundred paces. The lake is ex-
ceedingly shallow,—probably nowhere over ten feet deep,—but it covers an
area of nearly six square miles. It is slightly salt, but not too salt for use by
the camel train and horses.

Special interest attaches to the strongly marked, abandoned beaches
along the northern shore, which indicate a succession of much higher water
levels in former times (Plate XLII, B). If the lake were filled to the height
indicated by the upper beaches, it would overflow at the east end and estab-
lish a stream which would follow a general easterly course, marked out by a

broad valley in that direction, which doubtless was at one time the lake’s out-
let.

The reason for emphasizing this point, however, is its bearing on changed
climatic conditions. At the present time, and apparently for a long while in
the past, there has not been enough water furnished to the Tsagan Nor basin
to maintain a level of the lake high enough to overflow. In order to make
the beach terraces, there must have been a time when water supply was more
plentiful than now. This could happen only with a change toward greater
humidity. Each of the well-marked beach-levels must have been maintained
for along time. ‘The changes that have taken place, therefore, have not been
gradual or continuous, but they have come by steps or in cycles.

It is possible that there has been a time of greater aridity than the pres-
ent, and the very existence of the lake basin is somewhat of a support to that
hypothesis; for its broad, shallow, and rather oval form is not that of a river

SUCCESSIVE CLIMATES OF MONGOLIA 393

valley, but more that of a shallow deflation hollow. Probably it is indeed a
deflation hollow, whose floor must have been dry when the winds were exca-
vating it, and therefore the form of the basin of the present lake indicates a
preceding arid cycle.

This explanation would agree closely with observations made in the
granite areas farther to the north and northeast, which have already been
described (page 116) and interpreted as indications of a former stage of greater
aridity than the present. It would agree, also, with the evidence of the
trenched and terraced alluvial fans of Baga Bogdo.

Additional observations made in 1925 fully substantiate the inference
that the lake has been dry before. This evidence and argument will be pre-
sented in a later volume.

Glaciation

The entire failure of general glaciation and the restriction of glacial ice
to the high mountains, indicate a comparatively arid climate during the Ice
Age, or, at least, a distribution of seasonal precipitation which proved unfavor-
able for the continuous development of ice and snow. It would be possible
to have heavy rains, chiefly in the summer time, and a lighter precipitation
in winter, and yet wholly escape the development of notable glaciers. Inade-
quate or badly distributed seasonal precipitation would accomplish the same
result. Observations made in 1925 throw a flood of light on the problem of
climate in Pleistocene and Recent times, but this material cannot be made
available for this volume.

A vanished race

Near the easterly terminus of the great Altai Mountain ranges, in the
vicinity of Artsa Bogdo, where an old basalt lava flow has been undercut by
erosion so that large, flat-faced blocks lie about, we found drawings incised
on some of the rocks. They are unlike anything that the Mongols do, and
the subjects are in part different from those available in the desert of to-day.
In the illustration here used (Fig. 159), the unknown artist has represented a
reindeer or an elk, and possibly a moose, and a man with bow and arrow.
None of these animals inhabits the region now, and both the animals and the
men belong to a country with a different climate. The inference is that
when the drawings were made the mid-Gobi region, at least in the vicinity of
the easterly ranges of the Altai, had a climate favorable to these animals. It
must have been wooded in part, with more water. Both the unknown race
of hunters and the animals that they chased have moved on. Presumably
the climate changed toward greater aridity, and the land no longer supported
them.

394 GEOLOGY OF MONGOLIA

We do not know exactly when this happened; doubtless it was very long
ago. Probably the drawings are prehistoric—made by a people who wrote in
pictures and lived by the chase. The drawings emphasize, from a different
angle, the conclusion reached through many strictly geological evidences, that

FicurE 159.—Drawings cut upon a basalt block at Artsa Bogdo. These were made by a pre-Mongol
people. They represent two human beings, one with bow and arrow, a large deer, a fox, and four creatures of
doubtful identity, the largest of which resembles a moose.

central Asia has had changing climates for ages—sometimes favorable and
sometimes most unfavorable to the living creatures that occupied it. Per-
haps the changes were oft-repreated, and some of them may have been severe
enough to drive the occupying races to neighboring territory. Short inter-
vals of such exile would leave both a desire and an ability to return, but longer
periods of banishment might keep a race out permanently. Perhaps in this
way, changing climate has been vitally involved with the migrations of men
and other races of living things. One such exiled race hunted the deer with
the bow and arrow and indicated the climate of its time on blocks of basalt.

eee ee ee “a

SUCCESSIVE CLIMATES OF MONGOLIA 395

An abandoned dam

In a small valley ten or twelve miles north of Tsetsenwan, there is a well
preserved earth dam more than a quarter of a mile long and about forty
feet high. Except that it lacks a suitable spillway, it would compare favor-
ably with a modern earth dam. It has undoubtedly stood for centuries.
When it was made, why, or by whom, are quite undetermined; the Mongols
of the region know only that it was not made by them, and in their traditions
it was simply the work of a people who preceded them. No one would need
a dam in this region at the present time; there must have been more water
in the valley when the dam was built.

MEANING OF THE EVIDENCE

In assembling these items bearing upon the problem of climatic changes
it has been our purpose to show that there have been such changes, and
that the fluctuations have been many and of very unequal intensity and dura-
tion. Wet epochs that would favor the growth of crops of saplings in the Sain
Noin forest are surely of a different order of magnitude from the wet period
that favored the trenching of the great alluvial fan at Tiger Canyon. Lack of
' sufficient evidence has made it as yet impossible to correlate these climatic
fluctuations, or to assign dates to them. The significant inference is that
changes of climate—toward greater aridity on the one hand, and toward heav-
ier rainfall on the other—have taken place, and probably will continue to
take place, and that they have written their record upon the country and
upon its living creatures.

PRESENT CLIMATE

Travelers tell us that forty degrees below freezing is a common winter
temperature. Most of the wells freeze, and both men and cattle must depend
upon snowfall for water. In the latter part of April, at least in southeastern
Mongolia, a curious warm spell comes which lasts but a few weeks. It is fol-
lowed by renewed cold weather which the Russians call the Icy Saints’ Days,
and which lasts into June. The hot weather begins in the latter part of June,
and continues through July and most of August. Mr. Granger noted day tem-
peratures ranging between eighty degrees and one hundred degrees F. for this
period. The hottest night temperature was seventy-two degrees F., and most
nights ranged below sixty degrees. Snows and frosts may be expected any
time after September 1, but the true winter weather does not come, as a rule,
until mid-October or November. The temperatures cited above were taken
in the plains country at altitudes between 3,000 and 5,000 feet.

During the spring the winds are prevailingly from the west or north and

396 GEOLOGY OF MONGOLIA

are high and strong. In summer they are as a rule lighter and more variable,
and often from the south. The fall brings the northwest and north winds
again, and in winter the prevailing winds are from the north.

There is a light snowfall in the winter, with occasional rains in July and
part of August. Cloudbursts such as those described by Walther for tropical
deserts, are rare, if they ever occur, inthe Gobi. As always, the rains and snows
are heaviest in the mountains. Near the Arctic divide, the north side of the
ranges receives the heaviest rainfall. Probably this is true in the Altai region
as well. Rainfall increases again toward the Pacific divide, and here the
moisture must come from the Pacific Ocean, so that the east and south winds
should bring the rain. Forest patches appear on the well-watered northern
slopes of the Arctic divide, while the southern slopes bear only grass. The
grass-lands extend southward into the great basin, dying out very gradually
into the sparse vegetation of the desert. A similar grass-land extends along
the southern rim of the basin. It is nearly eighty miles broad from north
to south, and is cultivated by Chinese farmers, who raise oats, millet, and
potatoes.

Lacking all figures, we hazard a guess at the rainfall, based upon a com-
parison between the vegetation in Mongolia and that in similar latitudes in the ,
United States. It probably varies from about twenty inches in the rich
grass-lands to as little as eight or ten inches in the arid centers. Although we
believe that the Gobi is in one of its arid cycles, no part of it is comparable to
the extreme dryness of the Atacama desert of Chile, or the Hamada of Egypt.

PLATE XLII

A. SUN CRACKS AT TSAGAN NOR.

Shrinkage cracks in the sun-baked mud of a small playa near Tsagan Nor. The plates of mud are already nearly as hard as rock,
and represent some of the new deposits that are now forming.

B. THE ABANDONED BEACHES OF TSAGAN NOR. THREE BEACHES MARKED BY GROWTH OF SHRUBS.

PLATE XLIII.

A; A PRE-MONGOL GRAVE, MARKED BY A RECTANGULAR ENCLOSURE OF ROCK SLABS,

B. GAP THROUGH THE RUINED GREAT WALL BETWEEN CUINA AND MONGOLIA AT THE CREST OF WAN
CH'UAN PASS.

. CHAPTER XXII
SUMMARY OF GEOLOGIC HISTORY

TuIs is a preliminary summary. The work of the Expedition is far from
finished. In due time we shall have to make revisions. Even though some
of the statements may require considerable modification and doubtless many
additions, we present the following geologic story with confidence that it
will serve as a background for the studies that are to follow.

Our purpose is to assemble the historical facts established by our own and
other work to the present time, and to treat the problem largely from the dy-
namic standpoint, emphasizing in succession the different fundamental pro-
cesses that have made the Gobi region. It will be necessary sometimes to
group several series of formations and to treat them as a unit representing one
of the large segments of geological history characterized by a certain unity of
physical condition or result. The significance of a particular geologic process
that has operated through many different epochs may be seen clearly if treated
as a problem in itself. It is our wish to make the course of events stand out
as the center of interest, and those results are emphasized that lend them-
selves most readily to this end.

We have accordingly chosen to include in the summary the following
major elements of geologic history, as exhibited in this region:

1. Ancient eras characterized by metamorphism.

2. Paleozoic marine history.

3. Mid-Mesozoic sedimentation, initiating the continental history of
central Asia.
Development of basin structure, with its covering of later sediments.
A sedimentation summary.
The course of volcanism in Mongolia.
The deformation history of the Gobi region.
Prominent epochs of erosion.
Origin of the present topography.
Relation of geologic history to the development of man.

397

GND} 00 San eee

398 GEOLOGY OF MONGOLIA
ANCIENT ERAS CHARACTERIZED BY METAMORPHISM

The formations belonging to the ancient history of the region before Cam-
brian time are exceedingly complex, highly metamorphosed, and give evidence
of extensive modification (Fig. 161). Nevertheless, it is clear that the oldest
formations were sediments—a great series of them—including the usual lith-
ologic types represented by strata deposited in and with the aid of water.
Doubtless many of them were laid down in the sea, and in that distant time
there was probably an extensive marine history, now so obscured by later
transformations that it cannot be read in much detail. There are great
series of conglomerates, sandstones, shales, and limestones, repeatedly inter-
bedded with one another. Those types of rock which indicate marine rela-
tions are found more abundantly in the earliest rather than in the latest groups
of pre-Cambrian strata. The strata of the final stages of this early history
seem to indicate a continental rather than a marine origin.

There are at least three great eras represented by quite distinctly different
series of these meta-sediments, the oldest being by far the most changed from
its original condition, while the latest one is much less modified. These divi-
sions correspond roughly to those recognized in adjacent regions, particularly
in China, where almost the same succession has been established. They may
be tabulated from youngest to most ancient as follows:

1. The T’ai Shan Complex, as used by Willis in China, belonging to the
Archean era.

2. The Wu T’ai System, as used by Willis in China, belonging to the
early Proterozoic era.

3. The Khangai Series, probably equivalent to the Nan K’ou System of
von Richthofen and Willis and the Sinian System of Grabau, belonging to the
late Proterozoic era.

The T’ai Shan complex

It is evident from field observations that there is a very ancient series of
sediments which has been intimately invaded by igneous injections and intru-
sions of many kinds, chiefly of granitic type, and that the whole series, includ-
ing the igneous members, has been regionally metamorphosed, extensively
deformed, and exposed to destructive erosion. All succeeding formations
were laid down on that erosion floor. The system now consists of injection
gneisses, schists, and crystalline limestones which we correlate roughly with
the T’ai Shan Complex of Bailey Willis, as described for China.

The Wu T’ai system

The Wu T’ai System, which follows, is represented by a great succession
of schists, phyllites, and limestones that also were metamorphosed, folded, and

SUMMARY OF GEOLOGIC HISTORY 399

eroded in their turn before the succeeding Khangai Series was developed.
They are less extensively modified from their original condition and less inti-
mately injected with igneous materials contributed to their present composi-
tion than is the older series beneath; but they are affected to a much greater
degree in all these ways than any of the formations that follow. They must
have been folded into mountain ranges, as were the formations of the preced-
ing system, and it must have been from rocks of this kind, subsequently ex-
posed to erosion, that the Khangai graywacke-slate series of the next succeed-
ing period was formed.

The Khangai series

The formations, up to this point, have large developments of limestone,
suggesting repeated advances of the sea. With the opening of the next period,
however, and the beginning of the development of the graywacke-slate series,
there must have been a more pronounced continental control and supply.
There is a rather systematic change in the quality of the rocks of the series in
different, widely separated localities, suggesting something of the geography
of that time. Slates, with interbedded graywacke, predominate in southern
and central Mongolia, graywackes and sandstones, with thin interbedded
slates, are more common in northern Mongolia; while the assumed equivalent
in central China is largely limestone. This, we believe, is the first series to
have a continental significance, and it indicates that the sea lay to the south,
with continental areas to the north and west. The outstanding characteris-
tic of this ancient formation is its great extent and thickness, which is repre-
sented in the Khangai country and along the Tola River by many thousands
of feet of graywacke. There is probably more than 20,000 feet of these strata
still preserved. The condition at that time, therefore, must have been un-
usual. Graywacke is not an especially common type of rock, and any forma-
tion of such an enormous extent as this taxes one’s imagination to account for
it. During an incalculable period of time, conditions must have been reason-
ably constant over an immense area, to enable a formation of such striking
uniformity to be developed.

This is the youngest, and also the simplest, of the very ancient series of
more or less metamorphosed rocks. It is not everywhere heavily metamor-
phosed, however, and as it is not very intimately injected with volcanic prod-
ucts, although it carries many intrusions, its original character is easily deter-
mined. The series is folded into mountains, as were its predecessors, but the
folds are simpler and more open, especially in the north, and there has been less
internal, intimate deformation of the beds themselves. As a result, the gray-
wacke or sandstone beds are only moderately metamorphosed; the shale beds

400 GEOLOGY OF MONGOLIA

between are transformed into slates or phyllites, and the limestones have
become crystalline.

The triple character of this ancient section of geologic history is readily
established, but some of its detail is obscure, and only its major features are
as yet determined. It includes much the longest period of time, and it covers
the eras of profound transformations of simple rocks. These have lost their
original character, have been folded and faulted into new positions, sheared
and recrystallized into new internal structure, and indeed, in most cases, into
new mineralogical composition. They mark no less than three great sedi-
mentation eras, each followed by a mountain-making epoch. There is a
break between each two major series, or systems, representing erosion and a
lost interval of time, but the unconformities are obscure, and much more
work should be done with them.

The granite bathylith

The greatest igneous invasion recorded in Mongolia followed the moun-
tain-folding of the graywackes, but it is not strictly a part of the Khangai
history. It followed—and perhaps its activity helped—the deformations of
that time, but the igneous rocks of the great bathylith, which in the next pe-
riod, made great advances toward the surface through all these formations,
have a history of their own.

In spite of this complicated history, and in spite of the mountain structure
certainly represented, there is no remnant of the ancient relief at the present
time. The rocks, planed down to a low monotonous relief, constitute the
complex succession of formations of many kinds, which stand on edge for
hundreds of miles and form the ancient crystalline floor on which all other sed-
‘iments were laid down in central Asia.

PALZOZOIC MARINE HISTORY

The Paleozoic history of central Mongolia is as yet almost a blank, for
over great areas rocks of this age have not been identified. Only south of
Sair Usu have we found formations which give satisfactory evidence of the
conditions of that time. Even in these places, the land seems to have been
undergoing erosion during the early part of the Paleozoic era, as no marine
sediments have been identified older than the Mississippian or Dinantian
period. There is thus a great break between the ancient rocks of the meta-
morphic eras and the rocks of Paleozoic age. Even as we write this, we real-
ize that a very different correlation has been proposed by other observers in
adjacent districts. Most of the Russian geologists regard the graywacke

SUMMARY OF GEOLOGIC HISTORY 401

series as of mid-Paleozoic age. Some report the finding of Devonian fossils
in them, in localities farther north and east. To this we have little to say, as
we do not know the geologic structure of their country; but in central Mon-
golia these rocks, as far as we have seen them, are unfossiliferous, and have a
structural relation to other series and to the known Palzozoics themselves
that makes such a correlation appear improbable. Mid-Palzeozoic rocks of
such extent, and with the intimate relation to marine condition that seems
to be demanded of that age, should carry determinable fossils. Pre-Cambrian
rocks of the same type might well be normally without them. We have found
late Paleozoic, marine fossiliferous strata infolded or infaulted as patches
within graywacke areas, but this does not make the graywackes belong to their
age. Jurassic strata are found similarly infolded in the graywackes. All
that we can do is to place the Khangai series of graywackes and slates where
the evidence thus far seems to favor,—that is, in the late pre-Cambrian rather
than in mid-Palzozoic time.

In the interval between the deposition of the graywackes and the advance
of the sea in late Paleozoic time, this portion of central Asia must have been
dry land. During this time also, deep-seated igneous activity developed on
perhaps the most enormous scale known anywhere in the world, when the
Mongolian bathylith made its greatest advance and accomplished its greatest
work in massive invasion, in upward stoping, and in penetrating the enclosing
rock formations.

The known Paleozoic rocks are marine, and carry a characteristic fauna
which clearly indicates their correlation. Limestones and dolmitic limestones
predominate, while minor deposits of shale and sandstone are associated with
a basal conglomerate—a thick series, but not by any means so great as those
that preceded. The strata may have been much more extensive in former
time, both in thickness and in areal distribution. They surely must have
extended farther inland than they do now, even across the Arctic divide, and
at that time they probably covered all other formations of the region. Where
the limits were, however, is not thus far determined from the evidence avail-
able. It is clear only that they in their turn were folded, as the others had
been, into mountain structure, and subsequently were eroded. They formed
an important part of the southern mass that emerged from the sea at the close
of the Paleozoic era, to become the core of the continent of Asia (Grabau,
1923 d, pages 408, 468). Since Permian time, this portion of the continent
has never been resubmerged.

How great an interval of erosion followed is not certainly known, but it
is clear that a long period, represented only by an extensive unconformity,
intervened before the deposition of the succeeding formations that lie upon
the post-Permian erosion surface.

VOL. 11—26

402 GEOLOGY OF MONGOLIA

MID-MESOZOIC SEDIMENTATION INITIATING THE CONTINENTAL HISTORY OF CENTRAL ASIA

The Mesozoic era is represented in Mongolia by two formations of very
different age and structure, separated by one of the most pronounced uncon-
formities of the whole geologic column. The formation above the unconform-
ity is known to be of Lower Cretaceous age, and initiates an entirely new cycle
of events, different enough from those preceding to be regarded as a new divi-
sion of history. Therefore, this formation is treated with the overlying Ter-
tiary formations as a new structural unit, which we have referred to as the
“Later Sediments” of the Gobi region (Berkey and Granger, 1923).

The Mesozoic formation beneath the unconformity, has, at best, a some-
what obscure historical standing. It is clearly of continental type, but carries
almost no diagnostic fossils. The few traces found are obscure and poorly
preserved plant remains, which accompanied the making of coal. The form-
ation is enormously thick in certain localities and entirely wanting in other
extensive areas (Fig. 160). It may never have been developed over the
whole region, but there is a constancy in lithologic type and structure that
indicates a certain unity of physical history. The sediments are chiefly
coarse conglomerates and sandstones, most of which were rapidly accumulated
by running water and shifting currents. A considerable proportion is well
bedded and uniform in grain, indicating deposition in lakes. The formation
is wholly non-marine. The prevalence of coarse material, especially pebbles,
through strata thousands of feet thick in widely separated areas, supports the
hypothesis that these sediments were developed as isolated deposits in subsid-
ing areas adjacent to uplifted fault-blocks, the destruction of which must have
furnished the rapidly accumulating material. It is apparently impossible for
such materials to have been transported great distances from constant sources,
but it would be possible to supply material of this kind from a series of
nearby sources, during the progress of deformation that renewed the relief.
It is probable, therefore, that the formations known as Jurassic strata in the
Gobi region and in Siberia and China were never continuous, and that no two
localities bear exactly equivalent formations. Between the areas of deposi-
tion, there probably were divides whose destruction furnished the supply of
detritus. Some of these ridges have persisted to the present time, for the
erosion that followed was more destructive to the loose Jurassic sediments
than to the ancient divides of crystalline rocks. The only remnants of the
deposits, therefore, that are still preserved, lie in depressed areas, which have
been faulted or warped down in later time, and thus have been protected from
the general peneplanation that preceded the Cretaceous period. The land-
scape of Jurassic time was probably a series of mountains and intermontane
basins. But how they were distributed one can not make out with certainty,

SUMMARY OF GEOLOGIC HISTORY 403,

because there are so few remnants of the strata still preserved from erosion
and not now covered by later deposits.

In spite of the probability that these sediments do not exactly match in
different areas, they do all fall into a certain historical position. They lie in
the mid-Mesozoic. They follow a long period of erosion that is itself to be
subtracted from early Mesozoic time, and above them is another erosion break
of enormous proportions, preceding the Lower Cretaceous period. Between
these two great breaks occurred the sedimentation which, for convenience, is
referred to as the Jurassic period. It may possibly extend down into Triassic
time, but it surely does not continue to the end of the Jurassic. In the Meso-
zoic era, therefore, the continental character of central Mongolia became
firmly established, with two long periods of erosion and a period of continental
deposition represented before Lower Cretaceous time.

Folding and faulting and extensive igneous eruption closed the construc-
tive period, and pre-Cretaceous erosion beveled across all these structures in
making the great peneplane which reduced the Jurassic block-mountains,
together with ail preceding rock formations, to a surface of low monotonous
relief, which has become, for all succeeding time, the floor or basement on
which the later sediments were laid down. Viewed from this angle, the mid-
Mesozoic strata become a part of the complex floor. They are the closing
incident in a complex series of events that ended the mountain-folding of cen-
tral Asia. Since then the strata of central Mongolia have not again been
crumpled into mountain folds. All rocks of the ancient floor have been de-
formed in this way, but with the making of the great unconformity beneath
the Cretaceous beds in mid-Mesozoic time, the central core of the continent of
Asia became stabilized against lateral crowding, and its long history of re-
peated mountain-foldings has been succeeded by an unbroken history of simple
warpings and faultings, in which elevation and subsidence have dominated,

DEVELOPMENT OF BASIN STRUCTURE WITH COVERING OF LATER SEDIMENTS

Following the erosion epoch which closed Jurassic time, the region of cen-
tral Mongolia, which by this time had been reduced almost to a peneplane,
was lifted and warped into a broad basin with inland drainage, and a long
period of alternating localized denudation and deposition was commenced,
which has lasted to the present day. The extent and distribution of local
warpings of the earliest time may not be determined, but it is clear, from the
interruptions recorded in the deposits and the peculiar distribution of the sed-
iments marking the different horizons, that intermittent warping has charac-
terized the whole of post-Cretaceous time. The downwarped areas received
sediment washed from the upstanding masses, and thus became minor fills of

404 GEOLOGY OF MONGOLIA

sediment, separated from one another by warped or faulted divides of hard
rock (Fig. 160).

All the smaller basins and divides have a tendency to shift as the intermit-
tent warping is renewed so that many areas which were formerly covered have
subsequently been stripped; areas once covered with greater thicknesses, have

: en) HEC:
CES Na,

7] Cretaceous &Tertiary
: Sediments

Mountainous areas
Jurassic porphyry :

& conglomerate Shara Muru 77 a
ZA Late Paleozoic Ula Usu rumah | 7 Ye 0
wey 77 Yi) iy hy
= wines anne ee” g ay Yt: 5
ma ets ree JE ail prs ia:
MMM £2 vrcheororc Decca naira eM

100 50 °

ar

FicurE 160.—Sketch map of central Mongolia, showing geologic units along the traverses. An attempt
has been made to distinguish the different major groups of floor rocks along the route of travel, by different
patterns. The younger basin sediments along the same routes are stippled. In the untraversed regions, the
white areas represent lowland areas, as distinguished from the more mountainous tracts.

suffered partial denudation, while other bare areas have received deposits.
Thus it happens that the geologic column of any locality is always only a par-
tial one; it is always broken; and in most cases, it carries but a fraction of the
sedimentary representatives of the whole time interval.

The greatest break in the later sediments is that which marks the transi-
tion from Cretaceous to Tertiary age. All strata beneath this erosion break
are more deformed than are those which lie immediately above. Apparently,
the region has progressed to greater stability and simpler deformation with
the progress of time, even within the period of later sedimentary deposition.
Doubtless the dynamic cause is the same for all. There must be changes going

SUMMARY OF GEOLOGIC HISTORY 405

on beneath the surface at some unknown depth,—changes that tend in the
course of time to modify the supporting structure of the crust. It is prob-
ably, in essential quality a volcanic activity, and results in changes of dens-
ity as well as in a thinning of the roof, so that the readjustments are probably
isostatic. Basaltic outbreaks habitually accompany these deformations—
even those of latest time—suggesting that the hidden volcanic processes be-
neath are not only genetically connected with the volcanic outbreaks them-
selves, but are vitally connected with the deformations that accompany or
precede them. This type of deformation has been so widely distributed, in
both time and space, and has been repeated so many times, that it is the most
characteristic dynamic feature of Cretaceous and Cenozoic time. Deposition
in the depressed areas and erosion of the more elevated areas are merely inci-
dental accompaniments of the deformation.

The establishment of inland basin structure, and the renewal of warping
from time to time, have had profound effects upon the climate of central Asia,
tending to make it more arid as the uplifts grew. The same processes have
continued to the present day, and the present surface represents a consider-
able epoch of erosion, which has prevented deposition of beds of Pleistocene
age, except in certain minor areas. It is under such conditions that the ani-
mal and plant life existed whose remains now make these sedimentary rem-
nants important fossil fields.

The general dynamic history of the Cretaceous and Tertiary periods is,
therefore, readily pictured, but in detail it is much more complex, and the
building of a complete sedimentation column for the later sediments, as well as
the making of a complete story of the dynamic movements of that long period
of time, must await the assembling of a still larger body of data. At least the
major features of the story are well established; with them, perhaps, one must
be content until the work is finished.

A SEDIMENTATION SUMMARY

The oldest rocks known in the Gobi region are of sedimentary origin.
All these very ancient formations, however, are extensively metamorphosed
and, in most places, also extensively injected with igneous material. This
has so modified the original character that it is not possible to determine very
accurately the precise nature of the sediments themselves; but it is clear that,
from the very beginning of the geologic record, there have been strata of the
nature of sandstones and shales, limestones and conglomerates, just as there
have been in succeeding periods. The presence of limestones on a large scale
supports the interpretation that these strata were marine; but evidence of
any other sort is lacking throughout the whole of pre-Cambrian time, except

406 GEOLOGY OF MONGOLIA

in the very latest system, that of the graywacke-slate series, which is essen-
tially continental. There are thousands of feet of pre-Cambrian sediments,
which can be subdivided into at least three major divisions representing suc-
cessive time periods. Not a trace of organic life in the form of fossils is to
be found in any of them, unless it be in the uppermost slaty layers of the gray-
wacke-slate series, where we find obscure traces of forms that are believed to
be of organic origin. The determination of sequence and relative age, there-
fore, depends wholly on their structural relations in the field and their relative
complexity of petrographic condition,—it being assumed that, on the average,
the more complex petrographic types represent the more ancient systems,
and the simpler petrographic types, the younger. Originally even the most
complex injection gneisses must have been simple rocks, probably chiefly
sediments, and we must regard all the many changes in petrographic quality
as indications of as many steps in the subsequent history of the formation.

Stripped of these later complexities and additions, the whole group of
pre-Cambrian systems would look simple enough. But each older series of
rocks has had a longer experience, with increased confusion of its original
characters. They may be listed in groups, indicating their original, meta-
morphic, and injected condition, as follows:

FoLDED AND METAMORPHOSED

PRropucts IcNEous MEtA-PrRopuctTs

ORIGINAL SEDIMENTS

Conglomerates, quartz sandstones Quartzites, quartzite schists, Injection gneisses, banded gneisses
quartz-mica schists

Arkosic and lithic sandstones Graywackes Graywacke schists

Shales Slates, phyllites, mica schists Injection schists and gneisses,
banded gneisses

Limestones Crystalline limestones and dolo- | Silicated and _ silicified meta-
mites, marbles limestones and dolomites

The Paleozoic system is not largely represented in the Gobi region,
and probably never was represented by any formations preceding the Missis-
sippian, or, at the earliest, the Devonian. Basal conglomerates were found
in the Sair Usu region, immediately below the Mississippian. The sedimentary
succession is marine; the conglomerates and sandstones at the base are thin
and are succeeded by a thick series of limestones and dolomites in that district,
with less prominent shaly members; the chief sedimentary type is a granular,
fossiliferous limestone. At Jisu Honguer, the Permian beds begin with a

Se

SUMMARY OF GEOLOGIC HISTORY 407

basal conglomerate of well-rounded pebbles, followed by sandstones and shales
with intercalated lenses of limestone. The limestones contain the only fossils
thus far found.

At the close of Palzozoic time, the sedimentation habit was changed,
and marine conditions ended permanently. After a long interval of profound
erosion, sedimentation was established again in mid-Mesozoic time, with a
series of continental deposits. These are dominantly conglomerates and peb-
bly sandstones, and sandstones of strongly bedded character. In favorable
places, these beds total many thousands of feet,—perhaps twenty thousand
feet, in the best sections. There are no limestones, and there are few shales.
The character is that of alluvial fans, delta deposits, stream wash, and inter-
montane lake deposits. No fossil content has been recovered except traces of
plants and coal. It is a striking fact that the sedimentary type is so definite
in central Asia, and so much alike in localities scattered a thousand miles
apart. It seems to us that the formations never could have been continuous,
but that they were normally discontinuous, representing isolated sedimenta-
tion basins, which, because of the constancy of physical conditions over wide
areas, produced similar results in each locality where sedimentation was going
on. These conditions terminated with renewed deformation and a period
of erosion, which reduced the region to a low rolling surface. With these
mid-Mesozoic deposits, the whole sedimentation history of the formations that
constitute the ancient floor of the Gobi region of central Asia was completed.

All formations since that time have a simpler origin and structure. The
continental habit was retained, but it took on something of its present-day
features, in that sedimentation was carried on in interior basins which have
become in later times desert-like, and which, indeed, may have been desert-
like in many earlier epochs. All these strata lie on the eroded floor of the
mid-Mesozoic and earlier formations, and constitute a succession which con-
tinues from earliest Cretaceous to the present time. The sediments are
chiefly gravels, sands, silts, clays, and shales, with rare limestone layers.
Inland lakes were more common in the earlier epochs, so that muds which now
appear as fine paper-shales, were characteristic. These do not occur in such
prominence in the later epochs, where clay-sand mixtures predominate, though
sands and gravels arecommon. ‘These types have been repeated in successive
periods and in widely separated basins, so that there is an aggregate thickness
of more than 12,000 feet; but only a small fraction of this total appears in
most of the individual sediment-basins or gobis. The conditions which were
initiated at the beginning of Tertiary time have continued to the present,—
slow and moderate warping and sedimentation, followed by long intervals of
erosion. Alternations of these two sets of conditions succeeded one another
throughout Tertiary time, and the sedimentation habit still continues, almost

408 GEOLOGY OF MONGOLIA

as it began, covering altogether an enormously long period of comparatively
stable conditions.

The succession of sediments, marking this later history and constituting
the succession lying on the ancient erosion floor, is tabulated on page 371.

DEVELOPMENT OF VOLCANISM

As pointed out in the preceding section, the most ancient rocks identified
are interpreted as metamorphosed sediments, but with them are associated
very ancient igneous injections and intrusions, some of which must date
back to times .almost as ancient as the sediments themselves—for some of
the igneous materials are as complexly metamorphosed as are the sediments
with which they are associated. Granites are most common, but there are
diorites and rocks of still more basic type, such as pyroxenites and peridotites
that have been changed to serpentines and serpentinous schists. Although
very long volcanic history preceded the more spectacular intrusions of the
great Mongolian bathylith, the history of volcanism in central Mongolia is,
we believe, essentially a simple story, centering around the development of
the bathylith.

Deep-seated magmas

In early Paleozoic time, or, at least, after the deposition of the Khangai
graywacke series, there developed in central Mongolia a great underlying
bathylithic magma, whose uppermost portions in later solidification became
a granite. In hundreds of places, scattered over a total area of hundreds of
thousands of square miles, this bathylith is now exposed by erosion which
has cut deeply into the old roof. At many places the magmatic activity was
vigorous enough to penetrate far into the overlying strata. Doubtless, also,
great masses of the roof were engulfed and tended to sink into the plastic
magma, to be more or less completely absorbed and transformed. At favor-
able places, substances were given off from the magma, in the form of emana-
tions and solutions which, penetrating the overlying formations, attacked and
modified the rocks through which they passed. Large amounts of the hot
solutions were trapped within the enclosing formations, thus adding to their
original complexity. This process is probably responsible for the structural
confusion and complexity of composition that characterize the ancient series.

In still more ancient time, before the maximum development of the great
bathylith, there were igneous activities of a more limited sort, which we believe
were simply the forerunners of the same slowly developing volcanism. It
would not be unreasonable to believe that the igneous injections noted in
the earliest sediments, which were metamorphosed long before the time of
the Mongolian bathylith, actually came from the same deep-seated volcanic

SUMMARY OF GEOLOGIC HISTORY 409

centers, and that they represent the first steps in an age-long volcanic history
which should be regarded as a unit. In this view, the various evidences of
volcanism in the Gobi are but different exhibits, reaching separate expression
at successive stages in the normal development of a great subcrustal igneous
mass. The volcanism of early Paleozoic time is marked by the maximum
development of bathylithic granite. This has many petrographic variations,
but it is prevailingly a coarse-grained biotite granite of acid composition, and
usually of a massive structure. It extends, with amazing uniformity for
enormous lateral distances, and the local variations are certainly no greater
than one should expect as a result of differentiation within the mass itself, and
of syntectic modification produced by absorption of material from the roof.
It is one of the most extensive and uniform bathylithic masses known any-
where on the earth.

Subsequent volcanism takes an entirely different aspect; nevertheless,
it may be only another expression of the same history. All the succeeding
products may represent only the solidifying, differentiating, and residuary
activities of the bathylithic magma beneath. The chief later expressions are
three: (1) serpent-form dikes, (2) brittle porphyries, (3) basaltic and other
lavas and pyroclastics.

Serpent-form dikes

An immensely complicated series of dikes cuts the bathylithic granite
and adjacent portions of the roof. These are the so-called serpent-form dikes
that have been described in Chapter V. They are of great petrographic
variety, and are undoubtedly of different relative age, for they cut each other
in a most confusing way. Because of this great complexity, they are most
easily interpreted as special products given off during the differentiation and
cooling of the underlying mass. They are a type by themselves, not to be
confused with any others, and they probably developed early in the solidifying
stage.

The brittle porphyries

A series of brittle porphyry intrusives characterizes certain areas. They
are granite and syenite porphyries and, more rarely, dioritic and monzonitic
porphyries of great minor variety. Most of them are large, irregular intrusive
masses, either cutting Jurassic strata or occupying ground between Jurassic
strata and adjacent more ancient types. Some of them seem to be connected
with the deformation history of that time, whereas others must be much older.
They are remarkably similar over territory as widely separated as are the
individual deposits of Jurassic strata. It is most unlikely that such similarity
could have come about except under the influence of some regional control.

410 GEOLOGY OF MONGOLIA

The simplest one to suggest itself is the great bathylith. The porphyries
are probably only another expression of activity marking a new stage in its
history. They appear to represent deformation and outbreak in a period
succeeding the maximum development of granite. Since that time, nothing
similar to them has been produced on such a scale. Later field observations,
however, indicate that there was an immense development of porphyries and
flows preceding the maximum granite invasion. The porphyry problem re-
quires additional study in the field.

Basaltic and other lavas and pyroclastics

In later time, only one other type of volcanism is recorded. This is
represented by flows that are dominantly basalts, though rhyolites, trachytes,
and even pumiceous glasses also occur. Basalt flows have spread over thou-
sands of square miles. Like their immediate predecessors,—the brittle por-
phyries,—they seem to be intimately connected with the deformations of the
region. Although of Cretaceous and Tertiary age, they may be only another
expression of the closing stages in the life history of the great bathylith. The
upper portion is acid in composition, and was long ago solidified; the deeper
portions ought to be more basic, and there may still be highly heated portions
available at times for extrusion. Perhaps these localized remnants and their
activities are largely responsible for the crustal adjustments of later time.
Corresponding to the flows, there are extensive beds of ashes and tuffs, widely
distributed and repeated at several horizons. They seem to have the same
significance as the flows. With the basalts, the whole history of volcanism
has been run, from earliest times to the present. It is a fine example of an
igneous history that does not miss a single period from the bottom to the very
top of the column, wherever sediments are represented.

DEFORMATION HISTORY

The deformation processes that have affected this region have resulted
in a great variety of expressions. They have included continental epeirogenic
uplift and subsidence, mountain-folding, internal metamorphic reconstruction,
local fault-block displacement, and simple warping, and these major structural
effects have been accompanied by the usual minor structural modifications.

Epeirogenic movements

Epeirogenic movements have more than once submerged the region
beneath sea level so that marine sediments were deposited, and later have
raised it to continental position so that erosion could take place. Each un-
conformity, up to and including the Paleozoic era, must represent epeirogenic
movement of a profound character. The latest of these breaks is at the end

en

—_

ee eS ee

SUMMARY OF GEOLOGIC HISTORY 411

of the Paleozoic; another, at the end of the Proterozoic, follows the develop-
ment of the graywacke series; and a third, represented by the conglomerate
in the base of the graywacke series, apparently separates the Wu T’ai from
the Sinian system. A fourth is indicated between the T’ai Shan and the Wu
T’ai. In later time, there is only one major break—the so-called ‘‘Great
Unconformity,’’ between the folded conglomerates and sandstones of Jurassic
age and the overlying, simple, Lower Cretaceous strata. Doubtless there is
represented in this break important epeirogenic movement, but it is not of
the same significance as the previous ones, because it does not involve de-
pression beneath sea level. The great breaks are indicated in the tabulation
of major elements of Mongolian geology (Fig. 130, page 301).

Orogenic disturbances

All the strata, of every system up to and including the mid-Mesozoic
sediments of Jurassic age, have been folded. There are five such major units,
with unconformities of more or less profound significance between them. It
is not possible, however, with present information, to assert confidently that
separate mountain-folding affected each system as it was developed; but the
strikingly different structural condition of each, and the much more complex
condition of each older one, suggest emphatically that an additional mountain-
making experience is one of the reasons. In the older systems, folding is
complicated by metamorphism and very extensive igneous intrusion. This
applies particularly to the T’ai Shan system and to the Wu T’ai, in both of
which the folding is intense and essentially isoclinal. The next system, the
Sinian, is more openly folded. In all three of these ancient systems, judged
to have preceded Cambrian time, folding is the major deformation pro-
cess. The same type of deformation also affects the strata of Paleozoic age.

Probably regional folding came to an end in the early epochs of the Meso-
zoic era, but the Lower Jurassic strata are moderately folded and are limited
by large faults. The character of the sediments (see page 292) suggests that
they were laid down in separated basins between uplifted fault-blocks. After
the close of the period of active deposition, there probably was a renewal of
fault movements, which crowded and somewhat folded the Jurassic strata
between the blocks of oldrock. Thus the mid-Mesozoic disturbance is of a
mixed faulting-and-folding character. With it, folding comes wholly to an
end in central Asia, and all orogenic movements of later time are simple warp-
ings and faultings.

Warping
All the sediments from Lower Cretaceous time to the present lie on the
upturned, eroded edges of older strata which represent peneplaned mountain

412 GEOLOGY OF MONGOLIA

ranges. The strata of Cretaceous and Tertiary time are deformed also, but
never in this region by simple folding. In places they are disturbed so much
that they stand on edge, but this is due to sharp flexures or to drag accompany-
ing faulting, and is confined to narrow zones. Much commoner are the great
downwarped depressions and upwarped divides, which vary from only a few
miles to more than a hundred miles across. Judging from the attitude of the
strata which remain, there was more pronounced and more widely distributed
deformation in Cretaceous time than in the Tertiary, for the Cretaceous beds
are much the more disturbed. Large areas of Tertiary strata are very little
deformed. Inspection of a series of localities, scattered over the whole central
Mongolian region, discloses indisputable evidence that gentle warpings have
taken place, not quite continuously, but at frequent intervals, and have been
in operation virtually to the present time. The upwarpings have brought
portions of the ancient floor to the surface, and sometimes to elevations much
above the average level, so that a rugged topography is developed on them
by dissecting streams; the downwarps forming local basins have retained
downwashed sediments, thus making the gobis. The floor of a gobi is the
result of both erosion and warping, and its basin character may be due chiefly
to erosion, or chiefly to downwarping, or in some measure to both (Figs. 154
and 155, page 369). Where the sum of erosion and warping is downward or
negative, a basin containing sediments is preserved, but where the sum of all
warping and erosion is upward or positive, the ground is arched and the ancient
rocks lie bare, stripped of any sediment that may once have covered them.

Virtually no places thus far discovered have suffered continuous downwarp
so that they contain a complete unbroken section; probably, also, none has
recorded continuous upwarping. Much commoner has been a combination
of upwarpings and downwarpings, which has given opportunity for deposition,
followed by partial denudation. Every formation, up to and including strata
of Pliocene age, is affected by these movements, and even the erosion plane
that bevels the Pliocene beds, and doubtless represents a portion of Pleistocene
time, is itself warped. There is reason to believe that this kind of movement
still continues.

Thus a great change in the method of deformation appeared in north
central Asia in Cretaceous and Tertiary time. Folding characterizes the
pre-Cretaceous periods from beginning to end, whereas warping with local
faulting characterizes post-Jurassic time.

Faulting

Faults are conspicuous local features of the region, and doubtless have
accompanied the deformations of every era; but those of the earlier eras,
when folding predominated, are obscure and have no effect on the major

:

SUMMARY OF GEOLOGIC HISTORY 413

features of the present topography. Faulting must have been important in
Jurassic time; but the scarps have been planed away by subsequent erosion
which reduced the region to a general base-level. The block-faultings of
later geologic time, however, are much more conspicuous and important.

Some of the movements of Cretaceous and Tertiary time, which are in
part of block-fault type, are still preserved in the landscape, and account for
the most prominent mountain ranges of the Gobi region. The Altai moun-
tain ranges traversed by the Expedition, with many associated uplifts of
much less relief, are all of this type. The major units of the Altai system
which have been examined thus far by the Expedition are the Baga Bogdo,
the Artsa Bogdo, and the Gurbun Saikhan ranges. There are many minor
ones of similar habit, among them the Uskuk block and the unnamed range
east of Uskuk.

In general, the present-day mountains of Mongolia are of two types:
the fault-block, which owes its relief to direct uplift or tilting, and the eroded
arch. Although the internal structure of the fault-blocks is as complex as
the oldrock floor, and includes folded rocks, the mountains themselves are
not folded mountains. The other type of mountain is the result of the erosion
of great upwarped arches. They have been raised to such elevation and have
persisted through such enormous lengths of time that the ordinary processes
of erosion have carved mountain relief on them. Folded strata are included,
but the mountains are not due to folding. They are only the end-product
of erosion on one of the greater upwarps, which probably has had almost
continuous positive movement. This is the structure of the Khangai ranges
of Sain Noin and the Gangin Daba range of the Tola River country.

Metamorphic deformation

The rocks of the pre-Cambrian systems were so deeply buried when the
great mountain-making revolutions were taking place, that in addition to
the gross deformations of folding and thrusting, a series of intimate internal
changes was produced within the rocks themselves. These changes include
mashings, shearings, recrystallizations, and the long list of complex alterations
that are best understood as the result of dynamic metamorphism. The
changes characterize particularly the two most ancient systems, the T’ai
Shan and the Wu T’ai. They are much less pronounced in the later rocks,
although the weaker members of all the folded series are sheared and somewhat
recrystallized, so that a slaty cleavage cuts across the original bedding-planes.

Grouping of deformation products

From the standpoint of deformation, it is possible to recognize three
great types. The oldest includes the T’ai Shan and the Wu T’ai systems,

414 GEOLOGY OF MONGOLIA

which are folded and internally metamorphosed. The second type is charac-
terized by folding, with moderate internal deformation in the oldest portions
and some block-faulting in the youngest ones, but folding is their dominant
deformation feature. This type of deformation characterizes the graywacke-
slate series, the strata of Paleozoic age, and the mid-Mesozoic strata. The
third and latest type is characterized by repeated warpings, occasionally
accompanied by prominent block-faulting; either expression reaches, in ex-
treme cases, a relief of mountainous proportions. This type of deformation
belongs to post-Jurassic time and continues to the present, with some of its
most pronounced deformation effects belonging to very recent geologic his-

tory.

Epochs of erosion

The most prominent erosion breaks in the geologic column are those
between the major systems of the pre-Cambrian, that which represents
the early and mid-Paleozoic, and those which cover the early Mesozoic
and the late Jurassic (See table, page 301). Each of the great breaks un-
doubtedly represents an enormously long erosion interval, for during each
of them, relief features of mountainous character must have been leveled
down almost to the smoothness of a peneplane. This can be determined with
greatest certainty in the case of the Jurassic unconformity, which, because
it shows the greatest structural break in the whole series and forms the
ancient floor on which all later sediments were laid down, has been referred
to in the geologic column of Mongolia as the ‘‘Great Unconformity.”’ If we
are right, however, in correlating the Khangai series of graywackes with the
late pre-Cambrian or Sinian, there is a longer break between the Khangai
strata and those of Upper Paleozoic age than can be accounted for in the
Great Unconformity, for the total absence of deposits representing the Cam-
brian, Ordovician, Silurian, and Devonian periods must mark an enormous
erosion interval—longer than any later ones.

The significance of the break between marine Permian beds and the con-
tinental sediments of Jurassic time is likely to be overlooked, because, in the
limited distribution of the Palzozoic strata, little chance is given for direct
study of structural relations. But the mere fact that formations as thick as
the remnants preserved at Sair Usu and Jisu Honguer do not exist over im-
mense areas which formerly they must have covered, is in itself an argument
for a most profound erosion. Probably both the post-Permian break and
the early Paleozoic hiatus represent longer intervals than the Great Uncon-
formity itself.

There is no way of determining how profound may be the breaks be-
tween the pre-Cambrian systems. The difference in quality of the strata

Basin sediments, Lower Cretaceous to
Recent, unconformable upon the

OLDROCK FLOOR_
Spina tape ee and surface

Slows, tuffs and ashes

Jurassic sertes of continental sedi-
ments cut by porphyries

MIDDLE
MESOZOIC

Paleozoic series of marine sediments,
shales, [imestones,and sandstones, cut by
dikes. FUL Later intrusive rucks may pos-
sibly be derived from le bathylith, by re-
juvenation and differentiation in ils deep-
er parts

A?
Ps
=

LATE
PALFOZOIC
ae awa

Me Great Mongolian Bathylith —

a subjacent body, whose depth and dif |::;
ferentiation are suggested by the right- |. ::
fiand portion of the diagram

EARLY
PALE OZOIC

Kfiangai series of graywackes and slates,
jaspers, quartzites

83
S38
wet
hy ©
ois
oe
as
ate
z4
ee

The Wu Jai system, phyllites, schists, green-

stones and crystalline fimestones

PROTEROZOIC

The Jai Shan complex- gneisses, schists
and crystalline limestones, cut and satur
ated and even replaced by intrusives

FIGURE 161.—Generalized columnar section of the oldrock floor.
415

416 GEOLOGY OF MONGOLIA

themselves is a partial indication of great difference in age; but this may be
due in part to superposition, with a more pronounced metamorphic effect
in the deeper layers. It is clear only that there are breaks, that there is a
simpler structure in the higher systems than in those below, and that there
must be intervals between them.

Disconformities

All the earlier breaks are extensive as compared with the later ones.
They represent greater lengths of time, and mark periods of regional erosion
between more profound deformations than took place in the later epochs.
In post-Jurassic time, however, with a change in the deformation habit to
gentle warpings and occasional flexures and faults, the erosion breaks became
simpler, less extensive, and more numerous. We have mentioned that the
Cretaceous sediments are generally more deformed by tilting, flexing, and
faulting than are the overlying Tertiary beds. There is everywhere a break
of large time significance at the base of the Tertiary, representing the greatest
physical and faunal changes recorded in all the later sediments.

There are minor breaks within the Cretaceous and within the Tertiary.
The three well-recognized Cretaceous horizons—the Oshih-Ondai Sair, the
Iren Dabasu, and the Djadokhta—are characterized by distinct faunas, and
must be separated from one another by long intervals. In the Tertiary, the
breaks are of local, not of regional significance; some have continued for long
intervals, and some have been short, but no systematic relation between the
intervals at different localities has yet been discovered. For example, at
Iren Dabasu, slightly disturbed Cretaceous beds are followed by early Eocene,
upon which rest Oligocene gravels. But only twenty miles farther south,
the Oligocene beds are absent, and the early Eocene is followed by a later
Eocene horizon which is lacking at Iren Dabasu (Fig. 102, page 201). The
generalized diagram, figure 156, page 370, has the advantage of showing how
many breaks there are and of how variable length, at each camp where
formations have been named; but the diagram underestimates, rather than
emphasizes, the relative length of the intervals between the deposits. In
some places, the earlier Tertiary beds are more disturbed than the later ones,
and the angular divergence between the two suggests a profound unconformity.
But it does not have this significance; there is no regional unconformity sub-
sequent to Cretaceous time. The Tertiary breaks are local, and they vary
in area and in the time-interval which they record. Those portions of north
central Asia which have been continuously upwarped, such as the Khangai
mountain region, have a greater significance and probably mark the early
stages of a future unconformity. The strata, which are being laid down
now on the eroded lower flanks of the Khangai arch, preserve this uncon-

SUMMARY OF GEOLOGIC HISTORY 417

formity, and measure an erosion break that stretches from Jurassic time to
the present. This break may represent something akin to the Great Uncon-
formity; but the large majority of erosional breaks in the Gobi region affect
the sediments only, and are essentially local disconformities, of minor sig-
nificance in the history of the region.

ORIGIN OF PRESENT TOPOGRAPHY

In spite of the apparent monotony of the Gobi region, there is an im-
mense diversity of feature, due to widely different geologic processes and
structural control. The most noticeable features of the Gobi are smooth,
almost level stretches of country underlaid by simple sediments, which are
the true constructional plains. They are separated by more rugged divides,
composed of ancient deformed and crystalline rocks. This sort of monotony
is broken by mountains of magnificent proportions. All the surface elements
—heights, flats, and hollows—lie on the vast warped surface of a great inland
basin, with a north-south limit of more than six hundred miles and an east-
west extent of more than one thousand miles (Fig. 4, page 43).

Although it is true that this great basin region has been one of deposition
from the beginning of Cretaceous time to the present, only the minor surface
features are depositional. They include alluvial fans from the uplifted moun-
tain blocks; stream deposits along valleys, where sediments are dropped as
the waters disappear in the sands of the desert; and wind-blown sands, in
the form of dunes and thin sheets, shifting in low, bush-grown mounds over
broad stretches of open country. All are small and insignificant, and the
major features are of strictly erosional origin, the destructional work of wind,
water, and weather. They appear as erosion planes, dissected erosion planes,
and as uplifted, dissected areas that owe their relief to forces within the earth,
and their ruggedness and minor detail of feature to the destructive work
of surface agents. The great basin itself is a diastrophic product, and the
minor basins within it are of the same origin. The same general causes
account for the Altai fault-blocks and the great arch of the Khangai, and
for some of the minor relief structures such as volcanic cones and lava
fields.

On these primary relief elements, erosion has produced its usual variety
of relief. It has made mountainous country by deep erosion of great uplifted
arches; it has produced exceedingly rugged topography on upfaulted moun-
tain blocks, leaving only traces of the ancient smoothness that must have
marked the original uplifted floor; and it has beveled wide areas of both the
sediments and the harder rocks, wherever stability has been maintained for
a period sufficiently long, producing widespread planation that makes certain

VOL. II—27

418 GEOLOGY OF MONGOLIA

great stretches of Gobi country look as monotonously level as a floor. Even
these have been interfered with in some places by the inner forces of the earth,
so that some of the planes are warped into broad domes, while others have
sagged into basins.

The simplicity of the record of deposition is disturbed, also, in two other
ways. The present is an epoch of dissection. Stream courses have carved
channels across country, and many districts are intricately dissected into
badland topography, such as marks the work of rain in any dry region of
soft strata. In addition to this, the wind has gouged out hollows both in
the sediments and in the crystalline rock. The hollows are erratically dis-
tributed, are of varying shapes, sizes, and depths, and produce a variety of
erosional topography, which is not encountered except in desert regions.

The whole combination of features gives a variety to the Gobi region
not at first suspected. To the average traveler, it may seem to be a hopeless
waste, a discouraging and endless monotony of open country, but to the sci-
entist, who looks behind the scenes, it is a geological paradise—an exhibit of
processes, structures, and forms, probably unsurpassed elsewhere on the face
of the earth.

RELATION OF GEOLOGIC HISTORY TO THE DEVELOPMENT OF MAN

The Third Asiatic Expedition has often been referred to as the Expedition
in quest of early man, and there is a persistent demand for some explanation
of that side of its investigations.

No fossil traces of primitive man were found enclosed in any of the strata
of the Gobi region during the seasons of 1922 and 1923, but this fact does not
necessarily prove the futility of further search. A few stone artifacts have been
collected, some of which have a primitive palzolithic aspect, while the ma-
jority are surely neolithic types, but they were found lying on the surface,
not enclosed in sediments. For the present, therefore, they must be inter-
preted as proving the existence of primitive cultures; but they do not neces-
sarily prove the antiquity of man’s entry into this region, and they give no
evidence concerning man’s evolution. It may or may not be that the central
regions of Asia were favorable for the evolution of man in his early stages of
development. But the history of migrations, in so far as they are known,
and the geographic distribution of different races of people seem to favor the
assumption that central Asia may at least have been an important factor in
the dispersal of races. Much better results were obtained in 1925 when the
cultures of the Dune Dwellers of Shabarakh Usu were recovered from the
sand-filled valley in the district of Djadokhta; but a discussion of that dis-
covery must be reserved for later publications. It is sufficient at this time

PLATE XLIV.

ih
~~!

+

RRR

ANCIENT MONOLITH SET UP BY A PRE-MONGOL PEOPLE,

SUMMARY OF GEOLOGIC HISTORY 419

to say that later work on this problem has been very productive from both a
geological and archeological standpoint.

There is no object at this point in speculating on the place of man’s
origin. It is conceded that there is evidence elsewhere of the existence of
Pliocene man, and, if the other conditions were as favorable as the climate
during the Pliocene and Pleistocene, man may have existed in the Gobi region
during the Pleistocene and possibly also during part of the Pliocene. Among
the many suggestive thoughts offered by Joseph Barrell (1917) as guiding
hypotheses for our explorations in central Asia, his idea about primitive man
is especially ingenious. Man’s strong padded foot, his relatively long leg
and his erect posture, are all distinct departures from an adaptation to life
in the trees, and tend, instead, to fit him for running and for tramping long
distances; in short, for life on open plains where trees grow in patches along
the stream courses, rather than for life in a dense forest. Granting that the
more distant ancestors of men lived in trees and in jungles, it seems probable
that they would have remained arboreal in an environment of jungle and
forest. But in a region where forests were thinning, where open, treeless
plains were beginning to appear, and where the climate was changing toward
cooler and more arid conditions, it seems probable that arboreal types must
adapt themselves to the plains, or become extinct.

Central Asia probably offered just such a combination of opportunity
and penalty during Tertiary and Pleistocene times. It had a variable climate
swinging from arid to semiarid conditions and therefore was exceptionally
favorable to ranging animals which were tending toward the evolution of
fleet limbs and long endurance. It also formed the northern borderland of
the tropical forest country; and with the changing climatic cycles, the forests
must have advanced into central Asia and died out more than once. The grow-
ing barrier of the Himalayas and the elevation of the vast interior plateaus
profoundly altered living conditions for all the creatures that tried to maintain
themselves in the region. If the ancestors of man actually lived there, it
seems not unlikely that they must have developed adaptations to life in the
open plains and the abruptly upstanding hills, where patches of forest still
lingered along the water-courses. It is fair to search the country thoroughly
for proof or disproof of the working hypothesis that central Asia is the right
locality, and had, during the critical periods, the right environment for the
evolution of man.

Pliocene beds are exposed in few places, and it is possible that they may
carry important evidence, but their limited development is a distinct handicap.
There must have been sediments forming somewhere in adjacent regions while
the central Gobi was being eroded. The material carried out of the region,
both by streams and by wind, must have lodged somewhere under more favor-

‘420 GEOLOGY OF MONGOLIA

able conditions, and these deposits of adjacent territory should be much more
promising, not only for possible remains of man, but also for the remains
of all other forms of late Pliocene and Pleistocene life. It is the hope of the
Expedition that in the work of its additional years, for which traverses are
projected into adjacent districts, some of these more favorable conditions
and the deposits of later time may be discovered.

‘

_ BIBLIOGRAPHY

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BIBLIOGRAPHY

Throughout the text, references to works listed in the bibliography are cited by
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In the bibliography, complete works are printed in italics, thus: The Russian
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Aegypten. Unter mitwirkung mehrerer fachgenos-
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Paleontologischer Theil by different authors.

INDEX

Text illustrations are classified in the Index as figures, maps and sections,

by figure number: thus figure 5, on page 47, is entered as 47.

Aceratherium, 17, 200
Achenodont, 361
®olian cross-bedding, see Cross-bedding
Agates, 74, 151, 208, 254, 272, 334
Agriculture,
Chinese, 184, 187, 396
Plate VIII B
possibilities of, 59, 187
see also Grasslands, Irrigation
Ah Sah, acknowledgment to, x
Aioshi, acknowledgment to, x
Ala Shan desert, boundary of, 32
Ala Shan Mountains,
boundary of Ordos, 32
faulted margin of, 35
Alaska, past climates of, 374
Alaskite, 93, 99
Alexander, Edward J., acknowledgment to, viii
Algonkian deposits in Kentai Ola, 25, 298
Alluvial fans,

_ ancient, 110, 230, 236, 262, 31I-313, 315, 407
climatic significance of, 311-313, 389-391, 395
figure, 224
maps showing, 117, Plates XXVII, XXX
Plate XL B
recent, 115, 246, 262, 312, 313, 315, 329, 330
slopes of, 311, 312
terraces of, 390
see also Later sediments

Altai ranges, as boundary of “ Valley of the Lakes,’’ 308

displacement in, 20

fault blocks in, 9, 256, 306, 310, 413, 417
first impressions of, 138

maps showing, 43, 194, Plate IV

structure of, 34-35, 138, 158, 223, 226, 242-243,

252, 253, 260, 265, 308, 309-317, 320-321
trend of, 320-321

see also Altaids, Artsa Bogdo, Baga Bogdo, Gurbun

Saikhan, Ikhe Bogdo
Altaids, as defined by Suess, 27
Amblypoda (Amblypods), 208, 209, 361, 363

American Museum of Natural History, acknowledg-

ment to, vii
see also Specimens

Amphibolite, 136

Ambphicticeps shackelfordi, 234

Amur River, 21, 25, 306, 327
maps showing, 43, Plate IV
region of, 25-26

Amygdules, 74, 127, 253, 272, 334

Amynodon, 362

Amynodontide, 362

Andersson, Dr. J. G.,
cited on Cenozoic of China, 367
cited on Eneolithic culture, 13
cited on Oligocene, 31, 358
cited on Pleistocene, 317, 368, 385
cited on Pliocene, 317, 367, 370, 382

quoted on physiographic history of China, 347-348

Andesite, 96, 155, 157, 218, 228
conventional symbols for, Plate VII
section, 172

Andrews, Roy Chapman, leader, v, viii, 8, 84, 88,

157, 197

Plate XXI B
Andrewsarchus mongoliensis, 208, 360
Angara Series, in Khangai Mountains, 23

in Tannu Ola, 22

see also Jurassic period
Angara continental element, 32
Anthracotheriide, 208, 364

Anticlinal structures observed, 88, 93, 105, 109, 154,

156, 164, 170

see also Domes
Aphanite, see Dolerite
Archean era, see Archzozoic formations
Archeomeryr, 361

optatus, 362
Archzozoic era, see Archzeozoic formations
Archzozoic formations, 24, 25, 26, 296, 302, 398

maps showing, 290, 404

sections, 8, 299, 415

table showing, 301

see also Gneisses, T’ai Shan system
Arctic Divide,

altitude of, 307

animals and plants of, 130

figure, 82, 344

449

They are referred to by page number, not

450

Arctic Divide—Continued
in Gangin Daba Mountains, 82
glaciation of, 384
in Khangai Mountains, 125, 127, 128, 384
map showing, 43
Plates XIV, XL A
rainfall of, 326, 396
rivers from, 328
section, 81, 328
in Tannu Ola, 22
Ardyn Obo formation (Oligocene), age and correla-
tion of, 177, 367-369
climate indicated by, 380
conventional symbol for, Plate VI
description of, 174-177, 363, 369
figures, 176, 324
fossils of, 363-364
maps showing, 43, 176, 404
Plate XXI
problem of, 279
sections, 175, 177
upland gravels, 334
see also Camp Ardyn Obo
Ardynia precox, 360, 363
Ardynictis furunculus, 363
Ardynomys chihi, 363
olseni, 363
Argilintai Mountains, boundary of Ordos, 32
Argillite, 61, 84, 90, 94, I19, 122, 125, 132, 314, 315,
364, 375
conventional symbol for, Plate VI
see also Khangai Graywacke Series
Arguin Gol (river), 348
maps showing, 137, 140, 194
Argun River, course of, 25
drainage of eastern Gobi, 21, 306
maps showing, 43, Plate IV
volcanic rocks near, 28
Aridity, see Climate
Arishan,
area, extent of, 216
description of, 213
geological formations of, 216-220
hot springs of, 124, 126, 127, 213-215, 219, 220—
222
maps showing, 123, 133, 194, Plate XXVI
Plates XVII, XXIV, XXV
Sacred Mountain of, 124, 127, 132, 213, 215-217
sections, 134
Armor, see Bowlders, Desert armor
Arshanto formation (Eocene), climate indicated by,
379
conventional symbol for, Plate VI
correlation of, 211
description of, 205, 206-207, 359
fauna of, 207, 359
maps showing, 198, 404.
Artifacts, see Cultures

INDEX

Artiodactyla (Artiodactyls), 206, 208, 234, 235, 360,
361, 362, 364, 380
Artsa Bogdo range, description and history of, 313-
314
figure, 266
geologic structure, 253-255, 260, 262-268, 310, 413
journey across, 248-253
maps showing, 194, 304, 404
northern margin of, 151, 174, 261, 330
Plates XXXIV A, XXXIX B
tock formations of, 262-264
sections, 150, 253, 261, 263, 264
topographic features of, 267
Ashile, see Oshih
Asia Magazine, acknowledgment to, 8
Asia, structure lines of (map), 321
Asiatosaurus, 271, 354
Atacama Desert, comparison of, with Gobi Desert,
396
Aves, 235, 361

Badger, fossil, 382
Badlands, 268-270, 277, 278, 280, 353
figures, 200, 269, 270, 324, 339, 356
maps showing, 194, Plates XXIII,
XXVIII, XXIX
origin of, 338-340
Plates I, XXI A, XXII B, XXXII A, XXXIII A,
XXXV, XXXVI
see also Basins, Hollows, Later sediments
Baénomys ambiguus, 359
Baga Bogdo, 308
alluvial fans of, 390-393
boundary of Tsagan Nor basin, 224
compared with Artsa Bogdo, 260
description and history of, 138, 311-313
drainage of, 147, 311
figures, 224, 269
former sedimentary cover of, 238
geologic structure of, 223, 228, 239, 243, 244, 310,
413
maps showing, 194, 304, 404, Plate XXX
meaning of the name, 139
peneplane of, 242
Plates XVIII A, XL B
sections, 229
Bahadur, Abul Ghazi, cited on Tatar history, 15
Baiera furcata, 233
Baikal faults, 27, 320
Baikal, Lake,
drainage connections of, 90
maps showing, 43, Plate IV
regional structure of, 26, 27
Baikal trend, 319, 320
Baikal-Yablonovi mountain group, 309
Baluchitherium, 205, 364, 369, 381
grangert, 234
mongoliense, 235

XXVII,

INDEX

Banjien, acknowledgment to, x
Barbour, George B., acknowledgment to, viii
cited on Kalgan and vicinity, 31, 276
Barchans at Baga Bogdo, 351, see also Dunes
Barchin Series (Archean) in Kentai Ola, 25
Barley, cultivation of, 184
Baron Khara, inlier at Oshih, 269
Baron Saikhan, part of Gurbun Saikhan, 158, 314
tock formations of, 257
Baron Shiliuta, 333
figures, 239, 240
maps showing, 140, Plate XXVII
Barrell, Joseph, quoted on evolution of man, 419
Basalts, amygdules of, 74, 127, 253, 272, 334
bathylithic relations of, 405, 410
conventional symbols for, Plate VII
field observations of, 28, 48, 50, 60, 61, 66, 74, 78,
80, 96, 126, 148, 149, I51, 156, 158, 161, 189,
219-221, 235, 237, 238, 239, 241, 245, 253, 261,
262, 264, 266, 268, 272, 280, 281, 313, 355, 358,
365, 393
figures, 240, 242, 269
maps showing, Plates XXIII, XXVII, XXVIII,
XXIX, XXX
Plates VIII A, XVIII B, XXXI B, XXXV B
sections, 123, 129, 155, I7I
see also Lavas, Volcanic, Volcanoes
Basins, Ardyn Obo, 174-177, 279
south of Artsa Bogdo, 152, 248, 250, 254, 257, 266
Djadokhta, 156, 278, 358 ~
figures, 165, 199, 209, 266, 269, 310, 370
Guchu Burt, 141
Tren Dabasu, 196-199, 355
Jurassic, 407, 411
Los in Sumu, 283
major and minor, 9, 33, 303, 305, 306, 378
maps showing, 43, 137, 140, 176, 194, 198, 304,
321, 404
Ongin Gol, 161, 280
origin and structure of, 319, 321-322, 367-370, 403-
405, 412
Oshih, 268, 272, 278, 354
P’ang Kiang, 276
rock floor of, 341
sections, 8, 10, 42, 47, 51, 53, 57, 58, 62, 65, 69,
73) 77, 79, 95, 104, 134, 140, 145, 150, 155, 159,
160, 163, 167, 168, 171, 172, 175, 180, 185, 186,
188, 189, 201, 229, 253, 289, 316, 346, 367, 369,415
sedimentary, 52, 60, 61, 63, 64, 67, 76, 96, 135,
151, 164, 166, 169, 173, 174, 178, 275-280, 308,
313, 382
Shara Murun, 279
Tsagan Nor, 138, 142, 144, 223, 224, 238, 243-246,
277, 311, 392
near Tsetsenwan, 24
Ulan Nor, 280
Wan Ch’uan, 275, 355
see also Gobi, Gobi Basin, Later sediments, Tala

451

Bathylith, 52, 94, 98, 104, 156, 164, 166, 178, 181, 216,
217, 218, 223, 228, 260, 282, 284, 292, 298, 409,
410
problem of, 99-100, 284
see also Granites, Magma, Mongolian bathylith,
Roof-pendants, Xenoliths
Bato, acknowledgment to, x
Batrachia, 234
Beaches, ancient, at Tsagan Nor, 247, 350
as evidence of climatic change, 386, 392
Beale, Miss C. M., acknowledgment to, vii
Beaver, fossil, 382
Berkey, C. P., and Granger, W., cited on later sedi-
ments, 209, 402
Berkey, C. P., and Morris, F. K., cited on later sedi-
ments, 35, 366
cited on Mongolian bathylith, 93, 297
cited on oldrock floor, 288
see also Granger, Walter
Bilgoho, 181
Bird, fossil, 235
see also Aves
Bivalve, see Pelecypods
Black, Davidson, cited on human bones, 1 3
Black Butte, see Dzun Hsir
Backwelder, Eliot, cited on past climates, 374
see also Willis, Bailey
Block faults, see Mountains
Block-faulting, see Faults
Block mountains, see Mountains
Bogdanovich, K. I., cited on Central Asia, 17
routes of, Plate II
Bogdo Ola, in eastern Mongolia, 29
map showing, Plate IV
Bogdo Ola, in Tien Shan, topography and structure
of, 34
map showing, Plate IV
Bolkuk Gol (river), course of, 85
section along, 81
valley of, 82-86
see also Camp Bolkuk Gol
Boltai Urtu, 199, 209
figure, 199
map showing, 43, 198
section, 185
Bones, see Fossils
Bornhardt, W., cited on desert erosion, 325
Boro Sunit Temple, 196
Bos, 347
Bosses, observed, 94, 264
Bowlders, 70, 329
Plate X B
residuary, 66, 71, 122
Brachiopods, 255, 374
Breccias, fault, 97
igneous, 74, 78, 96, 108
Bridger formation, compared with Irdin Manha,
360

\

452 INDEX

Brontops gobiensis, 363 Camp Jurassic, compared with Tsetsenwan, 109
Brooks, C. E. P., cited on Pleistocene climate, 382, figure, 281
383 future studies recommended, 280

Bryan, Kirk, cited on desert erosion, 325 maps showing, 43, 275
Bryozoans, fossil, 374 Plate XI A
Buddha, 139, 162, 174, 194, 196, 215, 310] sections, 65, 281
Buksui Gol (river), 23 vicinity of, 67-70
Bunealurus parvulus, 234 see also Jurassic period

ulysses, 234 Camp Loh, 142, 225, 230, 234
Buriat official at Ude, 64 maps showing, Plates XXVIII, XXIX
Butler, F. B., acknowledgment to, ix see also Loh formation
Buttes, at Irdin Manha, 200 Camp North Dakota, 184

east of Uskuk, 329 section, 188 -

volcanic, 66 Camp Ondai Sair, 142

see also Baron Shiliuta, Volcanoes maps showing, 140, Plate XXVII

; section, 140

Cadurcotherium, 206, 364 see also Ondai Sair formation

ardynense, 263 Camp Ongin Gol, 115, 122
Cenolophus minimus, 362 map showing, 121

obliquus, 362 section, 121

proficiens, 361 see also Ongin Gol

progressus, 362 Camp Oshih, 268

promissus, 362
Cenopus, 206, 364
Calcite cement, 356
Calcium carbonate, see Limestone
Cambrian formations, 27, 32
Cambrian period, an erosion interval, 414
Camels, 141, 249, 250
Camelus, 235

see also Oshih hollow

Camp P’ang Kiang, 52

Camp Paradise Park, 252

Camp Porphyry Hills, 165
section, 167

Camp Rainy Gulch, map showing, 123
Plate XIII B

Camp Ardyn Obo, 176 section, 123
maps showing, 43, 275, 290, 404 vicinity of, 124-127
Camp Bolkuk Gol, vicinity of, 83, 88 Camp Snow Storm, 64
sections, 81, 89, 92 section, 65
Camp Canyon Brook, map showing, 117 Camp Tola River, 91
Plate XIII A section, 95
section, 113 see also Tola River
vicinity of, 113-115 Camp Tsagan Nor, 142, 143
Camp Desert Well, section, 172 see also Tsagan Nor
Plate XXXIV Camp Tsetsenwan, 97
Camp Five Antelope, section, 95 section, IOI
vicinity of, 91-94, 96 see also Tsetsenwan
Camp Forest, 128-129 Camp Volcanic Cliffs,
map showing, 129 section, 145
section, 129 Canide, 363
Camp Gorida, 136 Canyon Brook, see Camp Canyon Brook
map showing, 137 Caravan, 7, 75, 97, 136, 148, 166, 196, 248
section, 134 Carbon, paucity of, in later sediments, 379, 381
Camp Granite Hills in Chakhar Hills, 52 Carboniferous formations, deformation of, 22, 30, 33,
Plate IX B 34, 170
Camp Granite Monument, 183 reported in Mongolia, 298
section, 186 reported from surrounding regions, 17, 22, 30, 33, 34
Camp Gun Usu, section, 155 section, I7I
Camp Gurbun Saikhan, 250 table showing, 301
see also Gurbun Saikhan see also Dinantian formations, Paleozoic era,
Camp Irdin Manha, 208 Permian period, Sair Usu formation
see also Irdin Manha formation Carnivora (Carnivores), fossil, 177, 205, 208, 234,

Camp Jichirun, section, 155 360-361, 362, 363, 364, 380

INDEX

Carts, 189
Castor, 235
Cattle, 87, 258, 395
Cenozoic era, climate during, 378
orogenic movements during, 35, 405
Cenozoic formations, 358-368
structure lines of, 322
see also Tertiary period
Ceratopsian dinosaurs, 357
Cervid, 235
Cervus, 347
Chakhar Hills, eastern extension of, 306
figure, 342
general description of, 52
physiographic character of, 317
Plate IX
sections, 51, 53
see also Camp Granite Hills, Mongolian peneplane
Chalcedony, 74, 151, 176, 204, 205, 208, 248, 254,
272, 334, 356, 364
Chalicotheriide, 361, 362, 363
Chang Kwei Mo, acknowledgment to, x
Chelonia, 204, 206, 359, 361, 362, 364
Chernov, A., cited on Gashuin tala, 307
routes of, Plate II
Chi Shou Lun, acknowledgment to, x
Chihli, 16
China, Cretaceous climate of, 376
early cultures of, 13
fish-bearing shales of, 354
great plain of, 8
Jurassic deposits of, 292, 294, 376
loess of, 385
paucity of Miocene strata in, 346, 367
physiographic stages of, 347-348
pre-Cambrian rocks of, 291, 300, 302, 398, 399
see also Hu t'’o, Nan K’ou, Sinian, T’ai Shan,
Wu T’ai
Chinese, retreat of, through Tuerin, 75
settlements of, in Mongolia, 59, 183, 184, 187, 189,
396, Plate VIII B
telegraph station at Iren Dabasu, 196
see also Cultures
Chironomopsis gobiensis, 233
Cirques, figures, 83, 86, 344, 384
in Ikhe Bogdo, 311, 384
in Khangai Mountains, 130-131,-384
problematic, at Artsa Bogdo, 267
problematic, in Gangin Daba, 83
see also Glaciation
Clays, 23, 31, 63, 146, I5I, 157, 158, 177, 182, 200,
203, 204, 206, 207, 210, 230, 235, 236, 238, 271,
279, 306, 311, 315, 319, 323, 340, 347, 352, 353,
354, 355, 357, 358, 359, 360, 362, 363, 365, 366
374, 375, 376, 377, 379, 380, 381, 382, 407
Clay-galls in Irdin Manha formation, 207

Clements, J. Morgan, cited on graywackes, 24-25,
298

453

Climate, affecting Man’s evolution, 419
evidence of change of, 48, 118, 128-130, 148, 152,
162, 202, 247, 307, 311, 313, 331, 373-395
fluctuations of, in China, 347
fluctuations of, postulated by other explorers, 20
indicated by formational units, 373-382
present, in Mongolia, 395
Coal, 17, 89, 219, 228, 293, 376, 407
Cockerell, T. D. A., cited on Cretaceous fossils, 353
Coleoptera, 233
Colgate, S. Bayard, 153
acknowledgment to, x
Colodon inceptus, 363
Colony, Roy J., acknowledgment to, vii
Columbia University, acknowledgment to, vii
Comanchean period, 369, see also Cretaceous, Lower
Concretions, 157, 205, 207, 270, 272, 334, 356, 377
Plates XXI B, XXXVI B
Condylarthra, 359
Cone-in-cone structure, at Oshih, 271
Conglomerates, 22, 23, 24, 31, 49, 61, 63, 66, 104,
105, 106, 124, 127, 149, 152, 164, 165, 166, 170,
173, 174, 178, 216, 218, 219, 220, 224, 227, 228,
235, 236, 237, 238, 256, 259, 261, 262, 263, 265,
271, 282, 283, 291, 292, 293, 294, 298, 299, 313,
314, 315, 331, 334, 335, 353, 354, 355, 365, 374,
375, 376, 380, 398, 401, 402, 405, 406, 407
411
conventional symbol for, Plate VI
indicating unconformities, 119, 302
interbedded, interpretation of, 119
Corals, fossil, 255, 374
Cottonwood trees, 389
Creodonta (Creodonts), 359
Cretaceous formations, 40, 41, 152, 154, 156-158,
203-204, 210-212, 224-225, 230-232, 255, 257,
268-273, 277-279, 309, 314-315, 352-358, 366,
368-369, 376-377, 416
deformation of, 35, 41, 212, 244, 269-270, 310, 405,
410, 411, 412, 413
distribution of, 306, 307, 308, 309, 315, 358, 407
figures, 199, 269, 270, 356
gaps in sedimentary record of, 366, 416-417
marine, 17
maps showing, Plates XXIII, XXVII, XXVIII
Plates I, X A, XVIII B, XIX, XX B, XXXI A,
XXXV, XXXVI, XXXVII
problematic, 32, 49, 63, 68, 70, 71, 108, 151, 154,
161, 166, 169, 174, 178, 207, 255, 257, 261, 263-
266, 281, 313
sections, 47, 56, 58, 65, 69, 140, 150, 155, 159, 172,
201, 203, 229, 316
table showing, 371
see also Djadokhta, Dubshih, Iren Dabasu, Ochung-
chelo, Ondai Sair, and Oshih formations; also
Later sediments, Unconformity, Volcanism
Cretaceous period, climate during, 376-378
fossils of, 204, 233, 353, 354, 357

454

Cretaceous period—Continued
summaries of, 352-358, 366-372
uplift during, 240
Cretaceous period, Lower (Comanchean), beginning
of basin hab:t during, 10, 32, 40, 402
Cricetops dormitor, 234
Crinoids, fossil, 255
Crocodilia, (Crocodilian), 357, 377
Cross-bedding, zolian, in Djadokhta formation, 356,
377
figures, 54, 356
Plates XXI B, XXXVI B
torrential, 54, 177, 203, 204, 207, 231, 334-335, 36C»
363, 379
Cross-sections, geologic, preparation of, 6, 43, 143
Crush-zones, 97, I13, 135
see also Faults
Crustacea, 233
Crystalline rocks, 60, 64, 72, 154, 246
at Arishan, 216
at Artsa Bogdo, 260, 262-265
complex, 281-282, 299
at Gurbun Saikhan, 257-258
at Mt. Uskuk, 223, 226-228
pebbles of, 125, 255
see also Oldrock floor, Pre-Cambrian
Cuesta, 146, 219, 281
“false,’’ 391
figure, 281
see also Scarps
Culm formation, in Tannu Ola, 22
Cultures, primitive, 88, 248, 418
early Chinese, 13
Eneolithic, 13
figures, 115, 394
Mousterian, 13
Neolithic, 13, 418
Paleolithic, 13, 18, 248, 284, 418
Plates XLIII A, XLIV
pre-Mongol, 13-14, 114, 115, 152, 386, 393, 394
Cyclomylus lohensis, 234
Cymatophlebia mongolica, 233
Cynodictis, 363
elegans, 234
Cynodon teilhardi, 234
Cyprinide, 233
Czekanowskia, 233

Dalai Nor (Lake Kulun), 25, 327
map showing, Plate IV
volcanic rocks at, 28
Dalai tala, axis of, 321
description of, 306-307
map showing, 43
Pliocene formations in, 370
volcanic cones in, 318
Dam, ancient, near Tsetsenwan, 395
Davis, William M., cited on desert erosion, 325

INDEX

cited on drainage, 327
cited on explorations in Turkestan, 19
cited on graded rivers, 331
cited on peneplanes, 323, 347
routes of, Plate III
Da Ying Gol (river), 366
Da Ying Gol beds, at Hung Kureh, 366
Deer at Arctic Divide, 130
fossil, 381, 382
rock drawings of, 152, 393, 394
Deflation, see Erosion
Deforestation, 387-388
Deformation, of Gobi erosion plane, 230, 332, 333
of Jurassic strata, 293, 294
of Khangai graywacke series, 298
of later sediments, 411-412, 414, 418
local notes on, 113, 114, 154, 216, 224, 226, 239,
244, 245, 259, 261, 262, 263
pre-Cambrian, 398
processes of, 212, 284, 289, 405, 411-412, 413
products of, 413-414
summaries of, 410, 413
of talas, 321-322
see also Faults, Folding, Warping
Deinodont dinosaurs, 204, 355
Denudation, 386
Deperetella cristata, 362
Deposits, on Gobi upland, 230, 333
marine pre-Cambrian, 405
tables showing, 301, 371
wind-blown, 335
see also Alluvial fans, Sediments
Deposition, amount of, compared with erosion, 386
present, at Ulan Nor, 280
shifting locus of, 10, 20, 368-372
see also Sedimentation
Desert, types of, 303
see also Erosion
Desert armor, 326, 334
see also Bowlders, Gravels
Desert of Gobi, see Gobi Desert
Desert hollows, see Hollows
Desmatolagus, 362
gobiensis, 234
robustus, 234, 363
Desmatotherium fissum, 361
mongoliense, 208, 361
Devonian formations, 406
problematical, at Gurbun Saikhan, 255
reported by other geologists, 17, 22, 401
Devonian period, an erosion interval, 414
Diabase, 202
Didymoconus berkeyt, 234
colgatet, 234
Dikes, 48, 61, 71, 72, 82, 84, 91, 93, 96, 97, 105, I13,
II4, 116, 122, 124, 128, 132, 135, 139, 156, 164,
173, 178, 181, 183, 202, 216, 218, 264, 281, 291,
296, 298, 319

INDEX

Dikes—Continued
figure, 102
maps showing, 121, Plates XXIII, XXVI
Mesozoic, 291
sections, 47, 51, 53, 62, 65, 69, 73, 77, 79, 92, 95;
IOI, 102, 107, 113, I17, I2I, 134, 140, 155, 159,
167, 168, 171, 172, 175, 179, 186, 188, 253, 289,
291, 293, 295, 297, 415
“serpent form,’’ 98-102, I10, 112, 218, 409
see also Bathylith, Porphyry
Dinantian formations, at Gurbun Saikhan, 255
at Sair Usu, 173, 282, 400, 406
Dinosaur eggs, at Djadokhta, 157, 278, 279, 377
at Iren Dabasu, 204, 355
Plate XX B
Dinosauria (Dinosaurs), 31, 41, 59, 63, 157, 197, 203,
204, 206, 207, 210, 211, 271, 278, 354, 355, 357)
376, 377
see also Reptilia
Diorite, 16, 22, 50, 141, 178, 300, 313, 314, 408, 409
conventional symbol for, Plate VII
Disconformities, in later sediments, 211, 346, 359, 417
Plates XXIII, XXVIII, XXIX, XXX
sections, 201, 203, 316
summary of, 416
see also Unconformities
Diske formation (Pleistocene), 368
Dissection, checked by vegetation, 339
double, at Wan Ch’uan, 386
epoch of, 418
of Mongolian peneplane, 342, 344
Djadochtatherium matthew, 357
Djadokhta, artifacts at, 418
maps showing, 43, 404
overlying gravels at, 335
plain at, 254
Djadokhta formation (Cretaceous), 41, 156, 157-
158, 232, 257, 309, 315, 353, 358, 366, 368, 369,
416
climate indicated by, 377
conventional symbol for, Plate VI
description of, 356-357
dinosaur eggs in, 204
fauna of, 357
figure, 356
future studies of, recommended, 278-279
Plates I, XIX, XX B
sections, 155, 159, 316
Djargos range, 34
Dolerite, 93, 114, 120
Dolichorhinus kaiseni, 362
olseni, 361
Dolomite, 178, 262, 263, 264, 401, 406
conventional symbol for, Plate VII
Dolon Nor (lake), 28, 29
map showing, Plate IV
Domes, 418
at Camp Jurassic, 68, 70, 281

455

of Kentai Ola, 25
at Shiliuta east of Uskuk, 333
“Dragon bones,"’ 103, 365
Drainage, dendritic, 24, 309, 313
inland or centripetal, 21, 49, 307, 327, 403
local readjustments of, 240-241, 243, 244
superimposed, 85, 267
trellis, 25
Drawings on rocks, 114, 115, 152, 386, 393, 394
figures, 115, 394
Dubois, Eugen, cited on Pithecanthropus, 12
Dubshih formation (Lower Cretaceous), 156, 309
section, 155
Dunde Khara, 269
Dunde Saikhan range, 158, 314, 315
Dunes, 58, 63, 350, 417
at Tsagan Nor, 147, 230, 351
on Gobi erosion plane, 332, 335
map showing, Plate XXX
Plate XVIII A
Dune Dwellers of Shabarakh Usu, 418
Dune plum, maker of sand hummocks, 351
Dzun Usir, Tertiary sediments at, 235, 236
Dzun Saikhan range, 158, 314
Dzungaria, an extension of Gobi Basin, 33-34, 36
mapped by Renat, 15
maps showing, 43, Plate IV

Eder Gol (river), 23
map showing, Plate IV
Edsin Gol (river), 33, 307
map showing, Plate IV
Egin Gol (river), 23
Elephant, fossil, 385
Elephas, 347
Elms, 387
Emydid, 359, 364
Encyclopedia Britannica, quoted, 13
Eneolithic, see Cultures
Entelodon dirus, 206, 364
Eocene formations, 182, 205, 206, 210, 211, 234, 318,
332, 335, 358-362, 366, 369, 416
gaps in sedimentary record of, 367
see also Arshanto, Irdin Manha, Shara Murun,
Tukhum
Eocene period, climate during, 378-380
Epeirogenic movements, summary of, 410
Ephemeropsis melanurus, 233
trisetalis, 233
Epiaceratherium, 234
Epidote, 204, 334
Equisetacez, 233
Eras, tables showing, 301, 371
Erin Nor, terminal lake, 348
map showing, 194
Erlien, telegraph station, 55, 196
Erosion, epochs of, 10, 48, 98, 212, 241, 265, 346,
347, 368, 398, 399, 401, 403, 405, 407, 414

456

Erosion—Continued

evidence of, 67, 177, 178, 219, 235, 288, 294, 295,
305, 312

history of, at Mt. Uskuk, 243-244

by ice, 374

Plate XIX

stages of, 90, 201, 347

by water, 46, 115, 126, 135, 138, 177, 205, 247, 269»
280, 305, 312, 324, 325, 326, 328-330, 331, 335;
336, 338-340, 341, 347, 374, 383, 386, 387, 389,
390, 391, 400, 412, 418

by wind (deflation), 118, 141, 146, 148, 202, 246,
247, 305, 324, 325, 326, 335, 336-338, 347, 350

374, 386, 391, 418
see also Glaciation, Monadnocks, Peneplanes
Erosion planes, beveling later sediments, 161, 211,
216, 230, 236, 323-324, 328, 332-336, 345-348,
353, 359, 412, 417
beveling oldrock floor, 60, 71, 75, 96, 139, I41, I5I,
173, 211, 229, 259, 261, 262, 296, 313, 323, 330,
341, 342-348, 401, 417
Plates X B, XII A, XXII A, XXXVIIIA
Erosion remnants, 75, 96, 120, 122, 217, 224, 241, 289,
331
Ertemte formation (Pliocene), 370, 382
conventional symbol for, Plate VI
section, 51
Eruptives, see Volcanic rocks
Escarpment, see Scarps
Estheria middendorffi, 233
Eudinoceras mongoliensis, 208, 209, 361
Eumeryx culminis, 234
Eumys asiaticus, 234
Eurymylus laticeps, 359
Exploration, ancient, 12-14
modern, 14-20
Extrusives, see Volcanic rocks

Fairbanks, Helen R., acknowledgment to, viii
Faults, 30, 34, 46, 68, 71, 75) 96, 97, 104, 108-109,
114, 124, 125, 126-127, 135, 149, 154, 181-182,
215, 216, 218, 219, 220, 221, 236, 239, 240-241,
244-245, 257, 259, 262, 265, 266, 269, 270, 277,
282, 288, 292, 293, 294, 298, 307, 333, 353, 355)
403
age of, 270, 289, 295, 366, 378, 403, 411, 412-413
Baikal system of, 27, 320
figures, 98, 239, 242, 269, 270, 310
Khingan system of, 27, 320
maps showing, 133, 320, 321
Plates XI A, XXXI B, XXXV
sections, 8, 47, 53, 57, 62, 65, 69, 73, 79, 107, 123,
129, 134, 140, 150, 163, 167, 168, 171, 172, 175,
180, 199, 229, 253, 257, 261, 295, 316
topographic expression of, 412-413
trends of, 27, 30, 319, 320, 321, 322
see also Basins, Mountains

INDEX

Fauna, of Arctic Divide, 130
see also Fossils
Feldspar, 204, 341, 356, 374, 375, 377
Felsites, 45, 122, 148, 149
Fen Ho (physiographic stage), 347-348
Fishes, fossil, 353, 354, 360, 377, 379, 382
see also Pisces
“Flaming Cliffs,’ see Djadokhta, Hsanda Gol. Uskuk
Flatlands, 323
Flax, cultivation of, 184
Flint, 334
Flocks, 83, 88, 124, 125, 244, 249, 250, 252, 253, 258
Flora, of Arctic divide, 130
see also Vegetation
Folding, 34-35, 84, 88, 90, 104, 105-106, 108, 109,
113, I14, 124, 125, 132, 149, 164, 170, 181, 182,
217, 224, 226, 227, 258, 260, 263, 269, 282, 288,
291, 322, 366
periods of, 245, 265, 311, 344, 398, 399, 400, 401,
403, 411, 414
Forests, of Arctic Divide, 128-130, 396
Plates XIV A, XLA
see also Climate
Formations, tables of, 301, 371
Forster, Mary V., acknowledgment to, vii
Fossils (Fossil faunas and floras), 6, 7, 11, 40, 56,
144, 154, 225, 248, 306, 308, 317
collected by other explorers, 12, 17, 200, 209, 401
first discovery of, by the Expedition, 17, 59, 196-
197
marine organisms, 170, 173, 181-182, 255, 282-283,
295, 401
non-marine invertebrates, 233, 266, 376
plants, 63, 103, 169, 174, 233, 266, 376, 402
vertebrates, 52, 59, 70, 158, 177, 182, 196-197,
200, 203-205, 205-206, 208, 209-210, 211, 232-
233, 234-235, 253, 254-255, 270-271, 272, 275-
276, 278-279, 309, 314, 353-354, 355, 357, 358-
359, 360-361, 362, 363-364, 365, 371, 376-378,
379, 380, 381, 382, 385
Fox, rock drawing of, 152, 394
Friederichsen, Max, cited on the Tien Shan, 34
Frost, action of, 218, 305, 341, 374, 395

Gabbro, 22, 178
conventional symbol for, Plate VII
Gangin Daba Mountains, 21, 25, 84, 89, 282, 373
glacial evidence in, 85-86, 383
structure of, 413
topography of, 82
Gashato, maps showing, 194, 404
Gashato formation (Paleocene), 158, 309, 315, 367, 369
climate indicated by, 379
conventional symbol for, Plate VI
description of, 358
fossil fauna of, 358-359
problem of, 279
section, 316

INDEX

Gashuin Nor (lake), 307
map showing, Plate IV
Gashuin tala, 306, 307
map showing, 43
Gastropods, 255, 366, 381
Gazella, 235
Gedroits, Prince, cited on Great Khingan range, 28
Geologic column, tables of, 301, 370, 372, 415
Geologic data, method of recording, 39-44
Geologic history, continental, initiation of, 402
of granite bathylith, 400
map showing, 404
marine Paleozoic, 400
methods of recording, 41-44
of pre-Cambrian eras, 398
relation of, to Man’s development, 418
topical outline of, 397

Geological Survey of China, acknowledgment to, vii

Ghazi, Abul (Abu al), see Bahadur
Gilbert, G. K., cited on desert erosion, 325
Giraffide, 382
Glaciation, in Artsa Bogdo, 267, 268
in the Gangin Daba, 83, 85-86, 383-384
in Ikhe Bogdo, 311
in Khangai range, 130-131
map of, 383
summary of, 382-385
see also Cirques
Glires, 234, 359
Gneisses, 22, 71, 72, 80, 107, 108-109, III, 116, 119,
135-136, 138, 139, 141, 154, 156, 174, 177, 178,
184, 226, 227, 262, 282, 290, 291, 296, 300, 319,
320, 321, 322, 363, 373, 375, 398, 406
conventional symbols for, Plate VII

sections, 8, 47, 73, 79, I12, 117, 134, 140, 220, 257,

299, 415
table showing, 301
Goats, 313
Gobi, 307, 308, 362, 364, 366, 368, 370, 407
definition of, 306
meaning of, 58, 70
origin of, 412
sections, 47, 51, 53, 54, 62, 65, 69, 73, 77) 79,
95, 107, 134, 140, 145, 150, 155, 159, 160, 163,
168, 171, 172, 175, 177, 179, 180, 185, 186, 188,
189, 201, 203, 229, 253, 257, 261, 277, 281, 289,
295, 299, 310, 316, 367
trends of, 319, 322
Gobi Basin, 5, 196, 309, 352, 368
age of, 27
boundaries of, 21-36
description of, 9-10
entrance to, 49
general form of, 303
major subdivisions of, 306
maps showing, 43, 304
sections, 10, 367
structure lines in, 36

457

Gobt Desert, 9, 21, 190, 396
floor of, 59
general extent of, 303
Gobi erosion plane, 54, 56-58, 161, 177, 259, 268, 280,
307, 348, 386
age of, 346-347
ancient valleys of, 182
deformation of, 211, 244-245
figures, 176, 200, 209, 239, 240, 242, 245, 318, 324,
337
maps showing, 176, Plates XXIII,
SROXAV ATT: SKEET CRERER:
origin of, 324
Plates X A, XXI A, XXII B, XXXI A, XXXII A
problems of, 284
relation of, to P’ang Kiang hollow, 347
relation of, to Mongolian peneplane, 345
sections, 53, 57, 58, 77, 145, 155, 159, 160, 163,
175, 180, 185, 201, 346
summary of, 332-336
Gobi series, 40-41
Gobi stones, 74, 254, 272
Gobiohyus orientalis, 361
pressidens, 361
robustus, 361
Gochu formation (Pleistocene), 312, 368
section, 229
Gorida, 135-136, 138
maps showing, 137, 194
section, 134
Grabau, A. W., acknowledgment to, vii
cited on Dinantian, 255
cited on fish-bearing shales, 354
cited on Paleozoic era, 401
cited on Paleozoic fossils, 295
cited on the Permian, 181
cited on the Sinian, 300
quoted on Ondai Sair fossils, 366
Sinian system of, 227, 398
Grabens, 22, 23, 25, 27, 127, 220, 308, 309
see also Faults, Horsts, Mountains
Granger, Walter, 152, 193, 197, 209, 268
acknowledgment to, ix
cited on Cretaceous fossils, 255, 272, 353
discoverer of dinosaurs, 278
Plate XXI B
quoted on dinosaurs of Djadokhta, 377
quoted on fossils of Hsanda Gol, 365
quoted on Pleistocene, 235
quoted on temperatures, 395
see also Berkey, C. P., and Matthew, Wm. D.

XXVII,

Granger, Walter, and Berkey, C. P., cited on Iren

Dabasu, 211
cited on Later sediments, 197

Granites, 16, 22, 23, 50, 60, 61, 63, 64, 66, 71, 72, 74,

75, 76, 80, 91, 93, 94, 96, 97,.98, 99, 100, 102,
103, 104, 105, 108, 109, II0, III, I13, 114, 116,

118, 119, 122, 132, 135, 136, 138, 139, 141, 156,

458 INDEX

Granites—Continued
164, 165, 166, 173, 177, 178, 181, 183, 184, 199,
202, 204, 209, 216, 217, 218, 220, 223. 226, 228,
254, 258, 260, 262, 264, 269, 282, 291, 292, 296,
297, 298, 299, 300, 311, 313, 318, 323, 325, 334,
340, 341, 355, 363, 375, 392, 393, 398, 400, 408,

409, 410
conventional symbols for, Plate VII

figures, 138, 209, 318
maps showing, 137, 198, 290, 404, Plates XXIII,
XXVI
Plates IX, X B, XI B, XII, XVII A, XXV B
sections of, 8, 47, 51, 53, 58, 62, 65, 69, 73, 77, 79,
95, IOI, 102, 107, II2, 113, 117, 121, 134, 140,
150, 155, 159, 160, 163, 167, 168, I71I, 172, 175,
179, 180, 185, 186, 188, 229, 253, 261, 289, 291,
293, 295, 297, 299, 415
table showing, 301
see also Bathylith, Porphyries
Granophyre, 292
Graphite, 113
Grass, 313, 335, 350, 351
Grasslands, 21, 76-78, 84, 88, 125, 130, 183, 220, 244,
250, 252, 258, 268, 396
see also Meadows
Gravels, in alluvial fans, 311, 312, 328, 332, 389
in later sediments, 177, 197, 205, 206, 207, 237,
271, 279, 283, 315, 347, 352, 354, 358, 360, 362,
363, 364, 365, 379, 380, 381, 407, 416
residuary, 334
section, 334
in stream channels, 156, 252, 334-335, 348, 387
upland, 161, 200, 211, 271, 333-335, 339
Graywackes, 34, 52, 66, 135, 199, 205, 255, 334, 364,

375, 406
conventional symbols for, Plate VI
in Kentai Ola, 25
Plates XIII B, XXV B, XXXVIII B
sections, 53, 57, 65
see also Khangai graywacke series
Great Khingan range (Ta Hsing An Ling), 25, 31,
307, 322
boundary of Gobi basin, 28, 35, 303, 309
maps showing, 43, 275, 320, Plate IV
structure of, 16, 28, 29, 30
trend of, 309, 319, 322
see also Faults
Great Wall, the, 48, 187, 189, 275, 276
map showing, 304
Plate V B, XLIII B
Greenstones, 22, 72, 178, 300
section, 179
Gregory, W. K., cited on Cretaceous fauna, 353
Groeber, Paul, cited on Tien Shan, 34
routes of, Plate III
Guchu Burt, saltpan of, 141, 246
maps showing, 140, Plate XXVII
Plate XVII B

Gudger, E. W., acknowledgment to, vii

Giinz epoch, see Pleistocene period

Gurbun Saikhan range, 278, 307, 308, 309, 358
description and history of, 314-317
geologic structure of, 256, 260, 310, 317, 413
journey to, 250-252
maps showing, 43, 194, 404, Plate IV
rock formations of, 255, 256-258, 295, 297
sections, 257, 316
topography, 258
see also Camp Gurbun Saikhan

Gurley alidade, 193

Gypsum, 63, 210, 271, 362, 366, 376, 379, 380

Haberer, K. A., cited on primate tooth, 12
Hallong Ossu, 382
section, 51
Hamada Desert, 396
Han Hai series, 16, 40, 252, 257
age of, 35
fossils in, 17
Hapalodectes auctus, 360
Serus, 360
Hare, 382
Harpagolestes, 360
Hausen, H., cited on Tannu Ola, 22
Hedin, Sven, cited on von Strahlenberg, 15
, routes of, Plate III
Helaletide, 361, 362, 363
Helohyide, 361
Helwan, 326
Herds, of animals, 83, 87, 88, 125, 249, 250, 261, 313,
327
Hills, Leon B., acknowledgment to, viii
Himalaya ranges, 378, 382, 419
Hipparion, 235, 347, 381
Hobbs, W. H., cited on desert erosion, 325
Hogbacks, at Arishan, 219
at Camp Jurassic, 68
Hollows, undrained, 57-58, 61, 303, 307, 308, 327,
332, 351, 393, 417
at Arshanto, 207
figures, 54, 176, 199, 200, 269, 318, 324, 337,
340
at Guchu Burt, 141, 246
at Iren Dabasu, 60, 196, 199, 201, 379
at Irdin Manha, 56, 200
maps showing, I12, I17, 121, 140, 176, 194, 198,
Plates XXIII, XXVII, XXX
origin of, 116-118, 146, 326, 329, 336-341, 349, 350,
391, 418
at Oshih, 268, 269, 270, 271, 272, 314, 353, 354
at P’ang Kiang, 52-53, 54
problem of, 284
sections, 53, 57-58, 140, 175, 201, 229, 253, 277
at Shara Murun, 182
water-table in, 146
see also P’ang Kiang physiographic stage

es)

Nl as

INDEX

Holostai temple, 209
map showing, 198
Holti, 209
map showing, 198
Holy Mountain, see Arishan
Honan, Cretaceous beds of, 354
Hornfels, 34
Horse, 249, 258
fossil, 381, 382
rock drawings of, 152
Horsts, 23, 26, 220
Hot springs, see Arishan
Houldjin escarpment, description of, 200
figure, 199
fossils first found at, 59
maps showing, 198, Plate XXII1
Plate X A
sections, 58, 201, 203
Houldjin formation (Oligocene), age and correla-
tion of, 211, 367, 369
climate indicated by, 380
conventional symbol for, Plate VI
description of, 205, 364
figure, 199
fossils of, 205-206, 364
maps showing, 198, 404
sections, 201, 203
Hsanda Gol (river), 142, 230, 231, 235, 236, 333
course of, 142, 329, 333
maps showing, 194, Plates XX VII, XXVIII, XXIX
outcrops along, 230-232, 235, 236
Plate XXXI A
Hsanda Gol formation (Oligocene), 41, 152
age and correlation of, 367, 369
climate indicated by, 380-381
conventional symbol for, Plate VI
description of, 235-238, 364, 365
fossils of, 234, 381
history of, 270
maps showing, Plates XX VII, XXVIII, XXIX
Hsia, Wen Chiang, acknowledgment to, x
Hummocks, sand, 58, 335, 350, 351
see also Sand
Hung Kureh formation (Pliocene), 41, 144, 245, 347,
370
climate indicated by, 381
conventional symbol for, Plate VI
description of, 236, 311-312, 365-368
fossils of, 235
maps showing, 194, Plate XXX
Plate XVIII A
sections, 145, 229
Huns, migrations of, 13
Huntington, Ellsworth, cited on glaciation, 383
cited on peneplane in Turkestan, 347
cited on tamarisk mounds, 58, 351
cited on Turkestan, 19, 20
routes of, Plate III

459

Huronian system (pre-Cambrian), 373
Hu t’o series (pre-Cambrian), 300
Hwang Ho (Yellow River), 35, 307, 327

course of, 30-32

maps showing, 43, Plate IV
Hwei, Hsiu Yen, acknowledgment to, x
Hyenodon eminus, 363

irdinensis, 360

pervagus, 234
Hyznodontide, 208, 360, 362, 363
Hypertragulide, 362, 364
Hyracolestes ermineus, 359
Hyracodontide (Hyracodonts), 360, 361, 362, 363

Ibex, 313
Ice, erosive work of, 374
glacial, 305, 393
ground, 87, 350
Ice Age, see Pleistocene period
Ichang, 30
Igneous rocks, see Volcanic rocks, Intrusive rocks
Iguanodont dinosaurs, 204, 355, 357
Ikhe Bogdo, cirques of, 384
description of, 310
first impressions of, 139
maps showing, 194, 404
Implements, Paleolithic, see Cultures
In Shan range, boundary of Gobi basin, 31, 35
map showing, Plate IV
Indusia reisi, 233
Insecta (Insects), 63, 233, 376
Insectivora (Insectivores), 208, 234, 361, 381
Instruments, surveying, ix, 193
Intrusive rocks, 48, 113, 226, 228, 262, 288, 290-292,
294, 296, 313, 322, 398, 399, 405, 409
history of, 408-410
see also Bathylith, Dikes, Sills
Irdin Manha, 58, 200, 334
basin, extent of, 198-199
erosion forms at, 58
figure, 200
fossils found at, 59, 197
gravels at, 334
maps showing, 43, 198, 275, 304, 404
scarp at, 200
Irdin Manha formation (Eocene), age and correla-
tion of, 360, 367, 369
climate indicated by, 379, 380
conventional symbol for, Plate VI
description of, 206-208, 360
figures, 199, 200, 324
fossils of, 208, 360-361
sections, 57, 58, 201
Irdin Mankha (Obruchev’s term), 200
Iren Dabassun Nor (Obruchev’s term), 200
Iren Dabasu, 56, 60, 196, 197, 199, 307, 318
basin, extent of, 55, 198
description and significance of, 196-198, 307

460

Tren Dabasu—Continued
fossils of, 57, 204
granite inlier at, 318
hollow at, 60, 201-202
maps showing, 43, 198, 275, 290, 304, 404, Plates
IV, XXIII
Plate X A
rock formations at, 56, 60, 199, 202, 204-210
structure of, 210-212
topography of, 198-201
Iren Dabasu formation (Lower Cretaceous), 41, 225,
272, 352
age and correlation of, 210, 211-212, 355, 358, 366,
368, 416
climate indicated by, 376-377
conventional symbol for, Plate VI
description of, 203-204
figure, 199
fossils of, 204, 355
sections, 57, 58, 201, 203
Iren Nor, lake at Iren Dabasu, 60, 200, 355
map showing, Plate XXIIT
Iren Tala, general description of, 306, 307
gobis of, 366
map showing, 43
trend of, 321
Tris, 350
Irrigation, in former times, 184
possibilities of, 147
see also Agriculture
Tron-bearing quartzites, 263
Isostasy, problem of, 284, 352
Isostatic balance, unaffected by sediments, 370-372

Jasper (jasperite), 66, 82, 94, 208, 248, 254, 282, 334,
356, 357, 374, 375
conventional symbol for, Plate VI
Plate XXXIX A
Jenghiz Khan, 14, 15
Jia Jing Shando, well, 200, 208
Jisu Honguer formation (Permian), conventional
symbols for, Plate VI
description of, 181-182, 406
future studies recommended, 283
maps showing, 198, 275
remnant of, 414
sections, 180, 295
structure of, 182, 294-296, 298
table showing, 301
Johnson, Albert F., acknowledgment to, x
Johnson, Douglas W., acknowledgment to, vii
cited on plains, 323
Johnson plane table, 193
Jointing, in granite, 75, 135
in graywacke, 128
Plates XI B, XXXVIII B
Junipers, 310, 313

INDEX

Jurassic conglomerate series, age and correlation of,
104, 105-106, 402-403
climate indicated by, 375-376
coal in, 89
descriptions of, 103-104, 292-294, 402-403
fossils in, 103-104
maps showing, I2I, 126, 290, 404
outcrops of, 23, 24, 34, 67-68, 74, 96, 103-104,
105-109, 119, 124-125, 125-127, 132, 149, 165,
166, 178, 216, 218, 219, 224, 226, 228, 262, 263,
* 282, 283, 291, 313, 352
Plate XI A
. sections, 65, 107, 123, 129, 134, 150, 163, 167, 168,
I7I, 175, 281, 291, 293, 415
structure of, 103-104, 105, 164, 265
table showing, 301
trend of, 320
Jurassic formations noted by other geologists, 17,
22, 34, 233
Jurassic igneous rocks, 46, 148, 149, 164, 226, 269,
290, 291
age of, 174, 294
maps showing, 290, 404
relation of, to bathylith, 409
sections, 47, 145
Jurassic period, 24, 35, 74, 80,°96, 119, 148, 149, 165,
166, 178, 217, 226, 242, 265, 280, 281, 282, 283,
291, 313, 320, 352, 375, 414
deformation during, 344, 403, 413
possible age of Oshih formation, 354-355
problem of deformation during, 284
see also Angara series
Jutson, J. T., cited on desert erosion, 325

Kaisen, Peter C., acknowledgment to, x
Kalgan, 7, 28, 31, 45, 143, 153, 169, 170, 183, 187, 189,
196, 275, 276, 280, 355
location of, 9
maps showing, 43, 290, 304, 404, Plate IV
Kan, Ch’uen Pao, acknowledgment to, x
Kansu, fish-bearing shales of, 354
lowland of, 33
map showing, Plate IV
Kara Kul, 22
Kara Narin Ola (Narin Ola), boundary of Ordos, 31
map showing, Plate IV
Kara tala, 308 |
maps showing, 43, Plate IV
Karakoromys decessus, 234
Kashgar, 23
Kemp, James F., acknowledgment to, vii
Kentai Mountains, see Kentai Ola
Kentai Ola, 21, 25, 327
Arctic Divide in, 24
boundary of Gobi basin, 24-25, 35, 303, 309
graywackes in, 25, 298
map showing, Plate IV
part of Khangai, 22

INDEX

Kentai Shan, see Kentai Ola
Keratophyre, 23
Kerulen Gol (river), course of, 21, 25, 327
maps showing, 43, Plate IV
Keyes, Charles R., cited on desert erosion, 325
Khangai graywacke series, 61, 81, 82-83, 84, 88, 89,
90-91, 92, 93-94, 95, 96, 97, 100, IOI, 102, 103,
104, 105, 107, III, 121, 123, 125, 126, 127, 128,
129, 132, 134, 135, 156, 179, 180, 181, 182, 216—
217, 218, 220, 226, 257, 258, 282, 283, 289, 291,
293, 295, 297, 314-315, 406, 408, 411, 415
climate indicated by, 374
conventional symbol for, Plate VI
figures, 102, 384
maps showing, 198, 290, 404, Plate XXVI
Plates XIII B, XXV B, XXXVIII B, XXXIX A
problem of, 284
sections, 81, 92, 95, IOI, 107, I2I, 123, 129, 134,
179, 180, 295, 297, 415
structure and correlation of, 227, 255-256, 295-
296, 297-299, 399-400,.40I, 414
trend of, 320
type locality, 23-24, I19-120, 124
see also Proterozoic formations, Sinian system
Khangai Mountains, 21, 22, 24, 35, 120, 125, I3I,
161, 216, 280, 297, 298, 303, 308, 309, 329, 343,
373, 384, 386, 399, 413, 416, 417
boundary of Gobi basin, 21-22, 35, 303
drainage of, 24, 329, 386
glaciation of, 131, 384
maps showing, 43, 129, 137, 140, 275, Plate IV
tock formations of, 125, 297-298, 373, 399
sections, 129, 134, 140
structure of, 23, 24, 309, 413, 416-417
topography, 343, 384
Khara Khada range, see Shara Khada range
Kharangoin Tala (Obruchev’s term), 200
Khingan faults, 27, 320
Khingan line, 29-30
Khingan range, see Great Khingan range
Khingan trend, 319-322
Kholobolchi, map showing, 404
Pleistocene deposits at, 385
Khurul Obo, 333
figure, 159
section, 159
volcanic center of, 158, 319
Kiakhta, 23
map showing, Plate IV
Kirghiz tala, 308
map showing, 43, Plate IV
Kisin tala, 308
maps showing, 43, Plate IV
Klementz, D. A., quoted by Suess, 310
quoted on ‘‘Valley of the Lakes,’’ 308
routes of, Plate II
Kobdo, 136, 144, 152, 278

461

maps showing, 43, Plate IV
see also Trails
Koslov, P. K., cited on Han Hai beds, 257
routes of, Plate II
Kosso Gol (lake), 23
graben of, 348
map showing, Plate IV
Kropotkin, Prince, cited on Great Khingan range, 28
Kuen Lun ranges, trend of, 29, 320
K’uichow (Kweichow) formation (Cretaceous), 30,
354
Kuldsha coal basin, studied by Mushketov, 17
Kulun Nor (Khulun), see Dalai Nor
Kwei Ming Kwan, rock formations at, 61
section, 62
Kweihwating, map showing, Plate IV
see also Trails

Laccoliths, absence of, in Gobi basin, 319
in Transbaikalia, 26
Lacertilia, 357
Lakes, 76, 187, 201, 280, 305, 306, 385, 386
classes of, 348-349
as evidence of climatic change, 118, 202, 391-393
history of, 349-350 1
salt, 96, 141, 142, 143, 200, 224, 246-247, 307, 3II
terminal, 327, 328
see also Playas, Salt pans
see also Baikal, Dalai Nor, Dolon Nor, Erin Nor,
Gashuin Nor, Iren Nor, Kara Kul, Kholobolchi,
Kosso Gol, Lop Nor, Tsagan Nor, Ubsa Nor,
Ulan Nor
Lamas, 75, 88, 161, 162, 213, 214, 215
Plates XV, XXV
Lamaseries, 75, 88, 125, 127, 161, 162, 170, 343
Lanchowfu, 327
map showing, Plate IV
Larch (tamarack), 130, 389
Larsen, F. A., 88
acknowledgment to, x
Later sediments, 24, 33, 46-50, 54-55, 56, 59, 61,
63, 64, 67-71, 72, 80, 105, 108, III, 135, 136,
138, 141, 144-146, 149, I51, 154, 161, 164, 166,
169, 170, 173, 174-177, 178, 182-183, 189, 196-
213, 223-247, 253, 254-255, 261, 263, 264, 265,
266, 267, 268-273, 275-276, 277, 278, 279, 280,
281, 283, 306, 307, 308, 309, 313, 314, 315-317,
318, 323, 353
beveled by erosion plane, 332-335
climate indicated by, 373-385, 387
Cretaceous and Tertiary, 239
deformation of, 411-412, 414, 416
deposition of, 403-405, 407-408
at Djadokhta, 156-158, 278-279, 315
evidence of former extent of, 239-241
figures, 54, 165, 176, 199, 200, 209, 224, 239, 240,
242, 245, 269, 270, 281, 318, 324, 337, 338, 339,
340, 356, 370

462

Later sediments—Continued
maps showing, 117, 176, 198, 404, Plates XXIII,
XXVII, XXVIII, XXIX, XXX
observations by other explorers, 5
oldest (Lower Cretaceous), 32, 341, 344, 352, 411
Plates I, X A, XI A, XVII B, XVIII, XIX, XX,
SEX, XXIT B, SOCK SAN AGT, XSI,
XXXIV, XXXV, XXXVI, XLIA
problem of, 284
relation of, to isostatic balance, 370-372
relation of, to oldrock floor, 288-289, 352
sections, 47, 51, 53, 54, 62, 65, 69, 73, 77, 79) 95,
107, 134, 140, 145, 150, 155, 159, 160, 163, 168,
171, 172, 175, 177, 179, 180, 185, 186, 188, 189,
201, 203, 229, 253, 257, 261, 277, 281, 289, 295,
299, 310, 316, 367, 415
significance of, 10, 345-347, 368-372
stratigraphy of, 352-373
tables showing, 301, 371
thickness of, 56, 212, 366
Latite, 71
Lavas, 22, 28, 29, 31, 45, 68, 74, 78, 96, 127, 148, 151,
215, 219, 221, 224, 235, 236, 241, 242, 248, 253,
254, 264, 266, 271, 272, 273, 306, 308, 310, 313,
314, 319, 331, 334, 354, 358, 365, 375, 393, 409,
410, 417
figures, 240, 242, 269
Plates XVIII B, XXXI B, XXXV B
see also Andesites, Basalts, Mesas,
Trachytes
Lawson, Andrew C., cited on desert erosion, 325
Lesdain, Comte de, cited on Ordos, 32
Licent, Pére Emil, see Teilhard
Lignite in northern Siberia, 367
Limestone (crystalline), 23, 55, 66, 72, 113, 114, 139,
I4I, 149, I51, 154, 156, 165, 166, 168, 178, 183,
184, 187-189, 226, 227, 257, 258, 261, 262, 263,
264, 269, 291, 298, 299, 300, 313, 314, 331, 373,
398, 399, 400, 405
conventional symbols for, Plates VI, VII
sections, 8, 51, 53, 57, 65, 73, 140, 145, 150, 155,
167, 168, 175, 179, 185, 186, 188, 253, 257, 263,
299, 415
table showing, 301
Limestone (fossiliferous), 22, 32, 170, 173, I8I, 255,
282, 294, 374, 378, 401, 406, 407
conventional symbol for, Plate VI
sections, 171, 180, 295, 415
table showing, 301
Limestone (fresh-water), 56, 63, 169, 174, 271, 354
figure, 356
sections, 57, 172
Limestone (pebbles), 375
Lit-par-lit injections, 116
in ancient complex, 264
von Loczy, Gustav, cited on geology of Mongolia,
17
routes of, Plate III

Rhyolites,

INDEX

Loess, 45, 359; 377; 379
general absence of, 385

of northern China, 13, 347
Loh (camp manager), acknowledgment to, x
Loh formation (Miocene), age and correlation of,
365, 367, 369
climate indicated by, 381
conventional symbol for, Plate VI
description of, 365
extent of, 236
fossils of, 235
maps showing, 194, 404
Plates XXIX, XXX
sections, 145, 229
see also Camp Loh
Lop Nor (lake), 33, 36, 385
maps showing, 43, Plate IV
see also Tarim basin
Lophialetes expeditus, 361
minutus, 361
Lophiodontide (Lophiodonts), 205, 207, 208, 211,
359, 360, 361, 379
Lophiomeryx angare, 364
gobia, 364
Los in Sumu, figure, 165
map showing, 275
problems of, 283
Los Ola, figure, 330
Lung Shan range, map showing, Plate IV
structure of, 33
Lvov, Prince, cited on Amur region, 26
Lycopteride, 233
Lycoptera middendorffi, 233
Lyttonia, 181

Ma Lan, physiographic stage, 347-348
McGee, W. J., cited on sheet flood erosion, 325
Machatschek, Fritz, cited on Tien Shan, 34
Macropelobatis osborni, 234
Magma of Mongolian bathylith, 217, 219, 408
Mammalia (Mammals), fossil, 41, 59, 158, 205-206,
208, 234-235, 279, 309, 357, 359, 360, 362, 363-
364, 378, 379
living, 378
Man, development of, 4, 378, 418-420
migrations of, 12-14, 373
Neanderthal, 13
prehistoric, 18, 152, 248, 284, 386, 393-394, 395
see also Cultures
Manakin, M., cited on volcanoes, 28
Manganese, oxide of, near Baga Bogdo, 366
Mangu Khan, 14
Manus, fossil at Irdin Manha, 208
Manteoceras irdinensis, 361
Maps, of Altai region, 194
of Central Mongolia, 43, 275, 290, 304, 404
geological, Plates XXIII, XXVI, XXVII, XXVIII,
XXIX, XXX, in pocket

INDEX

Maps—Continued
of Iren Dabasu, Shara Murun and P’ang Kiang, 198
method of surveying, 193-195
Renat’s, of Central Asia, 15
route, I12, 117, 121, 123, 126, 129, 133, 137, 140,
214
von Strahlenberg’s, of northern Asia, 15
Marble, see Limestone (crystalline)
Marco Polo, 14
Marls, 22, 235, 238, 306, 382
Marsupial, fossil, 359
Mastodon (Mastodont), 235, 381, 382
Matthew, W. D., 29
acknowledgment to, vii
cited on Irdin Manha fauna, 360
cited on wind deposits, 207
quoted on Cretaceous fauna, 353
quoted on Iren Dabasu fauna, 355
quoted on Shara Murun fauna, 182
Matthew, W. D., and Granger, Walter
cited on Arshanto fauna, 207, 359
cited on Gashato fauna, 358-359
cited on Houldjin fauna, 364
cited on Hsanda Gol fauna, 369, 381
cited on Schlosseria, 359
cited on Teilhardia, 360
quoted on age of Loh fauna, 365
Meadows (upland parks), of Artsa Bogdo, 266, 267,
268, 313
figure, 266-267
of Gurbun Saikhan, 315
Melaphyre, 71, 99
Menodus mongoliensis, 363
Menotyphla, 359
Merin, acknowledgment to, ix
Mergen, volcanoes near, 28
map showing, Plate IV
Merzbacher, Gottfried, cited on Tien Shan, 34
Mesas, figures, 240, 269, 270
lava-capped, I15I, 152, 224, 241, 268, 271, 272, 313
maps showing, 140, Plate XX VII
Plate XVIII B
sandstone-capped, 182
section, 229
Mesonychide (Mesonychids), 208, 360, 362
Mesozoic era, climate during, 375-378
orogenic disturbances during, 35, 263, 295, 342, 346,
402-405, 411-414
Mesozoic formations, 26-27, 40, 67, 68, 206, 225, 263-
289, 294, 344, 352-358
faunas of} 225
intrusive rocks of, 262, 290, 291, 292
section, 415 ;
summaries of, 352-358, 368, 402-405, 407
see also Bathylith, Cretaceous period, Dikes, Juras-
sic period, Porphyries, Sills, Unconformities
Metadolomite, 178
Metals, see Mineral resources

463

Metamorphism, contact, 61, 91-93, 135, 217, 263,
264, 288, 297, 408
dynamic, 264, 410
products of, 60, 72, 80, 91-93, 114, 118, 149, 202,
217, 226, 260, 264, 281, 288, 290, 398-399, 405-
406, 411, 414 :
by silication and silicification, 264
summaries of, 398-399, 413-414
see also Limestone (recrystalline), Gneisses, Quartz-
ites, Schists, Serpentine, Slates
Metarhinus mongoliensis, 361
Miacide, 361
Miacis invictus, 361
Miao T’an, Chinese inn, 189
sections, 47, 189
Miarolitic structure in granite, 76
Millet, cultivation of, 184, 396
Mineral resources, paucity of, 60, 89, 263
Miocene formations, 144, 148, 230, 233, 236, 308,
349, 365, 367, 369
fossils of, 17, 200, 225, 235
gaps in sedimentary record of, 346, 367, 369, 381
see also Loh formation
Miocene period, climate during, 381
Miomeryx altaicus, 364
Mississippian period, see Dinantian formations
Molloy, Agnes, acknowledgment to, ix
Mollusks, 355, 376, 382
see also Gastropods, Pelecypods
Monadnocks, on desert floor, 317
on Mongolian peneplane, 313, 315, 342, 343, 348
on piedmont shelf, 261
Plate XIV
on pre-Cretaceous peneplane, 318
section, 342
Mongols, 8, 15, 75, 88, 138, 141, 152, 157, 174, 176,
182, 184, 187, 190, 196, 201, 236, 244, 248, 249,
250, 251, 258, 310, 313, 314, 326, 327, 388, 393,
395
Mongolia, explorations of, by other geologists, 14-20
geologic problems of, 274, 381
states (khans, eimaks) of, 251
Mongolian bathylith, 23, 52, 74, 76, 93, 96, 97-103,
104, I10, I13, I14, I16, 132-135, 139, 216-218,
223, 228, 260, 264
age of, 296, 401
contact metamorphism of, 297
end products of, 297
extent of, 71, 409
granites of, 297
as greatest igneous intrusion, 400
intrusives in, 110, 165
maps showing, 290, 404, Plates XXVI, XXVII,
XXVIII, XXX
outcrops of, 61, 118, 122, 156, 164, 166, 173, 178,
181, 216, 282
Plates, IX, XI B: XII, XXV B
relation of, to graywackes, 217, 298

464

Mongolian—Continucd
relation of, to other intrusives, 295
sections, 47, 51, 53, 62, 65, 69, 95, IOI, 102, 107,
112, 117, I2I, 134, 140, 163, 167, 168, 172, 175,
179, 185, 186, 188, 291, 297, 415
as source of Mesozoic intrusives, 292, 409
as source of later volcanics, 410
summary of, 408
table showing, 301
Mongolian peneplane, see Peneplanes
Mongolian plateau, edge of, at Wan Ch’uan, 46,
356
inland slope of, 196
Monuments, pre-Mongol, 13
see also Cultures
“Monument” granite (knobs, residuals), 75, 76, 178,
183, 325
figure, 138
Plates, XI B, XII B
Monzonite, 409
Mook, Charles C., quoted on Cretaceous fauna, 353
Moose, prehistoric drawing of, 398
Morris, Florence Eddowes, acknowledgment to, viii
Morris, Frederick K., see Berkey, Charles P.
Mount Tuerin, see Tuerin
Mount Uskuk, see Uskuk Mountain
Mountains, domed, 309, 417
fault-block, 50, 124, 414
maps showing, 43, 194, 275, 304
residual, 96, 217, 309, 317-318
trends of, 319-322
types of, in Gobi region, 309
volcanic, summary of, 318-319
see also Ala Shan, Altai, Altaids, Argilintai, Arishan,
Artsa Bogdo, Baga Bogdo, Baikal, Baron Saik-
han, Bogdo Ola, Chakhar, Djargos, Dunde Saik-
han, Dzun Saikhan, Great Kingan, Gurbun
Saikhan, In Shan, Kara Narin Ola, Kentai Ola,
Khangai, Los Ola, Lung Shan, Muni Ola, Nan
K’ou, Nan Shan, Oshigo Ola, Sailugem, Sayan,
Shara Khada, Shara Khata, Sheiten Ola, Tannu
Ola, Tien Shan, Tsingling Shan, Tuerin, Uskuk,
Western Hills
Mountain-making, periods of, 17, 34-35, 244-245,
411-413
types of, 17, 29, 34-36, 244-245, 305, 411-413
Mousterian, see Cultures
Multituberculata, 359
Muni Ola Range, boundary of Ordos, 31
map showing, Plate IV
Murin Gol (river), geologic formations of, 23
Mushketoy, I. V., cited on Tien Shan, 16
cited on volcanoes, 29
quoted on Great Khingan range, 31
Mustelids, 382

Nan K’ou Pass, rock formations of, 8-9, 50, 55
Nan K’ou range, boundary of Gobi basin, 28

INDEX

Nan K’ou series (pre-Cambrian), 61, 202, 227, 300
398
table showing, 301
see also Hu t’o series, Khangai graywacke series
Nan Shan range, boundary of Gobi basin, 31, 35, 303
structure of, 32-33
trend of, 320
Narin Ola range, see Kara Narin Ola
Necks, see Volcanoes
Neogene time (Cenozoic), 35
Neolithic, see Cultures
Nerchinsk, rock structures near, 27
map showing, Plate IV
Nerchinski-Savod, volcanic rocks near, 28
Ning-shan basin, faulting in, 30
Nomin River, rock structures near, 28
map showing, Plate IV
Notungulata, 359
Nummulites, 378

Oats, cultivation of, 184, 396
Obo, definition of, 174-176
use of, in mapping, 194
Obruchev, V. A., acknowledgment to, xiii, 17
cited on Altai ranges, 321
cited on Baikal trend, 319
cited on Gobi series, 40
cited on peneplanes, 347
cited on his traverse to Gurbun Saikhan, 307
explorations by, 5, 17-18
field names used by, 201
first discovery of fossils by, 209
map by, 320
quoted on discovery of fossils, 200-201
quoted on mountain-making, 34-35
quoted on Nan Shan, 32-33
quoted on Selenga River region, 23
quoted on Transbaikal ranges, 309
quoted on Transbaikalia, 26-27
routes of, Plate II
Ochungchelo formation (Lower Cretaceous), 309
Odoric, Father, journey of, 14
Okher, acknowledgment to, x
Olan formation (Pleistocene), 368
Oldham, cited on glaciation, 383
Oldrock floor, deformation of, 289, 413
exposures of, 55, 56, 60, 66, 173, 202, 225-229, 266
269, 281, 306
Plates X B, XII, XX A, XXII A, XXXVIII A
relation of, to later sediments, 283, 288-289, 344,
352, 353, 354, 355, 363, 368, 369, 411
rock formations of, 72, 289-302
sections, 51, 53, 57) 62, 73, 79, 150, 155, 167, 168,
175, 179, 185, 186, 299, 415
structure of, 289-302, 307, 319-322
table of formations of, 301
Oligocene formations, 31, 174-177, 211, 225, 229,
236-238, 308, 346, 349, 358, 363-365

INDEX

Oligocene—Continued
fossils of, 233-234, 360, 363-364
lavas of, 29, 31
see also Ardyn Obo, Houldjin, Hsanda Gol forma-
tions
Oligocene period, climate during, 380-381
gaps in sedimentary record of, 366-367, 369, 416
Olivine-aphanite, see Dolerite
Olsen, George, acknowledgment to, x
Olsenia mira, 362
Ondai Sair formation (Lower Cretaceous), 41
age and correlation of, 225, 353-354, 357, 358, 368,
369, 416
climate represented by, 376, 378
conventional symbol for, Plate VI
description of, 230-232
fossils of, 233
maps showing, 43, 140, 194, 404, Plates X XVII,
XXVIII
paper-shales of, 63, 271, 277, 278
sections, 140, 229
structure of, 224-225, 270
see also Camp Ondai Sair, Oshih
Ongin Gol (river), 3, 115, 120, 125, 127, 128, 158,
161, 219, 221, 280, 281, 292, 308, 328, 348
course of, 120-122, 125, 128, 161, 221, 328, 348
maps showing, I21, 129, 194, 404, Plate IV
Plate XX A
rock formations at, 120-122, 127, 158-161, 219,
221, 280, 281, 292, 308
sections, I2I, 129, 160
terraces of, 122, 162
see also Camp Ongin Gol, Khangai Mountains
Ongin Gol in Sumu (temple and lamasery), 161-162,
280
map showing, 194
Plate XX A
Ordos, as boundary of Gobi basin, 30-31, 36
maps showing, 275, 304, 321, Plate IV
Paleolithic cultures in, 13, 18, 284
Pleistocene fossils in, 18
structure of, 32
Ordovician period, an erosion interval, 414
Orkhon Gol (river), drainage of Khangai Mountains,
go
map showing, Plate IV
Orogenic disturbances, see Mountain-making
Osborn, Henry Fairfield, acknowledgment to, vii
cited on Cretaceous fauna, 353
discoverer of Eudinoceras, 209
quoted on Shara Murun fauna, 182
prediction by, 5 ;
Oshigo Ola (mountain), description of, 318
Oshih hollow, 152, 268, 278
figure, 269
inliers in, 269, 318
maps showing, 43, 194, 275, 404
topography of, 268-270

465

Oshih (Ashile) formation (Lower Cretaceous), 41,
152, 352
age and correlation of, 353, 366, 368, 416
climate indicated by, 376, 378
compared with other formations, 154, 232, 255, 257,
278, 309
conventional symbol for, Plate VI
description of, 270-273, 353-355
figures, 269, 270
former extent of, 273, 314
fossils of, 354
Plates XVIII B, XXXV, XXXVI, XXXVII
problem of, 278
structure of, 269-273, 357
Oshih Nuru, hill at Oshih, 268, 271
figure, 269
Plates XXXV B, XXXVI A
Ostracods, fossil, 205
Ostrich, fossil, 235, 381, 382
Ostrich-dinosaur at Iren Dabasu, 355
see also Struthiomimid dinosaurs
Otoburun, acknowledgment to, x
Oviraptor philoceratops, 357
Ovis, 347
Oxyenide, 363

Pachycynodon (Cynodon), 234
Pacific Divide, Plate VIII A
rainfall of, 326, 396
at Wan Ch’uan Pass, 9, 48-49
Paige, Sydney, cited on desert erosion, 325
Paleolithic, see Cultures
Paleoprionodon gracilis, 234
Paleoscaptor acridens, 234
rectus, 234
Palaostylops iturus, 359
Paleozoic era, climate during, 374-375
orogenic disturbances of, 26, 35, 320, 322, 401,
410-411, 414
problem of, 284
Paleozoic formations, 23, 28-29, 34, 170-173, 18I-
182, 255-256, 282-283, 291, 294-296, 297-298,
400-401, 406-407
fauna of, 23, 170-173, 255
intrusive rocks of, 296, 400-401, 408-409
maps showing, 198, 290, 404
sections, 171, 180, 295, 415
table showing, 301
see also Cambrian, Devonian, Dinantian, Mon-
golian bathylith, Ordovician, Permian, Uncon-
formities
Paleocene formations, 158, 279, 309, 358, 367, 369
see also Gashato formation
Paleocene period, climate during, 379
Pan Chiao physiographic stage, 347, 348
P’ang Kiang, maps showing, 43, 198, 275, 404
see also Camp P’ang Kiang

466

P’ang Kiang formation (Tertiary), 210, 346
conventional symbol for, Plate VI
description of, 53-55
figures, 199, 337
fossils of, 59
problem of, 276
sections, 53, 54, 57
P’ang Kiang physiographic stage, 56-58, 201
correlation of, 347, 348
figures, 54, 176, 200, 337
type locality of, 52-55, 336
see also Hollows
Pantolestes, 208, 361
Pantolestide, 208
Paper-shales, map showing, 275
outcrops of, 63, 231, 232, 233, 271-272, 276-277,
278, 353, 368, 376, 407
section, 62
Paracolodon curtus, 363
Paracynohyenodon morrisi, 208, 360
Paramys, 361 |
Park, upland, see Meadows
Parks, Flora Cook, acknowledgment to, viii
Passarge, Siegfried, cited on desert erosion, 325
Pea plants, makers of sand hummocks, 351
Pebbles, polished, 176, 208, 357, 379
Pediment, see Shelf
Pegmatites, 61, 63, 71, 139, 254, 300
Pei Shan, see Lung Shan
Pei T’ai physiographic stage, 347, 348
Peking, 7, 8, 28
bay of, 29-30
maps showing, 43, 275, 290, 304, 404, Plate IV
section, 8
Pelecypods, 272, 355, 376
Pelobatide, 234
Peneplanes, 10, 24, 28, 30
in China and Mongolia, compared, 347-348
figures, 98-99, 224-225, 342, 344-345
Khangai, 259, 343-344, 348
Mongolian, 24, 52, 240, 242, 259, 267, 307, 310,
311, 313, 314, 315, 317, 341, 342-343, 344, 345-
346, 348
observations of, by other explorers, 28, 30, 309
Pei T’ai, 347, 348
Plates VIII A, IX A, XII A, XIII A, XIV A
pre-Cretaceous (post-Jurassic), 211, 242, 244, 261,
289, 294, 317, 341-342, 344, 345-346, 402-403,
411
pre-Jurassic, 104, 164, 294
problem of¢ 283, 289, 323
relation of Khangai to Mongolian, 343-344
relation of Mongolian to Gobi erosion plane, 345-
346
relation of Mongolian to pre-Cretaceous, 345-346
sections, 168, 179, 224, 342, 345, 346
in Tien Shan, 347
in Transbaikalia, 28, 309, 347

INDEX

see also Disconformities, Erosion, Gobi erosion
plane, Monadnock, Physiographic stage, Un-
conformities
Pennsylvanian period, formations of 295, 296
see also Carboniferous formations
Peridotite, 408
Periods, tables showing, 301, 371
Perissodactyla (Perissodactyls), 205, 208, 234, 235,
360, 361, 362, 363, 364
Permian formations, 23, 34, 181-182, 283, 295-296,
406-407
fauna of, 182
sections, 180, 295
table showing, 301
see also Jisu Honguer formation
Permian period, 17, 23, 34, 181, 182, 283, 295, 296,
374, 401, 406
climate during, 374-375, 401
Pes, fossil, at Irdin Manha, 208
Phenacolophus fallax, 359
Phyllites, 72, 91, 94, 113, I19, 120, 181, 227, 282,
291, 299, 300, 314, 334, 374, 398, 400
conventional symbol for, Plate VII
Plates XXII A, XXXVIII A, XXXIX B
section, 179
Phyllocladites morrisi, 233
Physiographic history, of peneplanes, 343-344
problem of, 283, 385
significance of, 49-50
stages of, in China and Mongolia, 347-348
Picture-writing, see Drawings on rocks
Piedmont, see Shelf
Pievtsov, M., cited on Central Asia, 17
quoted on ‘Valley of the Lakes,” 22
routes of, Plate II
Pisces, 63, 233, 361
see also Fishes
Pithecanthropus erectus, 12
Plains, constructional landforms, 305, 323, 417
definition of, 323
map showing, 304
plain of China, 8, 304
plain of Kalgan basin, 9
traversed by Expedition, 149,153, 156, 161, 264, 268
Planation, 66, 71, 211, 242, 244, 259, 261, 294, 341,
342, 346, 370, 402, 417
evidence of, 242-243
of later sediments, 211-212, 259, 417
of oldrock floor, 66, 71, 211, 242, 244, 259, 261, 294,
402-403, 417
periods of, 341-348
of piedmont shelf, 261-262
relation of, to isostatic balance, 370-372
see also Disconformities, Erosion planes, Pene-
planes, Unconformities
Plant remains, fossil, 23, 63, 67, 103, 151, 169, 172,
174, 233, 271, 276, 293, 294, 366, 376, 377, 379,
381, 402, 407

INDEX

Plants, makers of sand hummocks, 351
Plateaus, constructional landforms, 305
definition of, 323
elevation of, 419
at Wan Ch’uan Pass, 30, 48-49
Playas, in hollows, 303, 350
maps showing, 112, Plates XXIII, XXVII
Plate XLII A
see also Hollows, Lakes, Salt pans
Playatuft, definition, 350
Pleistocene formations, 29, 152, 235, 280, 308, 317,
332, 349, 368, 382, 385
fossils of, 235, 382
see also Loess
Pleistocene period (Ice Age), climate during, 347, 382-
385, 393
deformation during, 30, 35-36, 230, 317, 333, 347,
348, 349, 412
erosion during, 230, 236, 336, 347, 348, 349, 405
glaciation during, 85-86, 382-385, 393
Giinz epoch of, 382
human record during, 13, 14, 18, 419-420
problem of, 284
Riss epoch of, 382
see also Cultures, Glaciation, Gobi erosion plane,
Man, Quaternary
Pliocene formations, 29, 144, 146, 225, 229, 230-233,
236, 306, 307, 308, 311, 317, 349, 365-366, 367,
369, 370, 381-382
fossils of, 235, 381-382
see also Ertemte formations, Hung Kureh
Pliocene period, climate during, 381-382
deformation during, 35-36
human record during, 14, 419-420
Plugs, see Volcanoes
Poplars, 389
Porphyries, 22, 28, 45, 50, 91, 96, 124, 166, 183, 291,
331, 374, 386, 387
alaskite, 99
andesite, 228
basalt, 127
“brittle,” 106, 224, 228, 409, 410
““complex’’ of, 106, 173, 174, 228, 292
conventional symbols for, Plate VII
diorite, 409
eruptive, 106, 228
granite, 23, 61, 71, 72, 74, 80, 94, 99, 108, 113, 116,
218, 292, 355, 375; 409
intrusive, 16, 98, 226, 228, 262, 290, 409
maps showing, 290, 404
monzonite, 409
quartz, 181, 292
sections, 107, 167, 168, 171, 172, 175, 291, 415
syenite, 72, 164, 292, 409
trachyte, 80, 173, 228, 269, 292, 355
see also Dikes, Jurassic igneous rocks
Post-Jurassic period, deformation during, 244, 245,
412, 414, 416

467

erosion interval of, 417
intrusive rocks of, at Uskuk, 228
peneplanation during, 35, 242, 403, 411
Post-Permian period, erosion during, 401, 414
mountain-making during, 17, 401
Post-Pleistocene period, see Recent period
Post-Pliocene period, deformation during, 239, 244,
245, 412
see also Pleistocene period
Post-Tertiary deformation, 320
see also Pleistocene period
Potanin, G. N., cited on Great Khingan Range, 28
routes of, Plate II
Potatoes, cultivation of, 396
Preaceratherium, 206, 364
Pre-Cambrian eras, 55, 149, 154, 202, 223, 227, 254,
263, 282, 291, 298, 299, 302, 320, 321, 368, 373,
398, 401, 405, 406, 413, 414
deformation during, 322, 411
erosion intervals of, 414-416
see also Archxozoic era, Proterozoic era
Pre-Cambrian formations, noted by other explorers,
23, 25, 298
section, 299
table showing, 301
trends of, 320, 322
see also Hu t’o series, Khangai graywacke series,
Nan K’ou series, Sinian system
Pre-Jurassic periods, deformation during, at Uskuk,
244, 245
Pre-Jurassic peneplane, 164
see also Unconformities
Pre-Tertiary erosion interval, 211, 342, 404, 416
see also Unconformities
Predentata (Predentates), 204, 354, 357
Prionessus lucifer, 359
Prievalski, N. M., cited on Great Khingan range, 28
quoted on Shara Khada range, 31
routes of, Plate II
Problems, 274, 282, 283, 298, 317, 344, 345, 367,
381, 385
of dominant erosion, 385
general, 283-284
of Khangai graywacke series, 298
of Miocene history, 367, 381
of Mongolian and Khangai peneplanes, 344-345
of special areas, 275-283
Proboscidea, 235
Processes, constructional, 303-305
destructional, 305
Propterodon irdinensis, 360
Prosciurus lohiculus, 234
Proterozoic era, deformation during, 411
Proterozoic formations, 24, 228, 300, 398
maps showing, 290, 404
section, 415
see also Hu t’o series, Khangai graywacke series,
Nan K’ou series, Sinian system, Wu T’ai system

468

Protiguanodon, 354
mongoliense, 233
Protitanotherium, 208
andrewsi, 362
grangeri, 361
mongoliense, 362
Protoceratops andrewsi, 157, 357, 377
Provinces, physiographic, 306, 308, 309, 317, 321
Psittacosaurus, 271, 354
beds, Plate XXXVI B
Pterodon hyenoides, 362
Pumice, 78
Pumpelly, Raphael, 20
cited on explorations in Turkestan, 18
cited on traverse of Mongolia, 16
routes of, Plate III
Pyroxenite, 258, 408

Quartz, 80, 374, 375
pebbles of, 176, 205, 208, 228, 293, 334, 357, 364
pegmatitic, 93
porphyries, 181, 292
sand, 203, 204, 219, 272, 293, 356
veins (stringers), 60, 63, 66, 89, 178, 181, 202, 254,
264
see also Graywackes, Sandstones, Schists
Quartzites, 22, 23, 34, 50, 66, 72, 82, 84, 94, 96, 113,
149, 178, 181, 183, 184, 199, 204, 205, 208, 209,
227, 228, 262, 263, 282, 291, 300, 355, 357, 364,
374, 375
conventional symbols for, Plates VI, VII
section, 185
Quaternary period, volcanoes of, 306, 319
see also Pleistocene period

Race, 386, 393, 394, 418
see also Man
Radlov, O., cited on pre-Mongol peoples, 13
“Ragged’’ Mountains of Gorida, 135, 138
figure, 138
map showing, 137
see also Gorida
Rainfall, 351
distribution of, 326-327, 330, 376
seasonal distribution of, 396
variations in, 148, 348, 376, 389, 390, 395
Raisz, Erwin J., acknowledgment to, viii
Recent period (post-Pleistocene period),
deposits of, 230, 280, 332, 385
fluctuations in climate during, 393-394
Red-beds, 151, 152, 157, 206, 207, 237, 359, 382
Reeds, Chester A., acknowledgment to, vii
Reis, Otto, cited on fish-bearing shales, 354
Rejuvenation, by climatic change, 48, 275
of igneous activity, 296
by uplift, 230, 243, 265
see also Climate, Deformation, Erosion, Mongolian
bathylith

INDEX

Remnants, Plate XXXII
sedimentary, 239-241
see also Erosion remnants
Renat, map of central Asia by, 15
Reptilia (Reptiles), 49, 70, 204, 206, 233, 253, 271,
308, 314, 357, 359, 361, 362, 364, 377
Residual (Residuary) hills, see Buttes, Erosion rem-
nants
Rhinoceros, 17, 200, 201
Rhinocerotidze (Rhinocerid, Rhinoceros), 177, 197,
205, 235, 279, 380, 382, 385
Rhyolites, 45, 74, 78, 148, 149, 157, 410
conventional symbols for, Plate VII
section, 155
von Richthofen, Ferdinand, 14
cited on Han Hai series, 16, 40
cited on Khingan line, 29-30
cited on Khingan trend, 319-322
cited on lavas, 31
explorations by, 5, 18
Nan K'ou series of, 227, 300, 301, 398
Richthofen range, boundary of Gobi Basin, 31, 33,
320
map showing, Plate IV
Riss epoch, see Pleistocene period
Rivers, 90, 120, 161, 327, 328, 331, 390
see also Streams
see also Amur, Arguin Gol, Argun, Buksui Gol, Da
Ying Gol, Eder Gol, Edsin Gol, Egin Gol,
Hsanda Gol, Hwang Ho, Kerulen Gol, Murin
Gol, Nomin, Ongin Gol, Orkhon Gol, Selenga
Gol, Sha-ho, Shilka, Sungari, Tarmil Gol, Tarim
Gol, Tatal Gol, Tatsin Gol, Telgir Merin Gol,
Tola Gol, Tsilin Gol, Uljitundur Gol, Wei Ho,
Yang Ho, Yangtze Kiang, Yenesei
Roberts, L. B., acknowledgment to, ix
Robinson, H. O., acknowledgment to, ix
Rodentia (Rodents), 59, 208, 235, 361, 362, 363, 381,
382
Romanovsky, G. D., cited on Turkestan, 17
routes of, Plate II
Roof-pendants, 24, 97, 100, 102, 105, I10, I14, 116,
181, 183, 217, 296

figure, 102 :
sections, 62, 95, IOI, 102, 107, I12, 113, 175, 179,
185, 186, 188

Rubbles, of alluvial fans, 283, 312, 319, 368
Russians, eastward migration of, 14, 15
quoted on graywackes, 255, 298, 400-401

Sacred Mountain, see Arishan
Sage, maker of sand hummocks, 351
Sailugem Mountains, boundary of Gobi Basin, 21, 303
map showing, Plate IV
Sain Noin (Khan), v, 3, 83, 87, 88, 124, 125, 127, 130,
132, 195, 213, 223, 227, 251, 275, 281, 292, 293,
297; 302, 343, 388, 395, 413
hot springs of, 132, 213

INDEX

Sain Noin (Khan)—( Continued)
lamasery at, 125, 343
maps showing, 126, 129, 194, 275, 304, Plate IV
province (khan, eimak) of, 213, 251, 297
section, 129
vicinity of, 24, 83, 87, 124-127, 227, 292, 293, 343)
388, 395, 413
Sair Usu, 164; 166, 170, 173, 256, 282, 283, 295, 298,
320, 333, 400, 406, 414
maps showing, 275, 304, 404, Plate IV
section, 168
vicinity of, 164, 166-167, 170-173
Sair Usu formation (Dinantian), 295, 298, 400, 406,
414
conventional symbols for, Plate VI
description of, 170-173, 256
problem of, 282-283
section, 171
strike of, 320
table showing, 301
see also Palzozoic period
Sairim formation (Lower Cretaceous),
section, 156
type locality, 272, 278, 354
see also Oshih formation
Sairim Gashato temple, 272, 278
map showing, 194
Salt pans, in hollows, 63, 141, 196, 224, 241, 246, 247,
303, 308, 355, 386, 392
Plate XVII B
see also Hollows, Playas
San Men gravels (Pleistocene), 348
Sand, 148, I5I, 200, 230, 235, 236, 237, 283, 306,
315, 323, 352, 362, 374, 382, 385, 407
alluvial, 31, 63, 156, 177, 203, 206, 238, 252, 271,
280, 311, 340, 354, 355, 358, 360, 363, 365, 366,
376, 379, 380
marine, 23
residual, 71, 157, 217, 218
wind-blown, 58, 146, 147, 149, 158, 161, 197, 207,
211, 230, 249, 253, 254, 272, 279, 325, 326, 332,
333, 335, 337) 350, 351, 354, 356, 357, 368, 369,
377, 381, 386, 417
see also Dunes, Graywackes, Hummocks, Sand-
stones
Sandstones, 22, 23, 24, 49, 63, 67, 68, 70, 103, 105,
106, 114, 124, 127, 146, 152, 157, 164, 165, 166,
169, 170, 174, 178, 181, 182, 202, 203, 204, 206,
207, 209, 210, 216, 218, 219, 220, 224, 226, 228,
231, 232, 237, 253, 254, 256, 261, 262, 263, 265,
266, 270, 271, 272, 291, 292, 293, 294, 297, 313,
325, 334, 353, 354, 355, 362, 373, 375, 379, 380,
398, 399, 401, 402, 405, 407, 41I
conventional symbols for, Plate VI

Sanjarav, acknowledgment to, x
Sarcodon pygma@us, 359

Sauropod dinosaurs, 233, 271, 354, 368
Saurornithoides mongoliensis, 357

469

Saxaul, maker of sand hummocks, 351
see also Tamarisk mounds
Sayan (Sayanski) ranges, boundary of Gobi basin,
303
maps showing, 43, 275, Plate IV
Sayan trend, 319
Scarps, erosion, 31, 60, 141, 157, 174, 177, 182, 196,
200, 209, 265, 275, 276, 279, 283, 314, 338, 339,
363, 365, 386, 390, 391
fault, 34, 253, 265, 269, 270
figures, 54, 98-99, 102, 176, 199, 200, 209, 224-225,
242, 267, 269, 281, 318, 324, 337, 338, 339,
340
maps showing, 117, 140, 176, 198, Plates XXVIII,
XXIX, XXX
Plates I, V A, X A, XXI
reversed, 339
sections, 47, 54, 57, 58, 65, 102, 159, 175, 177, 180,
201, 203, 209, 229, 281
Schists, 22, 23, 50, 72, 80, 82, 91, 94, 109, 113, I19,
135, 136, 139, 141, 149, 154, 156, 166, 170, 174,
178, 181, 183, 184, 226, 227, 254, 261, 262, 263,
264, 282, 290, 291, 299, 300, 3II, 313, 314, 319,
320, 322, 334, 373, 398, 408
conventional symbols for, Plate VII
sections, 117, 140, 168, 171, 185
Schizotherium avitum, 363
Schlosser, Max, cited on anthropoid tooth, 12
Schlosseria magister, 359
Schonstrom, geographic observations by, 15
Scoria, 78, 319
Sediments, see Later sediments
Sedimentation, ancient eras of, 400
Cretaceous and later, 403-405, 417
gaps in the record of, 366-368
lake deposits, 392
mid-Mesozoic, 402, 4.03
Paleozoic, 401
process of, 305
stream deposits, 331, 338, 390-391
summary of, 405-408
Selenga Gol (river), 23, 90
map showing, Plate IV
Selenite, 271
see also Gypsum
Selenomys mimicus, 234
Septarian nodules, 205
Sericite, 80, 178, 263
see also Schists, Phyllites
Serpentine, 258, 314, 408
conventional symbol for, Plate VII
Serridentinus mongoliensis, 235
Sha-ho (river), 30
Shabarakh Usu, dune dwellers of, 418
see also Djadokhta
Shackelford, J. B., 88
acknowledgment to, ix
discoverer of Protoceratops, 157

470

Shah Ola (hill), 271
map showing, 194
see also Oshih formation
Shales, 63, 114, 169, 181, 205, 210, 219, 231, 232,
263, 271, 276, 294, 297, 298, 354, 359, 374, 375;
379, 398, 401, 405, 407
conventional symbol for, Plate VI
Shamosuchus djadochtaensis, 357
Shansi, block mountains of, 29-30, 32 .
Shantung, fish-bearing shales of, 233, 354
map showing, Plate IV
Shara Khada range, as boundary of Gobi basin,
31
map showing, Plate IV
Shara Khata Mountains, 149
section, 145
Shara Murun, maps showing, 43, 198, 275, 304, 404,
Plate IV
section, 180
valley of, 182-183
see also Gobi erosion plane
Shara Murun formation (Eocene), 332, 335, 359, 363,
370
age and correlation of, 367, 369
climate indicated by, 380
conventional symbol for, Plate VI
description of, 182, 209-210, 362
extent of, 199
figure, 199
fossils of, 182, 208, 362
Plate XXII B
problem of, 279
sections, 180, 185
table showing, 272
Shargin tala, 308
Shaya Nor, see Iren Nor
Sheep, 87, 249, 258, 313
Sheiten Ola range, boundary of Ordos, 31
map showing, Plate IV
Shelf, 308, 315
figure, 330
origin of, 261-262, 265, 313-314, 330-332
sections, 77, 261, 345
see also Terrace
Shells, 203, 354, 355
Shensi, fish-bearing shales of, 354
Sherwood, George H., acknowledgment to, vii
Shilka River, 25
Shrines, 83, 170, 213
Siberia, 20, 32, 326
early settlements in, 15
edge of the forest of, 130
fish-bearing shales of, 354
Jurassic formations of, 402
Lower Cretaceous climate of, 376
Silica, 151, 315, 334
stacks, origin of, 272
Plate XX XVII

INDEX

Sills, 48, 61, 66, 74, 82, 93, 96, 102, 108, 113, 120
see also Dikes
Silurian formations, 27
Silurian period, an erosion interval, 414
Silt, 407
Simpson, George Gaylord, cited on Cretaceous mam-
mals, 353
Sinian system (pre-Cambrian), 227, 300, 373, 398,
41I, 414
table showing, 301
Sirimpil, acknowledgment to, x
Skulls, fossil, 208, 210, 279, 363, 380
Slates, 34, 50, 56, 60, 61, 63, 64, 66, 82, 91, 94, 97,
100, I19, 122, 178, 181, 199, 200, 202, 205, 216,
227, 282, 283, 291, 297, 298, 299, 313, 314, 315,
319, 323, 355; 373, 374, 375, 399, 400, 401
conventional symbol for, Plate VI
maps showing, Plates XXIII, XXVI
Plate XXII A
sections, 57, 58, 62, 65, 81, 89, 92, I2I, 123, 129,
134, 179, 201, 203
Snow, 64, 84, 125, 138, 183, 327, 395, 396
Soil, 90, 120, 184
alluvial, 83
residual, 50, 149, 220
Special studies listed, 195, 275
Specimens (American Museum catalogue numbers),
IIO, III, I14, 132, 149, 156, 165, 169, 174, 178,
219, 220, 257, 272
Springs, 87, 124, 127, 138, 214, 253, 348
Stacks, silica, at Oshih, 272
Plate XX XVII
Stocks, see Bosses
Storms, 64, 125, 183, 252, 326, 327, 330, 331, 336, 337,
351
von Strahlenberg, Philipp Johann, map of northern
Asia by, 15
Streams, 70, 76, 83, 113, 147, 148, 161, 202, 216, 236,
246, 265, 313, 348, 387
braided, 120
consequent, 327
courses of, 25, 33, 90, 240-241, 243
graded, 331, 336
immigrant, 327-328
of Khangai watershed, 24
misfit, 390
native, 327-329
see also Drainage, Erosion, Terraces, Valleys
Structure lines, 226, 319-322
map showing, 321
Struthiolithus, 235, 382
eggs of, 382
Struthiomimid dinosaurs, 204
Suess, Edouard, acknowledgment to, 17
Altaids of, 27
cited on Altai trend, 320
cited on boundary of Gobi basin, 22, 24, 36
cited on Great Khingan range, 28-29

INDEX

Suess, Edouard—Continued
cited on Ikhe Bogdo, 310
cited on Miocene lignites, 367
cited on Ordos, 31-32
cited on Rhinoceros, 200
cited on Sayan trend, 319
cited on Tannu Ola, 22
cited on ‘‘ Valley of the Lakes,’’ 308
views of, discussed by Obruchev, 34-35
Suma Khada range, 31
map showing, Plate IV
Sungari River, 30
Syenites, 16, 71, 72, 94, 164, 165, 269, 292, 409
conventional symbol for, Plate VII
poikilitic, 61
section, 163
Synclinal structure, 105, 108, 119, 124, 132, 154, 164,
226, 263, 265, 294
Synoplotherium, 360
Szechenyi, Bela, journey of, 17
routes of, Plate III

Ta Hsing An Ling, see Great Khingan range
T’ai Shan system (Archezozoic), deformation of, 413
component of oldrock floor, 291
outcrops of, 80, 109, III, 227
sections, 79, II2, 229, 415
summary of, 300-302, 398
table showing, 301
unconformity above, 411
see also Gneisses
Talas, 318, 323, 327
definition of, 306
of eastern and southern province, 306-307
trends of, 319, 321-322
of western province, 308-309
Tamarack (larch), 130, 389
Tamarisk mounds, 58, 351
Tamarisks, Plate XX XIII B
T’ang Hsien, physiographic stage, 347, 348
Tannu Ola range, boundary of Gobi basin, 21, 35,
303
boundary of Ubsa tala, 308
maps showing, 43, Plate IV
structure of, 22
trend of, 320
Tarim basin, extension of Gobi basin, 33-34
mapped by Renat, 15
see also Lop Nor
Tarmil Gol (river), tributary of Ongin Gol, 125,
127
maps showing, 123, 126, 129
Tarso-metatarsus, fossil, 361
Tatal Gol (river), Oligocene beds at, 365
map showing, Plate XXIX
Tatars, Bahadur’s history of, 15-16
Tataromys plicidens, 234
sigmodon, 234

471

Tatsin Gol (river), tributary of Tsagan Nor, 246, 328,

348
maps showing, 194, Plate IV

Teeth, fossil, 197, 201, 205, 208, 209, 210, 279, 380,

382

Teilhard de Chardin, Pére Pierre, quoted on pre-

historic man, 284

Teilhard de Chardin, Pére Pierre and Licent, Pére

Emil, cited on Paleolithic cultures, 13, 18
cited on Pliocene formations, 29, 306, 370, 382
cited on volcanic mountains, 29, 318

Teilhardia pretiosa, 360

Telegraph line, Kalgan-Urga, 189, 196, 336

Teleolophus medius, 361
Telmatherium berkeyi, 361

Telgir Merin Gol (river), map showing, Plate IV

rock formations at, 23

Temperatures, 84, 183, 220, 249, 341, 374, 395
Temples, Buddhist, 97, 161, 170, 196, 209, 272, 278,

280

Terraces, of abandoned beaches, 392-393

in alluvial fans, 312, 389-391, 393

in badlands, 209

of marginal shelf, 261

Plates XIII B, XIV B, XX A, XL B, XLII B
section, 342

in stream valleys, 90, 115, 122, 162, 31I, 313, 314,

315, 343, 348
see also Shelf

Tertiary period, climate during, 378-382, 419

erosion during, 241-243, 261, 332-333, 346, 347-
348, 407

lavas of, 30, 31, 236-238, 241, 334, 355, 410

orogenic movements during, 17, 20, 35, 36, 238,
243, 244, 265, 273, 295, 347-348, 405, 407, 410,
412, 413

sections, 47, 69, 209, 236

stratigraphic units of, 26-27, 31, 40, 41, 49, 52,
55, 63, 67, 68, 148, 152, 166, 174, 182, 204, 210,
225, 229, 233, 235, 236-238, 239, 241, 261, 263,
270, 277, 279, 281, 289, 306, 309, 314, 315, 332,
333, 335, 341, 353, 364, 366

structure lines of, 32, 319-321

summary of, 366-372

see also Cenozoic era

Testudo insolitus, 364

Theropoda (Theropods), 204, 354, 357
Third Asiatic Expedition, 23, 40, 48, 223, 228

purposes and problems of, 274
routes of, 43, 290, 304, 404, Plate III
summary of 1922 season’s work, 190
Tibet, explorations in, 15, 17
Tien Shan, explorations of, 16
glaciation of, 382
map showing, Plate IV

structure of, 34-35, 309-310
trend of, 320

472

Tiger Canyon, alluvial fans at, 311, 312, 395
map showing, Plate XXX
Plate XL B
trees at, 389-390
Ting, V. K., acknowledgment to, vii
Titanotheriide (Titanotheres), 177, 208, 360, 361,
362, 363
Tola Gol (river), 22, 82, 88, 89, 413
course of, 90-91
maps showing, 290, 304, 404, Plate IV
rock formations along, 84, 181, 227, 282, 297, 399
sections, 92, 95
topographic features of, 94-96, 342-343
see also Camp Tola River
von Toll, Baron E., cited on Miocene lignites, 367
Tolstikhin, N. J., cited on Selenga River region, 23
routes of, Plate II
Topography, character of, 76, 84-85, 94, 130, 154,
161-162, 173, 178, 181, 182, 187, 218, 235, 243-
244, 254, 258-259, 269, 351, 413
origin of local, 52-54, 58, 218, 258-259, 289, 306,
342-343, 397, 417-418
structural control of, 46, 49-50, 64-68, 94-95, 187,
266, 412
see also Basins, Erosion planes, Hollows, Moun-
tains, Peneplanes
Tortoise bones, fossil, 206, 362, 364, 377, 379, 380
see also Chelonia
Tourmaline, at granite contacts, 103, 218
Trachodont dinosaurs, 376
Trachy-andesites, 45
Trachytes, 45, 78, 80, 173, 228, 269, 292, 355, 410
conventional symbols for, Plate VII
section, 155
Tragulina, 361
Trails, character of, 4, 11, 58, 70, 78, 82, III, 122, 125,
130-I3I, 132, 146-147, 149, 156, 157, 158, 161,
174, 178, 189, 249, 253, 275
mode of formation of, 58, 147
Kweihwating-Kobdo, 143, 153, 261, 278
Sair Usu-Uliassutai, 132, 136, 139, 162, 164, 165,
166, 170, 279, 283, 292, 293, 333
Urga-Kalgan, 60, 84, 196, 232, 276, 280, 300, 302,
307, 317, 319, 342, 370, 376
Urga-Sain Noin, 90, 124
Transbaikalia, fault-block ranges of, 21-22, 25-28,
303, 309, 319
fish-bearing shales of, 354
map showing, Plate IV
Obruchev’s map of, 320
peneplane in, 347
Trap (basalt, diabase), 60, 202
Traverses, Ardyn Obo to Shara Murun, 177-182
Arishan to Gorida, 132-135
Artsa Bogdo to Djadokhta, 153-158
Artsa Bogdo to Gurbun Saikhan, 248-252
Bolkuk Gol] to Five Antelope Camp, 90-93
Bolkuk Gol to Urga, 88-89

INDEX

Camp Canyon Brook to Ongin Gol, 115-120
Camp Jurassic to Mount Tuerin, 70-75
Camp Volcanic Cliffs to Artsa Bogdo, 148-151
Djadokhta to Ongin Gol in Sumu, 158-162
Forest Camp to Arishan, 130-131
Gurbun Saikhan to Artsa Bogdo, 252-253
Tren Dabasu to Camp Jurassic, 60-67
Kalgan to Wan Ch’uan Hsien Pass, 45-49
major, 3
Ongin Gol to Sain Noin, 122-124
Ongin Gol in Sumu to Sair Usu, 162-169
Pacific Divide (Wan Ch’uan Hsien Pass) to P’ang
Kiang, 49-55
P’ang Kiang to Iren Dabasu, 55-59
Rainy Gulch to Arctic Divide, 127-128
Sair Usu to Ardyn Obo, 170-174
Shara Murun to Kalgan, 183-190
Tola River to Tsetsenwan, 94-97 4
Tsagan Nor to Camp Volcanic Cliffs, 144-148
Tsetsenwan to Camp Canyon Brook, 110-113
Tuerin to Bolkuk Gol, 76-83
Uliassutai Trail (Gorida) to Mount Uskuk, 138-141
Trees, 58, 128, 130, 351, 387, 388, 389, 390, Plates
XIV A, XXXIII B, XL
Triassic formations, noted by Mushketov, 17
Triassic period, as possible age of conglomerates, 104,
106, 294, 403
Trichopterella torta, 233
Trionychids, 361, 362, 364
Tsagan Nor (lake), 142, 311, 328, 385
ancient beaches of, 247, 392-393
description of, 246-247, 348
history of, 349-350
maps showing, 194, 275, 304, Plates IV, XXIX,
XXX
Plates XVIII, XXXIII B, XLII B
Roberts’ map of, ix r
sections, 145, 229
Tsagan Nor basin, 142-144, 149, 152, 195, 223, 229-
242, 276, 277, 365
maps. showing, 140, Plates XXVII, XXVIII,
XXIX, XXX
Plates XVIII A, XXXI, XXXII A, XXXIII,
XL B, XLII
Tsagan tala, structure of, 308-309, 366
Tsaganomys altaicus, 234
Tserin, acknowledgment to, x
Tsetsenwan, 302
abandoned dam near, 395
figures, 98, 102
local traverses from, 97, 98-104, IO5-I1I0
maps showing, 43, 275, 304, Plate IV
sections, IOI, 102, 107, II2, 293
vicinity of, 24, 97-104, 110, 218, 292, 297, 300
see also Camp Tsetsenwan
Tsetsenwan series, 292-293
table showing, 301
see also Jurassic conglomerate series

INDEX

Tsilin Gol (river), rock formations near, 370, 382
Tsingling Shan range, trend of, 320
Tuerin, Mount, 7, 84, 110
lamasery at, 75
maps showing, 43, 275, 290, 304, 404, Plate IV
Plate XI B
tock formations of, 74-76
sections, 73, 77
Tufa, Cretaceous, near Artsa Bogdo, 151
Tuffs, 45, 61, 74, 96, 108, 127, 178, 235, 237, 238, 253,
262, 264, 266, 375, 410
see also Volcanic rocks
Tugurik, 162
map showing, 194
section, 163
Tukhum formation (Eocene), 210, 362
description of, 359-360
fossils of, 360
map showing, 404
Tupaiodon minutus, 234
morrisi, 234
Turfan lowland in Tien Shan, 34
map showing, Plate IV
Turkestan, explored by other geologists, 17, 18, 19,
20
peneplane in, 347
Turkish tribes, monuments made by, 14
Turtle bones, fossil, 204, 359, 360, 361, 362, 377, 379,
382
see also Chelonia
Tushetu (province, khan, eimak), 251

Ubsa Nor basin, 22
maps showing, 43, Plate IV
see also Ubsa tala
Ubsa tala, extension of Gobi basin, 308
map showing, 43
see also “‘ Valley of the Lakes”
Ude, 293
maps showing, 43, 275, 290, 304, 404, Plate IV
rock formations near, 64, 297
section, 65
Ula Usu, maps showing, 304, 404, Plate XXII B
rock formations at, 209-210
section, 180
see also Shara Murun formation
Ulan Huataka Ama, at Gurbun Saikhan, 252
Ulan Nor (lake), 153, 157, 328, 348
maps showing, 194, 275, Plate IV
problem of, 280
Uliassutai, 297
maps showing, 43, 304, Plate IV
see also Trails
Uljitundur Gol (river), at Baga Bogdo, 147
map showing, Plate XXX
Unconformities, figures, 165, 199, 209, 242
physiographic, 343
Plates XXIII, XXVI, XXVII, XXIX, XXX

473

pre-Cretaceous (“‘Great Unconformity”), 106, 211,
229, 244, 288, 289, 290, 291, 294, 352, 354-355,
402, 403, 411, 414

pre-Jurassic (post-Permian),
411, 414

pre-Palzozoic, 296, 401, 411, 414

pre-Permian, 296, 401

pre-Sinian, 302, 398-399, 400

pre-Tertiary, 211, 223, 225, 229, 231, 236, 241, 244,
404

pre-Wu T’ai, 302, 400, 411

problem of, 282

sections, 47, 51, 53, 57, 58, 62, 65, 69, 73, 77, 79,
81, 107, 117, I2I, 123, 129, 134, 140, 145, 150,
155, 159, 160, 163, 167, 168, 171, 172, 175, 179,
180, 185, 186, 188, 189, 201, 203, 229, 253, 257,
261, 263, 277, 281, 289, 295, 299, 346, 415

tables showing, 301, 372

see also Conglomerates, Disconformities, Erosion
planes, Peneplanes

Urga, 197, 227, 282, 293, 297, 298, 320, 342

capital of Mongolia, 88

maps showing, 43, 275, 290, 304, 404, Plate IV

see also Trails

Urgo-Samoyeds, early migrations of, 13
Uriankhai basin, 13
Urulji Nuru, scarp at Oshih, 268, 271
figure, 269
Urumchi, in Dzungaria, 34
map showing, Plate IV
Uskuk Mountain, 141, 142, 205, 223, 247, 297, 329,
332, 353, 365, 366

ancient crystallines of, 224, 225-228

deformation of, 244-246, 270

erosional history of, 243-244, 341

fault block of, 138-139, 223-224, 231, 308, 311,
333, 413

figures, 239, 242

“Flaming Cliffs’ of, 275, 277

Jurassic formations of, 224

later sediments at, 224-225

lava cap of, 224

maps showing, 140, 194, 290, 404, Plates XXVII,
XXVIII

mid-Mesozoic formations of, 228-229, 292, 293

Plates XVII B, XXXI B, XXXII, XXXIII A

sections, 140, 229

see-also Guchu Burt

Ussov, M., cited on age of graywackes, 25, 298

cited on Kentai Ola, 25

routes of, Plate II

104, 294, 401, 410-

Valleys, abandoned, 147-148, 156, 158, 246, 395
aggraded, Plates XXXI A, XL B, XLI
ancient, 240-241, 266-268, 392
forms and adjustments of, 30, 52, 68, 75, 90-91,
120, 138, 141, 147-148, 156, 158, 161, 182, 216,
243-244, 246, 251, 255, 258, 266-267, 268-269,

474

Valleys, forms and ajustments of—Continued
272, 309, 311, 313, 315, 327-330, 331, 342-343,
383-384, 417
see also Erosion, Streams, Terraces
“Valley of Gems,"’ 208
“Valley of the Lakes,” 22, 308
maps showing, 43, Plate IV
Vegetation, distribution of, 76, 130, 249, 252, 326-
327, 396
maker of loess, 356
maker of sand hummocks, 351
protector against erosion, 48, 312, 326, 339, 351
see also Erosion, Flora, Forests, Scarps, Trees
Veins, 254, 264, 334, 357, 364
see also Mineral resources, Pegmatites, Quartz
Velociraptor mongoliensis, 357
Vertebrates, 182, 352, 360, 379
Villages of Mongol yurts, 88, 170, 184, 244, 258
Viverravus constans, 234
Volcanic ash, 61, 74, 108, 157, 237, 272, 315, 334, 374,
375, 410
Volcanic bombs, 78, 127, 157
Volcanic eruptives, 34, 262, 281, 319
Volcanic extrusives, 290
Volcanic flows, 96, 108, 126, 127, 148, 156, 157, 174,
221, 222, 241, 242, 245, 262, 266, 271, 306, 308,
310, 319, 334, 354, 358, 365, 375, 393, 410
Volcanic rocks, 24, 28, 48, 74, 97, 98, 108, 126, 127,
148, 174, 215, 238, 272, 314, 405
sections, 73, 145
Volcanism, development of, 408-410
effects of, 224, 245, 305, 309
evidences of, 66, 67, 68, 78, 266, 319
problem of, 284
relation of, to isostasy, 245
see also Bathylith
Volcanoes, 27, 28, 305, 306, 318, 319
cones of, 29, 67, 78, 80, 318, 319, 417
necks of, 48, 272, 319
plugs of, 48, 120, 219
section, 79
vents of, 66, 78-80, 219

Wadi Abu Shusha, cloudburst at, 326
Walther, Johannes, cited on cloudbursts, 326-327, 396
cited on desert erosion, 325
quoted on desert lakes, 348
Wan Ch’uan formation (Lower Cretaceous), 352
age and correlation of, 357-358
conventional symbol for, Plate VI
description of, 46-49, 355-356
Plate XLI A
problem of, 275-276
sections, 47, 188
Wan Ch’uan (Hsien) Pass, 196
entrance to Mongolia, 9
as evidencs of climatic change, 386-387
map showing, 275

INDEX

Plates VIII A, XLI, XLIII B
rock formations at, 45-49, 189
section, 47
Wang, Hung Ping, acknowledgment to, x
Warping, effects of, 262, 306, 308, 309, 322, 333, 357,
402
evidence of, 211-212, 247, 259, 288, 318, 333, 403-
405, 407
periods of, 244-245, 289, 348, 367, 403-405, 407,
410, 411-412, 414
sections, 367, 369
see also Deformation, Faults
Water, deposits formed by, 177, 208, 375, 379, 381,
398, 402, 417
ground-water, 87, 221, 350
presence of, 156, 187, 249, 250, 252, 253, 282, 315,
389
supply, 148, 151, 248
see also Erosion, Rainfall, Springs, Streams, Water-
table, Weathering
Water-table, 147, 196, 249, 328, 341, 348
Watersheds, 25, 244
see also Arctic Divide, Pacific Divide
Wealden (Lower Cretaceous), 366
Weather conditions, 64, 84, 124, 183, 248, 252, 330
see also Storms, Temperatures
Weathering, processes of, 118, 218, 305, 324-325, 330,
374, 375
products of, 66, 67, 218, 264, 374, 375, 417
Wei Ho (river), 32, 327
Wells, in Gobi basin, 64, 142, 165, 196, 200, 208,
209, 249, 250, 252, 253, 395
West Knolls, Pliocene deposits of, 366
map showing, Plate XXX
Western Hills, Plate V A
William of Rubrouck, journey of, 14
Willis, Bailey, cited on China, 18, 19
cited on T’ai Shan system, 80, 109, III, 302, 398
cited on Wu T’ai system, 25, 72, 82, 166, 300
quoted on Khingan line, 29-30
Willis, Bailey, and Blackwelder, Eliot, quoted on
Pei T’ai peneplane, 347-348
Wind, deposits by, 207, 335, 350-351, 359, 377,
417
prevailing directions of, 395-397
transporting power of, 118, 141, 148, 326, 341
see also Dunes, Erosion, Loess, Sand
Wong, Wenhow, acknowledgment to, vii
quoted on Cretaceous in Ordos, 32
Wu T’ai system (Proterozoic), component of oldrock
floor, 291
deformation of, 398-399, 411, 413
outcrops of, 24, 25, 72, 82, 114, 154, 166, 178, 183,
227
Plates XIII A, XXXIX B
sections, 73, 79, 113, 145, 150, 155, 168, 179, 185,
415
summaries of, 299-300, 398-399

Wu T’ai system—Continued
table showing, 301
unconformities limiting, 24, 411

Xenoliths, 24, 93, 110, 116, 139, 156, 183, 218
Plate XXV B
see also Bathylith, Roof-pendants

Yablonovi range, map showing, Plate IV

INDEX

Yaks, 313
Yang Ho (river), 9, 45
Yangtze Kiang (river), 30
Yellow River, see Hwang Ho
Yenisei River, 22
map showing, Plate IV
Yeniseians, ancient migrations of, 13
Yih, L. F., cited on Western Hills, 347
Yurts of Mongols, 64, 157, 213, 244, 250, 258

475

FAGAL

rie wast { :
Fe elwety aipaa’’ { , ey Oy od: Aaeie
ot intend steed. ; th 40 aed att,
OT HiserFien went Elis] Sacer, 10a ‘cma S
te weed, bere T ‘ae abt atcha tater &¢ de
to TOUS P atatt .pateoile ait a UAE ine
33} Zt DA Wabal taewinn innate snebeae tl | oe Haniiian inn . Seer teG wig
Peer AGT otetie ll no. ttle aS al we
Ris ie Hh? RASTA Shigtad. Jo's “Fier l. pAlpoa ana Opt ay

ed

= AO,
+ ata
Hy: dagiet on ig ie

@-vV 3NIT 3HL SNOOTY NOILD3S 91901039

HAG

GTO OSHA SITNURII

B

eso iW

ay 0p aygoindiuoy puns yuavey

45%
20'

45°)

THE AMERICAN MUSEUM OF NATURAL HISTORY

RECONNAISSANCE GEOLOGIC MAP

OF PART OF THE

TSAGAN NOR BASIN GEOLOGY OF MONGOLIA

HENRY FAIRFIELD OSBORN, President PLATE XXIX
10125) a0! a = =
LEGEND 560,
am SOUTHERN BLOCK OF USKUK MOUNTAIN

IGNEOUS AND

METAMORPHIC ROCKS
=

OLIGOCENE

Basalt flows
Surface lows of basalt interbedded with
the Hsanda Gol formation, and locally
faicumpanied by beds of ash and ruff

JURASSIC (?)

J\=

Complex of porphyries
rhyolite, trachyte, endesite and granite
Porphyries, associated with Jurassic (?)
conglomerate series Ly

2)

Ancient complex
Gneisses, schists and crystalline
limestones invaded by granite and
dikes of the great Mongolian bathylith J)

Faults

CENOZOIC

MESOZOIC

euchDEOZOle
EARLY PALAEOZOIC

ey 3S
dh i\ -

4s°

15°

are

tthe

15

SEDIMENTARY ROCKS
POST-PLEISTOCENE

Valley fill
Partly consolidated sand, deposited J

QUATERNARY

arene bach el oun ls
con sueamp mus,
and pd aes be

MIOCENE

pile

Yay oe

Loh formation

a U__|ase

= White sands and gravels, and =
alive greo anil ow clays 7 |10"

clayey sands and thin gravels N

z

OLIGOCENE a

Hsanda Gol formation
downward into sands and pists
%

Fossil Sites

CENTRAL ASIATIC EXPEDITIONS
ROY CHAPMAN ANDREWS, Leader

TRUE NORTH
MAGNETIC NORTH.

101°45"

me
Seale inoca Geology by Charles P. Berkey
ao os eaee t 5 A a 5 Miles Plane-table survey by Frederick K. Morris, 1922
Datum, Tsagan Nor in 1922

405 0 “4 2 3 4 5 Kilometers Tale outline, altitude, and position taken from the survey by L. B, Roberts,
Parts of the ially thi vi
Beare seid Se ur eee eats meen Tanean Nor and ata

Contour Interval, 100 feet Drawn by Erwin J. Ruisz

RECONNAISSANCE GEOLOGIC MAP
OR GHE GEOLOGY OF MONGOLIA

THE AMERICAN MUSEUM OF NATURAL HISTORY
HENRY FAIRFIELD OSBORN, President HUNG KUREH AREA PLATE XXX

i014"

101° 25" £ . — ,
a ; q pM x LEGEND

SEDIMENTARY ROCKS
POST-PLEISTOCENE

ps
Low flat covered
with swamp vegetation

PLEISTOCENE

==

Recent Sand, Comparable to the Shabarakh

QUATERNARY

High fans
pore bere hayos
erent

Gochu formation
sere a a
Ti iad eet

PLIOCENE

we

pany sie formation
i overlaid
erce paenere ee
light gray clays

MIOCENE

|

E
Loh formation
eames |
IGNEOUS AND
METAMORPHIC ROCKS

=

| a nung) koreh ——_———

CENOZOIC

DayYing Gol

GEOLOGIC SECTION ALONG THE LINE A-B

Rubbles ——————___

OR
EARLY PALAEOZOIC

Mongolian bathylith
Ohefly biotite granite, cut by

numerous dikes

PRE-CAMBRIAN

Ancient complex
Gneisses, schists crystalline
limestones bey ‘granite and
dikes of the great Mongolian bathylith

~
i

Faults

ARCHAEOZOIC LATE PROTEROZOIC

AND
PROTEROZOIC

LONGITUDE EAST FROM GREENWICH 101° 25" 3o'

CENTRAL ASIATIC EXPEDITIONS a os eo) A a= EY 4 3 Mites Geology . Berkey
ROY CHAPMAN ANDREWS, Leader FA 7 1932
OS 8, ‘ aS a 5 Kilometers Topography of Baga Bogdo range largely sketched, partly from photographs
= Datum, Teagan Nor in 1922 :
Lake outline, altitude, and position taken from the survey by L. B, Roberts,

Con! Interval, fe 1925 ©
san et Drawn by Erwin J. Raise

RECONNAISSANCE GEOLOGIC MAP

THE AMERICAN MUSEUM OF NATURAL HISTORY OF THE GEOLOGY OF MONGOLIA
HENRY FAIRFIELD OSBORN, President HSANDA GOL AREA PLATE XXVIII

101°55" 102°00°

a 3 a ees We UA UA RG is LEGEND
= “4 fone —. 45° SEDIMENTARY ROCKS
ED Oa re
tarp. Ondaiy Saar

feleler 4
000s

MIOCENE

en

Loh formation
Olives, and brown
ee erat are

Usayinos

OLIGOCENE

401g

Hsanda Gol formation
Red clays and dlayry sands, associated
ici leonfows and doweracard
into thick basal conglomerates and.sands

LOWER CRETACEOUS
Ondai Sair formation

as
x S
1
1:
2

HONSH gO

uleyunoW

White and yellow sands and gravels,
with gray to Nack poper-shales

GREAT UNCONFORMITY

JURASSIC (2)

pons lam rele series”
Shales, sands and thick conglomerates o
buff, brown oF black colors, locally mu
coaly beds and obscure plant remains

IGNEOUS AND
METAMORPHIC ROCKS
OLIGOCENE

Roath

Basalt flows

Surface flows of basalt interbedded with
the Hsanda Gol formation, and locally
accompanied by beds of ash and tuff

as”

@-v SNM 3HL SNOOTY NOILDSS 91901039

Mongolian bathylith
oa

PRE-CAMBRIAN

Ancient Complex
Gneisses, schists and crystalline
liens inode by rane and
dikes of the great Mongolian bathylith

J

fs

10145" 101°50" 10P55"

Longitude East from Greenwich aa
Scale igghor z iH Geology by Charles P. Berkey
CENTRAL ASIATIC EXPEDITIONS a a ee ee ee ee ree
TENS ae REC zseiee eBid dg Kilometers ; ; Drawn by Eris Bae

Contour Interval, 100 fect

CENOZOIC

MESOZOIC

CENOZOIC

MESOZOIC

ARCHAEOZOIC
EARLY PALAEOZOIC

RECONNAISSANCE GEOLOGIC MAP

OF THE
THE AMERICAN MUSEUM OF NATURAL HISTORY ae GEOLOGY OF MONGOLIA
HENRY FAIRFIELD OSBORN, President MOUNT USKUK AREA PLATE XXVII

102°00°

1014S"

LEGEND

SEDIMENTARY ROCKS
Extensive Lava Fields RECENT
+—— to the West F

2

UNCONFORMITY

Alluvial fans
OLIGOCENE (?) »
2
s
2
S Hsanda Gol formation (?)
iS e 4 : ee Conglomerates and red sandy clays

LOWER CRETACEOUS

id

Ondai Sair formation
Fine white gravels and sands, inter-
bedded with gray and black papersales.

GREAT UNCONFORMITY

JURASSIC (2)

Conglomerate series
colored conglomerates
and sanlstones.
IGNEOUS AND
METAMORPHIC ROCKS

OLIGOCENE
45°
30"
Basalt flows
u Remnants of a oncescontinnous
" cover of lava, which at Uskuk

useayuzsoN

“eg Dao

Peak included! at least 10 flows.

4

Q-9 3NI7 3HL ONOIW NOILD3S 91901039
4OHxST

TERTIARY

yeed YONSL

Plug or ike
dark fel thoid=
coe

y¥O°018

Mongolian bathylith
Een tose Li
argh Shiliuta <” PRE-CAMBRIAN

wae

oS

MOHSA WseYyn

i
=
ss 33
| =

ea

——<——
° nN a =
° ° ah
8 3 8 3 ote Faults
x“
Fossil sites

Lava Cap Uskuk Peak

GEOLOGIC SECTION ALONG THE LINE A-B

CENTRAL ASIATIC EXPEDITIONS eat pete Gealogy ies eee oe i Sorts
ROY CHAPMAN ANDREWS, Leader roatboo Stuns ;
Bae OS 1 2 3 4 5 Miles le Drawn by Erwin J. Raise
105 0 1 2 3 4 5s Kilometers H

Contour Interval, 190 feet

CENOZOIC

MESOZOIC

MESOZOIC

CENOZOIC

LATE PROTEROZOIC
EARLY PALAEOZOIC

ARCHAFOZOIC
PROTEROZOIC

RECONNAISSANCE GEOLOGIC MAP
OF THE

THE AMERICAN MUSEUM OF NATURAL HISTORY
HENRY FAIRFIELD OSBORN, President IREN DABASU AREA

mas Mm os HA
PR le atios

Lie wie fl HAY

‘ ts, HH i)
ye i

tly

Be i
ee

Greenwich ii"45"
Seale igaloas
CENTRAL ASIATIC EXPEDITIONS a a a a
ROY CHAPMAN ANDREWS, Leader 105 0 1 2 3 4 5 Kilometers

Contour Interval, 20 feet

GEOLOGY OF MONGCLIA
PLATE XXIII

LEGEND

SEDIMENTARY ROCKS
OLIGOCENE

Houldjin formation
Yellow gravels and sands

EOCENE

Irdin Manha formation
White and gray sands,
clays and gravela

Arshanto formation?

diatel and red ceva

LOWER CRETACEOUS

Tren Dabasu formation
Red. ane clays and.
thin light $ray sandstones

SINIAN (?)

Oldrock floor
Folded dark slates

IGNEOUS ROCKS

n2°5

Geology by Charles P. Berkey
Plane-table survey by Frederick K. Morris, 1922-1923
Drawn by Erwin J. Raise

“Se eS EE eee eee

CENOZOIC

LATE PROTEROZOIC MESOZOIC
OR PALAEOZOIC

CENOZOIC 7

LATE PROTEROZOIC
OR PALAEOZOIC

G-9 3NM 3HL SNOIY NOILOSS 919071039

ureyunoy) p2s2eS

Longitude Bast from Greenwich to1"so"

THE AMERICAN MUSEUM OF NATURAL HISTORY

@-vV 3NM 3HL SNOTY NOiL9aS 91901039

HENRY FAIRFIELD OSBORN, President

l a
| ies i i

i re Na

CENTRAL ASIATIC EXPEDITIONS if
ROY CHAPMAN ANDREWS, Leader le

SS
:

ZS It} A i

RECONNAISSANCE GEOLOGIC MAP

OF THE

ARISHAN AREA

i
fi Ss

di ‘
HTT

ces
| aN i

|
the mS
il ALi (i }

Sonle ‘os

os o 1 2

————— Sc es few p

Contour Interval, 20 feet

——
i A

ns

| in
oC
od ul us

it

IGEOLOGIC SEC

aMies Geology by Charles P. Berkey

.

Hi

GEOLOGY OF MONGOLIA
PLATE XXVI

_

Plane-table survey by Frederick K. Morris, 1922
acter and northeastern portions of the map sketched with the aid of 4
ints located by intersection

Za fade w oe longitude only approximate, as position was not determined

pies by ‘Wy Erwin J. Raise

LEGEND

SEDIMENTARY ROCKS
JURASSIC (?)

er

= Gonglonterata seri Ns

i
uy
{
He

UNCONFORMITY

SINIAN (7)

il series
Dark folded and fasted!
igraywackes and slates

IGNEOUS ROCKS

st
Mongolian bathylith
Mosnire tiotue
cat by nem dik

Faults

Jw

2
3
N
fo}
Ww
w
=
yy
oo
NON
fen
Ww
B.S
2°s
as
a4
35
>
ec
<
z
«
w
5
=<
|}
o

LATE PROTEROZOIC
EARLY PALAEOZOIC

TN