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THE MACMILLAN COMPANY
NEW YORK + BOSTON - CHICAGO
DALLAS + SAN FRANCISCO
MACMILLAN & CO., LimitepD
LONDON - BOMBAY + CALCUTTA
MELBOURNE
THE MACMILLAN CO. OF CANADA, Lrtp.
TORONTO
Digitized by the Internet Archive
in 2007 with funding from
Microsoft Corporation
/ww.archive.org/details/earthfeatu restheOOhobbuoft
Mount Balfour and the Balfour Glacier in the
elkirks.
PLaTE 1.
a a ria a ea
—. Mie ff », t ' a
EARTH FEATURES
THEIR MEANING Te
AN INTRODUCTION TO GEOLOGY
_ FOR THE STUDENT AND THE GENERAL READER .
BY
WILLIAM HERBERT HOBBS:
PROFESSOR OF GEOLOGY IN THE UNIVERSITY OF MICHIGAN
AUTHOR OF “EARTHQUAKES, AN INTRODUCTION TO
SEISMIC GEOLOGY”; “CHARACTERISTICS OF
EXISTING GLACIERS ”; ETC.
ar. aay Neto ot
THE MACMILLAN’ COMPANY
3) 1912
All rights reserved
ee
Copyriaut, 1912,
By THE MACMILLAN COMPANY,
Set up and electrotyped. Published March, rgr2.
CUBRARS
,, AUG 3 1 1967
Nortusov ress
J. 8. Cushing Co. — Berwick & Smith Co.
Norwood, Mass., U.S.A.
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De ee Bt tl a ee i a ae eee ou wt
TO THE MEMORY
OF
GEORGE HUNTIN GTON WILLIAMS
PREFACE
THE series of readings contained in the present volume give in
somewhat expanded form the substance of a course of illustrated
lectures which has now for several years been delivered each
semester at the University of Michigan. The keynote of the course
may be found in the dominant characteristics of the different earth
features and the geological processes which have been betrayed
in the shaping of them. Such a geological examination of land-
scape is replete with fascinating revelations, and it lends to the
study of Nature a deep meaning which cannot but enhance the
enjoyment of her varied aspects.
That there is a real place for such a cultural study of geology
within the University is believed to be shown by the increasing
number of students who have elected the work. Even more than
in former years the American travels afar by car or steamship, and
the earth’s surface features in all their manifold diversity are thus
one after the other unrolled before him. The thousands who each
year cross the Atlantic to roam over European countries may by
historical, literary, or artistic studies prepare themselves to derive
an exquisite pleasure as they visit places identified with past
achievement of one form or another. Yet the Channel coast, the
gorge of the Rhine, the glaciers of Switzerland, and the wild scenery
of Norway or Scotland have each their fascinating story to tell
of a history far more remote and varied. To read this history, the
runic characters in which it is written must first of all be mastered ;
for in every landscape there are strong individual lines of char-
acter such as the pen artist would skillfully extract for an outline
sketch. Such character profiles are often many times repeated in
each landscape, and in them we have a key to the historical record.
An object of the present readings has thus been to enable the
student to himself pick out in each landscape these more significant
lines and so read directly from Nature. In the landscapes which
Vill
Vill PREFACE
have been represented, the aim has been to draw as far as possible
upon localities well known to travelers and likely to be visited,
either because of their historical interest or their purely scenic
attractions. It should thus be possible for a tourist in America
or Europe to pursue his landscape studies whenever he sets out
upon his travels. The better to aid him in this endeavor, some
suggestions concerning the itinerary of journeys have been supplied
in an appendix.
Regarded as a textbook of geology, the present work offers some
departures from existing examples. Though it has been customary
to combine in a single text historical with dynamical and structural
geology, a tendency has already become apparent to treat the his-
torical division apart from the others. Again, a desire to treat the
science of geology comprehensively has led some authors into in-
cluding so many subjects as to render their texts unnecessarily
encyclopedic and correspondingly uninteresting to the general
reader. It is the author’s belief that there is a real need for a book
which may be read intelligently by the general public, and it must
be recognized that the beginner in the subject cannot cover the
entire field by a single course of readings. The present work has,
therefore, been prepared with a view to selecting for study those
dominant geological processes which are best illustrated by features
in northern North America and Europe. It is this desire to illus-
trate the readings by travels afield, which accounts for the promi-
nence given to the subject of glaciation; for the larger number of
colleges and universities in both America and Europe are surrounded
by the heavy accumulations that have resulted from former glacia-
tions.
Emphasis has also been placed upon the dependence of the domi-
nant geological processes of any region upon existing climatic con-
ditions, a fact to which too little attention has generally been given.
This explains the rather full treatment of desert regions, of which,
in our own country particularly, much may be illustrated upon the
transcontinental railway journeys.
More than in most texts the attempt has here been made to teach
directly through the eye with the efficient aid of apt illustrations
intimately interwoven with the text. For such success as has been
reached in this endeavor, the author is greatly indebted to two
students of the University of Michigan,—Mr. James H. Meier,
who has prepared the line drawings of landscapes, and Mr. Hugh M.
~
PREFACE ix
Pierce, who has draughted the diagrams. Though credit has in
most cases been given where illustrations have been made from
another’s photographs, yet especial mention should here be made
of the debt to Dr. H. W. Fairbanks of Berkeley, California, whose
beautiful and instructive photographs are reproduced upon many
a page.
As given at the University of Michigan, the lectures reflected
in the present volume are supplemented by excursions and by so
much laboratory practice as is necessary to become familiar with the
more common minerals and rocks, and to read intelligently the usual
topographical and geological maps. In the appendices the means
for carrying out such studies, in part with newly devised apparatus,
have been indicated.
The scope of the book precludes the possibility of furnishing the
reader with the sources for the body of fact and theory which is
presented, although much may be inferred from the names which
appear beneath the illustrations, and more definite knowledge will
be found in the references to literature supplied at the ends of
chapters. A large amount of original and unpublished material
is for a similar reason unlabeled, and it has been left for the pro-
fessional geologist to detect these new strands which have been
drawn into the web.
WILLIAM HERBERT HOBBS.
ANN ARBOR, MICHIGAN,
October 25, 1911.
CONTENTS
CHAPTER I
Tue CoMPILATION OF EartTH History
PAGE
The sources of the history — Subdivisions of geology — The study of earth |
features and their significance— Tabular recapitulation — Geological
processes not universal — Change, and not stability, the order of nature
— Observational geology versus speculative philosophy — The scientific
attitude and temper— The value of the hypothesis — Reading refer-
ences . ; ; ‘ ; ‘ ‘ . ; : ‘ ‘ ° -
CHAPTER II
Tue FIGURE OF THE EARTH
The lithosphere and its envelopes — The evolution of ideas concerning the
earth’s figure—The oblateness of the earth—The arrangement of
oceans and continents— The figure toward which the earth is tending
— Astronomical versus geodetic observations — Changes of figure dur-
ing contraction of a spherical body — The earlier figures of the earth
—The continents and oceans at the close of the Paleozoic era — The
flooded portions of the present continents — The floors of the hydro-
sphere and atmosphere — Reading references ; ; : ; . 8
CHAPTER III
Tue NATURE OF THE MATERIALS IN THE LITHOSPHERE
The rigid quality of our planet — Probable composition of the earth’s core
— The earth a magnet — The chemical constitution of the earth’s sur-
face shell — The essential nature of crystals — The lithosphere a com-
plex of interlocking crystals —Some properties of natural crystals,
minerals — The alterations of minerals — Reading references. ae
CHAPTER IV
“ THe Rocks oF THE Eartn’s SurFacE SHELL
The processes by which rocks are formed—The marks of origin — The
metamorphic rocks — Characteristic textures of the igneous rocks —
The classification of rocks —Subdivisions of the sedimentary rocks
xi
xii CONTENTS
PAGE
— The different deposits of ocean, lake, and river — Special marks of
littoral deposits —'The order of deposition during a transgression of
the sea — The basins of deposition of earlier ages — The deposits of the
deep sea— Reading references . : , = : : : 7 =U
CHAPTER V
CoNTORTIONS OF THE STRATA WITHIN THE ZONE OF FLOW
The zones of fracture and flow— Experiments which illustrate the frac-
ture and flow of solid bodies— The arches and troughs of the folded
strata — The elements of folds — The shapes of rock folds — The over-
thrust fold — Restoration of mutilated folds — The geological map and
section — Measurement of the thickness of formations — The detection
of plunging folds — The meaning of an unconformity — Reading refer-
ences . : ; ; ; ; . : , : : - . 40
CHAPTER VI
Tue ARCHITECTURE OF THE FRACTURED SUPERSTRUCTURE
The system of the fractures—The space intervals of joints—The dis-
placements upon joints: faults— Methods of detecting faults — The
base of the geological map— The field map and the areal geological
map — Laboratory models for study of geological maps— The method
of preparing the map — Fold vs. fault topography — Readingreferences 55 ~
CHAPTER VII
Tue INTERRUPTED CHARACTER OF EARTH MOVEMENTS: EARTH-
QUAKES AND SEAQUAKES
Nature of earthquake shocks— Seaquakes and seismic sea waves — The
grander and the lesser earth movements— Changes in-the earth’s
surface during earthquakes: faults and fissures—The measure of
displacement — Contraction of the earth’s surface during earthquakes
— The plan of an earthquake fault—The block movements of the
disturbed district — The earth blocks adjusted during the Alaskan
earthquake of 1899 ; 4 ; : ; : ‘ : ‘ 5 ee
CHAPTER VIII
Tue INTERRUPTED CHARACTER OF EARTH MOVEMENTS: EARTH-
QUAKES AND SEAQUAKES (concluded)
Experimental demonstration of earth movements— Derangement of water
flow by earth movement —Sand or mud cones and craterlets— The
earth’s zones of heavy earthquake — The special lines of heavy shock _
— Seismotectonic lines — The heavy shocks above loose foundations —
Construction in earthquake regions — Reading references . ‘ 2. OE
1g CONTENTS
CHAPTER IX
Tue Rise oF Mouten Rock To THE EARTH’S SURFACE; VOLCANIC
Mountains OF EXuDATION
Prevalent misconceptions about volcanoes—Early views concerning vol-
canic mountains — The birth of volcanoes— Active and extinct vol-
canoes — The earth’s volcano belts— Arrangement of volcanic vents
along fissures, and especially at their intersections— The so-called
fissure eruptions — The composition and the properties of lava— The
three main types of volcanic mountain — The lava dome — The basaltic
lava domes of Hawaii— Lava movements within the caldron of Kilauea
— The draining of the lava caldrons — The outflow of the lava floods .
CHAPTER X
Tue Rise or Morten Rock TO THE Eartn’s SurRFACE; VOLCANIC
Mountains oF Egectep MATERIALS
The mechanics of crater explosions — Grander volcanic eruptions of cinder
: cones — The eruption of Volcano in 1888 — The eruption of Taal
volcano on January 30, 1911 — The materials and the structure of cin-
der cones — The profile lines of cinder cones — The composite cone —
The caldera of composite cones— The eruption of Vesuvius in 1906
— The sequence of events within the chimney —The spine of Pelé
— The aftermath of mud flows — The dissection of volcanoes — The
formation of lava reservoirs — Character profiles — Reading references
CHAPTER XI
THe ATTACK OF THE WEATHER
The two contrasted processes of weathering — The réle of the percolating
water — Mechanical results of decomposition: spheroidal weathering
— Exfoliation or scaling — Dome structure in granite masses — The
prying work of frost — Talus — Soil flow in the continued presence of
thaw water — The splitting wedges of roots and trees — The rock man-
tle and its shield in the mat of vegetation — Reading references .
CHAPTER XII
Tue Lire Histories or RIveRsS
The intricate pattern of river etchings—The motive power of rivers—
Old land and new land —The earlier aspects of rivers—-The meshes
of the river network—The upper and lower reaches of a river con-
trasted —The balance between degradation and aggradation — The
accordance of tributary valleys—The grading of the flood plain —
PAGE
94
115
149
X1V CONTENTS
The cycles of stream meanders — The cut-off of the meander — Meander
scars — River terraces—The delta of the river—The levee — The
sections of delta deposits : ; : : é ‘ . F
CHAPTER XIII
EartTH FEATURES SHAPED BY RUNNING WATER
The newly incised upland and its sharp salients — The stage of adolescence
— The maturely dissected upland — The Hogarthian line of beauty —
The final product of river sculpture: the peneplain— The river cross
sections of successive stages— The entrenchment of meanders with
renewed uplift — The valley of the rejuvenated river — The arrest of
stream erosion by the more resistant rocks — The capture of one river by
another — Water and wind gaps — Character profiles — Reading refer-
ences. , ‘ : . - , ‘ : : , ‘ °
CHAPTER XIV
Tue TRAVELS OF THE UNDERGROUND WATER
The descent within the unsaturated zone — The trunk channels of descend-
ing water — The caverns of limestones — Swallow holes and limestone
sinks — The sinter deposits — The growth of stalactites — Formation
of stalagmites— The Karst and its features—A desert from the
destruction of forests — The ponore and the polje — The return of the
water to the surface — Artesian wells— Hot springs and geysers —
The deposition of siliceous sinter by plant growth — Reading references
CHAPTER XV
Sun AnD WIND IN THE LANDS OF INFREQUENT RAINS
The law of the desert — The self-registering gauge of past climates — Some
characteristics of the desert waste— Dry weathering: the red and
brown desert varnish — The mechanical breakdown of the desert rocks
— The natural sand blast— The dust carried out of the desert
CHAPTER XVI
THe FEATURES IN DesERT LANDSCAPES
The wandering dunes—The forms of dunes— The cloudburst in the
desert — The zone of the dwindling river — Erosion in and about the
desert — Characteristic features of the arid lands —The war of dune
and oasis—The origin of the high plains which front the Rocky
Mountains — Character profiles — Reading references . ‘ :
PAGE
158
169
180
197
209 '
CONTENTS XV
CHAPTER XVII
REPEATING PATTERNS IN THE EARTH RELIEF
PAGE
The weathering processes under control of the fracture system — The
fracture control of the drainage lines — The repeating pattern in drain-
age networks — The dividing lines of the relief patterns: lineaments
—The composite repeating patterns of the higher orders— Reading
references. , : : : : ; : ° eat . 223
CHAPTER XVIII
| Ture ForMS CARVED AND MOLDED BY WAVES
The motion of a water wave— Free waves and breakers— Effect of the
breaking wave upon a steep, rocky shore: the notched cliff— Coves,
sea arches, and stacks— The cut rock terrace—The cut and built
terrace on a steep shore of loose materials — The work of the shore
current — The sand beach — The shingle beach — Bar, spit, and bar-
rier— The land-tied island—A barrier series — Character profiles —
Reading references ; . . ; : , ; ‘ é . 281
CHAPTER XIX
Coast RECORDS OF THE RISE OR FALL OF THE LAND
The characters in which the record has been preserved — Even coast line
the mark of uplift — A ragged coast line the mark of subsidence — Slow
uplift of the coasts ; the coastal plain and cuesta— The sudden uplifts
of the coast — The upraised cliff—The uplifted barrier beach — Coast
terraces — The sunk or embayed coast — Submerged river channels —
Records of an oscillation of movement — Simultaneous contrary move-
' ments upon a coast — The contrasted islands of San Clemente and
Santa Catalina—The Blue Grotto of Capri—Character profiles —
Reading references ‘ : ; ‘ 4 » : ‘ ‘ . 245
CHAPTER XX
Tue GLACIERS OF MOUNTAIN AND CONTINENT
Conditions essential to glaciation — The snow-line — Importance of moun-
tain barriers in initiating glaciers— Sensitiveness of glaciers to tem-
perature changes — The cycle of glaciation — The advancing hemicycle
— Continental and mountain glaciers contrasted — The nourishment
of glaciers — The upper and lower cloud zones of the atmosphere . 261
CHAPTER XXI
THE ConTINENTAL GLACIERS OF POLAR REGIONS
The inland ice of Greenland — The mountain rampart and its portals —
The marginal rock islands— Rock fragments which travel with the
xvi CONTENTS
PAGE
ice — The grinding mill beneath the ice — The lifting of the grinding
tools and their incorporation within the ice — Melting upon the glacier
margins in Greenland — The marginal moraines —The outwash plain
or apron—The continental glacier of Antarctica — Nourishment of
continental glaciers — The glacier broom — Field and pack ice —The
drift of the pack — The Antarctic shelf ice — Icebergs and snowbergs
and the manner of their birth— Reading references . , ‘ . 202
CHAPTER XXII
Tue ConTINENTAL GLACIERS OF THE ‘'IcE AGE”?
Earlier cycles of glaciation — Contrast of the glaciated and nonglaciated
regions — The ‘‘driftless area’? — Characteristics of the glaciated
regions — The glacier gravings— Younger records over older: the
glacier palimpsest— The dispersion of the drift—The diamonds of
the drift Tabulated comparison of the glaciated and nonglaciated
regions — Unassorted and assorted drift— Features into which the
drift is molded — Marginal or ‘‘ kettle’? moraines — Outwash plains —
Pitted plains and interlobate moraines — Eskers — Drumlins — The
shelf ice of the ice age — Character profiles . ; : ‘ : . 297
CHAPTER XXIII
GuaciAL LAKES WHICH MARKED THE DECLINE OF THE Last IcE AGE
Interference of glaciers with drainage — Temporary lakes due to ice block-
ing — The ‘‘ parallel roads’’ of the Scottish glens — The glacial Lake
Agassiz — Episodes of the glacial lake history within the St. Lawrence
Valley — The crescentic lakes of the earlier stages— The early Lake
Maumee — The later Lake Maumee — Lakes Arkona and Whittlesey
— Lake Warren — Lakes Iroquois and Algonquin — The Nipissing
Great Lakes — Summary of lake stages — Permanent changes of
drainage effected by the glacier — Glacial Lake Ojibway in the Hud-
son’s Bay drainage basin — Reading references . , , g . 820
CHAPTER XXIV
Tue Uptitt oF THE Lanp At THE CLOSE OF THE IcE AGE
The response of the earth’s shell to its ice mantle — The abandoned strands
as they appear to-day —The records of uplift about Mackinac Island
— The present inclinations of the uplifted strands — The hinge lines of
uptilt — Future consequences of the continued uptilt within the lake
region —Gilbert’s prophecy of a future outlet of the Great Lakes to
the Mississippi — Geological evidences of continued uplift — Drowning
of southwestern shores of Lakes Superior and Erie — Reading refer-
ences . ; ; ; ; ; : . . 840
a- odh tas
Oh a
ta
CHAPTER XXV
NraGara Fatits A Crock or RECENT GEOLOGICAL TIME
7
pF» ae a alk OSD
2 ee oe
Features in and about the Niagara gorge— The drilling of the gorge —
‘a The present rate of recession — Future extinction of the American Fall
— The captured Canadian Fall at Wintergreen Flats —The Whirlpool
Basin excavated from the St. David’s gorge — The shaping of the
_ Lewiston Escarpment — Episodes of Niagara’s history and their corre-
eg lation with those of the glacial lakes— Time measures of the Niagara
' clock —'The horologe of late glacial time in Scandinavia — Reading
| references. ; : ‘ ‘ : :
CHAPTER XXVI
Lanp ScutrptuRE BY MountTAIN GLACIERS
Contrasted sculpturing of continental and mountain glaciers — Wind dis-
tribution of the snow which falls in mountains— The niches which
form on snowdrift sites— The augmented snowdrift moves down the
. valley: birth of the glacier—The excavation of the glacial amphi-
. theater or cirque — Life history of the cirque— Grooved and fretted
ws uplands — The features carved above the glacier — The features shaped
beneath the glacier — The cascade stairway in glacier-carved valleys —
‘ a The character profiles which result from sculpture by mountain glaciers
- — The sculpture accomplished by ice caps — The Norwegian tind or
= beehive mountain — Reading references
ce. i | CHAPTER XXVII
ie A Successive GLACIER Types OF A WANING GLACIATION
f Transition from the ice cap to the mountain glacier —The piedmont
ane glacier — The expanded-foot glacier — The dendritic glacier — The
gu radiating glacier — The horseshoe glacier — The inherited -basin glacier
— Summary of types of mountain glacier — Reading references .
CHAPTER XXVIII
ih. The glacier flow — Crevasses and scrace -- Bodies given up by the Glacier
*: des Bossons— The moraines — Selective melting upon the: glacier
surface —Glacier drainage — Deposits within the vacated valley —
Marks of the earlier occupation of mountains by glaciers — Reading
references . : . : ; : ; ‘
CONTENTS | XVii
PAGE
352
367
383
THE GLACIER’S SurFACE FEATURES AND THE DEPOSITS UPON ITS BED
890
XVill
CONTENTS
CHAPTER XXIX
A Stupy or LAKE Basins
Fresh water and saline lakes — Newland lakes — Basin-range lakes — Rift-
valley lakes — Earthquake lakes— Crater lakes — Coulée lakes —
Morainal lakes— Pit lakes —Glint or colk lakes — Ice-dam lakes —
Glacier-lobe lakes — Rock-basin lakes — Valley moraine lakes — Land-
slide lakes — Border lakes — Ox-bow lakes — Saucer lakes — Crescentic
levee lakes— Raft lakes — Side-delta lakes— Delta lakes — Barrier
lakes — Dune lakes — Sink lakes — Karst lakes: poljen — Playa lakes
— Salines — Alluvial-dam lakes— Résumé — Reading references
CHAPTER XXX
Tore EPHEMERAL EXISTENCE OF LAKES
Lakes as settling basins — Drawing off of water by erosion of outlet — The
pulling in of headlands and the cutting off of bays— Lake extinction
by peat growth — Extinction of lakes in desert regions — The rdle of
lakes in the economy of nature — Ice ramparts on lake shores — Read-
ing references
CHAPTER XXXI
Tue ORIGIN AND THE Forms OF MOUNTAINS
A mountain defined — The festoons of mountain arcs — Theories of origin
HOO nD
of the mountain arcs— The Atlantic and Pacific coasts contrasted —
The block type of mountain— Mountains of outflow or upheap—
Domed mountains of uplift; laccolites — Mountains carved from
plateaus — The climatic conditions of the mountain sculpture — The
effect of the resistant stratum — The mark of the rift in the eroded
mountains — Reading references . ; ° ° ° ‘ :
APPENDICES
. The quick determination of the common minerals ¢ ° ° P
Short descriptions of some common rocks . : . : ° 4
The preparation of topographical maps . . . .
. Laboratory models for study in the interpretation ‘of geological maps .
Suggested itineraries for pilgrimages to study earth features . .
INDEX . e e. e e e ° e e e e . e e.
PAGE
401
426
435
PLATE
ray OF PLATES
1. Mount Balfour and the Balfour Glacier in the Selkirks . Frontispiece
FACING PAGE
xix
2. A. Layers compressed in experiments and showing the effect of a com-
petent layer in the process of folding 44
B. Experimental production of a series of aie: ennai wilin
closely folded strata 44
C. Apparatus to illustrate ee action within the eyertarned iat
of a fold 44
8. A. An earthquake fault pene in Hownoes in 1906 witht vertipal ae
lateral displacements combined 72
B. Earthquake faults opened in Alaska in 1889 on “which vettical
slices of the earth’s shell have undergone individual adjustments 72
. 4, A. Experimental tank to illustrate the earth movements which are
manifested in earthquakes. The sections of the earth’s shell are
here represented before adjustment has taken place 82
B. The same apparatus after a sudden adjustment ; 82
C. Model to illustrate a block displacement in rocks which are tate
sected by master joints. ‘ 82
5. A. Once wooded region in China now idnbed ” deuce Gusta ae
forestation 156
B. ‘‘ Bad Lands”? in the Calorie Desert ‘ 156
_ 6. A. Barren Karst landscape near the famous Adewhors eroitees ‘ 188
B. Surface of a limestone ledge where joints have been widened through
solution +1185
7. A. Ranges of dunes pon, the shabytns ot the alseade Desert . 210
B. Sand dunes encroaching upon the oasis of Oued Souf, Algeria 210
8. A. The granite needles of sae! Peak in the Black Hills of South
Dakota . A . 216
B. Castellated erosion sbitinestoys't in El Gots Cafion, how tenis 216
9. Map of the High Plains at the eastern front of the Rocky Mountains. 220
10. A. View in Spitzbergen to illustrate the disintegration of rock under
the control of joints : 228
B. Composite pattern of the joint iedoturee within recent aitwvial
deposits of the Syrian Desert . . . . . 228
11. A. Ripple markings within an ancient sandstone . - ; . 282
B. Wave breaking as it approachesthe shore . . Pigs eae’ 18? gcc
XX LIST OF PLATES
PLATE FACING PAGE
12, A. V-shaped cafion cut in an upland recently elevated from the sea,
San Clemente Island, California . F . 256
B. A “‘hogback”’ at the base of the Bighorn Aounendug: Waban’ . 2656
13. A. Precipitous front of the Bryant Glacier outlet of the Greenland
inland ice. . 272
B. Lateral stream esids the Benedict Glacier batlee Giveniana eee
14. View of the margin of the Antarctic continental glacier in Kaiser
Wilhelm Land . : ° ° : . 282
15. A. An Antarctic ice foot with peat punt aii : ; . 290
B. A near view of the front of the Great Ross Barrier, ‘Aviseotien . 290
16. A. Incised topography within the ‘‘driftless area”? . ‘ ‘ . 3800
B. Built-up topography within the glaciated region. ° 300
17. A. Soled glacial bowlders which show differently directed ae appt
the same facet . 306
B. Perched bowlder upon a aernied edge of ‘aigerent foak type, eis
Park, New York . ; : : ‘ ; : . 806
C. Characteristic knob and basin sintace of a moraine ; 306
18. A. Fretted upland of the Alps seen from the summit of Mount Blane. 372
B. Model of the Malaspina Glacier and the fretted upland above it . 372
19. A. Contour map of a grooved upland, Bighorn Mountains, Wyoming 372
B. Contour map of a fretted upland, eae aces Quadrangle, Mon-
tana. ; 372
20. Map of the surface modeled by metnitian bine in the Siexré Nevadaa
of California ; 876
21. A. View of the Harvard Cision Aas aye ths phataciniene
terraces : ‘ ; . 894
B. The terminal moraine at he aot of a onan alee ; . 894
22. A. Model of the vicinity of Chicago, showing the position of the
outlet of the former Lake Chicago : ; P - 400
B. Map of Yosemite Falls and its earlier site near Raulo Peak : . 400
23. A. View of the American Fall at Niagara, showing the accumulation L
of blocks beneath . : ‘ ‘ ; : . 414
B. Crystal Lake, a landslide lake in alone : ; . 414 §
24. A. Apparatus for exercise in the preparation of fopoarabtie noe . 468 §
B. The same apparatus in use for testing the contours of a map . . 468
C. Modeling apparatus in use : ‘ F : . ; F . 468
ILLUSTRATIONS IN THE TEXT
. Diagram to show the measure of the earth’s surface irregularities
. Map to show the reciprocal relation of areas of land and sea
The tetrahedral form toward which the earth is tending
A truncated tetrahedron to show the reciprocal relation of projection
and depression upon the surface
. Approximations to earlier and present figures of ae enteh :
. Diagrams for comparison of coasts upon an upright and upon an in-
verted tetrahedron .
The continents, including anirpnase Sotlipna
Diagram to indicate the altitude of different parts of the inoecions
surface
Diagram to show how ine fbicetaist mcks sends into ne shetobrites
. Comparison of a crystalline with an amorphous substance
. ‘**Light figure’? seen upon etched surface of calcite . ‘ 5 °
. Battered sand grains which have developed crystal faces
. Unassimilated grains of quartz within a garnet crystal
. New minerals developed about the core of an augite crystal
A common rim of new mineral developed by reaction where earlier
minerals come into contact .
. Laminated structure of a sedimentary rock .
. Characteristic textures of igneous rocks
Diagram to show the order of sediments laid down during a ian
gression of the sea . °
19. Fractures produced by saiipromion ee a Bioek of moldex* 8 Wax
20. Apparatus to illustrate the folding of strata . ; ‘ , ‘ ‘
21. Diagrams of fold types . : 2 : : ’ , .
22. Diagrams to illustrate crustal sorteatile ; : ‘ d ° ‘
23. Anticlinal and synclinal folds ‘ ; , ; ; ‘ .
24. Diagrams to illustrate the shapes of rock folds
. Secondary and tertiary flexures superimposed upon the Sphnahy: ones
. A bent stratum to illustrate tension and compression upon opposite
sides
. A geological section = Giinaatol eotie roihredt
. Diagram to illustrate the nature of strike and dip . :
. Diagram to show the use of T symbols for strike and dip ideneahtbn: :
. Diagram to show how the thickness of a formation is determined
. Aplunging anticline. : ; ; s ; P ‘ ‘ ‘
xxi
PAGE
it
at
12
13
15
17
18
18
22
24
25
26
28
28
28
30
33
37
41
41
42
42
43.
44
44.
45.
47
47
48
49
50
XXll ILLUSTRATIONS IN THE TEXT
FIG PAGE
32. A plunging syncline . : . : ° - - 60
338. An unconformity upon the coast of Calttornia ; ; : 51
34. Series of diagrams to illustrate the episodes involved in the reduction
of an angular unconformity ; 52
35. Types of deceptive or erosional Ge someanmiicies : : . : . 53
36. A set of master joints in shale : ; A . 65
37. Diagram to show the manner of ee eren or: one set of coins by
another . ‘ 56
38. Diagram to show the eeosat cemtinatioae of joint series . 56
39. View of the shore in West Greenland : 57
40. View in Iceland which shows joint intervals of more ean one oder 57
41. Faulted blocks of basalt near Woodbury, Connecticut . : ‘ - 68
42. A fault in previously disturbed strata . 59
43. Diagram to show the effect of erosion upon a fault 60
44. A fault plane exhibiting drag 60
45. Map to show how a fault may be jndicuted = abecnt changes i in strike
and dip . : : : : =, wee
46. A series of parallel fants eareuled by offsets ; , - ; 61
47. Field map prepared from the laboratory table . = . . 64
48. Areal geological map based upon the field map . r 64
49. A portion of the ruins of Messina. : 67
50. Ruins of the Carnegie Palace of Peace at Ountaat Goats Rica . 68
51. Overturned bowlders from Assam earthquake of 1897 . Pei |
52. Post sunk into ground during Charleston earthquake . 69
53. Map showing localities where shocks have been reported at sea off
Cape Mendocino, California . = . wae
54. Effect of seismic water wave in Japan . ; 70
55. A fault of vertical displacement é Pavey f |
56. Escarpment produced by an earthquake fault i in indine A er see
57. A fault of lateral displacement . 72
58. Fence parted and displaced by isteral displaschient 4 on fault during
California earthquake : 72
59. Fault with vertical and lateral displacements eombikud ¢ 72
60. Diagram to show how small faults may be masked at the earth’s sur-
face , 3 73
61. ‘* Mole hill”? effect aoe buried pataaats. iui ‘ : " 73
62. Post-glacial earthquake faults : ‘ Wege sae aaa
63. Earthquake cracks in Colorado desert . . 4
64. Railway tracks broken or buckled at time of barthquaks 75
65. Railroad bridge in Japan damaged by earthquake . he
66. Diagrams to show contraction of earth’s crust i an Sarbanes 76
67. Map of the Chedrang fault of India s 16
68. Displacements along earthquake fault in Alaska . 17
Abrupt change in direction of throw upon an earthquake fault 77
. Map of faults in the Owens Valley, California, formed during earth-
quake of 1872.
402.
ILLUSTRATIONS IN THE TEXT XXdii
FIG,
71. Marquetry of the rock floor in the Tonopah district, Nevada.
72. Map of Alaskan coast to show adjustments of level during an earth-
quake
78. An Alaskan shore picsatea. payenisen feet guise ‘se enrotauake of
1899
74, Partially submerged foieat feet akarenter ‘of aot in oe ques
earthquake
75. Effect of settlement of the sibre at Part Reval during cues of
1907 ,
76. Diagrams to illustrate ne ae of halos nanae Spain tucices
77. Diagram to illustrate the derangements of water flow during an
earthquake
78. Mud cones aligned upon an Appaidtiake Austin] in een
79. Craterlet formed near Charleston, South Carolina, during the earth-
quake of 1886 ‘ F : ‘ ; :
80. Cross section of a craterlet . ; " e
81. Map of the island of Ischia to show the eolidentiation of snerhonnes
shocks .
82. A line of earth fracture iovenled in ‘he plat of ie relief
83. Seismotectonic lines of the West Indies
84, Device to illustrate the different effects of fradake in lem foee
and in loose materials .
85. House wrecked in San Francisco earth
86. Building wrecked in California earthquake by roof and poet Poor
battering down the upper walls
87. Breached volcanic cone in New Zealand showing the pending aoe
of the strata near the vent
88. View of the new Camiguin volcano formed | in 1871 in the Plilippines
89. Map to show the belts of active volcanoes . . .
90. A portion of the ‘‘ fire girdle’ of the Pacific ;
91. Volcanic cones formed in 1783 above the Skaptar fissure in osland ,
92. Diagrams to illustrate the location of volcanic vents upon fissure lines
93. Outline map showing the arrangement of volcanic vents upon the
island of Java ‘ ‘
94. Map showing the migration of ‘insides aie a fasare ;
95. Basaltic plateau of the moti woriers United States due to fisstize
eruptions of lava . 3 : ; ‘ ,
96. Lava plains about the Snake River j in Taaho ‘. i ‘ >
97. Characteristic profiles of lava volcanoes. : : ‘ ;
98. Adribletcone . ‘
99. Leffingwell Crater, a nian cone in the Guens Valley, California
100. Map of Hawaii and its lava volcanoes
‘101. Section through Mauna Loa and Kilauea
Schematic diagram to illustrate the moving wlsifort in the erator of
Kilauea. : : . : . -
103. View of the open lava Jalen of Haibasurant bah bcs . :
PAGE
79
79
80
80
80
83
84
84
85
85
87
87
88
88
90
91
96
97
98
98
99
100
100
101
102
102
108
. 104
104
106
106
107,
108
XXIV ILLUSTRATIONS IN THE TEXT
FIG.
104.
Map to show the manner of outflow of the lava from Kilauea in the
eruption of 1840
5. Lava of Matavanu flowing pon to ins sea ine due een of
1906
. Lava stream setae aa he sea ee a ie fennel
. Diagrammatic representation of the structure of lava voleanoes as a
result of the draining of frozen lava streams
. Diagram to show the formation of mesas by outflow of evan in valleys
and subsequent erosion
. Surface of lava of the Pahoehoe type .
. Three successive views to show the growth of the island of Sava
from lava outflow in 1906
. View of the voleano of Stromboli sowie the pxoeneee position of
the crater
. Diagrams to illustrate the siplions mith the cakae of Sironiboll
. Map of Volcano in the Xolian Islands
‘¢ Bread-crust ’’ lava projectile from the eruption of Volcano’ in 1888 .
‘¢ Cauliflower cloud’’ of steam and ash rising above the cinder cone
of Volcano
. Eruption of Taal volcano in 1911 seen fecal a distnva oie six es
. The thick mud veneer upon the island of Taal (after a Or ay
by Deniston)
. A pear-shaped lava Paeetile : ‘
. Artificial production-of a cinder cone .
. Diagram to show the contrast between a lava dimie andl a cinder cone
. Mayon volcano on the island of Luzon, Philippine Islands .
. A series of breached cinder cones due to migration of the eruption
along a fissure
. The mouth upon the inner cone of Mount iris foi which fismed
the lava of 1872
. A row of parasitic cones raised above a fesuite enna on ‘dhe Annies
of Etna in 1892
. View of Etna, showing the partesiiie cones upon its fizuike ?
. Sketch map of Etna to show the areas covered by lava and tuff re-
spectively
. Panum crater showing the Airs ;
. View of Mount Vesuvius before the eruption of 1906 .
. Sketches of the summit of the Vesuvian cone to bring out the changes
in its outline.
Night view of Vesuvius fon NapIos petite fie Aatheeak of "1906,
showing a small lava stream descending the central cone
. Scoriaceous lava encroaching upon the tracks of the Vesuvian railway
. Map of Vesuvius, showing the position of the lava mouths opened
upon its flanks during the eruption of 1906 .
. The ash curtain over Vesuvius lifting and disclosing the outlines of.
the mountain ; ; : s ; a z ‘
131
182
——- = Oe es
FIG.
134,
135.
136.
137.
138.
139.
140.
141.
142.
143.
144,
145.
146.
147.
148.
149.
150.
151.
152.
183.
+164.
155.
156.
157.
158.
159.
160.
161.
162.
168.
R164.
166.
166.
167.
ILLUSTRATIONS IN THE TEXT
The central cone of Vesuvius as it appeared after the eruption of 1906
A sunken road upon Vesuvius filled with indrifted ash
View of Vesuvius from the southwest during the waning stages of
the eruption .
The main lava stream aaeanciis Aeon aonconecase ‘
A pine snapped off by the lava and carried forward upon its duthices
Lava front pushing over and running around a wall in its path .
One of the ruined villas in Boscotrecase ;
Three diagrams to illustrate the sequence of events diting the" cone-
building and crater-producing periods . ;
The spine of Pelé rising above the chimney of the alee aie ihe
eruption of 1902 : ,
Successive outlines of the Pelé spine . :
Corrugated surface of the Vesuvian cone due to the nad few euleh
followed the eruption of 1906 ; : :
View of the Kammerbiihl near Eger in Bonaniin :
Volcanic plug exposed by natural dissection of a ene cone in
Colorado
A dike cutting beds of tuff in a pees discetea volune of south
western Colorado . ‘
Map and general view of St. Paul’s make. a Solow: cone aieecien
by waves , , : ‘
Dissection by beplesion of Little Paid in 1888 . ‘ :
The half-submerged volcano of Krakatoa before ‘gp after the ae
tion of 1883 . : ‘ ‘ ‘ : . : , ‘ ‘
The cicatrice of the Banat . :
Diagram to illustrate a probable cause of fopunktion of ae reservoirs
and the connection with volcanoes upon the surface
Effect of relief of load upon rocks by arching of a competent an
tion .. pe ‘
Character profiles dGnitediaa with veesioes
Diagrams to show the effect of aaa in prodseiug sptieraidal
bowlders : @ , ; -
Spheroidal weathering of an aidan ork ; : : ‘
Dome structure in granite mass . a ‘ ‘ ; ,
Talus slope beneath a cliff .
Striped ground from soil flow ‘
Pavement of horizontal surface due to soil ‘iow ‘
Tree roots prying rock apart on fissure , ;
Bowlder split by a growing tree . ; . ‘ ,
Rock mantle beneath soil and vegetable mat ‘ : ‘
Diagram to show the varying thickness of mantle rock ana the
different portions of a hill surface ‘ ‘ ;
Gullies from earliest stage of a river’s life . : .
Partially dissected upland . : ‘ F
Longitudinal sections of upper portion of a river ently .
XXV
PAGE
182
133
133
133
1338
134
134
135
136
137
138
139
140
140
141
141
142
142
148
144
146
150
151
152
153
154
154
154
155
155
156
160
160
161
XXV1 ILLUSTRATIONS IN THE TEXT
FIG.
168.
169.
170.
abate
172.
173.
174.
175.
176.
Ler.
178.
Lo:
180.
181.
182.
183.
184.
185.
186.
187.
188.
189.
190.
191.
192.
193.
194.
195.
196.
197.
198.
199.
200.
201.
202.
203.
204.
PAGE
Map and sections of a stream meander . : : ° . . 168
Tree undermined on the outer bank of ameander . : ‘ G4
Diagrams to show the successive positions of stream meanders . - 164
An ox-bow lake in the flood plain of a river. . : ‘ ; . 165
Schematic representation of a series of river terraces . : : . 165
‘¢ Bird-foot’’ delta of the Mississippi River ‘ 167
Diagrams to show the nature of delta deposits as exhibited in sec-
tions. . . : ° ‘ ; . 168
Gorge of the River Rhine near St. Gaara ‘ : 169
Valley with rounded shoulders characteristic of the eee of aaoles
cence . ; : : , : i , 5 ATO?
View of a maturely dissected dpiaale P : ; , f ree it
Hogarth’s line of beauty... 171
View of the oldland of New sass: ial Monkt Motaduook: rising
in the distance. : emee i |
Comparison of the cross sections a river valleval of different stages . mae
The Beavertail Bend of the Yakima River . : : : : er i
A rejuvenated river valley . oo, : i : : : . 174
Plan of a river narrows ; bea y € |
Successive diagranis to illustrate the origin of ae trellis ‘ienionce? FF eS TS
Sketch maps to show the earlier and present ipa near Harper’s
Ferry . : : ‘ ‘ ., 1
Section to illustrate five oy of ae cae. ; 177
Character profiles of landscapes shaped by stream erosion in iii
climates ; 177
Diagram to show the Sepak eee in ale poieon of the wee table 180
Diagram to show the effect of an impervious layer upon the descend-
ing water. . A . . » at
Sketch map to illustrate corrosion of finiastone along two series of
vertical joints : : ° ° « 26h
Diagram to show the relation of (neaeaare caverns to the river system
of the district ; , él. aye . . 182
Plan of a portion of Mammoth Gave, en : ; . 183
Trees and shrubs growing upon the bottoms of limestone Sica . 183
Diagrams to show the manner of formation of stalactites and stalag-
mites . ; : ; ‘ ; : . 185
Sinter formations in the Tneag e caverns. ° : ° ‘ . 186
Map of the dolines of the Karst region . ‘ ‘ : ° oy TBF
Cross section of a doline formed by inbreak ‘ ‘ 5 * . 187
Sharp Karren of the Ifenplatte . F : ; " . . 188
The Zirknitz seasonal lake . : ‘ ; . 189
Fissure springs arranged at ineectels of cae fretures , ‘ 190
Schematic diagrams to illustrate the different types of artesian walla’ 191
Cross section of Geysir, Iceland . : > , . . 192
Apparatus for simulating geyser action : ‘ ‘ ° ve 193
Cone of siliceous sinter about the Lone Star Geyser é i ‘ ioe
FIQG.
205. Former shore lines in the Great Basin : ; ‘ ‘
206. Map of the former Lake Bonneville . : : ‘
207. Borax deposits in Death Valley, California
208. Hollowed forms of weathered granite in a desert of Central Aals
209. Hollow hewn blocks in a wall in the Wadi Guerraui .
210. Smooth granite domes shaped by exfoliation
211. Granite blocks rent by diffission :
212, ‘‘Mushroom Rock”’ from a desert in Wyomiue :
213. Windkanten shaped by sand blast in the desert . ey
214. The ‘stone lattice ’’ of the desert
215. Shadow erosion in the desert
216. Cliffs in loess with characteristic vertical joan
217. A cafion in loess worn by traffic and wind .
218. Diagrams to illustrate the effects of obstructions in Rees ere
driven sand . :
219. Sand accumulating on either aide of a AS maa sceghotrebls see.
tion
220. Successive dindewnue is insets the Mater of ie ‘on of Kaen
upon the Kurische Nehrung .
221. View of desert barchans '
222. Diagrams to show the relationships of ici to sand saneiy ead wind
direction
223. Ideal section showing ‘the visite niguniat wall saat a pense ae
? the neighboring slope
_ 224. Dry delta at the foot of a range ao tbe onda of a rdoacre
q 225. Map of distributaries of streams which issue at the western base of
| the Sierra Nevadas .. ;
_ 226. A group of ‘‘demoiselles’’ in the “ bad ‘ands ide
_ 227. Amphitheater at the head of the Wadi Beni Sur :
_ 228. Mesa and outlier in the Leucite Hills of hog
— 229. Flat-bottomed basin separating dunes
Lop
— 231. Schematic diagram to show the zones of “deposition in their Sider
from the margin to the center of a desert . ‘ d ; :
. 932. Mounds upon the site of the buried city of Nippur_. : ‘ ;
p 238. Exhumed structures in the buried city of Nippur
a 235, Section across the lenticular threads of slinvial ‘deposits of the High
Plains .
_ 236. Distributaries of the foot hills taserinvowed sion an entias series
9 . Character profiles in the landscapes of arid lands ; - ‘
ILLUSTRATIONS IN THE TEXT XXVIl
XXVIll ILLUSTRATIONS IN THE TEXT
PAGE
249, Controlled drainage network of the Shepaug River in Connecticut . 226
243. A river network of repeating rectangular pattern ; , 226
244, Squared mountain masses which reveal a distribution of one in
block patterns of different orders. . : , : ; . 228
245. Island groups of the Lofoten Archipelago . ; — 229
246. Diagrams to illustrate the composite profiles of 8 sland on the
Norwegian coast . , . 229
247. Diagram to show the nature of ae icone in a pe re ee wave 2381
248. Diagram to illustrate the transformation of a free wave into a breaker 2382
249. Notched rock cliff and fallen blocks . : ° ; ; é . 288
250. A wave-cut chasm under control by joints . ; : . 283
251. Grand Arch upon one of the Apostle Islands in Lake Sapeor ; . 234
252. Stack near the shore of Lake Superior , , . 284
253. The Marble Islands, stacks in a lake of the piieh Apidae : . 2385
254. Squared stacks revealing the position of the joint planes on which
they were carved . : ; j : ; . 835
255. Ideal section cut by waves upon a ateat cle anos ; ‘ 236
256. Map showing the outlines of the island of Heligoland at different
stages inits history. . 236
257. Ideal section carved by waves upon a ices feiore of ioe roapariels . 287
258. Sloping cliff and boulder pavement at Scituate, Massachusetts . - 2387
259. Map to show the nature of the shore current and the forms which are
molded by it. ‘ ; F é $ . 2388
260. Crescent-shaped beach in tne lee “of a néadiand « ‘ ; : 285
261. Cross section of a beach pebble . ‘ é , ° . 239
262. A storm beach on the northeast shore of Ghesh Ray ; ; . 240
263. Spit of shingle on Au Train Island, Lake Superior . : A . 240
264, Barrier beach in front of a lagoon ; : ° . 241
-265. Cross section of a barrier beach with inten’) in its rear. , . 242
266. Cross section of a series of barriers and an outer bar . ¢ 242
267. A barrier series and an outer bar on Lake Mendota at Madisots
Wisconsin . : ‘ ; . 242
268. Series of barriers at the West saa of Lake Bacertue ; ; . 243
269. Character profiles resulting from wave action upon shores . ; . 248 -
270. The even shore line of a raised coast . : : : ‘ ; . 246
271. The ragged coast line produced by subsidence . ‘ . . 246
272. Portion of the Atlantic coastal plain at the base of the adiand i: . 246
273. Ideal form of cuestas and intermediate lowlands carved from acoastal —
plain. : > : ; 2 . 247
274. Uplifted sea cave on ‘ie aie of California ; ; ‘ : . 248
275. Double-notched cliff near Cape Tiro, Celebes . Les ° . 248 .
276. Uplifted stacks on the coast of California . x 249
277. Uplifted shingle beach across the entrance to a Format bay aah the
coast of California ; ‘ . 250
278. Raised beach terraces near Elie, Fife, ‘Boctlantis ‘ . =. 25a
279. Uplifted sea cliffs and terraces on he Alaskan coast . . . ‘ . 250
ee ee
ee ee
i
—— oe
*
| os
ae ae ve, hae. .
Pe Ae
FIG.
280.
281.
282.
283.
284.
285.
286.
287.
288.
289,
290.
291.
292.
298.
294.
295.
296.
297.
298.
299,
300.
301.
302.
303.
304.
305.
306.
307.
308.
309.
ILLUSTRATIONS IN THE TEXT
Diagrams to show how excessive sinking upon the sea floor will cause
the shore to migrate landward ;
A drowned river mouth or estuary upon a noite! Sah
Archipelago of steep rocky islets due to submergence
The submerged Hudsonian channel which continues the Pigeon
River across the continental shelf ;
Marine clay deposits near the mouths of the Maine rivers non pre-
serve a record of earlier subsidence and later elevation ;
View of the three standing columns of the Temple of Jupiter noe
at Pozzuoli
Three successive views to ack forth the eee oscillating of feqal on
the northern shore of the Bay of Naples
Relief map of San Clemente Island, California
Relief map of Santa Catalina Island, California . ‘ .
Cross section of the Blue Grotto, on the island of Capri. ;
Character profiles of coast elevation and subsidence .
Map showing the distribution of existing glaciers and the two poe
tant wind poles of the earth . ‘
An Alaskan glacier eee out at the tot of the ace which
nourishes it .
Surface of a glacier whose see jepers sonea with but sent ceatehing
from retaining walls ‘ :
Section through a mountain glacier . . ; .
Profile across the largest of the Icelandic ice caps
Ideal section across a continental glacier
View of the Eyriks Jékull, an ice cap of Iceland ;
The zones of the lower ree as revealed by recent kite and
balloon exploration
Map of Greenland, showing aie area of raladd ice and the toutes of
explorers
Profile in natural proporlisis across the wouthart ead of the gai
nental glacier of Greenland .
Map of a glacier tongue with dimple above
Edge of the Greenland inland ice, showing the naniiaks diminishing
in size toward the interior . ‘ A ’ ‘ ‘
Moat surrounding a nunatak in Victoria mee . : ‘ :
A glacier pavement of Permo-Carboniferous age in South Africa
Diagrams to illustrate the manner of formation of scape colks
Marginal moraine now forming at the wy of the continental glacier
of Greenland ,
Small lake between the ice front ‘and a moraine while it haus mostly
built. : ‘ .
View of a drained lake do tisas iebween the ice setont and an atau:
doned moraine :
Diagrams to show the manner of formation and the nenture of an
outwash plain and fosse . : . ‘ : ‘ .
. Lake and marsh district in northern Wiseonaln
. Cross section in natural proportion of the latest North Amerie
- Moraine with outwash apron in otk,
- Fosse between an outwash plain and a moraine .
340.
. Outline map of moraines and eskers in Finland .
ILLUSTRATIONS IN THE TEXT
. Map of the ice masses of Victoria Land, Antarctica . s ;
. Sections across the inland ice and the shelf ice of Antarctica
2. Diagram to show the nature of the fixed glacial anticyclone above
continental glaciers
. Snow deltas about the margins of a elacien tongue in Geers
. View of the sea ice of the Arctic region. .
. Map of the north polar regions, showing the area of drift i ice and te
tracks of the Jeannette and the Fram .
. The shelf ice of Coats Land with surrounding pack ice
. Tide-water cliff on a glacier tongue from which icebergs are born
. A Greenlandic iceberg after a long journey in warm latitudes
. Diagram showing one way in which northern icebergs are born from
the glacier tongue . : : ‘ nse ce ‘
. A northern iceberg surrounded = seaice .
. Tabular Antarctic iceberg separating from the shelf ice
. Map of the globe, showing the areas covered continental eaciens
during the ‘‘ ice age”’
. Glaciated granite bowlder Sy neied ue of a moraine ‘of Perio:
Carboniferous age, South Australia
. Map to show the glaciated and as regions CS North
America
. Map of the glaciated and nofmeintede areas ar northern evens
. An unstable erosion remnant characteristic of the ‘‘ driftless area’ .
. Diagram showing the manner in which a continental glacier obliter-
ates existing valleys
continental glacier
. Diagram showing the earlier and ine twice olaclae reodids together
upon the same limestone surface .
. Map to show the outcroppings of peculiar rock types in the raion
of the Great Lakes, and some localities where “drift copper”?
has been collected
. Map of the ‘‘ bowlder train ” on ion Hill, Rhode Island:
Shapes and approximate natural sizes of some of the seater sine from
the Great Lakes region .
Glacial map of a portion of the Great Lakes region
. Section in coarse till
. Sketch map of portions of ‘Michi au, Ohio, “ail Indiand, showing the
distribution of moraines
. Map of the vicinity of Devil’s Lake, Wisconsin: vanly sorerel by
the continental glacier .
View along an esker in southern Maine : é . . :
304
305
306
307
308 ©
310
312
313
313
314
515
315
Pe Pe ee ee ee ee
a oe
FIG.
842.
3438.
344,
345.
346.
347.
348.
349.
350.
351.
352.
353.
354.
355.
356.
357.
358.
359.
360.
361.
362.
363.
364.
365.
366.
367.
368.
369.
370.
371.
372.
3738.
374.
375.
376.
ILLUSTRATIONS IN THE TEXT XXX1
Sketch maps showing the relationships of drumlins and eskers .
View of a drumlin, showing an opening in the till
Outline map of the front of the Green Bay lobe to show the relation:
ships of drumlins, moraines, outwash plains, and ground moraine
Character profiles referable to continental glacier P
View of the flood plain of the ancient Illinois River near Pesiih
Broadly terraced valleys which mark the floods that once issued from
the continental glacier of North America ;
Border drainage about the retreating ice front south of Dats Erie
The ‘‘ parallel roads’’ of Glen Roy in the Scottish Highlands é
Map of Glen Roy and neighboring valleys of the Scottish Highlands .
Three successive diagrams to set forth the late glacial lake history of
the Scottish glens . ;
Harvesting time on the fertile rae of ie siacal Take Acaaie ;
Map of Lake Agassiz . . °
Map showing some of the padehes of Lake Apuats ail its outlet
Narrows of the Warren River where it ee between jaws of granite
and gneiss :
Map of the valley of the Waren Hives near F Miieapulls ‘
Portion of the Herman beach on the shore of the former Lake Meeele
Map of the continental glacier of North America when it covered the
entire St. Lawrence basin
Outline map of the early Lake Maumee ‘
Map to show the first stages of the ice-dammed lakes eihin oe
St. Lawrence basin
Outline map of the later Lake Navan ‘snd its catlet
Outline map of lakes Whittlesey and Saginaw
Map of the glacial Lake Warren . ‘
Map of the glacial Lake Algonquin ‘
Outline map of the Nipissing Great Lakes .
Probable preglacial drainage of the upper Ohio piri
Diagrams to illustrate the episodes in the recent history of a roe
necticut river ‘
The notched rock headland of Bavay Bluff ¢ on tans Michigan
View of Mackinac Island from the direction of St. Ignace .
The “ Sugar Loaf,’’ a stack of Lake Algonquin upon Mackinac Tana
Beach ridges in series on Mackinac Island . ;
Notched stack of the Nipissing Great Lakes at St. lunkse ;
Series of diagrams to illustrate the evolution of ideas concerning the
uplift of the lake region since the Ice Age
Map of the Great Lakes region to show the isobases and hinge lines of
uptilt
Series of diagrams to bidipate the gitare of the recovery of the direst
by uplift when unloaded of an ice mantle .
Portion of the Inner Sandusky Bay, for comparison of the shove line
of 1820 with that of to-day . ° s ‘ wih ths .
XXxii ILLUSTRATIONS IN THE TEXT
_ Ideal cross section of the Niagara Gorge to show the marginal terrace
. View of the bed of the Niagara River above the cataract where water
has been drained off . ‘ ; ;
9. View of the Falls of St. Anthony in 1851
. Ideal section to show the nature of the drilling process Honeah the
cataract
. Plan and section of the gorge, sbowine haw the depth is Spariioual
to the width .
. Comparative views of the Cupnaias Falls in 1827 ‘cate 1895
. Map to show the recession of the Canadian Fall
. Comparison of the present with the future falls
5. Bird’s-eye view of the captured Canadian Fall at once Flats
. Map of the Whirlpool Basin
7. Map of the cuestas which have aivaeal SO Estee a Aart in fixiiig
the boundaries of the lake basins .
. Bird’s-eye view of the cuestas south of Lakes Gutine. and re
. Sketch map of the greater portion of the Niagara Gorge to illustrate
Niagara history
. Snowdrift hollowing its bed ae nanetiOn :
. Amphitheater formed upon a drift site in northern Een
. The marginal crevasse on the highest margin of a glacier
. Niches and cirques in the Bighorn Mountains of Wyoming
. Subordinate cirques in the amphitheater on the west face of the
Wannehorn .
. ‘Biscuit cutting ’’ effect of plgeial Gee in ne Uinta ‘Mowstats
of Wyoming .
. Diagram to show the cause of the Tenerhalic curve of cols.
. A col in the Selkirks ;
. Diagrams to illustrate the forintion of sorab eases ool: md ovat
. The U-shaped Kern Valley in the Sierra Nevadas of California .
. Glaciated valley wall, showing the sharp line which separates the
abraded from the undermined rock surface .
. View of the Vale of Chamonix from the séracs of the Glacier eae
Bossons : :
. Map of an area near the eentinenid divide i in Coinrads
. Gorge of the Albula River in the Engadine cut through a rock bar
. Idealistic sketch, showing glaciated and nonglaciated side valleys
. Character profiles sculptured by mountain glaciers. : °
. Flat dome shaped under the margin of a Norwegian ice cap ‘
. Two views which illustrate successive stages in the shaping of tinds .
. Schematic diagram to bring out the relationships of the various types
of mountain glaciers
. Map of the Malaspina Glacier of iaics = ‘
- Map of the Baltoro Glacier of the Himalayas. .
. View of the Triest Glacier, a hanging glacieret . .
Map of the Harriman Fjord Glacier of Alaska . P
ILLUSTRATIONS IN THE TEXT _ XXX
FIG. PAGE
418. Map of the Rotmoos Glacier, a radiating glacier of Switzerland . . 386
414, Outline map of the Asulkan Glacier in the Selkirks, a horseshoe
glacier . ; 387
415. Outline map of the Tleciliowsat Gincier of the Selkirks, an inherited:
basin glacier . : < : 4 P . 388
416. Diagram to illustrate the sete aoe of leet : ; . 890
417. Diagram to show the transformation of crevasses into setae ; . 891
418. View of the Glacier des Bossons, showing the position of accidents
to Alpinists . . - 3892
| 419. Lines of flow upon the suztads of zis anes Ginetars in the
q Alps. ; . 893
420. Lateral and medial moraines sot om Mer i Cisse antl its frinntaticn, 393
| 421. Ideal cross section of a mountain glacier. : 394
422. Diagrams to illustrate the melting effects upon giseier tt ice of roe
: fragments of different sizes . ‘ ‘ ‘ ‘ . 3894
| 423. Small glacier table upon the Great Aletsch Glacier , F 895
_ 424, Effects of differential rights and subsequent re-freezing upon a Sates
: surface . ; : a a eee
425. Dirt cone with its paste in port randved ‘ ‘ ‘ . 3896
_ 426. Schematic diagram to show the manner of formation of adiee cornices 3897
_ 427. Superglacial stream upon the Great Aletsch Glacier . ; . 898
_ 428. Ideal form of the surface left on the site of a piedmont Sasena apron 3899
= 429. Map of the site of the earlier piedmont glacier of the Upper Rhine . 399
. 430. Diagram and map to bring out the characteristics of newland lakes . 402
431. View of the Warner Lakes, Oregon . ‘ . 402
432. Schematic diagram to illustrate the characteristics of ashe sine lakes 403
433. Schematic diagram of rift-valley lakes and the valley of the Jordan . 403
434. Map of the rift-valley lakes of East Central Africa . ‘ 404
435. Earthquake lakes formed in re in the flood plain of the Tower
Mississippi. ‘ ‘ . ; é ‘ . 404
436. View of a crater lake in Costa Rica : : : : . 405
437. Diagrams to illustrate the characteristics of Gentax tacos - i . 406
438. View of Snag Lake, a coulée lake in California . ; ; . 406
439. Diagrams to illustrate the characteristics of morainal lakes : . 407
440. Diagram to show the manner of formation of pitlakes . : - 408
441. Diagrams to illustrate the characteristics of pit lakes . : ‘ . 408
442. Diagram to show the manner of formation of glint lakes . : 409
443. Map of a series of glint lakes on the boundary of Sweden and MBraiy 409
444, Map of ice-dam lakes near the Norwegian boundary of Sweden . . 410.
445. Wave-cut terrace of a former ice-dam lake in Sweden ; : . 410
446. View of the Marjelen Lake from the summit of the Eggishorn . . 411
447. Diagrams to illustrate the arrangement and the characters of rock-
basin lakes. ; ‘ ‘ : - . 412
448. Convict Lake, a valley-moraine lake of California . : 413
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449. Lake basins produced by successive slides from the steep wills ue a
glaciated mountain valley . : 8 , , F we, 414
SS a > ac?
CL ok ee. ‘<
oui Ca en VT
XXXIV ILLUSTRATIONS IN THE TEXT
FIG.
450.
451.
452.
453.
454.
455.
456.
457.
458.
459.
460.
461.
462.
463.
464.
465.
. Various forms of ice ramparts
. Map of Lake Mendota, showing the poriion of the vinge which forihd
. A zone of diverse displacement in the wanert United States
. Section of an East African block mountain
. Tilted crust blocks in the Queantoweap valley . .
. View of the laccolite of the Carriso Mountain
. Map of laccolitic mountains
. Ideal sections of laccolite. and pramallis ‘
. The gabled facade largely developed in desert ‘Kaden ;
. Balloon view of the Mythen in Switzerland
. The battlement type of erosion mountain .
. Symmetrically formed low islands repeated in ranks shen Temagarnd
. Forms of crystals of a number of minerals .
Lake Garda, a border lake upon the site of a piedmont apron .
Diagrams to bring out the characteristics of ox-bow lakes .
Diagramatic section to illustrate the for mation of saucer-like bade:
between the levees of streams on a flood plain . . 7
Saucer lakes upon the bed of the former river Warren
Levee lakes developed in series within meanders in a delta ti
Raft lakes along the banks of the Red River in Arkansas and Louisiana
Map of the Swiss lakes Thun and Brienz . , ; ; ? .
Delta lakes formed at the mouth of the Mississippi : :
Delta lakes at the margin of the Nile delta . : ‘ , ‘.
Diagrams to illustrate the characteristics of barrier ince
Dune lakes on the coast of France
Sink lakes in Florida, with a schematic ‘Alsoeatn ‘0 fllusteate the
manner of their formation
Map of the Arve and the Upper Rhone
View of the Arve and the Rhone at their function
A village in Switzerland built upon a strath at the ead of "Bake
Poschiavo : :
View of the floating bog ane eiironndiae zones of venetian in a
small glacial lake . : :
. Diagram to show how small aes are idanatoned into peat bogs .
. Map to show the anomalous position of the delta in Lake St. Clair
. A bowlder wall upon the shore of a small lake . : ‘ ‘
. Diagrams to show the effect of ice shove in producing ice > ranperes
upon the shores of lakes ©
from ice expansion and the ice ramparts upon the shores
. The great multiple mountain arc of Sewestan, British India
. Diagrams to illustrate the theories of origin of mountain arcs
. Festoons of mountain arcs about the borders of the Pacific Ocean
. The interrupted Armorican Mountains common to western Europe
and eastern North America .
e
.
Lake, Ontario : ‘. j ; . 8
Forms of crystals of a SR a of eainewane:
FIG.
488.
489.
490.
491.
492.
493.
ILLUSTRATIONS IN THE TEXT XXXV
A student’s contour map ;
Models to represent outcrops of ee
Special laboratory table set with a problem in »geologea mapping
which is solved in Figs. 47 and 48
Three field maps to be used as suggestions in ane aboeatete
table for problems in the preparation of areal geological maps
Sketch map of Western Scotland and the Inner Hebrides to show
location of some points of special geological interest .. :
Outline map of a geological pilgrimage across the continent of Europe
PAGE
469
472
472
473
481
483
5
4
EXPLANATORY LIST OF ABBREVIATIONS FOR
JOURNAL NAMES IN READING REFERENCES
Am. Geol. : American Geologist.
Am. Jour. Sci.: American Journal of Science, New Haven.
Ann. de Géogr.: Annales de Géographie, Paris.
Ann. Rept. Geol. and Geogr. Surv. Ter.: Annual Report of the Geological and
Geographical Survey of the Territories (Hayden), Washington.
Ann. Rept. Geol. and Nat. Hist. Surv. Minn. : Annual Report of the Geological
and Natural History Survey of Minnesota, Minneapolis.
Ann. Rept. Mich. Geol. Surv.: Annual Report of the Michigan Geological Sur-
_ vey, Lansing.
Ann. Rept. U. S. Geol. Surv.: Annual Report of the United States Geological
Survey, Washington.
Bull. Am. Geogr. Soc.: Bulletin of the American Geographical Society, New
York:; -
Bull. Earthq. Inv. Com. Japan: Bulletin of the Earthquake Investigation Com-
mittee of Japan, Tokyo.
Bull. Geogr. Soc. Philadelphia: Bulletin of the Geographical Society of Phila-
delphia.
Bull. Geol. Soc. Am.: Bulletin of the Geological Society of America.
Bull. Mus. Comp. Zoél.: Bulletin of the Museum of Comparative Zodlogy,
Harvard College, Cambridge.
Bull. N. Y. State Mus. : Bulletin of the New York State Museum, Albany.
Bull. Soc. Belge d’Astronomie: Bulletin de la Société Belge d’Astronomie,
Brussels.
Bull. Soc. Belge Géol. : Bulletin de la Société Belge de Géologie, Brussels.
Bull. Soc. Sc. Nat. Neuchatel : Bulletin de la Société des Sciences Naturelles de
Neuchatel.
Bull. Univ. Calif. Dept. Geol. : Bulletin of the University of California, Depart-
ment of Geology, Berkeley.
Bull. U. S. Geol. Surv.: Bulletin of the United States Geological Survey,
Washington.
Bull. Wis. Geol. and Nat. Hist. Surv. : Bulletin of the Wisconsin Geological and
Natural History Survey, Madison.
C. R. Cong. Géol. Intern.: Comptes Rendus de la Congrés Géologique Inter-
nationale.
Dept. of Mines, Geol. Surv. Branch, Canada: Department of Mines, Geological
Survey Branch, Canada.
XXXVIli
XXXVill EXPLANATORY LIST OF ABBREVIATIONS
Geogr. Abh. : Geographische Abhandlungen.
Geogr. Jour. : Geographical Journal, London.
Geol. Folio U. S. Geol. Surv. : Geological Folio of the United States Geological
Survey.
Geol. Mag. : Geological Magazine, London (sections designated by decades).
Jour. Am. Geogr. Soc. : Journal of the American Geographical Society, New
York.
Jour. Coll. Sci. Imp. Univ. Tokyo: Journal of the College of Science of the
Imperial University, Tokyo, Japan.
Jour. Geol.: Journal of Geology, Chicago.
Jour. Sch. Geogr. : Journal of School Geography.
Livret Guide Cong. Géol. Intern.: Livret Guide Congrés Géologique Inter-
nationale. .
Mem. Geol. Surv. India: Memoirs of the Geological Survey of India, Calcutta.
Mitt. Geogr. Ges. Hamb.: Mitteilungen der Geographische Gesellschaft, Ham-
burg.
Mon. U. S. Geol. Surv.: Monograph of the United States Geological Survey,
Washington. '
Nat. Geogr. Mag.: National Geographic Magazine, Washington.
Nat. Geogr. Mon.: National Geographic Monographs, American Book Com-
pany, New York. 7
Naturw. Wochenschr. : Naturwissenschaftliche Wochenschrift.
Pet. Mitt.: Petermanns Mittheilungen aus Justus Perthes’ Geographischer
Anstalt, Gotha.
Pet. Mitt., Ergainzungsh. or Erg.: Petermanns Mittheilungen, Gotha (Ergin-
zungsheft or Supplementary Paper).
Phil. Jour. Sci.: Philippine Journal of Science, Manila.
Phil. Trans. : Philosophical Transactions of the Royal Society, London.
Proc. Am. Acad. Arts and Sci.: Proceedings of the American Academy of Arts
and Sciences.
Proc. Am. Assoc. Adv. Sci. : Proceedings of the American Association for the
Advancement of Science.
Proc. Am. Phil. Soc.: Proceedings of the American Philosophical Society,
Philadelphia.
Proc. Bost. Soc. Nat. Hist.: Proceedings of the Boston Society of Natural
History, Boston.
Proc. Ind. Acad. Sci. : Proceedings of the Indiana Academy of Science.
Proc. Linn. Soc. New South Wales: Proceedings of the Linnean Society of
New South Wales. .
Proc. Ohio State Acad. Sci.: Proceedings of the Ohio State Academy of Science.
Prof. Pap. U. S. Geol. Surv.: Professional Paper of the United States Geologi-
cal Survey, Washington.
Pub. Carneg. Inst. : Publication of the Carnegie Institution of Washington.
Pub. Mich. Geol. and Biol. Surv. : Publication of the Michigan Geological and
Biological Survey, Lansing.
Quart. Jour. Geol. Soc. Lond.: Quarterly Journal of the Geological Society,
London.
EXPLANATORY LIST OF ABBREVIATIONS xxxix
Rept. Brit. Assoc. Adv. Sci.: Report of the British Association for the Advance-
ment of Science.
' Rept. Geol. Surv. Mich. : Report of the Geological Survey of Michigan, Lansing.
Rept. Mich. Acad. Sci.: Report of the Michigan Academy of Science, Lansing.
Rept. Nat. Conserv. Com.: Report of the National Conservation Commission,
Washington.
_ Rept. Smithson. Inst. : Report of the Smithsonian Institution, Washington.
Sci. Bull. Brooklyn Inst. Arts and Sci. : Science Bulletin of the Brooklyn Insti-
tute of Arts and Sciences.
Scot. Geogr. Mag. : Scottish Geographic Magazine, Edinburgh.
Smith. Cont. to Knowl. : Smithsonian Contributions to Knowledge, Washington.
Tech. Quart.: Technology Quarterly of the Massachusetts Institute of Tech-
nology, Boston.
Trans. Am. Inst. Min. Eng.: Transactions of the American Institute of Mining
Engineers, New York.
Trans. Roy. Dublin Soc. : Transactions of the Royal Dublin Society.
Trans. Seis. Soc. Japan: Transactions of the Seismological Society of Japan,
Tokyo.
Trans. Wis. Acad. Sci.: Transactions of the Wisconsin Academy of Sciences,
Arts, and Letters, Madison.
U.S. Geogr. and Geol. Surv. Rocky Mt. Region: United States Geographical
and Geological Survey of the Rocky Mountain Region (Powell), Wash-
ington.
Zeit. d. Gesell. f. Erdk. z. Berlin: Zeitschrift der Gesellschraft fiir Erdkunde
zu Berlin. :
4 Zeit. f. Gletscherk: Zeitschrift fiir Gletscherkunde, Berlin.
EARTH FEATURES AND THEIR MEANING
CHAPTER I
THE COMPILATION OF EARTH HISTORY
The sources of the history. — The science which deals with the
chapters of earth history that antedate the earliest human writ-
ings is geology. The pages of the record are the layers of rock
which make up the outer shell of our world. Here as in old
manuscripts pages are sometimes found to be missing, and on
others the writing is largely effaced so as to be indistinct or even
illegible. An intelligent interpretation of this record requires a
knowledge of the materials and the structure of the earth, as
well as a proper conception of the agencies which have caused
change and so developed the history. These agencies in opera-
tion are physical and chemical processes, and so the sciences of
physics and chemistry are fundamental in any extended study of
geology. Not only is geology, so to speak, founded upon chemis-
try and physics, but its field overlaps that of many other im-
‘portant sciences. The earliest earth history has to do with the
form, size, and physical condition of a minor planet in the solar
system. The earliest portion of the story belongs therefore to
astronomy, and no sharp line can be drawn to separate this chap-
ter from those later ones which are more clearly within the domain
of geology. }
Subdivisions of geology. — The terms “cosmic geology”? and
“astronomic geology”? have sometimes been used to cover the
astronomy of the earth planet. The later earth history develops,
among other things, the varied forms of animal and vegetable life
which have had a definite order of appearance. Their study is
to a large extent zodlogy and botany, though here considered
from an essentially different viewpoint. This subdivision of our
science is called paleontological geology or paleontology, which .
PB 1
2 EARTH FEATURES AND THEIR MEANING
in common usage includes the plant as well as the animal world,
or what is sometimes called paleobotany. In order to fix the
order of events in geological history, these biological studies are
necessary, for the pages of the record have many of them been
misplaced as a result of the vicissitudes of earth history, and the
remains of life in the rock layers supply a pagination from which
it is possible to correctly rearrange the misplaced pages. As com-
piled into a consecutive history of the earth since life appeared
upon it, we have the division of historical geology; though. this
differs but little from stratigraphical geology, the emphasis in the
case of the former being placed on the history itself and in the
latter upon the arrangement of events — the pagination of the
record.
So far as they are known to us, the materials of which the
earth is composed are minerals grouped into various characteristic
aggregates known as rocks. Here the science is founded upon
mineralogy as well as chemistry, and a study of the rock materials
of the earth is designated petrographical geology or petrography.
The various rocks which enter into the composition of the earth’s
outer shell — the only portion known to us from direct observa-
tion — are built into it in an architecture which, when carefully
studied, discloses important events in the earth’s history. The
division of the science which is concerned with earth architecture
is geotectonic or structural geology.
The study of earth features and their significance. — The
features upon the surface of the earth have all their deep sig-
nificance, and if properly understood, a flood of light is thrown,
not only upon present conditions, but upon many chapters of the
earth’s earlier history. Here the relation of our study to topog-
raphy and geography is very close, so that the lines of separa-
tion are but ill defined. The terms ‘‘physiographical geology,”’
““physiography,”’ and ‘‘ geomorphology ”’ are concerned with the
configuration of the earth’s surface — its physiognomy — and with
the genesis of its individual surface features. It is this ge-
netical side of physiography which separates it from topography
and lends it an absorbing interest, though it causes it to largely
overlap the division of dynamical geology or the study of
geological processes. In fact, the difference between dynamical
geology and physiography is largely one of emphasis, the stress
THE COMPILATION OF EARTH HISTORY 3
being laid upon the processes in the former and upon the result-
ant features in the latter.
Under dynamical geology are included important subdivisions,
such as seismic geology, or the study of earthquakes, and vul-
canology, or the study of volcanoes. Another large subject,
glacial geology, belongs within the broad frontier common to both
dynamical geology and physiography. A relatively new sub-
division of geological science is orientational geology, which is
concerned with the trend of earth features, and is closely related
both to physiography and to dynamical and structural geology.
Tabular recapitulation. — In a slightly different arrangement
from the above order of mention, the subdivisions of geology are
as follows : —
Subdivisions of Geology
Petrographical Geology. Materials of the earth.
Geotectonic Geology. Architecture of the earth’s outer
shell.
Dynamical Geology. Earth processes.
Seismic Geology — earthquakes.
Vuleanology —voleanoes. Glacial
Geology — glaciers, ete.
Phystographical Geology. Karth physiognomy and _ its
genesis.
- Orientational Geology. The arrangement and the trend
of earth features.
In one way or another all of the above subdivisions of geology
are in some way concerned in the genesis of earth physiognomy,
and they must therefore be given consideration in a work which
is devoted to a study of the meaning of earth features. The
compiled record of the rocks is, however, something quite apart
and without pertinence to the present work. As already indicated
its subdivisions are :—
Astronomic Geology. Planetary history of the earth.
Statigraphic Geology. The pagination of earth records.
Historical Geology. The compiled record and its inter-
pretation.
Paleontological Geology. The evolution of life upon the earth.
In every attempt at systematic arrangement difficulties are
encountered, usually because no one consideration can be used
throughout as the basis of classification. Such terms as “ eco-
4 EARTH FEATURES AND THEIR MEANING
nomic geology ” and “‘ mining geology ” have either a pedagogical
or a commercial significance, and so would hardly fit into the
system which we have outlined.
-Geological processes not universal. — It is inevitable that the
geology of regions which are easily accessible for study should
have absorbed the larger measure of attention; but it should not
be forgotten that geology is concerned with the history of the
entire world, and that perspective will be lost and erroneous
conclusions drawn if local conditions are kept too often before
the eyes. To illustrate by a single instance, the best studied
regions of the globe are those in which fairly abundant precipita-
tion in the form of rain has fitted the land for easy conditions of
life, and has thus permitted the development of a high civilization.
In degree, and to some extent also in kind, geologic processes
are markedly different within those widely extended regions which,
because either arid or cold, have been but ill fitted for human
habitation. Yet in the historical development of the earth, those
geologic processes which obtain in desert or polar regions are none
the less important because less often and less carefully observed.
Change, and not stability, the order’ of nature. — Man is ever
prone to emphasize the importance of apparent facts to the dis-
advantage of those less clearly revealed though equally potent.
The ancient notion of the terra firma, the safe and solid ground,
arose because of its contrast with the far more mobile bodies of
water; but this illusion is quickly dispelled with the sudden quak-
ing of the ground. Experience has clearly shown that, both upon
and beneath the earth’s surface, chemical and physical changes
are going on, subject to but little interruption. ‘“‘ The hills rock-
ribbed and ancient as the sun” is a poetical metaphor; for the
Himalayas, the loftiest mountains upon the globe, were, to speak
in geological terms, raised from the sea but yesterday. Even
to-day they are pushing up their heads, only to be relentlessly
planed down through the action of the atmosphere, of ice, and of
running water. Even more than has generally been supposed, the
earth suffers change.- Often within the space of a few seconds,
to the accompaniment of a heavy earthquake, many square miles
of territory are bodily uplifted, while neighboring areas may be .
relatively depressed. Thus change, and not stability, is the order
of nature.
THE COMPILATION OF EARTH HISTORY 5
Observational geology versus speculative philosophy. — There
appears to be a more or less prevalent notion that the views which
are held by scientists in one generation are abandoned by those
of the next; and this is apt to lead to the belief that little is really
known and that much is largely guessed. Some ground there
undoubtedly is for such skepticism, though much of it may be
accounted for by a general failure among scientists, as well as
others, to clearly differentiate that which is essentially speculative
from what is based broadly upon observed facts. Even with
extended observation, the possibility of explaining the facts in
more than one way is not excluded; but the line is nevertheless
a broad one which separates this entire field of observation from
what is essentially speculative philosophy. To illustrate: the
mechanics of the action which goes on within volcanic craters is
now fairly well understood as a result of many and extended
observations, and it is little likely that future generations of
geologists will discredit the main conclusions which have been
reached. The cause of the rise of the lava to the earth’s surface
is, on the other hand, much less clearly demonstrated, and the
views which are held express rather the differing opinions than
any clear deductions from observation. Again, and similarly, the
_ physical history of the great continental glaciers of the so-called
~ “ice age”’ is far more thoroughly known than that of any existing
glacier of the same type; but the cause of the climatic changes
which brought on the glaciation is still largely a matter for specu
lation.
In the present work, the attempt will be, so far as possible, to
give an exposition of geologic processes and the earth features
which result from them, with hints only at those ultimate causes
which lie hidden in the background.
The scientific attitude and temper. — The student of science
should make it his aim, not only clearly to separate in his studies
the proximate from the ultimate causes of observed phenomena,
but he should keep his mind always open for reaching individual
conclusions. No doctrines should be accepted finally upon faith
merely, but subject rather to his own reasoning processes. This
should not be interpreted to mean that concerning matters of
which he knows little or nothing he should not pay respect to the
recognized authorities; but his acceptance of any theory should
6 EARTH FEATURES AND THEIR MEANING
be subject to review so soon as his own horizon has been sufficiently
enlarged. False theories could hardly have endured so long in the
past, had not too great respect been given to authorities, and in-
dividual reasoning processes been held too long in subjection.
The value of the hypothesis. — Because all the facts necessary
for a full interpretation of observed phenomena are not at one’s
hand, this should not be made to stand in the way of provisional
explanations. If science is to advance, the use of hypothesis is
absolutely essential; but the particular hypothesis adopted should
be regarded as temporary and as indicating a line of observation
or of experimentation which is to be followed in testing it. Thus
regarded with an open mind, inadequate hypotheses are eventu-
ally found to be untenable, whereas correct explanations of the
facts by the same process are confirmed. Most hypotheses of
science are but partially correct, for we now “ see through a glass
darkly ’’; but even so, if properly tested, the false elements in the
Penctheet: are one after the other eliminated as the embodied
truth is confirmed and enlarged. Thus ‘ working hypothesis”
passes into theory and becomes an integral part of science.
READING REFERENCES FOR CHAPTER I
The most comprehensive of general geological texts written in English is
Chamberlin and Salisbury’s ‘“‘Geology”’ in three volumes (Henry Holt,
1904-1906), the first volume of which is devoted exclusively to geological
processes and their results. An abridged one-volume edition of the work
intended for use as a college text was issued in 1906 (College Geology,
Henry Holt). Other standard texts are: —
Sir ARCHIBALD GEIKIE. Text-book of Geology, 4th ed. 2 vols. Lon-
don, 1902, pp. 1472.
W. B. Scorr. An Introduction to Geology. 2d ed. Macmillan, 1907,
pp. 816.
J. D. Dana. Manual of Geology. New edition. American Book Com-
pany, 1895, pp. 1087.
Joseph LeContre. Elements of Geology. (Revised by Fairchild.)
Appleton, 1905, pp. 667.
A very valuable guide to the recent literature of dynamical and struc-
tural geology is Branner’s ‘‘Syllabus of a Course of Lectures on Elemen-
tary Geology” (Stanford University, 1908).
On the relation of geology to aDOOn, a number of interesting books
have been written : —
James GeIkiE. Earth Sculpture or the Origin of Lanul-toces “New
York and London, 1896, pp. 397.
Bi
cH
‘i
5
a]
i
#
i
‘at
e
nce ae Ay
THE COMPILATION OF EARTH HISTORY <
Joun E. Marr. The Scientific Study of Scenery. Methuen, London,
1900, pp. 368. ; |
Sir A. Grerxie. The Scenery of Scotland. 3ded. Macmillan, London,
1901, pp. 540.
Srr Joun Lussocx. The Scenery of Switzerland and the Causes to which
it is Due. Macmillan, London, 1896, pp. 480.
Lorp Avesury. The Scenery of England. Macmillan, London, 1902,
pp. 534.
Sir A. Gerxie. Landscape in History, and Other Essays. Macmillan,
London, 1905, pp. 352.
N.S.SHater. Aspectsof the Earth. Scribners, New York, 1889, pp. 344.
G. pE La Nor et Emm. pe Marceriz. Les Formes du Terrain, Service
Géographique de l’Armée. Paris, 1888, pp. 205, pls. 48.
“W.M. Davis. Practical Exercises in Physical Geography, with Accom-
panying Atlas. Ginn and Co., Boston, 1908, pp. 148, pls. 45.
JoHNMuir. The Mountains of California. Unwin, London, 1894, pp. 381.
Upon the use and interpretation of topographic maps in illustration of
characteristic earth features, the following are recommended : —
R. D. Satissury and W. W. Atwoop. The Interpretation of Topo-
graphic Maps, Prof. Pap., 60 U.S. Geol. Surv., pp. 84, pls. 170.
D. W. Jounson and F. E. Marrues. The Relation of Geology to
Topography, in Breed and Hosmer’s Principles and Practice of Sur-
veying, vol. 2. Wiley, New York, 1908.
GénéraL Bertuaut. Topologie, Etude du Terrain, Service Géogra-
phique de l’Armée. Paris, 1909, 2 vols., pp. 330 and 674, pls. 265.
The United States Geological Survey issues free of charge a list of
100 topographic altas sheets which illustrate the more important physi-
ographic types. In his “ Traité de Géographie Physique,” Professor E. de
Martonne has given at the end of each chapter the important foreign
maps which illustrate the physiographic types there described.
“The Principles of Geology,’’ by Sir Charles Lyell, published first in three
volumes, appeared in the years 1830-1833, and may be said to mark the
beginning of modern geology. Later reduced to two volumes, an eleventh
edition of the work was issued in 1872 (Appleton) and may be profitably
read and studied to-day by all students of geology. Those familiar with
the German language will derive both pleasure and profit from a perusal
of Neumayr’s “‘ Erdgeschichte” (2d ed. revised by Uhlig. Leipzig and
Vienna, 2 vols., 1895-1897), and especially the first volume, ‘‘ Allgemeine
Geologie.’”’ A recent French work to be recommended is Haug’s “‘ Traité
de Géologie’’ (Paris, 1907).
Some texts of physical geography may well be consulted, especially
Emm. de Martonne’s “ Traité de Géographie Physique.’”’ Colin, Paris,
1909, pp. 910, pls. 48, and figs. 396.
Nore. An explanatory list of abbreviations used in the reading refer-
ences follows the List of Illustrations.
CHAPTER II
THE FIGURE OF THE EARTH
The lithosphere and its envelopes. — The stony part of the
earth is known as the lithosphere, of which only a thin surface
shell is known to us from direct observation. The relatively un-
known central portion, or ‘‘ core,’”’ is sometimes referred to as the
centrosphere. Inclosing the lithosphere is a water envelope, the
hydrosphere, which comprises the oceans and inland bodies of
water, and has a mass zs/z5 that of the lithosphere. If uniformly
distributed, the hydrosphere would cover the lithosphere to the
depth of about two miles, instead of being collected in basins as it
nowis. Though apparently not continuous, if we take into account
the zone of underground water upon the continents, the hydro-
sphere may properly be considered as a continuous film about the
lithosphere. It is a fact of much significance that all the ocean
basins are connected, so that the levels are adjusted to furnish a
common record of deposits over the entire surface that is sea-
covered.
Enveloping the hydrosphere is the gaseous envelope, the atmos-
phere, with a mass zypbo00 that of the lithosphere. The atmos-
phere is a mixture of the gases oxygen and nitrogen in parts by
volume of one of the former to four of the latter, with a relatively
small percentage of carbon dioxide. Locally, and at special
seasons, the atmosphere may be charged with relatively large
percentages of water vapor; and we shall see that both the carbon
dioxide and the vapor contents are of the utmost importance in
geological processes and in the influence upon climate. Unlike
the water which composes the hydrosphere, the gases of the
atmosphere are compressible. Forced down by the weight of
superincumbent gas, the layers of the atmosphere at the level of
the sea sustain a pressure of about fifteen pounds to the square
inch; but this pressure steadily decreases in ascending to higher
levels. From direct instrumental observation, the air has now
: | |
THE FIGURE OF THE EARTH 9
been investigated to a height of more than twelve miles from the
earth’s surface.
The evolution of ideas concerning the earth’s figure. — The
ideas which in all ages have been promulgated concerning the
figure of the earth have been many and varied. Though among
them are not wanting the purely speculative and fantastic, it will
be interesting to pass in review such theories as have grown directly
out of observation.
The ancient Hebrews and the Babylonians were dwellers of the
desert, and in the mountains which bounded their horizon they
saw the confines of the earth. Pushing at last westward beyond
the mountains, they found the Mediterranean, and thus arrived at
the view that the earth was a disk with a rim of mountains which
was floated upon water. The rare but violent rainfalls to which
they were accustomed — the desert cloudburst — further led them
to the belief that the mountain rim was continued upward in a
dome or firmament of transparent crystal upon which the heavenly
bodies were hung and from which out of ‘‘ windows of heaven ”’
‘the water “‘ which is above the earth ” was poured out upon the
earth’s surface. Fantastic as this theory may seem to-day, it
was founded upon observation, and it well illustrates the dangers
_of reasoning from observation within too limited a field.
As soon as men began to sail the sea, it was noticed that the
water surface is convex, for the masts of ships were found to remain
visible long after their hulls had disappeared below the horizon.
_ It is difficult to say how soon the idea of the earth’s rotundity was
acquired, but it is certainly of great antiquity. The Dominican
monk Vincentius of Beauvais, in a work completed in 1244, declared
that the surfaces of the earth and the sea were both spherical.
The poet Dante made it clear that these surfaces were one, and
in his famous address upon “ The Water and the Land,” which
was delivered in Verona on the 20th of January, 1320, he added
a statement that the continents rise higher than the ocean. His
explanation of this was that the continents are pulled up by the
attraction of the fixed stars after the manner of attraction of
magnets, thus giving an early hint of the force of gravitation.
The earth’s rotundity may be said to have been first proven
when Magellan’s ships in 1521 had accomplished the circumnavi-
gation of the globe. Circumnavigation, soon after again carried
10 EARTH FEATURES AND THEIR MEANING
out by Sir Francis Drake, proved that the earth is a closed body
bounded by curving surfaces in part enveloped by the oceans and
everywhere by the atmosphere. The great discovery of Copernicus
in 1530 that the earth, like Venus, Mars, and the other planets,
revolves about the sun as a part of a system, left little room for
doubt that the figure of the earth was essentially that of a sphere.
The oblateness of the earth. — Every schoolboy is to-day fa-
miliar with the fact that the earth departs from a perfect spherical
figure by being flattened at the ends of its axis of rotation. The
polar diameter is usually given as s$5 shorter than the equatorial
one. This oblateness of the spheroid was proven by geodesists
when they came to compare the lengths of measured degrees of
are upon meridians in high and in low latitudes.
The oblateness of the geoid is well understood from accepted
hypotheses to be the result of the once more rapid rotation of the
planet when its materials were more plastic, and hence more re-
sponsive to deformation. An elastic hoop rotating rapidly about
an axis in its plane appears to the eye as a solid, and becomes
flattened at the ends of its axis in proportion as the velocity of
rotation is increased. Like the earth, the other planets in the
solar system are similarly oblate and by amounts dependent on the
relative velocities of rotation.
The departure of the geoid from the spherical surface, owing to
its oblateness, is so small that in the figures which we shall use for
illustration it would be less than the thickness of a line. Since it
is well recognized and not important in our present consideration,
we shall for the time being speak of the figure of the earth in terms
of departures from a standard spherical surface.
The arrangement of oceans and continents. — There are other
departures from a spherical surface than the oblateness just re-
ferred to, and these departures, while not large, are believed to be
full of significance. Lest the reader should gain a wrong impres-
sion of their magnitude, it may be well to introduce a diagram
drawn to scale and representing prominent elevations and depres-
sions of the earth (Fig. 1).
Wrong impressions concerning the figure of the lithosphere are
sometimes gained because its depressions are obliterated by the
oceans. The oceans are, indeed, useful to us in showing where
the depressions are located, but the figure of the earth which we
¢
es Sree
ENCE
‘
continent as narrowing land masses to the southward
THE FIGURE OF THE EARTH 11
are considering is the naked surface of the rock. In a broad way,
the earth’s shape will be given by the arrangement of the oceans
and the continents. As
soon as we take up the Me Blone
: Fioor of Mediterroneaqn ——
study of this arrangement, “ar or Arlen
we find that quite signifi-
cant facts of distribution
are disclosed.
One of the most signifi- 20” of7ot4.cerrh’s_ recive ___
cant facts involved in the Qa
distribution of land and
sea, is a concentration of
the land areas within the F%9- 1:— Diagrams to afford
@ correct impression of the
northern and the seas _ measure of the inequalities
within the southern hemi- upon the earth’s surface com-
pared to the earth’s radius.
sphere. The noteworthy The shell represented in b is
exception is the occurrence 44, of the earth’s radius, and
of the great and high in a this zone is magnified
Antarctic continent cen- for comparison with surface
inequalities. .
tered near the earth’s
south pole; and there are extensions of the northern
Greatest JeEp~>——
of the equator. Hardly less significant than the ex-
istence of land and water hemispheres is the reciprocal
or antipodal distribution of land and sea (Fig. 2).
A third fact of significance is a dovetailing together of sea and
land along an east-
and-west direction.
While the seas are
generally § A-shaped
and narrow north-
ward, the land masses
are V-shaped and nar-
row southward, but
Vip : Eppes Yp,\ this occurs mainly in
eps sphere. Lastly, there
Fig. 2.— Map on Mercator’s projection to show the
reciprocal relation of the land and sea areas (after 8 SOME indication of
Gregory and Arldt). a belt of sea dividing
12 EARTH FEATURES AND THEIR MEANING
the land masses into northern and southern portions along the
course of a great circle which makes a small angle with the earth’s
equator. Thus the western continent is nearly divided by a
mediterranean sea, —the Caribbean, — and the eastern is in part
so divided by the separation of Europe from Africa.
The figure toward which the earth is tending. — Thus far in
our discussion of the earth’s figure we have been guided entirely
by the present dis-
tribution of land and
water. There are,
however, depres-
sions upon the sur-
face of the land, in
some cases extend-
ing below the level
of the sea, which are
not to-day occupied
by water. By far
the most notable of
these is the great
Caspian Depression,
Fig. 3.— The form toward which the figure of the earth which with its ex-
is tending, a tetrahedron with symmetrically truncated tension divides cen-
angles.
: tral and eastern Asia
upon the east from Africa and Europe upon the west. This
depression was quite recently occupied by the sea, and when
added to the present ocean basins to indicate depressions of the
lithosphere, it shows that the earth’s figure departs from the
standard spheroid in the direction of the form represented in
Fig. 3. This form approximates to a tetrahedron, a figure bounded
by four equal triangular faces, here with symmetrically truncated
angles. Of all regular figures with plane surfaces the tetrahedron
has the smallest volume for a given surface, and it presents more-
over a reciprocal relation of projection to depression. Every
line passing through its center thus finds the surface nearer than
the average distance upon one side and correspondingly farther
upon the other (Fig. 4). i .
Astronomical versus geodetic observations. — Confirmation of
the conclusions arrived at from the arrangement of oceans and
-southern than they are in the
ment to be the case, and the
ww a ur
iadiimnacs ieee usii>aiicina <tenetiin ee a a et
ei) Le 2 x Ree
piace Sere ae ee Fis
aR mE
THE FIGURE OF THE EARTH 13
continents has been secured in other fields. It was pointed out
that the earth’s oblateness was proven by comparison of the
measured degrees of latitude upon the earth’s surface in lower and
higher latitudes, the degree being found longer as the pole is
approached. Any variation from the spherical surface must ob-
viously increase the size of the measured degree of latitude in
proportion to the departure from the standard form, and so
the tetrahedral figure with one of its angles at the south pole
will require that the degrees
of latitude be longer in the
northern hemisphere. This
has been found by measure-
result is further confirmed by
pendulum studies upon the
‘distribution of the earth’s at-
traction or gravity. If less of
the mass of the earth is con-
centrated in the southern
hemisphere, its attraction as :
measured in vibrations of Fic. 4.—A truncated tetrahedron, showing
the pendulum should be cor- how the depression upon one side of the cen-
; ter is balanced by the opposite projection.
respondingly smaller.
Other confirmations of the tetrahedral figure of the earth have
been derived from a comparison of astronomical data, which assume
the earth to be a perfect spheroid, with geodetic measurements,
which are based upon direct measurements. Thus the arc meas-
ured in an east-and-west direction across Europe revealed a differ-
ent curvature near the angle of the tetrahedral figure from what
was found farther to the eastward.
Changes of figure during contraction of a spherical body. — If
we inquire why the earth in cooling should tend to approach the
tetrahedral figure, an answer is easily found. When formed,
the earth appears to have been a but slightly oblate spheroid,
or practically a sphere —the shape which of all incloses the
most space for a given surface. Cooled and solidified at the sur-
face to the temperature of the surrounding air, and the core
still hot and continuing to lose heat, the core must continue to
14 EARTH FEATURES AND THEIR MEANING
contract though the outer shell is no longer able to do so. The
superficial area being thus maintained constant while the volume
continues to diminish, the figure must change from the initial one
of greatest bulk to others of smaller volume, and ultimately, if the
process should continue indefinitely, to the tetrahedron, which of
all regular figures has the minimum volume for a given surface.
That a contracting sphere does indeed pass through such a
series of changes has been shown by the behavior of contracting
soap bubbles and of rubber balloons, as well as by experiments
upon the exhaustion of air contained in hollow metal spheres of
only moderate strength. In all these instances, the ultimate
form produced indicates an indenting of four sides of the sphere
which have the positions of the faces of a tetrahedron. The late
Professor Prinz of Brussels secured.some extremely interesting
results in which he obtained intermediate forms with six angles,
but unfortunately these studies were not prepared for publication
at the time of his death.
The earth’s departure from the spheroid in the direction of the
modified tetrahedron is, as we have seen, no hypothesis, but ob-
served fact revealed in (1) the concentration of the land about
a central ocean in the northern hemisphere; in (2) the antipodal
relation of the land to the water areas, and in (8) the threefold
subdivision of the surface into north and south belts by the two
greater oceans and the Caspian Depression.
The earlier figures of the earth. — The manner in which conti-
nent and ocean are dovetailed into each other in an east-and-west
direction has been generally adduced as additional evidence for
the tetrahedral figure as above described. Closer examination
shows that instead of being in harmony with this figure, it indi-
cates a departure from it, and, as we shall see, a significant depar-
ture which undoubtedly has its origin in the earlier history of
the planet. The mediterranean seas of both the eastern and the
western hemispheres likewise interfere with the perfection of the
tetrahedral figure and require an explanation.
Let us then examine in outline the past history of the world
with reference especially to the evolution of the continents and
to the times and the manners of surface change. It is now well |
known that there have been three major periods of great deforma-
tion of the earth’s shell. The first of these of which we have
Pye ee Le a Eee
——.
Se ee
THE FIGURE OF THE EARTH iS
record came at the end of the first great era of geologic history,
the so-called Eozoic era; a second great transformation came at
the close of the second or Paleozoic era; and a third began at the
end of the next or Mesozoic era, an adjustment which is apparently
‘continuing to-day. Each of these great surface deformations was
accompanied by. great volcanic eruptions of which we have the
evidence in the lavas'remaining for our inspection, and each was
followed by the formation of great glaciers which spread over
large areas of the existing continents.
Before the earliest of these great changes, the earth appears to
have approximated in its figure somewhat closely to the ideal
spheroid, for it was everywhere enveloped in the hydrosphere as a
universal ocean. ‘Toward the close of this period came the adjust-
ments which brought the lithosphere to protrude through the
hydrosphere in shield-like continents whose distribution, as shown
by the rocks of this period, is of great significance. Within the
northern hemisphere rose three land shields spaced at nearly
equal intervals and at nearly equal distances from the northern
pole. One of these was centered where now is Hudson Bay,
another about the present Baltic Sea, and the relics of the third
are found in northeastern Siberia. These earliest continents
have been referred to as the Laurentian, Baltic, and Angara shields.
Within the southern hemisphere shields appear to have developed
Ar eno oF EozoicERa Ar E10 OF Pazeozoic ERA Twe PRESENT
Fig. 5.— Approximations to earlier and present figures of the earth.
in somewhat similar grouping, namely, in South America, in South
Africa, and in Australia (Figs. 3 and 5).
These coigns or angles of a form into which the earlier spheroid
of the earth was being transformed have persisted through the
greater part of subsequent geologic time, and have been enlarged
by the growth of sediments about them as well as by the later
16 EARTH FEATURES AND THEIR MEANING
elevation and wrinkling of these deposits into marginal mountain
ranges.
The continents and oceans which arose at the close of the
Paleozoic era. — At the close of the second great era in the recorded
history of the earth, the now somewhat enlarged continents were
profoundly altered during a series of convulsive movements within
the surface shell of the lithosphere. When these convulsions were
over, there was a new disposition of land and sea, but one quite
different from the present arrangement. Instead of being ex-
tended in north-south belts, as they are at present, the continents
stretched out in broad east-west zones, one in the northern and
the other in the southern hemisphere. To the broad southern
continent of which so little now remains, the name ‘‘ Gondwana
Land ” has been given, and to the sea which divided the northern
from the southern continent the name “Ocean of Tethys.”’ The
northern continent stretched across the site of the present Atlantic
Ocean as the ‘‘ North Atlantis,” its northern shore to the west-
ward being somewhat farther south than the present northern
coast of North America, since life forms migrated in the north-
ern ocean from the site of Behring Sea to that of the present
North Atlantic.
This arrangement of land and water during the middle period
of the earth’s recorded history, when considered with reference
both to its earlier and to its later evolution, may perhaps be best
accounted for by the assumption that the lithosphere was then
shaped like Fig. 5 (middle). In this figure two truncated tetra-
hedrons are joined in a common plane of contact which may be
described as the twin plane. This medial depression upon the
lithosphere was occupied by the intercontinental sea, the Ocean of
Tethys.
Near the close of this second great era of the earth’s conti-
nental history, crustal convulsions, which were perhaps the most
remarkable in the entire record, resulted in the almost complete
disappearance of the southern continent and a concentration of
the land within the northern hemisphere as a somewhat inter-
rupted belt surrounding a central polar ocean (Figs. 3 and 5).
Upon the assumption of twin tetrahedrons in the intermediate
era of continental evolution, both the Ocean of Tethys of that
time and its present remnants, the Caribbean and Mediterranean
THE FIGURE OF THE EARTH 17
seas, are accounted for. The V-shaped continent extensions
and the A-shaped oceans of the southern hemisphere (Fig. 2) may
likewise be considered as relics of the now largely submerged tet-
rahedron of the southern hemisphere, since this had its apex to the
northward (Fig. 6).
Thus we see that the lithosphere can scarcely be regarded as a
perfect spheroid, since in the course of geologic ages it has under-
gone successive de-
partures from this
original form. In
its present state it
has been described
as tetrahedral, Zetrahedral faces with Apex toSourh
though we must
keep in mind that
the sharp angles
of that figure are
deeply truncated.
The soundings
first by Nansen Jerrahsdral Faces with Apex to North
and more recently Actudl lines tt Southerts Hermisphere.
by Peary in the 7
a Arctic basin, far Fic. 6.— Diagrams for comparison of shore lines upon
tetrahedrons which have an angle, the first at the south
to the north of the and the second at the north.
continental _ bor-
der, showed that this depression is characterized by profound
depths, and so have afforded confirmation of the tetrahedral fig-
ure. To match this depression at the northern extremity of the
earth’s axis, a high continent reaching to elevations in excess of
10,000 feet has been penetrated by Sir Ernest Shackleton at the
opposite extremity of this polar diameter. Considering its size
and its elevation, the Antarctic continent with its glacier mantle
is the largest protuberance upon the surface of the lithosphere.
In our study of the departures of the earth from the standard
spheroidal surface, we might even go a step farther and show how
the tetrahedron, which best represents the symmetry of the present
figure, is somewhat deformed by a flattening perpendicular to the
Pacific Ocean. To draw attention to this flattening of the earth,
it has sometimes been described as ‘“ potato-shaped,” since the
Cc
18 EARTH FEATURES AND THEIR MEANING
outline perpendicular to this face is imperfectly heart-shaped or
like a flattened “‘ peg top.”
The flooded portions of the present continents. — We are accus-
tomed to think of the continents as ending at the shores of the
oceans. If, however, we
are to regard them as
platforms. which rise
from the ocean depres-
sions, their margins
should be considerably
extended, for a _ sub-
merged shelf now prac-
_ tically surrounds all the
Fia. 7.— The continents with submerged portions oes as space oe
ne added (after Gilbert). uniform depth of 100
| fathoms or 600 feet.
The oceans thus more than fill their basins and may be thought of
as spilling over upon the continents. In Fig. 7, the submerged por-
tions of the continents have been joined to those usually represented,
thus adding about 10,000,000 square miles to their area, and giving
them one third, instead of one fourth, of the lithosphere surface.
The floors of the hydrosphere and atmosphere. — The highest
altitudes upon the continents and the profoundest deeps of the
* KOOOF?- 1
Xx e
+2GQO00 S S Ai 1
y x N
! a Ys
+/0,000 5 ~ Ne >
NK 8h §% Ny
Sea Lever.| = S “
-29000 {3 : ‘
E $ a é >
~JGQ00f2 +. :
Fig. 8.— Diagram to indicate the altitude of different parts of the
/ lithosphere surface.
ocean are each removed about 30,000 feet, or nearly 6 miles,
from the level of the sea. In comparison with the entire surface.
of the lithosphere, these extremes of elevation represerit such
small areas as to be almost inappreciable. Only about 35 of the
THE FIGURE OF THE EARTH 19
lithosphere surface rises more than 6000 feet above sea level,
and about the same proportion lies deeper than 18,000 feet below
the same datum plane (Fig. 8). Almost the entire area of the
lithosphere is included either in the so-called continental plateau
or platform, in the oceanic platform, or in the slope which separates
the two. The continental platform includes the continental shelf
above referred to, and represents about one third of the entire
area of the planet. This platform has a range of elevation from
6000 feet above to 600 feet below sea level and has an average
altitude of about 2300 feet. The oceanic platform slopes more
steeply, ranges in depth from 12,000 to 18,000 feet below sea level,
and comprises about one half the lithosphere surface. The
remaining portion of the surface, something less than one eighth
of all, is included in the steep slopes between the two platforms,
between 600 and 12,000 feet below sea. The two platforms and
the slope between them must not, however, be thought of as
continuous features upon the surface, but merely as representing
the average elevations of portions of the lithosphere.
READING REFERENCES FOR CHAPTER II
On the evclution of ideas concerning the earth’s figure : —
Suxrss. The Face of the Earth (Clarendon Press, 1906), vol. 2, Chapter 1.
_v. Zirtet. History of Geology and Paleontology (Walter Scott, Lon-
don, 1901), Chapters 1-2.
The departure of the spheroid toward the tetrahedron : —
W. Lowratan GREEN. Vestiges of the Molten Globe, Part 1. London, 1875.
(Now a rare work, but it contains the original statement of the idea.)
J. W. Gregory. The Plan of the Earth and Its Causes, Geogr. Jour.,
vol. 13, 1899, pp. 225-251 (the best general statement of the argu-
ments for a tetrahedral form).
W. Prinz. L’échelle reduite des ae géologiques, Bull. Soc. Belge
d’Astronomie, 1899.
B. K. Emerson. The Tetrahedral Barth and Zone of the Interconti-
nental Seas, Bull. Geol. Soc. Am., vol. 11, 1911, pp. 61-106, pls. 9-14.
M. P. Rupsxi. Physik der Erde (Tauchnitz, Leipzig, 1911), Chapters
1-3 (the best discussion of the geoid from the purely mathematical
standpoint, so far as the spheroid is concerned).
The earlier figures of the earth : —
Tu. Artpt. Die Entwicklung der Kontinente und ihrer Lebewelt. Engel-
mann, Leipzig, 1907. (Contains a valuable series of map plates,
showing the probable boundaries of the continents in the different
geological periods).
¥
a
:
UB
'
t
\
CHAPTER III
THE NATURE OF THE MATERIALS IN THE LITHOSPHERE
The rigid quality of our planet. — For a long time it was sup-
posed that the solid earth constituted a crust only which was
floated upon a liquid interior. This notion was clearly an out-
growth of the then generally accepted Laplacian hypothesis of
the origin of the universe, which assumed fluid interiors for the
planets, the crust being suggested by the winter crust of frozen
water upon the surface of our inland lakes. To-day the nebular
hypothesis in the Laplacian form is fast giving place to quite
different conceptions, in which solid particles, and not gaseous
ones, are conceived to have built up the lithosphere. The analogy
with frozen water has likewise been abandoned with the discovery
that frozen rock, instead of floating, sinks in its molten equivalent.
Yet even more cogent arguments have been brought forward
to show that whatever may be the state of aggregation within the
earth’s core — and it may be different from any now known to
us—it nevertheless has many of the properties recognized as
belonging to solid and rigid bodies. Provisionally, therefore, we
may regard the earth’s core as rigid and essentially solid. It was
long ago pointed out by the late Lord Kelvin that if our litho-
sphere were not more rigid than a ball of glass of the same size, it
would be constantly passing through periodic six-hourly distortions
of great amplitude in response to the varying attractions of the
moon. An equally striking argument emanating from the same
high authority is furnished by the well-known egg-spinning demon-
stration. For illustration, Kelvin was accustomed to take two
eggs, one boiled and the other raw, and attempt to spin them
upon their ends. For the boiled, and essentially solid, egg this is
easily accomplished, but internal friction of the liquid contents of
the raw egg quickly stops any rotary motion which is imparted to
it. Upon the same grounds it is argued that had the earth’s .
interior possessed the properties of a liquid, rotation must long
since have ceased.
20
NATURE OF THE MATERIALS IN THE LITHOSPHERE 21
A stronger proof of earth rigidity than either of these has been
lately furnished by the instrumental study of earthquakes. With
the delicate apparatus which is now installed for the purpose,
heavy earthquakes may be sensed which have occurred anywhere
upon the earth’s surface, the earth movement sending its own
message by the shortest route through the core of the earth to the
observing station. A heavy shock which occurs in New Zealand
is recorded in England, almost diametrically opposite, in about
twenty-one minutes after its occurrence. The laws of wave
propagation and their relation to the properties of the transmitting
medium are well known, and in order to explain such extraordinary
velocity it is necessary to assume that for such impulses the earth’s
interior is much more rigid than the finest tool steel.
Probable composition of the earth’s core. — In deriving views
concerning the nature of the earth’s interior we are greatly aided
by astronomical studies. The common origin long ago indicated
for the planets of the solar system and the sun has been confirmed
by the analysis of light with the aid of the spectroscope. It has
thus been found that the same chemical elements which we find in
the earth are present also in the sun and in the other stellar bodies.
Again, the group of planets of the solar system which are nearest
_to the sun — Mercury, Venus, the Earth, and Mars — have each
a high density, all except Mars, the most distant, having specific
gravities very closely 53, that of Mars being about 4. This
average specific gravity is also that of the solid bodies, the so-called
meteorites, which reach the surface of our planet from the sur-
rounding space. Yet though the earth as a whole is thus found
to have a specific gravity five and a half times that of water, its
surface shell has an average density of less than half this value,
or 2.7.
The study of meteorites has given us a possible clew to the
nature of the earth’s interior; for when both terrestrial and
_ celestial rock types are classified and placed in orderly arrange-
ment, it is found that the chemical elements which compose the
two groups are identical, and that these are united according to
the same physical and chemical laws. No new element has been
discovered in the one group that has not been found in the other,
and though some compounds of these elements, the minerals, oc-
cur in the earth’s crust that have not been found in meteorites,
22 EARTH FEATURES AND THEIR MEANING
and though some occur in meteorites which are not known from
the earth, yet of those which are common to both bodies there is
agreement, even to the minor details (Fig. 9). It is found, how-
ever, that the commonest of the minerals in the earth’s shell are
absent from meteorites, as the commoner constituents of meteor-
ites are wanting in the earth’s crust. This observation would go
far to show that we may in the two cases be examining different
Jerrestrial! Frocks.
Meteor ises ard raret Terrestrial FOCrs.
Fic. 9.— Diagram to show how terrestrial rocks grade into those of the meteorites.
1, oxygen; 2, silicon; 3, aluminium; 4, alkali metals; 5, alkaline earth metals;
6, iron, nickel, cobalt, etc. ; a, granites and rhyolites; 6, syenites and trachytes;
c, diorites and andesites; d, gabbros and basalts; e, ultra-basic rocks; f, basic
inclosures in basalt, etc.; g, iron basalts of west Greenland; h, iron masses of
Ovifak, west Greenland ; a’—d’, meteorites in order of density (after Judd).
portions of quite similar bodies; and this view is strikingly con-
firmed when the rocks of the two groups are arranged in the order
of their densities (Fig. 9). .
In a broad way, density, structure, and chemical composition
are all similarly involved in the gradations illustrated by the
diagram ; and it is significant that while there are terrestrial rocks
not represented by meteorites, the densest and the most unusual
of the terrestrial rocks are chemically almost identical with the
less dense of the celestial bodies. :
NATURE OF THE MATERIALS IN THE LITHOSPHERE 23
The earth a magnet.— The denser, and likewise the more
common, of the meteorite rocks — the so-called meteoric irons —
are composed almost entirely of the elements iron, nickel, and
cobalt. Such aggregates are not known as yet from terrestrial
sources, although transitional types appear to exist upon the
island of Disco off the west coast of Greenland. If it were pos-
sible to explore the earth’s interior, would such combinations of
the iron minerals be encountered? Apart from the surprising
velocity of transmission of earthquake waves, the strongest argu-
ment for an iron core to the lithosphere is found in the magnetic
property of the earth as a whole. The only magnetic elements
known to us are those of the heavy meteorites — iron, nickel,
and cobalt, — and the earth is, as we know, a great magnet whose
northern pole in British America and whose southern pole in
Antarctica have at last been visited by Amundsen and David,
respectively. The specific gravity of iron is 7.15, and those of
nickel and cobalt, which in the meteorites are present in relatively
small amounts, are 7.8 and 7.5, respectively. Considering that
the surface shell of the earth has a specific gravity of 2.7, these
values must be regarded as agreeing well with the determined
density of the earth (5.6) and the other planets of its group (Mer-
cury 5.7, Venus 5.4, Mars 4). |
The chemical constitution of the earth’s surface shell. — The
number of the so-called chemical elements which enter into the
earth’s composition is more than eighty, but few of these figure
as important constituents of the portion known to us. Nearly
one half of the mass of this shell is oxygen, and more than a quarter
is silicon. The remaining quarter is largely made up of aluminium,
iron, calcium, magnesium, and the alkalies sodium and potassium,
in the order named. These eight constituent elements are thus
the only ones which play any important réle in the composition
of the earth’s surface shell. They are not found there in the free
condition, but combined in the definite proportions characteristic
of chemical compounds, and as such they are known as minerals.
The essential nature of crystals.— A crystal we are accus-
tomed to think of as something transparent bounded by sharp
edges and angles, our ideas having been obtained largely from the
gem minerals. This outward symmetry of form is, however, but
an expression of a power which resides, so to speak, in the heart
24 EARTH FEATURES AND THEIR MEANING
or soul of the crystal individual — it has its own structural make-
up, its individuality. No more correct estimates of the compari-
son of crystal individualities would be obtained by the study of
outward forms alone of two minerals than would be gained by a
judgment of persons from the cut of their clothing. Too often
this outward dress tells only of the conditions by which both men
and crystals have been surrounded, and but little of the power
inherent in the individual. Many a battered mineral fragment
with little beauty to recommend it, when placed under suitable
conditions for its development, has grown into a marvel of beauty.
Few minerals are so mean that they have not within them this
inherent power of individuality which lifts them above the world —
of the amorphous and shapeless.
Just as the real nature of a person is first disclosed by his
: 3 ;
Gist (Ones) Acurphoey Scheie behavior under trying circum
conduct under stress of one sort
or another which brings out
its real character. By way of
illustration let us prepare a
sphere from the mineral quartz
—it matters not whether we
destroy the beautiful outlines of
the crystal or employ a bat-
tered fragment — and then pre-
pare a sphere of similar size and
shape from a noncrystalline or
amorphous substance like glass.
If now these two spheres be in-
troduced into a bath of oil and
raised to a higher temperature,
the glass globe undergoes. an
Fia. 10.—Comparison of a crystalline CDlargement without change of
with an amorphous substance when ex- its form; but the crystal ball
eras by heat and when attacked by reveals its individuality by ex-
oe panding into a spheroid in
which each new dimension is nicely adjusted to this more complex
figure (Fig. 10). “
We may, instead of submitting the two balls to the “ trial by
Dy
i Y/4
ele)
Ks PHY bof “
Pd
+‘.
(Glass} stances, so of a crystal it is its |
NATURE OF THE MATERIALS IN THE LITHOSPHERE 25
fire,” allow each to be attacked by the powerful reagent, hydro-
fluoric acid. The common glass under the attack of the acid
remains as it was before, a sphere, but with shrunken dimensions.
The crystal, on the other hand, is able to control the action of the
solvent, and in so doing its individuality is again revealed in a
beautifully etched figure having many curving outlines — it is as
though the crystal had possessed a soul which under this trial has
been revealed. This glimpse into the nature of the crystal, so as
to reveal its structural beauty, is still more surprising when the
crystal is taken from the acid in the
early stages of the action and held
close beneath the eye. Now the lit-
tle etchings upon the surface display
each the individuality of the sub-
stance, and joining with their neigh-
bors they send out a _ beautifully
symmetrical and entirely character-
istic picture (Fig. 11).
The lithosphere a complex of = eZ:
interlocking crystals. — To the lay- 2%}.7 7, Bee Sue wenger
man the crystal is something rare fy. 11. —*“Light figure” seen upon
and expensive, to be obtained from an etched surface of a crystal of
a jeweler or to be seen displayed in Cite (after Goldschmidt and
Wright).
the show cases of the great muse-
ums. Yet the one most striking quality of the lithosphere which
separates it from the hydrosphere and the atmosphere is its crys-
talline structure, — a structure belonging also to the meteorite, and
with little doubt to all the planets of the earth group. A snowflake
caught during its fall from the sky reveals all the delicate tracery
of crystal boundary; collected from a thick layer lying upon the
ground, it appears as an intricate aggregate of broken fragments
more or less firmly cemented together. And so it is of the litho-
sphere, for the myriads of individuals are either the ruins of former
crystals, or they are grown together in such a manner that crystal
facets had no opportunity to develop.
Such mineral individuals as once possessed the crystal form may
| ~ have been broken and their surfaces ground away by mutual attri-
tion under the rhythmic beating of the waves upon a shore or in
the continuous rolling of pebbles on a stream bed, until as bat-
26 EARTH FEATURES AND THEIR MEANING
tered relics they are piled away together in a bed of sand. Yet
no amount of such rough handling is sufficient to destroy the crys-
tal individuality, and if they are now surrounded with conditions
which are suitable for their growth, their individual nature again ©
becomes revealed in new crystal outlines. Many of our sand-
stones when turned in the bright sunlight send out flashes of light
to rival a bank of snow in early spring. These bright flashes
proceed from the facets of minute crystals formed about each
rounded grain of the sand, and if we examine them under a lens,
we may note the beauty of line formed with such exactness that
the most delicate instruments can detect no difference between
the similar angles of neighboring crystals (Fig. 12).
Fic. 12. — Battered sand grains which have taken on a new lease of life and have
developed a crystal form. a, a single grain grown into an individual crystal; b,
a parallel growth about a single grain; c, growth of neighboring grains until they
have mutually interfered and so destroyed the crystal facets — the common con-
dition within the mass of a rock (after Irving and Van Hise).
This individual nature of the crystal is believed to reside in a
symmetrical grouping of the chemical molecules of the substance
into larger and so-called ‘‘ crystal molecules.”” The erystal quality
belongs to the chemical elements and to their compounds in the
solid condition, but not to ordinary mixtures of them.
Some properties of natural crystals, minerals. — No two mineral
species appear in crystals of the same appearance, any more
than two animal species have been given the same form; and so
minerals may be recognized by the individual peculiarities of their
crystals. Yet for the reason that crystals have so generally been
prevented from developing or retaining their characteristic faces,
eal
Ae x
NATURE OF THE MATERIALS IN THE LITHOSPHERE 27
in the vast number of instances it is the behavior, and not the
appearance, of the mineral substance which is made use of for iden-
tification.
When a mineral is broken under the blow of a hammer, in-
stead of yielding an irregular fracture, like that of glass, it generally
tends to part along one or more directions so as to leave plane
surfaces. This property of cleavage is strikingly illustrated for
a single direction in the mineral mica, for two directions in feld-
spar, and for three directions in calcite or Iceland spar. Other
properties of minerals, such as hardness, specific gravity, luster,
color, fusibility, etc., are all made use of in rough determinations
of the minerals. Far more delicate methods depend upon the
behavior of minerals when observed in polarized light, and such
behavior is the basis of those branches of geological science known
as optical mineralogy and as microscopical petrography. An out-
line description of some of the common minerals and the means
for identifying them will be found in appendix A.
The alterations of minerals.— By far the larger number of
minerals have been formed in the cooling and consequent con-
solidation of molten rock material such as during a volcanic erup-
tion reaches the earth’s surface as lava. Beginning their growth
at many points within the viscous mass, the individual crystals
eventually may grow together and so prevent a development of
their crystal faces.
Another class of minerals are deposited from solution in iia
within the cavities and fissures of the rocks; and if this process
ceases before the cavities have been completely closed, the minerals
are found projecting from the walls in a beautiful lining of crys-
tal—the Krystallkeller or “ crystal cellar.’ It is from such
pockets or veins within the rocks that the valuable ores are ob-
tained, as are the crystals which are displayed in our mineral
cabinets.
There is, however, a third process by which minerals are formed,
and minerals of this class are produced within the solid rock as
a product of the alteration of preéxisting minerals. Under the
enormous pressures of the rocks deep below the earth’s surface,
they are as permeable to the percolating waters as is a sponge
at the surface. Under these conditions certain minerals are
dissolved and their material redeposited after traveling in the
28 EARTH FEATURES AND THEIR MEANING
solution, or solution and redeposition of mineral matter may go
on together within the mass of the same rock. One new mineral
may have been produced from the dissolved materials of a num-
ber of earlier species, or several new minerals may —
be the result of the alteration of a preéxisting min-
eral with a more complex chemical structure. Where
the new mineral has been formed “ in place,” it has
sometimes been able to utilize the materials of all
the minerals which before existed there, or it may
Fic. 13.—Crys- have been obliged to inclose within itself those earlier
tal of garnet onstituents which it could not assimilate in its own
developed in ;
a schist with structure (Fig. 13).
grains of At other times a crystal which is. imbedded in
pe on rock has been attacked upon its surface by the per-
cause not as- COlating solutions, and the dissolved
similated. materials have been deposited in place
as a crown of new minerals which steadily widens its
zone until the center is reached and the original
crystal has been entirely transformed (Fig. 14). It
is sometimes possible to say
that the action by which
these changes have been Fie. 14.—A
brought about has involved 4” ee ne
a nice adjustment of supply massof a rock
of the chemical constituents —_alteredin part
necessary to the formation . pes pai
of the new mineral or min- erals _ horn-
erals. In rocks which are blende and
a aggregates of several min- Note aed
Fic. 15.—A new mineral eral species, a newly formed _ inal outline of
(hornblende) forming as an mineral may appear only at the augite
intermediate “reaction . crystal.
rim” between the mineral the common margin of cer- °
having irregular fractures tain of these species, thus showing that
erage acs Pit they supply those chemical elements which
feldspar). were necessary to the formation of the
new substance (Fig. 15). Thus it is seen
that below the earth’s surface chemical reactions are constantly «
going on, and the earlier rocks are thus locally being transformed
into others of a different mineral constitution.
NATURE OF THE MATERIALS IN THE LITHOSPHERE 29
Near the earth’s surface the carbon dioxide and the moisture
which are present in the atmosphere are constantly changing
the exposed portions of the lithosphere into carbonates, hydrates,
and oxides. These compounds are more soluble than are the
minerals out of which they were formed, and they are also more
bulky and so tend to crack off from the parent mass on which
they were formed. As we are to see, for both of these reasons
the surface rocks of the lithosphere succumb to this attack from |
the atmosphere.
In connection with those wrinklings of the surface shell of the
' lithosphere from which mountains result, the underlying rocks
are subjected to great strains, and even where no visible partings
are produced, the rocks are deformed so that individual minerals
may be bent into crescent-shaped or S-shaped forms, or they are
parted into one or more fragments which remain imbedded within
the rock.
READING REFERENCES FOR CHAPTER III
Theories of origin of the earth : —
Tomson and Tait. Natural Philosophy. 2d ed. Cambridge, 1883,
pp. 422.
T.C. CHAMBERLIN. Chamberlin and Salisbury’s Geology, vol. 2, pp. 1-81.
Rigidity of the earth : —
Lorp Ketvin. The Internal Condition of the Earth as to Temperature,
Fluidity, and Rigidity, Popular Lectures and Addresses, vol. 2, pp.
299-318; Review of evidence regarding the physical condition of
the earth, ibid., pp. 238-272.
Hosss. Earthquakes (Appleton, New York, 1907), Chapters xvi and
Xvii.
Composition of the earth’s core and shell : —
O. C. Farrineron. The Preterrestrial History of Meteorites, Jour.
Geol., vol. 9, 1901, pp. 623-236. :
EK. 8. Dana. Minerals and How to Study Them (a book for beginners
in mineralogy). Wiley, New York, 1895.
On the nature of crystals : —
Victor Gotpscumiptr. Ueber das Wesen der Krystalle, Ostwalds Annalen
der Naturphilosophie, vol. 9, 1909-1910, pp. 120-139, 368-419.
CHAPTER IV
THE ROCKS OF THE EARTH’S SURFACE SHELL
The processes by which rocks are formed.— Rocks may be
formed in any one of several ways. When a portion of the molten
lithosphere, so-called magma, cools and consolidates, the product
is igneous rock. Either igneous or other rock may become dis-
integrated at the earth’s surface, and after more or less extended
travel, either in the air, in water, or in ice, be laid down as a sedi-
ment. Such sediments, whether cemented into a coherent mass
or not, are described as sedimentary or clastic rocks. If the fluid
from which they were deposited was the atmosphere, they are
known as subaérial or eolian sediments; but if water, they are
known as subaqueous deposits. Still another class are ice-deposited
and are known as glacial deposits.
But, as we have learned, rocks may undergo téanatocuations
through mineral alteration, in which case they are known as
metamorphic rocks.
When these changes
consist chiefly in the
production of more
soluble minerals at
the surface, accom-
panied by thorough
disintegration, due
to the direct attack
of the atmosphere,
the resulting rocks
are called residual
Fia. 16.— Laminated structure of sedimentary rock, rocks.
Western Kansas (aft h :
Tucker). cm aoa us dea The marks of ori-
- gin.—Each of the
three great classes of rocks, the igneous, sedimentary, and meta-
morphic, is characterized by both coarser and finer structures, in
the examination of which they may be identified. The igneous
30
¢
_—
IN cetietts once
4
BRC pat naar ries
THE ROCKS OF THE EARTH’S SURFACE SHELL oF
rocks having been produced from magmas, which are essentially
homogeneous, are usually without definite directional structures
due to an arrangement of their constituents, and are said to have
a massive structure. Sedimentary rocks, upon the other hand,
have been formed by an assorting process, the larger and heavier
fragments having been laid down when there was high velocity of
either wind or water current, and the smaller and lighter frag-
ments during intermediate periods. They are therefore more or
less banded, and are said to have a bedded or laminated structure
(Fig. 16).
Again, igneous rocks, being due to a process of crystallization,
are composed of mineral individuals which are bounded either
by crystal planes or by irregular surfaces along which neighboring
crystals have interfered with each other; but in either case the
grains possess sharply angular boundaries. Quite different has
been the result of the attrition between grains in the transpor-
tation and deposition of sediments, for it is characteristic of the
sedimentary rocks that their constituent grains are well rounded.
Kolian sediments have usually more perfectly rounded grains than
subaqueous deposits.
Glacial deposits, if laid down directly by the ice, are unstrati-
fied, relatively coarse, and contain pebbles which are both faceted
and striated. Such deposits are described as till or tillite. If
glacier-derived material is taken up by the streams of thaw
water and is by them redeposited, the sediments are assorted
or stratified, and they are described as fluvio-glacial deposits.
The metamorphic rocks.— Both the coarser structures and
the finer textures of the metamorphic rocks are intermediate
between those of the igneous and the sedimentary classes. A
metamorphosed sedimentary rock, in proportion to its alteration,
loses the perfect lamination and the rounded grain which were
its distinguishing characters; while an igneous rock takes on in
the process an imperfect banding, and the sharp angles of its
constituent grains become rounded off by a sort of peripheral
crushing or granulation. Metamorphic rocks are therefore
characterized by an imperfectly banded structure described as
schistosity or gneiss banding, and the constituent grains may be
either angular or rounded. If the metamorphism has not been
too intense or too long continued, it is generally possible to deter-
oe EARTH FEATURES AND THEIR MEANING
mine, particularly with the aid of the polarizing microscope,
whether the original rock from which it was derived was of igneous
or of sedimentary origin. There are, however, many examples
which have defied a reliable verdict concerning their origin.
Characteristic textures of the igneous rocks.— In addition to
the massiveness of their general aspect and the angular bound-
aries of their constituents, there are many additional textures
which are characteristic of the igneous rocks. While those that
have consolidated below the earth’s surface, the intrusive rocks,
are notably compact, the magmas which arrive at the surface of
the lithosphere before their consolidation reveal special structures
dependent either upon the expansion of steam and other gases
within them, or upon the conditions of flow over the earth’s sur-
face. Magmas which thus reach the surface of the earth are de-
scribed as lavas, and the rocks produced by their consolidation
are extrusive or volcanic rocks. The steam included in the lava
expands into bubbles or vesicles which may be large or small,
few or many. According to the number and the size of these
cavities, the rock is said to have a vesicular, scoriaceous, or pumi-
ceous texture.
Most lavas, when they arrive at the earth’s surface, contain
crystals which are more or less disseminated throughout the
molten mass. The tourist who visits Mount Vesuvius at the time
of a light eruption may thrust his staff into the stream of lava
and extract a portion of the viscous substance in which are seen
beautiful white crystals of the mineral leucite, each bounded by
twenty-four crystal faces. It is clear that these crystals must
have developed by a slow growth within the magma while it was
still below the surface, and when’ the inclosing lava has con-
solidated, these earlier crystals lie scattered within a groundmass
of glassy or minutely crystalline material. This scattering of
crystals belonging to an earlier generation within a groundmass
due to later consolidation is thus an indication of interruption in
the process of crystallization, and the texture which results is
described as porphyritic (Fig. 17 b). Should the lava arrive at
the surface before any crystals have been generated and consoli-
date rapidly as a rock glass, its texture is described as glassy *
(Fig. 17 c).
When the crystals of the earlier generation are numerous and
a eee Cte
THE ROCKS OF THE EARTH’S SURFACE SHELL 33
needle-like in form, as is very often the case, they arrange them-
selves “‘end on” during the rock flow, so that when consolida-
tion has occurred, the rock has akind of puckered lamination which
is the characteristic of the fluxion or flow texture. This texture
has sometimes been confused with the lamination of the sedi-
mentary rocks, so that wrong conclusions have been reached
Fic. 17. — Characteristic textures of igneous rocks. a, granitic texture characteristic
of the deep-seated intrusive rocks; b, porphyritic texture characteristic of the ex-
trusive and of the near-surface intrusive rocks ; c, glassy texture of an extrusive rock.
regarding origin. At other times the same needle-like crystals
within the lava have grouped themselves radially to form rounded
nodules called spherulites. Such nodules give to the rock a
spherulitic texture, which is nowhere better displayed than in the
beautiful glassy lavas of Obsidian Cliff in the Yellowstone Na-
tional Park.
Those intrusive rocks which consolidate deep below the earth’s
surface, part with their heat but slowly, and so the process of
crystallization is continued without interruption. Starting from
many centers, the crystals continue to grow until they mutually
intersect in an interlocking complex known as the granitic tex-
ture (Fig. 17 a).
Classification of rocks.—In tabular form rocks may thus be
classified as follows : —
Intrusive. Granitic or porphyritic texture.
Extrusive. Glassy or porphyritic texture;
often also with vesicular, scoriaceous, pumi-
ceous, fluxion, or spherulitic textures.
Igneous. Massive and
with sharply angular
grains.
34 EARTH FEATURES AND THEIR MEANING
‘Subaérial. Sands and loess.
Subaqueous. (See below.)
Sedimentary. Laminated | Glacial. Coarse, unstratified deposits with
and with rounded faceted pebbles. Till and tillite.
grains. Fluvio-glacial. Stratified sands and gravels
with ‘‘ worked over”’ glacial characters.
and with grains either changes.
Metamorphic. Schistose {Metamorphic proper. Due to below surface
angular or rounded. Residual. Disintegrated at or near surface.
Subdivisions of the sedimentary rocks. —While the eolian
sediments are all the product of a purely mechanical process of
lifting, transportation, and deposition of rock particles, this is
not always the case with the subaqueous sediments, since water
has the power of dissolving mineral substance, as it has also of
furnishing a home for animal and vegetable life. Deposited
materials which have been in solution in water are described as
chemical deposits, and those which have played a part in the life
process as organic deposits. The organic deposits from vege-
table sources are peat and the coals, while limestones and marls
are the chief depositories of the remains of the animal life of the
water. The tabular classification of the sediments is as follows : —
Classification of Sediments.
Subaqueous ' Conglomerate, sand-
Deposited by water. stone and shale.
Subaérial or Eolian Sandstone and loess.
’ Deposited by wind.
p.
sis aglaa asa Glacial Till and tillite.
Deposited by ice.
Fluvio-glacial Sands and gravels.
Glacier-water deposits.
‘Calcareous tufa Deposited in springs
and rivers.
Chasiect ; Odlitic limestone Deposited at the
mouths of rivers
between high and
q low tide.
Formed of plant re- Peats and coals.
mains. FA
Formed of animal re- Limestones and
mains. marls.
Organic
THE ROCKS OF THE EARTH’S SURFACE SHELL 35
Winds are under favorable conditions capable of transporting
both dust and sand, but not the larger rock fragments. The dust
deposits are found accumulating outside the borders of des-
erts as the so-called loess (Fig. 216), though the sand is never
carried beyond the desert border, near which it collects in wide
belts of ridges described as dunes. When this sand has been
cemented into a coherent mass, it is known as eolian sandstone.
A section of the appendix (B) is devoted to an outline description
of some of the commoner rock types.
The different deposits of ocean, lake, and river.—Of the sub-
aqueous sediments, there are three distinct types resulting:
(1) from sedimentation in rivers, the fluviatile deposits; (2) from
sedimentation in lakes, the lacustrine deposits; and (8) from sed-
imentation in the ocean, marine deposits. Again, the widest
range of character is displayed by the deposits which are laid
down in the different parts of the course of a stream. Near the
source of a river, coarse river gravels may be found; in the middle
course the finer silts; and in the mouth or delta region, where the
deposits enter the sea or a lake, there is found an assortment of
silts and clays. Except within the delta region, where the area
of deposition begins to broaden, the deposits of rivers are stretched
out in long and relatively narrow zones, and are so distinguished
_ from the far more important lacustrine and marine deposits.
Lakes and oceans have this in common that both are bodies
of standing as contrasted with flowing water; and both are sub-
ject to the periodical rhythmic motions and alongshore currents
due to the waves raised by the wind. About their margins, the
deposits of lake and ocean are thus in large part wrested by the
waves from the neighboring land. Their distribution is always
‘such that the coarsest materials are laid down nearest to the shore,
and the deposits become ever finer in the .direction of deeper
water. Relatively far from shore may be found the finest sands
and muds or calcareous deposits, while near the shore are sands,
and, finally, along the beach, beds of beach pebbles or shingle.
When cemented into coherent rocks, these deposits become shales
or limestones, sandstones, and conglomerates, respectively.
As regards the limestones, their origin is involved in consid-
erable uncertainty. Some, like the shell limestone or coquina
of the Florida coast, are an aggregation of remains of mollusks
36 EARTH FEATURES AND THEIR MEANING
which live near the border of the sea. Other limestones are de-
posited directly from carbonate of lime in solution in the water.
A deposit of this nature is forming in southern Florida, both as
a flocculent calcareous mud and as crystals of lime carbonate
upon a limestone surface. Again, there is the reef limestone ~
which is built up of the stony parts of the coral animal, and,
lastly, the calcareous ooze of the deep-sea deposits. .
The marine sediments which are derived from the conti-
nents, the so-called terrigenous deposits, are found only upon the
continental shelf and upon the continental slope just outside it.
Of these terrigenous deposits, it is customary to distinguish:
(1) littoral or alongshore deposits, which are laid down between
high and low tide levels; (2) shoal water deposits, which are found
between low-water mark and the edge of the continental shelf; and
(3) aktian or offshore deposits, which are found upon the conti-
nental slope. The littoral and shoal water deposits are mainly
gravels and sands, while the offshore deposits are principally
muds or lime deposits.
Special marks of littoral deposits.—The marks of ripples are
often left in the sand of a beach, and may be preserved in the sand-
stone which results from the cementation of such deposits (pl. 11 A).
Very similar markings are, however, quite characteristic: of the
surface of wind-blown sand. For the reason that deposits are
subject to many vicissitudes in their subsequent history, so that
they sometimes stand at steep angles or are even overturned,
it is important to observe the curves of sand ripples so as to dis-
tinguish the upper from the lower surface.
In the finer sands and muds of sheltered tidal flats may be pre-
served the impressions from raindrops or of the feet of animals
which have wandered over the flat during an ebb tide. When
the tide is at flood, new material is laid down upon the surface
and the impressions are filled, but though hardened into rock,
these surfaces are those upon which the rock is easily parted,
and so the impressions are preserved. In the sandstones of the
Connecticut valley there has been preserved a quite remarkable
record in the footprints of animals belonging to extinct species,
which at the time these deposits were laid down must have been |
abundant upon the neighboring shores.
Between the tides muds may dry out and crack in intersecting
=
Se
OS ge Bh eS
= _— 4
apis —-
a wl
THE ROCKS OF THE EARTH’S SURFACE SHELL 37
lines like the walls of a honeycomb, and when the cracks have been
filled at high tide, a structure is produced which may later be
recognized and is usually referred to as ‘‘ mud-crack”’ structure.
This structure is of special service in distinguishing marine de-
posits from the subaérial or continental deposits.
A variation in the direction of winds of successive storms
may be responsible for the piling up of the beach sand in a pecul-
iar “‘ plunge and flow ”’ or “ cross-bedded ” structure, a structure
which is extremely common in littoral deposits, though simu-
lated in rocks of eolian origin. j
The order of deposition during a transgression of the sea. —
Many shore lines of the continents are almost constantly migrat-
ing either landward or seaward. When the shore line advances
Fig. 18. — Diagram to show the order of the sediments laid down during a trans-
gression of the sea.
over the land, the coast is sinking, and marine deposits will be
_ formed directly above what was recently the ‘‘ dry land.” Such
an invasion of the land by the sea, due to a subsidence of the coast,
is called a transgression of the sea, or simply a transgression.
Though at any moment the littoral, shoal water, and offshore
deposits are each being laid down in a particular zone, it is evi-
dent that each must advance in turn in the direction of the shore
and so be deposited above the zones nearer shore. Thus there
comes to be a definite series of continuous beds, one above the other,
provided only that the process is continued (Fig. 18). At the
very bottom of this series there will usually be found a thin bed
of pebbly beach materials, which later will harden into the so-
called basal conglomerate. If the size of the pebbles is such as to
make possible an identification, it will generally be found that these
represent. the ruins of the rock over which the sea has advanced
upon the land.
Next in order above the basal conglomerate, will follow the
coarser and then the finer sands, upon which in turn will be laid
down the offshore sediments — the muds and the lime deposits.
38 EARTH FEATURES AND THEIR MEANING
Later, when cemented together, these become in order, coarser
and finer sandstones, shales, and limestones. The order of super-
position, reading from the bottom to the top, thus gives the order
of decreasing age of the formations.
A subsequent uplift of the coast will be accompanied by a
recession of the sea, and when later dissected by nature for our
inspection, the order of superposition and the individual character
of each of the deposits may be studied at leisure. From such
studies it has been found that along with the inorganic deposits
there are often found the remains of life in the hard parts of such
invertebrate animals as the mollusks and the crustacea. These
so-called fossils represent animals which were gradually developed
from simpler to more and more complex forms; and they thus
serve the purpose of successive page numbers in arranging the
order of disturbed strata, at the same time that they supply
the most secure foundation upon which rests the great doctrine
of evolution.
The basins of earlier ages.—It was the great Viennese geolo-
gist, Professor Suess, who first pointed out that in mountain regions
there are found the thickest and the most complete series of the
marine deposits; whereas outside these provinces the forma-
tions are separated by wide gaps representing periods when no
deposits were laid down because the sea had retired from the
region. The completeness of the series of deposits in the mountain
districts can only be interpreted to mean that where these but
lately formed mountains rise to-day, were for long preceding ages
the basins for deposition of terrigenous sediments. It would
seem that the lithosphere in its adjustment had selected these
earlier sea basins with their heavy layers of sediment for zones of
special uplift.
The deposits of the deep sea. — Outside the continental slope,
whose base marks the limit of the terrigenous deposits, lies the
deeper sea, for the most part a series of broad plains, but varied by
more profound steep-walled basins, the so-called “‘ deeps ” of the
ocean. As shown by the dredgings of the Challenger expedi-
tion and others of more recent date, the deposits upon the ocean
floor are of a wholly different character from those which are «
derived from the continents. Except in the great deeps, or
between depths of five hundred and fifteen hundred fathoms,
THE ROCKS OF THE EARTH’S SURFACE SHELL 39
these deposits are the so-called ‘‘ ooze,’”’ composed of the cal-
careous or chitinous parts of alge and of minute animal organisms.
The pelagic or surface waters of the ocean are, as it were, a great
meadow of these plant forms, upon which the minute crustacea,
such as globigerina, foraminifera, and the pteropods, feed in count-
less myriads. The hard parts of both plant and animal organisms
descend to the bottom and there form the ooze in which are some-
times found the ear bones of whales and the teeth of sharks.
In the deeps of the ocean, none of these vegetable or animal
deposits are being laid down, but only the so-called ‘ red clay,”
which is believed to represent decomposed volcanic material
deposited by the winds as fine dust on the surface of the ocean, or
the product of submarine volcanic eruption. From the absence
of the ooze in these profound depths, the conclusion is forced upon
us that the hard parts of the minute organisms are dissolved while
falling through three or four miles of the ocean water.
READING REFERENCES FOR CHAPTER IV
J. 8. Ditter. The Educational Series of Rock Specimens collected and
distributed by the United States Geological Survey, Bull. 150
U.S. Geol. Surv., 1898, pp. 1-400.
L. V. Pirrsson. Rocks and Rock Minerals. Wiley, New York, 1908.
Str Joun Morray. Deep-sea Deposits, Reports of the Challenger
expedition, Chapter iii.
L. W. Cotter. Les dépéts marins. Doin, Paris, 1907 (Encyclopédie
Scientifique).
CHAPTER V
CONTORTIONS OF THE STRATA WITHIN THE ZONE OF
FLOW
The zones of fracture and flow. — It is easy to think of the
atmosphere and the hydrosphere as each sustaining at any point
the load of the superincumbent material. At the sea level the
weight of air upon each square inch of surface is about fifteen
pounds, whereas upon the floor of the hydrosphere in the more
profound deeps the load upon the square inch must be measured
in tons. Near the lithosphere surface the rocks support by their
strength the load of rock above them, but at greater depths they
are unable to do this, for the load bears upon each portion
of the rock with a pressure equivalent to the weight of a rock
column which extends upward to the surface. The average
specific gravity of rock is 2.7, and it is thus easy to calculate the
length of the inch square column which has a weight equivalent
to the crushing strength of any given rock. At the depth repre-
sented by the length of such a column, rocks cannot yield to pres-
sure by fracture, for the opening of a crack implies that the rock
upon either side is strong enough to prevent the walls from clos-
ing. At this depth, rock must therefore yield to pressure not by
fracture, as it would at the surface, but by flow after the manner
of a liquid; and so the zone below this critical level is referred to
as the zone of flow.
In contrast, the near-surface zone is called the zone of fracture.
But different rocks possess different strengths, and these are
subject to modifications from other conditions, such, for example,
as the proximity of an uncooled magma. The zone of flow is
therefore joined to the zone of fracture, not upon a definite surface,
but in an intermediate zone described as the zone of fracture and
flow.
Experiments which illustrate the fracture and flow of solid
bodies. — A prismatic block prepared from stiff molders’. wax, ~
if crushed between the jaws of a testing machine, yields a system
40
CONTORTIONS OF. THE STRATA At.
of intersecting fractures which are perpendicular to the free sur-
faces of the block and take two directions each inclined. by half
of a right angle to the direction of compression
(Fig. 19). This experiment may illustrate the
manner in which fractures are produced by
the compression within the zone of fracture
of the lithosphere, as its core continues to
contract. |
To reproduce the conditions within the zone
of flow, it will be necessary to load the lateral
surfaces of the block instead of leaving them
unconstrained as in the above-described ex-
periment. The experiment is best devised as
in Fig. 20. Here a series of layers having
varying degrees of rigidity is prepared from
beeswax as a base, either stiffened by ad-
mixture of varying proportions of plaster of
Paris, or weakened by the use of Venice turpen-
tine. Such a series of layers may represent
rocks of as widely different characters as lime-
stone and shale. The load which is to rep-
resent superincumbent rock is supplied in the
experiment by a deep layer of shot.
When compression is applied to the layers
from the ends, these normally solid materials,
instead of fracturing, are bent into a series
-Fia. 19.— Two inter-
secting parallel series
of fractures produced
upon each free sur-
face of a prismatic
block of stiff molders’
wax when broken by
compression from the.
ends (after Daubrée
and Tresca).
of folds. The stiffer, or more competent, layers are found to be
less contorted than are the weaker layers,
particularly if the
Section on line ab Section on line cd
Fic. 20.— Apparatus to illustrate the folding of strata within the zone of flow
(after Willis).
42 EARTH FEATURES AND THEIR MEANING
latter have been protected under an arch of the more competent
layer (pl. 2 A).
The arches and troughs of the folded strata. — Every series
of folds is made up of alternating arches and troughs. The arches
of the strata the geologist calls anticlines or anticlinal folds, and
the troughs he calls synclines or synclinal folds (Fig. 21). When a
stratum is merely dropped in a
Ds Ge a bend to a lower level without
producing a complete arch or a
complete trough, this half fold
ee ees is termed a monocline. .
Fic. 21.—Diagrams representing a, an Any flexuring of the strata
anticline ; 6, a syncline ; and c, a mono- implies a reduction of their
cline.
surface area, or, considering a
single section, a shortening. If the arches and troughs are low
and broad, the deformation of the strata is slight, the shorten-
ing is comparatively small, and the folds are described as open
(Fig. 22 b). If they be relatively both
high and narrow, the deformation is
considerable, a larger amount of crustal
shortening has gone on, and the folds
are described as close (Fig.22c). This
closing up of the folds may continue
until their sides have practically the
same slope, in which case they are said
to be isoclinal (Fig. 22 d).
The elements of folds. — Folds must
always be thought of as having ex-
tension in each of the three dimensions
of space (Fig. 23), and not as properly
included within a single plane like the
cross sections which we so often use in
illustration. A fold may be conceived
of as divided into equal parts by a plane
@e —
é ————_—
at | SRY. Ee
PN eb A
Fic. 22.—A comparison of
folds to express increasing
degrees of crustal shortening
or progressive deformation
within the zone of flow: a,
stratum before folding; b,
open folds; c, close folds ;
which passes along the middle of either the arch or the trough,
and is called the axial plane. The line in which this plane inter-
sects the arch or the trough is the axis, which may be called the»
crestline in an anticline, and the troughline in asyncline.
In the case of many open folds the axis is practically hori-
d, isoclinal folds.
CONTORTIONS OF THE STRATA 43
zontal, but, in more complexly folded regions this is seldom true.
The departure of the axis from the horizontal is called the pitch,
and folds of this type are described as pitching folds or plunging
A a pPITGH
Fig. 23. — Anticlinal and synclinal folds in strata (after Willis).
folds. The axis is in reality in these cases thrown into a series
of undulations or ‘ longitudinal folds,” and hence pitch will
vary along the axis.
The shapes of rock folds. — By the axial plane each fold is
divided into two parts which are called its limbs, which may have
either the same or different average inclinations. To describe
now the shapes of rock folds and not the degree of compression of
the district, some additional terms are necessary. Anticlines
or synclines whose limbs have about the same inclinations are
known as upright or symmetrical folds. The axial plane of the
symmetrical fold is vertical (Fig. 24). If this plane is inclined to
the vertical, the folds are unsymmetrical. So soon as the steeper
of the two limbs has passed the vertical position and inclines in
the same direction as the flatter limb, the fold is said to be over-
turned. 'The departure from symmetry may go so far that the
axial plane of the fold lies at a very flat angle, and the fold is then
said to be recumbent. The observant traveler by train along any
of the routes which enter the Alps may from his car window find
illustrations of most of these types of rock folds, as he may also,
44 EARTH FEATURES AND THEIR MEANING
though generally less easily, in passing through the Appalachian
Symrmerrical
\ \ .
\ \
\ \
\ \
\ \
Uns yrrrmnerrical
\
5
% i
oi
‘
Overrurmed
So
=~
SON
ae =
“ N
SS ~
Frecurnbentr
Fig. 24. — Diagrams to illustrate
the different shapes of rock folds.
Mountains.
In regions which have been closely
folded the larger flexures of the strata —
may be found with folds of a smaller
order of magnitude superimposed
upon them, and these in turn may
show crumplings of still lower orders.
It has been found that the folds of
the smaller orders of magnitude pos-
sess the shapes of the larger flexures,
and much is therefore to be learned
from their careful study (Fig. 25).
It is also quite generally discovered
that parallel planes of ready parting,
which are described as rock cleavage,
take their course parallel to the axial
plane within each minor fold. As
was long ago shown by-the pioneer
British geologists, these planes of
cleavage are essentially parallel and
follow the fold axes throughout large
areas. .
The overthrust fold. — Whenever
a stratum is bent, there is a tendency for its particles to be
separated upon the convex side of the bend, at the same time
that those upon the con-
cave side are crowded
closer together — there
is tension in the former
case and compression
in the latter (Fig. 26).
Within an unsymmet-
rical or an overturned
fold, the peculiar dis-
tortions in the different
sections of the stratum
are less simple and are
best illustrated by
Fia. 25.— Secondary and tertiary flexures superim-
posed upon the primary ones.
a —_—-----
PLATE ¢2:
A. Layers compressed in experiments and showing the effect of a competent layer
in the process of folding (after Willis).
—————eE
B. Experimental production of a series of parallel thrusts within closely folded strata
(after Willis).
q
,
C. Apparatus to illustrate shearing action within the overturned limb of a fold.
Se A
CONTORTIONS OF THE STRATA 45
pl.2C. This apparatus shows two similar piles of paper sheets,
upon the edges of each of which a series of circles has been drawn.
When now one of the piles is bent into an unsymmetrical fold, it
is seen that through an accommodation by the paper sheets sliding
each over its neighbor large distortions of the circles have occurred.
In that steeper limb which with closer folding will be overturned
the circles have been drawn
out into long and narrow
ellipses, and this indicates -
that those rock particles
which before the bending
were included in the circle |
have been moved past each Fic. 26.— A bent stratum to illustrate tension
upon the convex and compression upon the
other in the manner of the concave side (after Van Hise).
blades of a pair of shears.
Such extreme “ shearing” action is thus localized in the under-
turned limb of the fold, and a time must come with continuation
of the compression when the fold will rupture at this critical place
along a plane parallel to the longest axis of the ellipses or nearly
parallel to the axial plane of the anticline. Such structures prob-
ably occur in the zone of combined fracture and flow, up into
which the beds are forced in cases of close compression. Relief
thus being found upon this plane of fracture, the upper portion
— of the fold will now ride over. the lower, and the displacement is
described .as.a. thrust or overthrust.
In the long series of experiments conducted by Mr. Bailey
Willis of the United States Geological Survey, all the stages be-
tween the overturned fold and the overthrust fold were reproduced.
Where a series of folds was closely compressed, a parallel series of
thrusts developed (pl. 2 B), so that a series of slices cutting across
neighboring strata was slid in succession, each over the other,
like the scales upon a fish or the shingles upon a roof. Quite
remarkable structures of this kind have been discovered in rocks —
of such closely folded districts as the Northwest Highlands of Scot-
land, where the overriding is measured in miles. Near the thrust
planes the rocks show a crushing of the grains, and the planes them-
selves are sometimes corrugated and: polished by the movement.
Restoration of mutilated folds. — Since flexuring of the rocks
takes place within the zone of flow at a distance of several miles
46 EARTH FEATURES AND THEIR MEANING
below the earth’s surface, it is quite obvious that the results of the
process can be studied only after some thousands of feet of super-
incumbent strata have been removed. We are a little later to see
by what processes this lowering of the surface is accomplished,
but for the present it may be sufficient to accept the fact, realizing —
that before foldings in the strata can reach the surface, they must
have passed through the upper zone of fracture.
It might perhaps be supposed that the anticlines would appear
as the mountains upon the surface, and occasionally this is true;
as, for example, in the folded Jura Mountains of western Europe.
More generally, the mountains have a synclinal structure and the
valleys an anticlinal one; but as no general rule can be applied,
it is necessary to make a restoration of the truncated folds in each
district before their character can be known. |
The geological map and section. — The earth’s surface is in
most regions in large part covered with soil or with other inco-
herent rock material, so that over considerable areas the hard rocks.
are hidden from view. Each locality at which the rock is found
at the earth’s surface “in place” is described as an outcropping
or exposure. In a study of the region each such exposure must
be examined to determine the nature of the rock, especially for
the purpose of correlation with neighboring exposures, and, in
addition, both the probable direction in which it is continued along
the surface —the strike—and the inclination of its beds —
the dip. If the outcroppings are sufficiently numerous, and rock
type, strike and dip, may all be determined, the folds of the dis-
trict may be restored with almost as much accuracy as though
their curves were everywhere exposed to view. A cross section
through the surface which represents the observed outcrops with
their inclinations and the assumed intermediate strata in their
probable attitudes‘is described as a geological section (Fig. 27). A
map upon which the data have been entered in their correct loca-
tions, either with or without assumptions concerning the covered
areas, is known as a geological map.
If the axes of folds are absolutely horizontal, and the surface
of the earth be represented as a plain, the lines of intersection of
the truncated strata with the ground, or with any horizontal sur- -
face, will give the directions of continuation of the individual
strata. This strike direction is usually determined at each expo-
CONTORTIONS OF THE STRATA 47
sure by use of a compass provided with a spirit level. When that
edge of the leveled compass which is parallel to the north-south
line upon the dial is held against the sloping rock stratum, the
yy
Lif by
r
LLEGL 8 WU) oS
4 Gai NNN
c i Cc
Fig. 27.—A geological section based upon observations at outcrops, but with
the truncated arches restored.
angle of strike is measured in degrees by the compass needle. If
the cardinal directions have been placed in their correct positions
upon the compass dial, the needle will point to the northwest
when the strike is northeast, and vice versa (Fig. 28 a). Upon
—SS \ \\ Z
——s =| Ss SN \\ Mt
NN She hi 7 ty A
vali ao ‘l
rll
Fig. 28. — Diagram to illustrate the manner of oe acl te strike of rock beds
at an outcropping. a, a compass which has the cardinal directions in their
natural positions; b, a compass with the east and west initials reversed upon the
dial ; c, home-made clinometer in position to determine the dip.
the geologist’s compass it is therefore customary to reverse the
initials which represent the east and west directions, in order that
the correct strike may be read directly from the dial (Fig. 28 6).
By the dip is meant the inclination of the stratum at any expo-
sure, and this must obviously be measured in a vertical plane
48 EARTH FEATURES AND THEIR MEANING
along the steepest line in the bedding plane. The dip angle is
always referred to a horizontal plane, and hence vertical beds have
a dip of 90°. The device for measuring this angle of dip, the
clinometer, is merely a simple pendulum which serves as an indi-
eator and is centered at the corner of a graduated quadrant. A
home-made variety is easily constructed from a square piece of
board and an attached paper quadrant (Fig. 28 c), but the geolo-
gist’s compass is always provided with a clinometer attachment
to the dial.
Since the strike is the intersection of the bedding plane with a
horizontal surface, and the dipis the intersection with that partic-
ular vertical plane which gives the steepest inclination, the strike
and dip are perpendicular to each other. To represent them
upon maps, it is more or less customary to use the so-called T
symbols, the top of the T giving the direction of the strike and the
shank that of the dip. If meridians are drawn upon the map, the
direction or attitude of the T can be found by the use of a simple
protractor; and when entered upon the map, the exact angle of
the strike may be supplied by a figure near the top of the T, and
the dip angle by a figure at the end of the shank. It is the custom,
also, to make the length of the shank inversely proportional to
the steepness of the dip, so that in a broad way the attitudes of
the strata may be taken in at a glance (Fig. 29). It is further of
advantage to make the top of the
\ T a double line, so that some
symbol or color may show the
4 so~ correlations of the different expo-
45 a sures. To illustrate, in Fig. 29,
the symbol marked a represents
4 e ° an outcrop of limestene, the strike
15 of which is 50° east of north (N.
- 60° E.), and the dip of which is
Fic. 29.— Diagram to show the use 49° southeast. In the same figure
of T symbols to indicate the dip and } represents a shale outcrop in hori-
usin ease 3 eas zontal beds, which have in conse-
quence a universal strike and a dip of 0°. An exposure of limestone
in vertical beds which strike N. 60° E. is shown at c, ete.
Measurement of the thickness of formations. — When forma-
tions still lie in horizontal beds, we may sometimes learn their
CONTORTIONS OF THE STRATA 49
thickness directly either from the depth of borings to the under-
lying rock, or by measurements upon steep cafion walls. If the
beds stand vertically, the matter is exceedingly simple, for in this
case the thickness is the width of the outcrops of the formation
between the beds which bound it upon either side. In the general
case, in which the beds are 3
neither horizontal nor ver- ! fi a
tical, the thickness must be ; P
obtained indirectly from the
width of the exposures and
the angle of the dip. The
factor by which the ex-
posure width must be mul- \
tiplied is known as the sine fy¢, 30.— Diagram to show how the thickness
of the dip angle (Fig. 30), of a formation may be obtained from the
which is given with sufficient angle of the dip and the width of the ex-
accuracy for most purposes piu
in the following table. It is obvious that in order to obtain
the full thickness of a formation it is necessary to measure from
the contact with the adjacent formation upon the one side to a
similar contact with the nearest formation upon the other.
Natural Sines
0° -00 35° OT 70° 94
5° .09 40° 64 75° 97
10° ay 4 45° 71 80° .98
168) 8 50° a i § 85° 1.00
20° 34 55° 82 90° 1.00
25° 42 GO’: FS ac
30° ~=—..50 65° 91
The detection of plunging folds. — When the axis of a fold is
horizontal, its outcrops upon a plain will continue to have the same
strike until the formation comes to an. end. Upon a generally
level surface, therefore, any regular progressive variation in the
strike direction is an indication that the folds have a plunging
or pitching character. Many serious mistakes of interpretation
have been made because of a failure to recognize this evidence of
plunging folds. The way in which the strikes are progressively
modified will be made clear by the diagrams of Figs. 31 and 32,
E
50 EARTH FEATURES AND THEIR MEANING
the first representing a pitching anticline and the second a pitch- |
ing syncline. In both these reciprocal cases the strikes of the
cm
AEA
NAVAN AN
A
\\
ANI
AW
\
\
——. “"
er
Fig. 31.— Combined surface and sectional views of a plunging anticline (after Willis).
beds undergo the same changes, and the dip directions serve to
distinguish which of the two structures is present in a given case.
There is, however, one further difference in that the hard layers
Fia. 32.— Combined surface and sectional views of a plunging syncline (after Willis).
oe ieee
CONTORTIONS OF THE STRATA 51
of the plunging anticline, where they disappear below the surface
in the axis, will present a domed surface sloping forward like the
back of a whale as it rises above the surface of the sea. Plunging
folds in series will thus appear in the topography as a series of
sharply zigzagging ranges at those localities where the harder
layers intersect the surface. Such features are encountered in
eastern Pennsylvania, where the hard formations of the Appala-
chian Mountain system plunge northeastward under the later
formations. The pitch of the larger fold is often disclosed by that
of the minor puckerings superimposed upon it.
The meaning of an unconformity. — The rock beds, which are
deposited one above the other during a transgression of the sea,
SSS =
——————S Tie am => *
Fig. 33. — See between a lower and an upper series of beds upon the coast
of California. Note how the hard layer stands in relief upon the connecting
surface (after Fairbanks).
are usually parallel and thus represent a continuous process of
deposition. Such beds are said to be conformable. Where, upon
the other hand, two series of deposits which are not parallel to
each other are separated by a break, they are said to form un-
conformable series, and the break or surface of junction is an un-
conformity (Fig. 33).
52 EARTH FEATURES AND THEIR MEANING
Here it is evident that the sediments which compose the lower
series of beds have been folded in the zone of flow, though the
upper series has evidently escaped this vicissitude. Furthermore,
the surface which delimits the lower series from the upper is some-
what irregular and shows a hard layer standing in relief, as it |
would if it had opposed greater resistance to the attacks of the
atmosphere upon it. |
In reality, an unconformity between formations must be in-
terpreted to mean that the lower series is not only older than the
upper, as shown by the order of superposition, but that the time
of its deposition was separated from that of the upper by a hiatus
in which important changes took place in the lower series. The
stages or episodes in the history of the beds represented in
Fig. 33 may be read as follows (see Fig. 34 a-e) :—
(a) Deposition
of the lower series
during a transgres-
sion of the sea.
(b) Continued
subsidence and
burial of the lower
series beneath
overlying sedi-
ments, and flexur-
ing in the zone of
flow.
(c) Elevation of
the combined de-
posits to and far
above sea level and
removal by erosion
Fia. 34. — Series of diagrams to illustrate in succession the f + thick
episodes involved in the historical development of an 0! VS ICKNESSES
angular unconformity. The vertical arrows indicate of the upper sedi-
the direction of movement of the land, and the horizontal ments
arrows the direction of shore migration. ( d) A new sub
sidence of the truncated lower series and deposition of the upper
series across its eroded surface.
(e) A new elevation of the double series to its present position
above sea level.
Se =
ate — apis aie.
CONTORTIONS OF THE STRATA 53
From this succession of episodes it is seen that a break of this
kind between two series of deposits involves a double oscillation
of subsidence followed by elevation — a large depression followed
by a large elevation, a smaller subsidence followed by elevation.
The time interval which must have been represented by these re-
peated operations is so vast as at first to stagger the mind in con-
templating it. When, as in this instance, the dips of the lower
series of beds differ from those of the upper, we have to do with
an angular unconformity. It may be, however, that the lower
series was not so far depressed as to enter the zone of flow, and
that its beds meet those of the upper series with apparent con-
formity. Such an unconformity is often extremely difficult to
recognize, and it is described as a deceptive or erosional uncon-
formity.
With a deceptive unconformity the clew to its real nature is
usually some fact which indicates that the lower series of sedi-
ments had been raised above the _____.
level of the sea: before the upper === =
series was deposited upon it. +
This may be apparent either in
the irregularity of the surface on
_ which the two series are joined,
in some evidence of the action
of waves such as would be fur-
nished by a basal conglomerate
in the upper series, or some in-
dication of different resistance of
different rocks of the lower series -
to attacks of the atmosphere
upon them (Figs. 33 and 35 a-c).
In most cases, at least, the
lowest member of the upper
series will be a different type of ‘
Cc: Depression orf surface over
rock from the uppermost mem- a- weak rock, and projection over
ber of the lower series, hence the b- strong rock.
frequent occurrence of the dis- Fie.35.— Types of deceptive or erosional
cordant cross bedding in sand- Renee:
stone should not deceive even the novice into the assumption
of an unconformity.
54 EARTH FEATURES AND THEIR MEANING
READING REFERENCES TO CHAPTER V
The zones of fracture and flow :—
C. R. Van Hist. Principles of North American Precambrian Geology,,.
16th Ann. Rept. U.S. Geol. Surv., 1895, Pt. I, pp. 581-603.
Baitey Wituis. Mechanics of Appalachian Structure, 13th Ann. Rept..
U.S. Geol. Surv., 1893, Pt. II, pp. 217-253.
A. Davusrée. Etudes Synthétiques de Géologie Expérimentale. Paris,.
1879, pp. 306-328, pl. IT.
W. Prinz. Quelques remarques générales & propos de l’essai de carte:
tectonique de la belgique, etc., Bull. Soc. Belge Geol., vol. 18, 1904,,
p. 143, pl. V.
Analysis of folds : —
Van Hise and WILuIs as above; p—E Marcerir et Herm; Les disloca--
tions de l’écorce terrestre (in French and German languages). Zurich,
1888.
Geological maps : —
Wa. H. Hosss. The Mapping of the Crystalline Schists, Jour. Geol.,
vol. 10, 1902, pp. 780-792, 858-890.
CHAPTER VI
THE ARCHITECTURE OF THE FRACTURED SUPER-
STRUCTURE
The system of the fractures. — In referring to experiments made
upon the fracture of solid blocks under compression (p. 41), it was
shown that two series of parallel fractures develop perpendicular
to each free surface of
the block, and _ that
these series are each of
them inclined by half
of a right angle to the
direction of compres-
sion, and thus perpen-
dicular to each other.
The fragments into
which a block with one
free surface would thus
tend to be divided
should be square prisms
perpendicular to the
free surface. It would
be interesting, if it were
Pei Sa is Bat au 7 aie ip ee
ote er : ’
F Wess 1) rap Wy any
practicable, to learn sent Wen As pela
from Pepermnent how Ria a “det ig
: ‘ ASN ’ yy
these prisms would be Ree wil A . i
further fractured by a Fig. 36.— A set of master joints developed in shale
. * upon the shores of Cayuga Lake near Ithaca,
continuation of the com- . New York (after U. S. G. 8.).
pression. From me-
chanical considerations involving the resolution of forces with refer-
ence to the ready-formed fractures, it seems probable that the next
series of fractures to form would bisect the angles of the first double
series or set. Wherever rocks are found exposed in their original
55
56 EARTH FEATURES AND THEIR MEANING
attitudes, they are, in
fact, seen to be inter-
sected by two parallel
series of fractures
which are perpendicu-
lar to the earth’s sur-
face and to each other
and are described as
joints. In many cases
more than two series of
WY
such fractures are
< 4 found, yet even in
N
these cases two more
va
perfectly developed
series are prominent
Fic. 37. — Diagram to show how sets of master joints
and almost exactly ;
perpendicular to each |
other as well as to the |
differing in direction by half a right angle may earth’s surface. This
abruptly replace each other.
omnipresent double series or
set of joints is the well-known
set of master joints, and very
often it is found developed
practically alone (Fig. 36).
Over large areas, the direction
of the set of master joints
may remain practically con-
stant, or this set may quite
suddenly give place to a sim-
ilar set which is, however,
turned through half a right
angle from the first (Fig.
37). Not infrequently two
such sets of master joints
are found together bisecting
each other’s angles within the
Same rocks, and to them
aus i
SS
- ae
= \ BYAS %
=== <ESR SE
AZ a
x
a4
Fig. 38.— Diagram to show the different ~
combinations of the series composing two
double sets of master joints, and in a, a, a
additional disorderly fractures.
ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 57
are sometimes added additional though less perfect series of joint
planes.
Studied throughout a considerable district, the various series
which make up these two sets of master joints may be seen locally
—
» SSS SS
A Sw >
Ss >
equal spacing of the joints (after Kornerup).
developed in different combinations as well as in association with
additional fissure planes which are not easily reduced to any simple
law of arrangement
(Fig. 38 a, a, a).
Only rarely are reg-
ular joint series ob-
served which do not
stand perpendicular
to the original atti-
tude of the rock
beds. In afewlocal-
ities, however, rec-
tangular joint sets
have been discov-
ered which divide
the rock into prisms
parallel to the
earth’s surface and
Fig. 40.— View of an exposed hillside in Iceland upon
which the snow collected in crannies along the joints
brings out to advantage both the larger and the smaller
intervals of the joint system (after Thoroddsen).
with the joint series inclined to it- each by half a right angle.
Where the rock beds have been much disturbed, the complex of
58 EARTH FEATURES AND THEIR MEANING
joints may be such as to defy all attempts at orderly arrange-
ment.
The space intervals of joints. ‘The same kind of subequal spac- ©
ing which characterizes the fractures near the surface of the block
in Daubrée’s experiment (Fig. 19, p. 41) is found simulated by the
rock joints (Fig. 39). Such unit intervals between fractures may
be grouped together into larger units which are separated by frac-
tures of unusual perfection. We may think of such larger space
units as having the smaller ones superimposed upon them (Fig. 40).
The displacements upon joints — faults. —In the vast majority
of cases, the joint fractures when carefully examined betray no
evidence of any appreciable movement of the two walls upon each
other. Generally the rock layers are seen to cross the joints with-
out apparent displacement. Joints are therefore planes of dis-
junction only, and not planes of displacement.
Within many districts, however, a displacement may be seen
to have occurred upon certain of the joint planes, and these are
then described as faults. Such displacements of necessity imply
Fic. 41. — Faulted blocks ef basalt divided by joints near Woodbury, Connecticut.
To show the structure of the rock, some of the foliage has been removed in prepar-
ing the sketch from a photograph.
a differential movement of sections or blocks of the earth’s crust,
the so-called orographic blocks, which are bounded by the joint:
planes and play individual réles in the movement. A simple case
of such displacements in rocks intersected by a single set of mas-
ter joints is represented in the model of plate 4C. The most promi-
nent fault represented by this model runs lengthwise through the
middle, and the displacement which is measured upon it not only
varies between wide limits, but is marked by abrupt changés at
the margins of the larger blocks. This vertical displacement upon
a ee
\ | tional technical terms (Fig. 42).
ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 59
the fault is called its throw. Though not illustrated by the model,
horizontal displacements may likewise occur, and these will be
more fully discussed when the subject of earthquakes is considered
in the following chapter. An actual example of blocks displaced
by vertical adjustment is represented in Fig. 41, a simple type of.
faulting which has taken place in rocks but slightly disturbed from
their original attitude, but intersected by a relatively simple sys-
tem of master joints. In those regions where the beds have been
folded and perhaps overthrust before their elevation into the zone
of fracture, and which are further intersected by disorderly fissure
planes, the results are far more complex.’ In such cases the
planes of individual displacement may not be vertical, though
they are generally steeper than 45°. For their description it is
necessary to make use of addi-
The inclination of a sloping fault
plane measured against the ver-
tical is called the hade of the fault.
The total displacement is measured
along the plane of the fault from a
point upon one limb to the point
from which it was separated in fia. 42.—A fault in previously dis-
] ‘the other. The additional terms turbed strata. AB, displacement ;
_ are made sufficiently clear by the
AC, throw ; BD, stratigraphic throw ;
: BC, heave; angle CAB, hade.
diagram.
Methods of detecting faults. — The first effect of a fault is usually
to produce a crack at the surface of the earth; and, provided there
is a vertical displacement or throw, an escarpment which rises
upon the upthrown side of the fault. In general it may be said
that escarpments which appear at the earth’s surface as plane
surfaces probably represent planes of fracture, though not neces-
; _ Sarily planes of faulting. In many cases the actual displacements
lie buried under loose rock débris near to and paralleling the es-
carpment, and in some cases as a result of the erosional processes
working upon alternately hard and soft layers of rock, the escarp-
_ ment may later appear upon the downthrown side or limb of the
7 fault (Fig. 43). As an illustration of a fault escarpment, the
_ fagade of El Capitan and many other rock faces of the Yosemite
_ valley may be instanced.
60 EARTH FEATURES AND THEIR MEANING
When we have further studied the erosional processes at the
earth’s surface, it will be appreciated that faults tend to quickly
bury themselves from sight, where-
as fold structures will long remain
in evidence. Many faults will thus
be overlooked, and too great weight
is likely to be ascribed to the folds
in accounting for the existing atti-
tudes and positions of the rock
masses. Faults must therefore be
sought out if mistakes of interpreta-
tion are to be avoided.
The most satisfactory evidence of
a fault is the diszovery of a rock bed
which may be easily identified, and
Fic. 43.— Diagrams to show how which is actually seen displaced on
ee ip tere! ee a plane of fracture which intersects
through erosion, appear upon phe it (Fig. 42, p. 59). When such an
downthrown side. easily recognizable layer is not to be
found, the plane of. displacement
may perhaps be discovered as a narrow zone composed of angular
fragments of the rock cemented together by minerals which form
out of solution in water. Such a fractured rock zone which
follows a plane of faulting is
a fault breccia. If the fault
breccia, or vein rock, is much
stronger than the rock on
either side, it may eventually
stand in relief at the surface
like a dike or wall. At other
times the displacement pro-
duces little fracture of the
walls, but they slide over each
other in such a manner as to
yield either a smoothly cor-
rugated or an evenly polished
surface which is described as
cc > = ?
slickensides. It may be, Fie. 44.— A fault plane exhibiting ‘‘ drag.”
however, that during the move- The opening is artificial (after Scott).
a ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 61
i ment either one or both of the walls have “‘ dragged,’’ and so are
‘> curled back in the immediate neighborhood of the fault plane
_ sig. 44). | ,
When, as is quite generally the case, the actual plane of dis-
placement of a fault is not open to inspection, the movement may
be proven by the observation of
abrupt, as contrasted with grad-
ual, changes in the strikes and dips
of neighboring exposures (Fig. 45) ;
{ or by noting that some easily rec-
- ognized formation has been
sharply offset in its outcrops (Fig.
ha 46).
There are in addition many in-
dications rather than proofs of the
presence of faults, which must be
taken account of in every general
study of the geology of a district.
Thus the outcrops of all neighbor-
4 ing formations may terminate
7 | abruptly upon a straight line which fig. 45.— Map to show how a fault
___ intersects all alike. Deep-seated may beindicated in abrupt changes
‘1 ; | fissure sprin os may be align ed in of the strike and dip of neighboring
as ee exposures.
a striking manner, and so indicate
bi | the course of a prominent fracture,
, though not necessarily of a fault.
& Much the same may be said of the
4 dikes of cooled magma which have
a been injected along preéxisting frac-
a g tures.
i Fic. 46.— A series of parallel ioe wannes Of: Sie. geological’ TAP. =~
faults indicated by successive Modern topographic maps form an im-
offsets in the course of an portant part of the library of the serious
easily recognizable rock for- student of physiography; they are the
mation. : j
gazetteer of this branch of science.
Every civilized nation has to-day either completed a topographic
_ __ atlas of its territory, or it is vigorously prosecuting a survey to
- furnish maps which represent the relief with some detail, and pub-
lishing the results in the form of an atlas of quadrangles. Thus
62 EARTH FEATURES AND THEIR MEANING
a relief map will erelong be obtainable of any part of the civilized
world, and may be purchased in separate sections. Nowhere is this
work being taken up with greater vigor than in the United States,
where a vast domain representing every type of topographic pecul-
iarity is being attacked from many centers. Here and elsewhere
the relief of the land is being expressed by so-called contours. or
lines of equal altitude upon the earth’s surface. It is as though
a series of horizontal planes, separated by uniform intervals of 20
or 40 or 100 feet, had been made to intersect the surface, and the
intersection curves, after consecutive numeration, had been dropped
into a single plane for printing.
Where the slopes are steep, the contour lines in the topographic
map will appear crowded together and so produce a deep shade
upon the map; whereas with relatively flat surfaces white patches
will stand out prominently upon the map. More and more the
topographic map is coming into use, and for the student of nature
in particular it is important to acquire facility in interpreting the
relief from the topographic map. To further this end, a special
model has been devised, and its use is described in appendix C.
Usually before any satisfactory geological map can be prepared,
a contoured topographic map of the district to be studied must
be available. :
' The field map and the areal geological map. — As the atlas of
topographic maps is the physiographic gazetteer, so geological
maps together constitute the reference dictionary of descriptive
geology. Not only are topographic maps of many districts now
generally available, but more and more it has become the policy
of governments to supply geological maps in the same quadrangle
form which is‘the unit of the topographic map. The geological
map is, however, a complex of so many conventional symbols,
that without some practical experience in the actual preparation
of one, it is exceedingly difficult for the student to comprehend
its significance. A modern geological map is usually a rectangular
sheet printed in color, upon which are many irregular areas of in-
dividual hue joined to each other like the parts of a child’s pic-
ture puzzle.
The colored areas upon the geological map are each supposed>
to indicate where a certain rock type or formation lies immediately
below the surface, and this distribution represents the best judg-
ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 63
ment of the geologist who, after a study of the district, has prepared
the map. Unfortunately the conventions in use are such that his
observation and his theory have been hopelessly intermingled
in the finished product. Armed with the geological map, the
student who visits the district finds spread out before him, it may
be, a landscape of hill and valley, of green forest and brown farming
land, which is as different as may be from the colored puzzle which
he holds in his hand. Hidden under the farm vegetation or masked
by the woods are scattered outcroppings of rock which have been
the basis of the geologist’s judgment in preparing the map. Ex-
perience shows that in order to bridge the wide gap between the
geology in the landscape and the patches of color upon the map
something more than mere examination of the colored sheet is
necessary. We shall therefore describe, with the aid of laboratory
models, the various stages necessary to the preparation of a geo-
logical map, and every student should be advised to follow this by
practical study of some small area where rocks are found in out-
crop.
Though the published areal geological map represents both fact
and theory, the map maker retains an unpublished field map or
map of observations, upon which the final map has been based.
This field map shows the location of each outcrop that has been
studied, with a record of the kind of rock and of such observations
as strike, dip, and pitch. Our task will therefore be to prepare:
(1) a field map; (2) an areal geological map; and (3) some typical
geological sections. )
Laboratory models for the study of geological maps. — In order
to represent in the laboratory the disposition of rock outcrops
in the field, special laboratory tables are prepared with removable
covers and with fixed tops, which are divided into squares num-
bered like the township sections of the national domain (Fig. 47).
To represent the rock outcrops, blocks are prepared which may
be fixed in any desired position by fitting a pin into a small augur
hole bored through the table. The outcrop blocks for the sedi-
mentary rock types are so constructed as to show the strike and
dip of the beds. (See Appendix D.)
The method of preparing the map.— To prepare the map, use
is made of a geological compass with clinometer attachment, a
protractor, and a map base divided into sections like the top of
64 EARTH FEATURES AND THEIR MEANING
the table, and on the scale of one inch to the foot. Each exposure
represented upon the table is “ visited ”’ and then located upon the
base map in its proper position and attitude. The result is the
field map (Fig. 47), which thus represents the facts only, unless
wtp eh ig tf + ty)
Vlo? Pel NP NT Qt FT ? 7
(CRISS e eee
) Yo N " y Q Q @)
> wi ity ic \e
Ser L gy S ~ eels 4 2
ONO | hf | it tele \e wa
O Zia de lee
~ y ci %
O OSK IE n> | HOP ‘Ke 41k
Fig. 47.— Field map prepared from a laboratory table.
there have been uncertainties in the correlation of exposures or
in determining the position of the bedding plane.
To prepare the areal geological map from the field map, it is
first necessary to fix the boundaries which separate formations at
Formations
Sedimentary
Oraft lp
Sandstone ay
\~
l
eee
=
I=‘ 7\
VA wed
’ at z
X
a
Ls
j
e
4
c
a
a
“1
Conglomte
Sandstone
a
TIN I CI BT S.C O MET OULY AIA SANS APL
=e BLY A Pp LLL DLT eS FSS ESS SSss .
PHS Str
Fig. 48.— Areal oalogonl map constructed from the field map ie Fig. 47, with two
selected geological sections.
the surface; and now perhaps for the first time it is realized how
large an element of uncertainty may enter if the exposures were
widely separated. It is clear that no two persons will draw these
lines in the same positions throughout, though certain portions
ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 65
of them—where the facts are more nearly adequate — may cor-
respond. In Fig. 48 is represented the areal geological map con-
structed from the field map, with the doubtful area at one side left
blank.
Some conclusions from this map may now be profitably con-
sidered. The complexly folded sandstone formation at the left
of the map appears as the oldest member represented, since its
area has been cut through by the intrusive granite which does not
intrude other formations, and is unconformably overlaid by the
limestone and its basal layer of conglomerate. The limestone in
turn is unconformably overlaid by the merely tilted sandstone
beds at the right of the map. These three sedimentary forma-
tions clearly represent decreasing amounts of close folding, from
which it is clear that each earlier formation has passed through
an episode not shared by that of next younger age. Of the other
intrusive rocks, the dike of porphyry is younger than all the other
formations, with the possible exception of the upper sandstone.
Offsetting of the formations has disclosed the course of a fault,
and from its relations to the dikes we may learn that of these the
porphyry is younger and the basalt older than the date of the
faulting.
The dashed lines upon the map (AB and CD) have been selected
as appropriate lines along which to construct geological sections
(Fig. 48, below map), and from these sections the exposed thick-
nesses of the different formations may be calculated. In one in-
stance only, that of the conglomerate, can we be sure that this
exposed thickness measures the entire formation.
Fold versus fault topography. — The more resistant or “stronger ”’
rock beds, as regards attacks of the atmosphere, in the course
of time come to stand in relief, separated by depressions which
overlie the “‘ weaker ’’ formations. Simple open folds which are
not plunging exercise an influence upon topography by producing
generally long and straight ridges. More complex flexures, since
they generally plunge, make themselves apparent by features
which in the map are represented by curves. Fracture structures, :
_ and especially block displacements, are differentiated from these
_ curving features by the dominance of straight or nearly rectilinear
_ lines upon the map. The effect of erosion is to reduce the asperity
of features and to mold them with flowing curves. The frac-
F
66 EARTH FEATURES AND THEIR MEANING
ture structures are for this reason much more likely to be over-
looked, and if they are not to elude the observer, they must be
sought out with care. Fold and fracture structures may both be
revealed upon the same map.
READING REFERENCES TO CHAPTER VI
Joint systems : —
Joun Puiuurrs. Observations made in the Neighborhood of Ferrybridge
in the Years 1826-1828, Phil. Mag., 2d ser., vol. 4, 1828, pp. 401-409 ;
Illustrations of the geology of Yorkshire, Pt. II, The Limestone Dis-
trict. London, 1836, pp. 90-98.
SamvuEL Haucuton. On the Physical Structure of the Old Red Sand-
stone of the County of Waterford, considered with reference to cleav-
age, joint surfaces, and faults, Trans. Roy. Soe. London, vol. 148,
1858, pp. 333-348.
W.C. Broaeer. Spaltenverwerfungen in der Gegend Langesund-Skien,
Nyt Magazin for Naturvidernskaberne, vol. 28, 1884, pp. 253-419.
Wma. H. Hosss. The Newark System of the Pomperaug Valley, Con-
necticut, 21st Ann. Rept. U.S. Geol. Surv., Pt. III, 1901, pp. 85-143.
Geological map : —
Wa. H. Hosss. The Interpretation of Geological Maps, School Science
and Mathematics, vol. 9, 1909, pp. 644-653.
Par Pe
veges Pe
=
ews
~< 9 -
we - or
ee art, 6
E-, wipes: na Sher te
sire BPE
at
“>
nt. fy
iy
aon, “5
Cm omagin cap My
CHAPTER VII
‘THE INTERRUPTED CHARACTER OF EARTH MOVE-
MENTS: EARTHQUAKES AND SEAQUAKES
Nature of earthquake shocks. — Man’s belief in the stability of
Mother Earth — the terra firma — is so inbred in his nature that
even a light shock of earthquake brings a rude awakening. The
terror which it inspires is no doubt largely to be explained by this
a
ASiLoode
Fig. 49.— View of a portion of the ruins of Messina after the earthquake of
December 28, 1908.
bE ig EN:
G BAL Sy Y
VGA Aa,
disillusionment from the most fundamental of his beliefs. Were
he better advised, the long periods of quiet which separate earth-
quakes, and not the lighter shocks which follow all grander dis-
turbances, would occasion him concern.
67
68 EARTH FEATURES AND THEIR MEANING
Earthquakes are the sensible manifestations of changes in level
or of lateral adjustments of portions of the continents, and the
seismic disturbances upon the sea — seaquakes and seismic sea
waves — relate to similar changes upon the floor of the ocean.
During the grander or catastrophic earthquakes, the changes
are indeed terrifying, and have usually been accompanied by losses
to life and property, which are only to be compared with those of
great conflagrations or of inundations on thickly populated plains.
The conflagration has all too frequently been an aftermath of
the great historic earthquakes. The earthquake of December 28,
1908, in southern Italy, destroyed almost the entire population of
a great city, and left of its massive buildings only a confused heap
of rubble (Fig. 49). Two years later a heavy earthquake resulted
in great damage to cities in Costa Rica (Fig. 50), while two years
: = i ———_——, =
wr
-—~s
Fie. 50.— Ruins of the Carnegie Palace of Peace at Cartago, Costa Rica, de-
stroyed when almost completed by the great earthquake of ae 4, 1910 (after
a photograph by Rear-Admiral Singer, U.S.N.).
earlier our own country was first really awakened to the danger
in which it stands from these convulsive earth throes; though, as
we shall see, these dangers can be largely met bhectpr proper
methods of construction.
Earthquakes are usually preceded for a brief instant by sub-
terranean rumblings whose intensity appears to bear no relation
to the shocks which follow. The ground then rocks in wavelike
P
{
5
ie J
77.
EARTHQUAKES AND SEAQUAKES 69
motions, which, if of large amplitude, may induce nausea, prevent
animals from keeping upon their feet, and wreck all structures
not specially adapted to withstand them. Heavy bodies are some-
times thrown up from the ground (Fig. 51), and at other times
Fic. 51. — Bowlders thrown into the air and overturned during the Assam
earthquake of 1897 (after R. D. Oldham).
similar heavy masses are, apparently because of their inertia, more
deeply imbedded in the earth. Thus gravestones and heavy stone
posts are often sunk more deeply in the ground and are surrounded
by a hollow and perhaps by small
open cracks in the surface (Fig. 52).
When bodies are thrown upward, it
would imply that a quick upward . :
movement of the ground had been frye. 52.— Heavy post sunk deeper
suddenly arrested, while the burial into the ground during the
of heavy bodies in the earth is prob- SY eee ta ane press:
ably due to a movement which
begins suddenly and is less abruptly terminated.
Seaquakes and seismic sea waves.— Upon the ocean the quakes
which emanate from the sea floor are felt on shipboard as sudden
joltings which produce the impression that the ship has struck upon
a shoal, though in most instances there is no visible commotion in
70 EARTH FEATURES AND THEIR MEANING
the water. The distribution of these shocks, as indicated either
by the experiences of neighboring ships at the time of a particular
shock, or by the records of vessels which at different times have
sailed over an area of frequent seismic disturbance, appears to be
limited to narrow zones or lines (Fig.
53). The same tendency of under-sea
disturbances to be localized upon defi-
nite straight lines has been often illus-
trated by the behavior of deep-sea
cables which are laid in proximity to
one another and which have been
: known to part simultaneously at points
Wis B= Mea ahiowiie thes: ranged upon a straight line.
calities at which shocks have Far grander disturbances upon the
been reported at sea off Cape floor of the ocean have been revealed
seems eat ica by the great sea waves — the so-called
“tidal waves,’’ properly referred to as tsunamis — which recur in
those sea districts which adjoin the special earthquake zones upon
the continents (p. 86). The forerunner of such a sea wave approach-
Pe a ae ee
ae Lee
te
: ss - ve
Fie. 54. — Effect of a seismic water wave at Kamaishi, Japan, in 1896 (after E. R.
Scidmore).
ing the shore is usually a sudden withdrawal of the water so as to
lay bare a portion of the bottom, but this is well-recognized to be _
the premonition of a gigantic oncoming wave which sweeps all before
it and is only halted when it has rolled over all the low-lying coun-
4 found to have moved with reference to
EARTHQUAKES AND SEAQUAKES vgl
try and encountered a mountain wall. Such seismic waves have
been especially common upon the Pacific shore of South America
and upon the Japanese littoral (Fig. 54). These waves proceed
from above the great deeps upon the ocean bottom, and clearly
result from the grander earth movements to which these depres-
sions owe their exceptional depth. The withdrawal of the water
from neighboring shores may be presumed to be connected with
a descent of the floor of the depression and the consequent draw-
ing-in of the ocean surface above. The later high wave would
thus represent the dispersion of the mountain of water which is
raised by the meeting of the waters from the different sides of the
depression.
The grander and the lesser earth movements. — Upon the
land the grander and so-called catastrophic earthquakes are
usually the accompaniment of important changes in the sur-
face of the ground that will be discussed in later sections.
Those shocks which do little damage to structures produce no
visible changes in the earth’s surface, except, it may be, to shake
down some water-soaked masses of earth upon the steeper slopes.
Still other movements, and these too slight to be felt even in
the night when the animal world is at rest, may yet be distin-
_ guished by their sounds, the unmistakable rumblings which are
characteristic alike of the heaviest and the lightest of earth-
quake shocks.
Changes in the earth’s surface during earthquakes — faults and
fissures. — Each of the grander among historic earthquakes has
been accompanied by noteworthy changes in the configuration of
the earth’s surface within the district
_ where the shocks were most intense.
A section of the ground is usually
_ another upon the other side of a verti-
_ cal plane which is usually to be seen;
_ we have here to do with the actual
_ making of a fault or displacement such
as we find the fossil examples of within Fie. 55.—A fault of vertical
_ therocks. The displacement, or throw, Sigenepotapnt.
_ upon the fault plane may be either upward or downward or
_ laterally in one direction or the other, or these movements may be
Sat a
ft ay i
A i We
72
combined.
Fic. 56.— Escarpment produced by an
earthquake fault of vertical displace-
ment which cut across the Chedrang
River and thus produced a waterfall,
Assam earthquake of 1897 (after R. D.
EARTH FEATURES AND THEIR MEANING
A movement of adjacent sections of the ground
upward or downward with refer-
ence to each other (Fig. 55) has
been often observed, notably
at Midori after the great Jap-
anese earthquake of 1891, and
in the Chedrang valley of Assam
after the earthquake of 1897.
(Fig. 56).
A lateral throw, unaccom-.
panied by appreciable vertical
displacement (Fig. 57), is espe-
cially well illustrated by the
fault in California which was
ra formed during the earthquake
of 1906 (Fig. 58). A combination of the two types of displace=-
ment in one (Fig. 59) is exempli- | eee
Fig. 57.— A fault of lateral displacement. Fig. 58, -—— Renée parted and diantaaan
fifteen feet by a transverse fault
formed during the California earth-
quake of 1906 (after W. B. Scott).
fied by the Baishiko fault of
Formosa at the place shown in
plate 3 A.
The measure of displacement. — To
afford some measure of the displacements
which have been observed upon earth-
quake faults, it may be stated that the
maximum vertical throw measured upon
the fault in the Neo valley of Japan (1891)
was 18 feet, in the Chedrang valley of
Assam (1897) 35 feet, and of the Alaskan
coast (1899) 47 feet. Large sections of
land were bodily uplifted in these cases
within the space of a few seconds, or
Sa }
Fic. 59. — Fault with verti-=
eal and lateral displace-
ments combined.
PLATE 3.
A. An earthquake fault opened in Formosa in 1906, with vertical and lateral dis-
placements combined (after Omori).
eee
SS ee
;
I
4
B. Earthquake faults opened in Alaska in 1889, on which vertical slices of the
earth’s shell have undergone individual adjustments (after Tarr and Martin).
Re
oJ
a
EARTHQUAKES AND SEAQUAKES ee
at most a few minutes, by the amounts given. The largest re-
corded lateral displacement measured upon an earthquake fault
is about 21 feet upon the California
rift after the earthquake of 1906;
though an amount only slightly less <+:: cea et Say ae te :
than this is indicated in the shifting 22 -)ih gc vlepus ins
of roads and arroyas dating from the °° 5+, 24 °.079824 9335
Stage 4th Vials
earthquake of 1872 in the Owens valley, :-:°.~
California. Fault lines once established .%*.7:°
are planes of special weakness and
become later the seat of repeated
movements of the same kind.
The greater number of earthquake
faults are found in the loose rock cover
which so generally mantles the firmer F'6- 60. — Diagram to show how
car Q small faults in the rock base-
rock basement, and itis almost certain rent may be masked at the
that the throws within the solid rock — surface through adjustments
are considerably larger than those Within the loose rock mantle.
which are here measured at the surface, owing to the adjustments
which so readily take place in the looser materials. Those lighter
shocks of earthquake which are accompanied by no visible dis-
placements at the surface do,
however, in some instances affect
SS. in a measure the flow of water
upon the surface, and thus indi-
er ae cate that small changes of sur-
eae ye Bees ee face level have occurred without
iWipikye? breaks sufficiently sharp to be
+, ae-b
if] |
‘Ub Fly hr Ba
So ii
perceived (Fig. 60). Intermedi-
strates ate between thesteepescarpment
yy and the masked displacement
just described is the so-called
: “mole-hill”’ effect, —a rounded
Fic. 61. -— Diagram to show the appear- and variously cracked slope or
ance of a ‘mole hill” above a buried ridge above the position of a
earthquake fault (after Kotd). buried fault (Fi g 61)
The escarpments due to earthquake faults in loose materials
3 at the earth’s surface can obviously retain their steepness for a
____ few years or decades at the most; for because of their verticality
a
WE
74
EARTH FEATURES AND THEIR MEANING
they must gradually disappear in rounded slopes under the action
of the elements.
: aa \w Wik 1
Sy or ii sia a il ae
r Wy
ioe oes ——— rt Nie
“uy Gi na ae ae us Soh one ae A
Ms wis ie
NUnit yt We eal wl M ae Ti
svi nh ie Wat
Savill 4
nN Ne Mi Agee
oe oN : “ a7
~, “%e aN : ‘ Ss Epo ar
tee Reeds
st Ss
Fia. 62. — Post-glacial earthquake faults of small
but cumulative displacement, eastern New
York (after Woodworth).
tained unaltered for many centuries.
Smaller displacements within a rock which
rapidly disintegrates under
the action of frost and sun
will likewise before long be
effaced. In those excep-
tional instances where a
resistant rock type has had
all altered upper layers
planed away until a fresh
and hard surface is ex-
posed, and has further
been protected from the
frost and sun beneath a
thin layer of soil, its origi-
nal surface may be re-
Upon such a surface the
lightest of sensible shocks, or even the smaller earth movements
which are not perceived at the time, may leave an almost indelible
record. Such records particu-
larly show that the movements
which they register occur upon
the planes of jointing within the
rock, and that these ready
formed cracks have probably
been the seats of repeated and
cumulative adjustments (Fig.
62).
Contraction of the earth’s
surface during earthquakes. —
The wide variations in the
amount of the lateral displace-
ment upon earthquake faults,
like those opened in California
in 1906, show that at the time of
a heavy earthquake there must
be large local changes in the
density of the surface materials.
Fie. 63. — Bastheaiass cracks in Colorado
desert (after a photograph by Sauenyery
Literally, thousands of fig-
sures may appear in the lowlands, many of them no doubt a
EARTHQUAKES AND SEAQUAKES (es
secondary effect of the shaking, but others, like the quebradas of
the southern Andes or the “‘ earthquake cracks ”’ in the Colorado
desert (Fig. 63), may have a deeper-seated origin. Many facts
go to show, however, that though local expansion does occur in
— le eae
—
me > ae
as
o |
Scale:
o 0.2. 3 @ 8 6 76:4: wrt.
Fie. 64.— Diagrams to show how railway tracks are either broken or buckled
locally within the district visited by an earthquake.
some localities, a surface contraction is a far more general conse-
quence of earth movement. In civilized countries of high indus-
trial development, where lines of metal of one kind or another run
for long distances beneath or upon the surface of the ground, such
general contraction of the surface may be easily proven. Com-
*
Fic. 65.— The Biwajima railroad bridge in Japan after the earthquake of 1891
(after Milne and Burton).
paratively seldom are lines of metal pulled apart in such a way
as to show an expansion of the surface; whereas bucklings and
kinkings of the lines appear in many places to prove that the area
within which they are found has, as a whole, been reduced.
Water pipes laid in the ground at a depth of some feet may be
bowed up into an arch which appears above the surface; lines of
76 EARTH FEATURES AND THEIR MEANING
curbing are raised into broken arches, and the tracks of railways
are thrown into local loops and kinks which imply a very consid-
erable local contraction of the surface (Fig. 64). With unvarying
regularity railway or other bridges which cross rivers or ravines,
‘tf the structures are seriously damaged, indicate that the river
banks have drawn nearer together at the time of the disturbance.
In such cases, whenever
the bridge girder has re-
mained in place upon its
abutments, these have
either been broken or back-
tilted as a whole in such a
Fic. 66. — Diagrams to show how the compres- manner as to indicate an
sion of a district and its consequent contraction approa ch of the founda-
during an earthquake may close up the joint F
spaces within the rock basement and concen- tions which was prevented
trate the contraction of the overlying mantle at the top by the stiffness
where this is partially cut through and so : :
weakened in the valley sections. of the gir der (F 1g. 65).
The simplest explana-
tion of such an approach of the banks at the sides. ofthe valleys
cut in loose surface material is to be found in a general closing up
of the joint spaces within the underlying rock, and an adjust-
ment of the mantle upon the floor mainly in the valley sections
(Fig. 66). |
The plan of an earthquake fault.— In our consideration of earth-
quake faults we have thus far given our attention to the displace-
adie ° “] Mile
Fia. 67.— Map of the Chedrang fault which made its appearance during the Assam
earthquake of 1897. The figures give the amounts of the local vertical displace-
ment measured in feet (after R. D. Oldham).
ment as viewed at a single locality only. Such displacements are,
however, continued for many miles, and sometimes for hundreds
of miles; and when now we examine a map or plan of such a line «
of faulting, new facts of large significance make their appearance.
This may be well illustrated by a study of the plan of the Chedrang
et ee ne ee
}
{
,
;
"
u
;
3
EARTHQUAKES AND SEAQUAKES rire
fault which appeared at the time of the Assam earthquake of
1897 (Fig. 67). From this map it will be noticed that the upward.
or downward displacement upon the perpendicular plane of the
fault is not uniform, but is subject to large and sudden changes.
Thus in order the measurements in feet
are 32, 0, 18, 35,0, 8, 25, 12, 8, 2, 0. N
The fault formed in 1899 upon the
shores of Russell Fjord in Alaska (Fig.
68) reveals similar sudden changes of
throw, only that here the direction of <i
the movement is often reversed; or, 1S
otherwise expressed, the upthrow is S
suddenly transferred from one side of SS
the fault to the other. Such abrupt Dom :
changes in the direction of the dis- Arg
placement have been observed upon :
l 2 3
of throw upon an earthquake fault which MILES.
was formed in the Owens valley, Califor- Fic. 68.— Map giving the
nia,in 1872. The observer looks directly displacements in feet
along the course of the fault from the left measured along an earth-
foreground to the cliff beyond and to the quake fault formed in
left of the impounded water (after a Alaska in 1899 (after Tarr
photograph by W. D. Johnson). and Martin).
many earthquake faults, and a particularly striking one is repre-
sented in Fig. 69.
The block movements of the disturbed district.— The displace-
ments upon earthquake faults are thus seen to be subdivided into
sections, each of which differs from its neighbors upon either side
and is sharply separated from them, at least in many instances.
These points of abrupt change of displacement are, in many cases
at least, the intersection points with transverse faults (Fig. 69).
78
EARTH FEATURES AND THEIR MEANING
Such points of abrupt change in the degree or in the direction of
the displacement may be, when
Ouest
ares cA
wictetgaennainit erry ENN
x
BY
g
NY
8
> i
|
eee |
il
.4
,
f
wate i 1"
a
“
“Sn,
Fic. 70. — Map of the faults within an area
of the Owens valley, California, formed
in part during the earthquake of 1872,
and in part due to early disturbances.
In the western portions the displace-
ments cut across firm rock and alluvial
deposits alike without deviation of di-
rection (after.a map by W. D. Johnson).
looked at from above, abrupt
turning points in the direction
of extension of the fault, whose
course upon the map appears as
a zigzag line made up of straight
sections connected by sharp
elbows (Fig. 70).
Such a grouping of surface
faults as are represented upon
the map is evidence that the
area of the earth’s shell, which
is included, has at the time of
the earthquake been subject to
adjustments as a series of sepa-
rate units or blocks, certain of
the boundaries of which are the
fault lines represented. The
changes in displacement meas-
ured upon the larger faults
make it clear that the observed
faults can represent but a frac-
tion of the total number of
lines of displacement, the others
being masked by variations in
the compactness of the loose
mantling deposits. Could we
but have this mantle removed,
we should doubtless find a rock
floor separated into parts like
an ancient Pompeiian pavement,
the individual blocks in which
have been thrown, some upward
and some downward, by vary-
ing amounts. Less than a
hundred miles away to the east-
ward from the Owens valley, a
portion of this pavement has
been uncovered in the extensive
®
Pet ce
EARTHQUAKES AND SEAQUAKES 79
operations of the Tonapah Min-
7 ing District, so that there we
q may study in all its detail the
elaborate pattern of earth mar-
quetry (Fig. 71) which for the
floor of the Owens valley is as
yet denied us.
The earth blocks adjusted
during the Alaskan earthquake
of 1899.—For a study of the
] adjustments which take place
; 7 between neighboring earth blocks
Scale. 12M ie.
a ; Fia. 7 1. — Marquetry of the rock floor
i) during a great earthquake, the of the Tonapah Mining District,
ie recent Alaskan disturbance has Nevada (after Spurr).
offered the advantage
that the most affected
district was upon the
seacoast, where changes
of level could be referred
to the datum of the sea’s
surface. Here a great
island and large sections
of the neighboring shore
underwent movements
both as a whole in large
blocks and in adjust-
ments of their subordi-
nate parts among them-
selves (Fig. 72). Some
sections of the coast were
here elevated by asmuch
as 47 feet, while neigh-
boring sections were up-
lifted by smaller amounts
(Fig. 73), and certain
smaller sections were
even dropped below the
Fic. 72.— Map of a portion of the Alaskan coast to . d OPP
show the adjustments in level during the earth- level of the Sea. The
quake of 1899 (after Tarr and Martin). amount of such subsid-
Sopot
oP
or me
(a
Gn
80 EARTH FEATURES AND THEIR MEANING
ence is, however, difficult to ascertain, for the reason that the
former shore features are now covered with water and thus removed
, fromobservation. In favor-
able localities the minimum
amount of submergence may
sometimes be measured upon
forest trees which are now
flooded with sea water. In
Fig. 74 a portion of the
coast is represented where
the beach sand is now ex-
a tended back into the spruce
Fig. 73.— View on en Island, Disen- forest, a distance of a hun-
chantment Bay, Alaska, revealing the shore df
that rose seventeen feet above the sea during dred feet or more, and where
the earthquake of 1899, and was found with sedgy beach grass is growing
barnacles still clinging to the rock (after among trees whose roots are
Tarr and Martin). .
now laved in salt water.
At the front of this forest the great storm waves overturn the
trees and pile the wreckage in front of those that still remain
standing.
Upon the glaciated rock surfaces of the Alaskan coast, excep-
tionally favorable opportunities are found for study of the intricate
i. Faq. 75. —Bettlemeart of a section of the
ae ak shore at Port Royal, Jamaica, during
Fig. 74. — Partially submerged forest the earthquake of January 14, 1907,
upon the shore of Knight Island, Alaska, adjacent to a similar but larger settle-
due to the sinking of a section of the ment of the near shore during the
coast during the earthquake of 1899 earthquake of 1692 (after a photo-
(after Tarr and Martin). : graph by Brown).
pattern of the earth mosaic which is under adjustment at the time
of an earthquake. Upon Gannett Nunatak the surface was found
divided by parallel faults into distinct slices which individually —
underwent small changes of level (plate 3 B).
—
alto). “Eee ae
Big
. 4
a
*
.
Bey
: fo
.
\ a
: ree
1
a
7 y
CHAPTER VIII
THE INTERRUPTED CHARACTER OF EARTH MOVE-
MENTS: EARTHQUAKES AND SEAQUAKES (Concluded)
Experimental demonstration of earth movements. — The study
of the Alaskan earthquake of 1899 showed that during this adjust-
ment within the earth’s shell some of the local blocks moved up-
ward and by larger amounts than their neighbors, and that still
others were actually depressed so that the sea flowed over them.
It must be evident that such differential vertical movements of
neighboring blocks at the earth’s surface can only take place
if lateral transfers of material are made beneath it. From under
those strips of coast land which were depressed, material must
have been moved so as to fill the void which would otherwise have
formed beneath the sections that were uplifted. If we take into
consideration much larger fractions upon the surface of our planet,
we are taught by the great seaquakes which are now registered
upon earthquake instruments at distant stations that large down-
ward movements are to-day in progress beneath the sea much more
than sufficient to compensate all extensions of the earth’s surface
within those districts where the land is rising in mountains. From
under the offshore deeps of the ocean to beneath the growing
mountains upon the shore, a transfer of earth material must be
assumed to take place when disturbances are registered.
Within the time interval that separates the sudden adjustments
of the surface which are manifested in earthquakes, the condition
of strain which brings them about is steadily accumulating, due,
as we generally assume, to earth contraction through loss of its
heat. It seems probable that the resistance to an immediate ad-
justment is found in the rigidity of the shell because of the com-
pression to which it is subjected. To illustrate: a row of blocks
well fitted to each other may be held firmly as a bridge between
the jaws of a vice, because so soon as each block starts to fall a
large resistance from friction upon its surface is called into exist-
ence, a force which increases with the degree of compression.
G 81
82 EARTH FEATURES AND THEIR MEANING
It is thus possible upon this assumption crudely to demonstrate
the adjustment of earth blocks by the simple device represented in
plate4A. The construction of this experimental tank is so simple
that little explanation is necessary. Wooden blocks of different
heights are supported in water within a tank having a glass front,
and are kept in a strained condition at other than their natural
positions of flotation by the compression of a simple vice at the
top. Held firmly in this position, they may thus represent the
neighboring blocks within the earth’s outer shell which are sup-
ported upon relatively yielding materials beneath, and prevented
from at once adjusting themselves to their natural positions through
the compression to which they are subjected. Held as they now
are, the water near the ends of the tank is forced up beneath the
blocks to higher than its natural level, and thus tends to flow from
both ends toward the center. Such a movement would permit
the end blocks to drop and force the middle ones to rise. The end
blocks are, let us say, the sections of Alaskan coast line which sunk
during the earthquake, as the center blocks are the sections which
rose the full measure of 47 feet. Upon a larger scale the end blocks
may equally well be considered as the floor of the great deeps off
the Alaskan coast, whose sinking at the time of the earthquake
was the cause of the great sea wave. Upon this assumption the
center blocks would represent the Alaskan coast regarded as a
whole, which underwent a general uplift.
Though we may not, in our experiment, vary the tendency to
adjustment by any contractional changes in either the water or
the blocks, we may reduce the compression of the vice, which leads
to the same general result. As the compression of the vice is
slowly relaxed, a point is at last reached at which friction upon
the block surfaces is no longer sufficient to prevent an adjustment
taking place, and this now suddenly occurs with the result shown in
plate 4B. In the case of the earth blocks, this sudden adjustment
is accompanied by mass movements of the ground separated by
faults, and these movements produce successional vibrations that
are particularly large near the block margins, and other frictional
vibrations of such small measure as to be generally appreciated by
sounds only. The jolt of the adjustments has thrown some blocks &
beyond their natural position of rest, and these sink and rise-sub-
sequently in order to readjust themselves with lighter vibrations,
rc hawt Ot te mee a es
PLATE 4.
by apypoare =
A. Experimental tank to illustrate the earth movements which are
manifested in earthquakes. The sections of the earth’s shell are here
represented before adjustment has taken place.
& B. The same apparatus after a sudden adjustment.
C. Model to illustrate a block displacement in rocks which are intersected
by master joints.
- «
2p ay Se <
=? a ?
ie a ee; ae
_ the water in the tank, a “lake” will
EARTHQUAKES AND SEAQUAKES 83
‘which may be repeated and continued for some time. In the case
of the earth these later adjustments are the so-called aftershocks
which usually continue throughout a considerable period follow-
ing every great earthquake. Gradually they fall off in intensity
and frequency until they can no longer be felt, and are thereafter
continued for a time as rumblings only.
Derangement of water flow by earth movement.— The water
which supported the blocks in our experiment has represented
the more mobile portion of the earth’s substance beneath its outer
zone of fracture. The surface water layers in the tank may, how-
ever, be considered in a different
way, since their behavior is remark-
ably like that of the water within
and upon the earth’s surface during
an earth adjustment. At the instant
when adjustment takes place in the
tank, water frequently spurts upward
from the cracks between the sinking
end blocks; and if in place of one
of the higher center blocks we insert
one whose top is below the level of
Fig. 76.—Diagrams to_ illustrate
é the draining of lakes during
be formed above it. When the ad- earthquakes. iad
justment occurs, this lake is im-
mediately drained by outflow of the water at its bottom along
one of the cracks between the blocks (Fig. 76).
Such derangements of water flow as have been illustrated by
the experiment are among the commonest of the phenomena
which accompany earthquakes. Lakes and swamp lands have
during earthquakes been suddenly drained, fountains of water
have been seen to shoot up from the surface and have played for
some minutes or hours before their sudden disappearance in a suck-
ing down of the water with later readjustment. During the great
earthquake of the lower Mississippi valley in 1811, known as the
New Madrid earthquake, the earlier Lake Eulalie was completely
drained, and upon the now exposed bed there appeared parallel
fissures on which were ranged funnel-like openings down which
the water had been sucked. In other sections of the affected
region the water shot up in sheets along fissures to the tops of high
84 EARTH FEATURES: AND THEIR MEANING
trees. Areas where such spurting up of the water has been ob-
served have in most cases been shown to correspond to areas of
depression, and such areas have sometimes been left flooded with
water. During the Indian earthquake of 1819 an area of some
200 square miles suddenly sank and was transformed into a lake, —
Sand or mud cones and craterlets.—From a very moderate
depth below the surface to that of several miles, all pore spaces
arthquake Springs
ding, Sand-eones & Grater iets Swamp drained
q an
free
ae Ci
. >
a
PP Os
be
iP
“ES.
tees
ae
2
tun
D
SE ROROUS SEs | Se DN
eal es bes Lo
Fig. 77. — Diagram to illustrate the derangements of flow of water at the time of
an earthquake; water issuing at the surface over downthrown rocks, and being
sucked down in upthrown blocks.
and all larger openings within the rock are completely filled with
water, the “ trunk lines ” of whose circulation is by way of the
joints or along the bedding planes of the rocks. The principal
reservoirs, so to speak, of this water inclosed within the rock are
Fig. 78.— Mud cones aligned upon a fissure opened at Moraza, Servia, during
the earthquake of April 4, 1904 (after Michailovitch).
the porous sand formations. When, now, during an earthquake a
block of the earth’s shell is suddenly sunk and as suddenly arrested
q
4
EARTHQUAKES AND SEAQUAKES 85
in its downward movement, the effect is to compress the porous
layers and so force the contained water upward along the joints to
the surface, carrying with it large quantities of the sand (Fig. 77).
; : ’ mu Ws.) mn 6 li
Bot || sep i N\\ HUTA ese i
a ee calli . ml Neg mel s) _
FA Ss wy \ Hi
Ae i tal
wal
"9 _ ‘
seria Ta ae,
Ss Sash ESS e AE
= SWE Se pg
Fie. 79.— One of the many craterlets formed near Charleston, South Carolina,
during the earthquake of August 31, 1886. The opening is twenty feet across,
and the leaves about it are encased in sand as were those upon the branches
of the overhanging trees to a height of some twenty feet (after Dutton),
Ejected at the surface this water appears in fountains usually
arranged in line over joints, or even in continuous sheets, and the
sand collecting about
the jets builds up lines ~ WZ ious tye ————
of sand or mud cones = = Ww 2 Y f= SSS
sometimes described as
“mud voleanoes”’ (Fig. 1)
78). The amount of
—
—=——
sand thus poured out ——— SS
is sometimes so vrealhe, i OSES
that blankets of quick- ——————|
>
sand are spread over :
P Fig. 80.— Cross section of a craterlet to show the
large sections of the trumpet-like form of the sand column.
country. Most fre-
quently, however, the sand is not built above the general level
of the surface, but forms a series of craterlets which are largely
86 EARTH FEATURES AND THEIR MEANING
shaped as the water is sucked down at the time of the readjustment:
with which the play of such earthquake fountains is terminated
(Fig. 79). Subsequent excavations made about such craterlets.
have shown them to have the form of a trumpet, and that in the
sand which so largely fills them there are generally found scales of ©
mica and such light bodies as would be picked out from the hetero-
geneous materials of the sand layers and carried upward in the
rush of water to the surface (Fig. 80).
The earth’s zones of heavy earthquake. —Since earthquakes
give notice of a change of level of the ground, the special danger
zones from this source are the growing mountain systems which
are usually found near the borders of the sea. Such lines of moun-
tains are to-day rising where for long periods in the past were the
basins of deposition of former seas. They thus represent the
zones upon the earth’s surface which are the most unstable —
which in the recent period have undergone the greatest changes.
of level.
By far the most unstable belt upon the earth’s surface is the
rim surrounding the Pacific Ocean, within which margin it has
been estimated that about 54 per cent of the recorded shocks of
earthquake have occurred. Next in importance for seismic in-
stability is the zone which borders both the Mediterranean Sea
and the Caribbean—the American Mediterranean—and is ex-
tended across central Asia through the Himalayas into Malaysia.
_Both zones approximate to great circles upon the earth’s surface.
and intersect each other at an angle of about 67°. It has been
estimated that about 95 per cent of the recorded continental earth-
quakes have emanated from these belts.
The special lines of heavy shock.— Within any earthquake
district the shocks are not felt with equal severity at all places,
but there are, on the contrary, definite lines which the disturbance
seems to search out for special damage. From their relations to
the relief of the land these lines would appear to be lines of fracture
upon the boundaries of those sections of the crust that play in-
dividual réles in the block adjustment which takes place. More
or less masked as these lines are beneath the rounded curves of
the landscape, they are given an altogether unenviable prominence |
with each succeeding earthquake. At such times we may think
of the earth’s surface as specially sensitized for laying bare its
EARTHQUAKES AND SEAQUAKES 87
hidden structure, as is the sensitized plate under the magical in-
fluence of the X rays.
When, at the time of an earthquake, blocks are adjusted with
reference to their neighbors, the movements of oscillation are
greatest in those marginal portions
of direct contact. Corners of blocks
— the intersecting points of the im-
portant faults — should for the same
reason be shaken with a double
violence, and this assumption ap--
pears to be confirmed by observation.
Upon the island
of Ischia, off the
Bay of Naples,
the shocks from
1883.
recent earth- 5. 81.—Map of the island of
quakes have
Ischia to show how the shocks
been strangely of recent earthquakes have been
concentrated concentrated at the crossing
point of two fractures (after
near the town of
Casamicciola,
Mercalli and Johnston-Lavis).
which was last destroyed in 1883. This un-
fortunate city lies at the crossing point of
important fractures whose course upon the
island is marked by numerous springs and
suffiont (Fig. 81).
Seismotectonic lines. — The lines of im-
‘ . portant earth fractures, as will be more clearly.
shown in the sequel (p. 227), are often indi-
cated with some clearness by straight lines in
the plan of the surface relief (Fig. 82). Lines
of this nature are easily made out upon the
map of the West Indies, and if we represent
w-—-—-—Epicenirum of 1883.
enrssesnemueeeee INteNnSe Aestructive Area 1796
e 1828
1881.
qo Moe. sau
Fic. 82.— A line of earth
fracture indicated in
the plan of the relief,
which may at any time
become the seat of
movement and result-
ant shock.
upon it by circles of different diameters the
combined intensities of the recorded earth-
quakes in the various cities, it appears that
the heavily shaken localities are ranged upon
lines stamped out in the relief, with the most severely damaged
places at their intersections (Fig. 83).
These lines of exceptional
88 EARTH FEATURES AND THEIR MEANING
Fig. 83. —Seismotectonic lines of the West Indies.
instability are known as seismotectonic lines — earthquake struc-
ture lines.
The heavy shocks above loose foundations. — It is character-
istic of faults that they soon bury themselves from sight under
loose materials, and are thus made difficult of inspection. The
escarpment which is the direct consequence of a vertical displace-
ment upon a fault tends to migrate from the place of its formation,
rounding the surface as it does so and burying the fault line beneath
its deposits (Fig. 43, p. 60).
This is not, however, the sole reason why loose foundations
should be places of special danger at the time of earth shocks, for
the reason that earthquake waves are sent out in all directions
from the surfaces of displacement through the medium of the un-
derlying rock. These waves travel
distances with only a gradual dissipa-
tion of their energy, but with their
entry into the loose surface deposits
their energy is quickly used up in
local vibrations of large amplitude,
= and hence destructive to buildings.
thaed eis honasnisake The essential difference between
different effects upon the trans- firm rock and: such loose materials as
mission and the character of are found upon a river bottom or in
shocks which are produced by ,; “4:
firm rock and by loose materials. the “made land” about our cities
' may be illustrated hy the simple
device which is represented in Fig. 84. Two similar metal pans
are suspended from a firm support by bands of steel and “elastic”
braid of similar size and shape, and carry each a small block of
wood standing upon its end. Similar light blows are now admin-
istered directly to the pans with the effect of upsetting that block
within the firm rock for considerable.
+>
EARTHQUAKES AND SEAQUAKES 89
which is supported by the loose braid because of the large range
or amplitude of movement that is imparted to the pan. The
“elastic ” braid, because of these large vibrations of which it is
susceptible, may represent the loose materials when an earthquake
Wave passes into them. In the case of the steel support, the
energy of the blow, instead of being dissipated in local swingings
of the pan, is to a large extent transmitted through the elastic
metal to materials beyond. The steel thus resembles in its high
elasticity the firmer rock basement, which receives and transmits
the earthquake shocks, but except when ruptured in a fault is
subject to vibrations of small amplitude only.
Construction in earthquake regions. — Wherever earthquakes
have been felt, they are certain to occur again; and wherever
mountains are growing or changes of level are in progress, there
no record of past earthquakes is required in order to forecast the
future seismic history. Although the future earthquakes may be
predicted, the time of their coming is, fortunately or unfortunately, —
still hidden from us. If one’s lot is to be cast in an earthquake
country, the only sane course to pursue is to build with due regard
to future contingencies.
The danger from destructive fires may to-day be largely met
by methods of construction which levy an additional burden of
cost. Though the danger from seismic disturbances can hardly
be met as fully as that from fire, yet it is true that buildings may
be so constructed as to withstand all save those heaviest shocks
in the immediate vicinity of the lines of large displacement. Here,
also, a considerable additional expense is involved in the method
of construction, in the case of residences particularly.
From what has been said, it is obvious that much of the danger
from earthquakes can be met by a choice of site away from lines
of important fracture and from areas of relatively loose foundation.
The choice of building materials is next of importance. Those
buildings which succumb to earthquakes are in most cases racked
or shaken apart, and thus they become a prey to their own inherent
properties of inertia. Each part of a structure may be regarded
as a weight which is balanced upon a stiff rod and pivoted upon
the ground. When shocks arrive, each part tends to be thrown
into vibration after the manner of an inverted pendulum. In
proportion, therefore, as the weights are large and rest upon long
90 EARTH FEATURES AND THEIR MEANING
supports, the danger of overthrow and of tearing apart is increased.
In general, structures are best constructed of light materials whose
weight is concentrated near the ground. Masonry structures,
and especially high ones, are, therefore, the least suited for resisting
earthquakes, of which the late complete destruction of the city
of Messina is a grewsome reminder. Despite repeated warnings
in the past, the buildings of that stricken city were generally con-
structed of heavy rubble, which in addition had been poorly ce-
mented (Fig. 49, p. 67). Such structures are usually first ruptured
at the edges and corners, since here the vibrations which tend to
US lovderbucte
Fia. 85. — House wrecked in San Francisco earthquake of 1906 because the floors
and partitions were not securely fastened to the walls (after R. L. Humphrey).
tear the building asunder are resisted by no supports and are
reénforced from neighboring walls.
An advantage of the first importance is evidently secured if the
rods of the pendulum, of which the building is conceived to be com-
posed, have sufficient elasticity to be considerably distorted with-
out rupture and to again recover their original position. This is
the supreme advantage of structural steel for all large buildings,
which is coupled, however, with the disadvantage that the
riveted fastenings are apt to be quickly sheered off under the
vibrations. Large and high buildings, when sufficiently elastic,
have fortunately the property of destroying the earth waves
by interference before they have traveled above the lower.
stories. t
For large structures in which wood cannot be used, strongly
——w
oe eee
ah acy
Oe ee
’
wi, ee Sg
oe 2%
x jee es
=
EARTHQUAKES AND SEAQUAKES 91
reénforced concrete is well adapted, for it has in general the same
advantages as steel with somewhat reduced elasticity, but with a
more effective binding together of the parts. This requirement
of thorough bracing and tying together of the several parts of a
building causes it to vibrate, not as many pendulums, but as one
body. If met, it removes largely the danger from racking strains,
and for small structures particularly it is the requirement which
is most easily complied with. For such buildings it is therefore
necessary that the framework should be built in a close network
A
Z
See Wz?
NON
AALEEEEAINN
{9 Jouderback a
Fig. 86.— Building wrecked at San Mateo, California, during the late earth-
quake. The heavy roof and upper floor, acting as a unit, have battered down
the upper walls (after J. C. Branner).
with every joint firmly braced and with all parts securely tied to-
gether. Especial attention should be given to the fastenings of
floor and partition ends. The house shown in Fig. 85 could not
have been subjected to heavy shocks, for though the walls are
thrown down, the floors and partitions have been left near their
original positions.
This tendency of the walls, floors, partitions, and roof to act
as individual units in the vibration, is one that must be reckoned
with and be met by specially effective bracing and tying at the
junctions. Otherwise these larger parts of the structure may act
like battering rams to throw over the walls or portions of them
(Fig. 86).
92 EARTH FEATURES AND THEIR MEANING
_Reapina REFERENCES FOR CHAPTERS VII anv VIII
General works : —
Joun Mitne. Seismology. London, 1898, pp. 320.
C. E. Durron. Earthquakes in the Light of the New Seismology. Put-
nam, New York, 1904, pp. 314.
A. Strpera. Handbuch der Erdbebenkunde. Braunschweig, 1904, pp.
362.
Count F. pe Montsssus pE Batuore. Les Tremblements de Terre,
Géographie Séismologique. Paris, 1906, pp. 475; La Science Séismo-
logique. Paris, 1907, pp. 579. ;
WituraM Herpert Hosss. Earthquakes, an Introduction to Seismic
Geology. Appleton, New York, 1907, pp. 336.
C.G. Knorr. The Physics of Earthquake Phenomena. Clarendon Press,
Oxford, 1908, pp. 283.
E. Rupoupx. Ueber Submarine Erdbeben und Eruptionen, Beitrage zur
Geophysik, vol. 1, 1887, pp. 133-365 ; vol. 2, 1895, pp. 537-666 ; vol. 3,
1898, pp. 273-536.
Descriptive reports of some important earthquakes : —
C. E. Durron. The Charleston Earthquake of August 31, 1886, 9th
Ann. Rept. U.S. Geol. Surv., 1889, pp. 203-528.
B. Koré. On the Cause of the Great Earthquake in Central Japan, 1891,
Jour. Coll. Sei. Imp. Univ., Tokyo, Japan, vol. 5, 1893, pp. 295-353,
pls. 28-35.
Joun Mine and W. K. Burton. The Great Earthquake of Central
Japan. 1891, pp. 10, pls. 30.
R. D. OtpHam. Report on the Great Earthquake of 12th June, 1897,
Mem. Geol. Surv. India. Vol. 29, 1899, pp. 379, pls. 42.
A. C. Lawson, and others. The California Earthquake of April 18, 1906,
Report of the State Earthquake Investigation Commission, three
quarto vols. (Carnegie Institution of Washington) ; many plates and
figures.
Italian Photographic Society, Messina and Reggio before and after the
Earthquake of December 28, 1908 (an interesting collection of pic-
tures). Florence, 1909.
R. S. Tarr and L. Martin. Recent Changes of Level in the Yakutat
Bay Region, Alaska, Bull. Geol. Soc. Am., vol. 17, 1906, pp. 29-64,
pls. 12-23. |
Witi1am Hersert Hosss. The Earthquake of 1872 in the Owens
Valley, California, Beitrige zur Geophysik, vol. 10, 1910, pp. 352-
385, pls, 10-23. !
Faults in connection with earthquakes : —
~
‘Witt1am H. Hosss. On Some Principles of Seismic Geology, Beitraige zur
Geophysik, vol. 8, 1907, Chapters iv-v.
~~
SS a ee
EARTHQUAKES AND SEAQUAKES 93
Expansion or contraction of the earth’s surface during earthquakes : —
Witui1aM H. Hosss. A Study of the Damage to Bridges during Earth-
quakes, Jour. Geol., vol. 16, 1908, pp. 636-653 ; The Evolution and
the Outlook of Seismie Geology, Proc. Am. Phil. Soc., vol. 48, 1909,
pp. 27-29.
Earthquake construction: — _
Joun Mitne. Construction in Earthquake Countries, Trans. Seis. Soc.,
Japan, vol. 14, 1889-1890, pp. 1-246.
F. pe Montessus DE Bauuore. L’art de batir dans les pays 4 tremble-
ments de terre (34th Congress of French Architects), L’ Architecture,
193 Année, 1906, pp. 1-31.
GitBeRT, HumpHrey, SeweLi, and Sous. The San Francisco Earth-
quake and Fire of April 18, 1906, and their Effects on Structures and
Structural Materials, Bull. 324, U. S. Geol. Surv., 1907, pp. 1-170,
pls. 1-57.
Wituram H. Hosss. Construction in Earthquake Countries, The En-
gineering Magazine, vol. 37, 1909, pp. 1-19.
Lewis AupEN Estes. LEarthquake-proof Construction, a discussion of the
effects of earthquakes on building construction with special reference
to structures of reénforeced concrete, published by Trussed Concrete
Steel Company. Detroit, 1911, pp. 46.
CHAPTER IX
THE RISE OF MOLTEN ROCK TO THE EARTH’S
SURFACE
VOLCANIC MOUNTAINS OF EXUDATION
Prevalent misconceptions about volcanoes. — The more or less
common impression that a volcano is a ‘ burning mountain ”
or a “smoking mountain ”’ has been much fostered by the school
texts in physical geography in use during an earlier period. The
best introduction to a discussion of volcanoes is, therefore, a disil-
lusionment from this notion. Far from being burning or smoking,
there is normally no combustion whatever in connection with a
voleanic eruption. The unsophisticated tourist who, looking out
from Naples, sees the steam cap which overhangs the Vesuvian
crater tinged with brown, easily receives the impression that the
material of the cloud is smoke. Even more at night, when a bright
glow is reflected to his eye and soon fades away, only to again glow
brightly after a few moments have passed, is it difficult to remove
the impression that one is watching an intermittent combustion
within the crater. The cloud which floats away from the crest of
the mountain is in reality composed of steam with which is ad-
mixed a larger or smaller proportion of fine rock powder which
gives to the cloud its brownish tone. The glow observed at night
is only a reflection from molten lava within the crater, and the
variation of its brightness is explained by the alternating rise and
fall of the lava surface by a process presently to be explained.
Not only is there no combustion in connection with volcanic
eruptions, but so far as the volcano is a mountain it is a product
of its own action. The grandest of volcanic eruptions have pro-
duced no mountains whatever, but only vast plains or plateaus
of consolidated molten rock, and every volcanic mountain at
some time in its history has risen out of a relatively level surface.
When the traditional notions about volcanoes grew up, it was »
supposed that the solid earth was merely a “ crust ” enveloping
still molten material. As has already been pointed out in an ear-
94
\
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 95
lier chapter, this view is no longer tenable, for we now know that the
condition of matter within the earth’s interior, while perhaps not
directly comparable to any that is known, yet has properties most
resembling known matter in a solid state; it is much more rigid
than the best tool steel. While there must be reservoirs of molten
rock beneath active volcanoes, it is none the less clear that they
are small, local, and temporary. This is shown by the compara-
; tive study of volcanic outlets within any circumscribed district.
: It is perhaps not easy to frame a definition of a volcano, but
F' its essential part, instead of being a mountain, is rather a vent or
channel which opens up connection between a subsurface reservoir
of molten rock and the surface of the earth. An eruption occurs
whenever there is a rise of this material, together with more or less
steam and admixed gases, to the surface. Such molten rock ar-
riving at the surface is designated lava. ‘The changes in pressure
upon this material during its elevation induce secondary phenom-
af ena as the surface is approached, and these manifestations are
ul often most awe inspiring. While often locally destructive, the
| geological importance of such phenomena is by reason of their
terrifying aspect likely to be greatly exaggerated.
Early views concerning volcanic mountains. — As already pointed
out, a volcano at its birth is not a mountain at all, but only, so to
speak, a shaft or channel of communication between the surface
and a subterranean reservoir of molten rock. By bringing this
melted rock to the surface there is built up a local elevation which
may be designated a mountain, except where the volume of the
material is so large and is spread to such distances as to produce a
plain (see fissure eruptions below).
In the early history of geology it was the view of the great Ger-
man geologist von Buch and his friend and colleague von Hum-
& boldt, that a volcanic mountain was produced in much the same
i manner as is a blister upon the body. The fluids which push up
the cuticle in the blister were here replaced by fluid rock which
% elevated the sedimentary rock layers at the surface into a dome or
mound which was open at the top—the so-called crater. This
i “elevation-crater ’”’ theory of volcanoes long held the stage in
i geological science, although it ignored the very patent fact that
#
the layers on the flanks of volcanic cones are not of sedimentary
rock at all, but,.on the contrary, of the volcanic materials which
96 EARTH FEATURES AND THEIR MEANING
are brought up to the surface during the eruption. The observa-
tional phase of science was, however, dawning, and the English
geologists Scrope and Lyell
were able to show by study of
— mound about the volcanic vent
was due to the accumulation
of once molten rock which had
been either exuded or ejected.
Fic. 87.—Breached volcanic cone near : ;
Auckland, New Zealand, showing the Making use of data derived
bending down of the sedimentary strata from New Zealand, Scrope
in the neighborhood of the vent (after
Heaphy snd Sorepe): showed that, instead of being
elevated during the formation
of a volcanic mountain, the sedimentary strata of the vicinity
may be depressed near the volcanic vent (Fig. 87).
The birth of volcanoes. — To confirm the impression that the
formation of the volcanic mountain is in reality a secondary phe-
nomenon connected with eruptions, we may cite the observed birth
of a number of volcanoes. On the 20th of September, 1538, a
new volcano, since known as Monte Nuovo (new mountain), rose
on the border of the ancient Lake Lucrinus to the westward of
Naples. This small mountain attained a height of 440 feet, and
is still to be seen on the shore of the bay of Naples. From Mexico
have been recorded the births of several new volcanoes: Jorullo
in 1759, Pochutla in 1870, and in 1881 a new volcano in the Ajusco
Mountains about midway between the Gulf of Mexico and the
Pacific Ocean. The latest of new volcanoes is that raised in Japan
on November 9, 1910, in connection with the eruption of Usu-san.
This “‘ New Mountain ” reached an elevation of 690 feet.
As described by von Humboldt, Jorullo rose in the night of the
28th of September, 1759, from a fissure which opened in a broad
plain at a point 35 miles distant from any then existing volcano.
The most remarkable of new volcanoes rose in 1871 on the island
of Camiguin northward from Mindanao in the Philippine archi-—
pelago. This mountain was visited by the Challenger expedition
in 1875, and was first ascended and studied thirty years later
by a party under the leadership of Professor Dean C. Worcester, —
the Secretary of the Interior of the Philippine Islands, to whom
the writer is indebted for this description and the accompanying
voleanic mountains that the
See
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 97
illustration of this largest and most interesting of new-born vol-
canoes. As in the case of Jorullo, the eruption began with the
formation of a fissure in a
level plain, some 400 yards
distant from the town of
Catarman (Fig. 88). The
eruption continued for four ane VERO ETELE
years, at the end of which 9-2" = Sis eS
time the height of the sum- Fic. 88.— View of the new Camiguin volcano
; : from the sea. It was formed in 1871 over
mit was estimated by the a nearly level plain. The town of Catarman
Challenger expedition to be appears at the right near the shore (after an
1900 feet. At the time of unpublished photograph by Professor Dean
the first ascent in 1905, Ee orereiery:
the height was determined by aneroid as 1750 feet, with sharp
rock pinnacles projecting some 50 or 75 feet higher.
Active and extinct volcanoes.— The terms “active” and
“‘ extinct ’”’ have come into more or less common use to describe
respectively those volcanoes which show signs of eruptive activity,
and those which are not at the time active. The term “‘ dormant ”
is applied to volcanoes recently active and supposed to be in a
doubtfully extinct condition. From a well-known volcano in
the vicinity of Naples, volcanoes which no longer erupt lava or
cinder, but show gaseous emanations (fumeroles) are said to be in
the solfatara condition, or to show solfataric activity.
Experience shows that the term “ extinct,’’ while useful, must
always be interpreted to mean apparently extinct. This may be
illustrated by the history of Mount Vesuvius, which before the
Christian era was forested in the crater and showed no signs of
activity ; and in fact it is known that for several centuries no erup-
tion of the voleano had taken place. Following a premonitory
earthquake felt in the year 63, the mountain burst out in grand
explosive eruption in 79 a.p. This eruption profoundly altered
the aspect of the mountain and buried the cities of Pompeii, Stabeii,
and Herculaneum from sight. Once more, this time during the
middle ages, for nearly five centuries (1139 to 1631) there was
complete inactivity, if we except a light ash eruption in the year
1500. During this period of rest the crater was again forested,
but the repose was suddenly terminated by one of the grandest
eruptions in the mountain’s history.
H
98 EARTH FEATURES AND THEIR MEANING
The earth’s volcano belts. — The distribution of volcanoes is
not uniform, but, on the contrary, volcanic vents appear in definite
zones or belts, either upon the margins of the continents or included
within the oceanic areas (Fig. 89). The most important of these
Fig. 89. — Map showing the location of the belts of active volcanoes.
belts girdles the Pacific Ocean, and is represented either by chains
or by more widely spaced volcanic mountains throughout the
Cordilleran Mountain system of South and Central America and
Mexico, by the volcanoes of the Coast and Cascade ranges of North
America, the festooned volcanic chain of the Aleutian Islands, and
the similar island ares off the eastern coast of the Eurasian con-
tinent. The belt is further continued through the islands of
Malaysia to New Zealand, and on the Pacific’s southern margin
are found the volcanoes of Victoria Land, King Edward Land,
and West Antarctica.
This volcano girdle is by no means a perfect one, for in addi-
tion to the principal festoons of the western border there are many
Fic, 90.— A portion of the “fire girdle” of the Pacific, showing the relation of the
chains of volcanic mountains to the deeps of the neighboring ocean floor.
+
Were. WS ay POPs +! Ste 5
se
ee ee a ee |
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 99
secondary ones, and still other arcs are found well toward the
center of the oceanic area. Another broad belt of volcanoes bor-
ders the Mediterranean Sea, and is extended westward into the
Atlantic Ocean. Narrower belts are found in both the northern
and southern portions of the Atlantic Ocean, on the margins of
the Caribbean Sea, etc. The fact of greatest significance in the
distribution seems to be that bands of active volcanoes are to be
found wherever mountain ranges are paralleled by deeps on the
neighboring ocean floor (Fig. 90). As has been already pointed
out in the chapter upon earthquakes, it is just such places as these
which are the seat of earthquakes; these are zones of the earth’s
crust which are undergoing the most rapid changes of level at the
present time. Thus the rise of the land in mountains is proceeding
simultaneously with the sinking of the sea floor to form the neigh-
boring deeps.
Arrangement of volcanic vents along fissures and especially at
their intersections. — Within those districts in which volcanoes
~ ot ~o ~ et et
” OF OOD PEERY IST: MY A Do 3 ‘
Brae te CO Oeret SLES ee wine 8 6B «um «= a Ts, 5 elim eda cal
Fig. 91.— Volcanic cones formed in 1783 above the Skaptar fissure in Iceland
(after Helland).
are widely separated from their neighbors, the law of their arrange-
ment is difficult to decipher, but the view that volcanic vents are
aligned over fissures is now supported by so much evidence that
illustrations may be supplied from many regions. An excep-
tionally perfect line of small cones is found along the Skaptdr
cleft in Iceland, upon which stands the large volcano of Laki.
This fissure reopened in 1783, and great volumes of lava were
exuded. Over the cleft there was left a long line of volcanic
cones (Fig. 91). There are in Iceland two dominating series of
parallel fissures of the same character which take their directions
respectively northeast-southwest and north-south. Many such
fissures are traceable at the surface as deep and nearly straight
clefts or gjds, usually a few yards in width, but extending for many
miles. The Eldgj4 has a length of more than 18 English miles
and a depth varying from 400 to 600 feet. On some of these
fissures no lava has risen to the surface, whereas others have at
numerous points exuded molten rock. Sometimes one end only
of a fissure, the more widely gaping portion, has supplied the
100 EARTH FEATURES AND THEIR MEANING
conduits for the molten lava. This is well illustrated by the
cratered monticules raised by the common ant over the cracks
ae which separate the blocks of cement
a = sidewalk, the hillocks being located
=% where the most favorable channel was
found for the elevation of the mate-
rials. :
h Those places upon fissures which be-
Fic. 92. — Diagrams to illustrate come lava conduits appear to be the
the location of voleanic vents (16. where the cleft gapes widest so as
upon fissure lines. a, openings ; :
caused by lateral movement of to furnish the widest channel. Wher-
fissure walls ;b, openingsformed ever a differential lateral movement of
esac sh als hence the walls has occurred, openings will
be found in the neighborhood of each minor variation from a
straight line (Fig. 92a). Wherever there are two or more series
of fissures, and this would appear to be the normal condition,
places favorable for lava conduits occur at fissure intersections.
Within such veritable volcano gardens as are to be found in Ma-
laysia, the law of volcano distribution became apparent so soon as
W
Fia. 93. — Outline map of the eastern portion of the island of Java, displaying the ar-
rangement of volcanic vents in alignment upon fissures with the larger mountains
at fissure intersections (after Verbeek).
accurate maps had been prepared. Thus the outline map of a por-
tion of the island of Java (Fig. 93) shows us that while the vol-
canoes of the island present at first sight a more or less irregular
band or zone, there are a number of fissures intersecting in a net-_
work, and that the volcanoes are aligned upon the fissures with
the larger cones located at the intersections. So also in Iceland,
~~ heey eet Sot
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 101
the great eruption of Askja in 1875 occurred at the intersection
of two lines of fissure.
Outside these closely packed volcanic regions, similar though
less marked networks are indicated; as, for example, in and near
the Gulf of Guinea. If now, instead of reducing the scale of our
voleano maps, we increase it, the same law of distribution is no —
less clearly brought out. The monticules or small volcanic cones
which form upon the flanks of larger volcanic mountains are like-
wise built up over fissures which on numerous occasions have
been observed to open and the cones to form upon them.
Still further reducing now
the area of our studies and
considering for the moment the
“frozen ”’ surface of the boil-
ing lava within the caldron of
Kilauea, this when observed at
night reveals in great perfec-
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tion the sudden formation of MAN
i oy the erust with th He
bth Catteries iMod S Fia. 94.— Map of the Puy Pariou in the
appearance of miniature vol- Auvergne of central France. The seat of
canoes rising successively at eruption has migrated along the fissure
more or less regular intervals tind i, oan Pe eee
along them.
It not infrequently happens that after a volcanic vent has
become established above some conduit in a fissure, the conduit
migrates along the fissure, thus establishing a new cone with more
or less complete destruction of the old one (Fig. 94).
The so-called fissure eruptions. — The grandest of all volcanic
eruptions have been those in which the entire length and breadth
of the fissures have been the passageway for the upwelling lava.
Such grander eruptions have been for the most part prehistoric,
and in later geologic history have occurred chiefly in India, in
Abyssinia, in northwestern Europe, and in the northwestern
United States. In western India the singularly horizontal pla-
teaus of basaltic lava, the Dekkan traps, cover some 200,000
Square miles and are more than a mile in depth. The underlying
basement where it appears about the margins of the basalt is
in many places intersected by dikes or fissure fillings of the same
material. No cones or definite vents have been found.
102 EARTH FEATURES AND THEIR MEANING
The larger portion of the northwestern British Isles would
appear to have been at one time similarly blanketed by nearly
horizontal beds of basaltic lava, which beds extended north-
westward across the sea through the Orkney and Faroe islands
to Iceland. Remnants of this vast plateau are to-day found in all
the island groups as well as in large areas of northeastern Ireland,
and fissure fillings of the same material occur throughout large
areas of the British Isles. In many cases these dikes represent
once molten rock which may never
have communicated with the surface
at the time of the lava outpouring, yet
they well illustrate what we might ex-
pect to find if the basalt sheets of
Iceland or Ireland were to be removed.
The floods of basaltic lava which in
the northwestern United States have
yielded the barren plateau of the Cas-
Fia. 95.— Basaltic plateau of the e¢gde Mountains (Fig. 95) would appear
northwestern United States due
to fissure eruptions of lava. +0 offer another example of fissure erup-
tion, though cones appear upon the
surface and perhaps indicate the position of lava outlets during the
later phases of the eruptive period. The barrenness and desola-
tion of these lava plains is suggested by Fig. 96.
oO
uw
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Sere a et
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bs eo tee
se “.
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pus es
7 2th eae eel® area ne
.
.
ee
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-
Pipa es : A ‘ o.
.
a . Ai 'o sf On OO a oS of
Bg Wee Peer rau ae re
©.
Fia. 96.— Lava plains about the Snake River in Idaho.
Though the greater effusions of lava have occurred in pre-
historic times, and the manner of extrusion has necessarily been
largely inferred from the immense volume of the exuded materials
and the existence of basaltic dikes in neighboring regions, yet in
4
,
{
a ile
}
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 103
Iceland we are able to observe the connection between the dikes
and the lava outflows. Professor Thoroddsen has stated that in
the great basaltic plateau of Iceland, lava has welled out quietly
from the whole length of fissures and often on both sides without
giving rise to the formation of cones. At three wider portions of
the great Eld cleft, lava welled out quietly without the formation
of cones, though here in the southern prolongation of the fissure,
where it was narrower, a row of low slag cones appeared. Where
the lava outwellings occurred, an area of 270 square miles was
flooded. .
The composition and the properties of lava. — In our study of
igneous rocks (Chapter IV) it was learned that they are com-
posed for the most part of silicate minerals, and that in their
chemical composition they represent various proportions of silica,
Fig. 97. — Characteristic profiles of lava voleanoes. 1, basaltic lava mountain ;
2, mountain of siliceous lava (after Judd).
alumina, iron, magnesia, lime, potash, and soda. The more
abundant of these constituents is silica, which varies from 35 to
70 per cent of the whole. Whenever the content of silica is rela-
tively low, — basic or basaltic lava, — the cooled rock is dark in
color and relatively heavy. It melts at a relatively low tempera-
ture, and is in consequence relatively fluid at the temperatures
which lavas usually have on reaching the earth’s surface. Further-
more, from being more fluid, the water which is nearly always
present in large quantity within the lava more readily makes its
escape upon reaching the surface. Eruptions of such lava are
for this reason without the violent aspects which belong to extru-
sions of more siliceous (more “ acidic’’) lavas. For the same
104 EARTH FEATURES AND THEIR MEANING
reason, also, basaltic lava flows more freely and can spread much
farther before it has cooled sufficiently to consolidate. This is
equivalent to saying that its surface will assume a flatter angle of
slope, which in the case of basaltic lava seldom exceeds ten degrees
and may be less than one degree (Fig. 97).
Siliceous lavas, on the other hand, are, when consolidated, rela-
tively light both in color and weight and melt at relatively high
temperatures. They are, therefore, usually but partly fused and
of a viscous consistency when they arrive at the earth’s surface.
Because of this viscosity they offer much resistance to the libera-
tion of the contained water, which therefore is released only to
the accompaniment of more or less violent explosions. The lava
is blown into the air and usually falls as consolidated fragments
of various degrees of coarseness.
It must not, however, be assumed that the temperature of lava
is always the same when it arrives at the surface, and hence it
may happen that a siliceous lava is exuded
at so high a temperature that it behaves
like a normal basaltic lava. On the other
hand, basaltic lavas may be extruded at
unusually low temperatures, in which case
their behavior may resemble that of the
- normal siliceous lavas. If, however, as is
generally the case, the energy of explosion
of a basaltic lava is relatively small, any
ejected portions of the liquid lava travel
to a moderate height only in the air, so that on Fetes: they are
still sufficiently pasty to
adhere to rock surfaces
and thus build up the
remarkably steep cones
and spines known as
“spatter cones” or. |.
“driblet cones” (Fig. [agente 2s.-
98). When, on the other | a a
hand, the energy of ex- — se
plosion is great, as is nor-
Fic. 98.—A driblet cone
(after J. D. Dana).
mally the case with sili- Fie. 99.—View of Leffingwell crater, a cinder
cone in the Owens valley, California (after an
ceous lavas, the portions unpublished photograph by W. D. Johnson).
sty Pts ek
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 105
of ejected lava have been fully consolidated before their fall to the
surface, so that they build up the same type of accumulation as
would sand falling in the same manner. The structures which
they form are known as tuff, cinder, or ash cones (Fig. 99).
Whenever the contained water passes off from siliceous lavas
without violent explosions, the lava may flow from the vent, but
in contrast to basaltic lavas it travels a short distance only before
consolidating. The resulting mountain is in consequence propor-
tionately high and steep (Fig. 97). Eruptions characterized by
violent explosions accompanied by a fall of cinder are described
as explosive eruptions. Those which are relatively quiet, and in
which the chief product is in the form of streams of flowing lava,
are spoken of as convulsive eruptions.
The three main types of volcanic mountain. — If the eruptions
at a volcanic vent are exclusively of the explosive type, the ma-
terial of the mountain which results is throughout tuff or cinder,
and the volcano is described as a cinder cone. If, on the other
hand, the vent at every eruption exudes lava, a mountain of solid
rock results which is a lava dome. It is, however, the exception
for a voleano which has a long history to manifest but a single
kind of eruption. At one time exuding lava comparatively
quietly, at another the violence with which the steam is liberated
yields only cinder, and the mountain is a composite of. the two
materials and is known as a composite volcanic cone.
The lava dome. — When successive lava flows come from a
crater, the structure which results has the form of a more or less
perfect dome. If the lava be of the basaltic or fluid type, the
slopes are flat, seldom making an angle of as much as ten degrees
with the horizon and flatter toward the summit (Fig. 101, p. 106).
If of siliceous or viscous lava, on the other hand, the slopes are
correspondingly steep and in some cases precipitous. To this
latter class belong some of the Kuppen of Germany, the puys of
central France, and the mamelons of the Island of Bourbon.
The basaltic lava domes of Hawaii. — At the “ crossroads of
the Pacific” rises a double line of lava volcanoes which reach
from 20,000 to 30,000 feet above the floor of the ocean, some
of them among the grandest volcanic mountains that are known.
More than half the height and a much larger proportion of the
bulk of the largest of these are hidden beneath the ocean’s surface.
106 EARTH FEATURES AND THEIR MEANING
The two great active vents are Mokuaweoweo (on Mauna Loa)
and Kilauea, distinct volcanoes notwithstanding the fact that their
lava extravasations have been merged in a single mass. The rim
of the crater of Mauna Loa is at an elevation of 13,675 feet above
the sea, whereas that of Kilauea
is less than 4000 feet and ap-
pears to rest upon the flank of
the larger mountain (Figs. 100
and 101). Although one crater
is but 20 miles distant from the
other and nearly 10,000 feet
lower, their eruptions have ap-
parently been unsympathetic.
Nowhere have still active lava
mountains been subjected to
such frequent observations ex-
tending throughout a long pe-
riod, and the dynamics of their
Fra. 100.— Map of Hawaii and the lava eruptions are fairly well under-
volcanoes of Mokuaweoweo (Mauna gtood, To put this before the
Loa) and Kilauea (after the government ‘i ;
Waele Alecander): reader, it will be best to con-
sider both mountains, for
though they have much in common, the observations from one are
strangely complementary to those of the other. The lower crater
pe ee Mauna Loa
——
mmemancn ooone een onorne rene ;
Scale of Miles.
: 50
Fia. 101.— Section through Mauna Loa and Kilauea.
being easily accessible, Kilauea has been often visited, and there
exists a long series of more or less consecutive observations upon
it, which have been assembled and studied by Dana and Hitch-
cock. The place of outflow of the Kilauea lavas has not generally
been visible, whereas Mokuaweoweo has slopes rising nearly 14,000
feet above the sea and displays the records of outflow of many
eruptions, some of which were accompanied by the grandest of
volcanic phenomena. °
Lava movements within the caldron of Kilauea. — The crater
of these mountains are the largest of active ones, each being in
a
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RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 107
excess of seven miles in circumference. In shape they are irregu-
larly elliptical and consist of a series of steps or terraces descend-
ing to a pit at the bottom, in which are open lakes of boiling lava.
Enough is known of the history of Kilauea to state that the steep
cliffs bounding the terraces are fault walls produced by inbreak
of a frozen lava surface. The cliff below the so-called ‘“ black
ledge”? was produced by the falling in of the frozen lava surface
at the time of the outflow of 1840, the lava issuing upon the
eastern flank of the mountain and pouring into the sea near
Nanawale. Since that date the floor of the pit below the level
of this ledge has been essentially a movable platform of frozen
lava of unknown and doubtless variable thickness which has risen
and descended like the floor of an elevator car between its guiding
ways (Fig. 102). The floor has, however, never been complete,
for one or more open lakes are
always to be seen, that of Hale-
maumau located near the south-
western margin having been much
the most persistent. Within the iy
cus ; Fig. 102. — Schematic diagram to illus-
open lakes the boiling lava is ap- trate the moving platform of frozen
parently white hot at the depth lava which rises and falls in the crater
CRATER
MOLTEN LAVA
of but a few inches below the of Kilauea.
surface, and in the overturnings of the mass these hotter portions
are brought to the surface and appear as white streaks marking
the redder surface portions. From time to time the surface
freezes over, then cracks open and erupt at favored points along
the fissures, sending up jets and fountains of lava, the material of
which falls in pasty fragments that build up driblet cones. Small
fluid clots are shot out, carrying a threadlike line of lava glass
behind them, the well-known ‘“ Pele’s hair.”” Sometimes the open
lakes build up congealed walls, rising above the general level of
the pit, and from their rim the lava spills over in cascades to
spread out upon the frozen floor, thus increasing its thickness from
above (Fig. 103). At other times a great dome of lava has been
pushed up from the pit of Halemaumau under a frozen shell, the
molten lava shining red through cracks in its surface and exuding
so as to heal each widely opened fissure as it forms.
At intervals of from a few years to nine or ten years the crater
has been periodically drained, at which times the moving plat-
108 EARTH FEATURES AND THEIR MEANING
form of frozen lava has sunk more or less rapidly to levels far
below the black ledge and from 900 to 1700 feet below the crater
rim. Following this descent a slow progressive rise is inaugurated,
which has sometimes gone on at a rate of more than a hundred
feet per year, though it is usually much slower than this. When
|
Ni
Kilauea, the molten lava shown cascading over the raised lava walls on to the
floor of the pit (after Pavlow).
the platform has reached a height varying from 700 to 350 feet
below the crater rim, another sudden settlement occurs which
again carries the pit floor downward a distance of from 300 to 700
feet.
The draining of the lava caldrons.— The changes which go on
within the crater of Mokuaweoweo, though less studied than
those of Kilauea, appear to be in some respects different. Here
every eruption seems to be preceded by a more or less rapid influx
of melted lava to the pit of the crater, this phenomenon being
observed from a distance as a brilliant light above the crater —
the reflection of the glow from overhanging vapor clouds. The
uprising of the lava has often been accompanied by the formation
of high lava fountains upon the surface, and the molten lava ~
sometimes appears in fissures near the crater rim at levels well
above the lava surface within the pit.
LS SO
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y
which opened five
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 109
Although in many cases the lava which has thus flooded the
crater has suddenly drained away without again becoming visible,
it is probable that in such cases an outlet has been found to some
submarine exit, since under-ocean discharge effects have been
observed in connection with eruptions of each of the volcanoes.
Inasmuch as no earthquakes are felt in connection with such
outflows as have been described, it is probable that the hot lava
fuses a passageway for itself into some open channel underneath
the flanks of the mountain. Such a course is well illustrated by
the outflow of Kilauea
in 1840, when, it will \N
be remembered, oc- INNS
curred the great down-
plunge of the crater
that yielded the pit -
below the black ledge.
At this time the lava
first made its appear-
ance upon the flanks
of the mountain at the
bottom of a small pit
or inbreak erater Fic. 104.— Map showing the manner of outflow of
lava from Kilauea during the eruption of 1840.
The outflowing lava made its appearance succes-
miles southeast of the sively at the points A, B, C, m, n, and finally at a
main crater of Kilauea point below n, from whence it issued in volume and
Z ets flowed down to the sea at Nanawale (after J. D.
(Fig. 104). Within Dana).
—= fr //
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WZ
Le ae
this new crater the
lava rose, and small ejections soon followed from fissures formed in
its neighborhood. Some time after, the lava sank in the first new
crater, only to reappear successively at other small openings (Fig.
104, B, C, m, n) and finally to issue in volume at a point eleven
miles from the shore and flow thereafter wpon the surface of the
mountain until it had reached the sea. Only the slightest earth
tremors were felt, and as no rumblings were heard, it is evident
that the lava fused its way along a buried channel largely open at
the time (see below, p. 112). 3
In a majority of the eruptions of Mokuaweoweo, when the
, outflowing lavas have become visible, the molten rock has ap-
parently fused its way out to the surface.of the mountain at.
EO EARTH FEATURES AND THEIR MEANING
points from 1000 to 3000 feet below the bottom of the crater,
and this discharge has corresponded in time to the lowering
of the lava surface within the crater. There are, however,
three instances upon record in which the lava issued from definite
rents which were formed upon the mountain flanks at compara-
tively low levels. In contrast to the formation of fused outlets,
these ruptures of a portion of the mountain’s flank were always
accompanied by vigorous local earthquakes of short duration. In
one instance (the eruption of 1851) such a rent appeared under
the same conditions but at an elevation of 12,500 feet, or near
the level of the lava in the crater.
The outflow of the lava floods. —In order to properly com-
prehend these and many otherwise puzzling phenomena connected |
=
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—————SSSSS—————aaaa
TS So er
ae rr
=
sea during the eruption of 1906. The course may be followed by the jets of steam
escaping from the surface down to the great steam cloud which rises where the
fluid lava discharges into the sea (after H. I. Jensen).
with volcanoes, it is necessary to keep ever in mind the quite
remarkable heat-insulating property of congealed lava. So soon
as a thin crust has formed upon the surface of molten rock, the
heat of the underlying fluid mass is given off with extreme slow-
ness, so that lava streams no longer connected with their internal
lava reservoirs may remain molten for decades.
We have seen that for Mokuaweoweo and Kilauea, lava either
quietly melts its way to the surface at the time of outflow, or —
else produces a rent for its egress to the accompaniment of vigor-
ous local earthquakes. In either case if the lava issues at a point
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RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 111
far below the crater, gigantic lava fountains arise at the point of
outflow, the fluid rock shooting up to heights which range from
250 to 600 or more feet above the surface. A certain proportion
of this fluid lava is sufficiently cooled to consolidate while travel-
ing in the air, and falling, it builds up a cinder cone which is left
as a location monument for the place of discharge. From this
outlet the molten lava begins its journey down the slope of the
mountain, and quickly freezes over to produce a tunnel, beneath
the roof of which the fluid lava flows with comparatively slow
further loss of heat. Save for occasional steam jets issuing from
its surface, it may give little indication of its presence until it has
reached the sea (Fig. 105).
If sufficient in volume and the shore be not too distant, the
stream of lava arrives at the sea, where, discharging from the
alavatunnel. Eruption of Matavanu on Savaii in 1906 (after Sapper).
mouth of its tunnel, it throws up vast volumes of steam and in-
duces ebullition of the water over a wide area (Fig. 106). Pro-
fessor Dana, who visited Hawaii a few months only after the
great outflow of 1840, states that the lava, upon reaching the
112 EARTH FEATURES AND THEIR MEANING
ocean, was shivered like melted glass and thrown up in millions of
particles which darkened the sky and fell like hail over the sur-
rounding country. The light was so bright that at a distance of
forty miles fine print could be read at midnight.
Protected from any extensive consolidation by its congealed
cover, the lava within a stream may all drain away, leaving behind
an empty lava tunnel, which in the
case of the Hawaiian volcanoes
sometimes has its roof hung with
: beautiful lava stalactites and its
Fig. 107.— Diagrammatic repre- aH 3 ;
sentation of the structure of the floor studded with thin lava spines.
flanks of lava volcanoes as are- Later lava outflows over the same
sult of the draining of frozen lava or neighboring courses bury such
ieee tunnels beneath others of similar
nature, giving to the mountain flanks an elongated cellular struc-
ture illustrated schematically in Fig. 107. These buried channels
may in the future be again utilized for outflows similar in char-
acter to that of Kilauea in 1840.
While the formation of lava stalactites of such perfection and
beauty is peculiar to the Hawaiian lava tunnels, the formation of
the tunnel in connection with lava outflow is the rule wherever a dis-
sipation at the end has permitted of drainage. A few hours only
after the flow has begun, the frozen surface has usually a thickness
of a few inches, and this cover may be walked over with the lava still
molten below. At first in part supported by the molten lava, the
tunnel roof sometimes caves in so soon as drainage has occurred.
Wherever basaltic lava has spread out in valleys on the surface
of more easily eroded
material, either cinder
or sedimentary forma-
tions, the softer inter-
vening ridges are first
ae iy Fia. 108.— Diagram to show the manner of forma-
cami away by the tion of mesas or table mountains by the outflow
eroding agencies, leav- . of lava in valleys and the subsequent more rapid
ing the lava as cappings erosion of the intervening ridges. AR, earlier river
5 x pping valley ; R’R’, later valleys.
upon residual eleva-
tions. Thus are derived a type of table mountain or mesa of the
sort well illustrated upon the western slopes of the Sierra Ne-
vadas in California (Fig. 108).
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 113
. \\
‘ IY) VY \\ \\\ \
NK WS SN A \N:
SS
Sas : WY
$1 Se SY ST 7
SS
» WZ ee “
Cig? ie
Sr S S
== x.
ESS
WS
Fia. 109.— Surface of lava of the Pahoehoe type.
The surface which flowing lava assumes, while subject to con-
siderable variation, may yet be classified into two rather distinct
types.
On the one hand there is the billowy surface in which
ellipsoidal or kidney-shaped masses, each with dimensions of from
one to several feet, lie merged
in one another, not unlike an
irregular collection of sofa
pillows. ‘This type of lava has
become known as the Pahoehoe,
from the Hawaiian occurrence
(Fig. 109). A variation from
this type is the “ corded” or
“ropy”’ lava, the surface of
which much resembles rope as
it is coiled along the deck of
a vessel, the coils being here the .
lines of scum or scorie arranged
in this manner by the currents
at the surface of the stream
(Fig. 123, p. 124). A quite
different type is the block lava
(Aa type) which usually has a
ragged scoriaceous surface and
consists of more or less separate
fragments of cooled lava (Fig.
131, p. 130).
I
oe
rd panes
Fig. 110.—Three successive views to
illustrate the growth of the Island of
Savaii from the outflow of lava at
Matavanu in the year 1906. a, near the
beginning of the outflow; b, some weeks
later than a; c, some weeks later than
b (after H. I. Jensen).
114 EARTH FEATURES AND THEIR MEANING
Wherever lava flows into the sea in quantity, it extends the
margin of the shore, often by considerable areas. The outflow of
Kilauea in 1840 extended the shore of Hawaii outward for the
distance of a quarter of a mile, and a more recent illustration of
such extension of land masses is furnished by Fig. 110.
et se ot a a ee 6 oe ee a 4 — ait
PRS Riga eee ee Sol eee oe
CHAPTER X
THE RISE OF MOLTEN ROCK TO THE EARTH’S
SURFACE
VOLCANIC MOUNTAINS OF EJECTED MATERIALS
The mechanics of crater explosions. —If we now turn from
the lava volcano to the active cinder cone, we encounter an entire
change of scene. In place of the quiet flow and convulsive move-
ments of the molten lava, we here meet with repeated explosions
of greater or less violence. If we are to profitably study the
manner of the explosions, considering the volcanic vent as a great
experimental apparatus, it would be well to select for our purpose
a volcano which is in a not too violent mood. The well-known
cinder cone of Stromboli in the Eolian group of islands north of
Sicily has, with short and unimportant interruptions, remained in
a state of light explosive activity since the beginning of the Chris-
tian era. Rising as it does some three thousand feet directly out
of the Mediterranean, and displaying by day a white steam cap
and an intermittent glow by night, its summit can be seen for a
distance of a hundred miles at sea and it has justly been called
the ‘ Lighthouse of the Mediterranean.”” The “flash” interval
of this beacon may vary from one to twenty minutes, and it may
show, furthermore, considerable variation of intensity.
For the reason that the crater of the mountain is located at
one side and at a considerable distance below the actual summit,
the opportunity here afforded of looking into the crater is most
favorable whenever the direction of the wind is such as to push
aside the overhanging steam cloud (Fig. 111). Long ago the
Italian vulcanologist Spallanzani undertook to make observations
from above the crater, and many others since his day have profited
by his example.
Within the crater of the volcano there is seen a lava surface
lightly frozen over and traversed by many cracks from which
vapor jets are issuing. Here, as in the Kilauea crater, there are
open pools of boiling lava. From some of these, lava is seen
115
116 EARTH FEATURES AND THEIR MEANING
welling out to overflow the frozen surface ; from others, steam is
ejected in puffs as though from the stack of a locomotive. Within
others lava is seen heaving up and down in violent ebullition, and
at intervals a great bubble of steam is ejected with explosive vio-
lence, carrying up with it a considerable quantity of the still
molten lava, together with its scumlike surface, to fall outside the
crater and rattle down the mountain’s slope into the sea. Fol-
lowing this explosion the lava surface in the pool is lowered and
the agitation is renewed, to culminate after the further lapse of a
few minutes in a second explosion of the same nature. The rise
of the lava which
precedes the ejection
appears at night as a
brighter reflection or
glow from the over-
hanging steam cloud
—the flash seen by
the mariner from his
vessel. |
What is going on
within the crater of
tee eerie Stromboli we may
Fia. 111.— The voleano of Stromboli, showing the perhaps best; slime
excentric position of the crater (after a sketch by te
Judd). trate by the boiling
of a stiff porridge
over a hot fire. Any one who has made corn mush over a hot
camp fire is fully aware that in proportion as the mush becomes
thicker by the addition of the meal, it is necessary to stir the
mass with redoubled vigor if anything is to be retained within the
kettle. The thickening of the mush increases its viscosity to such
an extent that the steam which is generated within it is unable to
make its escape unless aided by openings continually made for it
by the stirring spoon. If the stirring motion be stopped for a
moment, the steam expands to form great bubbles. which soon
eject the pasty mass from the kettle.
For the crater of Stromboli this process is illustrated by the
series of diagrams in Fig. 112. As the lava rises toward the
surface, presumably as a result of convectional currents within
the chimney of the volcano, the contained steam is relieved from
Scams --
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 117
pressure, so that at some depth below the surface it begins to
separate out in minute vesicles or bubbles, which, expanding as
they rise, acquire a rapidly accelerating velocity. Soon they flow
together with a quite sudden increase of their expansive energy,
and now shooting upward with further accelerated velocity, a
layer of liquid lava with its cover of scum is raised on the surface
of a gigantic bubble and thrown high into the air. Cooled during
their flight, the quickly congealed lava masses become the tuff or
volcanic ash which is the material of the cinder cone.
Fig. 112.— Diagrams to illustrate the nature of eruptions within the crater of
Stromboli.
Grander volcanic eruptions of cinder cones. — Most cinder and
composite cones, in the intervals between their grander eruptions,
if not entirely quiescent, lapse into a period of light activity
during which their crater eruptions appear to be in all essential
respects like the habitual explosions within the Strombolian
crater. This phase of activity is, therefore, described as Strom-
bolian. By contrast, the occasional grander eruptions which have
punctuated the history of all larger volcanoes are described in
the language of Mercalli as Vulcanian eruptions, from the best
studied example.
Just what it is that at intervals brings on the grander Vul-
canian outburst within a volcano is not known with certainty ;
but it is important to note that there is an approach to periodicity
118 EARTH FEATURES AND THEIR MEANING -
in the grander eruptions. It is generally possible to distinguish
eruptions of at least two orders of intensity greater than the
Strombolian phase; a grander one, the examples of which may
be separated by centuries, and one or more orders of relatively
moderate intensity which recur at intervals perhaps of decades,
their time intervals subdividing the larger periods marked off by
the eruptions of the first order. ?
The eruption of Volcano in 1888. — In the Eolian Islands to
the north of Sicily was located the mythical forge of Vulcan.
From this locality has come our word “ volcano,” and both the
island and the mountain bear no other
name to-day (Fig. 113). There is in the |
structure of the island the record of a.
somewhat complex volcanic history, but
the form of the large central cinder cone
was, according to Scrope, acquired during
the eruption of 1786, at which time the
crater is reported to have vomited ash for
a period of fifteen days. Passing after
this eruption into the solfatara condition,
Scake or Mies. with the exception of a light eruption in
one re ae 1873, the volcano remained quiet until
cano in the Eolian group 1886. So active had been the fumeroles
of islands. The smaller within the crater during the latter part of
si as this period that an extensive plant had
Vulcanello (after Juda), been established there for the collection
especially of boracic acid. In 1886 occurred
a slight eruption, sufficient to clear out the bottom of the crater,
though not seriously to disturb the English planter whose vine-
yards and fig orchards were in the valley or atrio near the point d
upon the map (Fig. 113), nearly a mile from the crater rim. On
the 3d of August, 1888, came the opening discharge of an eruption,
which, while not of the first order of magnitude, was yet the greatest
in more than a century of the mountain’s history, and may serve us
to illustrate the Vulcanian phase of activity within a cinder cone.
During the day, to the accompaniment of explosions of consider-
able violence, projectiles fell outside the crater rim and rolled
down the steep slopes toward the atrio. These explosions were
repeated at intervals of from twenty to thirty minutes, each
Ay:
Sy
no }
pon
<
S AN
Ai CIWS
—— SS aK S
SS}
ee
Ss ee ee
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 119
beginning in a great upward rush of steam and ash, accompanied
by a low rumbling sound. During the following night the erup-
tions increased in violence, and the anxious planter remained on
watch in his villa a mile from the crater. Falling asleep toward
morning, he was rudely awakened by a rain of projectiles falling
upon his roof. Hastily snatching up his two children he ran
toward the door just as a red hot projectile, some two feet in di-
ameter, descended through the roof, ceiling, and floor of the drawing
room, setting fire to the building. A second projectile similar to
the first was smashed into fragments at his feet as he was emerg-
ing from the house, burning one of the children. Making his
escape to Vulcanello at the extremity of the island, the remainder
of the night and the following day, until rescue came from Lipari,
were spent just beyond the range
of the falling masses.
When the writer visited the
island some months later, the
eruption was still so vigorous that
the crater could not be reached. @uiy\
The ruined villa, smashed and »“: i :
charred, stood with its walls half a as peepee he eee
buried in ash and lapilli, among __ in 1888 (after Mercalli).
which were partly smashed pumi-
ceous lava projectiles. The entire atrio about the mountain lay
buried in cinder to the depth of several feet and was strewn with
projectiles which varied in size from a man’s fist to several feet
in diameter (Fig. 114). The larger of these exhibited the peculiar
*“‘ bread-crust ”’ surface and had generally been smashed by the
force of their fall after the manner of a pumpkin which has been
thrown hard against the ground. One of these projectiles fully
three feet in diameter was found at the distance of a mile and a
half from the crater. Though diminished considerably in inten-
sity, the rhythmic explosions within the crater still recurred at
intervals varying from four minutes to half an hour, and were
accompanied by a dull roar easily heard at Lipari on a neighboring
island six miles away. Simultaneously, a dark cloud of ‘‘ smoke,”
the peculiar “ cauliflower cloud’ or pino mounted for a couple
of miles above the crater (Fig. 115), and the rise was succeeded
by a rain of small lava fragments or lapilli outside the crater rim.
120 EARTH FEATURES AND THEIR MEANING
A, CUeants 2 pre
: = ihe . .
one: ~ rer - fee ory
. : oe FO ars a
=~ . Pe Anat Uk. BD me os es Lz
fs
*
. : . . on & Cee
ba wig © LG re rpietseWin tele st Otc eS eer et
7 fa vine
Fia. 115. — Peculiar ‘cauliflower cloud’”’
or pino composed of steam and ash,
rising above the cinder cone of Volcano
There seems to be no good
reason to doubt that Vulcanian
cinder eruptions of this type
differ chiefly in magnitude from
the rhythmic explosion within
the crater of Stromboli, if we
except the elevation of a con-
siderable quantity of acces-
sory and older tuff which is
derived from the inner walls
of the crater and carried up-
ward into the air together
during the waning phases of the explo-
sive eruption of 1888 (after a photo-
graph by B. Hobson).
with the pasty cakes of fresh
lava derived from the chimney.
It is this accessory material
which gives to the pino its dark or even black appearance.
The eruption of Taal volcano on Januaey 30, 1911. — The
recent eruption of the cinder
cone known as Taal volcano
is of interest, not only because
so fresh in mind, but because
two neighboring vents erupted
simultaneously with explosions
of nearly equal violence (Fig.
116). This Philippine vol-
cano lies near the center of a
lake some fifteen miles in
diameter and about fifty miles
south of the city of Manila.
After a period of rest extend-
ing over one hundred and fifty
years, the symptoms of the
coming eruption developed
rapidly, and on the morning
of January 30 grand explo-
sions of steam and ash oc. -=: a?
curred simultaneously in the bee ateee, “a fs
neighboring craters, and the Fig. 116.— Double explosive eruption of
, Taal volcano on the morning of Janu
condensed moisture brought — 30, 1911. : if:
oa
ne a ee eee
_
ei
. ae
ed ae
A
s
ie
i,
oy
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 121
down the ash in an avalanche of scalding mud which buried the
entire island. Almost the entire population of the island, num-
bering several hundreds,
was literally buried in the
blistering mud (Fig. 117);
and the gases from the ex-
plosions carried to the dis-
tant shores of the lake
added to this number many
hundred victims.
The shocks which accom-
panied the explosions raised
a great wave upon the sur-
face of the lake, which, ad-
vancing upon the shores,
washed away structures for
a distance of nearly a half
mile.
The materials and the
structure of cinder cones.
— Obviously the materials
which compose cinder cones
island of Taal (after a photograph by
are the cooled lava frag- —peniston).
ments of various degrees of
coarseness which have been ejected from the crater. If larger
than a finger joint, such fragments are referred to as volcanic
projectiles, or, incorrectly, as ‘‘ voleanic bombs.’”’ Of the larger
masses it is often true that the force of expulsion has not been
| applied opposite the cen-
WU Witte ter of mass of the body.
fl! \ Wego ;
if ow CB : Thus it follows that they
——s
undergo complex whirl-
ing motions during their
flight, and being still
semiliquid, they develop
curious pear-shaped or
SS ZF less regular forms (Fig.
yA 118). When crystals
Fig. 118.—A pear-shaped lava projectile. have already separated
A ee
TY
yy
122 EARTH FEATURES AND THEIR MEANING
out in the lava before its rise in the chimney of the volcano, the
surrounding fluid lava may be blown to finely divided volcanic
dust which floats away upon the wind, thus leaving the crystals
intact to descend as a crystal rain about the crater. Such a
shower occurred in connection with the eruption of Etna in 1669,
and the black augite crystals may to-day be gathered by the
handful from the slopes of the Monti Rossi (Fig. 125, p. 125).
The term lapilli, or sometimes rapilli, is applied to the ejected
lava fragments when of the average size of a finger joint. This is
the material which still
partially covers the un-
exhumed portions of the
city of Pompeii. Vol-
canic sand, ash, and dust
are terms applied in
order to increasingly
fine particles of the
ejected lava. The finest
material, the volcanic
dust, is often carried
for hundreds and some-
times even for thou-
sands of miles from the
crater in the high-level
currents of the atmos-
phere. Inasmuch as
et a ee ea ‘ this material is de-
1G. . — Artificial production of the structure o :
a cinder cone with use of colored sands carried up posited far from the
in alternation by a current of air (after G. Linck). crater and in layers
more or less horizontal,
such material plays a small réle in the formation of the cinder
cone. The coarser sands and ash, on the other hand, are the
materials from which the cinder cone is largely constructed.
The manner of formation and the structure of cinder cones
may be illustrated by use of a simple laboratory apparatus (Fig.
119). Through an opening in a board, first white and then
colored sand is sent up in a light current of air or gas supplied
from suitable apparatus. The alternating layers of the sand
form in the attitudes shown; that is to say, dipping inward or
Sees
i
Yr
fA
A
.
.
a
s
ee ee ee ee
’ other hand, builds a high cone
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 123
toward the chimney of the volcano at all points within the crater
rim, and outward or away from it at all points outside (Fig. 119).
If the experiment is carried so far that at its termination sand
slides down the crater walls into the chimney below, the inward
dipping layers will be truncated, or even removed entirely, as
shown in Fig. 119 6.
The profile lines of cinder cones. — The shapes of cinder cones
are notably different from those of lava mountains. While the
Fig. 120.— Diagram to show the contrast between a lava dome and acinder cone.
AAA, cinder cone; BabC, lava dome; DE, line of low cinder cones above a fissure
(after Thoroddsen).
latter are domes, the mountains constructed of cinder are conical
and have curves of profile that are concave upward instead of
convex (Fig. 120). In the earlier stages of its growth the cinder
cone has a crater which in proportion to the height of the moun-
tain is relatively broad (Fig. 99, p. 104).
Speaking broadly, the diameter of the crater is a measure of
the violence of the explosions within the chimney. A single series
of short and violent explosive
eruptions builds a low and
broad cinder cone. A _long-
continued succession of moder-
ately violent explosions, on the
with crater diameter small if
compared with the mountain’s
altitude, and the profile afforded
is a remarkably beautiful sweep-
ing curve (Fig. 121). Toward Luzon, P.I. A remarkably perfect
; gh cinder cone.
the summit of such a cone the
loose materials of which it is composed are at as steep an angle
as they can lie, the so-called angle of repose of the material;
whereas lower down the flatter slopes have been determined by the
distribution of the cinder during its fall from the air. When one
124 EARTH FEATURES AND THEIR MEANING
makes the ascent of such a mountain, he encounters continually
steeper grades, with the most difficult slope just below the crest.
The composite cone. — The life
histories of volcanoes are generally
so varied that lava domes and the
pure types of cinder cones are less
common than volcanoes in which
paroxysmal eruptions have alternated
Fic. 122.—A series of breached
cinder cones where the place of
eruption has migrated along the
underlying fissure. The Puys with explosions, and where, therefore,
Noir, Solas, and La Vache in the the structure of the mountain repre-
Mont Dore Province of central
RreacatntienSGupen: sents a composite of lava and cinder.
Such composite cones possess a skele-
ton of solid rock upon which have been built up alternate sloping
layers of cinder and lava. In most respects such cones stand in
an intermediate position be-
tween lava domes and cinder
cones.
Regarded as a retaining wall
for the lava which mounts in
the chimney, the cinder cone
is obviously the weakest of
all. Should lava rise in a
cinder cone without an ex-
plosion occurring, the cone is
at once broken through upon
one side by the outwelling
of the lava near the base.
Thus arises the characteristic
breached cone of _ horseshoe
form (Fig. 122). : —
Quite in contrast with the 9 aR
OK: uf :
€
weak cinder cone is the lava _ R22 PLD
dome with its rock walls and
uel Fig. 123.— The bocca or mouth upon the
relatively flat slopes: Coli= Année cone of Mount Vesuvius from which
sidered as a retaining wall for flowed the lava stream of 1872. This
lava it is much the stron gest lava stream appears in the foreground
A f with its characteristic ‘‘ropy’”’ surface.
type of volcanic mountain,
and it is likely that the hydrostatic pressure of the lava within
the crater would seldom suffice to rupture the walls, were it not
~~
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 125
that the molten rock first fuses its way into old stream tunnels
buried under the mountain slopes (see ante, p. 112). Composite
cones have a strength as retaining walls for lava which is inter-
mediate between that of the
other types. Their Vulcanian Ma
eruptions of the convulsive ri
type are initiated by the forma-
tion of a rent or fissure upon
the mountain flanks at eleva-
tions well above the base, the
opening of the fissure being
generally accompanied by a
local earthquake of greater or Fia. 124.—A row of parasitic cones raised
less violence From one or above a fissure which was opened upon
; the flanks of Mount Etna during the
more such fissures the lava eruption of 1892 (after De Lorenzo).
issues usually with sufficient
violence at the place of outflow to build up over it either an
enlarged type of driblet cone, referred to as a “‘ mouth,” or bocca!
(Fig. 123), or one or more cinder cones which from their position
upon the flanks of the larger volcano are referred to as parasitic
| pepe :
Fia. 125.— View looking toward the summit of Etna from a position upon the
southern flank near the village of Nicolosi. The two breached parasitic cones
seen behind this village are the Monti Rossi which were thrown up in 1669 and
from which flowed the lava which overran Catania (after a photograph by
Sommer).
cones (Fig. 124). The lava of Vesuvius more frequently yields
bocchi at the place of outflow, whereas the flanks of Etna are
1 Italian for mouth ; plural bocchi.
126 EARTH FEATURES AND THEIR MEANING
pimpled with great numbers of parasitic cinder cones, each the
monument to some earlier eruption (Fig. 125).
It is generally the case that a single
eruption makes ‘but a relatively small
contribution to the bulk of the mountain.
From each new cone or bocca there pro-
ceeds a stream of lava spread in a rela-
tively narrow stream extending down the
slopes (Fig. 126).
The caldera of composite cones. —
Because of the varied episodes in the
Fic. 126.—Sketch map of history of composite cones, they lack the
Etna, showing the indi- regular lines characteristic of the two
vidualsurfacelavastreams .
(in black) and the tuff Simpler types. The larger number of the
covered surface (stippled). more important composite cones have
been built up within an outer crater of
relatively large diameter, the Somma cone or caldera, which
surrounds them like a gigantic ruff or collar. This caldera is
clearly in most cases at least the relic of an earlier explosive
crater, after which successive eruptions of lesser violence have
built a more sharply conical structure. This-can only be inter-
preted to mean that most larger and long-active volcanoes have
yous xe WP agin ree nee
a" ae = ti A ANS NN Ss S S
y ‘ ‘ ris mu + mit AO \
ve, y! pee I oft: ay At: SAN SSS
MU Te his \
Z in
o ena: é: a a TV)
: coat Bae See pk Pm
NS
shew
yy SS pet ‘
Fig. 127. — Panum crater, showing the caldera and the later interior cones
(after Russell).
been born in the grandest throes of their life history, and that a .
larger or smaller lateral migration of the vent has been responsible
for the partial destruction of the explosion crater. Upon Vesu-—
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 127
vius we find the crescent-like rim of Monte Somma; on Etna it
is the Val del Bove, etc. It is this caldera of composite cones
which gave rise to the theory of the ‘ elevation crater”’ of von
Buch (see ante, p. 95, and Fig. 127).
The eruption of Vesuvius in 1906.— The volcano Vesuvius
rises on the shores of the beautiful bay of Naples only about ten
miles distant from the city of Naples. The mountain consists of
the remnant of an earlier broad-mouthed explosion crater, the
. Monte Somma, and an inner, more conical elevation, the Monte
_ Vesuvio. Before the eruption of 1906 this central cone was sharply
conical and rose to
a height of about
4300 feet above
the surface of the
bay, or above the
highest point of the
ancient caldera.
The base of this
inner cone is at an
elevation of some-
thing less than half
that of the entire Fic. 128.— View of Mount Vesuvius as it appeared from
: : the Bay of Naples shortly before the eruption of 1906
ABAD, and is 5 al The horn to the left is Monte Somma.
rated from the en-
circling ring wall of the old crater by the aétrio, to which corre-
sponds in height a perceptible shelf or piano upon the slope toward
the bay of Naples (Fig. 128).
_ An active composite cone like that of Vesuvius is for the greater
part of the time in the Strombolian condition; that is to say, light
crater explosions continue with varying intensity and interval,
except when the mountain has been excited to the periodic Vul-
canian outbreaks with which its history has been punctuated.
The Strombolian explosions have sufficient violence to eject small
fragments of hot lava, which, falling about the crater, slowly built
up a rather sharp cone. The period of Strombolian activity has,
therefore, been called the cone-producing period. Just before each
new outbreak of the Vulcanian type, the altitude of the mountain
has, therefore, reached a maximum, and since the larger explosive
eruptions remove portions of this cone at the same time that
128 EARTH FEATURES AND THEIR MEANING
they increase the dimensions of the crater, the Vulcanian stage in
contrast to the other has been called the crater-producing period.
In this period, then, the material ejected during the explosions does
not consist solely of fresh lava cakes, but in part of the older débris
derived from the crater walls, whence it is avalanched upon the-
chimney after each larger explosion. The over-
men NS Yonging cloud, which during the Strombolian
period has consisted largely of steam and is
a \ noticeably white, now assumes a darker tone,
ae the ‘‘smoke’”’ which characterizes the Vulcanian
eruption.
5, Peay On several historical occasions the cone of
Vesuvius has been lowered by several hundred
Ang. of feet, the greatest of relatively recent truncations
having occurred in 1822 and in 1906. Between
ee Vulcanian eruptions the Strombolian activity is
by no means uniform, and so the upward growth
spts™} of the cone is subject to lesser interruptions and
truncations (Fig. 129).
son The Vesuvian eruption of 1906 has been
selected as a type of the larger Vulcanian erup-
mecnet tion of composite cones, because it combined the
{om explosive and paroxysmal elements, and because
Kt] - it has been observed and studied with greater
thoroughness than any other. The latest pre-
vious eruption of the Vulcanian order had
occurred in 1872. Some two years later the
Fic. 129.— A series
of consecutive
sketches of the
summit of the
Vesuvian cone,
showing the modi-
fications in its out-
line (after Sir Wil-
liam Hamilton).
period of active cone building began and pro-
ceeded with such rapidity that by 1880 the new
cone began to appear above the rim of the crater
of 1872. From this time on occasional light
eruptions interrupted the upbuilding process,
and as the repairs were not in all cases com-
pleted before a new interruption, a nest of cones, each smaller
than the last, arose in series like the outdrawn sections of an old-
time spyglass. At one time no less than five concentric craters
were to be seen.
For a brief period in the fall of 1904 Vesuvius had been in alinost
absolute repose, but soon thereafter the Strombolian crater ex-
:
OF piRe fa twee ay
= aga
—_
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 129
plosions were resumed. On May 25, 1905, a small stream of lava
began to issue from a fissure high up upon the central cone, and
from this time on the lava continued to flow down to the valley or
atrio, separating the inner cone from the caldera remnant of Monte
Somma. Seen in the night, this stream of lava appeared from
Fig. 130.— Night view of Vesuvius from Naples before
the outbreak of 1906. A small lava stream is seen
descending from a high point upon the central cone
(after Mercalli).
Naples like a red hot wire laid against the mountain’s side (Fig.
130). With gradual augmentation of Strombolian explosions
and increase in volume of the flowing lava stream, the same condi-
tion continued until the first days of April in 1906. The flowing
lava had then overrun the tracks of the mountain railway and
accumulated in considerable quantity within the aério (Fig. 131).
On the morning of April 4, a preliminary stage of the eruption
was inaugurated by the opening of a new radial fissure about 500
K
130 EARTH FEATURES AND THEIR MEANING
feet below the summit of the cone (Fig. 132 a), and by early after-
noon the cone-destroying stage began with the rise of a dark “ cauli-
flower cloud ” or pino to replace the lighter colored steam cloud.
The cone was beginning to fall into the crater, and old lava débris
was mingled in the ejections with the lava clots blown from the
still fluid material within the chimney. From now on short and
snappy lightning flashes played about the black cloud, giving out
a sharp staccato “ tack-a-tack.”” The volume and density of the
cloud and the intensity of the crater explosions continued to in-
crease until the culmination on April 7. On April 5 at midnight a
Fig. 131.—Scoriaceous lava encroaching upon the tracks of the Vesuvian railway
(after a photograph by Sommer).
new lava mouth appeared upon the same fissure which had opened
near the summit, but now some 300 feet lower (Fig. 182 6). The
lava now welled out in larger volume corresponding to its greater
head, and the stream which for ten months had been flowing from
the highest outlet upon the cone now ceased to flow. The next
morning, April 6, at about 8 o’clock, lava broke out at several
points some distance east of the opening b, and evidently upon
another fissure transverse to the first (Fig. 132 c). The lava sur-
face within the chimney must still have remained near its old
level, — effective draining had not yet begun, — since early upon
the following morning a small outflow began nearly at thé top of
the cone upon the opposite side and at least a thousand feet higher.
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 131
The culmination of the eruption came in the evening of April 7
when, to the accompaniment of light earthquakes felt as far as
Naples, lava issued for the first time in great volume from a mouth
more than halfway
down the mountain side
(Fig. 1382 f), and thus
began the drainage of
the chimney. At about
the same time with loud
detonations a huge
black cloud rose above
the crater in connection
with heavy explosions,
and a rain of cinder was
general in the region
about the mountain but
especially within the
northeast quadrant.
Those who were so for-
tunate as to be in Pom-
peii had a clear view of
the mountain’s summit
where red hot masses of
lava were thrown far
into the air. The direc-
tion of these projections
was reported to have
been not directly up-
ward, but inclined
toward the northeast
quadrant of the moun-
tain; but since with a
northeast surface wind
the heaviest deposit of
ash and dust should
BD ANS
oy ag
A YY
Mit
pes
Be),
and order of formation of the lava mouths upon
its flanks during the eruption of 1906 (after
Johnston-Lavis).
have been upon the southwestern quadrant of the mountain, it
is evident that the material was carried upward until it reached
the contrary upper currents of the atmosphere, to be by them dis-
tributed.
132 EARTH FEATURES AND THEIR MEANING
When the heavy curtain of ash, which now for a number of
succeeding days overhung all the circum-Vesuvian country, began
SERS TSS SS PS GS MY SEARS
Fig. 133. — The ash curtain which had overhung Vesuvius lifting and disclosing
the outlines of the mountain on April 10, 1911 (after De Lorenzo).
to lift (Fig. 133), it was seen that the summit of the cone had been
truncated an average of some 500 feet (Fig. 134). All the slopes
and much of the surrounding country had the aspect of being
buried beneath a cocoa-
colored snow of a depth
a to the northeastward of
e several feet, where it had
drifted into all the hollow
ways so as almost to
efface them (Fig. 135).
More than thrice as
heavy as water, the weak
roof timbers of the houses
Fic. 134.— The central cone of Vesuvius as it rl
appeared after the eruption of 1906, but with at the base of the moun- i
the earlier profile indicated. The truncation tain gave way beneath r
represents a lowering of the summit by some the added load upon
five hundred feet, with corresponding increase in :
the diameter of the crater (after Johnston- them, thus making many
Lavis). victims. Inasmuch, how-.
ever, as the ash-fall par--
takes of the same general characters as in eruptions from cinder
cones, we may here give our attention especially to the streams of
; Sys 4
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 133
oe ee Oe ee oe
lava which issued upon the
opposite flank of the moun-
tain (Fig. 136).
The main lava stream
descended the first steep
ES en ha 1 a ae ee
mile in twenty-five minutes,
about the strolling speed of
a pedestrian, but this rate
was gradually reduced as
the stream advanced far-
ther from the mouth. Tak-
‘Fig. 137.— The main lava stream of
1906 advancing upon the village of
Boscotrecase.
drifted cocoa-colored ash from the Vesu-
vian eruption of 1906.
slopes with the velocity of a Fie. 136.— View of Vesuvius taken from the
southwest during the waning stages of the
eruption of 1906. In the middle distance
may be discerned the several lava mouths
aligned upon a fissure, and the courses of
the streams which descend from them. In
the foreground is the main lava stream with
scoriaceous surface (after W. Prinz).
ing advantage of each depression of the surface, the black stream
advanced slowly but relentlessly toward the cities at the south-
west base of the mountain. With a motion not unlike that of a
heap of coal falling over itself down a slope, the block lava
Fig. 138.— An Italian pine snapped off
by the lava and carried forward upon
its surface as a passenger (after Haug).
134 EARTH FEATURES AND THEIR MEANING
advances without burning
the objects in its path
(Fig. 137). The beautiful
pines are merely charred
where snapped off and
are carried forward upon
the surface of the stream
(Fig. 188). When a real
obstruction, such as a
Rae - bridge or a villa, is en-
Fig. 139. — Lava front both aang over and eeedsn th t ,
running around a wall which lies athwart its countered, e stream 1S
course (after Johnston-Lavis). at first halted, but the
rear crowding upon the
van, unless a passage is found at the side, the lava front rises
higher and higher until by its weight the obstruction is forced to
give way (Figs. 139 and 140).
The sequence of events within the chimney. — The thorough
study of this Vesuvian eruption has placed us in a position to infer
with some confidence in our conclusions the sequence of events
within the chimney and |
crater of the volcano, both
before and during the erup-
tion. Anticipating some
conclusions derived from the
observed dissection of vol-
canoes, which will be dis-
cussed below, it may be
stated that what might be
termed the core of the com- ;
. : Fic. 140.— One of the villas in Boscotrecase
osit — ; : :
P ; e cone — the chimney which was ruined by the Vesuvian lava flow
—is a more or less cylin- of 1906. The fragments of masonry from
drical plug of cooled lava the ruined walls traveled upon the lava
ach during thé “achive current, where they sometimes became
. incased in lava.
period of the vent has an |
interior bore of probably variable caliber. This plug in its
lower section appears in solid black in all the diagrams of Fig.
141. During the cone-building period (Fig. 141 a and b) the plug.
is obviously built upward along with the cone, for lava often flows
out at a level a few hundred feet only below the crater rim. By
a
F -
a)
4 - -
~ - i
re Ot te om
sd , “| g oes
ee ad ee ee
ee ee
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 135
what process this chimney
building goes on is not well
understood, though some light
is thrown upon it by the post-
eruption stage of Mont Pelé in
1902-1903 (see below).
Both the older and newer
sections of this plug or chimney
are furnished some _ support
against the outward pressure
of the contained lava by the
surrounding wall of tuff; and
they are, therefore, in a condi-
tion not unlike that of the
inner barrel of a great gun over
which sleeves of metal have
been shrunk so as to give sup-
port against bursting pressures.
On the other hand, when not
sustaining the hydrostatic pres-
sure of the liquid lava within,
the chimney would tend to be
crushed in by the pressure
of the surrounding tuff. Its
strength to withstand bursting
pressures is dependent not
alone upon the thickness of its
rock walls, but also upon its
internal diameter or caliber.
A steam cylinder of given
thickness of wall, as is well
known, can resist bursting
pressures in proportion as its
internal diameter is small. So
in the volcanic chimney, any
tendency to remelt from within
the chimney walls must weaken
them in a twofold ratio.
We are yet without accurate
CS ee)
iW 11) sy
Fie. 141.— Three diagrams to illustrate
the sequence of events within the crater
of a composite cone during the cone-
building and crater-producing periods.
a and b, two successive stages of the
cone building or Strombolian period;
c, enlargement of the crater, truncation
of the cone, and destruction of the upper
chimney during the relatively brief
crater-producing or Vulcanian period.
136 EARTH FEATURES AND THEIR MEANING
temperature observations upon the lava in volcanic chimneys,
but it seems almost certain that these temperatures rise as the
Vulcanian stage is approaching, and such elevation of temperature
must be followed by a greater or less re-fusion of the chimney
walls. The sequence of events during the late Vesuvian eruption ©
Fic. 142.— The spine of Pelé rising above the chimney of the volcano after
the eruption of 1902 (after Hovey).
is, then, naturally explained by progressive re-fusion and conse-
quent weakening of the chimney walls, thus permitting a radial
fissure to’open near the top and gradually extend downwards. _
Thus at first small and high outlets were opened insufficient to — F
drain the chimney, but later, on April 7, after this fissure had : %
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 137
been much extended and a new and larger one had opened at a
lower level, the draining began and the surface of lava commenced
rapidly to sink.
When the rapid sinking of the lava surface occurred, the lower
lava layers were almost immediately relieved of pressure, thus
causing a sudden expansion of the contained steam and resulting
in grand crater explosions. The partially re-fused and fissured
upper chimney, now unable to withstand the inward pressure of
Seale of Feet.
Fic. 143.— Outlines of the Pelé spine upon successive dates. The full line repre-
sents its outline on December 26,1902; the dotted-dashed line is a profile of
January 3, 1902; while the dotted line is that of January 9, 1903. The dark
line is a fissure( after E. O. Hovey).
the surrounding tuff walls, since outward pressures no longer
existed, crushed in and contributed its materials and those of
the surrounding tuff to the fragments of fresh lava rising in
volume in the grand explosions (Fig. 141). In outline, then,
these seem to be the conditions which are indicated by the
sequence of observed events in connection with the late Vesuvian
outbreak.
The spine of Pelé. — The disastrous eruption of Mont Pelé
upon Martinique in the year 1902 is of importance in connection
with the interesting problem of the upward growth of volcanic
chimneys during the cone-building period of a volcano. After
the conclusion of this great Vulcanian eruption, a spine of lava
138 EARTH FEATURES AND THEIR MEANING
grew upward from the chimney of the main crater until it had
reached an elevation of more then a thousand feet above its base,
a figure of the same order of magnitude as the probable height of
the upper section of the Vesuvian chimney previous to the erup-
tion of 1906. The Pelé spine (Fig. 142) did not grow at a uniform
rate, but was subject to smaller or larger truncations, but for a
period of 18 days the upward growth was at the rate of about 41
feet per day. Later, the mass split upon a vertical plane revealing =
a concave inner surface, and was somewhat rapidly reduced in e }
altitude to 600 feet (Fig. 143), only to rise again to its full height
of about 1000 feet some three months later.
While apparently unique as an observed phenomenon, and not
free from uncertainty as to its interpretation, the growth of this
obelisk has at least shown us that a mass of rock can push its way
up above the chimney of an active volcano even when there are no
walls of tuff about it to sustain its outward pressures.
The aftermath of mud flows. — When the late Vulcanian ex-
plosions of Vesuvius had come to an end, all slopes of the moun-
| tain, but especially
the higher ones,
were buried in
thick deposits of |
the cocoa-colored |
ash, included in
which were larger
and smaller pro-
jectiles. As this
Fic. 144.—Corrugated surface of the Vesuvian cone material is ex- a i
after the mud flows which followed the eruption in 1906 ll
tremely porous, It
(after Johnston-Lavis). ‘4
greedily sucks up ©
the water which falls during the first succeeding rains. When ~
nearly saturated, it begins to descend the slopes of the mountain =
and soon develops a velocity quite in contrast with that of the
slow-moving lava. The upper slopes are thus denuded, while
the fields and even the houses about the base are invaded by these
torrents of mud (lava d’ acqua). Inasmuch as these mud flows are
the inevitable aftermath of all grander explosive eruptions, the
Italian government has of late spent large sums of money in the 4
Se ee ee
é ee ee ee ee
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 139
It was streams of this sort that buried the city of Herculaneum
after the explosive eruption of 79 a.D.
After the mud flows have occurred, the Vesuvian cone, like all
similar volcanic cones under the same conditions, is found with
deep radial corrugations (Fig. 144), such as were long ago de-
scribed as “ barrancoes ”’ and supposed to support the “ elevation
crater’ theory of volcano formation.
The dissection of volcanoes.— To the uninitiated it might ap-
pear a hopeless undertaking to attempt to learn by observation
the internal structure of a volcano, and especially of a complex
volcano of the composite type. The earliest successful attempt
appears to have been made by Count Caspar von Sternberg in
order to prove the cor-
rectness of the theory
of his friend, the poet
Goethe. Goethe had
claimed that a little
hill in the vicinity of
Eger, on the borders
of Bohemia, was an ex-
tinct volcano, though
the foremost geologist Sapte
of the time, the fa- Fig. 145.— The Kammerbithl near Eger, showing
the tunnel completed in 1837 which proved the
volcanic nature of the mountain (after Judd).
mous Werner, had pro-
mulgated the doctrine
that this hill, in common with others of similar aspect, originated
in the combustion of a bed of coal. The elevation in question,
{ 7 which is known-as the Kammerbihl, consists mainly of cinder,
and Goethe had maintained that if a tunnel were to be driven
horizontally into the mountain from one of its slopes, a core or
plug of lava would be encountered beneath the summit. The
excavations, which were completed in 1837, fully verified the
poet’s view, for a lava plug was found to occupy the center of
the mass and to connect with a small lava stream upon the side
of the hill (Fig. 145).
It is not, however, to such expensive projects that reference
is here made, but rather to processes which are continually going
on in nature, and on a far grander scale. The most important
dissecting agent for our purpose is running water, which is con-
140 EARTH FEATURES AND THEIR MEANING
tinually paring down the earth’s surface and disclosing its buried
structures. How much more convincing than any results of
sada Fea ae
teas eae
Fia. 146.— Volcanic plug exposed by natural dissection of a
_ volcanic cone in Colorado (U.S. G. S.).
artificial excavation, as evidence of the internal structure of a
volcano, is the monument represented in Fig. 146, since here the
lava plug stands in relief like a
gigantic thumb still surrounded by
a remnant of cinder deposits. Such
exposed chimneys of former volcanoes
are found in many regions, and have
become known as volcanic necks,
pipes, or plugs.
Not infrequently the beds of tuff
composing the flanks of the voleano,
upon dissection by the same process,
bring to light walls of cooled lava
standing in relief (Fig. 147) — the
\ . filling of the fissure which gave outlet
eds cutting bedsof to the flanks of the mountain at the —
of southwestern Colorado (after "Me Of the eruption. Study of ex:
Howe, U.S. G. S.). posed dikes formed in connection —
with recent eruptions of Vesuvius
has shown that in many instances they are still hollow, the lava
having drained from them before complete consolidation.
aT a
OO, eee ee ee,
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 141
Another agent which is effective in uncovering the buried struc-
tures of volcanoes is the action of waves on shores. Always a
relatively vigorous erosive agency, the softer structures of vol-
canic cones are removed with especial facility by this agent. On
the shores of the island of Volcano, the little
cone of Vulcanello has been nearly half
carried away by the waves, so as to reveal
with especial perfection the structure of the
cinder beds as well as the internal rock
skeleton of the mass. Here the character-
istic dips of lava streams, intercalated as
they now are between tuff deposits and the
lava which consolidated in fissures, are both
revealed.
In mid-Atlantic a quite perfect crater, the
St. Paul’s Rocks, has been cut nearly in
half so as to produce a natural harbor Fic. 148.— Map and gen-
(Fig. 148) eral view of St. Paul’s
; p Rocks, a volcanic cone
In still other instances we may thank the dissected by waves.
volcano itself for opening up the interior of
the mountain for our inspection. The eruption in 1888 of the
Japanese volcano of Bandai-san, by removing a considerable part
of the ancient cone, has afforded us a section completely through
-the mountain. The summit and one side of the small Bandai was
carried completely away, and there was substituted a yawning
crater eccentric to the former mountain and having its highest
wall no less than 1500
feet in height (Fig. 149).
St 7 In two hours from the
: <a. i , first warning of the ex-
YW yl Y/J IJ, ~ B plosion the catastrophe
was complete and the
2 heat eruption over.
_ Fig. 149. — Dissection by explosion of Little
Bandai-san in 1888 (after Sekiya). The eruption of Kra-
katoa in 1883, probably
| _ the grandest observed volcanic explosion in historic times, left
_ 4 voleanic cone divided almost in half and open to inspection
_ (ig. 150). Rakata, Danan, and Perbuatan had before con-
stituted a line of cones built up round individual craters sub-
142 EARTH FEATURES AND THEIR MEANING
sequent to the partial destruction of an earlier caldera, portions
of which were still existent in the islands Verlaten and Lang.
By the eruption of 1883 all the exposed parts and considerable
submerged portions of the two smaller cones were entirely de-
stroyed, and the larger one, known as Rakata, was divided just
outside the plug so as to leave a precipitous wall rising directly
from the sea and
showing lavastreams
‘ in alternation with |
somewhat thicker
tuff layers, the whole -
knit together by nu-
merous lava dikes.
in the Sunda Straits before and after the eruption of In order to eet.
1883 (after Verbeek). our dissecting pro-
cess down to levels
below the base of the volcanic mountain, it is usually necessary to
inspect the results of erosion by running water. Here the plug or .
chimney, instead of being surrounded by tuff, is inclosed by the
country rock of the region, which is commonly a sedimentary
formation. Such exposed lower sections of volcanic chimneys are
numerous along the northwestern shores of the British Isles.
Where aligned upon
a dislocation or note-
worthy fissure in the
rocks, the group of [fs 2
plugs has been re- | a
ferred toasascaror fas pe Me “ é %
cicatrice (Fig. 151). |8 Aeanaote Zone a
Associated with the [macy esemee HU li y
plugs of the cicatrice — Fyg, 151. —The cicatrice of the Banat (after Suess). _
are not infrequently 4
dikes, or, it may be, sheets of lava extended between layers of 4
sediment and known as sills. 4
If we are able to continue the dissection process to still greater —
‘depths, we encounter at last igneous rock having a texture known —
as granitic and indicating that the process of consolidation was _
not only exceedingly slow but also uninterrupted. This rock —
is found in masses of larger dimensions, and though generally of —
MA
A
a, OK WN
| ity
l
er ee
. ~*~ r , _ .
a es
- *
out, such reservoirs as exist
must be local and temporary, able cause of formation of lava reser-
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 143
more or less irregular form, no one dimension is of a different order
of magnitude from the others. Such masses are commonly de-
scribed as bosses, or, if especially large, as batholites (Fig. 152).
Wherever the rock beds appear as though they had been forced
up by the upward pressure of the igneous mass, the latter takes
the form of a mushroom and has been described as a laccolite
(Figs. 479-481, pp. 441-442). Evidence seems, however, to accumu-
late that in the greater number of cases the molten rock has fused
its way upward, in part assimilating and in part inclosing the rock
which it encountered. This pro-
cess of upward fusion has been
likened to the progress of a red
hot iron burning its way through
a board. Will ‘
The formation of lava reser- ses = Y= =
voirs. — The discarding of the <<. “i \\ -
earlier notion that the earth has = Ea NK
a liquid interior makes it proper SSS
in discussing the subject of vol- VEEL Pe:
canoes to at least touch upon
the origin of the molten rock
material. As already pointed
ie. 152. — Diane to thaseniat a ears
or it would be difficult to see voirs, and to show the connection
how the existing condition of between such reservoirs and the vol-
Re: ; canoes at the surface.
earth rigidity could be main- |
tained. From the rate at which rock temperatures rise, at,
increasing depths below the surface, it is clear that all rocks would
be melted at very moderate depths only, if they were not kept in a
solid state by the prodigious loads which they sustain. Any relief
from this load should at once result in fusion of the rock.
Now the restriction of active volcanoes to those zones of the
earth’s surface within which mountains are rising, and where
in consequence earthquakes are felt, has furnished us at least a
clew to the origin of the lava. Regarded as a structure capable
of sustaining a load, the competency of an arch is something quite
remarkable, so that the arching up of strong rock formations into
anticlines within the upper layers of the zone of flow, or of com-
144 EARTH FEATURES AND THEIR MEANING
bined fracture and flow, would be sufficient to remove the load
from relatively weak underlying beds, which in consequence would
be fused and form local reservoirs of lava (Figs. 152 and 153).
It has been further quite generally observed that lines of vol-
canoes, in so far as they betray any relation in position to neigh-
boring mountain ranges, tend to appear upon the rear or flatter
limb of unsymmetrical arches, or where local tension would favor
the opening of channels toward the surface. Moreover, wherever
recent block movements of surface portions of the earth’s shell
have been disclosed in the neighborhood of volcanoes, the latter
appear to be connected with downthrown blocks, as though the lava
had, so to speak, been squeezed out from
beneath the depressed block or blocks.
We must not, however, forget that the
igneous rocks are greatly restricted in the
range of their chemical composition. No
: sca igneous rock type is known which could
Fic. 153.— Result of experi- be formed by the fusion of any of the
ment with layers of com- Gorbonate rocks such as limestone or
position to illustrate the ; ihe ;
effect of relief of load upon Golomite, or of the more siliceous rocks,
rocks by arching of com- guch as sandstone or quartzite. There
ir formation (after yomains only the argillaceous class of
sediments, the shales and slates, and so _
soon as we examine the composition of these rocks we are struck by
the remarkable resemblance to that of the class of igneous rocks.
For purposes of comparison there is given below the composite or
average constitution of igneous rocks in parallel column, with the
average attained by combining the analyses of 56 slates and shales,
the latter recalculated with water excluded:
AVERAGE IGNEous Rock
AVERAGE SHALE
(Clark) (Washington)
SiOz GE25 61.69 63.34
oon ae 15.94 16.56
e203 ; 1.88 4.41
FeO 31 }6-31| gee}453| 3445} 7.8
MgO 4.47 4.90 3.54
CaO 5.038 5.02 3.00
NaeO 3.64 4.09 1.29
KO 287 3.35 3.5273
TiOe .62 48 53
100.00 100.00 100.00
bh TR My ait 8 LOD: >
kz
—
. - Yale
On nto Es
GPT oT REF fet) Tore CTS
i Ae gE ht = 4
a
= AE mt
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 145
This close resemblance is probably of deep significance, for the
reason that shales and slates are structurally the weakest of all
rocks and for the further reason that they rather generally di-
rectly underlie the carbonate rocks, which are by contrast the
strongest (see ante, p. 37). For these reasons shales and slates are
the only rocks which are likely to be fused by relief from load
through the formation of anticlinal arches within the earth’s zone
of flow. If this view is well founded, lavas and other igneous
rocks are in large part fused argillaceous sediments formed in con-
nection with the process of folding, or are refused rocks of igneous
origin and similar composition.
Character profiles. — The character profiles of features con-
nected in their origin with volcanoes are particularly easy to
recognize, and in a few cases in which they might be confused with
others of a different origin, an examination of the materials of
the features should lead to a definitive judgment.
The lava plains which result from massive outflows of basalt
might perhaps strictly be regarded as lack of feature, so great may
be their continuous extent. Wherever definite vents exist, a
broad flat dome is the usual result of the extravasation of a basal-
tic lava. The puys of France and many of the Kuppen of Ger-
many, being formed from less fluid lava, have afforded profiles
with relatively small radius of curvature.
In its youthful stage, the cinder cone usually presents a broad
summit sag and relatively short side slopes, whereas the cone of
later stages is apt to present long sweeping and upwardly concave
curves with both the gradient and the radius of curvature increas-
ing rapidly toward the summit. In contrast, too, with the earlier
stage, the crest is relatively small. A marked reduction in the
high symmetry of such profiles is noted wherever a breaching by
lava outflow has occurred (Fig. 154).
With the composite cone, complexity and corresponding lack
of symmetry is introduced, especially in the partially ruined
caldera, and by the more or less accidental distribution of parasitic
cones, as well as by migrations of the central cone. Peculiarly
similar acuminated profiles result from spatter-cone formation,
from the formation of a superchimney spine, and by the uncover-
ing of the chimney through denudational processes — the volcanic
neck.
L
146 EARTH FEATURES AND THEIR MEANING
Another important feature resulting from denudation is /the
Mesa or table mountain with its protecting basalt cap above softer
rocks. Its profile most resembles that of table mountains due to
differential erosion of alternately strong and weak horizontally
je Plair LS, Gree Gare
Basalr Plat NS Srage/ .
Core
(Ade. Stage)
Sreache. Cones
Oe ete ge
Cornposire Core fF Same a
Neck
Fia. 154.— Character profiles connected with volcanoes.
bedded rocks, such as compose the upper portion of the section in
the Grand Cafion of the Colorado. Here, however, in place of a
single unusually strong top layer there are found several strong
layers in alternation with weaker ones so as to produce additional
steps in the profile.
READING REFERENCES TO CHAPTERS IX AND X
General works : —
Pavutett Scropr. The Geology of the Extinct Volcanoes of Central
France. John Murray, London, 1858, pp. 258. (An epoch-making
work of early date which, like the following reference, may be studied
to advantage to-day.)
Sir Cuarues Lyewty. Principles of Geology, vol. 1, Chapters xxiii-xxv.
Metcuior Neumayr. Erdgeschichte, vol. 1, Allgemeine Geologie, revised
edition by v. Uhlig, 1897, pp. 133-277 (a storehouse of valuable infor-
mation clearly presented).
J. D. Dana. Characteristics of Voleanoes, with Contributions of Facts
and Principles from the Hawaiian Islands. Dodd, Mead, and Com-
pany, New York, 1890, pp. 397. 7
Tempest ANDERSON. Volcanic Studies in Many Lands, being reproduc-
tions of photographs by the author with explanatory notes. John
Murray, London, 1903, pp. 200, pls. 105.
T. G. Bonney. Voleanoes, their Structure and Significance. John -
Murray, London, 1899, pp. 331.
ee 2 oe ee oe 1 eee eee ree ee we en + re Ow Oe EP eee
me 4 x‘ x o ;
RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE 147
I. C. Russetyu. Volcanoes of North America. Macmillan, New York,
1897, pp. 346.
Euis£e Recuus. Les voleans de la terre, Belgian Society of Astronomy;
Meteorology, and Physics of the Globe, 1906-1910 (a valuable de-
secriptive geographical and bibliographical work of reference).
G. Mercatui. I vuleani attivi della terre. Hoepli, Milan, 1907, pp. 421.
(A most valuable work, beautifully illustrated, but in the Italian
language. )
Arrangement of voleanic vents : —
Ta. THoroppsEeN. Die Bruchlinien und ihre Beziehungen zu den Vul-
kanen, Pet. Mitt., vol. 51, 1905, pp. 1-5, pl. 5.
R. D. M. Verseex. Various volumes and atlases of maps covering the
Dutch East Indies and fully cited in the following reference (p. 21).
WitiramM H. Hosss. The Evolution and the Outlook of Seismic Geology,
Proc. Am. Phil. Soc., vol. 48, 1909, pp. 17-27.
Birth of voleanoes : —
F. Omorr. The Usu-san Eruption and Earthquake and Elevation Phe-
nomena, Bull. Earthq. Inv. Com., Japan, vol. 5, No. 1, 1911, pp. 1-37,
pls. 1-13.
Fissure eruptions : —
Tu. THoroppsen. Island, IV, Vulkane, Pet. Mitt., Erganzungsh. 153, -
1906, pp. 108-111.
A. Gerxiz. Text-book of Geology, 4th ed., pp. 342-346.
Lava domes of Hawaii : —
J. D. Dana. Characteristics of Volcanoes (as above).
C. H. Hircucocx. Hawaii and Its Voleanoes. Honolulu, 1909, pp. 314.
Eruption of Matavanu voleano in 1906 : —
Karut Sapper. Der Matavanu-Ausbruch auf Savaii, 1905-1906, Zeit.
d. Gesell. f. Erdk. z. Berlin, vol. 19, 1906, pp. 686-709, 4 pls.
H. J. Jensen. The Geology of Samoa, and the Eruptions in Savaii, Proc.
Linn. Soe., New South Wales, vol. 31, 1906, pp. 641-672, pls. 54-64.
Tempest ANDERSON. The Volcano of Matavanu in Savaii, Quart. Jour.
Geol. Soc., London, vol. 66, 1910, pp. 621-639, pls. 45-52.
Eruption of Voleano in 1888:— __.
H. J. Jounston-Lavis. The South Italian Voleanoes. Naples, 1891,
pp. 342, pls. 16.
Eruption of Taal voleano in 1911: —
W. E. Pratt. The Eruption of Taal Voleano, January 30, 1911, Phil.
Jour. Sci., vol. 6, No. 2, Sec. A, 1911, pp. 63-86, pls. 1-14.
F. H. Nosue. Taal Voleano, album of views of 1911 eruption, Manila,
1911, pp. 1-48.
The volcano of Etna: —
G. vom Ratu. Der Aetna. Bonn, 1872, pp. 1-33. (A beautiful piece of
descriptive writing from both the geological and scenic standpoints.)
148 EARTH FEATURES AND THEIR MEANING
SARTORIUS VON WALTERSHAUSEN. Der Aetna. Leipzig, 1880, 2 quarto
vols., pp. 371 and 548.
The eruption of Vesuvius in 1906: —
H. J. Jounston-Lavis. Geological Map of Monte Somma and Vesuvius,
with a short and concise account, ete. Geo. Philip & Son, London,
1891.
H. J. Jounsron-Lavis. The Eruption of Vesuvius in April, 1906, Trans.
Roy. Dublin Soe., vol. 9, 1909, Pt. VIII, pp. 189-200, pls. 3-23 (the
most authoritative work upon the subject).
T. A. Jaccar, Jr. The Volcano Vesuvius in 1906, Tech. Quart., vol. 19,
1906, pp. 105-115.
W. Prinz. L’éruption du Vesuv d’avril, 1906, Ciel et Terre, 27e Année,
1906, pp. 1-49.
Frank A. Perret. Notes on the Electrical Phenomena of the Vesuvian.
Eruption, April, 1906, Sci. Bull., Brooklyn Inst. Arts and Sci., vol. 1,
No. 11, pp. 307-312; Vesuvius, Characteristics and Phenomena of
the Present Repose Period, Am. Jour. Sci., vol. 28, 1909, pp. 413-430.
Wituiam H. Hosss. The Grand Eruption of Vesuvius in 1906, Jour.
Geol., vol. 14, 1906, pp. 636-655.
The spine of Pelée: — :
E. O. Hovey. The New Cone of Mont Pelé and the Gorge of the Riviére
Blanche, Martinique, Am. Jour. Sci., vol. 16, 1903, pp. 269-281, pls.
11-14. ;
A. Heruprin. The Tower of Pelée. Philadelphia, 1904, pp. 62, pls. 22.
A. Lacroix. La montagne Pelée et ses éruptions, Acad. des Sciences,
Paris, 1904, Chapter iii.
Karu Sapper. In den Vulkangebieten Mittelamerikas und Westindiens,
Stuttgart, 1905, pp. 172-178. .
A.C. Lang. Absorbed Gases of Vulcanism, Science, N.S., vol. 18, 1903,
p. 760.
G. K. Ginpert. The Mechanism of the Mont Pelée Spine, zbid., vol. 19,
1904, pp. 927-928.
I. C. Russeiyt. Pelé Obelisk once More, ibid., vol. 21, 1905, pp. 924-931.
The dissection of voleanoes : —
J. W. Jupp. Volcanoes, Chapter v.
S. Sexya and Y. Krxucur. The Eruption of Bandai-San, Trans. Seis.
Soc., Japan, vol. 13, Pt. 2, 1890, pp. 140-222, pls. 1-9.
R. D. M. Verserx. Krakatau. Batavia, 1885, pp. 557, pls. 25.
Royat Socrery, The Eruption of Krakatoa and Subsequent Phenomena.
London, 1888, pp. 494.
G. K. Gitpert. Report on the Geology of the Henry Mountains, U.S.
aes and Geol. Surv., Rocky Mt. Region, Washington, 1877, pp.
2-60. .
Sir A. Gerxizr. Ancient Volcanoes of Great Britain, vol. 2 especially.
D. W. Jounson. Volcanic Necks of the Mount Taylor Region, New
Mexico, Bull. Geol. Soe. Am., vol. 18, 1907, pp. 303-324, pls. 25-30.
a
Pique See et .
eal
SBM OF
CHAPTER XI
THE ATTACK OF THE WEATHER
The two contrasted processes of weathering. — It has already
been pointed out that change and not stability is the order of
nature. Within the earth’s outer shell and upon it rock altera-
tion goes on continually, and from some portions of its surface the
changed material is as constantly migrating to neighboring or
even far distant regions. Before such transportation can begin
the hard rock must first be broken down and reduced to fragments
which the transporting agencies are competent to move.
To accomplish this breaking down, or degeneration, of the rock
masses, either a wide range in temperature or chemical reaction is
essential. In the atmosphere are found such active chemical
agents as oxygen and carbon dioxide, the so-called carbonic acid
gas; and these agents in the presence of water react chemically
with the minerals of the rocks and form other minerals such as the
hydrates and carbonates, which are lighter in weight and more
soluble. This chemical attack upon the outer shell of the litho-
sphere is described as decomposition.
On the other hand the rock may succumb to changes which are
purely mechanical and are due either to the stresses set up by dif-
ferences between surface and interior temperatures, or to the prying
action of the frost in the crevices. Such purely mechanical de-
generation of the rocks is in contrast with decomposition and is
described as disintegration. The two processes of decomposition
and disintegration may, however, go on together; and the changes
of volume that are caused by decomposition may result directly
in considerable disintegration, as we are to see.
The réle of the percolating water. — In order to effect chemical
change or reaction, it is essential that the substances which are
to react must be brought into such intimate contact with each
other as it is seldom possible to attain except by solution. The
chemical reactions which go on between the gaseous atmosphere
and the solid lithosphere are accomplished through solution of the
149 ;
150 EARTH FEATURES AND THEIR MEANING
gases in water. This water, derived from rain or snow, percolates
into the ground or descends along the crevices in the rocks, carry-
ing with it a certain measure of dissolved air. This air differs
from that of the surrounding atmospheric envelope by containing
Fie. 155.— Successive dia-
grams to show the effect of
decomposition and resulting
disintegration upon joint
blocks so as to produce
spheroidal bowlders' by
weathering.
relatively large amounts of oxygen and
of the other active element carbon diox-
ide. It follows from the important réle
thus performed by the percolating water
that the process of decomposition will
be relatively important in humid re-
gions where the atmospheric precipita-
tion is sufficient for the purpose.
Within hot and dry regions there is
a larger measure of rock disintegration,
and distinct chemical changes unlike
those of humid regions take place in the
higher temperatures and with the more
concentrated saline solutions. The dis-
cussion of such changes will be deferred
until desert conditions are treated in
another chapter.
Mechanical results of decomposition
— spheroidal weathering. — From an
earlier chapter it has been learned that
the rocks of the earth’s outermost shell
are generally intersected by a system of
vertical fissures which at each locality
tend to divide the rock into parallel and
upright rectangular prisms. It is these
joints which offer relatively easy paths
for the descent of the water into the
rocks. In rocks of sedimentary origin
there are found, in addition to the vertical joints, planes of bed-
ding originally horizontal, and in the intrusive and volcanic rocks
a somewhat similar parting, likewise parallel to the surface of the
ground. The combined effect of the joints and the additional
parting planes is thus to separate the rock mass into more or
less perfect squared blocks (Fig. 155, upper figure) which stand
in vertical columns.
ee
5
Pred
‘ a
- —_ | lee. 4
- "is > , -
- 7 -_s | a
—— So are
nat ee “
Se eS
im ep a wey =. 6-3 Lee, *
ieee Penton SS it CIR, 5
vo
8
4
i
-
- ae atl i at —— eS es
aaa Ree pee fe Sipps gee i a ae ees
5 .
THE ATTACK OF THE WEATHER 151
The water which percolates downward upon the joints, finds
its way laterally along the parting planes, and so subjects the en-
tire surface of each block to simultaneous attack by its reagents.
Though all parts of the surface of each block are alike subject to
attack, it is the angles and the edges which are most vigorously
acted upon. In the narrow crevices the solutions move but slug-
gishly, and as they are soon impoverished of their reagents in the
attack upon the rock, fresh solution can reach the middle of the
faces from relatively few directions. The edges are at the same
time being reached from many more directions, and the corners
from a still larger number.
The minerals newly formed by these chemical processes of
hydration and carbonization are notably lighter, and hence more
bulky than the minerals from whose constituents they have been
largely formed. Strains are thus set up which tend to separate
the bulkier new material from the core of unaltered rock below.
As the process continues, distinct channels for the moving waters
~ are developed favorable to action at the edges and corners of the
blocks. Eventually, the squared block is by this process trans-
formed into a spheroidal core of still unaltered rock wrapped in
layers of decomposed material, like the outer wrappings of an onion.
These in turn are usually imbedded in more thoroughly disinte-
grated material from which
the shell structure has dis-
appeared (Fig. 156).
Exfoliation or scaling. — A
fact of much importance to
geologists, but one far too sS@h. 7 fo JRO Pe ge
often overlooked, is that rocks “4.-. > aS ri |i
are but poor conductors for + WR. | P
heat. It results from this ase ;
that in the bright sun of a Fia. 156. — Spheroidal weathering of an
; ; : igneous rock.
summer’s day a thin skin, as
it were, upon the rock surface may be heated to a relatively high
temperature, although the layer immediately below it is prac-
tically unaffected. The consequent expansion of the surface layer
causes stresses that tend to scale it off from the layer below,
which, uncovered in its turn, develops new strains of the same
sort. This process of exfoliation acquires exceptional importance
152 EARTH FEATURES AND THEIR MEANING
in desert regions where the rock surfaces are daily elevated to
excessively high temperatures (see Chapter XV).
Dome structure in granite masses. — In large granite masses,
such as are to be found in the ranges of the Sierra Nevada of Cali-
fornia, a peculiar dome structure is sometimes found developed:
upon a large scale, and has had an important influence upon the
breaking down of the rock and
upon the shaping of the mountain
(Fig. 157). Suchastructure, made
up as it is of prodigious layers,
can have little in common with
the veneers of weathered miner-
als which are the result of exfoli-
Ber ae ation, and it is quite likely that
BTS WAS ate the dome structure is in some
FE eee seas fi aaa way connected with the relief of
goeeaa oe these massive rocks from their
ere rn mane load —the rock which once rested
upon them, but has been carried away by erosion since the uplift
of the range. }
The prying work of frost. — In all countries where winter tem-
peratures range below the freezing point of water, a most potent
agent of rock disintegration is the frost which pries at every crevice
and cranny of the surface rock. Important in the temperate zones,
in the polar regions it becomes almost the sole effective agent of
rock weathering. There, as elsewhere, its efficiency as a disinte-
grating agent is directly dependent upon the nature of the crevices
within the rock, so that the omnipresent joints are able to exer-
cise a degree of control over the sculpturing of the surface features
which is hardly to be looked for elsewhere (see plate 10 A).
Talus. — Wherever the earth’s surface rises in steep cliffs, the
rock fragments derived from frost action, or by other processes of
disintegration, as they become detached either fall or slide rapidly
downward until arrested upon a flatter slope. Upon the earlier
accumulations of this kind, the later ones are deposited, until their
surface slopes up to the cliff face as steeply as the material will lie
— the angle of repose. Such débris accumulations at the base of
a cliff (Fig. 158) are known as talus, and the slope is described as
a talus slope, or in Scotland as a “ scree.”
. - —— -
i aa oka ak ae ot
tag SaaS E Thien ports oe b=
tiles _ =
ee ac
jt xt in tite ee 2.
HBP esd uence seal
1 a
y;
THE ATTACK OF THE WEATHER 153
Soil flow in the continued presence of thaw water. — So soon
as the rocks are broken down by the weathering processes, they are
easily moved, usually to lower levels. In part this transportation
may be accomplished by gravity slowly acting upon the disinte-
grated rock and causing
it to creep down the slope.
Yet even in such cases
water is usually present
in quantity sufficient to fill
the spaces between the
grains, and so act as a
lubricant to facilitate the eet
“i Pass
avs NS
Hi
P Hf ica
rela fi i He
y i ma ih
iii i Wy?
migration. et : ae:
Upon a large scale rocks ee cee po See te
. . . . ois y pee
which were either origi- ="). ite om ten naga
nally incoherent or have
been made so by weather- i A
ae ae Beene ae a
ing, after they have be-
come saturated with
water, may start into sudden motion as great landslides or ava-
lanches, which in the space of a few moments materially change the
face of the country, and by burying the bottom lands leave dis-
aster and misery in their wake.
Within the subpolar regions, where a large part of the surface
is for much of the year covered with snow, the underlying rocks
are for long periods saturated with thaw water, and in alternation
are repeatedly frozen and thawed. Essentially similar conditions
are met with in the high, snow-capped mountains of temperate or
torrid regions. .For the subpolar regions particularly it is now
generally recognized that somewhat special processes of soil flow,
described under the name solifluction, are characteristic. The
exact nature of these processes is as yet imperfectly understood, but
there can be little doubt concerning the large réle which they have
played in the transportation of surface materials. Such soil flow
is clearly manifested under different aspects, and it is likely that
by this comprehensive term distinct processes have been brought
together.
Possibly the most striking aspect of the soil al in subpolar
regions is furnished by the remarkable “ stone rivers ” and “ rock
Fia. 158.— Talus ee baneath a cliff.
154
glaciers’; though the more generally
EARTH FEATURES AND THEIR MEANING
characteristic are peculiar
stripings or other markings which appear upon the surface of the
not
Fig. 159.—Striped ground feetin soil flow
of chipped rock fragments upon a slope,
Snow Hill Island, West Antarctica (after
Otto Nordenskidld).
159). The direction of the furrows is
slope, and the striping is marked in pro-
portion as the slope is steep. Where the
bottom is reached, the furrows are re-
placed by a sort of mosaic pavement
of hexagonal repeating figures, each of
which may be an area of the surface six
feet or more across (Fig. 160, and Fig.
390, p. 368). The depressions which
separate the ‘ blocks ”’
are often filled with clay, while the in-
closed surfaces are made up of coarsely
chipped stone.
of the pavement’
ground and thus betray the
movements of the underlying
materials.
Upon slopes it is
uncommon for the surface
to be composed of angular rock
fragments riven by the frost
and crossed by broad parallel
furrows as though a gigantic
plow had gone over it (Fig.
always up and down the
Fig. 160.— Pavement of hori-
zontal surface due to soil
flow, Spitzbergen (after Otto
Nordenskiéld).
The splitting wedges of roots and trees. — In the mechanical
TARY ea ee ea ty = a ae a
Z
eee ”
DDL
= 161. — ene roots entering finieca rock and
prying its sections apart.
breakdown of the rocks
within humid regions a
not unimportant part is
sometimes taken by the
trees, which insinuate the
tenuous extremities of their
rootlets into the smallest
cracks, and by continued
growth slowly wedge even
the firmer rocks apart (Fig.
161). In a similar manner
the small tree trunk grow-
ing within a crevice of the
rock may in time split its parts asunder (Fig. 162).
THE ATTACK OF THE WEATHER 155
The rock mantle and its shield in the mat of vegetation. —
Through the action of weathering, the rocks, as we have seen,
lose their integrity within a surface layer, which, though it may be
as much as a hundred fect or more
in thickness, must still be accounted
a mere film above the underlying bed
rock. The mechanical agents of the
breakdown operate only within a few
feet of the surface, and the agents of
rock decomposition, derived as they
are from the atmosphere, become
inert before they have descended to
any considerable depth. The Surface
layer of incoherent rock is usually
referred to as the rock mantle (Fig.
4 wey,
163). Where the rock mantle is rel- yg, 162, — A large glacial bowlder
atively deep, as it is in the states split by a growing tree near East
Lansing, Michigan (after a pho-
south of th hio in the eastern
e Ohio in tograph by Bertha Thompson).
United States, there is found, deep
below the outer layer of soil, a partially decomposed and disin-
tegrated rock, of which the unaltered minerals lie unchanged in
position but separated by the new minerals which have resulted
from the breakdown of their more
susceptible associates. Whilethus
in a certain sense possessing the
original structure, this altered ma-
terial is essentially incoherent and
easily succumbs to attack by the
pick and spade, so that it is only
at considerably greater depths
that the unaltered rock is en-
countered.
broken rock, above which is soil and Because of the tendency of
eymtatiemat, Const oCalforis mantle rock to creep down upon
slopes it is generally found thicker
upon the crests and at the bases of hills and thinnest upon their
slopes (Fig. 164).
In the transformation of the upper portion of the mantle rock
into soil, additional chemical processes to those of weathering
156 EARTH FEATURES AND THEIR MEANING
are carried through by the agency of earthworms, bacteria, and
other organisms, and by the action of humus and other acids de- _
rived from the decomposition of vegetation. The bacteria par-
ticularly play a part in the formation of carbonates, as they do |
also in changing ~
the nitrogen of
the air into ni-
trates which be-
come available
Fig. 164.— Diagram to show the varying thickness of aS plant food. |
mantle rock upon the different portions of a hill surface Within the
(after Chamberlin and Salisbury).
humid _ tropical
regions ants and other insects enter as a large factor in rock
decomposition, as they do also in producing not unimportant
surface irregularities.
How important is the cover of vegetation in retaining the rock
mantle and the upper soil layer in their respective positions, as
required for agricultural purposes, may be best illustrated by the
disastrous consequences of allowing it to be destroyed... Wherever,
by the destruction of forests, by the excessive grazing of animals,
or by other causes, the mat of turf has been destroyed, the sur-
face is opened in gullies by the first hard rain, and the fertile layer
of soil is carried from the slopes and distributed with the coarser
mantle upon the bottom lands. Thus the face of the country is
completely transformed from fertile hills into the most desolate
of deserts where no spear of grass is to be seen and no animal food
to be obtained (plate 5 A). The soil once washed away isnotagain
renewed, for the continuation of the gullying process now effec- __
tively prevents its accumulation. Anima
READING REFERENCES TO CHAPTER XI
Decomposition and disintegration : —
Grorce P. Merritt. The Principles of Rock Weathering, Jour. Geol., a
vol. 4, 1896, pp. 704-724, 850-871. Rocks, Rock Weathering, and
Soils. Macmillan, New York, 1897, Pt. iii, pp. 172-411. |
Auexis A. Junien. On the Geological Action of the Humus Acids, Proc. _
Am. Assoc. Adv. Sci., vol. 28, 1879, pp. 311-410. a
Corrosion of rocks : — . “
Cc. W. Hayes. Solution of Silica under Atmospheric Conditions, Bull.
Geol. Soc. Am., vol. 8, 1897, pp. 213-220, pls. 17-19. a
|
|
|
PLATE 5
A. Once wooded region in China now reduced to desert through deforestation
(after Willis).
B. ‘‘ Bad Lands”’ in the Colorado Desert (after Mendenhall).
THE ATTACK OF THE WEATHER 157
M. L. Fuuuer. Etching of Quartz in the Interior of Conglomerates,
Jour. Geol., vol. 10, 1902, pp. 815-821.
C. H. Smytu, Jr. Replacement of Quartz by Pyrites and Corrosion of
Quartz Pebbles, Am. Jour. Sci. (4), vol. 19, 1905, pp. 282-285.
Dome structure of granite masses : —
G. K. Gitspert. Domes and Dome Structure of the High Sierra, Bull.
Geol. Soc. Am., vol. 15, 1904, pp. 29-36, pls. 1-4.
RautpH ArRNouLD. Dome Structure in Conglomerate, zbid., vol. 18, 1907,
pp. 615-616.
Soil flow : —
J. GuNNAR ANDERSSON. Solifluction, a Component of Subaérial Denuda-
tion, Jour. Geol., vol. 14, 1906, pp. 91-112.
Orro NorDENSKIOLD. Die Polarwelt und ihre Nachbarlander, Leipzig,
~ 1909, pp. 60-65.
Ernest Howr. Landslides in the San Juan Mountains, Colorado, ete.,
Prof. Pap., 67 U. S. Geol. Surv., 1909, pp. 1-58, pls. 1-20.
G. E. Mircueiui. Landslides and Rock Avalanches, Nat. Geogr. Mag.,
vol. 21, 1910, pp. 277-287.
~ Wiuuram H. Hosss. Soil Stripes in Cold Humid Regions and a Kindred
Phenomenon, 12th Rept. Mich. Acad. Sci., 1910, pp. 51-53, pls. 1-2.
Relation of deforestation to erosion : —
N.S. SHater. Origin and Nature of Soils, 12th Ann. Rept. U.S. Geol.
Surv., 1891, Pt. 1, pp. 268-287.
4 ' WJ McGer. The Lafayette Formation, ibid., pp. 430-448.
_ _F.H. Kine. Soils. Macmillan, New York, 1908, pp. 50-54.
_ Batwey Wiis. Water Circulation and Its Control, Rept. Nat. Conserv.
Com., 1909, vol. 2, pp. 687-710.
WJ McGesz. Soil erosion, Bull. 71, U. S. Bureau of Soils, 1911. pp, 60,
pls. 33.
CHAPTER XII
THE LIFE HISTORIES OF RIVERS
The intricate pattern of river etchings. — The attack of the
weather upon the solid lithosphere destroys the integrity of its
surface layer, and through reducing it to rock débris makes it the
natural prey of any agent competent to carry it along the surface.
We have seen how, for short distances, gravity unaided may pile
up the débris in accumulations of talus, and how, when assisted by
thaw water which has soaked into the material, it may accomplish
a slow migration by a peculiar type of soil flow. Yet far more
potent transporting agencies are at work, and of these the one of
first importance is running water. Only in the hearts of great
deserts or in the equally remote white deserts of the polar regions
is the sound of its murmurings never heard. Every other part of
the earth’s surface has at some time its running water coursing
in valleys which it has itself etched into the surface. It is this
etching out of the continents in an intricate pattern of anastomos-
ing valleys which constitutes the chief difference between the land
surface and the relatively even floor of the oceans.
The motive power of rivers. — Every river is born in throes
of Mother Earth by which the land is uplifted and left at a higher
level than it was before. It is the difference of elevation thus
brought about between separated portions of the land areas that
makes it possible for the water which falls upon the higher portions
to descend by gravity to the lower. This natural “ head ” due to
differences of elevation is the motive power of the local streams,
and for each increase in elevation there is an immediate response
in renewed vigor of the streams. The elevated area off which the
rivers flow is here termed an upland.
The velocity of a stream will be dependent not only upon the
difference in altitude between its source and its mouth, but upon
the distance which separates them, since this will determine the —
grade. The level of the mouth being the lowest which the stream
158
re — ——==—
THE LIFE HISTORIES OF RIVERS 159
can reach is termed the base level, and the current is fixed by the
slope or declivity. The capacity to lift and transport rock débris
is augmented at a quite surprising rate with every increase in
current velocity, the law being that the weight of the heaviest
transportable fragment varies with the sixth power of the velocity
of the current. Thus if one stream flows twice as rapidly as
another, it can transport fragments which are sixty-four times as
heavy.
Old land and new land. — The uplifts of the continents may
proceed without changes in the position of the shore lines, in
which case areas, already carved by streams but no longer actively
modified by them, are worked upon by tools freshly sharpened
and driven by greater power. The land thus subjected to active
stream cutting is described as old land, and has already had
engraved upon it the characteristic pattern of river etchings,
albeit the design has been in part effaced.
If, upon the other hand, the shore line migrates seaward with
the uplift, a portion of the relatively even sea floor, or new land,
is elevated and laid under the action of the running water.
As we are to see, stream cutting is to some extent modified when
a river pattern is inherited from the uplift. The uplift, whether
of old land only or of both old land and new land, marks the
starting point of a new river history, usually described as an
erosion cycle.
The earlier aspects of rivers. — Though geologists have some-
times regarded the uplift of the continents as a sort of upwarping
in a continuous curved surface, the discussions of river histories
and the pictorial illustrations of them have alike clearly assumed
that the uplift has been essentially in blocks and that the ele-
vated area meets the lower lying country or the sea in a more or
less definite escarpment. The first rivers to develop after the
uplift may be described as gullies shaped by the sudden down-
rush of storm waters and spaced more or less regularly along the
margin of the escarpment (Fig. 165). These gullies are relatively
short, straight, and steep; they have precipitous walls and few,
if any, tributaries.
With time the gully heads advance into the upland as they
take on tributaries; and so at length they in part invest it and
dissect it into numerous irregularly bounded and _flat-topped
160 EARTH FEATURES AND THEIR MEANING
tables which are separated by cafions (Fig. 166). At the same
time the grade of the channel is becoming flatter, and its precipi-
tous walls are being replaced by curving slopes, as will be more
Fig. 165. — Two successive forms of gullies from the earliest stage of a
river’s life (after Salisbury and Atwood).
fully described in the sequel. It is because of this progressive
reduction of grades with increasing age that the early stages of
a river’s life are much the most turbulent of its history. The
Fic. 166.— Partially dissected upland (after Salisbury and
; Atwood). ei
water then rushes down the steep grades in rapids, and is often —
at times opened out in some basin to form a lake where differ-
ences of uplift have been characteristic of neighboring sections.
i
7
a
va
j
:
r
a= et
SII
THE LIFE HISTORIES OF RIVERS 161
For several reasons such basins in the course of a stream are rela-
tively short lived (Chapter XXX), and they disappear with the
earlier stages of the river history. |
The meshes of the river network. — From the continued throw-
ing out of new tributaries by the streams, the meshes in the
river network draw more closely together as the stages of its his-
tory advance. The closeness of texture which is at last developed
upon the upland is in part determined by the quantity of rainfall,
so that in New Jersey with heavy annual precipitation the meshes
in the network are much smaller than they are, for example,
upon the semiarid or arid plains of the western United States.
Its design will, however, in either case more or less clearly express
the plan of rock architecture which is hidden beneath the surface
(Chapter XVII).
The upper and lower reaches of a river contrasted. — From
the fact that the river progressively invades new portions of the
upland and lays the acquired sections under more and more
thorough investment, it has near its headwaters for a long time
a frontier district which may be regarded as youthful even though
the sections near its mouth have reached a somewhat advanced
stage. The newly acquired sections of river valley may thus .
possess the steep grade and precipitous walls which are charac-
teristic of early gullies and cafions and are in contrast with
the more rounded and flat-bottomed sections below. Lateral
streams, from the fact that they are newer than the main or trunk
= aa
Fie. 167.— Characteristic longitudinal sections of the upper portion of a river
valley and its tributaries (after scaled sections by Nussbaum).
stream to which they are tributary, likewise descend upon somewhat.
steeper grades (Fig. 167).
The balance between degradation and aggradation. — We have
seen that the power to transport rock fragments is augmented at
a most surprising rate with every increase in the current velocity.
While the lighter particles of rock may be carried as high up as
the surface of the water, the heavier ones are moved forward
5 upon the bottom with a combined rolling and hopping motion
aided by local eddies. Those particles which come in contact
M
162 EARTH FEATURES AND THEIR MEANING FE:
with the bottom or sides of the channel abrade its surface so as
ever to deepen and widen the valley. This cutting accomplished
by partially suspended débris in rapidly moving currents of water is a:
known as corrasion and the stream is said to be incising its valley. bd
As the current is checked upon the lower and flatter grades, - j
some of its load of sediment, and especially the coarser portion,
will be deposited and so partially fill in the channel. A nice
a
ee
san alia
a
ee ree ES
PIERO eo PR pt:
balance is thus established between degradation and the con-
trasted process known as aggradation. The older the river valley —
the flatter become the grades at any section of its course, and .s
thus the point which separates the lower zone of aggradation
from the upper one of degradation moves steadily upstream with
the lapse of time.
The accordance of tributary valleys. — It is a consequence of |
the great sensitiveness of stream corrasion to current velocity a
that no side stream may enter the trunk valley at a level above ;
that of the main stream — the tributary streams enter the trunk
stream accordantly. Each has carved its own valley, and any
abrupt increase in gradient of the side streams near where they
enter the main stream would have increased the local corrasion
at an accelerated rate and so have cut down the channel to the
level of the trunk stream.
The grading of the flood plain. — All rivers are subject to
seasonal variations in the volume of their waters. Where there
are wet and dry seasons these differences are greatest, and for a —
large part of the year the valleys in such regions may be empty
of water, and are in fact often utilized for thoroughfares. In the
temperate climates of middle latitudes rivers are generally flooded
in the spring when the winter snows are melted, though they
may dwindle to comparatively small streams during the late
summer. In the upper reaches of the river the current velocities
are such that the usual river channel may carry all the water of
flood time; but lower down and in the zone of aggradation, where
the current has been checked, the level of the water rises in flood
above the banks of its usual channel and spreads over the sur-
rounding lowlands. As a deposit of sediment is spread upon the
surface, the succession of the annual deposits from this source
raises the general level as a broad floor described as the flood plain
of the river. mA
Parone
on
=~ 2. er
vt
ln Oe T=
iene aaelalae ait
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ia
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i ees,
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POS
THE LIFE HISTORIES OF RIVERS 163
|
The cycles of stream meanders. — The annual flooding with
f water and simultaneous deposition of silt is not, however, the
| f only grading process which is in operation upon the flood plain.
i It is characteristic of swift currents that their course is main-
. tained in relatively straight lines because of the inertia of the
rapidly moving water. In proportion as their currents become
iy sluggish, rivers are turned aside by the smallest of obstructions ;
and once diverted from their straight course, a law of nature
¢ becomes operative which increases the curvature of the stream
i at an accelerated rate up to a critical point, when by a change,
i sudden and catastrophic, a new and direct course is taken, to be
if in its turn carried through a similar cycle of changes. This
it so-called meandering of a stream is accompanied by a transfer of
Fi sediment from one bend or meander of the river to those below
1 and from one bank to the other. Inasmuch as the later meanders
" cross the earlier ones and in time occupy all portions of the plain
ti to the same average extent, a process of rough grading is accom-
1 plished to which the annual overflow deposit is supplementary.
Ne The course of the current in consecutive meanders and the
it cross sections of the channel which result directly from the mean-
le dering process will be made clear from examination of Fig. 168.
:% So soon as diverted from its direct course, the current, by its
' inertia of motion, is
| thrown against the
| outer or convex side
re so as to scour or
| 4 corrade that bank.
| Upon the concave
|
or inner side of the
curvethereisin con- F!¢- 168.—Map and sections of a stream meander.
P ¥ The course of the main current is indicated by the
. 4 sequence an area Of — gashed line. ;
slack water, and here
the silt scoured from higher meanders is deposited. The scouring
of the current upon the outer bank and the filling upon the inner
thus gives to the cross section of the stream a generally unsym-
metrical character (Fig. 168 ab). Between meanders near the
point of inflection of the curve, and there only, the current is cen-
tered in the middle of the channel and the cross section is sym-
metrical (Fig. 168 cd).
164 EARTH FEATURES AND THEIR MEANING
The scour upon the convex side of a meander causes the river
to swing ever farther in that direction, and through invasion of
the silted flood plain to migrate across it. Trees which lie in its
path are undermined and fall out-
ward in the stream with tops di- ©
rected with the current (Fig. 169).
Whenever the flood plain is for-
ested, the fallen trees may be so
numerous as to lie in ranks along
nae ore the shore, and at the time of the
oH Nall vi ~ next flood they are carried down-
dle saint stream to jam in narrow places
| ae along the channel and give the er-
roneous impression that the flood
; has itself uprooted a section of for-
Fia. 169.— Tree inpart undermined gt, (see p. 418).
upon the outer bank of a meander.
The cut-off of the meander. —
As the meander swings toward its extreme position it becomes
more and more closely looped. Adjacent loops thus approach
nearer and nearer to each other, but in the successive positions
a nearly stationary point is established near where the river
makes its sharpest turn (Fig. 170, G, and
Fig. 454, p. 417). At length the neck of land
which separates meanders is so narrow that
in the next freshet a temporary jamming of
logs within the channel may direct the waters
across the neck, and once started in the new
direction a channel is scoured out in the
soft silt. Thus by a breaking through of -
the bank of the stream, a so-called “ cre- Fie. 170.— Diagrams to
vasse,” the river suddenly straightens its Show the successive
APA ‘ ‘ positions of stream
course, though up to this time it has steadily jeanders and the
become more and more sharply serpentine. relatively stationary
After the cut-off has occurred, the old chan- Point: near the sharp-
nel may for a time continue to be used by the bam |
stream in common with the new one, but the advantage in velocity
of current being with the cut-off, the old channel contains slacker
water and so begins to fill with silt both at the beginning’and
the end of the loop. Eventually closed up at both ends, thisloop __
‘THE LIFE HISTORIES OF RIVERS 165
or ‘ oxbow” is entirely separated from the new channel, and
once abandoned of the stream is transformed into an oxbow
lake (Fig. 171 and p. 415).
Meander scars. —Swinging as it occasionally does in its
meanderings quite across the flood plain and against the bank of
the earlier degrading river in
this section, the meander at
times scours the high bank
which bounds the flood plain,
and undermining it in the same
manner, it excavates a recess
of amphitheatral form which is
known as a meander scar (Fig. ree
(172). Atlengththe entire bank Fie. 171.— An oxbow lake in the flood
is scarred in this manner so as Ce eee
to present to the stream a series of concave scallops separated by
sharp intermediate salients of cuspate form.
River terraces. — Whenever the river’s history is interrupted
by a small uplift, or the base level is for any reason lowered, the
stream at once begins to sink its channel into the flood plain.
Once more flowing upon a low grade, it again meanders, and so
produces new walls at a lower level, but formed, like the first, of
intersecting meander scars. Thus there is produced a new flood
plain with cliff and ter-
race above, which is
known as a river terrace.
A succession of uplifts
tj or of depressions of the
Fra. 172. — Schematic’ representation of a series base level yields terraces
of river terraces. a, 6, c, e, successive terraces jn geries, as they appear
oF eae d,d,d, terrace slopesformed —..-hematically represented
in Fig. 172. Such ter-
races are to be found well developed upon most of our larger
rivers to the northward of the Ohio and Missouri. The highest
terrace is obviously the remnant of the earliest flood plain, as the
lowest represents the latest.
The delta of the river. — As it approaches its mouth the river
moves more and more sluggishly over the flat grades, and swings
in broader meanders as it flows. Yet it still carries a quantity
Reet Paes ve a: aes sis
Ve | SSS os
Sa a *®ye * :
tee
wir.
te
166 EARTH FEATURES AND THEIR MEANING
of silt which is only laid down after its current has been stopped
on meeting the body of standing water into which it discharges.
If this be the ocean, the salinity of the sea water greatly aids ina
quick precipitation of the finest material. This clarifying effect
upon the water of the dissolved salt may be strikingly illustrated |
by taking two similar jars, the one filled with fresh and the other
with salt water, and stirring the same quantity of fine clay into
each. The clay in the salt water is deposited and the water
cleared long before the murkiness of the other has disappeared.
By the laying down of the residue of its burden of sediment
where it meets the sea, the river builds up vast plains of silt and
clay which are known as deltas and which often form large local
extensions of the continents into the sea. Whereas in its upper
reaches the river with its tributary streams appears in the plan
like a tree and its branches, in the delta region the stream, by
dividing into diverging channels called distributaries (Fig. 458,
p. 420), completes the resemblance to the tree by adding the
roots. From the divergence of the distributaries upon the delta
plain the Greek capital letter A is suggested and has supplied the
name for these deposits. Of great fertility, the delta plains of
rivers have become the densely populated regions of the globe,
among which it is necessary to mention only the delta of the
Nile in Egypt, those of the Ganges and Brahmaputra in India,
and those of the Hoang and Yangtse rivers in China.
The levee. — When the snows thaw upon the mountains at
the headwaters of large rivers, freshets result and the delta regions
are flooded. At such times heavily charged with sediment, a
thin deposit of fertile soil is left upon the surface of the delta
plain, and in Egypt particularly this is depended upon for the
annual enrichment of the cultivated fields. Though at this time
the waters spread broadly over the plain, the current still continues
to flow largely within the normal channel, so that the slack water
upon either side becomes the locus for the main deposit of the
sediment. There is thus built up on either side of the channel a
ridge of silt which is known as a levee, and this bank is steadily
increased in height from year to year (Fig. 452).
To prevent the danger of floods upon the inhabited plains, — 4g \
artificial levees are usually raised upon the natural ones, and in a
country like Holland, such levees (dikes) involve a large expendi- |
THE LIFE HISTORIES OF RIVERS 167
ture of money and no small degree of engineering skill and ex-
perience to construct.’ So important to the life of the nation is
the proper management of its dikes, that in the past history of
China each weak administration has been marked by the develop-
ment of graft in this important department and by floods which
have destroyed the lives of hundreds of thousands of people.
Wherever there has been a markedly rapid sinking upon a
delta region, and depressions are common in delta territory, no
doubt as a result of the loading down
of the crust, the river may present the
paradoxical condition of flowing at a
higher level than the surrounding coun-
try. Between the levees of neighboring
distributaries there are peculiar saucer-
shaped depressions of the country which
easily become filled with water. At the y/
extremity of the delta the levee may be #4
the only land which shows above the Fic. 173.—"Bird-foot” delta
of the Mississippi River.
ocean surface, and so present the pecul-
iar ‘‘ bird-foot ”’ outline which is characteristic of the extremity
of the Mississippi delta, though other processes than the mere
sinking of the deposits may contribute to this result (Fig. 173).
The sections of delta deposits. — If now we leave the plan of
the delta to consider the section of its deposits, we find them so
characteristic as to be easily recognized. Considered broadly,
the delta advances seaward after the manner of a railroad embank-
ment which is being carried across a lake. Though the greater
portion of the deposit is unloaded upon a steep slope at the front,
a smaller amount of material is dropped along the way, and a
layer of extremely fine material settles in advance as the water
clears of its finely suspended particles (Fig. 174). Simultaneous
deposits within a delta thus comprise a nearly horizontal layer
of coarser materials, the so-called top-set bed; the bulk of the
deposit in a forward sloping layer, the so-called fore-set bed ;
and a thin film of clay which is extended far in advance, the
bottom-set bed (Fig. 174, 2). If at any point a vertical section is
made through the deposits, beds deposited in different periods
are encountered; the oldest at the bottom in a horizontal posi-
tion, the next younger above them and with forward dip, and the
168 EARTH FEATURES AND THEIR MEANING
youngest and coarsest upon the top in nearly horizontal position
(Fig. 174, 3).
It has been estimated that the surface of the United States
is now being pared down by erosion at the average rate of an
inch in 760 years.
Pr
© The derived ma-
’ ox, . . .
Lia ouse = terial is being
JPEN ee es a Stic Nae ee ee ee ee : f
Oe ey eS deposited in the
flood plain and
delta regions of its
Jecorser of) Se principal rivers.
aa ay ee ee Some 513 million
Sorrom oe" “7 ~—tons of suspended
matter is in the
United States car-
ried to tidewater
each year, and
about half as much
more goes out to
sea as dissolved
1 Profle of a Lel?*a.
& lerrical Section acrass the beds. matter. If this
Fie. 174. Diagrams to show the nature of delta de- material were re-
posits as exhibited in section. moved from the
Panama Canal cutting, an 85-foot sea-level canal would be ex-
cavated in about 73 days. The Mississippi River alone carries
annually to the sea 340 million tons of suspended matter, or
two thirds of the entire amount removed from the area of the
United States as a whole. It is thus little wonder that great
deltas have extended their boundaries so rapidly and that the
crust. is so generally sinking beneath the load.
CHAPTER XIII
EARTH FEATURES SHAPED BY RUNNING WATER
The newly incised upland and its sharp salients. — The suc-
cessive stages of incising, sculpturing, and finally of reducing an
uplifted land area, are each of them possessed of distinctive
characters which are all to be read either from the map or in the
lines of the landscape. Upon the newly uplifted plain the incis-
ing by the young rivers is to be found chiefly in the neighbor-
hood of the margins. In this stage the valleys are described as
V-shaped cafions, for the valley wall meets the upland surface
in sharp salients (plate 12 A), and the lines of the landscape are
throughout made up from straight elements. Though the land-
scapes of this stage present the grandest scenery that is known
and may be cut out in massive proportions, often with rushing
river or placid lake to enhance the effect of crag and gorge, they
lack the softness and grace of
outline which belong only to the
maturer erosion stages. The
grand cafion of the Colorado
presents the features character-
istic of this stage in the grandest a
and most sublime of all exam- & “SSS
ples, and the castled Rhine isa “i.
gorge of rugged beauty, carved Fic. 175.—Gorge of the River Rhine
out from the newly elevated 3 en ee ea eae
plateau of western Prussia,
through which the water swirls in eddying rapids (Fig. 175).
The stage of adolescence. —As the upland becomes more
largely invaded as a consequence of the headward advance of
the cafions and their sending out of tributary side cafions, the
sharp angles in which the cafion walls intersect the plain become
gradually replaced by well-rounded shoulders. Thus the lines in
the landscape of this stage are a combination of the straight
169
yh’
£70 EARTH FEATURES AND THEIR MEANING
line with a simple curve convex toward the sky (Fig. 176). In
this stage large sections of the original plateau remain, though
cut into small areas by the ex-
tensions of the tributary valleys.
The maturely dissected up--
land. — Continued ramifications
by the rivers eventually divide
the entire upland area into sep-
Fig. 176.— V-shaped valley with well- arated parts, and the rounding
rounded shoulders characteristic of of the shoulders of valleys pro- .
the stage of adolescence. Allegheny geeds simultaneously until of the
plateau in West Virginia. aie 3 Q
original upland no easily recog-
nizable compartments are to be found. Where before were flat
hilltops are now ridges or watersheds, the well-known divides.
The upland is now said to be completely dissected or to have
arrived at maturity. The streams are still vigorous, for they
make the full descent from the upland level to base level, and
yet a critical turning point of
their history has been reached, .
and from now on they are to
show a steady falling off in eff- seer
ciency as sculpturing agents. | We.
Viewed from one of the hill- p46, 177.—View of a maturely dissected
tops, the landscape of this stage upland from one of its hilltops, Kla-
bears a marked resemblance to ™ath Mountains, California (after a
: t photograph by Fairbanks).
a sea in which the numberless |
divides are the crests of billows, and these, as distance reduces
their importance in the landscape, fade away into the even line
of the horizon.(Fig. 177).
The Hogarthian line of beauty. — Since the youthful stage of
the upland, when the lines of its landscape were straight, its
character rugged, and its rivers wild and turbulent, there has
been effected a complete transformation. The only straight line
to be seen is the distant horizon, for the landscape is now molded
in softened outlines, among which there is a repeated recurrence
of the line of beauty made famous by Hogarth in his “ Analysis of
Beauty.” As well known to all art students, this is a sinuous — 7
line of reversed or double curvature —a curve which passes
insensibly at a point of inflection from convex to concave (Fig.
EARTH FEATURES SHAPED BY RUNNING WATER 171
178). The curve of beauty is now found in every section of the
hills, and it imparts to the landscape a gracefulness and a measure
of restfulness as well, which are not to be found in the landscapes
of earlier stages in the erosion cycle. In the bottoms of the
valleys also the initial windings of the
rivers within their narrow flood plains
add silver beauty lines which stand
out prominently from the more som-
ber background of the hills.
Considered from the commercial Fic. 178.— Hogarth’s line of
viewpoint, the mature upland is one pea:
of the least adaptable as a habitation for highly civilized man.
Direct lines of communication run up hill and down dale in
monotonous alternation, and almost the only way of carrying a
railroad through the region, without an expenditure for trestles
which would be prohibitive, is to follow the tortuous crest of a
main divide or the equally winding bed of one of the larger valleys.
The final product of river sculpture — the peneplain. — When
maturity has been reached in the history of a river, its energies
are devoted to a paring down of the valley slopes and crests so
as to reduce the general level. From this time on hill summits
no longer fall into a common level — that of the original upland
_ —for some mount notably higher than others, and with increas-
ing age such differences become accentuated. There is now also
a larger aggradation of the valleys to form the level floors of
flood plains, out of which at length the now slight elevations rise
upon such gentle slopes that the process of land sculpture ap-
proaches its end. Gradually
the vigor of the stream has
faded away, and can now only
be renewed through a fresh
uplift of the land, or, what
would amount to the same
aS “t thing, a depression of the base
Fic. 179.— View of the old land of New level. Upland and river have
England, with Mount Monadnockrising yeached old age together, and
in the distance. . j
: the approximation to a new
plain but little elevated above base level is so marked that the
name peneplain is applied to it. Scattered elevations, which be-
Point of
Inflection
172 EARTH FEATURES AND THEIR MEANING
cause of some favoring circumstance rise to greater heights above
the general level of the peneplain, are known as monadnocks after
the type example of Mount Monadnock in New Hampshire (Fig.
179).
The river cross sections of successive stages. — To the suc- ©
cessive stages of a river’s life it has been common to carry over
the names from the well-marked periods of a human life. If
neglecting for the moment the general aspect of the upland, we
fix our attention up-
on the characteristic
cross sections of the
Mntoncy Yourh river valley, we find
that here also there
CO a are clearly marked
Adolescence Maturity characters to distin-
guish each stage of the
ae ae ee NIZA river’s life (Fig. 180).
In infancy the steep,
Old Age Comyoor/sor
Fic. 180. — Comparison of the cross sections of river
valleys for the different stages of the erosion cycle.
narrow, and_ sharp-
angled cafion is a char-
acteristic ; with youth
the wider V-form has already developed ; in adolescence the angles
of the cafion are transformed into well-rounded shoulders, and the
valley broadens so as in the lower reaches to lay down a flood
plain; in maturity the divides and the double curves of the line
of beauty appear; while in the decline of old age the valleys are
extremely broad and flat and are floored by an extended flood
plain.
The entrenchment of meanders with renewed uplift. — Upon
the reduced grades which are characteristic of the declining stage
of a river’s life, the current has little power to modify the surface
configuration. On the old land of this stage a renewed uplift
starts the streams again into action. This infusion of driving
power into moving water, regarded as a machine capable of ac-
complishing certain work, is like winding up a clock that has
run down. Once more the streams acquire a velocity sufficient
to enable them to cut their valleys into the land surface, and |
so a new erosional cycle may be inaugurated upon the old land
surface — the peneplain. After such an uplift has been accom-
Eh Pe Oe ee
~
Te o,
nen
=
ro
sek
woes
cecal
*
ae
or
T>
i a eal ae ee coat
EARTH FEATURES SHAPED BY RUNNING WATER 173
plished and the rivers have sunk their early valleys within the
new upland, we may look out from this now elevated surface
and the eye take in but a single horizontal line, since we view
the plain along its edge.
By: the uplift the meanders of the earlier rivers may become
entrenched in the new upland, the wide lobes of the individual
meanders being now separated by mountains where before had
been plains of silt only. The New River of the Cumberland
plateau and the Yakima River of central Washington (Fig. 181)
furnish excellent American examples of intrenched meanders, as
~
VE
fy
3
=) % U s - ss a A ke fied gf
Fig. 181.— The Beavertail Bend of the Yakima Cafion in central Washington
(after George Otis Smith).
"
PR
oo
N
the Moselle River does in Europe. Upon the course of the latter
river near the town of Zell a tunnel of the railroad a quarter of
a mile in length pierces a mountain in the neck of a meander
lobe in which the river itself travels a distance of more than six
miles in order to make the same advance. The Kaiser Wilhelm
tunnel in the same district penetrates a larger mountain included
in a double meander of the river. Although intrenched, river
meanders are still competent to scour and so undermine the
outer bank, and with favoring conditions they may by this process
erode extended ‘ bottoms ” out of the plateau. (See Lockport
quadrangle, U. 8S. G. S.) 3
The valley of the rejuvenated river. — Whenever a new uplift
occurs before an erosional cycle has been completed, the rivers
become intrenched, not in a peneplain, but in the bottoms of
broad valleys. The sweeping curves which characterize mature
174 EARTH FEATURES AND THEIR MEANING
landscapes may thus be brought into striking contrast with the
straight lines of youthful cafions which with V-sections descend
from their lowest levels
(Fig. 182). The full
cross section of such a
valley shows a central V
whose sharp shoulders
are extended outward
and upward in the soft-
ened curves of later ero-
sion stages.
The arrest of stream
Fig. 182. — A rejuvenated river valley (after a erosion by the more re-
photograph by Fairbanks).
sistant rocks. — The ca-
pacity of a river to erode and carry away the rock material
that lies along its course is dependent not only upon the ve-
locity of the current, but also upon the hardness, the firmness
of texture, and the solubility of the material. Particularly in
arid and semiarid regions, where no mantle of vegetation is at
hand to mask the surfaces of the firmer rock masses, differences
of this kind are stamped deeply upon the landscape. The rock
terraces in the Grand Cajfion of the Colorado together represent
the stronger rock formations of the region, while sloping talus
accumulations bury the weaker beds from sight.
Each area of harder rock which rises athwart the course of a
stream causes a temporary arrest in the process of valley erosion
and is responsible for a noteworthy local contraction of the river
valley. The valley is carved less widely as well as less deeply,
and since a river can never corrade
below its base, a ‘‘ temporary base a ae be, sth rae wh
level” is for a time established _e eat le BB aa
above the area of harder rock.
Owing to the contraction of the
valley under these conditions, the ==” Ves
locality is described as a river ae Wes Neo e Slowim
narrows (Fig. 183). The narrows ““"
‘upon the Hudson River occur in
the Highlands where the river leaves a broad expanse occupied
Fig. 183. — Plan of a river narrows.
iby softer sediments to traverse an island-like area of hard crystal- 7 bt
EARTH FEATURES SHAPED BY RUNNING WATER 175
line rocks. Within the narrows of a river the steep walls, charac-
teristic of youth and the turbulent current as well, are often retained
long after other portions of the river have acquired the more restful
lines of river maturity. The picturesque crag and the generally
rugged character of river narrows render them points of special
interest upon every navigable river.
The capture of one river’s territory by another. — The effect
of a hard layer of rock interposed in the course of a stream is
thus always to delay the advance of the erosional process at all
levels above the obstruction. When a stream in incising its
valley degrades its channel through a veneer of softer rocks into
harder materials below, it is technically described as having dis-
covered the harder layer. Where several neighboring streams flow
by similar routes to their common base level, those which dis-
cover a harder rock will advance their headwaters less rapidly
into the upland and so will be at a disadvantage in extending
their drainage territory. A stream
which is not thus hindered will in the
course of time rob the others of a por-
tion of their territory, for it is able to
erode its lower reaches nearer to base
level and thus acquire for its upper
reaches, where erosion is chiefly accom-
plished, an advantage in declivity. The
divide which separates its headwaters
from those of its less favored neighbor
will in consequence migrate steadily in-
to the neighbor’s territory. The divide
is thus a sort of boundary wall separat-
ing the drainage basins of neighboring
streams, and any migration must extend
the territory of the one at the expense Se eae
of the other. As more and more terri- dete tp itubtrate repauted
tory is brought under the dominion of river piracy and the devel-
the more favored stream, there will come opment of “trellis drainage,”
a time when the divide in its migration he Pecaeaay of
will arrive at the channel of the stream that is being robbed, and
so by a sudden act of annexation draw off all the upper waters
into its own basin. By this capture the stream whose territory has
176 EARTH FEATURES AND THEIR MEANING
been invaded is said to have been beheaded. By this act of piracy
the stronger stream now develops exceptional activity because of
the local steep grades near the point of capture, and with this
newly acquired cutting power the invader is competent to ad-
vance still further and enter the territory of the stream that lies ©
next beyond. The type of drainage network which results from ,
repeated captures of this kind is known as “trellis drainage ”
(Fig. 184), a type well illustrated by the rivers of the southern
Appalachians. |
In general it may be said that, other conditions being the
same, of two neighboring streams which have a common base
level, that one which takes the longest route will lose territory
to the other, since it must have the flatter average slope. Stream
capture may thus come about without the discovery of hard
rock layers which are more unfavorable to one stream than an-
other.
Water and wind gaps. — In the Allegheny plateau rivers cross
the range of harder rocks in deep mountain narrows which upon
the horizon appear as gateways through the barrier of the moun-
tain wall. Such gate-
ways are sometimes
referred to as ‘water
gaps,” of which the
Delaware Water Gap
is perhaps the best
known example,
though the Potomac
crosses the Blue
Ridge at the historic
Fia. 185. —Sketch maps to show the earlier and the
b]
: r’s Ferry through
present drainage condition about the Blue Ridge Harpe yt ous
near Harper’s Ferry. a similar portal. The ©
valley of the tributary
Shenandoah has been the scene of an interesting episode in the
struggle of rival streams which is typical of others in the same
upland region. The records which may be made out. from the
landscapes show clearly that in an earlier but recent period,
when the general surface stood at a higher level which has been
called the Kittatinny Plain, the younger Potomac of that “time
and a younger but larger ancestor of Beaverdam Creek each
ee
4
Ti
3
1 y
EARTH FEATURES SHAPED BY RUNNING WATER 177
crossed the Blue Ridge of the time through similar water gaps
(Fig. 185, map, and Fig. 186). The Potomac of that time was,
however, the more
deeply intrenched, eR AN
and possessing an p 8h
advantage in slope 3 RS
it was able to 2. Bi De og eee e
advance the divide = Farrer Sieve!” oF HF rariy ‘~~ PLAIN ~~
at the head of its Zere) oF SHENANDOAH x7 PLAIN
tributary, the Fic. 186.—Section to illustrate the history of Snickers
Shenandoah, into Gap.
the territory of Beaverdam Creek. Thus the beheading of the
Beaverdam by the Shenandoah was accomplished (Fig. 185, second
map) and its upper waters annexed to the Potomac system. .
With the subsequent lowering of the general level of the country
which yielded the present Shenandoah Plain, the former water gap
of Beaverdam Creek was abandoned of its stream at a high level
in the range. Known as Snickers Gap, it may serve as a type of
the “‘ wind gaps ” of similar origin which are not altogether un-
common in the Appalachian Mountain system (Fig. 186).
Character profiles. — For humid regions the landscapes possess
characters which, speaking broadly, depend upon the stage of the
erosion cycle. For the earliest stages the straight line enters
-as almost the only element in the design; as the cycle advances
to adolescence the rounded forms begin to replace the angles of
YOUTH \ aie OLES aN
MATURITY Metts ie ee
OLD AGE..°
PESUVENATION
Fig. 187. — Character profiles of landscapes shaped by stream erosion in humid
climates.
N
178 EARTH FEATURES AND THEIR MEANING
the immature stages, and with full maturity the lines of beauty
alone are characteristic. As this critical stage is passed irregu-
larity of feature and ever more flattened curves are found to cor-
respond to the decline of the river’s vital energies. There are
thus marks of senility in the work of rivers (Fig. 187).
Reaping REFERENCES FoR CHAPTERS XII anp XIII
General : — |
Sir JoHn Puayrarr. Illustrations of the Huttonian Theory of the Earth.
Edinburgh, 1802, pp. 350-371.
J. W. Powretu. Exploration of the Colorado River of the West and its
Tributaries. Washington, 1875, pp. 149-214.
G. K. Gitspert. Report on the Geology of the Henry Mountains. Wash-
ington, 1877, pp. 99-150. (A classic upon the work of rivers.)
C. E. Durtron. Tertiary History of the Grand Cajfion District (with
atlas), Mon. 2, U.S. Geol. Surv., 1882, pp. 264.
W.M. Davis. The Rivers and Valleys of Pennsylvania, Nat. Geogr. Mag.
vol. 1, 1889, pp. 203-219; The Triassic Formation of Connecticut,
18th Ann. Rept. U. S. Geol. Surv., Pt. ii, 1898, pp. 144-153.
Sir A. Grerxiz. The Scenery of Scotland. London, 1901, pp. 1-12.
I. C. Russety. Rivers of North America. Putnam. New York, 1898,
pp. 327.
M. R. Campsetu. Drainage Modifications and their Interpretation,
Jour. Geol., vol. 4, 1896, pp. 567-581, 657-678.
Henry Gannett. Physiographic Types, U. S. Geol. Surv., Topographic
Atlas, Folios 1-2, 1896, 1900.
W. M. Davis. The Geographical Cycle, Geogr. Jour., vol. 14, 1899,
pp. 481-504.
The flood plain : —
Henry Gannett. The Flood of April, 1897, in the Lower Mississippi,
Scot. Geogr. Mag., vol. 13, 1897, pp. 419-421.
W.-M. Davis. The Development of River Meanders, Geol. Mag., Decade
iv, vol. 10, 1903, pp. 145-148.
W.S. Tower. The Development of Cut-off Meanders, Bull. Am. Geogr.
Soc., vol. 36, 1904, pp. 589-599.
River terraces : —
W. M. Davis. The Terraces of the Westfield River, Massachusetts, Am.
Jour. Sci., vol. 14, 1902, pp. 77-94, pl. 4; River Terraces in New Eng-
land, Bull. Mus. Comp. Zodl., uel 38, 1902, pp. 281-346.
River deltas : —
G. K. Gitpert. The Topographic Features of Lake Shores, 5th Ann.
ahs. =
a one
i ae Po na
— 2 = 2 a : ‘
2 - en se cet ==
oP - he
ay ee or a a = 2 a ee a a cee ee ie ons
» aes y —= a ee ¥ - ~ - a ‘J “a ot ~__
ae em AS * L— ‘ok oe =”, f = ee So ee ~ Se? = ~ " PS: tn, _ =
2 anne et 5 Ai i ea oD Se “ addins ee : eo eo oe a oe a ee ee
. - - - i ss - eeieetnteer - = a i = ~~ e sos
a ial lil . eel - oe _ tne
a
et ane
= oe
ety Dist
2 CeO TS thee rae Rigs
EARTH FEATURES SHAPED BY RUNNING WATER 179
Rept. U.S. Geol. Surv., 1885, pp. 104-108; Lake Bonneville, Mon. I,
U.S. Geol. Surv., 1890, pp. 153-167.
Charts of Mississippi River Commission.
G. R. Crepner. Die Deltas, ihre Morphologie, geographische Ver-
breitung und Entstehungsbedingungen, Pet. Mitt. Ergh. 56, 1878,
pp. 1-74, pls. 1-3.
The peneplain : —
W. M. Davis. Plains of Marine and Subaérial Denudation, Bull. Geol.
Soe. Am., vol. 7, 1896, pp. 377-398; The Peneplain, Am. Geol., vol.
23, 1899, pp. 207-239.
Intrenchment of meanders : —
W. M. Davis. The Seine, the Meuse, and the Moselle, Nat. Geogr. Mag.,
vol. 7, 1896, pp. 189-202.
Stream capture : —
N. H. Darton. Examples of Stream Robbing in the Catskill Mountains,
Bull. Geol. Soe. Am., vol. 7, 1896, pp. 505-507, pl. 23.
Co.LuiER Coss. A Recapture from a River Pirate, Science, vol. 22, 1893,
p. 195.
Wiiuram H. Hosss. The Still Rivers of Western Connecticut, Bull.
Geol. Soc. Am., vol. 18, 1902, pp. 17-22, pl. 1.
IsatAn Bowman. A Typical Case of Stream Capture in Michigan, Jour.
Geol., vol. 12, 1904, pp. 326-334.
CHAPTER XIV
THE TRAVELS OF THE UNDERGROUND WATER
The descent within the unsaturated zone. — Of the moisture ©
precipitated from the atmosphere, that portion which neither
evaporates into the air nor runs off upon the surface, sinks into
the ground and is described as the ground water. Here it descends
by gravity through the pores and open spaces, and at a quite
moderate depth arrives at a zone which is completely saturated
with water. The depth of the upper surface of this saturated zone
varies with the humidity of the climate, with the altitude of the
earth’s surface, and with many other similarly varying factors.
Within humid regions its depth may vary from a few feet to a few
hundred feet, while in desert areas the surface may lie as low as a
thousand feet or more.
The surface of the zone of the lithosphere that is saturated
with water is called the water table, and though less accentuated it
conforms in general to the relief of the country (Fig. 188). Its
Fia. 188. — Diagram to show the seasonal range in the position of the water table
and the cause of intermittent streams.
depth at any point is found from the levels of all perennial streams
and from the levels at which water stands in wells.
During the season of small precipitation the water table is
lowered, and if at such times it falls below the bed of a vailey,
the surface stream within the valley dries up, to be revived when,
after heavier precipitation, the water table has in turn been raised.
Such streams are said to be intermittent, and are especially char-
acteristic of semiarid regions (Fig. 188).
180
=
~
Taf tC
sig
2
x ee sss
EF,
Mee
ire We.
Se
THE TRAVELS OF THE UNDERGROUND WATER 181
Wherever in descending from the surface an impervious layer,
such as clay, is encountered, the further downward progress of the
water is arrested. Now conducted in a lateral direction it issues
at the surface as a spring at the line of emergence of the upper sur-
face of the impervious layer (Fig. 189).
Fig. 189. — Diagram to show how an impervious layer conducts the descending
water in a lateral direction to issue in surface springs.
The trunk channels of descending water. — While within the
unconsolidated rock materials near the surface of the earth, it is
clear that water can circulate in proportion as the materials are
porous and so relatively pervious. As the pore spaces become
minute and capillary, the difficulty of permeation through the
materials becomes very great. Thus in the noncoherent rocks
it is the coarse gravel and the layers of sand which serve as the
underground channels, while the fine clays have the effect of an
impervious wall upon the circulating waters. In coarse sand as
much as a third of the volume of the material is pore space for the
absorption and transmission of water. Even under these favor-
able conditions the movement of the water is exceedingly slow
and usually less than a fifth of a mile a year.
Within the hard rocks it is the sandstones which have the largest
pore spaces, but in
nearly all consolidated
rocks there are addi-
tional spaces along
certain of the bedding
planes, the joint open-
ings (Fig. 190), and
the crushed zones of
displacement, so that
these parting planes
become the trunk
channels, so to speak, Fig. 190. — Sketch map of the Oucane de Chabriéres
BE a neala ting near Chorges in the High Alps, to illustrate the cor-
rosion of limestone along two series of vertical joints.
water. It is along (after Martel).
182 EARTH FEATURES AND THEIR MEANING
such crevices that in the course of time the mineral matter carried
in solution by the water is deposited to produce the ore veins
and the associated crystallized minerals.
The caverns of limestones. — Where limestone formations have /
a nearly flat upper surface, a large part of the surface water enters.
the rock by way of the joint spaces, which it soon widens by solu-
tion into broad crevices with well-rounded shoulders. At joint
intersections solution of the limestone is so favored that the water
may here descend in a sort of vertical shaft until it meets a bedding
plane extending laterally and offering more favorable conditions
for corrosion. Its journey now begins in a lateral direction, and
solution of the rock continuing, a tunnel may be etched out and
extended until another joint is encountered which is favorable to
its further descent into the formation.. By this process on alter-
nating shafts and galleries the water descends to near the surface
of the water table by a series of steps, and is eventually discharged
into the river system of the district (Fig. 191). Within the larger
caverns the water at the lowest level
usually flows as a subterranean river
to emerge later into the light from be-
neath a rock arch. |
Fra. 191. — Diagram to show the From the plan of a system of con=
relation of cavernsinlimestone necting caverns it may often be ob-
to the river system of the dis- served that the galleries of the several
Ur eg LN nce levels are alike directed along two
holes”’ upon the surface.
rectangular directions which indicate
the master joint directions within the limestone formation. This
is especially clear from the map of the galleries in the explored
portions of the Mammoth Cave (Fig. 192).
Swallow holes and limestone sinks. — Above the caverns of
limestone formations there are selected points where the water
has descended in the largest volume, and here funnel-shaped
depressions have been dissolved out from the surface of the rock.
In different districts such depressions have become known as
“sinks,” “ swallow holes,” entonnoirs, and Orgeln. Wherever the
depressions have a characteristic circular outline, there can be
little doubt that they are the product of solution by the descend-
ing water, and have relatively small connections only with the
aibicesaian caverns. They have thus naturally collected upon
saliiaiies
ao S ip !
THE TRAVELS OF THE UNDERGROUND WATER § 183
their bottoms the insoluble clay which was contained in the impure
limestone as well as a certain amount of slope wash from the sur-
Fig. 192. — Plan of a portion of Mammoth Cavé, Kentucky (after H. C. Hovey).
face. Inasmuch as the clays are impervious to water, the bottoms
of these swallow holes are better supplied with moisture than the
surrounding rock surfaces, and
by nourishing a more vigorous
plant growth are strongly im-
pressed upon the landscape
(Fig. 193).
Certain of the depressions
above caverns are, however,
less regular in outline, and their
bottoms are occupied by a
mass of limestone rubble. In
some instances, at least, these
~~
pee a os ae
Fie. 193.— Trees and shrubs growing
luxuriantly upon the bottoms of sinks
within a limestone country (after a
photograph by H. T. A. de L. Hus).
184 EARTH FEATURES AND THEIR MEANING
depressions appear to be the result of local incaving of the cavern
roofs. An incaving of this nature may close up an earlier gallery in
the cavern and divert the cave waters to a new course. The de-
struction of the roofs of caverns through this process of incaving may
continue until only relatively small remnants are left. From long
subterranean tunnels the caves are thus transformed into subaérial
rock bridges that have become known as “ natural bridges.”” The
best-known American example is the Natural Bridge near Lex-
ington, Virginia. Much grander natural bridges have been formed
in sandstone by a totally different process, and must not be con-
fused with these limestone remnants of caverns.
The sinter deposits. — Just as water can dissolve the calcare-
ous rocks with the formation of caverns, it can under other con-
ditions deposit the material which has thus been taken into solu-
tion. Its power to hold carbonate of lime in solution is dependent
upon the presence of carbonic acid gas within the water. Water
charged with gas and dissolved lime carbonate is said to be “‘ hard,”
and if the gas be driven off by boiling or otherwise, the dissolved
lime is thrown out of solution and deposited in a form well known
to all housekeepers. :
Hard water flowing in a surface stream, if dashed into spray
at a cascade, may deposit its lime carbonate in an ever thickening
veneer wherever the spray is dashed about the falls. This material,
when cut in section, has waving parallel layers and is known as
travertine or calcareous sinter. Some of the most remarkable de-
posits of this nature may be seen at the cascade of Tivoli near
Rome, and most of the Roman buildings have been constructed
from travertine that has been quarried in the vicinity.
The growth of stalactites. — Water, after percolating slowly
through the crevices of limestone, where it becomes charged with
the carbonic acid gas and with dissolved carbonate of lime, may
trickle from the roof of a cavern. Emerging from the narrow
crevice, it may give off some of its contained gas and is usually
subject to evaporation, with the result that the lime carbonate is
left adhering to the rock surface from which evaporation took
place. If the water collects upon the cavern roof so slowly that
it can entirely evaporate before a drop can form, the entire content
of carbonate will be left adhering to the roof. Evaporation is
most rapid near the margins and over the center of each drop as it
ete
falls to the floor, and, spattering as
THE TRAVELS OF THE UNDERGROUND WATER 185
develops, and the deposit which is left thus takes the form of tiny
white rings at those points upon the crevice where there is the
easiest passage for the trickling water. To the outer surface of
these rings water will first adhere and then evaporate, as it will
also slowly ooze through the passage in the ring, but here without
evaporation until it reaches the lower surface. A pendant struc-
ture will, therefore, develop, growing outward in all directions by
the deposition of concentric layers which are thickest near the roof,
and downward into the form of a rock “ icicle ” through evapora-
tion of the water which collects near the tip. These pendant
sinter formations are known as stalactites and are thus formed of
concentric layers arranged like a series of nested cornucopias with
a perforation of nearly uniform caliber along the axis of the struc-
ture (Fig. 194).
Formation of stalagmites. — Wherever the water percolates
through the roof of the cavern so rapidly that it cannot entirely
evaporate upon the roof, a portion
it strikes, builds up a relatively
thick cone of sinter known as a
stalagmite, and this is accurately
centered beneath a stalactite upon
the roof. In proportion as the
cavern is high, the dropping water
is widely dispersed as it strikes the
floor, with the formation of a corre- Fie. 194.— Diagrams to show the
spondingly thick and blunt stalag- Dae tins carpe eine tate:
mite. As this rises by growth to- beneath parallel crevices upon the
ward the roof, it often develops 0ofs of caverns (in part after von
upon its summit a distinct crater- peenbeel)-
like depression (Fig. 194, lower figure). When the process is
long continued, stalactites and stalagmites may grow together
to form columns which may be ranged with their neighbors
like the pipes of an organ, and like them they give out clear
tones when struck lightly with a mallet. At other times the
columns are joined to their neighbors to form hangings and dra-
peries of the most fantastic and beautiful design (Fig. 195).
In remote antiquity limestone caverns afforded a refuge to many
species of predatory birds and animals as well as to our earliest
186 EARTH FEATURES AND THEIR MEANING
ancestors. The bones of all these denizens of the caves lie en-
tombed within the clays and the sinter formations upon the cavern
floors, and they tell tbe story of a fierce and long-continued war-
fare for the possession of these natural strongholds. The evidence
is clear that these cave men with their primitive weapons were ©
]
n\\
1”
Ae )
1 PRSS yy Be
)
ae \X \ Wh yy Key
\\ Ni Ait) 2 ih HH
NW “e 3 ;
eA
Wy’
* i
. Yer’
NEAT
. : UREN :
Fig. 195. — Sinter formations in the Luray caverns, Virginia.
able at times to drive away the cave bears, lions, and hyenas, and
to set up in the cavern their simple hearths, only in their turn to
be conquered by the ferocity of their enemies. Some of the Euro-
pean caves have yielded many wagonloads of the skeletons of
these fierce predatory animals, together with the simple weapons
of the primitive man.
The Karst and its features. — Most so-called limestones have a
large admixture of argillaceous materials (clays) and of siliceous
or sandy particles. Such impurities make up the bulk of the clays
and muds which are left behind when the soluble portions of the
limestone have been dissolved. .
Swallow holes we have found to be characteristic features within
such districts. When limestones are more nearly pure, as in the
OT, GM es gi
THE TRAVELS OF THE UNDERGROUND WATER 187
Karst region east of the Adriatic Sea, similar features are devel-
oped, but upon a grander scale, and certain additional forms are
encountered. In place of |
the sink or swallow hole,
there appears the “ karst
funnel ” or doline, a deep,
bowl-shaped depression
having a flat bottom.
Such funnels may be 30
to 3000 feet across and
from 6 to 300 feet in depth
(Fig. 196). Though in
one or two _ instances
known to be the result
of the break down of
cavern roofs (Fig. 197),
yet like the swallow holes
of other regions these
larger funnels appear gen-
erally to be the work of
solution by the descending waters. Where they have been opened
in artificial cuttings along railroads or in mines, the original rock
is found intact at the bottom, with
small crevices only going down to
lower levels. Over the bottoms of
the dolines there is spread a layer
of fertile red clay, the terra rossa,
= like that which is obtained as a
| | ALA residue when a fragment of the
Fig. 197. — Cross section of the do- limestone has been dissolved in
line formed by inbreak of a cavern laboratory experiments.
pont, be Stara Apnenks doling.” “A desert from the destruction of
in Carinthia (after Martel).
forests. — Between the dolines is
found a veritable desert with jutting limestone angles and little
if any vegetation. The water which falls upon the surface either
runs off quickly or goes down to the subterranean caverns by which
so much of the country is undermined: Hence it is that the gar-
dens which furnish the sustenance for the scattered population
are all included within the narrow limits of the doline bottoms.
Fig. 196. — Map of the dolines of the Karst re-
gion near Divaéa.
188 EARTH FEATURES AND THEIR MEANING
Although to-day so largely a barren waste, we know that the Karst
upon the Adriatic was in remote antiquity a heavily forested re-
gion and that it supplied the myriads of wooden piles upon which
the city of Venice is supported. The vessels which brought to
this port upon the Adriatic its ancient prosperity were built from
wood brought from this tract of modern desert. In the days of
Venetian grandeur the fertile terra rossa formed a veneer upon
the rock surface of the Karst and so retained the surface waters
for the support of the luxuriant forest cover. After deforestation
this veneer of rich soil was washed by the rains into the dolines
or into the few stream courses of the region, thus leaving a barren
tract which it avill be all but impossible to reclaim (plate 6 A).
Upon the steeper slopes
, over the purer limestones,
* . the rain water runs away,
>“ guided by the joints within
. the rock. There is thus
etched out a more or less
complete network of nar-
row channels (Fig. 190,
p. 181), between which the
remnants rise in sharp
__ blades to produce a struc-
Fic. Ao) Seance ae ae aaa in ture often simulated upon
the fissured surface of a
glacier that has been melted in the sun’s rays (Fig. 401). These
almost impassable areas of karst country are described as Schratten
or Karrenfelder (Fig. 198).
The ponore and the polje. — To-day large areas of the Karst
are devoid of surface streams, nearly all the surface water finding
its way down the crevices of the limestone into caverns, and there.
flowing in subterranean courses. The foot traveler in the Karst
country is sometimes suddenly arrested to find a precipice yawn-
ing at his feet, and looking down a well-like opening to the depth
of a hundred feet or more, he may see at the bottom a large river —
which emerges from beneath the one wall to disappear beneath
the other. These well-like shafts are in the Austrian Karst known
as Ponores, while to the southward in Greece they are called
Katavothren.
i i: ae int) = P — —— . . ~
\ = ——s ~~ ~_ De ie —, P tial - —» és — —
" a Pa ee a ee ovine as ea ee Ln
a ee NO I A sa TTD Nal ETA t+ ee LE ee RGR I AR Bed 9 Smee BY, e.g ee Saeki
PLATE 6.
A. Barren Karst landscape near the famous Adelsberg grottoes.
(Photograph by I. D. Scott.)
B. Surface of a limestone ledge where joints bave been widened through solution.
Syracuse, N.Y.
(Photograph by I. D. Scott.)
THE TRAVELS OF THE UNDERGROUND WATER 189
Elsewhere the karst river may emerge from its subterranean
course in a broader depressed area bounded by vertical cliffs, from
which it later disappears beneath the limestone wall. Such de-
pressions of the karst are known as poljen, and appear in most
cases to be above the downthrown blocks in the intricate fault
mosaic of the region. Some of these steeply walled inclosures
have an area of several hundred square miles, and especially at
the time of the spring snow melting they are flooded with water
and so transformed into seasonal lakes (Fig. 199 and p. 422). It
appears that at such times the cave
galleries of the region with their local
narrows are not able to carry off all
the water which is conducted to them ;
and in consequence there is a tempo-
rary impounding of the flood waters in
those portions of the river’s course
which are open to the sky and more
extended. The rush of water at such
times may bring the red clay into the
subterranean channels in sufficient Fic. 199.—The Zirknitz seasonal
quantity to clog the passages. The ee kat chins Sate
Zirknitz Lake usuaily has high water
two or three times a year, and exceptionally the flooding has con-
tinued for a number of years. It has thus in some districts been
necessary to afford relief to the population through the construc-
tion of expensive drainage tunnels.
The conditions which are typified in the Karst area to the east
of the Adriatic Sea are encountered also in many other lands; as,
for example, in the Vorarlberg and Swiss Alps, in Lebanon, and
in Sicily.
The return of the water to the surface.— Water which has de-
scended from the surface and been there held between impervious
layers, may be under the pressure of its own weight or “ head ”;
and will later find its way upward, it may be to the surface or
higher, where a perforation is discovered in its otherwise imper-
vious cover. Such local perforations are produced naturally by
lines of fracture or faulting (widened at their intersections),
and artificially through the sinking of deep wells. The water,
which at ordinary times reaches the surface upon fissures, is usually
190 EARTH FEATURES AND THEIR MEANING
concentrated locally at the intersections of the fracture network,
where it issues in lines of fissure springs (Fig. 200) ; but at the time
of earthquakes the water may rise above the surface in lines of
fountains (p. 83), or occasionally as sheets of water which may
mount some tens of feet into the air.
In contrast to the flow of surface springs, which varies with the
season through wide ranges both in its volume and in temperature
of the water, the volume of
fissure springs is but slightly
affected by the seasonal pre-
cipitation, and the water tem-
perature is maintained rela-
tively constant. Rock is but
a poor heat conductor, and the
seasonal temperature changes
descend a few feet only into the
ground. Thus water which
rises from depths of a few hun-
Qo V6%e 4% dred feet only is apt to be icy
Fic. 200. — Fissure springs arranged upon cold, while from greater depths
lines of rock fracture at intersections, the effect of the earth’s internal
Pomperaug valley, Connecticut. i ; :
heat is apparent in a uniform
but relatively higher temperature of the water. Such “ warm ”
or thermal springs are apt to contain considerable mineral matter
in solution, both because the water is far traveled and because its
higher temperature has considerably increased its solvent properties.
It has long been recognized that lines of junction of different
rock formations at the base of mountain ranges are localities fa-
vorable for the occurrence of thermal springs. These junction
lines are usually within zones where by movement upon fractures
the widest openings in the rock have formed, and the catchment
area of the neighboring mountain highland has supplied head for
the ground water. A map of the hot springs within the Great
Basin of the western United States would present in the main a
map of its principal faults.
Artesian wells. — From the natural fissure spring an artesian
well differs in the artificial character of the perforation of the im-
pervious cover to the water layer. The water of artesian wells
may flow out at the surface under pressure, or it may require
7, ‘- im os.
THE TRAVELS OF THE UNDERGROUND WATER 191
pumping to raise it from some lower level. Ideal conditions are
furnished where the geological structure of the district is that of a
broad basin or syncline. The water which falls in a neighboring
upland is here impounded between two parallel, saucer-like walls
and will flow under its head if the upper wall be perforated at
some low level (Fig. 201, 3).
os ene woe
Pr 4
Fic. 201. — Schematic diagrams to illustrate the different types of artesian wells,
(1) A non-flowing well; (2) flowing wells without basin structure caused by
clogging of the pervious formation; (3) flowing wells in an artesian basin. The
dotted lines are the water levels within the pervious layers (after Chamberlin).
A monoclinal structure may furnish artesian conditions when
the generally pervious layer has become clogged at a low level so
as to hold back the water (Fig. 248, 2). Pumping wells may be
used successfully even when such clogging does not exist, for the
slow-moving underground water flows readily in the direction of
all free outlets (Fig. 201, 1).
Hot springs and geysers. — Thermal springs whose temperature
approaches the boiling point of water are known as hot springs.
A geyser is a hot spring which intermittently ejects a column of
water and steam. Both hot springs and geysers are to be found
only in volcanic regions, and appear to be connected with uncooled
masses of siliceous lava. In two of the three known geyser regions,
Iceland and New Zealand, the volcanoes of the neighborhood are
still active, and the lavas of the Yellowstone National Park date
from the quite recent geological period which immediately pre-
ceded the so-called “ Ice Age.”
Wherever found, geysers are in the low levels along lines of drain-
192 EARTH FEATURES AND THEIR MEANING
age where the underground water would most naturally reappear
at the surface. Their water has penetrated to considerable depths
below the surface, but has been chiefly heated by ascending steam
or other vapors. The water journey has been chiefly made along
fissures, as is shown by the cool springs which often issue near
them. Though some hot springs and geysers may disappear from _
a district, others are found to be forming, and there is no good
reason to think that geysers are rapidly dying out, as was at
one time supposed.
The action of a geyser was first satisfactorily explained by the
great German chemist Bunsen after he had made studies of the
Icelandic geysers, and the mechanics of the eruption was later
strikingly illustrated in the laboratory by an artificial geyser con-
structed by the Irish physicist Tyndall. In many respects this
action is like that of the Strombolian eruption within a cinder
cone, since it is connected with the viscosity of the fluid and the
resistance which this opposes to the liberation of the developing
vapor. In the case of the geyser, a column of heated water stands
within a vertical tube and is heated near the bottom of the column.
Though the water may at its surface have the normal boiling
temperature and be there in quiet ebullition, the boiling point
for all lower levels is raised by the
I) Ce Sy weight of the column of superin-
aes .- cumbent liquid, and so for a time
i; es 10 the formation of steam within the
Observed B= i ;
Temp. Tem. |2o mass is prevented. In Fig. 202
Oe eS: == usc ‘so ig shown a cross section of the
= = =a rah es Icelandic Geysir from which our
ae mae ‘oo [eo name for such phenomena has been
= | fro derived, and to this section have
zd ““"" been added the actual observed
Fic. 202.—Cross section of Geysir, temperatures of the water at the
Iceland, with simultaneously ob- ,.
served temperatures recorded at the different levels as well as the tem-
left, and the boiling temperaturesfor peratures at which boiling can
the same levels at the right (after take place at these levels. From
warped): this it will be seen that at a depth
of 45 feet the water is but 2° Centigrade below its boiling point.
A slight increase of temperature at this level, due to the con-
stantly ascending steam, will not only carry this layer above the
ae ee Fg ee ae ee
ee a i ee a rire
int
a a ea
oe
a"
THE TRAVELS OF THE UNDERGROUND WATER 193
boiling point, but the expansion of the steam within the mass will
elevate the upper layers of the water into zones where the boiling
points are lower, and thus bring about a sudden and violent ebul-
lition of all these upper portions. Thus is explained the almost
universal observation that just before geysers erupt the hot water
rises in the bowls and generally overflows them.
The water ejected from the geyser is considerably cooled in the
air, and after its return to the tube must be again heated by the
ascending vapors before another eruption can
occur. The measure of the cooling, the time
necessary to fill the tube, and the supply of
rising steam, all play a part in fixing the period
which separates consecutive eruptions. If the
top of the tube be narrowed from its average
caliber, as is commonly observed to be true of
the geysers within the Yellowstone National
Park, the escape of the steam is further hin-
dered, and frequent geyser eruption promoted.
An artificial geyser for demonstration of the
phenomenon in the lecture room is represented
in Fig. 203. The cut has been prepared from
a photograph of an apparatus designed by
Professor B. W. Snow of the University of
Wisconsin. In this design the tube is con-
tracted so as to have a top diameter one fourth
only of what’ it is at the bottom, where heat is
directly applied by multiple Bunsen lamps.
The water once sufficiently heated, this arti-
ficial geyser erupts at regular intervals of time -
which are dependent upon the dimensions of Fic. 203.—Apparatus
the apparatus and the quantity of heat applied. J ee
In case of natural geysers a considerable ture room (by cour-
quantity of heat escapes between eruptions in __ tesy of Professor B.
steam which issues quietly from the bowl of |: 52°”:
the geyser. If this heat be retained by plugging the mouth
of the tube with a barrowful of turf, as is sometimes done
with the geyser Strokr in Iceland, eruption is promoted and so
takes place earlier. Another method of securing the same result
is to increase the viscosity of the water through the addition of.
Oo
194 EARTH FEATURES AND THEIR MEANING
soap, as was accidentally discovered by a Chinaman who was uti-
lizing the geyser water in the Yellowstone Park for laundry opera-
tions. After this discovery it became a common custom to
“soap” the Yellowstone geysers in order to make them play; ©
but this method was prohibited under heavy penalty after the dis-
astrous eruption of the Excelsior Geyser.
The deposition of siliceous sinter by plant growth. — Geysers
are known only from areas of siliceous volcanic lava, and this may
perhaps have its cause in the easier
solution of the geyser tube from
UO ae ise {| such materials. The silica dis-
Seki --| solved in the heated waters is
fee - | again deposited at the surface to
form siliceous sinter or geyserite.
This material forms terraces sur-
rounding the geysers or is built up
into mounds which are often quite
symmetrical, such as those of the
a AA “| Bee Hive and Lone Star geysers
oe “lof the Yellowstone Park (Fig. 204).
Fic. 204.— Cone of siliceous sinter The greater part of this sepa-
built up about the mouth of the : “7:
Lone Gis “Gopedet ah Valowes TOWN of silica from the heated
stone National Park. geyser waters is due to the action
of plants or alge that are: able
to grow in the boiling waters and which produce the beautiful |
colors in the linings to the hot springs. The wonderful variety
of the tints displayed is accounted for by the fact that the alge —
take on different colors at different temperatures. The silica
is deposited from the water in the gelatinous hydrated form, which,
however, dries in the sun to a white sand. The growth within the
pools goes: on in a manner similar to that of a coral reef, the alge
dying below and there becoming encased in the rock lining while
still continuing to grow upon the surface. Whereas sinter of this
nature, when deposited by evaporation alone, can produce a maxi-
mum thickness of layer of a twentieth of an inch each year, the
growth from alga deposition within limited areas may be as much
as eight inches during the same period.
|
a
if
Ms
i
4 ;
a
i,
i
THE TRAVELS OF THE UNDERGROUND WATER 195
READING REFERENCES FOR CHAPTER XIV
General : —
F. H. Kine. Principles and Conditions of the Movements of Ground
Water, 19th Ann. Rept. U.S. Geol. Surv., 1899, Pt. ii, pp. 59-294,
pls. 6-16.
C.S. Sticuter. The Motions of the Underground Waters, Water Supply
Paper No. 67, U.S. Geol. Surv., 1902, pp. 1-106, pls. 1-8; Field
Measurements of the Rate of Movement of Underground Waters,
ibid., No. 140, 1905, pp. 1-122, pls. 1-15.
M. L. Futter. Occurrence of Underground Water, ibid., No. 114, 1905,
pp. 18-40, pls. 4; Bibliographic review and index of papers relating
to underground waters published by the United States Geological
Survey, 1879-1904, ibid., No. 120, 1905, pp. 1-128.
Caverns : —
EH. A. Marrtex. Les abimes, les eaux souterraines, les cavernes, les
sources, la spéleologie. Delagrave, Paris, pp. 578. (Lavishly illus-
trated.)
H.C. Hovey. Celebrated American Caverns. Cincinnati, 1896, pp. 228 ;
The Mammoth Cave of Kentucky. Louisville, 1897, pp. 111.
J. W. Breve. Cycle of Subterranean Drainage in the Bloomington
Quadrangle, Proc. Ind. Acad. Sci., 1910, pp. 1-31.
- Karst conditions : —
J. Cvisic. Das Karstphinomen, Geogr. Abh., vol. 5, 1893.
Emite Cuarx. La topographie du desert de platé (Hautes Savoie), Le
Globe, vol. 34, 1895, pp. 1-44, pls. 1-16, pp. 217-330.
W. v. Knesext. Hoéhlenkunde mit Beriicksichtigung der Karstphino-
mene. Vieweg, Braunschweig, 1906, pp. 222.
A. Grunp. Die Karsthydrographie, Studien aus Westbosnien, Geogr.
Abh., vol. 7, No. 3, 1903, pp. 200.
Euite Cuarx—pu ‘Bors et Anpri Cuarx. Contribution a l’étude des
lapies en Carniole et au Steinernes Meer, Le Globe, vol. 46, 1907,
pp. 17-56, pls. 26.
P. Arspenz. Die Karrenbildungen geschildert am Beispiele der Karren-
felder bei der Frutt in Kanton Obwalden (Schweiz). Deutsch. Alpen-
zeitung, Munich, 1909, pp. 1-9.
F. Karzer. Karst und Karsthydrographie. Sarejevo, 1909, pp. 95.
M. Neumayr. Erdgeschichte, vol. 1, pp. 500-510.
EK. p—e Martonne. ‘Traité de Géographie Physique, pp. 462-472 (excel-
lent summaries in this and the last reference).
E. A. Marte. The Land of the Causses, Appalachia, vol. 7, 1893, pp.
on 18-149, pls. 4-13.
4% Fissure springs : —
_ -~A. ©. Pzatz. Natural Mineral Waters of the United States, 14th Ann.
Rept. U. S. Geol. Surv., Pt. ii, 1894, pp. 49-88.
ae
oo
em -
=e
wie
te Ee Oe ee, |) ee
Ba eg a EELS
ee
196 EARTH FEATURES AND THEIR MEANING
Wi.t1am H. Hosss. The Newark System of the Pomperaug Valley.
Connecticut, 21st Ann. Rept. U.S. Geol. Surv., Pt. iii, 1901, pp. 91-93.
Artesian wells : —
T. C. CHAMBERLIN. Requisite and Qualifying Conditions of Artesian.
Wells, 5th Ann. Rept. U. S. Geol. Surv., 1885, pp. 131-173.
Hot springs and geysers : —
A. C. Prate. Yellowstone Park, Thermal Springs, 12th Ann. Rept..
Geol. and Geogr. Surv. Ter. (Hayden), Pt. ii, See. ii, pp. 63-454.
(many plates and maps).
W.H. Weep. Geysers, Rept. Smithson. Inst., 1891, pp. 163-178.
ArNoLpD Hacue and W. H. Weep (on hot springs and geysers of Yellow-.
stone National Park), C. R. Cong. Géol. Intern., Washington, 1891,
pp. 346-363.
W. H. Weep. Formation of Travertine and Siliceous Sinter by the.
Vegetation of Hot Springs, 9th Ann. Rept. U.S. Geol. Surv., 1889,
pp. 613-676, pls. 78-87.
M. Nevumayrr. Erdgeschichte, vol. 1, pp. 500-510.
Arnotp Hacur. Soaping Geysers, Trans. Am. Inst. Min. Eng., vol. 17,.
1889, pp. 546-553.
Joon TynpDALL. Heat as a Mode of Motion, New York, 1873, pp. 115--
121 (artificial geyser).
re
CHAPTER XV
SUN AND WIND IN THE LANDS OF INFREQUENT
RAINS
The law of the desert. — It is well to keep ever in mind that
there is no universal law which dominates Nature’s processes in
all the sections of her realm. Those changes which, because often
observed, are most familiar, may not be of general application,
for the reason that the areas habitually occupied by highly civi-
lized races together comprise but a small portion of the earth’s
surface. In the dank tropical jungle, upon the vast arid sand
plains, and in the cold white spaces near the poles, Nature has
instituted peculiar and widely different processes.
The fundamental condition of the desert is aridity, and this
necessitates an exclusion from it of all save the exceptional rain
cloud. Thus deserts are walled in by mountain ranges which
serve as barriers to intercept the moisture-bringing clouds. They
are in consequence saucer-shaped depressions, often with short
mountain ranges rising out of the bottoms, and such rain as falls
within the inclosure is largely upon the borders. Of this rainfall
none flows out from the desert, for the water is largely returned
to the atmosphere through evaporation.
_ The desert history is thus begun in isolation from the sea from
which the cloud moisture is derived, a balance being struck be-
tween inflow and evaporation. Yet if deserts have no outlets,
it is not true that they have no rivers. These are occasionally
permanent, often periodic, but generally ephemeral and violent.
The characteristic drainage of deserts comes as the immediate
~ result of sudden cloudburst. As a consequence, the desert stream
flows from the mountain wall choked with sediment, and entering
the depressed basin, is for the most part either sucked down into the
floor or evaporated and. returned to the atmosphere. The dis-
solved material which was carried in the water is eventually left
197
198 EARTH FEATURES AND THEIR MEANING
in saline deposits, and the great burden of sediment accumulates
in thick stratified masses which in magnitude outstrip the largest
deltas in the ocean. |
The self-registering gauge of past climates. — From the ini-
tiation of the desert in its isolation from the lands tributary to the
sea, its history becomes an individual and independent one. An
increasing quantity of rainfall will be marked by larger inflow to
the basin, and the lakes which form in its lowest depression will,
as a consequence, rise and expand over larger areas. A contrary
climatic change will bring about a lowering of the lakes and leave
behind the marks of former shorelines above the water level (Fig.
205). Deserts are thus in a sense self-registering climatic gauges
whose records go back far beyond the historic past. From them
S <A
SY —s ~~ .. oe : —& —S 3 >
WSS a LTE &b = . SS SOT Ss
: 4 LANs were
Fig. 205. — Former shore lines on the mountain wall surrounding the desert of the
Great Basin. View from the temple in Salt Lake City (after Gilbert).
it is learned that there have been alternating periods of larger and
smaller precipitation, which are referred to as pluvial and inter-
plural periods.
From such records it is learned that the Great Basin of the
western United States was at one time occupied by two great desert
lakes, the one in the eastern portion being known as Lake Bonne- .
ville (Fig. 206). With the desiccation which followed upon the
series of pluvial periods, which in other latitudes resulted in great
continental glaciers and has become known as the Glacial Period,
this former desert lake dried up to the limits of Great Salt Lake and
a few smaller isolated basins. Between 1850 and 1869 the waters
of Great Salt Lake were rising, while from 1876 to 1890 their level
was falling, though subject to periodic fluctuations, and in recent
years the waters of the lake have risen so high as to pass all records
since the occupation of the country. As a consequence the so- ’ f |
called Salt Lake “ cut-off”? of the Union Pacific Railway, con- Mf
structed at great expense across a shallow portion of the lake, has
SUN AND WIND IN LANDS OF INFREQUENT RAINS 199
been overflowed by its waters. The Sawa Lake in the Persian
Desert, which disappeared some five hundred years ago, again
came into existence in 1888 so as to cover the caravan route to
Teheran.
The record in the rocks of the distant past reveals the fact that
in some former deserts barriers were, in the course of time, broken
down, with the result that an invading
sea entered through the breached wall.
The result was the sudden destruction
of land life, the remains of which are
preserved in ‘‘ bone beds,’”’ now covered
by true marine deposits. A still later
episode of the history was begun when
the sea had disappeared and land ani-
mals again roamed above the earlier
desert. Such an alternation of marine
deposits with the remains of land plants
and animals in the. deposits of the Paris
Basin, led the great Cuvier to his belief
that geologic history was comprised of
a succession of cataclysms in which life
was alternately destroyed and re-created
in new forms — a view which later, under ger os
the powerful influence of Lyell and ey Mites.
Darwin, gave way to that of more |.
gradual changes and the evolution of fie. 206.— Map of the former
life forms. Lake Bonneville (dotted
shores), and the boundaries
80
——— ]
Some characteristics of the desert
wastes. — The great stretches of the
arid lands have been often compared to
the ocean, and the Bedouin’s camel is
of the Great Salt Lake of
1869 (smaller area) and that
of the present (after Berg-
haus).
known as “‘ the ship of the desert.” Though a deceptive resem-
blance for the most part, the comparison is not without its value.
Both are closed basins, and it is in this respect that the desert and
the ocean may be said to most resemble each other, for none of
the water and none of the sediment is lost to either except as
boundaries are, with the progress of time, transposed or destroyed.
Flatness of surface and monotony of scenery both have in common,
and the waters and the sand are in each case salt ; yet the ocean,
200 EARTH FEATURES AND THEIR MEANING
from the tropics to the poles, has the same salts in essentially the
same proportions, while in the desert the widest variations are
found both in the salts which are present and in their relative
quantities.
Upon the borders of the ocean are found ridges of yellow sand
heaped up by the wind, but these ramparts are small in comparison
to those which in deserts are found upon the borders (plate 7 A).
The desert is a land of geographic paradoxes. As Walther has
pointed out, we have rain in the desert which does not wet, springs
which yield no brooks, rivers without mouths, forests preserved
in stone, lakes without outlets, valleys without streams, lake basins
without lakes, depressions below the level of the sea yet barren
of water, intense weathering with no mantle of disintegrated rock,
a decomposition of the rocks from within instead of from without,
and valleys which branch sometimes upstream and sometimes
down.
Within the deserts curious mushroom-like remnants of erosion
afford a local relief from the searching rays of the desert sun.
Pocket-like openings large enough for a hermit’s habitation are
hollowed out by the wind from the disintegrated rock masses.
Amphitheaters open out from little erosion valleys or wadi, and
isolated outliers of the mountains stand like sentinels before their
massive fronts. , |
Because of the general absence of clouds above a desert, no
shield such as is common in humid regions is provided against the
blinding intensity of the sun’s rays. Sun temperatures as high
as 180° Fahrenheit have been registered over the deserts of
western Africa. Every one is familiar with the fact that a
blanket of thick clouds is a prevention of frosts at night, for, with
the setting of the sun and the consequent radiation of heat from
the earth, these rays are intercepted by the clouds, returned and
re-returned in many successive exchanges. Over desert regions
the absence of any such blanket of moisture is responsible for the a
remarkable falls of temperature at sunset. Though shortly before
temperatures of 100° Fahrenheit or greater may have been
measured, it is not uncommon for water to freeze during the
following night. Much the same conditions of sudden tempera-
ture change with nightfall are experienced in high mountains when
one has ascended above the blanketing clouds.
SUN AND WIND IN LANDS OF INFREQUENT RAINS 201
Dry weathering — the red and brown desert varnish. — In
desert lands the fierce rays of the sun suck up all the available
moisture, and the water table may be hundreds of feet below the
surface. Roots of trees a hundred feet or more in length have
been found to testify to the
fierce struggle of the desert
plant with the arid conditions.
In humid regions the meteoric
water dissolves the more
soluble sodium salts near the
surface of the rock and carries
them out to the ocean, where se EN 2+
‘ they add to the saltness of the Fie. 207.— Borax deposits upon the floor
sea. In the desert the rare of Death valley, California (after a pho-
be ; tograph by Fairbanks).
precipitations prevent an out-
flow, but the sun’s strong rays suck out with the moisture the
salts from within the rock, and evaporating upon the surface, the
salts are left as a coat of ‘ alkali,”’ which is in part carried away on
the wind and in part washed off in one of the rare cloudbursts.
In either case these constituents find their way to the lowest de-
pressions of the basin,
where they contribute
to the saline deposits
a of the desert lakes (Fig.
MAW'"N SS ws Certain of the saline
wy” 4s S \\ constituents of the
rocks, as they are thus
drawn out by the sun’s
rays, fuse with the rock
at the surface to form a
WR dense brown substance
Fic. 208.— Hollowed forms of weathered granite in with smooth surface
a desert of central Asia (after Walther).
ES a t \\ \ \ hi
‘ \ \\ ANN i \ \\\
\ itil NYA
\ \\ H Hiss Day
SRA
coat, known as desert
varnish. Within the interior a portion of the salts crystallize
within the capillary fissures, and like water freezing within a pipe,
they rend the walls apart. As a direct consequence of this
disintegrating process the interior of rock masses may crumble
into sand; and if the hard shell of varnish be broken at any
202 EARTH FEATURES AND THEIR MEANING
point, the wind makes its entrance and removes the interior por-
tion so as to leave a hollow shell — the characteristic ‘‘ pocket
rock” (Fig. 208) of the desert. The nummulitic limestone of
Mokkatan and many of the great hewn blocks of Egyptian lime- |
stone sound hollow under the tap of the hammer, and when
broken, they reveal a shell a few inches only in thickness (Fig.
209).
The brown desert varnish is one of the most characteristic
marks of an arid country. It is found in all deserts under much
the same conditions, and
is especially apt to be pres-
ent in sandstone. When
scratched, the surface of
the rock becomes either
cherry-red, indicating an-
nace Verano. hydrous ferric oxide, or it
ee is yellowish, due to the
aad
ey
Fia. 209. — Hollow hee faain in a wall in the pee saa OnE vo
Wadi Guerraui (after Walther). we know as iron rust.
Thus it is seen that the
sands of deserts, in contrast to those yielded by other processes
within humid regions, have a characteristic red color, and this
may vary from brownish red upon the one hand to a rich carmine
upon the other.
The mechanical breakdown of the desert rocks. — The chemi-
cal changes of decomposition within desert rocks are, as we have —
seen, largely due to the action of concentrated solutions of salts
at high temperatures. That there is a certain mechanical rending
of these rocks, due to the “ freezing ”’ of salts within the capil-
lary fissures, has been already mentioned. A further strain
effect arises in rocks like granite, which are a mixture of different
minerals. Heated to a high temperature during the day and
cooled through a considerable range at night, the different minerals
alternately expand and contract at different rates and by dif-
ferent relative amounts, so that strains are set up, tending to
‘tear them apart. The effect of these strains is thus a surface
crumbling of rocks. ‘
But rock is, as already pointed out, a relatively poor conductor
of heat, and hence it is a relatively thin skin only which passes
RE pum sess ves
Pe vio me -
+e Ay amN. , ; AK Pen
»
owes
on
hate ae
— —_ oa . eee
a rian’ oa i wae nandias Gas f: Guin) -* 6 Se ee
= a ee pans —s
se — ‘
te ie
ey
SUN AND WIND IN LANDS OF INFREQUENT RAINS 203
through the daily round of wide temperature range. This outer
shell when heated is expanded, and so tends to peel off, or ex-
foliate, like the outer skin of an onion. The process is therefore
described as exfoliation. In all rocks of homogeneous texture the
continued action of this process results in convexly spherical sur-
faces, the material scaled off in the process remaining as a slope
or talus which surrounds the projecting knob (Fig. 210). Naked,
—
=
3 Fic. 210.— Smooth granite domes shaped by exfoliation and surrounded by a rim of.
talus. Gebel Karsala, Nubian Desert (after Walther).
these projecting domes rise above the rim of débris at their bases.
Not a particle of dust adheres to the fresh rock surface — no
dirt interferes with its glaring whiteness. Yet close at hand lie
masses of débris into which wells may be carried to depths of
more than six hundred feet without encountering either solid
rock or ground water. The bare walls of granite sometimes mount
upwards for thousands of feet into the air, as steep and as inac-
cessible as the squared towers of the Tyrolean Dolomites.
Rock is such a poor conductor of heat that special strains are
set up at the margin of sunlight and shade. This localization of
the disintegration on the margin of the shaded portions of rock
masses is known as shadow weathering (see Fig. 215, p. 206).
There is, however, still another mechanical disintegrating
process characteristic of the desert regions, which is likewise
dependent upon the sudden changes of temperature. Rains,
204 EARTH FEATURES AND THEIR MEANING
though they may not occur for a year or more, come as sudden
downpours of great volume and violence. Rock masses, which
are highly heated beneath the desert sun, if suddenly dashed
with water, may be rent apart by the differential strains set up.
near the surface. That rocks may be easily rent as a result of
sudden chilling is well known to our Northern farmers, who are
accustomed to rid themselves of objectionable bowlders by first
building a fire about them and then dashing water upon their
surface. Thus split into fragments, even the larger bowlders
may be handled and so removed from the farming land. The
natural process of rock rending by the occasional cloudburst may
be described as diffission. Blocks as much as twenty-five feet in
diameter have been observed
in the desert of western
Texas, soon after being —
broken into several frag-
ments at the time of a down-
pour of rain (Fig. 211).
The natural sand blast. —
Because of the saucer-like
shape, the vast expanse, and
Fia. 211.— Granite blocks in the Sierrade the absence of wind breaks,
los Dolores of Texas, rent into several the potency of wind as a
fragments by the dash of rain (after ‘ sen
Walihad. geological agent is in desert
areas not easily overesti-
mated. While most of its work is accomplished with the aid of
tools, it has been proven that even without this help, considerable
work is done through the friction of the wind alone, particularly .
when moving as powerful eddies in cracks and crannies. This
wear of the wind, unaided by cutting tools, is known as deflation.
The greater work of the wind is, however, accomplished with
the aid of larger or smaller rock particles, the sand and dust,
with which it is so generally charged above the deserts. Un-
protected by any mat of vegetation the materials of the desert
surface are easily lifted and are constantly migrating with the
wind. The finest dust is raised high into the air, and is carried
beyond the marginal barriers, but none of the sand or~coarser _
materials ever passes beyond the borders. ‘
The efficiency of this sand as a cutting tool when carried by the
SUN AND WIND IN LANDS OF INFREQUENT RAINS 205
wind is directly proportioned to the size of the grain, since with
larger fragments a heavier blow is struck when carried at any
given velocity. These more effective grains are, however, not lifted
far above the ground, but advance with a squirming or hopping
motion, much as do the larger pebbles upon the bottom of a river
at the time of a spring freshet. To quote Professor Walther:
‘“ Whoever has had the oppor-
tunity to travel over a surface
of dune sand when a strong wind
is blowing has found it easy to
convince himself of the grinding
action of the wind. At such
times the ground becomes alive,
everywhere the sand is creeping Fic. 212.— ‘Mushroom rock” from a
over the surface with snake-like desert in Wyoming (after Fairbanks).
squirmings, and the eye quickly
tires of these writhing movements of the currents of sand and
cannot long endure the scene.”
A. direct consequence of this restriction of the more effective
cutting tools to the layer of air just above the ground, is the
strong tendency to cut away all projecting masses near their
bases. The “‘ mushroom rocks,” which are so characteristic of
desert landscapes, have been shaped in this manner (Fig. 212).
| Another product of the desert
KE B sand blast isthe so-called Wind-
| GAS kante (wind-edge) or Dreikante
(three-edge), a pebble which is
usually shaped in the form of
a pyramid (Fig. 213).
SS Whenever a rock face, open
RR a to direct attack by the drifting
Fic. 213. — Windkanten shaped by the sand, is constituted of parts
desert sand blast (after Chamberlin and which have different hardness
Salsbury). :
the blast of sand pecks away
at the softer places and leaves the harder ones in relief. Thus is
produced the well-known “stone lattice’ of the desert (Fig. 214).
Particularly upon the neck of the great Sphinx have the flying
sand grains, by removing the softer layers, brought the sedi-
mentary structures of the sandstone into strong relief.
206 EARTH FEATURES AND THEIR MEANING
When guided both by planes of sedimentation and planes of joint-
ing, forms of a very high degree of ornamentation are developed.
Some of the most remarkable forms are due to the protection af-
forded to the sun-exposed surfaces by the shell of desert varnish.
In the shaded portions of projecting masses there is no such pro-
tection, and here the sand blast insinuates itself into every crack
and cranny. In this it is aided
by shadow weathering due to
the differential strains set up at
the border of the expanded sun-
heated surface. As a result,
projecting rock masses are some-
times etched away beneath and
give the effect of a squatting
animal. These forms, due to
shes
Fic. 214.—The “stone lattice” of the shadow erosion, have also been
desert, the work of the natural sand likened to projecting faucets.
blast (after Walther). (Fig 91 5) '
Worn by its impact upon neighboring sand grains while in trans-
port, but much more as it is thrown against the ground or hard
rock surfaces, the wind-driven or eolian sand is at last worn into
smoothly rounded granules which approach the form of a sphere.
Compared to the sur-
face which sea sand
acquires by attrition, ” oR
this shaping process is Pi it’ GSN
much the more effi-
cient, since in the
water the beach sand
is buoyed up and is
more effectively cush-
ionedagainstits neigh- Fic. 215.— Projecting rock carved by the drifting
boring erains. The sand into the form of a couchant animal as a result
3 of shadow weathering and erosion. Cut in granite
grains of beach sand on the north Indian Desert (after Walther).
when examined under
a microscope are found to be much more irregular in form and
usually display the original fracture surfaces only in part abraded. |
The dust carried out of the desert. — When, standing upon the>
mountain wall that surrounds a desert, the traveler gazes out to
=
SUN AND WIND IN LANDS OF INFREQUENT RAINS 207
ae
windward over the great depression, his field of view is generally
obscured by the yellow haze of the dust clouds moving across
the margins. Upon the mountain
flanks and extending far outside
the borders, this cloud of dust
i settles as a shrouding mantle of
impalpable yellow powder, which
is known as loess. These deposits
are continually deepening, and
have sometimes accumulated until
they are hundreds or even thou-
sands of feetin thickness. Before
i reaching its final resting place the
i dust of this deposit may have
settled many times, and has cer- ed
; tainly been in part redistributed fxg, 216.— Cliffs in loess 200 feet in
by the streams near the desert height which exhibit the characteris-
margin. In it are the ingredients tic vertical jointing (after von Rich-
y tofen).
which are necessary for the nour-
ishment of plants, and it constitutes the most important of natural
soils. Continually fed by new deposits from
the desert, and refertilized from below by a
natural process so soon as the upper layers
become impoverished, it requires no artificial
fertilization. Without artificial aids the loess
of northern China has been tilled for thousands .
of years without any signs of exhaustion.
Though easily pulverized between the fingers,
loess is none the less characterized by a perfect
vertical jointing and stands on vertical faces
as does the solid rock (Fig. 216), but it is ab-
Be solutely devoid of layers or bedding. Its ca-
Fig. 217.— A cafion ; : , ; ‘
Re ts wary to pacity of standing in vertical cliffs the loess owes
trafficandwind. A to a never failing content of lime carbonate
highway in north- which acts as a cement, and to a peculiar porous
ern China (after ;
von Hishtaten). structure caused by capillary canals that run
vertically through the mass, branching like
rootlets and lined with carbonate of lime. This texture once
destroyed, loess resolves itself into a common sticky clay.
RON Sarr pe pean eee
208 EARTH FEATURES AND THEIR MEANING
By the feet of passing animals or by wheels of vehicles, the loess
is crushed, and a portion is lifted and carried away by the wind.
Thus in the course of time roadways sink deep into the mass as
steep-walled cafions (Fig. 217). A portion of the now structure-
less clay remaining upon the roadway is at the time of the rains
transformed into a thick mud which makes traveling all but
impossible, though before its structure has been destroyed the
loess is perfectly drained to the bottom of its deposits.
The particles which compose the loess are sharply angular
quartz fragments, so fine that all but a few grains can be rubbed
into the pores of the skin. Fine scales of mica, such as are easily
lifted by the wind, are disseminated uniformly throughout the
mass. The only inclosures which are arranged in layers consist
of irregularly shaped concretions of clay. These show a striking
resemblance to ginger roots and are called by the Chinese “stone
ginger,” though they are elsewhere more generally known by
their German name of Loessmdnnchen, or loess dolls. These
concretions are so disposed in the loess that their longer axes are
vertical, and they were evidently separated from the mass and
not deposited with it.
CHAPTER XVI
THE FEATURES IN DESERT LANDSCAPES
The wandering dunes. — Over the broad expanse of the desert,
sand and dust, and occasionally gypsum from the saline deposits,
are ever migrating with the wind; on quiet days in the eddying
“sand devils,” but especially during the terrifying sand storms
such as in the windy season darken the air of northern China and
southern Manchuria. This drift of the sand is halted only when
an obstruction is encountered — a projecting rock, a bush, or a
bunch of grass, or again the buildings of a city ora town. The
manner in which the sand is ar- aoe on
rested by obstacles of different ee oe
kinds is of great interest and im-
portance, and is utilized in raising
defenses against its encroachments.
If the obstacle is unyielding but
allows some of the wind to pass
through it, no eddies are produced
and the sand is deposited both to
windward and to leeward of the
obstruction to form a fairly sym-
metrical mound (Fig. 218 a). An Fia. 218. — Diagrams to illustrate the
obstruction which yields to the effects of obstructions of different
wind causes the sand to deposit types in arresting wind-driven sand.
J ‘ ; a, An unyielding obstruction which
in a mound which is largely to permits the wind to pass through it ;
leeward of the obstruction (Fig. 6, a flexible and perforated obstruc-
218.5). A solid wall, on the other tion; c, an unyielding closed barrier
hand, by inducing eddies, is at eee
_ first protected from the sand and mounds deposit both to wind-
ward and to leeward (Fig. 218 c and Fig. 219).
Except when held up by an obstruction, the drifting sand travels
to leeward in slowly migrating mounds or ridges which are known
as dunes. Their motion is due to the wind lifting the sand from
P 209
210 EARTH FEATURES AND THEIR MEANING
the windward side and-carrying it over the crest, from where it
slides down the leeward slope and assumes a surface which is
the angle of repose of the material. In contrast with this the
windward slope is
notably gradual, be-
ing shaped in con-—
formity to the wind
currents.
The dunes which
are raised upon sea-
shores, like those of
the desert, are con-
Fic. 219.—Sand accumulating both to windward and stantly migrating,
to leeward of a firm and impenetrable obstruction. those upon the shores
The wind comes from the left (after a photograph f the. North & t
by Bastin). 0 € NOr ca a
the average rate of
about twenty feet per year. Relentlessly they advance, and de-
spite all attempts to halt them, have many times overwhelmed
the villages along the coast. Upon the great barrier beach known
as the Kurische Nehrung, on the southeastern shore of the Baltic
Sea, such a burial of villages has more than once occurred, but
as in the course of time further migration of the dune has pro-
ceeded, the ruins of the buried villages have been exhumed by
this natural excavating process (Fig. 220).
—— £2
7800 ft
7839 a
_ he STITT ere Ta Pee re PR
7869 a>
Scale of Miles. s
Fia. 220.— Successive diagrams to show how the town of Kunzen was buried, and
subsequently exhumed in the continued migration of a great dune upon the
Kurische Nehrung (after Behrendt).
The forms of dunes. — The forms assumed by Et are de-
pendent to a very large extent upon the strength of the wind _
and the available supply of sand. With small quantities ef
PLATE 7.
———O or lc OTT
B. Sand dunes encroaching upon the oasis of Wed Souf, Algeria (after T. H.
Kearney).
THE FEATURES IN DESERT LANDSCAPES 211
sand and with moderate winds, sickle-shaped dunes known as
barchans (Fig. 221) are formed, whose convex and flatter slopes
are toward the wind and whose steep concave leeward slopes are
ie a a
Fig. 221.— View of desert barchans (after Haug).
‘ maintained at the angle of repose. The barchan is shaped by the
wind going both over and around the dune, constantly removing
sand from the windward side and depositing it to leeward. With
larger supplies of sand and winds which are not too violent a
series of barchans is built up, and these are arranged transversely
to the wind direction (Fig. 2226). If the winds are more violent,
the minor depressions in the crests of the dunes become wind
channels, and the sand is then trailed out along them until the
arrangement of the ridges is parallel to the wind (Fig. 222 c).
The surfaces of dunes are
generally marked by beau-
tiful ripples in the sand,
which, seen from a little
distance, may give the ap-
pearance of watered silk
(plate 7 A). 7
Under normal condi- Fia. 222.— Diagrams to show the relationships
tions dunes are not sta- in form and in orientation of dunes to the sup-
tionary but continue to ply of sand and to the strength of the wind.
; E a, barchans formed by small supplies of sand
wander with the prevail- and moderate winds; b, transverse dune ridges,
ing winds until they have formed when supply of sand is large and winds
reached the outer edge of are moderate; c, dune ridges formed with large
end of vegeta pe oe anor ai violent winds (after Walther
near the base of the foot- ; |
hills at the margin of the desert. Here the grasses and other
desert plants arrest the first sand grains that reach them, and
they continue to grow higher as the sands accumulate. Some of’
. = — ST ee areas A ad - .
= + Oa rer eet ae _ =
> - FT _
nals coe ae
ee
a es
Fie » laa te
Seer “f
iter ts nee A EG
ae LS ~
nips
-) as ater}
-
made
212 EARTH FEATURES AND THEIR MEANING
the desert plants, like the yuccas, have so adapted themselves to
desert conditions that they may grow upward with the sand for
many feet and so keep their crowns above its surface.
- The cloudburst in the desert.— Such clouds as enter the
desert through its mountain ramparts, and those derived from
evaporation from the hot desert soil, usually precipitate their
moisture before passing out of the basin. Above the highly
heated floor the heavy rain clouds are unable to drop their bur-
den. The rain can sometimes be seen descending, but long
before it has reached the ground it has again passed into vapor,
Fig. 223.—Ideal section across the rising mountain wall surrounding a desert
and a part of the neighboring slope (after R. W. Pumpelly).
and through repetition of this process the clouds become so charged
with moisture that when they encounter a mountain wall and
are thus forced to rise, there is a sudden downpour not equaled
in the humid regions. Desert rains are rare, but violent beyond
comparison. Often for a year or more there is no rainfall upon
the loose sand or porous clay, and the few plants which survive
must push their roots deep down until they have reached the
zone of ground water. When the clouds burst, each small cafion
or wed (pl. wad) within the mountain wall is quickly occupied
by a swollen current which carries a thick paste of sediment and
drowns everything before it. Ere it has flowed a mile, it may
be that the water has disappeared entirely, leaving a layer of mud.
and sand which rapidly dries out with the reappearance of the sun.
As the mountains upon seacoasts are generally rising with
reference to the neighboring sea bottom, so the mountains which |
hem in the deserts are generally growing upward with reference
to the inclosed desert floor. The marginal dislocations which
separate the two are often in evidence at the foot of the steep
slope (Fig. 223), and these may even appear as visible earth-
— ——— ——— =
Pe ae ee Ss eee ee ee ee pee PRS
: ae 4 5
THE FEATURES IN DESERT LANDSCAPES 213
quake faults to indicate that the uplift is more accelerated than
the deposition along the mountain front.
The zone of the dwindling river. — The rapid uplift so generally
characteristic of desert margins gives to the torrential streams
which develop after each cloud-
burst such an unusual velocity
that when they emerge from the
mountain valleys on to the desert
floor, the current is suddenly
checked and the burden of sedi-
ment in large part deposited at
the mouth of the valley so as to
form a coarse delta deposit which
is described as a dry delta (Fig. 224). Dependent upon its steep-
ness of slope, this delta is variously referred to as an alluvial fan
or apron, or as an alluvial cone. Over the conical slopes of the
delta surface the stream is broken up into numerous distributaries
which divide and subdivide as do the roots of a tree. In the
Mohammedan countries described as wadi, these distributaries
the foot of a mountain range upon
the borders of a desert.
9 Seale of Miles. 25 oe \
Fie. 225.— Map of the distributaries of neighboring streams which emerge at the
western base of the Sierra Nevadas in California (after W. D. Johnson).
upon dry deltas are on the Pacific coast of the United States
referred to as ‘‘ washes ” (Fig. 225). :
Fast losing their velocity after emerging from the mountains,
the various distributaries drop first of all the heavy bowlders,
214 EARTH FEATURES AND THEIR MEANING
then the large pebbles and the sand, so that only the finer sand.
and the silt are carried to the margin of the delta. As they
enlarge their boundaries, the neighboring deltas eventually
coalesce and so form an alluvial bench or “ gravel piedmont ”’ at:
the foot of the range. Only the larger streams are able to entirely
cross this bench of parched deposits with its coarsely porous
structure, for the water is soon sucked up by the thirsty ma-
terials. Encountering in its descent more clayey layers, this water
is conducted to the surface near the margin of the bench and
may there appear as a line of springs. At this level there develops,
therefore, a zone of vegetation, though there is no local rain.
The alluvial bench grows upward by accretion of layers which
are thickest at the mountain end, so that the steepness of the
bench increases with time.
Erosion in and about the desert. — The violent cloudburst that.
is characteristic of the arid lands is a most potent agent in model-
ing the surface of the ground wherever the rock materials are
not too firmly coherent. Under the:dash of the rain a peculiar
type of “ badland”’ topography is developed (plate 5B and Fig. 226).
Such a rain-cut surface is a veritable maze
of alternating gully and ridge, a country
worthless for agricultural purposes and offer-
ing the greatest difficulty in the way of pene-
trating it. When composed of stiff clay with
scattered pebbles and bowlders, the effect of
the “rain erosion’ is to fashion steep clay
pillars each capped by a pebble and described
as “ demoiselles ” (Fig. 226). |
Behind the mountain front the valleys out
of which the torrents are discharged are usu-
re tae arene ally short with steep side walls and a rela-
“demoiselles” in the tively flat bottom, ending headward in an |
“bad lands” (after a amphitheater with precipitous walls (Fig.
oy aph by Fair- 997). In the western United States such
; valleys are referred to as ‘“ box cafions,” but
in Mohammedan countries the name “wed” applies to the river
valley within the mountains and to the distributaries as well.
Characteristic features of the arid lands. — It is characteristic
of erosion and deposition within humid regions that all outlines a
Oe se ee
_
z
et.
ee
~~ Pepi s<
gee npn Rene eat
Sy am
PPE toe
ors
Sa
Toke ot
rn eT NE Ee
— = ba es earina
THE FEATURES IN DESERT LANDSCAPES 215
become softened into flowing curves, due to the protective mat
of vegetation. In arid lands those massive rocks which are
without structural planes of separation, partly as a consequence
of exfoliation, develop broad domes which are projected upon
the horizon as great semicircles,
broken in half it may be by
displacement. The same massive
rocks where intersected by vertical
joint planes yield, on the contrary,
sharp granite needles like those of
Harney Peak (plate 8 A). Similarly,
schistose or bedded rocks, when tilted
at a high angle, may yield forms
which are almost identical. Ex-
amples of such needles are found in
the Garden of the Gods in Colorado.
At lower levels, where the flying
sand becomes effective as an erod-
ing agent, flat bedded rocks become ct i uceanealy’ se bg
etched into shelves and cornices, and —_—walther).
if intersected by joints, the shelves” -
and cornices are transformed into groups of castellated towers and
pinnacles of a high degree of ornamentation. These fantastic
erosion remnants are usually referred to as “chimneys” and may
be seen in numbers in the bad lands of Dakota, as they may in
Colorado either in Monument Park or in the new Monolithic
National Park (plate 8 B).
Where wind erosion plays a smaller réle in the sculpture, but
where after an uplift a river has made its way, horizontally bedded
rocks are apt to be carved into broad rock terraces, nowhere shown
upon so grand a scale as about the Grand Cafion of the Colorado.
Each harder layer has here produced a floor or terrace which
ends in a vertical escarpment, and this is separated from the
next lower layer of more resistant rock by a slope of talus which
largely hides the softer intermediate beds. The great Desert of
Sahara is shaped in a series of rock terraces or steppes which
descend toward the interior of the basin.
A single harder layer of resistant rock comes often to form
the flat capping of a plateau, and is then known as a mesa, or
216 EARTH FEATURES AND THEIR MEANING
table mountain. Along its front, detached outliers usually stand
like sentinels before the larger mass, and according to their rela-
tive proportions, these are referred to either as small mesas or
as the smaller buttes (Fig. 228).
2 oad hey
soonest “ye A
Pes Re eS: a oA the
ih be
Fig. 228. — Mesa and outlying butte in the Leucite Hills of Wyoming (after Whit-
man Cross, U.S. G. S.).
The war of dune and oasis.— In every desert the deposits
are arranged in consecutive belts or zones which are alternately
the work of wind and water. Surrounding the desert and upon
the flanks of the mountain wall there is found (1) the deposit of
loess derived from the dust that is carried out of the desert by
the wind. Immediately within the desert border at the base of
the mountains is (2) the zone of the dwindling river with its
sloping bench of coarse rubble and gravel. Next in order is
(3) the belt of the flying sand, a zone of dune ridges often sepa-
rated by narrow, flat-bottomed basins (Fig. 229) into which the
Fia. 229.—Flat-bottomed basin separating dunes — bajir or takyr (after Ells ‘a
worth Huntington).
_ a £
strongest streams bring the finer sands and silt from the moun-
tains. Lastly, there is (4) the central sink or sinks, into which
all water not at once absorbed within the zone of alluviation or _
in the zone of dunes is finally collected. Here are the true lacus- _ |
PLATE 8.
A. The granite needles of Harney Peak in the Black Hills of South Dakota (after
Darton).
B. Castellated erosion chimneys in El Cobra Cafion, New Mexico.
(Photograph by E. C. Case.)
THE FEATURES IN DESERT LANDSCAPES 217
trine deposits of clay and separated salts (Fig. 230 and Fig. 207,
p. 201). The lake deposits fill in all the original irregularities
of the desert floor, out of which the tops of isolated ranges of
mountains now project like islands out of the surface of the sea.
The several zones of de-
' posits in their order from
the margin to the center
of the desert are given
schematically in Fig. 231.
The zone of vegetation,
as already stated, lies near
the foot of the alluvial
bench, so that here are
found the oases about Fie. 230.—Billowy surface of the salt crust on
a a the central sink in the Lop Desert of central
which have clustered the Asia (after Ellsworth Huntington).
cities of the desert from
the earliest records of antiquity until now. Just without the line
of oases is the wall of dunes held back from further advance only
by the vegetation which in turn is dependent upon the rains in
the neighboring mountains. With every diminution in the water
supply, the dunes advance and encroach upon the oases (plate 7 B) ;
while with every considerable increase in this supply of moisture
s
Margitial
Displaceme.
N
0
B |
ry
°
h
»
° Zone of
5 Alluviation
SS Ba POD Za: é ;
ee ee LPS PSE
Lacustrine Dune Goawe Boulder V/
Deposits ee eh
Seds
Fia. 231.— Schematic diagram to show the zones of deposition in their order from
the margin to the center of a desert.
the alluvial bench advances over the dunes and acquires a strip
of their territory. Thus with varying fortunes a war is con-
tinually waged between the withering river and the flying sand,
and the alternations of climate are later recorded in the dove-
218 EARTH FEATURES AND THEIR MEANING
tailing together of the eolian and alluvial deposits at their com-
mon junction (Fig. 231).
In addition to the smaller periodic alternations of pluvial and
interpluvial climate —
the “ pulse of Asia”? —
the record of the Asiatic
deserts indicates a pro-
gressive desiccation of
the entire region, which
has now given the vic-
tory to the dune. The
ancient history of the
cities of the plains sup-
plies the records of many
that have been buried
in the dunes. To-day,
where once were pros-
perous cities, nothing is
to be seen at the surface but a group of mounds (Fig. 232). Ex-
humed after much painstaking labor, the walls and palaces of
these ancient cities
have once more been oe ee
brought to the light
of day, and much
has thus been
learned of the civi-
lization of these
early times (Fig.
233). Quite re- ss
cently the mounds Ss SS
which cover be- Wes
tween one and two
hundred buried vil-
lages have been
found upon the bor-
ders of the Tarim
basin of central Asia, where they were lost to history when
they were overwhelmed in the early centuries of the Christian
Era.
Fia. 232. — Mounds upon the site of the buried
city of Nippur (after the cast by Muret).
3 <<
>
Fig. 233.— Exhumed structures in the buried city of
Nippur (after Hilprecht).
THE FEATURES IN DESERT LANDSCAPES 219
The origin of the high plains which front
the Rocky Mountains. —To the eastward of
the great backbone of the North American
continent stretches a vast plain gently in-
clined away from the range and generally
known as the High Plains region (plate 9).
The tourist who travels westward by train
ascends this slope so gradually that when he
has reached the mountain front it is difficult
to realize that he has climbed to an altitude
of five thousand feet above the level of the
sea. That he has also passed through several
climatic zones — a humid, a semiarid, and
an arid —and has now entered a semiarid
district, is more easily appreciated from study
of the vegetation (Fig. 234). The surface of
the High Plains, where not cut into by rivers,
is remarkably even, so that it might be com-
pared to the quiet surface of a great lake.
The materials which compose the surface
veneer of these plains are coarse conglomer-
ates, gravels, and,sands, and the so-called
“mortar beds,” which are nothing but sands
cemented into sandstone by carbonate of lime.
The pebbles in all these deposits are far-
traveled and appear to have been derived
from erosion of those crystalline rocks which
compose the eastern front of the Rocky
Mountains. These different materials are
not arranged in strictly parallel beds, as are
the deposits of a lake or sea; but the beds
are made up of long threads of lenticular
cross section which are interlaced in the most
intricate fashion and which extend down the
slope, or outward from the mountain front
(Fig. 235). It is thus shown that the High
Plains are a bench or plain of alluviation
formed at the front of the Rocky Mountains
during an earlier series of pluvial periods, and that subsequent -
*(mosuUYyOr "q *M 107JB) J 9AOG SOTIOZ O1VVUITTD OY} PUB 90¥.110} OY} JO UOT}ISOd oY} BULMOYS ‘sUIETY YSTH OY} Ssos0w WOTPOG — “FEZ “OI
220 EARTH FEATURES AND THEIR MEANING
uplift has produced the modern river valleys which are cut out of
the plain. The plexus of long threads of the coarser materials are
the courses of dwindling rivers which interlaced over the former
Dery ‘4 “e
esa VE EOES
————
SEES A Ne aoe
= ef Vvalcin~
SSS
SS :
SS eee EE ———
compose the veneer of the High Plains (after W. D. Johnson).
plain, and which in time were buried under other channel deposits
of the same nature but in different positions (Fig. 236). The
pluvial periods in which this
bench was formed produced
in other latitudes the great
continental glaciers which
wrought such marvelous
changes in northern North
Fie. 236.— Distributaries of the foothills America and in northern
superimposed upon an earlier series (after Europe. e
sr i ck Character profiles. — In
contrast with the profiles in the landscapes of humid regions (see
Fig. 187, p. 177), those of arid lands are marked by straighter
— Pie Sa
Arock Jerraces :
Need/es :
Alluvial
Mushroom
Castrellared
Chim ioasie Dernoisell/e Loess 7
Cur
Fia. 237.— Character profiles in the landscapes of arid lands.
PLATE 9.
THE HIGH PLAINS
o ‘n Scale of miles :
ee ———— ae —
250 200
rr
THE FEATURES IN DESERT LANDSCAPES Ze
elements (Fig. 237). Almost the only exception of importance is
furnished by the domes of massive granite monoliths, which are
sometimes broken in half by great displacements. Below the
horizon the secondary lines in the landscape betray the same
straightness of the component elements by the gabled slopes
‘of talus which are many times repeated so as almost to repro-
duce the lines in a house of cards, since the sloping lines are
maintained at the angle of repose of the materials (Fig. 482, p. 443).
Wherever the waves of desert lakes have made an attack upon the
rocks and have retired the projecting spurs, other gables charac-
terized by slightly different slopes are introduced into the landscape.
READING REFERENCES FOR CHAPTERS XV AND XVI
General : —
JOHANNES WALTHER. Das Gesetz der Wiistenbildung in Gegenwart und
- Vorzeit. Berlin, 1900, pp. 175, many plates. (This extremely valua-
ble work is now out of print, but both a revised edition and an Eng-
lish translation are promised for 1912.) .
SreGFrriep PassarGe. Die Kalihari. Berlin, 1904, pp. 662.
W. M. Davis. The Geographic Cycle in an Arid Climate, Jour. Geol.,
vol. 13, 1905, pp. 381-407.
ExLuswortH Huntineton. The Pulse of Asia. New York and Boston,
1907, pp. 415. |
Sven Hepin. Scientific Results of a Journey through Central Asia, 1899-
1900. Stockholm, 1904-1905, vols. 1 and 2, pp. 523 and 717, pls.
56 and 76.
JOSEPH Barret: Relative Geological Importance of Continental, Lit-
toral and Marine Sedimentation, Jour. Geol., vol. 14, 1906, pp. 316—
356, 429-457, 524-568.
E. F. Gautier. Etudes sahariennes, Ann. de Géogr., vol. 16, 1907, pp.
46-69, 117-138.
The self-registering gauge of past climates : —
G. K. Giusert. Lake Bonneville, Mon. I, U. S. Geol. Surv., Chapter vi,
pp. 214-318.
T. F. Jamizson. ‘The Inland Seas and Salt Lakes of the Glacial Period,
Geol. Mag. decade III, vol. 2, 1885, pp. 193-200.
J. EK. Tatmace. The Great Salt Lake, Present and Past. Salt Lake City,
1900, pp. 116, plates.
E. Huntineton. Some Characteristics of the Glacial Period in Non-
glaciated Regions, Bull. Geol. Soc. Am., vol. 18, 1907, pp. 351-388,
pls. 31-39.
T. C. CuamBertin. The Future Habitability of the Earth, Rept.
Smithson. Inst., 1910, pp. 371-389.
222 EARTH FEATURES AND THEIR MEANING
The red and brown desert varnish : —
I. C. Russeiy. Subaérial Decay of Rocks and Origin af the Red Color
of Certain Formations. Bull. 52, U.S. Geol. Surv., 1889, pp. 65, pls. 5.
Erosion in the desert : —
J. A. Uppren. Erosion, Transportation, and Sedimentation performed by
the Atmosphere, Jour. Geol., vol. 2, 1894, pp. 318-331.
S. Passarcre. Die pfannenférmigen Hohlformen der stidafrikanischen
Steppen, Pet. Mitt., vol. 57, 1911, pp. 57-61, 130-135.
The dust carried out of the desert : —
F. von Ricutoren. China, Ergebnisse eigene Reisen und darauf ge-
griindeten Studien, Berlin, 1877, vol. 1, pp. 56-125. |
E. Hintaarp. The Loess of the Mississippi Valley, Am. Jour. Sci., (3),
vol. 18, 1879, pp. 106-112.
T. C. CHAMBERLIN and R. D. Sauissury. Preliminary Paper on the
Driftless Area of the Upper Mississippi Valley, 6th Ann. Rept. U.S.
Geol. Surv., 1885, pp. 278-307.
E. E. Free. The movement of soil material by the wind, with a bibli-
ography of eolian geology by S. C. Stuntz and E. E. Free, Bull. 68,
U. S. Bureau of Soils, 1911, pp. 272, pls. 5.
M. Neumayr. Erdgeschichte, vol. 1, pp. 510-514.
E. p—E MarTonNnE. Géographie physique, pp. 663-668.
Dunes : —
VauGcHan CornisH. On the Formation of Sarid-dinae, Geogr. Jour.,
vol. 9, 1897, pp. 278-309 (a most important paper).
F. Sotcrer and Others. Diinenbuch. Enke, Cheer 1910, pp. 373.
The zone of the dwindling river : —
E. Huntineton. The Border Belts of the Tarim Basin, Bull. Am. Geogr.
Soc., vol. 38, 1906, pp. 91-96; The Pulse of Asia, pp. 210-222, 262-279.
The war of dune and oasis : —
R. Poumpeuiy. Explorations in Turkestan, Expedition of 1904, etce.,
Pub. 73, Carneg. Inst., Washington, vol. 1, pp. 1-13.
E. Huntineton. The ann of Kharga, Bull. Am. Geogr. Soc., vol. 42.
1910, pp. 641-661.
Tu. H. Knarney. The Country of the Ant Men, Nat. Geogr. Mag., vol.
22, 1911, pp. 367-382.
Features of the arid lands : —
C. EK. Durron. Tertiary History of the Grand Cafion District, with
Atlas, Mon. II, U. 8. Geol. Surv., 1882, pp. 264, pls. 42, maps 23.
G. SwrernrurtH. Map Sheets of the Eastern Egyptian Desert. Berlin,
1901-1902, 8 sheets. |
The origin of the high plains : —
W. D. Jounson. The High Plains and their Utilization, 21st Ann. Rept.
U.S. Geol. Surv., Pt. iv, 1901, pp. 601-741.
CHAPTER XVII
REPEATING PATTERNS IN THE EARTH RELIEF
The weathering processes under control of the fracture system.
— In an earlier chapter it was learned that the rocks which com-
pose the earth’s surface shell are intersected by a system of
joint fractures which in little-disturbed areas divide the surface
beds into nearly square perpendicular prisms (Fig. 36, p. 55),
more or less modified by additional diagonal joints, and often
also by more disorderly fractures. Throughout large areas these
fractures may maintain nearly constant directions, though either
one or more of the master series may be locally absent. This
distinctive architecture of the surface shell of the lithosphere has
exercised its influence upon the various weathering processes, as it
has also upon the activities of running water and of other less
common transporting agencies at the surface.
Within high latitudes, where frost action is the dominant
weathering process, the water, by insinuating itself along the
joints and through repeated freezings, has broken down the rock
in the immediate neighborhood of these fractures, and so has
impressed upon the surface an image of the underlying pattern
of structure lines (plate 10 A).
In much lower latitudes and in regions of insufficient rainfall,
the same structures are impressed upon the relief, but by other
weathering processes. In the case of the less coherent deposits
in these provinces, the initial forms of their erosional surface have
sometimes been determined by the dash of rain from the sudden
' cloudburst. Thus the “ bad lands ” may have their initial gullies
directed and spaced in conformity with the underlying joint struc-
tures (Fig. 238).
In such portions of the temperate regions as are favored by a
humid climate, the mat of vegetation holds down a layer of soil,
and mat and soil in coéperation are effective in preventing any
223
224 EARTH FEATURES AND THEIR MEANING
such large measure of frostwork as is characteristic of the sub-
polar regions or of high levels in the arid lands. In humid regions
the rocks become a prey espe-
cially to the processes of solu-
cal decomposition, and these
processes, although guided by
the course of the percolating
ground water along the frac-
ture planes, do not afford such
striking examples of the con-
trol of surface relief.
Those limestones which
a Mt slowly pass into solution in
: ti I ARR BN" | the percolating water do,.how-
teats SSNs ever, quite generally indicate
= ' a localization of the solution
ee along the joint channels (Fig.
Fia. 238. - —— ‘Rain sculpturing under control 239 and plate 6 B). Though
by joints. Coast of southern California in other rocks not so apparent,
(after a photograph by Fairbanks). yet solutions generally take
their courses along the same channels, and upon them is localized
the development of the newly formed hydrated and carbonate
minerals, as is well illustrated by
the phenomenon of spheroidal
weathering (Fig. 155, p. 150).
The fracture control of the drain-
age lines. — The etching out of sm
the earth’s architectural plan in [*™
the surface relief, which we have
seen begun in the processes of |5
weathering, is continued after the ©
transporting agents have become which shows the effects of solution
effective. It is often easy. to see along neighboring joints in a sagging
that a river has taken its course 0f the upper beds (after Gilbert, U.
8. G. S.).
in rectangular zigzags like the
elbows of a jointed stove pipe, and that re walls are formed
of joint planes from which an occasional squared buttress pro-
jects into the channel. This structure is rendered in the plan of
tion and accompanying chemi- .
a
ri
Ai yi
i
i |
a
a
if ‘
a )
4 7
a
Fig. 239. — Oakes of flaggy limestone
REPEATING PATTERNS IN THE EARTH RELIEF 225
the Abisko Cafion of northern Lapland (Fig. 240). We are later
to learn that another great transporting agent, the water wave,
makes a selective attack upon the litho- ea
sphere along the fractures of the joint es « on
system (Fig. 250, p. 233 and Fig. 254,
p. 235).
Where the scale of the example is large,
as in the cases which have been above
cited, the actual position and directions
of the joint wall are easily compared with
the near-by elements of the river’s course,
so that the connection of the drainage
lines with the underlying structure is at
once apparent. In many examples where
the scale is small, the evidence for the con-
Fig. 240.— Map of the
j joint-controlled Abisko
trolling influence of the rock structure in — Cafion in northern Lap-
determining the courses of streams may [224 (after Otto Sjogren).
be found in the peculiar character of the drainage plan. To
illustrate: the course of the Zambesi River, within the gorge below
the famous Victoria Falls, not only makes repeated turnings at a
right angle, but its tributary streams, instead of making the usual
- sharp angle where they join the
main stream, also affect the right
angle in their junctions (Fig. 241).
The repeating pattern in drainage
networks. — It is a characteristic of
the joint system that the fractures
; within each series are spaced with
~p approximation to uniformity. If
* the plan of a drainage system has
a been regulated in conformity with
Pinay ine Seticune the architecture of the underlying
Falls (alter Tieraplugk): rock basement, the same repeating
rectangles of the master joints may
be expected to appear in the lines of drainage — the so-called
drainage network.
Such rectangular patterns do very generally appear in the
drainage network, though they are often masked upon modern |
maps by what, to the geologist, seems impertinent intrusion of the
Q
226 EARTH FEATURES AND THEIR MEANING
black lines of overprinting which indicate railways, lines of high-
way, and other culture elements. On river maps, which are
printed without culture, the pattern is much more easily recog-
nized (Figs. 242 and 243). Wherever the relief is strong, as is
v S
Smiles
Fia. 242. — Controlled
drainage network of Fia. 243.—A river network of repeating rectangular pat-
the Shepaug River tern. Near Lake Temiskaming, Ontario (from the map
in Connecticut. by the Dominion Government).
the case in the Adirondack Mountain province of the State of
New York, individual hills may stand in relief between the bound- .
ing streams which compose the rectangular network, like the |
squared pedestals of monuments. Such a type of relief carved in
repeating patterns has been described as ‘‘ checkerboard topog-
raphy.”
The dividing lines of the relief patterns — lineaments. — The
repeating design outlined in the river network of the Temiska-
ming district (Fig. 243) would appear in greater perfection if we
could reproduce the relief without at the same time obscuring
OT ona PB ke
REPEATING PATTERNS IN THE EARTH RELIEF 227
the lines of drainage; for where the pattern is not completely
closed by the course of the stream, there is generally found either
a dry valley or a ravine to complete the design. If these are
not present, a bit of straight coast line, a visible line of frac-
ture, a zone of fault breccia, or the boundary line separating
different formations may one or more of them fill in the gaps of
the parallel straight drainage lines which by their intersection
bring out the pattern. These significant lines of landscapes
which reveal the hidden architecture of the rock basement are
described as lineaments (Fig. 82, p. 87). They are the character
lines of the earth’s physiognomy.
It is important to emphasize the essentially composite ex-
pression of the lineament. At one locality it appears as a drain-
age line, a little farther on it may be a line of coast; then, again,
it is a series of aligned waterfalls, a visible fault trace, or a recti-
linear boundary between formations; but in every case it is some
surface expression of a buried fracture. Hidden as they so gen-
erally are, the fracture lines must be searched out by every means
at our disposal, if we are not to be misled in accounting for the
positions and the attitudes of disturbed rock masses.
As we have learned, during earthquake shocks, as at no other
time, the surface of the earth is so sensitized as to betray the
position of its buried fractures. As the boundaries of orographic
blocks, certain of the fractures are at such times the seats of
especially heavy vibrations; they are the seismotectonic lines
of the earthquake province. Many lineaments are identical
with seismotectonic lines, and they therefore afford a means of
to some extent determining in advance the lines of greatest dance
ger from earthquake shock.
The composite repeating patterns of the higher orders. — Not
only do the larger joint blocks become impressed upon the earth
relief as repeating diaper patterns, but larger and still larger com-
posite units of the same type may, in favorable districts, be found
to present the same characters. Attention has already been
more than once directed to the fact that the more perfect and
prominent fracture planes recur among the joints of any series at
more or less regular intervals (Fig. 40, p. 57, and Fig. 41, p. 58).
Nowhere, perhaps, is this larger order of the repeating pattern
more perfectly exemplified than in some recent deposits in the
228 EARTH FEATURES AND THEIR MEANING
Syrian desert (plate 10 B). It is usually, however, in the older
sediments that such structures may be recognized; as, for ex-
ample, in the squared towers and buttresses of the Tyrolean
Dolomites (Fig. 244). Here the larger blocks appear in the thick
Fic. 244.— Squared mountain masses which reveal a distribution of the joints in.
block patterns of different orders of magnitude. The Pordoirange of the Sella
group of the Dolomites, seen from the Cima di Rossi (after Mojsisovics).
bedded lower formation, the dolomite, divided into subordinate
sections of large dimensions; but in the overlying formations
in blocks of relatively small size, yet with similarly perfect sub-
equal spacing. )
The observing traveler who is privileged to make the journey
by steamer, threading its course in and out among the many is-
lands and skerries of the Norwegian coast, will hardly fail to be.
struck by the remarkable profiles of most of the lower islands —
(Fig. 245). These profiles are generally convexly scalloped with a —
noteworthy regularity, and not in one unit only, but in at least two a s
with one a multiple of the other (Fig. 246). Asthe steamer passes
near to the islands, it is discovered that the smaller recognizable ’ a,
units in the island profiles are separated by widely gaping joints i
which do not, however, belong to the unit series, but to a larger
composite group (Fig. 246 b). Frostwork, which depends for its
PLATE 10.
A. View in Spitzbergen to illustrate the disintegration of rock under the control of
joints.
(Photograph by O. Haldin.)
B. Composite pattern of the joint structures within recent alluvial deposits.
(Photograph by Ellsworth Huntington.)
REPEATING PATTERNS IN THE EARTH RELIEF 229
action upon open spaces within the rocks, has here been the cause
of the excessive weathering above the more widely gaping joints.
High northern latitudes are thus especially favorable for re-
vealing all the details in the architectural pattern of the litho-
Fig. 245.—Island groups of the Lofoten archipelago off the northwest coast of
Norway, which reveal repeating patterns of the relief in two orders of magnitude
(after a photograph by Knudsen).
sphere shell, and we need not be surprised that when the modern
maps of the Norwegian coast are examined, still larger repeating
patterns than any
that may be seen
in the field are to
be made out. The
Norwegian coast
was long ago shown
b
to be a complexly py. 246. — Diagrams to illustrate the composite profiles
faulted region, and _ of theislands on the Norwegian coast. a; distant view;
these larger divi- b, near view, showing the individual joints and the more
: f widely gaping fractures beneath each sag in the profile.
sions of the relief
pattern, instead of being explained as a consequence solely of
selective weathering, must be regarded as due largely to fault
displacements. of the type represented in our model (plate 4 C).
Yet whether due to displacements or to the more numerous
joints, all belong to the same composite system of fractures
expressed in the relief.
230 EARTH FEATURES AND THEIR MEANING
READING REFERENCES FOR CHAPTER XVII
Wituram H. Hosss. The River System of Connecticut, Jour. Geol.,
vol. 9, 1901, pp. 469-485, pl. 1; Lineaments of the Atlantic Border
Region, Bull. Geol. Soc. Am., vol. 15, 1904, pp. 483-506, pls. 45-47 ;
The Correlation. of Fracture Systems and the Evidences for Plan-.
etary Dislocations within the Earth’s Crust, Trans. Wis. Acad. Sci.,
—ete., vol. 15, 1905, pp. 15-29; Repeating Patterns in the Relief and
in the Structure of the Land, Bull. Geol. Soc. Am., vol. 22, 1911, pp.
123-176, pls. 7-13.
py pe — i. oe
ee :
a = =
CHAPTER XVIII
THE FORMS CARVED AND MOLDED BY WAVES
The motion of a water wave. — The motions within a wave
upon the surface of a body of water may be thought of in two
different ways. First: of all, there is the motion of each particle
of water within an orbit of its own; and there is, further, the for-
ward motion of propagation of the wave considered as a whole.
The water particle in a wave has a continued motion round and
round its orbit like that of a horse circling a race course, only that
here the track is in a
vertical plane, directed
along the line of propa-
gation of the wave (Fig.
247). Each particle of
water, through its fric-
tion upon neighboring
particles, is able to
transmit its motion both
along the surface and
downward into the water
below. The force which
starts the water in mo-
tion and develops the : Saad
, cAI a Fia. 247.— Diagram to show the nature of the
wave, is the friction of motions within a free water wave.
wind blowing over the
water surface, and the size of the orbit of the water particle at
any point is proportional to the wind’s force and to the stretch of
water over which it has blown. The wind’s effect is, therefore,
cumulative — the wave is proportional to the wind’s effect upon
all water particles in its rear, added to the local wind friction.
The size or height of the wave is measured by the diameter of the
orbit of motion of the surface particle, and this is the difference
in height between trough and: crest. The distance from crest
to crest, or from trough to trough, is called the wave length.
Though the wave motion is transmitted downward into the water,
231
cy
r '
y
N)
§ we Hl 1. I if
ce 3 wets i i
|
|
i eee
“7,
ee ee ae ee sereck a wecoeee
»,
he or a a ae ee ae oF wae mn o
'
‘
Se ae
232 EARTH FEATURES AND THEIR MEANING
there is a continued loss of energy which is here not compensated
by added wind friction, and so the orbital motion grows smaller and
smaller, until at the depth of about a wave length it has completely
died out. This level of no motion is called the wave base. In ©
quiet weather the level of no motion is practically at the water’s
surface, and inasmuch as the geological work of waves is in large ~
part accomplished during the great storms, the term “wave base”’
refers to the lowest level of wave motion at the time of the heavi- _
est storms. Upon the ocean the highest waves that have been
measured have an amplitude of about fifty feet and a wave © a ‘
length of about six hundred feet.
Free waves and breakers. — So long as the depth of the water
is below wave base, there is obviously no possibility of interfer-
ence with the wave through friction upon the bottom. Under
these conditions waves are described as free waves, and their forms
are symmetrical except in so far as their crests are pulled over
and more or less dissipated in the spray of the “ Bis oe : caps” at
the time of high winds.
As a wave approaches a shore, which ceudiae has a gentle
outward sloping surface, there is interposed in the way of a free
forward movement the friction upon the bottom. This friction
begins when the depth of water is less than wave base, and its
effect is to hold back the wave at the bottom. Carried slowly
es Aa AER att Aen wee A
Her Beep Wii Ml
Fra. 248.— Diagram to illustrate the transformation of a free wave into a breaker oe
as it approaches the shore.
upward in the water by the friction of particle upon particle,
the effect of this holding back is a piling up of the water, which in- — a
creases the wave height as it diminishes the wave length, and also
interferes with wave symmetry (Fig. 248). Moving forward
at the top under its inertia of motion and held back at the bottom
PLATE 11.
A. Ripple markings within an ancient sandstone (courtesy of U. S. Grant).
B. A wave breaking as it approaches the shore.
(Photograph by Fairbanks.)
|
'
\
Pea ae
ie ‘i A
THE FORMS CARVED AND MOLDED BY WAVES 233
by constantly increasing friction, a strong turning motion or
couple is started about a horizontal axis, the immediate effect
of which is to steepen the forward slope of the wave, and this con-
tinues until it overhangs,
and, falling, “‘ breaks ”’ into
surf. Such a_ breaking
wave is called a “‘ comber ”’
or ‘‘ breaker ”’ (plate 11 B).
Effect of the breaking
wave upon a steep rocky
shore — the notched cliff. —
If the shore rises abruptly
from deeper water, the top «% : ISS
of the breaking wave is Fia. 249. — Notched rock cliff cut by waves and
hurled against the cliff with the fallen blocks derived from the cliff through
the force of a battering ram. undermining. Profile’ Rock at Farwell’s
. Point near Madison, Wisconsin.
During storms the water of
shore waves is charged with sand, and each sand particle is driven
like a stone cutter’s tool under the stroke of hishammer. The effect
is thus both to chip and to batter away the rock of the shore to
the height reached by the wave, undermining it and notching
the rock at its base (Fig. 249). When the notch has been cut
in this manner to a sufficient depth, the overhanging rock falls
by its own weight in blocks which
are bounded by the ever present
joints, leaving the upper cliff face
essentially vertical.
Coves, sea arches, and stacks.
—It is the headland which is
most exposed to the work of the
waves, since with change of wind
direction it is exposed upon more
than asingle face. The study of
headlands which have been cut
by waves shows that the joints
within the rock play a large réle
in the shaping of shore features:
Fic. 250.—A wave-cut chasm under
control by joints, coast of Maine (after The attack of the waves under
Tarr). the direction of these planes of
234 EARTH FEATURES AND THEIR MEANING
ready separation opens out indentations of the shore (Fig. 250) or
forms sea caves which, as they extend to the top of the cliff by the
process of sapping, yield the coves which are so common a feature
upon our rock-bound shores
(Fig. 259, p. 238). With contin-
uation of this process, the caves
formed on opposite sides of the
headland may be united to form
a sea arch (Fig. 251).
A later stage in this selective
wave carving under the control
of joints is reached when the
bridge above the arch has
fallen in, leaving a detached
rock island with precipitous
Fia. 251. — The sea arch known as the walls. Such an offshore island
Grand Arch upon one of the Apostle of rock with precipitous sides
Islands in Lake Superior (after a pho- is known as a stack (Fig.
tograph by the Detroit Photographic :
Gncansy. 252), or sometimes as a
‘‘ chimney,” though this latter
term is best restricted to other and similar forms which are the
product of selective weathering (p. 300).
Whenever the rock is less firmly consolidated, and so does not
stand upon such steep planes,
the stack is apt to have a
more conical form, and may
not be preceded in its forma- |
tion by the development of
the sea arch (Fig. 260, p. 239).
In the reverse case, or where
the rock possesses an unusual £
tenacity, the stack may be
largely undermined and stand
supported like a table upon
thick legs or pillars of rock
(Fig. 253). In Fig. 254 is
seen a group of stacks upon the coast of California, which show
with clearness the control of the joints in their formation, but
unlike the marble of the South American example the forms
Fig. 252.—Stack near the shore of Lake
Superior.
THE FORMS CARVED AND MOLDED BY WAVES 235
are not rounded, but retain
their sharp angles.
The cut rock terrace.—
When waves first begin their
attack upon a steep, rocky
shore, the lower limit of the
action is near the wave base.
The action at this depth is,
however, less efficient, and as
the recession of the cliff is one
of the most rapid of erosional
Se
we =
#ig..253.-— The Marble Islands, stacks in
Lake Buenos Aires, southern Andes
(after F. P. Moreno).
processes, the rock floor outside the receding cliff comes to slope
gradually downward from the cliff to a maximum depth at the
Fig. 254. — Squared stacks which reveal the position of the joint planes which have
controlled in the process of carving by the waves. Pt. Buchon, California
(after a photograph by Fairbanks).
edge of the terrace, approximately equal to wave base (Fig. 255).
This cut terrace is extended seaward or lakeward, as the case may
be, in a built terrace constructed from a portion of the rock débris
acquired from the cliff.
236 EARTH FEATURES AND THEIR MEANING
The broken wave, after rising upon the terrace under the inertia
of its motion until all its energy has been dissipated, slides out-
| ward by gravity, and though
checked and overridden by
succeeding breakers, it con-
tinues its outward slide as
the ‘ undertow” until it
reaches the end of the ter-
race. Here it suddenly en-
ters deep water, and losing
Fic. 255.—Ideal section of a steep rocky its velocity . drops its burden
shore carved by waves into a notched cliff of r ock, and builds the ter-
and cut terrace, and extended by a built race seaward after the man-
ee ner of construction of an
embankment. As we are to see, the larger portion of the wave-
quarried material is diverted to a different quarter. :
To -gain some conception of the importance of wave cutting
as an eroding process, we may consider the late history of Heli-
goland, a sandstone island off the mouth of the Elbe in the North
Sea (Fig. 256). From a periphery of 120 miles, which it possessed
in the ninth cen-
tury of the Chris-
tian era, the
island has reduced
its outline to 45
miles in the four-
teenth century, 8
miles in the seven-
teenth, and to
about 3 miles at
the beginning of
the twentieth cen-
tury. The German
government,which . oS eee nse
recently acquired
this little remnant Fic. 256.— Map showing the outlines of the Island of
¢ Bavlanisd Heligoland at different stages in its recent history. The
rom England, has peripheries given are in miles.
expended large | e
sums of money in an effort to save this last relic.
THE FORMS CARVED AND MOLDED BY WAVES 237
The cut and built terrace on a steep shore of loose materials.
— In materials which lack the coherence of firm rock, no vertical
cliff can form; for as fast as undermined by the waves the loose
materials slide down
and assume a surface
of practically constant —
ar a *: ves) snes - ie
se m o aca OT ha # he Ii
rials (Fig. 257). The pets, ie li
terrace below this Fia. 257.—Cut and built terrace with bowlder pave-
sloping cliff will not ‘ment shaped by waves on a steep shore formed of
5 é loose materials.
differ in shape from
that cut upon a rocky shore; but whenever the materials of the
shore include disseminated blocks too large for the waves to handle,
they collect upon the terrace near where they have been exhumed,
thus forming what’ has been called a ‘‘ bowlder pavement ” (Fig.
258).
The edge of the cut and built terrace is, as already mentioned,
maintained at the depth of wave'base. If one will study the sub-
merged contours of any of our
inland lakes, it will be found
that these basins are sur-
rounded by a gently sloping
marginal shelf,—the cut and
built terrace,—and that the
depth of this shelf at its outer
edge is proportioned to the
size of the lake. Upon Lake
Mendota at Madison, Wiscon-
sin, the large storm waves have
; a length of about twenty feet,
Fig. 258.—Sloping cliff and terrace with which is the depth of the outer
bowlder pavement exposed at low tide edge of the shore terraces (Fig.
oaminee! ap: shore at Scituate, Mass- 267, p. 242). The shelf sur-
rounding the continents has,
with few local exceptions, a uniform depth of 100 fathoms, or about
the wave base of the heaviest storm waves.
The work of the shore current. —In describing the formation
of the built terrace, it was stated that the greater part of the rock
ines eee
hay Te
PLinioh or ey
238 EARTH FEATURES AND THEIR MEANING
material quarried upon headlands by the waves is diverted from
the offshore terrace. This diversion is the work of the shore cur-
rent produced by the wave.
At but few places upon a shore will the storm waves beat per-
pendicularly, and then for but short periods only. The broken
wave, as it crawls ever more slowly up the beach, carries the sand
with it in a sweeping curve, and by the time gravity has put a stop
to its forward movement, it is directed for a brief instant parallel
tothe shore. Soon, however, the pull of gravity upon it has started
the backward journey in a more direct course down the slope of
irectiowr of
Prevailing
Storm Wird
Fig. 259. — Map to show the mature of the shore current and the forms which are
molded by it.
the terrace; and here encountering the next succeeding. breaker,
a portion of the water and the coarser sand particles with it are
_ again carried forward for a repetition of the zigzag journey. This
many times interrupted movement of the sand particles may be
observed during a high wind upon any sandy lee shore. The “ set”
of the water along the shore as a result of its zigzag journeyings
is described as the shore current (Fig. 259), and the effect upon
sand distribution is the same as though a steady current moved
parallel to the shore in the direction of the average trend of the
moving particles.
The sand beach. — The first effect of the shore nonnene Is to
deposit some portion of the sand within the first slight recess upon
‘the shore in the lee of the cliff. The earlier deposits near the cliff
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THE FORMS CARVED AND MOLDED BY WAVES 239
gradually force the shore current farther from the shore and
so lay down a sand selvage to the shore, which is shaped in the
form of an are or crescent and known as a beach (Fig. 259 and
Fig. 260).
Pa eee ee eee _
; =i OE ee
= 4
Fig. 260.— Crescent-shaped beach formed in the lee of a headland. Santa
Catalina Island, California (after a photograph by Fairbanks).
The shingle beach. — With heavy storms and an exceptional
teach of the waves, the shore currents are competent to move, not
the sand alone, but pebbles, the area of whose broader surface may
be as great as the palm of one’s hand. Such rock fragments are
shaped by the continued wear against their neighbors under the
restless breakers, until they have a len-
ticular or watch-shaped form (Fig. 261).
_ Such beach pebbles are described as shingle, °
and they are usually built up into distinct F1¢- 261.— Cross section
ridges upon the shore, which, under the CE nT Cone
fury of the high breakers, may be piled several feet above the level
of quiet water (Fig. 262). Such storm beaches have a gentle
240 EARTH FEATURES AND THEIR MEANING
forward slope graded by the shore current, but a steep back-
ward slope on the angle of repose. Most storm beaches have
been largely shaped by the last great storm, such as comes only
at intervals of a number of years.
Wherever the shore upon which
a beach is building makes a
sudden landward turn at the en-
trance to a bay, the shore cur-
rents, by virtue of their inertia
3 of motion, are unable longer to
Fic. 262.—Storm beach of coarse follow the shore. The débris
shingle about four feet in height at which they carry is thus trans-
te bug of Brat muon he morte ord into. deeper water in &
Michigan. direction corresponding to a con-
tinuation of the shore just before
the point of turning (see Fig. 259, p. 238). The result is the
formation of a bar, which rises to near the water surface and is ex-
tended across the entrance to the bay through continued growth
at its end, after the manner of constructing a railway embank-
ment across a valley.
Over the deeper water near the bar the waves are at first not
generally halted and broken, as they are upon the shore, and so
the bar does not at once
build itself to the surface,
but remains an invisible
bar to navigation. From
its shoreward end, how-
ever, the waves of even
moderate storms are
broken, and the bar is
there built above the water
surface, where it appears SS ——
NPY —
as a oe TOW raps of sand Fic. 263.— Spit of shingle on Au Train Island,
or shingle which gradually Lake Superior (after Gilbert).
thins in approaching its
apex. ‘This feature is the well-known spit (Fig. 263) which, as it
grows across the entrance to the bay, becomes a barrier or barrier
beach (Fig. 264).
a
us
See ae el
Bar, spit, and _ barrier. — -
ves
ae
———
ws ia
oe
Taare *
By
a
a
<p teeta ae
ype oe :
Pan _
cee 5 Os Seine
a eh
,
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THE FORMS CARVED AND MOLDED BY WAVES 241
The continuation of the visible in the usually invisible bar, is
at the time of high winds made strikingly apparent, for the wave
base is below the crest of the bar, and at such times its crescentic
course beyond the spit can be followed by the eye in a white arc
of broken water.
The construction of a barrier across the entrance to a bay trans-
forms the latter into a separate body of water, a lagoon, within
which silting up and peat
formation usually lead to an
early extinction (p.429). The
formation of barriers thus
tends to straighten out the
irregularities of coast lines,
and opens the way to a
natural enlargement of the
land areas. While the coasts
of the United Kingdom of
Great Britain have been :
losing some four thousand Fia. 264. — Barrier beach in front of alagoon
acres through wave erosion, on Lake Mendota at Madison, Wisconsin.
there has been a gain by The shallow lagoon behind the barrier is
; F A filling up and is largely hidden in vege-
growth in quiet lagoons which gation.
amounts to nearly seven
times that amount. As evidence of the straightening of the shore
line which results from this process, the coast of the Carolinas or
of Nantucket (Fig. 459) may serve for illustration.
The land-tied island. — We have seen that wave erosion oper-
ates to separate small islands from the headlands, but the shore
currents counteract this loss to the continents by throwing out
barriers which join many separated islands to the mainland. Such
land-tied islands are a common feature on many rocky coasts,
and upon the New England coast they usually have been given the
name of ‘‘ neck.” The long are of Lynn Beach joins the former
island of Nahant, through its smaller neighbor Little Nahant,
to the coast of Massachusetts. A similar land-tied island is
Marblehead Neck. The Rock of Gibraltar, formerly an island,
is now joined to Spain by the low beach known as the “ neutral
ground.” The Spanish name, tombola, has sometimes been em-
ployed to describe an island thus connected to the shore.
fe ;
242 EARTH FEATURES AND THEIR MEANING
A barrier series. — The cross section of a barrier beach, like
that of a storm beach upon the shore, slopes gently upon the for-
ward side, and more steeply
7 yy, Ty j ji i ————=———— at the angle of repose upon
Yi, i} MMi phd Wy) 77, the rear or landward margin ~
YU ML I ae ae
Yep yy ppp aly pg a ify Wy (Fig. 265). The thinning
VM Speco cee ee
Fia. 265. — Cross section of a barrier beach
: a the barrier throws out to sea-
with lagoon in its rear.
ward raises the level of the
lake bottom (Fig. 266), and when coast irregularities are favor-
able to it, new spits will develop upon the shore outside the
4 Barrier
Borrier
Cobre
Wf i ar jj =
‘Original/ /Bortom
ly / IMAL UIT TTA /
Fia. 266.— Cross section of a series of barriers and an outer bar.
earlier one, and a new bar, and in its turn a barrier, will be found
outside the initial one, taking a course in a direction more or less
parallel to it (Fig. 267).
‘ Scale in Miles.
Fic. 267. — Formation of barrier series and an outer bar in University Bay of
Lake Mendota, at Madison, Wisconsin. The water contour interval is fiye feet,
and the land contour interval ten feet (based on a map by the Wisconsin Geologi-
cal Survey).
THE FORMS CARVED AND MOLDED BY WAVES 243
So soon as the first barrier is formed, processes are set in opera-
tion which tend to trans-
form the newly formed la-
goon into land, and so with
a series of barriers, a zone
of water lilies between the
outer barrier and the bar,
a bog, and a land platform
may represent the succes-
sive stages in this acquisi-
tion of territory by the *
lands. A noteworthy ex- ae ae
ample of barrier series
and extension of the land
behind them, is afforded by Fig. 268. —Series of barriers at the western end
the bay at the western end of Lake Superior (after Gilbert).
of Lake Superior (Fig. 268).
Character profiles.— The character profiles yielded by the
work of waves are easy of recognition (Fig. 269). The vertical
cliff with notch at its
base is varied by the
PR TAP
Notched stack of sugar-loaf
Frock .
hie form carved in softer
rocks, or the steeper
Stack Worchew uotched variety cut
Strack from harder masses.
Sea caves and sea
Sea Arch : -
S/opiing arches yield varia-
CLF Aa tions of a curve com-
mon to the undercut
ei forms. Wherever the
B er 8
arrier Beach materials of the shore
are loosely consoli-
dated only, the slop-
ing cliff is formed at the anole of repose of the materials. The.
barrier beach, though projecting but a short distance above the
waves, shows an unsymmetrical curve of cross section with the
steeper slope toward the land.
Fia. 269. — Character profiles resulting from wave
action upon shores.
|
244 EARTH FEATURES AND THEIR MEANING
READING REFERENCES FOR CHAPTER XVIII
G. K. Giupert. The Topographic Features of Lake Shores, 5th Ann.
Rept. U.S. Geol. Surv., 1885, pp. 69-123, pls. 3-20; Lake Bonne-
ville, Mon. I, U. S. Geol. Surv., 1890, Chapters ii-iv, pp. 23-187.
VAUGHAN CornisH. On Sea Beaches and Sand Banks, Geogr. Jour., vol.
11, 1898, pp. 528-548, 628-658. |
F. P. Guuutver. Shore Line Topography, Proc. Am. Acad. Arts and
Sci., vol. 34, 1899, pp. 149-258.
N.S. SHauter. The Geological History of Harbors, 13th Ann. Rept. U.S.
Geol. Surv., 1893, pp. 93-209.
Str A. Guerxiz. The Scenery of Scotland, 1901, pp. 46-89.
W.H. Wueever. The Sea Coast. Longmans, London, 1902, pp. 1-78.
G. W. von Zann. Die zerstérende Arbeit des Meeres an Steilkiisten nach
Beobachtungen in der Bretagne und Normandie in den Jahren 1907
und 1908, Mitt. d. Geogr. Ges. Hamb., vol. 24, 1910, pp. 193-284,
pls. 12-27.
eT a a ae ee
CHAPTER XIX
COAST RECORDS OF THE RISE OR FALL OF THE
LAND
The characters in which the record has been preserved. —
The peculiar forms into which the sea has etched and molded its
shores have been considered in the last chapter. Of these the
more significant are the notched rock cliff, the cut rock terrace,
the sea cave, the sea arch, the stack, and the sloping cliff and ter-
race, among the carved features; and the barrier beach and built
terrace, among the molded forms. It is important to remember
that the molded forms, by the very manner of their formation,
stand in a definite relationship to the carved features; so that
when either one has been in part effaced and made difficult of de-
termination, the discovery of the other in its correct natural posi-
tion may remove all doubt as to the origin of the relic.
In studies of the change of level of the land, it is customary to
refer all variations to the sea level as a zero plane of reference.
It is not on this account necessary to assume that the changes
measured from this arbitrary datum plane are the absolute up-
ward or downward oscillations which would be measured from the
earth’s center; for the sea, like the land, has been subject to its
‘changes of level. There need, however, be no apology for the
use of the sea surface as a plane of reference; for it is all that we
have available for the purpose, and the changes in level, even if
relative only, are of the greatest significance. It is probable that
in most cases where the coast line is rising from uplift, some por-
tion of the sea basin not far distant is becoming deepened, so that
the visible change of level is the algebraic sum of the two effects.
Even coast line the mark of uplift. — It was early pointed out
in this volume (p. 158) that the floor of the sea in the neighborhood
of the land presents a relatively even surface. The carving by
waves, combined with the process of deposition of sediments, tends
to fill up the minor irregularities of surface and preserve only the
245
246 EARTH FEATURES AND THEIR MEANING
features of larger scale, and these in much softened outlines. Upon
the continents, on the contrary, the running water, taking advan-
tage of every slight difference in elevation and
searching out the hidden structure planes
within the rock, soon etches out a surface of the
most intricate detail. The effect of elevation
of the sea floor into the light of day will there-
fore be to produce an even shore line devoid of
harbors (Fig. 270). If the coast has risen f
along visible planes of faulting near the sea
margin, the coast line, in addition to being even, |
will usually be made up of notably straight ele-
ments joined to one another. |
A ragged coast line the mark of subsid-
ence. — When in place of uplift a subsidence
occurs upon the coast,
the intricately etched
surface, resulting from
AEE HMC gr erosion beneath the
coast of Florida, with Sky, comes to be in-
even shore line char- yaded by the sea
ee of araised slong each trench and
furrow, so that a most
ragged outline is the result (Fig. 271).
Such a coast
has m any Fia. 271.— Ragged coast line
harbors of Alaska, the effect of sub-
; , sidence.
while the
uplifted coast is as remarkable for its
=| lack of them.
Slow uplift of the coast — the
coastal plain and cuesta. — A gradual
uplift of the coast is made apparent
in a progressive retirement of the sea
across a portion of its floor, thus ex-
posing this even surface of recent
Fic. 272.—Portion of Atlantic sediments. The former shore land
coastal plainand neighboringold- . ‘ ‘. % “
land of the Appalachian Moun- Will be easily recognized by its-etched
tains. surface, which will now come into
COAST RECORDS OF THE RISE OR FALL OF LAND 247
sharp contrast with the new plain. It is therefore referred to as
the oldland and the newly exposed coastal plain as the newland
(Fig. 272).
But the near-shore deposits upon the sea floor had an initial
dip or slope to seaward, and this inclination has been increased
in the process of uplift. The streams from the oldland have
trenched their way across these deposits while the shore was ris-
ing. But the process being a slow one, deposits have formed
upon the seaward side of the plain after the landward portion was
above tide, and the coastal plain may come to have a “ belted ”
or zoned character. The streams tributary to those larger ones
which have trenched the plain may encounter in turn harder and
softer layers of the plain deposits, and at each hard layer will be
deflected along its margin so as to fi
enter the main streams more nearly fe Sse
at right angles. They will also, as & :
time goes on, migrate laterally sea- R&
ward through undermining of the
harder layers, and thus will be
shaped alternating belts of lowland
separated by escarpments in the
harder rock from the residual higher slopes. Belts of upland of
this character upon a coastal plain are called cuestas (Fig. 273).
The sudden uplifts of the coasts. — Elevations of the coast
which yield the coastal plain must be accounted among the
slower earth movements that result in changes of level. Such
movements, instead of being accompanied by disastrous earth-
quakes, were probably marked by frequent slight shocks only,
by subterranean rumblings, or, it may be, the land rose gradually
without manifestations of a sensible character.
Upon those coasts which are often in the throes of seismic dis-
turbance, a quite different effect is to be observed. Here within
the rocks we will probably find the marks of recent faulting with
large displacements, and the movements have been upon such a
scale that shore features, little modified by subsequent weathering,
stand well above the present level of the seas. Above such coasts,
then, we recognize the characteristic marks of wave action, and
the evidence that they have been suddenly uplifted is beyond
question.
and intermediate lowlands carved
from a coastal plain (after Davis).
248 EARTH FEATURES AND THEIR MEANING
Fra. 274.— Uplifted sea cave, ten feet above the water upon the coast of Califor-
nia; the monument to a former earthquake (after a photograph by Fairbanks).
Fig. 275.— Double-notched cliff near Cape Tiro, Celebes (after a photograph by
Sarasin).
+ hat
COAST RECORDS OF THE RISE OR FALL OF LAND 249
The upraised cliff.— Upon the coast of southern California
may be found all the features of wave-cut shores now in perfect
preservation, and in some cases as much as fifteen hundred feet
above the level of the sea. These features are monuments to the
grandest of earthquake disturbances which in recent time have
visited the region (Fig. 274). Quite as striking an example of
similar movements is afforded by notched cliffs in hard limestone
upon the shore of the Island of Celebes (Fig. 275). But the coast
of California furnishes the other characteristic coast features in the
high sea arch and the stack as additional monuments to the recent
Fig. 276. — Jasper rock stacks uplifted on the coast of California (after a photo-
graph by Fairbanks).
uplift. Let one but imagine the stacks which now form the Seal
Rocks off the Cliff House at San Francisco to be suddenly raised
high above the sea, and the forms which they would then present
would differ but little, from those which are shown in Fig. 276.
The uplifted barrier beach. — Within the réentrants of the
shore, the wave-cut cliff is, as we know, replaced by the barrier
beach, which takes its course across the entrance to a bay. After
an uplift, such a barrier composed of sand or shingle should be
connected with the headlands, often with a partially filled lagoon
behind it. Its cross section should be steep in the direction of
the lagoon, but quite gradual in front (Fig. 277).
250
EARTH FEATURES AND THEIR
MEANING
{Fia. 277.— Uplifted shingle beach across the entrance to a former bay upon
the coast of southern California (after a photograph by Fairbanks).
Coast terraces. — Upon those shores where to-day high moun- —
tains front the sea, the coast may generally be seen to rise inaseries
Fia. 278. — Raised beach terraces near Elie,
Fife, Scotland.
Fia. 279. — Uplifted sea cliffs and terraces on the coast of Russell Fjord, Ala
PP
*
of terraces (Fig. 278). This
is notably true of those coasts
which are to-day racked by
earthquakes, such as is the
eastern margin of the Pacific
mw
. -
A
from Alaska to Patagonia. —
The traveler by steamer along _
the coast from San Francisco
.
(after Tarr and Martin). - ;
Alaska we are fortunate in having the history of the later stages
this uplift (Fig. 279). As described in a former chapter, portic
of this shore rose in the month of September of the year 1899
3 COAST RECORDS OF THE RISE OR FALL OF LAND 251
some places as high as forty-seven feet, to the accompaniment of
PY a terrific earthquake and sea wave. Above the terrace which
ie Mrs.
Fig. 280.— Diagrams to show how excessive sinking upon the sea floor will cause
the shore to migrate landward as it is uplifted.
marks the beach line of 1899 there is a higher terrace of similar
form now overgrown with trees, but none the less clearly to be rec-
a ognized as a shore line of the past century
3 which preceded in the long sequence the
uplift of 1899.
As was noted in our study of earth-
quakes, the recent instrumental records of
distant earthquakes tell us that the move-
ments upon the sea floor are many times
larger than those upon the continents, and
Qh.
ae. s
& that while the mountainous coasts are gen- :
erally rising, the deeps of the sea are sink- of
ing. The effect of this over-balance of ‘y
sinking, or resultant shrinking of the earth’s
shell, may be to compress the mountain
district and so cause the shore line to move
landward at the same time that it moves
upward (Fig. 280).
The sunk or embayed coast. — When
now, upon the other hand, a section of the
coast line sinks with reference to the sea,
the water invades all the near-shore val-
leys, thus “ drowning ” them and yielding
the “drowned river mouth” or estuary.
Rl ey
Fig. 281. — A drowned river
mouth or estuary upon a
coastal plain.
If the relief of the shore was slight, as it generally is upon a
coastal plain, slight depression only will produce broad estuaries,
PAS EARTH FEATURES AND THEIR MEANING
Fic. 282.— Archipelago of steep rocky islets due to large submergence of a coast [
having strong relief. Entrance to Esquimalt Harbor, Vancouver Island (after :
a photograph by Fairbanks).
ee eee = Such as Chesapeake Bay at the |
a Oe drowned mouth of the Susque- —
- — hanna (Fig. 281). am
—> : —— If, on the other hand, the relief
“= (f the shore is strong and the sub-
sidence is large, the entire coast
line will be transformed into an
archipelago of steep-walled rocky __
islets which rise abruptly from the
sea (Figs. 282 and 284). Aplateau
which is intersected by deep and _
steep-walled valleys of U-section
(p. 341) under large submergence
yields the fjords so characteristic
of Scandinavia or Alaska. A rag-
ged coast line, fringed with islands —
as a result of submergence, is de-
scribed as an embayed coast. ‘a
Submerged river channels. —
The sinking of a coast of small
Fic. 283.—The submerged Hudso- relief may be sufficient to comma
nian channel which continues the ; a
Hudson River across the continental Pletely submerge river valleys, —
shelf. whose channels then begin to fill
|
h
()
Ai
a»
a7
7
.
= ‘
~aa
ro, oe
i a
‘
COAST RECORDS OF THE RISE OR FALL OF LAND 253
with sediment and whose courses can only be followed in sound-
ings. One of the most interesting of such channels is that which
continues the Hudson River across the continental shelf into the
deeper sea (Fig. 283).
Records of an oscillation of movement. — Because a coast
is deeply embayed is no ground for assuming that a subsidence
is now in prog-
ress, or is, in
fact, the latest
movement re-
corded upon the
coast. In many
cases it is easy
to see that such
is not the case.
The coast of
Maine is_ per-
haps as typical
of an embayed
shore line as
any that might
be cited, but a
study of the Fic. 284.— Marine clay deposits near the mouths of the rivers
river valleys in Of Maine which preserve a record of earlier subsidence (after
Stone).
the neighbor-
hood shows clearly that the present submergence of their mouths
is a fraction only of an earlier one which has left a record of its
existence in beds of marine clay which outline the earlier and far
deeper indentations (Fig. 284).
If now we give a closer examination to the coast, it is found
that there are marks of recent uplift in an abandoned shore line
now far above the reach of the waves. There is here, then, the
record, first of subsidence and consequent embayment, and, later,
of an uplift which has reduced the raggedness of the coast outline,
exposed the clay deposits, and raised the strands of the period of
deep subsidence to their present position.
In countries which possess a more ancient civilization than our
own, the record of such oscillations in the level of the ground has
sometimes been entered upon human monuments, so that it is
254 EARTH FEATURES AND THEIR MEANING
possible to date more or less definitely the periods of subsidence . 4
or elevation. At the little town of Pozzuoli, upon the shore of
the Bay of Naples, is found one of the most instructive of these ag |
records.
In the ruins of the ancient temple of Jupiter Serapis are three
marble monoliths 40 feet in height, curiously marked by a
: roughened surface between the
heights of 12 and 21 feet above
inspection shows that this rough-
ened surface has been produced
the lithodomus, which lives in the
waters of the Bay of Naples, and
to be found within the cavities
their pedestals (Fig. 285). Closer :
by a marine, rock-boring mollusk, — % 4
the shells of this animal are still
Fig. 285. — View of the three standing upon the surface of the cohimns A oy ’
columns of the temple of Jupiter Se-
rapis at Pozzuoli, showing the dark
and rough band nine feet in width have been many times recited
Pe Ao, a, aay since these interesting monu-
ments were first geologically ex-
sie by Babbage and Lyell, it may be stated that a record is’
here preserved, first of subsidence amounting to some 40 feet, and
of subsequent elevation, of the low coast land on which stood the
temple in the old Roman city of Puteoli (Fig. 286).
Without recounting details which — a
At the time of deepest submergence the top of the lithodomus
zone upon the column stood at the level of the water in the Bay of =
Naples, the smoother lower zone being buried at the time in the __
sand at the bottom, and thus made inaccessible for the lithodomi. =
It is to be added that studies made in the environs of Pozzuoli __
have fully confirmed the changes of level revealed by the columns, — =
through the discovery of now elevated shore lines which are re- P
ferable to the period of deep submergence.
Simultaneous contrary movements upon a coast.—In our a
study of the changes in the level of the ground that take place __ s
during earthquakes, it was learned that neighboring sections of
the earth’s crust may be moved at different rates or even in Op-
posite directions, notwithstanding the fact that the general méve- __
ment of the province is one of uplift. Thus during the Alaskan
te’ ww,
orem? by dom,
‘a
eh,
ery
* *sewes wall S000 99M e,
ae eee
ek
A
)
Jaa
i)
4
‘
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ee
a
Di
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= ‘ ;
oe ; Bap 4
ae =a . eg Sep Bite
= 085 ES a ee a Ses oe ak
Fi AD 8 > ak oe aes ‘ ~ nd -. a m4 mA
7 Pre Sy were ‘ . — vay
Pe ; Se le it I a ee e670 A hye LAR
be As ee eee ee es on ty ELOY)” £ Sate A Ce eae 7s AP 51a a
= - Y."s re . gee
T = vt hei ache
7 f 4 . *
*
ae :
- .
~ amas Fl
—. ° =
eee aS te —= tee
=a | os =; - QO
aks <1"
Opis)
2 ln fe eons 2 —— ed
i a
a Peek OO od
2 fare
[ee
oo al Py yeerh, N ° $ vy Z ' |
Noe" Sie RTS 13.5 Gan ee '
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Fig. 286: — Pozzuoli in the 3rd, 9th, and 20th Centuries.
256 ~ EARTH FEATURES AND THEIR MEANING
earthquake of 1899, although portions of the coast line were elevated
by as much as forty-seven feet, neighboring sections were raised by
smaller amounts, and some small sections were sunk and so far
submerged that the salt water and the beach sand were washed =
about the roots of forest trees. a
A region racked by heavy earthquakes, where the present con-
figuration of the ground speaks strongly for a movement of some-
what similar nature, but with average movement of elevation much
greater to the northward than in the opposite direction, is the ex-
tended coast line of Chili. This country is characterized by a
great central north and south valley which separates the coast Er
range from the high chain of the Cordilleras to the eastward. To
the southward the floor of this valley descends, and has its con-
tinuance in the Gulf of Corcovado behind the island of Chiloe and =
the Chonos archipelago. The known recent uplift of the coast of
Chili, particularly in the northern sections and during the earth-
quakes of the eighteenth, nineteenth, and twentieth centuries,
lends great interest to this topographic peculiarity. Indications
are not lacking that, during the earthquake of Concepcion in — 4
1835, and of Valparaiso in 1907, the measure of uplift was greater
to the north than it was to the south. =
The contrasted islands of San Clemente and Santa Catalina. —
Perhaps the most striking example of simultaneous opposite move-
“ In 5
x
mt vs
& aus ac
Fia. 287.— Map of San Clemente Island, California, showing the characteristic
topography of recent uplift (after U. S. Coast and Geodetic Survey). me
ments observable in neighboring portions of the earth’s crust :
is furnished by the coast of southern California. The coast itself a
at San Pedro and the island of San Clemente, some fifty miles off “ay
this point, in common with most portions of the neighboring coast Be x
land, have been rising in interrupted movements from the sea, and :
offer in rare perfection the characteristic coast terraces (Fig. 287 bis!
PLATE 12.
A. V-shaped cafion cut in an upland recently elevated from the sea, San Clemente
Island, California (after W. S. Tangier-Smith).
B. A “‘hogback”’ at the base of the Bighorn Mountains, Wyoming (after Darton).
—
er — Fs =e
COAST RECORDS OF THE RISE OR FALL OF LAND) 257
and Fig. 278, p. 250). Midway between these two rising sections
of the crust, and less than twenty-five miles distant from either, is
the island of Santa Catalina, which has been sinking beneath the
waves, and apparently at a similarly rapid rate (Fig. 288). The
psyr - \" foes ’ ? ‘ od
a Paes
AGA i ’
Ape \}
Fic. 288.— Map of Santa Catalina Island, California, showing the characteristic
surface of an area which has long been above the waves, and the entire absence of
coast terraces (after U. 8. C. and G.§.).
topography of the island shows the intricate detail of a maturely
eroded surface, while that of the neighboring San Clemente shows
only the widely spaced, deep cafions of the infantile stage of erosion
(Fig. 165 and pl. 12 A). While Santa Catalina has been sinking,
San Pedro Hill has risen 1240 feet, and San Clemente, 1500 feet.
It is characteristic of a sinking coast line that the cliff recession is
abnormally rapid, and evidence for this is furnished by the shores
of Santa Catalina, upon which the waves are cutting the cliffs
back into the beds of cafions, and so causing small falls to develop
at the cafion mouths.
The Blue Grotto of Capri.— We may now return to the Bay
of Naples for additional evidence that oscillations of level in
neighboring portions of the same coast are not necessarily syn-
chronous, and that near-lying sections may even move in opposite
directions at the same time, as has already been shown for the isl-
ands off the California coast. For the Pozzuoli shore of the bay
it was learned that within the Christian Era a complete cycle of
downward, followed by later upward, movement has been largely
accomplished. Across the bay, and less than 20 miles distant, is
the Blue Grotto of Capri, a sea cave cut in limestone above an
earlier cave of the same nature which is now deep below the water
surface. It is the refracted sunlight which enters the cave through
s
258 EARTH FEATURES AND THEIR MEANING
this lower submerged opening and has been robbed on the way of
all but its blue rays which gives to the famous grotto its special
charm (Fig. 289).
It is known that the former, and now submerged, sea cave was
in use by Roman patricians as a -
cool retreat from the oppressive
hot wind known as the sirocco,
and that an artificial entrance or
window was cut where is now the
only accessible entrance to the
grotto. In the ancient writings,
no mention is made, however, of
Fic. 289.—Cross section of the Blue the remarkable blue illumination
Grotto on the Island of Capri, show- for which it is now famous, and
Se adr a oe oe the conditions at the time, as we
grotto, and the higher artificial win- May see, were not such as to make
dow now widened by wave action this possible. Later subsidence
ee Roane): of the coast has brought the
ancient window to the sea level, where it has been considerably _
enlarged by the waves. The earlier grotto, abandoned. as its
entrance was closed, was rediscovered in 1826 by the painter and —
poet, August Kopisch. :
A grotto with green illumination (the Grotto Verde) is situated —
upon the opposite side of the island, and a blue grotto, having its —
origin in similar conditions to those of the famous Blue Grotto,
is found upon the island of Busi off the Dalmatian coast. zs
Character profiles. — In the landscape of a coast which has been
slowly uplifted the characteristic line is the profile of the cuesta,
with short perpendicular element joined to a gently sloping and
longer section and continued in the horizontal portion correspond-
ing to the lowland (Fig. 290). Rapidly uplifted coasts offer in
contrast the lines characteristic of wave erosion and deposition,
but at higher levels and in repeated sections. Most prominent
of all is the staircase constructed of coast terraces, with either
vertical or sloping risers and with outwardly inclining and gently
graded treads. Near the steep riser in the staircase may some-
times be seen the sugar-loaf outline of the stack cut in softer ma-
terial, or the obelisk-like pillar undercut at its base, which is carved
in firmer rock masses. With excessively rapid uplift, the double-
oa
a
COAST RECORDS OF THE RISE OR FALL OF LAND) 259
notched cliff or the double sea arch may appear in the landscape.
Upon a submerged coast the most significant lines in the view
are those of the rock islet and the steep-walled fjord.
Cuesta
=.
ik
S&S Coasr
ie Terraces High
J Strack
| High
eS Norched Stack
DQovb/e
po 4 Norched Cliff
tetbta Abandoned
“ Sarrier
Sea rch
ee ES LE Ae TT IOm
te aie Rs SUBSIDENCE
. ‘s/er
AND
SUBSIDENCE. LATER ELEVATION.
Fig. 290.—Character profiles in coast landscapes where there has been either
elevation or depression.
a READING REFERENCES FoR CHapreR XIX
‘ga General : —
Sir Cu. Lyeuu. Principles of Geology, vol. 2, pp. 180-197.
Ep. Suess. The Face of the Earth, Clarendon Press, Oxford, 1906, vol.
2, Chapters i and xiv, pp. 1-29, 535-556.
Rosert Sieger. Seenschwankungen und Strandverschiebungen in Scan-
dinavien, Zeit. d. Gesell. f. Erdk., Berlin, vol. 28, 1893, pp. 1-106,
393-688, pl. 7.
Elevated shore lines : —
F. B. Taytor. The Highest Old Shore Line of Mackinac Island, Am.
Jour. Sci., vol. 43, 1892, pp. 210-218.
Tuomas L. Watson. Evidences of Recent Elevation of the Southern
Coast of Baffins Land, Jour. Geol., vol. 5, 1897, pp. 17-33.
J. W. Gotptawait. The Abandoned Shore Lines of Eastern Wisconsin.
Bull. 17, Wis. Geol. and Nat. Hist. Surv., 1907, pp. 134, pls. 1-37.
Evidences of depression : —
oF W. 8B. Scorr. Introduction to Geology, New York, 1907, pp. 33-36.
bey W J McGer. The Gulf of Mexico as a Measure of Isostacy, Am.
: Jour. Sci. (3), vol. 44, 1892, pp. 177-192.
2 es ee
ct To ey * 7
260 EARTH FEATURES AND THEIR MEANING
A. Linpenxout. Notes on the Submarine Channel of the Hudson
River, ete., Am. Jour. Sci. (3), vol. 41, 1891, pp. 489-499, pl. 18.
J. W. Spencer. The Submarine Great Cafion of the Hudson River,
ibid. (4), vol. 19, 1905, pp. 1-15; Submarine Valleys off the American
Coast and in the North Atlantic, Bull. Geol. Soc. Am., vol. 14, 1903,
pp. 207-226, pls. 19-20.
F. Nansen. The Bathymetrical Features of the North Polar San: with a
Discussion of the Continental Shelves and Previous Oscillations of
Shore Line, Norwegian North Polar Expedition, vol. 4, 1904, pp. 99- ‘ie
231, pl. 1.
W. v. Kneset. Hoéhlenkunde, etc., Braunschweig, 1906, pp. 175-177 |
(the blue grotto of Capri).
Oscillation of movement : —
C. Lyretu. Principles of Geology, vol. 2, pp. 164-176 (Temple of Jupiter
Serapis).
KE. Ray LankestTerR. Extinet Animals, New York, 1905, pp. 31-42. .
H. W. Farrpanks. Oscillations of the Coast of California during the ~
Pliocene and Pleistocene, Am. Geol., vol. 20, 1897, pp. 213-245.
G. H. Stone. Mon. 34, U.S. Geol. Surv., 1899, pp. 56-58, pl. 2.
Baitey Wiuuis. Ames Knob, North Haven, Maine. Bull. Geol. Soe. : 3
Am., vol. 14, 1903, pp. 201-206, pls. 17-18.
Simultaneous contrary movements on a coast : — “4
A.C. Lawson. The Post-Pliocene Diastrophism of the Coast of Southern 4
California, Bull. Univ. Calif. Dept. Geol., vol. 1, 1893, PP. 115-160,
pls. 8-9.
W. S. Taneier-SmitH. A Geological Sketch of San Clemente Island,
18th Ann. Rept. U.S. Geol. Surv., Pt. ii, 1898, pp. 459-496, pls. 84-96.
R. S. Tarr and L. Martin. Recent Changes of Level in the Yakutat
Bay Region, Alaska, Bull. Geol. Soc. Am., vol. 17, 1906, pp. 29-64, |
pls. 12-23.
CHAPTER XX
THE GLACIERS OF MOUNTAIN AND CONTINENT
Conditions essential to glaciation. — Wherever for a suffi-
ciently protracted period the annual snowfall of a district is in
excess of the snow that is melted, a residue must remain from
each season to be added to that of succeeding ones. Eventually
so much snow will have accumulated that under its own weight
and in obedience to its peculiar properties, a movement will begin
within the mass tending to spread it and so to reduce the slope
of its upper surface (Frontispiece plate). The conditions favorable
to glaciation are, therefore, heavy precipitation and low annual
temperature. If the precipitation is scanty, the small snowfall
is soon melted; and if the temperature be too high, the moisture
is precipitated not in the form of snow but as rain. It is impor-
tant here to keep in mind that snow is a poor heat conductor
and itself protects its deeper layers from melting.
The snow-line.— Because of the low temperatures glaciers
should be most abundant or most extensive in high latitudes and
in high altitudes. The largest are found in polar and sub-polar
regions, and they are elsewhere encountered only at considerable
elevations. The largest glaciers are the vast sheets of ice which
inwrap the continents of Greenland and Antarctica, but glaciers
of large size are to be found upon other large land masses of the
Arctic, as well as in Alaska, in the southern Andes, and in New
Zealand. Much smaller glaciers are characteristic of certain
highlands within temperate and tropical regions, but because
of specially favorable conditions both of altitude and _ precipi-
tation the Himalayas, although in relatively low latitudes, nourish
glaciers of large proportions. In general, it may be said that
the nourishing grounds of glaciers are largely restricted to those
areas where snow covers the ground throughout the year. The
lower margin of such areas is designated the snow line, and varies
but little from the line on which the average summer tempera-
ture is at the freezing point of water —the so-called swmmer
261
262 - EARTH FEATURES AND THEIR MEANING
isotherm of 32° Fahrenheit. Within the tropics this line may
rise as high as 18,000 feet above the sea, whereas in polar lati-
tudes it descends to sea level.
Importance of mountain barriers in initiating glaciers. — The
precipitation within any district depends, however, not alone ~
upon the amount of moisture which is brought to it in the clouds,
but upon the amount which is abstracted before the clouds have
passed over it. The capacity of space to hold moisture increases
with its temperature, and hence any lowering of this temperature
will reduce the capacity. If lowered sufficiently, the point of
complete saturation will be reached and further cooling must
result in precipitation. Hence, anything which forces an air.
current to rise into more rarefied zones above, will lower the pres-
sure upon it and so bring about a cooling effect in which no heat
is abstracted. This so-called adiabatic refrigeration of a gas —
may be illustrated by the cool current which issues in a jet from
a warm expanded rubber tire after the cock has been opened; or __
even better, by the instant solidification at extreme low tempera- ie
tures of such normal gases as carbonic acid when they are allowed _
to issue under heavy pressure from a small orifice. ae
As applied to moisture-laden and near-surface winds, the
effective agents of adiabatic cooling are the upland areas upon —
the continents, and especially the ranges of mountains. These —
barriers force the moving clouds to rise, cool, and deposit their —
moisture. It is, therefore, the highland barriers which face the
on-coming, moisture-laden winds that receive the heaviest pre-
cipitation. Within temperate regions, because of the prevalence
of westerly winds, those barriers which face the western shores —
receive the heaviest fall. Within the tropics, on the other hand,
it is the barriers facing the eastern shores which, because of the —
easterly “‘ trades,” are most favorable to precipitation. ae
Thus it is in the Sierra Nevadas of California, and not in the _
Rockies or the Appalachians, that the glaciers of the United States
are found. The highland of the Swiss Alps lying likewise athwart —__
the ‘“ westerlies” of the temperate zone acquires the moisture
for nourishment of its glaciers from the western ocean — here
the Atlantic (Fig. 291). Within the tropics the conditions are
reversed, and it is in general the ranges which lie nearer the eastern
coasts that are the more favored. If no barrier is found upon
THE GLACIERS OF MOUNTAIN AND CONTINENT 263
this coast, the clouds may travel over vast stretches of country
before being arrested by mountains and robbed of their moisture.
Thus in tropical Brazil the glaciers are found in the Andes upon
the Pacific coast though nourished by clouds from the Atlantic.
Fia. 291. — Map showing the distribution of existing glaciers, and the two im-
portant wind poles of the earth.
Sensitiveness of glaciers to temperature changes. — How
sensitive is the adjustment between snow precipitation and tem-
perature may be strikingly illustrated by the statement on ex-
cellent authority that if the average annual temperature of the air
within the Scottish Highlands should be lowered by only three
degrees Fahrenheit, small glaciers would be the result; and a
moderate temperature fall within the region surrounding the
Laurentian lakes of North America would bring on glaciation,
otherwise expressed as a depression of the snow line of the region.
The cycle of glaciation. — Though to-day buried beneath its
ice mantle, it is known that Greenland had more than once in earlier
geological ages a notably mild climate, and in some future age
it may revert to this condition. In other regions, also, we have
- evidence that such a rotation of climatic changes has been suc-
cessively accomplished, the climate having steadily increased in
severity towards a culminating point, and been followed by a
reverse series of changes. Such a complete period may be called
a cycle of glaciation. While the climate is steadily becoming
more rigorous, we have to do with an advancing hemicycle of
264 EARTH FEATURES AND THEIR MEANING
glaciation, but after the culminating point has been reached, the
period of amelioration of climate is the receding hemicycle.
The advancing hemicycle. — There is little reason to doubt.
that whatever be the cause of the climatic changes which bring
on glacial conditions, these changes come on by insensible grada-
tions. The first visible evidence of the increased severity of
the climate is the longer persistence of the winter snows, at first
}
Lede ‘
“Za VY eo eS
BULB suscreh ae r = a
of
A:
- ft
nN \s
0
LOWER COPPER RIVER
=
S
SS .
aN
Mey.
SF ee
SAC
AG
iP A =
Fig. 292. — An Alaskan glacier spreading out at the foot of the range which
nourishes it.
within the more elevated districts. In such positions drifts must — | a
eventually continue throughout the warm season and so con-
tribute to the snow accumulations of the succeeding winter. This 3 |
point once reached, small glaciers are inevitable, even should the
average temperature fall no further, for the snow left over in
each season must steadily increase the depth of the deposits until
the weight brings about an internal motion of the mass from higher ~ ae
. to lower levels.
The inherited depressions of the upland — the gentle hollows
at the heads of rivers — will first be filled, and so the valleys
aie
=tun ee
ae fy. ee ae
’
THE GLACIERS OF MOUNTAIN AND CONTINENT 265
below become the natural channels for the outflow of the early
glaciers. With a continued lowering of the annual temperature
and consequent increased snowfall, the early glaciers become
more and more amply nourished. Snow and ice will, therefore,
cover larger areas of the upland, and the glaciers will push their
fronts farther down the valleys before they are wasted in the
warm air of the lower levels. As the valleys become thus more
completely invested by the glacier they are likewise filled to greater
and greater depths, and they may thus submerge portions of the
walls that separate adjacent valleys. Reaching at last the front
of the upland area, the glaciers may now be so well nourished at
their heads that they push out upon the flatter foreland and with-
out restraint from retaining walls spread broadly upon it (Fig. 292).
The culmination of the progressive climatic change may ere
this have been reached and milder conditions have ensued. If,
however, the severity of the climate should be still further in-
creased, the expanded fronts of neighboring glaciers will coalesce
to form a common ice fan or apron along the foot of the upland
(Plate 18B). This could hardly take place without a still further
deepening of the ice within the valleys above, and, probably, a
progressive submergence of the lower crests in the valley walls.
“tn
; NG 2 ee ee
—_—— oe “
—— =—
from retaining walls. Surface of the Folgefond, an ice cap of southern Norway.
266 EARTH FEATURES AND THEIR MEANING
This may even continue until all parts of the upland area have
been buried. The snow and ice now take the form of a covering
cap or carapace, and the upper portions being no longer restrained
at the sides, now spread into a broad dome, as would a viscous _
liquid like thick molasses when poured out upon the floor (Fig.
293). The lower zones of the mass and the thinner marginal
portions still have their motion to a greater or less extent con-
trolled by the irregularity of the rock floor against which they rest.
The reverse series of changes in the glacier is inaugurated by an
amelioration of the climate, and here, therefore, the advancing
hemicycle becomes merged in the receding hemicycle of glacia-_
tion. ;
Continental and mountain glaciers contrasted.— The time
when the rock surface becomes submerged beneath the glacier
is, as regards both the surface forms and the erosive work, a criti-
cal point of much significance; for the ice cap and larger conti-
nental glacier obviously protect the rock surface from the action
of those chemical and mechanical processes in which the atmosphere
enters as chief agent, and which are collectively known as weath-
ering processes. Until submergence is accomplished, larger or
smaller portions of the rock surface project either through or
between the ice masses and are, therefore, exposed to direct
attack by the weather (see below, p. 370).
Snow which falls in the mountains is not allowed to remain
long where it falls. By the first high wind it is swept off the
more elevated and exposed surfaces and collected under eddies in
any existing hollows, but especially those upon the lee slopes of.
the range. We are to learn that glaciers carve the mountains by
enlarging the hollows which they find and producing great basins
for the collection of their snows; but with the initiation of gla-
ciation the inherited hollows are in most cases the unimportant
depressions at the heads of streams. Whatever they may be
and however formed, the snow first fills those hollows which are
sheltered from the wind, and as it accumulates and becomes
distributed as ice, assumes a surface of its own that is dependent
upon the form and the position of the basin which it occupies
(see Fig. 294).
When the quantity of accumulated snow is so great that all
hollows of the rock surface are filled, its own surface is no longer
THE GLACIERS OF MOUNTAIN AND CONTINENT 267
controlled by retaining rock walls, and it now assumes a form
largely independent of the irregularities in the upland. Expe-
ff,
YY a YYy
Fic. 294. — Section through a mountain glacier (in solid black), showing how its
surface is determined by the irregularities in the rock basement (after Hess).
rience shows that this surface is approximately that of a flat dome
or shield, and as it covers all the upland, save where the ice thins
upon its margins, this type of glacier is called an ice cap (Fig.
295). All types of glacier in which rock projects above the
highest levels of the ice and snow are known as mountain glaciers.
~2000— ca.i900m
ee eM ees ee ~SEnde 2 -
a ' ee | Me +1500 -
—1000 ¢'s +—1000—
ok a ae '
—568 500 -
Om Ni am
Fig. 295. — Profile across the largest of the Icelandic ice caps, with the vertical
scale greatly exaggerated (after Thoroddsen and Spethmann).
The flat domes of ice which mantle the continents of Green-
land and Antarctica, though resembling in form the smaller ice
cap, are yet because of their vast size so distinct from them, par-
ticularly in the manner of their nourishment, that they belong in
a separate class described as inland ice or continental glaciers.
Though they have some affinities with ice caps, they are most
sharply differentiated from all types of mountain glaciers. Of them
it is true that the lithosphere projects through them only in the
neighborhood of their margins (Fig. 296), whereas in the case of
Biss Sige ie > cs lad ARR iro
Fia. 296. — Ideal section across a continental glacier, with the vertical scale and
the projecting rock masses of the marginal zone greatly magnified.
268 EARTH FEATURES AND THEIR MEANING
mountain glaciers rock may project at any level but always above
the highest snow surface. Ice caps may be regarded as interme-
diate between the two main classes of mountain and continental
glaciers (Fig. 297). Because of the large réle which continental
fl oe —
Fig. 297.— View of the Eyriks-Jékull, an ice-cap of Iceland (after Grossman).
glaciers have played in geological history, it is thought best to con-
sider them first, leaving for later discussion the no less interest-
ing but less important mountain glaciers.
The nourishment of glaciers. — The life of a glacier is depend-
ent upon the continued deposition of snow in aggregate amount
in excess of that which is lost by melting or by other depleting
processes. Whenever, on the other hand, the waste exceeds the °
precipitation, the glacier is in a receding condition and must
eventually disappear, if such conditions are sufficiently long con-
tinued. The source of the snow is the water of the ocean evapo-
rated into the atmosphere and transported over the land in the
form of clouds. We are to learn that the changes which this.
moisture undergoes before its delivery to the glacier are notably
different for the classes of continental and mountain glacier.
The upper and lower cloud zones of the atmosphere. — Be-
fore we can comprehend the nature of the processes by which gla-
ciers are nourished, it will be necessary to review the results of
recent studies made upon the earth’s atmospheric envelope. It
must be kept in mind that the sun’s rays are chiefly effective in
warming the atmosphere through being first absorbed by some
solid body such as rock or water and their heat then communicated
by contact to the immediately adjacent air layers. The layers thus
warmed being now lighter than before, they rise and are replaced
by colder air, which in its turn is warmed and likewise set in up-
ward motion. Such currents developed in the air by contact
with warmer solid bodies constitute the process known as con-
vection.
wee SS Te CC LC TT
THE GLACIERS OF MOUNTAIN AND CONTINENT 269
To a relatively small degree the atmosphere is heated by the
direct absorption of the sun’s rays which pass through it. Since
air has weight, it compresses the lower layers near the earth, and
hence as we ascend from the earth’s surface the air becomes con-
tinually lighter. Convection currents must, therefore, adjust
themselves by the air expanding as it rises. But expansion of a
gas always results in its cooling, as every one must have observed
KILOMETERS CENTIGRADE
Re econo: Bere ee MA
| OR
Po DAVE fo iv E
ae iN iG
Cerling Of convective zone
a
a
a
aay
= (7 miles abhouwe sea level.)
e “U. UPRER ECONG EAE D:VAPOR®
-
3
wn
4 nr
Fic. 298.— The zones of the inwus atmosphere as revealed by recent kite and
balloon explorations.
who has placed his finger in the air current which escapes from
the open valve of a warm rubber tire. Dry air is cooled a degree
Fahrenheit for every six hundred feet of ascent in the atmos-
phere. At a height of about seven miles above the earth’s sur-
face all rising air currents have cooled to about 68° below the
zero of the Fahrenheit scale, and exploration with balloons has
shown that the currents rise no farther. At this level they
270 EARTH FEATURES AND THEIR MEANING
move horizontally, just as rising vapor spreads out in a room be-
neath the ceiling. Above this level, as far as exploration has gone,
or to a height of more than twelve miles, the temperature remains
nearly constant, and this upper zone is, therefore, called the zso-
thermal or the advective zone—the uniform temperature zone
of the lower atmosphere. Beneath the convective ceiling the
process of convection is characteristic, and this zone is therefore
described as the convective zone (Fig. 298).
A large part of the moisture which rises from the ocean’s sur-
face is condensed to vapor before it has ascended three miles, and in
this form it makes its transit over land as fleecy or stratiform
clouds — the so-called cumulus and stratus clouds and their many
intermediate varieties (see Frontispiece). This lower layer within
the convective zone is, therefore, a moist one overlaid by a rela-
tively drier middle layer of the convective zone. That mois-
ture which rises above the lower cloud layer is congealed by adia-
batic cooling to fine ice needles visible as the so-called cirrus
clouds which float as feathery fronds beneath the convectiv.
ceiling (see frontispiece at right upper corner of picture). TH
we have within the convective zone an upper layer more or less
charged with water in the form of ice needles. It is the clouds
of the lower zone whose moisture in the form of vapor supplies
the nourishment of mountain glaciers, and the high: cirrus clouds
_ whose congealed moisture, after interesting transformations, is
responsible for the continued existence of continental glaciers.
As we are to see, there are other noteworthy differences be-
tween continental and mountain glaciers, in the manner of their
sculpture of the lithosphere, so that long after they have disap-
peared the characters of each are easily identified in their handi-
work. How the lower clouds are forced upward and so compelled
to give up their moisture to feed the mountain glaciers, and how
the upper clouds are pulled downward to nourish the glaciers of
continents, can be best understood after the characteristics of
each glacier class have been studied.
CHAPTER XXI
THE CONTINENTAL GLACIERS OF POLAR REGIONS
The inland ice of Greenland.—In Greenland and in Antarc-
tica the land is almost or quite buried under a cover of snow and
ice—the so-called ‘‘inland ice”’
— which always assumes the
surface of a very flat dome or
shield. In Greenland there is
found a marginal ribbon of
land generally from five to
twenty-five miles in width
(Fig. 299), but in Antarctica
all the land, with the excep-
tion of a few mountain peaks,
is inwrapped in a mantle of
ice which is also extended upon
the sea in a broad shelf of snow
andice. Neither of these vast
glaciers has been explored ex-
cept in its marginal portion,
yet such is the symmetry of
the profiles along the routes
traversed, and such the flat-
ness and monotony of the snow
surface within the margins,
that there is little reason to
doubt that the profile made Fic. 299.— Map of Greenland showing the
along Nansen’s route in south- 27° of inland-ice and the routes of differ-
ent explorers.
ern Greenland would, save only
for magnitude, fairly represent a section across the middle of the
continent (Fig. 300).
The mountain rampart and its portals. —As soon as we ex-
amine the coastal belt we observe that the “‘ Great Ice” of
271
272 EARTH FEATURES AND THEIR MEANING
Greenland is held in by a wall of mountains and so prevented from
spreading out to its natural surface in the marginal portions.
Through portals of the inclosing mountain ranges — the ouf-
lets —it sends out tongues of ice which in many respects resemble
certain types of mountain glaciers.
Fic. 300. — Profile in natural proportions across the southern end of the continental
glacier of Greenland, constructed upon an arc of the earth’s surface and based
upon Nansen’s profile corrected by Hess. The marginal portions of the profile
are represented below upon a magnified scale in order to bring out the characters
of the marginal slopes.
Such measurements as have been made upon the inland ice
of Greenland at points back from, but yet comparatively near to,
the outlets, show that it has here a surface rate of motion amount-
ing to less than an inch per day, and it is highly probable that at
moderate distances from the margin this amount diminishes to
zero. Upon the outlets, on the contrary, surface rates as high as
59 feet per day have been measured, and even 100 feet per day has
been reported. We are thus justified in saying that glacier flow
within the outlets is from 700 to 1000 times as great as it is upon
the near-by inland ice, and that the glacier is in a measure drained
through the portals of the inclosing ranges. Back from these
outlet streams of ice, or tongues, the surface of the inland ice is
depressed to form a dimple or “ basin of exudation ” as is the sur-
face of areservoir above the raceway when the water is being rapidly
drawn away (Fig. 301).
Fissures in the ice, the so-called crevasses, are the recognized —
marks of ice movement, and these are always concentrated at the
steep slopes of the ice surface in the neighborhood of its margins.
Upon the Greenland ice, crevasses are restricted in their distribu-
tion to a zone which extends from seven to twenty-five miles
within the ice border.
The marginal rock islands. — From its margin the ice surface
rises so steeply as to be climbed only with difficulty, but this
- ee ee ee, ee
PLATE 13.
A. Precipitous front of the Bryant glacier outlet of the Greenland inland-ice (after
Chamberlin).
B. Lateral stream beside the Benedict glacier outlet, Greenland (after R. E. Peary).
STORES EE UE See
oe ae
. i ©
War . TR ee
pe ee ee, = ' rs
ey ‘e
. Kay eat
THE CONTINENTAL GLACIERS OF POLAR REGIONS 273
gradient steadily diminishes until at a distance of between seventy-
five and a hundred miles its slope is less than two degrees. Where
crossed by Nansen near latitude 64° N. the broad central area of
ice was so nearly level as to
appear to be a plain.
As we pass across the irregu-
lar ice margin in the direction
of the interior, larger and larger
proportions of the land’s sur-
face are submerged, until only
projecting peaks rise above
the ice as islands which are
described as nunataks (Fig.
302).
Though not a universal ob-
servation, it has been often
noted that the absorption of
the sun’s rays by rock masses
projecting through the snow
results in a radiation of the
heat and a lowering by melting
of the surrounding snow and
ice. For this reason nunataks
are often surrounded by a deep
trench due to a melting of the
snow. Such a depression in
the ice surface about the mar-
gin of a nunatak, from its re-
semblance to a trench about
an ancient castle, has been
designated a moat (Fig. 303).
For the same reason, the out-
let tongues of ice which descend
in deep fjords between walls of
rock are melted away from
the walls and a lateral stream
of water is sometimes found
to flow between ice and rock
(pl. 13 B).
Tr
Contours
On ieee
on lang —-—-—
° i 2 3
Kilometers
=
Fie. 301.— Map of a glacier tongue, with
dimple showing above and due to in-
draught of theice. Umanakfjord, Green-
land (after von Drygalski).
274 EARTH FEATURES AND THEIR MEANING
Rock fragments which travel with the ice. — Rock surfaces
which are exposed to the atmosphere are in high latitudes broken
: ? down through the freez-
———- ! ing of water within their
| crevices. The frag-
. RAN ) < ==
ER
A AN ments resulting from
ans this rending process fall
upon the glacier surface
and are carried forward
as passengers in the di-
é FTAs rection of the ice mar-
OS ee . gin. They are either
Fig. 302.—Edge of the Greenland inland ice, visible as long and nar-
showing the nunataks diminishing in size toward yow ridges or trains fol-
the interior. The lines upon the ice are medial
moraines starting from nunataks (after Libbey).
lowing the directions of
the steepest slope (Fig.
302), or they become buried under fresh falls of snow and only
again become visible where summer melting has lowered the glacier
surface in the vicinity of its margin. These longitudinal trains of
rock fragments upon the glacier surface always have their starting
point at the lower margin of one of the nunataks, and are known
as medial moraines (Fig. 301, p. 273, and Fig. 302). Inside
the zone of nunataks the glacier surface is, however, clear of rock
débris except where dust has
been blown on by the wind,
and this extends for a few
miles only. The material of
the medial moraines is a col-
lection of angular blocks whose
surfaces are the result of frost
rending, for in their travel
above the ice they are sub-
jected to no abrading pro
cesses.
A contrasted type of surface
moraines upon the Greenland F%- 30
glacier, instead of being par-
allel to the direction of ice movement, is directed transversely. or
parallel to the margins. The materials of these moraines are
3. — Moat surrounding a nunatak in
Victoria Land (after Scott).
fos “<a vit rae
THE CONTINENTAL GLACIERS OF POLAR REGIONS 275
more rounded fragments of rock which have come up from the
bottom layers, and we shall again refer to the origin of such
moraines after the subglacial conditions have been considered.
The grinding mill beneath the ice. — If, now, we examine the
front of a glacier tongue which goes out from the inland ice, we
find that while the upper portion is white and mainly free from rock
débris (plate 13 A), the lower zone is of a dark color and crowded
with layers of pebbles and bowlders which have been planed,
polished, and scratched in a quite remarkable manner. The ice
front is itself subject to forward and retrograde migrations of short
period, but it is easily seen that in the main its larger movement
has been a retrograde one. The ground from which it has lately
withdrawn is generally a hard rock floor unweathered, but smooth,
polished, and scratched in the same manner as the bowlders which
are imbedded within the ice. It is perfectly apparent that the
latter have been derived from some portion of the rock basement
upon which the glacier still rests, and that floor and bowlders have
alike been ground smooth by mutual contact under pressure.
This erosion beneath the ice is accomplished by two processes;
namely, plucking and abrasion. Wherever the rock over which
the glacier moves has stood up in projecting masses and is riven
_ by fissure planes of any kind, the ice has found it easy to remove
it in larger or smaller fragments by a quarrying process described
as plucking. The rock may be said to be torn away in blocks which
are largely bounded by the preéxisting fissure planes. Over rela-
tively even surfaces plucking has little importance, but where
there are noteworthy inequalities of surface upon the glacier bed,
those sides which are away from the oncoming ice (lee side) are
degraded by plucking in such a manner as sometimes to leave
steep and ragged fracture surfaces. The tools of the ice thus ac-
quired in the process of plucking are quickly frozen into the lowest
ice layers, and being now dragged along the floor they abrade in
the same manner as does a rasp or file. These tools of the ice are
themselves, worn away in the process and are thus given their
characteristic shapes. Just as the lapidary grinds the surface of
a jewel into facets by imbedding the gem in a matrix, first in one
and then in another position, each time wearing down the pro-
jecting irregularities through contact with the abrading surface;
so in like manner the rock fragment is held fast at the bottom of
276 EARTH FEATURES AND THEIR MEANING
the glacier until ‘‘ soled ” or “‘ shod,”’ first upon one side and then
upon another. Accidental contact with some obstruction upon
the floor may suffice to turn the fragment and so expose a new sur-
face to wear upon the abrading floor. Minor obstructions com-
ing in contact with one side of the fragment only, may turn it in ©
its own plane without overturning. Evidence of such interrup-
tions can be later read in the different directions of striz upon
the same facet (plate 17 A).
The floor beneath the glacier is reduced by the abrading process
to a more or less smooth and generally flattened or rounded sur-
face — the so-called glacier pavement (Fig. 304). To accomplish
this all former mantle rock
due to weathering processes
must first be cleared away,
and the firm unaltered rock
beneath is wherever suscep-
tible of it given a smooth
polish although locally
scored and scratched by the
grinding bowlders. The
earlier projections of the
surface of the floor, if not
Fig. 304. — A glacier pavement of Permo-Car-
boniferous age in South Africa. The striz x
running in the direction of the observer are entirely planed away, are
prominent and a noteworthy gouging ofthe at least transformed into
surface is to be noted to the right in the
middle distance (after Davis). rounded shoulders of rock,
which from their resem~
blance to closely crowded backs in a flock of sheep have been
called ‘“‘sheep backs” or ‘‘roches moutonnées.”? Thus the effect
of the combined action of the processes of plucking and abrasion
is to reduce the accent of the relief and to mold the contours of
the rock in smoothly flowing curves, generally of large radius.
The lifting of the grinding tools and their incorporation
within the ice. — Wherever the ice is locally held in check by the
projecting nunataks, relief is found between such obstructions,
and there the flow of the ice has a correspondingly increased ve-
locity (Fig. 305 6). If the obstructions are not of large dimensions,
the ice which flows around the outer edges is soon joined to that
which passes between the obstructions and so normal conditions
of flow are restored below the nunataks. The locally rapid flow
-
i lS Seto
eae eee =
ON LS en
a oe ee eee. ee
THE CONTINENTAL GLACIERS OF POLAR REGIONS 277
of the ice is, therefore, restricted to a relatively short distance, the
passageway between the nunataks, and the conditions are thus
to be likened to the fall of water at a raceway due to the sudden
descent of its surface from the level of the reservoir to the level of
the stream in the outlet. As is well known, there is under these
conditions a prodigious scour upon the bottom which tends to dig
a pit just above and below the dam — a scape colk — and carry
the materials up to the surface below the pit. Such a tendency
was well illustrated by the behavior of the water at the opening
‘of the Neu Haufen dam below the city of Vienna (Fig. 305a). In
Scale 1 : 5760 (1 = 80").
® » rd =e
a
Fig. 305.— a, Map showing pit excavated by the current below the opening in a
dam. 6, Nunataks and surface moraines on the Greenland ice. Dalager’s
Nunataks (after Suess).
the case of ice, material from the bottom may by the upward cur-
rent be brought up to the surface of the glacier at the lower edge
of the colk and thus produce a type of local surface moraine of:
horseshoe form with its direction generally transverse to the direc-
tion of ice movement (Fig. 305 b).
Any obstruction upon the pavement of the glacier apparently
exerts a larger or smaller tendency to elevate the bowlders and
pebbles and incorporate them within the ice. Rock débris thus
incorporated is described as englacial drift. In the case of Green-
land glaciers this material seems at the ice front to be largely re-
stricted to the lower 100 feet (plate 13 A).
Near the front of the inland ice the increased slope of the upper
surface greatly increases the flow of the upper ice layers in com-
278 EARTH FEATURES AND THEIR MEANING
parison with those nearer the bottom, so that the upper layers
override the lower as they would an obstruction. The englacial
drift is either for this reason or because of rock obstructions
brought to the surface, where it yields parallel ridges corresponding —
in direction to the glacier margin. Such transverse surface mo-—
raines are thus in many respects analogous to those which ap-
pear about the lower margins of scape colks. In contrast to the
longitudinal or medial surface moraines the materials of the trans-
verse moraines are more faceted and rounded — they have been
abraded upon the glacier pavement.
Melting upon the glacier margins in Greenland. — During the
short but warm summer season, the margins of the Greenland ice
are subject to considerable losses through surface melting. When
the uppermost ice layer has attained a temperature of 32° Fahren-
heit, melting begins and moves rapidly inward from the glacier
margin. In late spring the surface of the outer marginal zone is.
saturated with water, and this zone of slush advances inward with
the season, but apparently never transgresses the inner border of
what we have generally referred to as the marginal zone of the ice
characterized by relatively steep slopes, crevasses, and nunataks.
Upon the ice within this zone are found streams large enough to be
designated as rivers and these are connected with pools, lakes, and
morasses. The dirt and rock fragments imbedded in the ice are
melted out in the lowering of the surface, so that late in the season
the ice presents a most dirty aspect. At the front of the great
mountain glaciers of Alaska, a more vigorous operation of the same
process has yielded a surface soil in which grow such rank forests
as entirely to mask the presence of the ice beneath.
In addition to the visible streams upon the surface of the Green-
land ice, there are others which flow beneath and can be heard by
putting the ear to the surface. All surface streams eventually |
encounter the marginal crevasses and plunge down in foaming
cascades, producing the well known “ glacier wells” or “ glacier
mills.” The progress of the water is now throughout in tunnels
within the ice until it again makes its appearance at the glacier
margin.
The marginal moraines. — Study of both the Greenland and
Antarctic glaciers has shown that if we disregard the smaller and
short-period migrations of the ice front, the general later move-
eS
ss
FR OT PO A ah
SF,
as,
hd
eee oi es 0
ewes
“we
ot
Ki
7
h > ;
‘
-
maaan Sante fi
THE CONTINENTAL GLACIERS OF POLAR REGIONS 279
ment has been a retrograde one — we live in a receding hemicycle
of glaciation. The earlier Greenland glacier has now receded so
as to expose large areas of
the former glacier pave-
ment. In places this
pavement is largely bare,
indicating a relatively rapid pas : ee
retirement of the ice front, f aes
but at all points at which
the ice margin was halted
there is now found a ridge
of unassorted rock mate- (“SSS ==
rials which were dropped Fie. 306.— Marginal moraine now forming at.
2 ‘ : the edge of Greenland inland ice, showing a
by the ice as it melted (Fig. smooth rock pavement outside it. A small
306). Such ridges, com- lake with a partial covering of lake ice occu-
posed of the unassorted pies a hollow of this pavement (after von
materials described as tall, strikes”
come to have a festooned arrangement largely concentric to the ice
margin, and are the marginal or terminal moraines (see Fig. 336,
p. 312). Marginal moraines, if of large dimensions, usually have a
hummocky surface, and are apt to be composed of rock fragments
of a wide range of size from rock flour (clay) to large bowlders
(plate 17 A), which may represent many types since they have
| been plucked by the glacier
or gathered in at its surface
from many widely separated
localities.
As the glacier front retires
from the moraine which it
has built up, the water which
emerges from beneath the
ice is impounded behind the
FR, = new dam so as to form a
Fic. 307.—Small lake impounded between lake of crescentic outline
the ice front and a moraine which it has (Fig. 307). Such lakes are
recently built. Greenland (after von Dry-
galski). particularly short-lived, for
. the reason that the water
finds an outlet over the lowest point in the crest of the moraine
and easily cuts a gorge through the loose materials, thus draining
280 EARTH FEATURES AND THEIR MEANING
the lake (Fig. 308). Thereafter, the escaping water flows in a
braided stream across the late lake bottom and thence at the
bottom of the gorge through the moraine.
LORS
Fia. 308. — View of a drained lake bottom between the woraine-covered ice front
in the foreground and an abandoned marginal moraine in the middle distance. The
water flows from the ice front in a braided stream and passes out through the mo-
raine in a narrow gorge. Variegated glacier, Alaska (after Lawrence Martin).
The outwash plain or apron. — The water which descends from
the glacier surface in the glacier wells or mills, eventually arrives
at the bottom, where it follows a sinuous course within a tunnel
melted out in the ice. Much of this water may issue at the ice
front beneath the coarse rock materials which are found there,
and so be discovered with the ear rather than by the eye. The
water within the tunnels not flowing with a free surface but being
confined as though it were in a pipe, may, however, reach the
glacier margin under a hydrostatic pressure sufficient to carry it
up rising grades. Inasmuch as it is heavily charged with rock
débris and is suddenly checked upon arriving at the front it de-
posits its burden about the ice margin so as to build up plains of
assorted sands and gravels, and over this surface it flows in ever
shifting serpentine channels of braided type (Fig. 308).. Such
plains of glacier outwash are described as outwash plains or out-
wash aprons. 2
Rising as it does under hydrostatic pressure the water issuing
at the glacier front may find its way upward in some of the cre-
¥
;
i
M,
‘
4
j
‘4
(
y
,
THE CONTINENTAL GLACIERS OF POLAR REGIONS 281
vasses and so emerge at a level considerably above the glacial
floor. It may thus come about that the outwash plain is built
up about the nose of the glacier so as partially to bury it from
Fic. 309. — Diagrams to show the manner of formation and the structure of an
outwash plain, and the position of the fosse between this and the moraine.
sight. When now the ice front begins a rapid retirement, a de-
pression or fosse (Fig. 309 and Fig. 339, p. 314) is left behind the
outwash plain and in front of the moraine which is built up at the
next halting place.
The continental glacier of Antarctica. — In Victoria Land, upon
the continent of. Antarctica, so far as exploration has yet gone,
the continental glacier is held back by a rampart of mountains,
as has been shown to be true of the inland ice of Greenland. The
same flat dome or shield has likewise been found to characterize
its upper surface (Fig. 310).
The most noteworthy differences between the inland ice masses
of Greenland and Antarctica are to be ascribed to the greater
severity of the Antarctic climate and to the more ample nourish-
ment of the southern glacier measured by the land area which it
has submerged. There is here no marginal land ribbon as in Green-
land, but the glacier covers all the land and is, moreover, extended
upon the sea as a broad floating terrace — the shelf ice (Fig. 311).
This barrier at its margin puts a bar to all further navigation,
rising as it does in some cases 280 feet above the sea and descend-
ing to even greater depths below (plate 15 B).
In that portion of Antarctica which was explored by the German
expedition, the inland ice is not as in Victoria Land restrained
within walls of rock, but is spread out upon the continent so as to
assume its natural ice slopes, which are therefore much flatter
282 EARTH FEATURES AND THEIR MEANING
than those examined in Greenland and Victoria Land. Here in
Kaiser Wilhelm Land the ice rises at its sea margin in a cliff which
is from 130 to 165 feet in height, then upon a fairly steeply curving
Sour
Fig. 310. — Map showing the inland ice of Victoria Land bordered by the shelf
ice of the Great Ross Barrier. The arrows show the direction of the prevailing
winds (based on maps by Scott and Shackleton).
slope to an elevation of perhaps a thousand feet. Here the grades
have become relatively level, and on ever flatter slopes the surface ~
appears to continue into the distant interior (plate 14). Near
the ice margin numerous fissures betray a motion within the mass
tm!
“(PISTeBAIC, “A “GY Joye) puULT WUPOYTIM Jostey Ur Je~R[s [eyUaUTZUOD oNOIRIUY 94} JO UIsIvUI 94} JO MaTA
a
THE CONTINENTAL GLACIERS OF POLAR REGIONS 283
which exact measurements indicate to be but one foot per day, and
at a distance of a mile and a quarter from the margin even this
slight value has diminished by fully one eighth. It can hardly
be doubted that at moderate distances only within the ice margin,
the glacier is practically without motion.
Rain or general melting conditions being unknown in Antarctica,
a striking contrast is offered to the marginal zone of the Greenland
continent. This is to a large extent explained by the existence
Shackleton’s
Furthest South
9
2
io
err Br]
Sits eiaae}
VESSSHEHESon
Névé Fiela
tate ~
SCLLCOGS
in
BUGO'
| hone
gr ae
g
j
1
|
:
i
7°
B
eo
gh
| iat cae
a
ee
Great Ice Barrier
SF LS SS SS ST SAT
ase B2° ‘gi° ‘g0° 79° 78°
“Lat. 72° 25's Long.155°16"
Boum idl
of Feet —— . Sea Level
er Ried
Fig. 311.— Sections across the inland ice of Victoria Land, Antarctica, with the
shelf ice in front (after Shackleton).
upon the northern land mass of a coast-land ribbon which becomes
. quickly heated in the sun’s rays, and both by warming the air and
by radiating heat to the ice it causes melting and prodyces local
air temperatures which in summer may even be described as hot.
About Independence Bay in latitude 82° N. and near the north-
ernmost extremity of Greenland, Peary descended from the in-
land ice into a little valley within which musk oxen were lazily
grazing and where bees buzzed from blossom to blossom over a
gorgeous carpet of flowers.
Nourishment of continental glaciers. — Explorations upon and
about the glaciers of Greenland and Antarctica have shown that
the circulation of air above these vast ice shields conforms to a
quite simple and symmetrical model subject to spasmodic pulsa-
284 EARTH FEATURES AND THEIR MEANING
tions of a very pronounced type. Each great ice mass with its
atmospheric cover constitutes a sort of refrigerating air engine
and plays an important part in the wind system of the globe.
(See Fig. 291, p. 263). Both the domed surface and the low tem-
perature of the glacier are essential to the continuation of this
pulsating movement within the atmosphere (Fig. 312). The air
layer in contact with the ice is during a period of calm cooled, con- °
tracted, and rendered heavier, so that it begins to slide downward
and outward upon the domed surface in all directions. The ex-
treme flatness of the greater portion of the glacier surface — a
ee a ae et ieee S~ ep
gem nes G.L Ac | A Ls eee?
, Le ee Te,
ANTIGCY CLONE
P <_<" 2° oi : | me ioe Pray . Wives
tr kr betta owe’ ted kay — OTe RR SOE SESS ye a oe
| AEE RE Ce Geen WA Fl RSet Re ARSE AV ste sg °
CONTINENTAL GLACIER
Fig. 312.— Diagram to show the nature of the fixed glacial anticyclone above
continental glaciers and the process by which their surface is shaped.
fraction only of one degree — makes the engine extremely slow
in starting, but like all bodies which slide upon inclined planes,
the velocity of its movement is rapidly accelerated, until a blizzard
is developed whose vigor is unsurpassed by any elsewhere experi-
enced.
The effect of such centrifugal air currents above the glacier is
to suck down the air of the upper currents in order to supply the
void which soon tends to develop over the central portion of the
glacier dome. ‘This downward vortex, fed as it is by inward-blow-
ing, high-level currents, and drained by outwardly directed sur-
face currents, is what is known as an anticyclone, here fixed in
position by the central embossment of the dome.
The air which descends in the central column is warmed by
compression, or adiabatically, just as air is warmed which is forced
into a rubber tire by the use of a pump. The moisture congealed
in the cirrus clouds floating in the uppermost layer of the convec-
tive zone, is carried down in this vortex and first melted and in
turn evaporated, due to the adiabatic effect. This fusion and
evaporation of the ice by its transformation of latent, to sensible,
heat, in a measure counteracts, and so retards, the adiabatic ele-
|
{i Ad
|
'
:
;
_.
as
~~
t%
>
re
Pal
4
iy
a
¥
menses
EE
Ailes
THE CONTINENTAL GLACIERS OF POLAR REGIONS 285
vation of temperature within the column. Eventually the warm
air now charged with water vapor reaches the ice surface, is at
once chilled, and its burden of moisture precipitated in the form of
fine snow needles, the so-called ‘‘ frost snow,” which in accompani-
ment to the sudden elevation of temperature is precipitated at the
termination of a blizzard.
The warming of the air has, however, had the effect of damping
as it were, the engine stroke, and, as the process is continued, to
start a reverse or upward current within the chimney of the anti-
cyclone. The blizzard is thus suddenly ended in a precipitation
of the snow, which by changing the latent heat of condensation
to sensible heat tends to increase this counter current.
The glacier broom. — During the calm which succeeds to the
blizzard, heat is once more abstracted from the surface air layer,
and a new outwardly directed engine stroke is begun. The tem-
pest which later develops acts as a gigantic centrifugal broom which
Fic. 313.— Snow deltas about the margins of the Fan glacier outlet of Greenland
(after Chamberlin).
sweeps out to the margins of the glacier all portions of the latest
snowfall which have not become firmly attached to the ice surface.
The sweepings piled up about the margin of continental glaciers
have been described as fringing glaciers, or the glacial fringe. The
northern coast of Greenland and Grant Land are bordered by a
fringe of this nature (plate 14 A, and Fig. 315, p. 288). It is by the
286 EARTH FEATURES AND THEIR MEANING
operation of the glacier broom that the inland ice is given its charac-
teristic shield-like shape (Fig. 312). The granular nature of the
snow carried by the wind is well brought out by the little snow
deltas about the margins of Greenland ice tongues (Fig. 313).
Obviously because of the presence of the vigorous anticyclone, no
snows such as nourish mountain glaciers can be precipitated upon
continental glaciers except within a narrow marginal zone, and,
as shown by Nansen rock dust from the coastland ribbon and
from the nunataks
of Greenland, is car-
ried by a few miles
inside the western
margin, and not
at all within the
eastern.
Field and pack
zt cn ice. — Within polar
— Redan regions the surface
Fie. 314. —Sea ice of the Arctic region in lat. 80° 5’ N. of ine se ae
and long. 2° 52’ E. (after Duc d’Orleans). during the long
winter season, the
product being known as sea-ice or field-ice (Fig. 314). This ice
cover may reach a thickness by direct freezing of eight or more
feet, and by breaking up and being crowded above and below
neighboring fragments may increase to a considerably greater
thickness. Ice thus crowded together and more or less crushed is
described as pack ice or the pack.
The pack does not remain stationary but is continually drifting
with the wind and tide, first in one direction and then in another,
but with a general drift in the direction of the prevailing winds.
Because of the vast dimensions of the pack, the winds over widely
separated parts may be contrary in direction, and hence when cur-
rents blow toward each other or when the ice is forced against a
land area, it is locally crushed under mighty pressures and forced
up into lines of hwmmocks —the so-called pressure ridges. At
other times, when the winds of widely separated areas blow away
from each other, the pack is parted, with the formation of lanes or
leads of open water.
If seen in bird’s-eye view the lines of hummocks would accord-
THE CONTINENTAL GLACIERS OF POLAR REGIONS 287
ing to Nansen be arranged like the meshes of a net having roughly
squared angles and reaching to heights of 15 to 25, rarely 30, feet
above the general surface of the pack. The ice within each mesh of
the network is a floe, which at the times of pressure is ground against
its neighbors and variously shifted in position. At the margin of
the pack these floes become separated and float toward lower lati-
tudes until they are melted.
The drift of the pack. — The discovery of the drift in the Arctic
pack is a romantic chapter in the history of polar exploration, and
has furnished an example of faith in scientific reasoning and judg-
ment which may well be compared with that of Columbus. The
great figure in this later discovery is the Norwegian explorer
Fridtjof Nansen, and to the final achievement the ill-fated Jean-
nette expedition contributed an important part.
The Jeannette carrying the American exploring expedition
was in 1879 caught in the pack to the northward of Wrangel Island
(Fig. 315), and two years later was crushed by the ice and sunk to
the northward of the New Siberian Islands. In 1884 various
articles, including a list of stores in the handwriting of the com-
mander of the Jeannette, were picked up at Julianehaab near the
southern extremity of Greenland but upon the western side of
Cape Farewell. Nansen, having carefully verified the facts,
concluded that the recovered articles could have found their way
to Julianehaab only by drifting in the pack across the polar sea,
and that at the longest only five years had been consumed in the
transit. After being separated from the pack the articles must
have floated in the current which makes southward along the east
coast of Greenland and after doubling Cape Farewell flows north-
- ward upon the west coast. It was clear that if they had come
through Smith Sound they would inevitably have been found
upon the other shore of Baffin Bay. In confirmation of this view
there was found at Godthaab, a short distance to the northward
of Julianchaab (Fig. 315), an ornamented Alaskan ‘“‘ throwing
stick’ which probably came by the same route. Moreover,
large quantities of driftwood reach the shores of Greenland which
have clearly come from the Siberian coast, since the Siberian
larch has furnished the larger quantity. |
Pinning his faith to these indubitable facts, Nansen built the
Fram in such a manner as to resist and elude the enormous pres-
288 EARTH FEATURES AND THEIR MEANING /
NORTH
ft Nl 4
FROM .\%/GLACIER FRINGE
70°
Fig. 315. — Map of the north polar regions, showing the area of drift ice and the
tracks of the Jeannette and the Fram (compiled from various maps). ~*~
THE CONTINENTAL GLACIERS OF POLAR REGIONS 289
sures of the ice pack, stocked her with provisions sufficient for
five years, and by allowing the vessel to be frozen into the pack
north of the New Siberian Islands, he consigned himself and
his companions to the mercy of the elements. The world knows
the result as one of the most remarkable achievements in
the long history of polar exploration. The track of the Fram,
charted in Fig. 315, considered in connection with that of the
Jeannette, shows that the Arctic pack drifts from Bering Sea west-
ward until near the northeastern coast of Greenland.
Special casks were for experimental purposes fastened in the
ice to the north of Behring Strait by Melville and Bryant, and two
of these were afterwards recovered, the one near the North Cape
in northern Norway, and the other in northeastern Iceland (see
map, Fig. 315). Peary’s trips northward in 1906 and 1909 from
the vicinity of Smith Sound have indicated that between the Pole
and the shores of Greenland and Grant Land the drift is through-
out to the eastward, corresponding to the westerly wind. Upon
this border the great area of Arctic drift ice is in contact with
great continental glaciers bordered by a glacier fringe. Admiral
Peary has shown that instead of consisting of frozen sea ice, the
pack is here made up of great floes from 20 to 100 feet in thickness
and that these have been derived from the glacier fringe.
Whenever the blizzards blow off the inland ice from the south,
leads are opened at the margin of the fringe and may carry strips
from the latter northward across the lead. With favorable con-
ditions these leads may be closed by thick sea ice so that with
the occurrence of counter winds from the north they do not entirely
return to their original position. A continuance of this process
may have resulted in the heavy floe ice to the northward of Green-
land, which, acting as an obstruction, may have forced the thinner
drift ice to keep on the European side of the Arctic pack.
About the Antarctic continent there is a broad girdle of pack
ice which, while more indolent in its movements than the Arctic
pack, has been shown by the expeditions of the Belgica and the
Pourquoi-Pas to possess the same kind of shifting movements.
In the southern spring this pack floats northward and is to a large
extent broken up and melted on reaching lower latitudes.
The Antarctic shelf ice.—It has been already pointed out
that the inland ice of Antarctica is in part at least surrounded by
U
290 EARTH FEATURES AND THEIR MEANING
a thick snow and ice terrace floating upon the sea and rising to
heights of more than 150 feet above it (plate 15 Band Fig. 316).
The visible portions of this shelf-ice are of stratified compact
f
2 LD We Gay “<r — oS —
—— ‘LO ECS er
a a — ee _ ‘ AS Sa —_— -“Z
Fia. 316.— The shelf ice of Coats Land with the surrounding pack ice showing
in the foreground (after Bruce).
snow, and the areas which have thus far been studied are found
in bays from which dislodgment is less easily effected. The origin
of the shelf ice is beli¢ved to be a sea-ice which because not easily
detached at the time of the spring ‘ break-up ” is thickened in
succeeding seasons chiefly by the deposition of precipitated and
drifted snow upon its
surface, so that it is
bowed down under
the weight and sunk
to greater and greater
depths in the water.
To some extent, also, —
it is fed upon its inner
margin by overflow
of glacier ice from
the inland ice masses.
Icebergs and snowbergs and the manner of their birth. — Green-
land reveals in the character of its valleys the marks of a large
subsidence of the continent — the serpentine inlets or fjords by
Fie. 317.— Tidewater cliff at the front of a glacier
tongue from which icebergs are born.
PLATE 15.
Me i RN i
rags
a 7ST PURE rn EIAs SFE rm ane
— P - , = " a oT =
_ <T = . 2 copes to “4
THE CONTINENTAL GLACIERS OF POLAR REGIONS 291
which its coast is so deeply indented. Into the heads of these fjords
the tongues from the inland ice descend generally to the sea level
and below. The glacier ice is thus directly attacked by the waves
as well as melted in the water, so that it terminates in the fjords
in great cliffs of ice (Fig. 317). It is also believed to extend
- beneath the water surface as
a long toe resting upon the
bottom (Fig. 319).
The exposed cliff is notched
and undercut by the waves in
the same manner as a rock cliff,
Bene Bopper DOmlens overnice Fia. 318.— A Greenlandic iceberg after a
the lower so that at frequent in- vcs aetunney iawers tatiudes.
tervals small masses of ice from
this front separate on crevasses, and toppling o over, fall into the
water with picturesque splashes. Such small bergs, whose birth
may be often seen at the cliff front of both the Greenland and
Alaskan glaciers, have little in common with those great floating
islands of ice that are drifted by the winds until, wasted to a frac-
tion only of their former proportions, they reach the lanes of trans-
atlantic travel and become a serious menace to navigation (Fig. 318).
Northern icebergs of large dimensions are born either by the lifting
of a separated portion of the extended glacier toe lying upon the
bottom of the fjord, or else they separate bodily from the cliff
“SAS » = =A, af Te ei a
ale i jase ili
li eae
ila au ue iil
= TaD ZZ — 3 .
rae
4
Fig. 319.— Diagram showing one way in which northern icebergs may be born
from the glacier tongue (after Russell).
itself, apparently where it reaches water sufficiently deep to float it.
In either case the buoyancy of the sea water plays a large réle in its
separation.
If derived from the submerged glacier toe (Fig. 319), a loud noise
is heard before any change is visible, and an instant later the great
292 EARTH FEATURES AND THEIR MEANING
mass of ice rises out of the water some distance away from the
cliff, lifting as it does so a great volume of water which pours off on
all sides in thundering cascades and exposes at last a berg of the
deepest sapphire blue. The commotion produced in the fjord is
prodigious, and a vessel in close proximity is placed in jeopardy.
Even larger bergs are sometimes seen to separate from the ice
cliff, in this case an instant before or simultaneously, with a loud
report, but such bergs float away with comparatively little com-
motion in the water.
The icebergs of the south polar region are usually built upon a.
far grander scale than those of the Arctic regions, and are, further,
both distinctly tabular in form and bounded by rectangular out-
lines (Fig. 321). Whereas the large bergs of Greenlandic origin
are of ice and blue in color, the tabular bergs of Antarctica might.
better be described as snowbergs, since they are of a blinding white-
Fig. 320.— A northern iceberg surrounded by sea ice.
ness and their visible portions are either compacted snow or alter-
nating thick layers of compact snow and thin ribbons of blue ice,
the latter thicker and more abundant toward the base. All such
bergs have been derived from the shelf ice and not from the inland
ice itself. Blue icebergs which have been derived from the inland
ice have been described from the one Antarctic land that has been
explored in which that ice descends directly to the sea.
= “as
ee eT
ae
siemenee me tre omy are etey torawrre ms ——— : > ms
{
|
{
ero
PE PS I Pe
peg «
aaa oes
THE CONTINENTAL GLACIERS OF POLAR REGIONS 293
In both the northern and southern hemispheres those bergs
which have floated into lower latitudes have suffered profound
transformations. Their exposed surfaces have been melted in the
sun, washed by the rain, and battered by the waves, so that they
lose their relatively simple forms but acquire rounded surfaces in
place of the early angular ones (Fig. 318, p. 291). Sir John Murray,
who had such extended opportunities of studying the southern ice-
wane ran
bt Ee)
- 8 wees, ~~ aie TX eee
» . — —_
Rats hy dy “) FS bY KOC
— tt Aes > \
jae te ag ‘ ‘f, mY ’ }
i
>a ~ a ced \
~ a= — bp
—=~_ . aS | nett ft
- =
- Fie. 321. — Tabular Antarctic iceberg separating from the shelf ice (after Shackleton).
| bergs from the deck of the Challenger, has thus described their
beauties :
‘“Waves dash against the vertical faces of the floating ice island as
against a rocky shore, so that at the sea level they are first cut into ledges
and gullies, and then into caves and caverns of the most heavenly blue,
from out of which there comes the resounding roar of the ocean, and into
which the snow-white and other petrels may be seen to wing their way
through guards of soldier-like penguins stationed at the entrances. As
these ice islands are slowly drifted by wind and current to the north, they
tilt, turn and sometimes capsize, and then submerged prongs and spits are
thrown high into the air, producing irregular pinnacled bergs higher, pos-
sibly, than the original table-shaped mass.”’
READING REFERENCES FOR CHAPTERS XX AND XXI
General : —
I. C. Russetyu. Glaciers of North America. Ginn, Boston, 1897, pp.
210, pls. 22.
CHAMBERLIN and SauisBury. Geology, vol. 1, pp. 232-308.
294. EARTH FEATURES AND THEIR MEANING
H. Hess. Die Gletscher, Braunschweig, 1904, pp. 426 (illustrated).
Wituram H. Hosss. Characteristics of Existing Glaciers. . Macmillan,
1911, pp. 301, pls. 34.
Special districts of mountain glaciers : —
James D. Forses. Travels Through the Alps of Savoy and other Parts =
of the Pennine Chain with Observations on the Phenomena of Glaciers.
Edinburgh, 1845, pp. 456, pls. 9, maps 2.
A. Pencx, E. Brickner, et L. pu Pasquier. Le systéme glaciare des
alpes, etc., Bull. Soc. Se. Nat. Neuchatel, vol. 22, 1894, pp. 86.
E. Ricuter. Die Gletscher der Ostalpen. Stuttgart, 1888, pp. 306,
7 maps.
James D. Forses. Norway and Its Glaciers, ete. Edinburgh, 1853, pp.
349, pls. 10, map.
I. C. Russetu. Existing Glaciers of the United States, 5th Ann. Rept.
U.S. Geol. Surv., 1885, pp. 307-355, pls. 32-55; Glaciers of Mt.
Ranier, 18th Ann. Rept. U.S. Geol. Surv., 1898, pp. 349-423, pls.
65-82.
W.H. SHerzer. Glaciers of the Canadian Rockies and Selkirks, Smith.
Cont. to Knowl. No. 1692, Washington, 1907, pp. 135, pls. 42.
H. F. Rei. Studies of Muir Glacier, Alaska, Nat. Geogr. Mag., vol. 4,
1892, pp. 19-84, pls. 1-16.
I. C. Russetyt. Malaspina Glacier, Jour. Geol., vol. 1, 1893, pp. 219-
245.
G. K. Gitsert. Harriman Alaska Expedition, vol. 3, Glaciers, 1904,
pp. 231, pls. 37.
W. M. Conway. Climbing and Exploration in the Karakoram Hima-
layas, Maps and Scientific Reports, 1894, map sheets I-II.
Fanny Buttock Workman and Wiuu1am Hunter WorkKMAN. ‘The His-
par Glacier, Geogr. Jour., vol. 35, 1910, pp. 105-182, 7 pls. and map.
The cycle of glaciation : —
Wituram H. Hosss. The Cycle of Mountain Glaciation, Geogr. Jour.,
vol. 36, 1910, pp. 146-163, 268-284.
Upper and lower cloud zones of the atmosphere : —
R. Assmann, A. Berson, and H. Gross. Wissenschaftliche Luftfahrten
ausgefiihrt vom deutschen Verein zur Férderung der Luftschiffahrt
in Berlin, 1899-1900, 3 vols.
E. Goup and W. A. Harwoop. The Present State of our Knowledge of
the Upper Atmosphere as Obtained by the Use of Kites, Balloons,
and Pilot-ballons, Rept. Brit. Assoc. Adv. Sci., 1909, pp. 1-55.
W. H. Moore. Descriptive Meteorology, Appleton, New York, 1910,
pp. 95-136.
Witu1am H. Hosss. The Pleistocene. Glaciation of North America
Viewed in the Light of our Knowledge of Existing Contitiental
Glaciers, Bull. Am. Geogr. Soc., vol. 42, 1911, pp. 647-650.
=
phe etise s, *
THE CONTINENTAL GLACIERS OF POLAR REGIONS 295
The continental glacier of Greenland :—
F. Nansen. The First Crossing of Greenland, 2 vols, Longmans, Lon-
don, 1890 (the scientific results are contained in an appendix to
volume 2, pp. 443-497).
R. E. Peary. A Reconnaissance of the Greenland Inland Ice, Jour. Am.
Geogr. Soc., vol. 19, 1887, pp. 261-289; Journeys in North Green-
land, Geogr. Jour., vol. 11, 1898, pp. 213-240.
T. C. CuamBeruin. Glacier Studies in Greenland, Jour. Geol., vol. 2,
1894, pp. 649-668, 768-788, vol. 3, pp. 61-69, 198-218, 469-480, 565-
582, 668-681, 833-843, vol. 4, pp. 582-592, 769-810, vol. 5, pp. 229-
245; Recent glacial studies in Greenland (Presidential address),
Bull. Geol. Soc. Am., vol. 6, 1895, pp. 199-220, pls. 3-10.
R.S. Tarr. The Margin of the Cornell Glacier, Am. Geol., vol. 20, 1897,
pp. 189-156, pls. 6-12.
R. D. Satispury. The Greenland Expedition of 1895, Jour. Geol., vol. 3,
1895, pp. 875-902.
E. v. Dry@atski. Grénland Expedition der Gesellschaft fiir Erdkunde zu
Berlin 1891-1893, Berlin, 1897, 2 vols., pp. 551 and 571, pls. 53,
maps 10.
Wiituram H. Hosss. Characteristics of the Inland Ice of the Arctic
Regions, Proc. Am. Phil. Soc., vol. 49, 1910, pp. 57-129, pls. 26-30.
The Antarctic continental glacier : —
R. F. Scorr. The Voyage of the Discovery. London, 2 vols., 1905.
EK. H. SHacxteton. The Heart of the Antarctic. London, 2 vols., 1910.
EK. von Dryaatski. Zum Kontinent des eisigen Siidens, Deutsche Siid-
polar-Expedition, Fahrten und Forschungen des ‘‘ Gauss,’’ 1901-1903,
Berlin, 1904, pp. 668, pls. 21.
Orto NoORDENSKIOLD and J. S. ANpeRsson. Antarctica or Two Years
Amongst the Ice of the South Pole. London, 1905, pp. 608, illus-
trated.
EK. Pururpprr. Ueber die fiinf Landeis-Expeditionen, etc., Zeit. f. Glet-
scherk., vol. 2, 1907, pp. 1-21.
Nourishment of continental glaciers : —
Witiram H. Hosss. Characteristics of the Inland Ice of the Arctic
Regions, Proc. Am. Phil. Soc., vol. 49, 1910, pp. 96-110; The Ice
Masses on and about the Antarctic Continent, Zeit. f. Gletscherk.,
vol. 5, 1910, pp. 107-120; Characteristics of Existing Glaciers. New
York, 1911, pp. 143-161, 261-289. Pleistocene Glaciation of North
America Viewed in the Light of our Knowledge of Existing Con-
tinental Glaciers, Bull. Am. Geogr. Soc., vol. 43, 1911, pp. 641-659.
Field and pack ice : —
Emma DE Lona. The Voyage of the Jeannette, the ship and ice journals
of George W. de Long, ete. Berlin, 1884, 2 vols., chart in back of
vol. 1.
296 EARTH FEATURES AND THEIR MEANING
Rosert E. Peary. The Discovery of the North Pole (for further refer-
ences on both sea and pack ice and Antarctic shelf ice, consult Hobbs’s
Characteristics of Existing Glaciers, pp. 210-213, 242-244.
Icebergs : —
WrvitLte Tuomson. Challenger Report, Narrative, vol. 1, 1865, Pt. i,
pp. 431-4382, pls. B—-D. . |
I. C. Russetyt. An Expedition to Mt. St. Elias, Nat. Geogr. Mag., vol. 3,
1891, pp. 101-102, fig. 1. .
H. F. Rei. Studies of Muir Glacier, Alaska, ibid., vol. 4, 1892, pp. 47-
48.
E. von DryaGatski. Grénland-Expedition, ete., vol. 1, pp. 367-404.
M. C. Eneextu. Ueber die Entstehung der Eisberge, Zeit. f. Gletscherk.,
vol. 5, 1910, pp. 112-132.
CHAPTER XXII
THE CONTINENTAL GLACIERS OF THE “ICE AGE”’
Earlier cycles of glaciation. — Our study of the rocks compos-
ing the outermost shell of the lithosphere tells us that in at least
three widely separated periods of its history the earth has passed
through cycles of. glaciation during which considerable portions
of its surface have been submerged beneath continental glaciers.
The latest of these occurred in the yesterday of geology and has
Fig. 322. Map of the globe showing the areas which were covered by the con-
tinental glaciers of the so-called ‘‘ice-age’’ of the Pleistocene period. The arrows
show the directions of the centrifugal air currents in the fixed anticyclones above the
glaciers.
often been referred to as the “ ice age,” because until quite re-
cently it was:supposed to be the only one of which a record was
preserved.
This latest ice age represents four complete cycles of glaciation,
for it is believed that the continental ice developed and then
completely disappeared during a period of mild climate before the
next glacier had formed in its place, and that this alternation of
climates was no less than three times repeated, making four cycles
in all. At nearly or quite the same time ice masses developed in
297
298 EARTH FEATURES AND THEIR MEANING
ae ae ee
northern North America and
in northern Europe, the em-
= bossments of the ice domes |
wo’ being located in Canada and
in Scandinavia respectively :
(Fig. 322). There appears to q
2 have been at this time no ex-
Fra. 323. — Glaciated granite bowlder t ‘ laaiait {th th
which has weathered out of a moraine COSIVS 8 ACLAPLON © © sibs rs
of Permo-Carboniferous age upon which ern hemisphere, though in the
it rests. South Australia (after How- yext earlier of the known great
chin). , Pie he ‘
periods of glaciation — the so-
called Permo-Carboniferous — it was the southern hemisphere, and
not the northern, that was affected (Fig. 323 and Fig. 304, p. 276).
=
Yuli
a a\ | \
Wear &
a
ta
oo
¥ ORIFTLESS AREA
‘Ngsts TERNAL MORAINE,
-
Fig. 324. — Map to show the glaciated and nonglaciated regions of North America
(after Salisbury and Atwood).
THE CONTINENTAL GLACIERS OF THE “ICE AGE” 299
From the still earlier glacial period our data are naturally much
more meager, but it seems probable that it was characterized by
glaciated areas within both the northern and the southern hemi-
spheres.
Contrast of the glaciated and nonglaciated regions. — Since
we have now studied in brief outline the characteristics of the exist-
ing continental glaciers, we are in a position to review the evidences
AIA
‘e
\
\Y
_ .
\ ay
ZO
AS aN as
“I
\
Fic. 325.— Map of the glaciated and nonglaciated areas of northern Europe. The
strongly marked morainal belts respectively south and north of the Baltic depres-
sion represent halting places in the retreat of the latest continental glacier (com-
piled from maps by Penck and Leverett).
of former glaciers, the records of which exist in their carvings, their
gravings, and their deposits.
An observant person familiar with the aspects of Nature in both
the northern and southern portions of the central and eastern |
United States must have noticed that the general courses of the
Ohio and Missouri rivers define a somewhat marked common border .
of areas which in most respects are sharply contrasted (Fig. 324).
Hardly less striking is the contrast between the glaciated and the
nonglaciated regions upon the continent of Europe (Fig. 325).
It is the northern of the two areas which in each case reveals the
characteristic evidences of glaciation, while there is entire absence
300 EARTH FEATURES AND THEIR MEANING
of such marks to the southward of the common border. Within
the American glaciated region there is, however, an area surrounded
like an island, and within this district (Fig. 324) none of the marks
characteristic of glaciation are to be found. This area is usually
referred to as the ‘ driftless area,’ and occupies portions of the -
states of Wisconsin, Illinois, Minnesota, and Iowa. Even better
than the area to the southward of the Ohio and Missouri rivers, it
permits of a comparison of the nonglaciated with the drift-cov-
ered region. }
The ‘driftless area.””— Within this district, then, we have
preserved for our study a landscape which remains largely as it was
before the several ice
invasions had so pro-
foundly transformed the
general surface of the
surrounding country.
Speaking broadly, we
may say that it rep-
resents an uplifted and
in part dissected plain,
which to the south and
east particularly reveals
the character of nearly
mature river erosion
(Fig. 177, p.170). The
rock surface is here
everywhere mantled by
decomposed and disin-
; tegrated rock residues
Fig. 326.—‘‘Stand Rock” near the “Dells” of the Of local origin. The
‘ Wisconsin river, an unstable erosion remnant char- goluble constituents of
acteristic of the driftless area of North America the rock, such as the
p
(after Salisbury and Atwood).
carbonates, have been
removed by the process of leaching, so that the clays no longer
effervesce when treated with dilute mineral acid.
Wherever favored by joints and by an alternation of harder
and softer rock layers, picturesque unstable erosion remnants or
“‘ chimneys ” may stand out in relief (Fig. 326). Furthermore, the
driftless area is throughout perfectly drained — it is without lakes
PLATE 16.
4 Aca al
vA WORE ATONON Ro KG Re)
m Scale, 5 Miles.
tn
A. Incised topography within the “‘ driftless area’’ (U. S. Geol. Survey).
'
{
vt
a
.)
#/.
y T ty Nei
vir hs
bier Tt D
S mee
"2 GE
th
"
’
<A)
ZENA
> iN
[eX = \3 a é iS
e Scale. 3 Miles.
B. Built-up topography within glaciated region (U. S. Geol. Survey).
Sip als Rls eV tay
THE CONTINENTAL GLACIERS OF THE “ICE AGE” 301
or swamps — since all valleys are characterized throughout by
forward grades. The side valleys enter the main valleys as do the
branches a tree trunk; in other words, the drainage is described as
arborescent. Insofar as an¥ portions of a plane surface now remain
in the landscape, they are found at the highest levels (plate 16 A).
The topography is thus the result of a partial removal by erosion
of an upland and may be described as incised topography. Nowhere
within the area are there found rock masses foreign to the region,
but all mantle rock is the weathered product of the underlying
ledges.
Characteristics of the glaciated regions. — The topography of
the driftless area has been described as incised, because due to the
partial destruction of an uplifted plain; and this surface is, more-
over, perfectly drained. The
characteristic topography of the
“ drift ”’ areas is by contrast built
up; that is to say, the features of
the region instead of being carved
out of a plain are the result: of
Fic. 327.— Diagram showing the man- molding by the process of deposi-
ner in which a continental glacier ob- tion (plate 16B). Inso far as a
literates existing valleys (after Tarr). plane is recognizable, it is to be
found not at the highest, but at
the lowest level —a surface represented largely by swamps and
lakes — and above this plain rise the characteristic rounded hills
of various types which have been buzlt wp through deposition. The
process by which this has been accomplished is one easy to compre-
hend. As it invaded the region, the glacier planed away beneath
its marginal zone all weathered mantle rock and deposited the
planings within the hollows of the surface (Fig. 327). The
effect has been to flatten out the preéxisting irregularities of the
surface, and to yield at first a gently undulating plain upon which
are many undrained areas and a haphazard system of drainage
(Fig. 328). All unstable erosion remnants, such as now are to be
found within the driftless area, were the first to be toppled over by
the invading glacier, and in their place there is left at best only
rounded and polished ‘“ shoulders ” of hard and unweathered rock
— the well-known roches moutonnées.
The glacier gravings. — The tools with which the glacier works
302 EARTH FEATURES AND THEIR MEANING
are never quite evenly edged, and instead of an in all respects
perfect polish upon the rock pavement, there are left furrowings,
gougings, and scratches. Of whatever sort, these scorings indi-
cate the lines of ice movement and are thus indubitable records
graven upon the rock floor. When mapped over wide areas, a
‘
Oe
i
Si
Fig. 328.— Lake and marsh district in northern Wisconsin, the effect of glacial
deposition in former valleys (after Fairbanks).
most interesting picture is presented to our view, and one which
supplements in an important way the studies of existing continental
glaciers (Fig. 334, p. 308, and Fig. 336, p. 312).
It has been customary to think of the glacier as everywhere
eroding its bed, although the only warrant for assuming degra-
dation by flow of the ice is restricted to the marginal zone, since
here only is there an appreciable surface grade likely to induce
flow. Both upon the advance and again during the retreat of a
glacier, all parts of the area overridden must be subjected to this
action. Heretofore pictured in the imagination as enlarged
models of Alpine glaciers, the vast ice mantles were conceived to
have spread out over the country as the result of a kind of viscous
flow like that of molasses poured upon a flat surface in cold
weather. The maximum thickness of the latest American glacier
of the ice age has been assumed to have been perhaps 10,000 feet
near the summit of its dome in central Labrador. From this
:
THE CONTINENTAL GLACIERS OF THE “ICE AGE” 303
point it was assumed that the ice traveled southward up the
northern slope of the Laurentian divide in Canada, and thence
to the Ohio river, a distance of over 1300 miles. If such a mantle
of ice be represented in its natural proportions in vertical section,
to cover the distance from center to margin we may use a line
six inches in length, and only $5 of an inch thick. Upon a reduced
scale these proportions are given in Fig. 329. Obviously the
force of gravity acting within a viscous mass of such proportions
Fig. 329.— Cross section in approximate natural proportions of the latest North
American continental glacier of Pleistocene age from its center to its margin.
would be incompetent to effect a transfer of material from the
center to the periphery, even though the thickness should be
doubled or trebled. Yet until the fixed glacial anticyclone above
the glacier had been proven and its efficiency as a broom recog-
nized, no other hypothesis than that of viscous flow had been
offered in explanation. The inherited conception of a universal
plucking and abrasion on the bed of the glacier is thus made un-
tenable and can be accepted for the marginal portion only.
Not only do the rock scorings show the lines of ice movement,
but the directions as well may often be read upon the rock. Wher-
ever there are pronounced irregularities of surface still existing on
the pavement, these are generally found to have gradual slopes
upon the side from which the ice came, and relatively steep falls
upon the lee or ‘“‘ pluck ” side. If, however, we consider the irregu-
larities of smaller size, the unsymmetrical slopes of these protruding
portions of the floor are found to be reversed — it is the steep slope
which faces the oncoming ice and the flatter slope which is upon the
lee side. Such minor projections upon the floor usually have their
origin in some harder nodule which deflects the abrading tools and
causes them to pass, some on the one side and some upon the other.
By this process a staple-shaped groove comes to surround the
nodule, leaving an unsymmetrical elevated ridge within, which is
steep upon the stoss side and slopes gently away to leeward.
Younger records over older — the glacier palimpsest. — Many
important historical facts have been recovered from the largely
effaced writing upon ancient palimpsests, or parchments upon
which an earlier record has been intentionally erased to make room
304 EARTH FEATURES AND THEIR MEANING
for another. In the gravings upon the glacier pavement, earlier
records have been likewise in large part effaced by later, though in
favorable localities the two may be read together. Thus, as an
example, at the great limestone quarries of Sibley, in south-
eastern Michigan, the glaciated rock surface wherever stripped of .
its drift cover is a smoothly polished and relatively level floor
with striz which are directed west-northwest. Beneath this gen-
eral surface there are, however, a number of elliptical depres-
sions which have their longer axes directed south-southwest, one
being from twenty-five to thirty feet long and some ten feet in
depth (Fig. 330). These boat-shaped depressions are clearly the
| remnants of an earlier:
more undulating sur-
face which the latest
glacier has in large
part planed away,
since the bottoms of
the depressions are no
less perfectly glaciated
but have their strize
directed in general
near the longer axis of
the troughs. Palimp-
wee sest-like there are
a here also the records
Fig. 330.— Limestone surface at Sibley, Michigan. of apie’ e than one
graving.
The dispersion of the drift. — Long before the “ ice age”’ had
been conceived in the minds of Agassiz and his contemporaries,
it had been remarked that scattered over the North German plain
were rounded fragments of rock which could not possibly have been
derived from their own neighborhood but which could be matched —
with the great masses of red granite in Sweden well known as the
“Swedish granite.””’ Buckland, an English geologist, had in 1815
accounted for such “ erratic” blocks of his own country, here of
Scotch granite, by calling in the deluge of Noah; but in the late
thirties of the nineteenth century, Sir Charles Lyell,-with the results
of English Arctic explorers in mind, claimed that such traveled
blocks had been transported by icebergs emanating from the polar
THE CONTINENTAL GLACIERS OF THE “ICE AGE” 305
regions. A relic of Buckland’s earlier view we have in the word
“ diluvium ”’ still occasionally used in Germany for glacier trans-
ported materials ; while the term “ drift ”’ still remains in common
use to recall Lyell’s iceberg hypothesis, even though the original
meaning of the term has been abandoned. Drift is now a generic
term and refers to all deposits directly or indirectly referable to the
continental glaciers.
In general the place of derivation of the glacial drift may be said
to be some point more distant from and within the former ice mar-
gin at the time
when it was de-
posited ; in other oe
words, the dis-
persion of the
drift was cen- ,
trifugal with ref- : -
erence to the ‘4 ¢------ Le
glacier.
Wherever
rocks of unusual
and therefore »
easily recogniz- ~~,
able character
can be shown to
occur in place
and with but lim-
ited areas, the Fia. 331.— Map to show the outcroppings of peculiar rock
dispersion of types in the region of the Great Lakes, and some of the
localities where ‘‘float copper’’ has been collected (float
copper localities after Salisbury).
such material is
easy to trace.
The areas of red Swedish and Scotch granite have been used to
follow out in a broad way the dispersion of drift over northern
Kurope. Within the region of the Great Lakes of North America
are areas of limited size which are occupied by well marked rock
types, so that the journeyings of their fragments with the conti-
nental glacier can be mapped with some care. Upon the northern
shore of Georgian Bay occurs the beautiful jasper conglomerate,
whose bright red pebbles in their white quartz field attract such
general notice. At Ishpeming in the northern peninsula of Michi-
x
boUO EARTH FEATURES AND THEIR MEANING
gan is found the equally beautiful jaspilite composed of puckered
alternating layers of black hematite and red jasper. On Keweenaw.
Peninsula, which protrudes into Lake Superior from its southern
shore, is found that remarkable occurrence of native copper within
a series of igneous rocks of varied types and colors. Fragments -
of this copper, some weighing several
hundreds of pounds each and masked
in a coat of green malachite, have under
the name of “ drift’’ or “‘ float ”’ copper
been collected at many localities within
a broad ‘‘ fan ” of dispersal extending
almost to the very limits of glaciation
(Fig. 331).
Some miles to the north of Provi-
dence in Rhode Island there is a hill
known as Iron Hill composed in large
part of black magnetite rock, the so-
called Cumberlandite. From this hill
as an apex there has been dispersed a
great quantity of the rock distributed
as a well marked ‘ bowlder train ”
within which the size and the fre-
quency of the dispersed bowlders is in
inverse ratio to the distance from the
parent ledge (Fig. 332). Similar
though less perfect trains of bowlders
are found on the lee side of most pro-
ORE ET IV SG ce er jecting masses of resistant rocks within
train” from Iron Hill, R.I. the area of the drift.
(based upon Shaler’s map, but § Large bowlders when left upon a
with the directions of glacial <4. :
wistenaddasl)’ ledge of notably different appearance
$
described as ‘‘ perched bowlders.”” Resting as they sometimes do
upon a relatively small area, they may be nicely balanced and
thus easily given a pendular or rocking motion. Such ‘“ rocking
stones ”’ are common enough, especially among the New England —
hills (plate 17 B). Many such bowlders have made somewhat
remarkable peregrinations with many interruptions, having-been
easily attract attention, and have been ~ i
carried first in one direction by an earlier glacier to be later trans-
PLATE 17.
B. Perched bowlder upon a striated ledge of different rock type, Bronx Park, New
York (after Lungstedt).
C. Characteristic knob and basin surface of a moraine.
THE CONTINENTAL GLACIERS OF THE “ICE AGE” 9307
ported in wholly different directions at the time of new ice inva-
sions.
The diamonds of the drift. — Of considerable popular, even if
not economic, interest are the diamonds which have been sown
in the drift after long and interrupted journeyings with the ice
from some unknown home far to the northward in the wilderness
of Canada. The first stone to be discovered was taken by work-
men from a well opening near the little town of Eagle in Wisconsin
in the year 1876. Its nature not being known, it remained where
it was found as a curiosity only, and it was not until 1883 that it
was taken to Milwaukee and sold to a jeweler equally ignorant
of its value, and for the merely nominal sum of one dollar. Later
recognized as a diamond of the unusual weight of sixteen carats,
SH PAP
Fig. 333. —Shapes and approximate natural sizes of some of the more important
diamonds from the Great Lakes region of the United States. In order from left
to right these figures represent the Eagle diamond of sixteen carats, the Saukville
- diamond of six and one half carats, the Milford diamond of six carats, the Oregon
' diamond of four carats, and the Burlington diamond of a little over two carats.
it was sold to the Tiffanys and became the cause of a long litiga-
tion which did not end until the Supreme Court of Wisconsin had |
decided that the Milwaukee jeweler, and not the finder, was en-
titled to the price of the stone, since he had been ignorant of its
value at the time of purchase.
_ An even larger diamond, of twenty-one carats weight, was found
at Kohlsville, and smaller ones at Oregon, Saukville, Burlington,
and Plum Creek in the state of Wisconsin ; at Dowagiac in Michi-
gan; at Milford in Ohio, and in Morgan and Brown counties in
Indiana. The appearance of some of the larger stones in their
natural size and shape may be seen in Fig. 333.
While the number of the diamonds sown in the drift is undoubt-
"edly large, their dispersion is such that it is little likely they
can be profitably recovered. The distribution of the localities at
which stones have thus far been found is set forth upon Fig. 334.
Obviously those that have been found are the ones of larger size,
ft
iG!
L
'
i
ii
HI |
oH |
Fia. 334.—Glacial map of a portion of the Great Lakes region, showing the ungla-
ciated area and the areas of older and newer drift. The driftless area, the mo- é
raines of the later ice invasion, and the distribution of diamond localities upon
the latter are also shown. With the aid of the directions of strie some attempt
has been made to indicate the probable tracks of more important diamonds, which
tracks converge in the direction of the Labrador peninsula.
308
—_
a ol a ad
THE CONTINENTAL GLACIERS OF THE “ICE AGE”
since these only attract attention.
309
In 1893, when the finding of
the Oregon stone drew attention to these denizens of the drift,
the writer prophesied that other stones would occasionally be dis-
covered under essentially the same conditions, and such discoveries
are certain to continue in the future.
Tabulated comparison of the glaciated and nonglaciated re-
gions. — It will now be profitable to sum up in parallel columns
the contrasted peculiarities of the glaciated and the unglaciated
regions.
UNGLACIATED REGION
GLACIATED REGION
TOPOGRAPHY
The topography is destructional ;
the remnants of a plain are found at
the highest levels or upon the hill
tops; hills are carved out of a high
plain; unstable erosion remnants are
characteristic.
The topography is constructional ;
the remnants of a plain are found
at the lowest levels in lakes and
swamps; hills are molded above a
plain in characteristic forms; no
unstable erosion remnants, but only
rounded shoulders of rock.
DRAINAGE
The area is completely drained,
and the drainage network is arbores-
cent.
The area includes undrained
areas, — lakes and swamps, — and
- the drainage system is haphazard.
ROCK MANTLE
The exposed rock is decomposed
and disintegrated to a considerable
depth; it is all of local derivation
and hence of few types — homogene-
ous; the fragments are angular;
soils are leached and hence do not
contain carbonates.
No decomposed or disintegrated
rock is ‘‘in place,’’ but only hard,
fresh surface; loose rock material
is all foreign and of many sizes and
types — heterogeneous; rock bowl-
ders and pebbles are faceted and
polished as well as striated, usually
in several directions upon each
facet; soils are rock flour — the
grist of the glacial mill.
ROCK SURFACE
Rock surface is rough and irreg-
ular.
Rock surface is planed or grooved,
and polished. Shows glacial striz.
Unassorted and assorted drift. — The drift is of two distinct
types; namely, that deposited directly by the glacier, which is
310° EARTH FEATURES AND THEIR MEANING
without stratification, or unassorted; and that deposited by water
flowing either beneath or from the ice, and this like most fluid de-
posited material is assorted or stratified. ‘The unassorted material
is described as till, or sometimes as ‘‘ bowlder clay”; the as-
sorted is sand or gravel, sometimes with small included bowlders, »
and is described as kame gravel. To recall the parts which both
the glacier and the streams have played in its deposition, all water-
deposited ‘materials in connection with glaciers are called fluvio-
— glacial. | |
Till is, then, characterized by a noteworthy lack of homogeneity,
both as regards the size and the composition of its constituent
parts. As many as twenty
een yee Ry tes Seer . .
cis somatig www Gifferent rock types of varied .
pe” dcol
wy oO textures and colors may some-
4
«>, times be found in a single
co*., exposure of this material, and
the entire gamut is run from
the finest rock flour upon the
one hand to bowlders whose
diameter may be measured
in feet (Fig. 335).
In contrast with those de-
rived by ordinary stream
Fia. 335. — Section in coarse till. Note the action, the pebbles and
rang ng ofthe matrly the In of bowldere of the till are fac-
howiders. eted or “ soled,”’ and usually
show striations upon their
faces. Ifa number of pebbles are examined, some at least are sure
to be found with striations in more than one direction upon a
single facet. As a criterion for the discrimination of the material
this may be an important mark to be made use of to distinguish
in special cases from rock fragments derived by brecciation and
slickensiding and distributed by the torrents of arid and semiarid -
regions.
Inasmuch as the capacity of ice for handling large masses is
greater than that of water, assorted drift is in general less coarse,
and, as its name implies, it is also stratified. From ordinary -
stream gravels, the kame gravels are distinguished by the form of
their pebbles, which are generally faceted and in some cases
THE CONTINENTAL GLACIERS OF THE “ICE AGE” 3811
striated. In proportion, however, as the materials are much
worked over by the water, the angles between pebble faces be-
come rounded and the original shapes considerably masked.
Features into which the drift is molded. — Though the pre-
existing valleys were first filled in by drift materials, thus reducing
the accent of the relief, a continuation of the same process resulted
in the superimposition of features of characteristic shapes upon
the imperfectly evened surface of the earlier stages. These
features belong to several different types, according as they were
built up outside of, at and upon, or within the glacier margin.
The extra-marginal deposits are described as outwash plains or
aprons, or sometimes as valley trains; the marginal are either
moraines or kames; while within the border were formed the till
plain or ground moraine, and, locally also, the drumlin and the
esker or os. These characteristic features are with few exceptions
to be found only within the area covered by the latest of the ice
invasions. For the earlier ones, so much time has now elapsed
that the effect of weathering, wash, and stream erosion has been
such that few of the features are recognizable.
Marginal and extra-marginal features are extended in the direc-
tion of the margin or, in other words, perpendicular to the local
‘ice movement; while the intra-marginal deposits are as note-
-worthy for being perpendicular to the margin, or in correspondence
with the direction of local ice movement. Each of these features
possesses characteristic marks in its form, its size, proportions,
surface molding and orientation, as well as in its constituent
materials. It should perhaps be pointed out that the existing
continental glaciers, being in high latitudes, work upon rock ma-
terials which have been subjected to different weathering processes
from those characteristic of temperate latitudes. Moreover, the
melting of the Pleistocene glaciers having taken place in relatively
low latitudes, larger quantities of rock débris were probably released
from the ice during the time of definite climatic changes, and hence
heavier drift accumulations have for both of these reasons resulted.
Marginal or “ kettle’? moraines. — Wherever for a protracted
period the margin of the glacier was halted, considerable deposits
of drift were built up at the ice margin. These accumulations
form, however, not only about the margin, but upon the ice sur-
face as well; in part due to materials collected from melting down
312 EARTH FEATURES AND THEIR MEANING
of the surface, and in part by the upturning of ice layers near the
margin (see ante, p. 277).
An important rdéle is played by the thaw water which emerges
at the ice margin, especially within the reéntrants or recesses of
the outline. The materials of moraines are, therefore, till with
large local deposits of kame gravel, and these form in a series of
ridges corresponding to the temporary positions of the ice front.
Their width may range from a few rods to a few miles, their height
may reach a hundred feet or more,
and they stretch across the country
for distances of hundreds or even
thousands of miles, looped in ares
or scallops which are always convex
outward and which meet in sharp
cusps that in a general way point
toward the embossment of the
former glacier (Fig. 334, p. 308, and
Fig. 336). These festoons of the
moraines outline the ice lobes of
the latest ice invasion, which in
North America were centered over
the depressions now occupied by the
tg f . - Laurentian lakes. . There was, thus,
Vig” # a Lake Superior lobe, a Lake Mich-
Fie. 336. —Sketch map of portions jgan lobe, etc. With the aid of
of Michigan, Ohio, and Indiana, those moraine maps we may thus
showing the festooned outlines of | ; ” : ‘ f
the moraines about the former ice 10 imagination picture in broad lines
lobes, and the directions of ice the frontal contours of the earlier
movement as determined by the ojaciers. At specially favorable lo-
strie upon the rock pavement ey c
(after Leverett). calities where the ice front has
| crossed a deep valley at the edge of
the Driftless Area, we may, even in a rough way, measure the slope
of the ice face. Thus near Devils Lake in southern Wisconsin the
terminal moraine crosses the former valley of the Wisconsin River,
and in so doing has dropped a distance of about four hundred feet
within the distance of a half mile or thereabouts (Fig. 337).
The characteristic surface of the marginal moraine is responsible
for the name “ kettle’ moraine so generally applied to it. The
“ kettles ”’ are roughly circular, undrained basins which lie among
<* [ee
Ss ae
Se ee ee ee eee
THE CONTINENTAL GLACIERS OF THE “ICE AGE” 313
Fig. 337.— Map of the vicinity of Devils Lake, Wisconsin, located within a reén-
trant of the “kettle’? moraine upon the margin of the Driftless Area. The lake
- lies within an earlier channel of the Wisconsin River which has been blocked at
’ both ends, first by the glacier and later by its moraine. The stippled area upon
the heights and next the moraine represents the clay deposits of a former lake
(based on map by Salisbury and Atwood).
SRS SS NENA S
SAWS SN
5 7
= \ aioe
a ee eae
5 x wes
feos C Ta) ‘ On ‘ f= went Mey tn. im" *~
ab = RAR oa LACE Dodie gas Pe
——— SS S Tare DALES He, walt apne wr weds ae ST ee any ee © car
peer? Sn ON A eG Bea Ge am yon oe een
erage! at Wis in A yea seat oS rs a ae : oy AY
Se wu! OT ah ce”) Ce deca ies Be Rs oh ae j ean, Ah.
¢ <1 — . er ee vot :
_ re PS Dateg eg: = A PV
-s
Fig. 338. — Moraine with outwash apron in front, the latter in part eroded bya
river. Westergdtland, Sweden (after H. Munthe).
314 EARTH FEATURES AND THEIR MEANING
hummocks or knobs, so that the surface has often been referred
to as “ knob and basin ” topography (plate 17 C).
Kames.— Within reéntrants or recesses of the ice margin the
drift deposits were especially heavy, so that high hills of hummocky
surface have been built up, which are described as kames. Most
of the higher drift hills have this origin. They rarely have any
principal extension along a single direction, but are composed in
large part of assorted materials. In contrast with other portions
of the morainal ridges they lack the prominent basins known as
kettles. Other kames are high hills of assorted materials not in
direct association with mo-
raines and believed to have
been built up beneath glacier
wells or mills (p. 278).
Outwash plains. — Upon the
outer margin of the moraine
m= is generally to be found a plain
he a \-| of glacial ‘“ outwash” com-
aye a — === posed of sand or gravel de-
Fig. 339. — Fosse between anoutwashplain posited by the braided streams
Gn the foreground) and the moraine, (Fig. 308, p. 280) flowing from
which rises to the left in the middle dis- : pee
tance. Ann Arbor, Michigan. the glacier margin. Such
plains, while notably flat (Fig.
338), slope gently away from the moraine. Between the outwash
plain and the moraine there is sometimes found a pit, or fosse
(Fig. 309, p. 281), where a part of the ice front was in part buried
in its own outwash (Fig. 339).
Pitted plains and interlobate moraines. — Where glacial outwash
is concentrated within a long and narrow reéntrant, separating
glacial lobes, strips of high plain are sometimes built up which
overtop the other glacial deposits of the district. The sand and
gravel which compose such plains have a surface which is pitted by
numerous deep and more or less circular lakes, so that the term
“pitted plain ”’ has been applied to them. The surface of such a
plain steadily rises toward its highest point in the angle between
the ice lobes. Though consisting almost entirely of assorted
materials, and built up largely without the ice margins, such.
gently sloping pitted platforms are described as interlobate mo-
raines. Upon a topographic map the course of such an inter-
—
THE CONTINENTAL GLACIERS OF THE “ICE AGE” 315
lobate moraine may often be followed by the belts of small pit
a lakes (see Fig. 336).
Eskers. — Intra-morainal features, or those developed beneath
the glacier but relatively near its margin, include the “ serpentine
kame,” esker, or, |
as it is called in
Scandinavia, the os
(plural osar) (Fig.
340). These di-
minutive ridges ee , ! ae
have a width sel- i a 44 ge - Tei rae oe 8c,” e
dom exceeding a
few rods, and a
height a few tens
of feet at most, but with slightly sinuous undulations they may be
followed for tens or even hundreds of miles in the general direction
of the local ice movement (Fig. 341). They are composed of
0
Fig. 340. — View looking along an esker in southern
"Maine (after Stone).
Sco/e of miles.
o
Fia. 341.— Outline map showing the eskers of Finland trending southeasterly to-
ward the festooned moraines at the margin of the ice. The characteristic lakes
of a glaciated region appear behind the moraines (after J. J. Sederholm).
316 EARTH FEATURES AND THEIR MEANING
poorly stratified, thick-bedded sands, gravels, and ‘‘ worked over ”’
materials, and are believed to have been formed by subglacial
rivers which flowed in tunnels beneath the ice. Inasmuch as the
deposits were piled against the ice walls, the beds were disturbed
at the sides when these walls disap-
peared, and the stratification, which
was somewhat arched in the beginning,
has been altered by sliding at both
margins. As already stated, eskers
have not a general distribution within
the glaciated area, but are often found
in great numbers at specially favored
localities. Formed as they are beneath
the ice, it is believed that many have:
their materials redistributed so soon as
uncovered at the glacier margin, be-
cause of the vigorous drainage there.
They are thus to be found only at those
favored localities where for some reason
border drainage is less active, or where
the ice ended in a body of water. |
Drumlins. — A peculiar type of small
hill likewise found behind the marginal
moraine in certain favored districts has
the form of an inverted boat or canoe,
the long axis of which is parallel to
Mi agen eatery aed ee the direction of ice movement, as is
“"Contgur interval 20 feet that of the esker (Fig. 342). Unlike
Fic. 342.—Small sketch maps the esker, this type of hill is composed
showing the relationships in f till df hot f d in Txeland
size, proportions, and orienta- a a 2 a re ene dopa ie rere
tion of drumlins and eskers in it is called a drumlin, the Irish word
southern Wisconsin. The es- meaning a little hill (Fig. 343). Drum-
kers are in solid black (after 1j fief ans
Alden}. _ lins are usually found in groups more
or less radial and not far behind the
outermost moraine, to which their radiating axes are perpendicular.
The manner of their formation is involved in some uncertainty,
but it is clear that they have been formed beneath the margin of »
the glacier, and have been given their shape by the last glacier
which occupied the district.
THE CONTINENTAL GLACIERS OF THE “ICE AGE” 317
The mutual relationships of nearly all the molded features
resulting from continental glaciation may be read from Fig. 344.
The shelf ice of the ice age. —Shelf ice, such as we have become
familiar with in Antarctica as a marginal snow-ice terrace floating
Fig. 343. — View of a drumlin, showing an opening in the till.. Near Boston, Mas-
sachusetts (after Shaler and Davis).
upon the sea, no doubt existed during the ice age above the Gulf
of Maine (see Fig. 324, p. 298), and perhaps also over the deep sea
to the westward of Scotland. Though the inland ice probably
covered the North Sea, and upon the American side of the Atlantic —
s ) °
By NU f 1
va = Son yada
WN | y A . < e
S D) ‘ @,
on Huh an \ *G et
£53 H lyn 4
At ym
: WAS
Z V Py ' _ N \ ts ye A / ee
YU iL, A ‘ 5
40 Ser ay ie
y
f (} J “ ae YY
\
\
aft
§ id KPALMYR
ro) Ss)
Al / y NEW) P oe
EN LSEAST
a = = B . oo 4
= aces 2 aoe we i :
Fig. 344. Gadling, map a the front of the Green Bay lobe of the latest continental
glacier of the United States. Drumlins in solid black, moraines with diagonal
hachure, outwash plains and the till plain or ground moraine in white (after
Alden).
the Long Island Sound, both these. basins are so shallow that
the ice must have rested upon the bottom, for neither is of
sufficient depth to entirely submerge one of the higher European
cathedrals.
318 EARTH FEATURES AND THEIR MEANING
Character profiles. — All surface features referable to continental ;
glaciers, whether carved in rock or molded from loose materials, .
present gently flowing outlines which are convex upward (Fig. |
345). The only definite features carved from rock are the roches
moutonnées, with their flattened shoulders, while the hillocks upon
Pee) He AG Sy wi I OY EY 22 if FRI SOS) :
a Sm
Mt ona INE
Fig. 345. — Character profiles referable to continental glacier.
ORIN GAN
moraines and kames, and the drumlins as well, approximate to
the same profile. The esker in its cross sections is much the same,
though its serpentine extension may offer some variety of curvature
when viewed from higher levels.
ee ee eee ee ee eee
READING REFERENCES FOR CHAPTER XXII
General : —
JAMES GEIKIE. The Great Ice Age. 3d ed. London, 1894, pp. 850,
maps 18. .
CHAMBERLIN and SauisBuryY. Geology, vol. 3, 1906, pp. 327-516.
FRANK Leverett. The Illinois Glacial Lobe, Mon. 38, U. S. Geol. Surv.,
1899, pp. 817, pls. 34; Glacial formations and Drainage Features of
the Erie and Ohio Basins, Mon. 41, zbid., 1902, pp. 802, pls. 25;
Comparison of North American and European Glacial Deposits, Zeit.
f. Gletscherk., vol. 4, 1910, pp. 241-315, pls. 1-5.
Former glaciations previous to Ice Age: —
A. Stranan. The Glacial Phenomena of Paleozoic Age in the Varanger
Fjord, Quart. Jour. Geol. Soc., London, vol. 53, 1897, pp. 137-146, pls.
8-10.
Bartey Wiuuis and Exiot BLAacKWELDER. Research in China, Pub. 54,
Carnegie Inst. Washington, vol. 1, 1907, pp. 267-269, pls. 37-38.
A. P. Coteman. A Lower Huronian Ice Age, Am. Jour. Sci. (4), vol. ii
1907, pp. 187-192.
W. M. Davis. Observations in South Africa, Bull. Geol. Soc. Am., ae
17, 1906, pp. 377-450, pls. 47-54. .
Davin Wurtz. Permo-Carboniferous Climatic Changes in South America,
Jour. Geol., vol. 15, 1907, pp. 615-633.
THE CONTINENTAL GLACIERS OF THE “ICE AGE” 319
Driftless and drift areas : —
T. C. CHamBeruin and R. D. Sauispury. Preliminary Paper on the
Driftless Areas of the Upper Mississippi Valley, 6th Ann. Rept. U. S.
Geol. Surv., 1885, pp. 199-322, pls. 23-29.
R. D. Sauispury. The Drift, its Characteristics and Relationships,
Jour. Geol., vol. 2, 1894, pp. 708-724, 837-851.
R. H. Wuirseck. Contrasts between the Glaciated and the Driftless
Portions of Wisconsin, Bull. Geogr. Soc., Philadelphia, vol. 9, 1911,
pp. 114-123.
Glacier gravings : —
T. C. CHAMBERLIN. The Rock Scorings of the Great Ice Invasions, 7th
Ann. Rept. U. S. Geol. Surv., 1888, pp. 147-248, pl. 8.
The dispersion of the drift : —
R. D. Satispury. Notes on the Dispersion of Drift Copper, Trans. Wis.
Acad. Sci., ete., vol. 6, 1886, pp. 42-50, pl.
N. S. SHater. The Conditions of Erosion beneath Deep Glaciers,
based upon a Study of the Bowlder Train from Iron Hill, Cumberland,
Rhode Island, Bull. Mus. Comp. Zodél. Harv. Coll., vol. 16, No. 11,
1893, pp. 185-225, pls. 1-4 and map.
Wituram H. Hosss. The Diamond Field of the Great Lakes, Jour.
Geol., vol. 7, 1899, pp. 375-388, pls. 2 (also Rept. Smithson. Inst.,
1901, pp. 359-366, pls. 1-3).
Glacial features : —
-T. C. CHAMBERLIN. Preliminary Paper on the Terminal Moraine of the
Second Glacial Epoch, 3d Ann. Rept. U.S. Geol. Surv., 1883, pp.
291-402, pls. 26-35.
G. H. Sronse. Glacial Gravels of Maine and their Assdciated Deposits,
Mon. 34, U. 8S. Geol. Surv., 1899, pp. 489, pls. 52.
W. C. Atpmn. The Delaven Lobe of the Lake Michigan Glacier of the
Wisconsin Stage of Glaciation and Associated Phenomena. Prof. Pap.
No. 34, U.S. Geol. Surv., 1904, pp. 106, pls. 15; The Drumlins of
Southeastern Wisconsin, Bull. 273, U. S. Geol. Surv., 1905, pp. 46,
pls. 9.
W. M. Davis. Structure and Origin of Glacial Sand Plains, Bull. Geol.
Soc. Am., vol. 1, 1890, pp. 196-202, pl. 3; The Subglacial Origin
of Certain Eskers, Proc. Bost. Soc. Nat. Hist., vol. 35, 1892, pp. 477-
499. ‘
F. P. Gutiiver. The Newtonville Sand Plain, Jour. Geol., vol. 1, 1893,
pp. 803-812. —
CHAPTER XXIII
GLACIAL LAKES WHICH MARKED THE DECLINE OF
THE LAST ICE AGE
Interference of glaciers with drainage. — Every advance and
every retreat of a continental glacier has been marked by a com-
plex series of episodes in the history of every river whose territory
it has invaded. Whenever the valley was entered from the direc-
tion of its divide, the
effect of the advanc-
ing ice front has gen-
erally been to swell
the waters of the river
into floods to which
= tuomer i) )) ere Sa
fe St
\
Fig. 346. — The Illi.ois River where it passes through
the outer moraine at Peoria, Illinois, showing the
flood plain of the ancient stream as an elevated
terrace into which the modern stream has cut its
gorge (after Goldthwait).
the present streams
bear little resemblance
(Fig. 346). Because
of the excessive melt-
ing, this has been even
more true of the ice
retreat, but here when
the ice front retired up the valley toward the divide. A sufficiently
striking example is furnished by the Wabash, Kaskaskia, Illinois,
and other streams to the southward of the divide which surrounds
the basin of the Great Lakes (Fig. 347).
Wherever the relief was small there occurred in the immediate
vicinity of the ice front a temporary diversion of the streams by the
parallel moraines, so that the currents tended to parallel the ice
front. This temporary diversion known as “ border drainage ”
was brought to a close when the partially impounded waters had,
by cutting their way through the moraines, established more perma-
nent valleys (Fig. 348).
320
SEE
o Sie pe in wat,
et Sore S5e- PE Rares
aaa :
cl
GLACIAL LAKES d21
Temporary lakes due to ice blocking. — Whenever, on the con-
trary, the advancing ice front entered a valley from the direction
of its mouth, or a re-
treating ice front retired
down the valley, quite
different results fol-
lowed, since the waters
were now impounded
by the ice front serving
asadam. Though the Ay
histories of such block- Ae ee
ing of rivers are often hit’ X
quite complex, the prin- yy 4
ciples which underlie
them are in reality sim-
ple enough. Of the
lakes formed during ad-
vancing hemicycles of
glaciation, and of all
save the latest reced- Fic. 347.—Broadly terraced valleys outside the
ing hemicycle, no satis- divide of the St. Lawrence basin, which remain to
factory records are pre- mark the floods that issued from the latest con-
tinental glacier during its retreat (after Leverett).
served, for the reason
that the lake beaches and the lake deposits were later disturbed
and buried by the overriding ice sheets. We have, however, every
o 50° 1ooM les.
LAD
LRIP
Fig. 348.— Border drainage about the retreating ice front south of Lake Erie.
The stippled areas are the morainal ridges and the hachured bands the valleys
of border drainage (after Leverett).
reason to suppose that the histories of each of these hemicycles
were in every way as complex and interesting as that of the one
which we are permitted to study.
Y
B22 EARTH FEATURES AND THEIR MEANING
As an introduction to the study of the ice-blocked lakes of North
America, and to set forth as clearly as may be the fundamental
principles upon which such lakes are dependent, we shall consider
in some detail the late glacial history of certain of the Scottish
glens, since their area is so small
and the relief so strong that rela-
tionships are more easily seen; it
is, so to speak, a pocket edition
of the history of the more ex-
SS tended glacial lakes.
ak aad The “ parallel roads” of the
Fia. 349. — The “parallel roads” of Scottish glens.—In a number
oe eee oe oe aaa of neighboring glens within the
southern highlands of Scotland
there are found faint terraces upon the glen walls which under the
name of the “ parallel roads ”’ .(Fig. 349) have offered a vexed
problem to scientists. Of the many scientists who long attempted
to explain them, though in vain, was Charles Darwin, the father
of modern evolution. He offered it as his view that the “ roads ”
Fig. 350. — Map of Glen Roy and neighboring valleys of the Scottish highlands with
the so-called ‘‘roads’’ entered in heavy lines. Glens Roy, Glaster, and Spean |
have three ‘“‘roads,’’ two ‘‘roads,”’ and one “‘road,’’ respectively (after Jamieson).
were beaches formed at a time when the sea entered the glens
and stood at these levels. When, however, Jamieson’s studies —
had discovered their true history, Darwin, with a frankness~char-
acteristic of some of the greatest scientists, admitted how far astray
GLACIAL LAKES 320
he had been in his reasoning. Let us, then, first examine the facts,
and later their interpretation. The map of Fig. 350 will suffice
to set forth with sufficient clearness the course of the several
“roads.” These “ roads” are found in a number of glens tribu-
tary to Loch Lochy, and of the three neighboring valleys, Glen
Roy has three, Glen Glaster two, and Glen Spean one “ road.”
The facts of greatest significance in arriving at their interpretation
relate to their elevations with reference to the passes at the valley
heads, their abrupt terminations down-valleyward, and the mo-
rainic accumulations which are found where they terminate. The
single ‘‘road”’ of Glen Spean is found at an elevation of 898
feet, a height which corresponds to that of the pass or col at the
head of its valley and to the lowest of the “‘ roads ” in both Glens
Glaster and Roy. Similarly the upper of the two “roads” in
Glen Glaster is at the height of the pass at its head (1075 feet)
and corresponds in elevation to the middle one of the three “ roads ”’
in Glen Roy. Lastly, the highest of the “roads” in Glen Roy is
found at an elevation of 1151 feet, the height of the col at the head
of the Glen. In the neighboring Glen Gloy is a still higher ‘‘ road ”’
corresponding likewise in elevation to that of the pass through
which it connects with Glen Roy.
- To come now to the explanation of the “‘ roads,” it may be said
at the outset that they are, as Darwin supposed, beach terraces
cut by waves, not as he believed of the ocean, but of lakes which
once filled portions of the glens when glaciers proceeding from
Ben Nevis to the southwestward were blocking their lower por-
tions. The several episodes of this lake history will be clear from
a study of the three successive idealistic diagrams in Fig. 351.
To derive the principles underlying this history, it is at once
seen that all changes are initiated by the retirement of the ice front
to such a point that rt ‘unblocks for the waters of a lake an outlet that
as lower than the one in service at the time. This is the principle
which explains nearly all episodes of glacial lake history. Thus,
when the ice front had retired so as to open direct connections
between Glen Roy and Glen Glaster, the col at the head of Glen
Roy was abandoned as an outlet, and the waters fell to the level
fixed for Glen Glaster. A still further retirement at last opened
direct connection between Glen Glaster and Glen Spean, so that
the lake common to Glens Glaster and Roy fell to the level of the
324 EARTH FEATURES AND THEIR MEANING
Fig. 351. — Three successive diagrams to set forth in order the late glacial lake
history of the Scottish glens.
GLACIAL LAKES 325
col which was the outlet of the Spean valley at the time. This
stage continued until the ice front had retired so far that the waters
drained naturally down the river Spean to Loch Lochy and thence
to the ocean.
Only in their far grander scale and in the lesser relief of the land
over which they formed, do the complex histories of the great
3 ——
i ah het t SO N00 on Sw... sr.
vt
all
Fig. 352. — Harvesting time on the fertile floor of the glacial Lake Agassiz ice
Howell).
ice-blocked lakes of North America differ from these little valley
lakes whose beaches may be visited and the relationships worked
out, thanks to Jamieson, in a single day’s strolling.
The glacial Lake Agassiz. — The grandest of the temporary lakes
referable to blocking by the continental glaciers of the ice age
must be looked for in the largest
valleys that lay within the terri-
tory invaded and which normally
drain toward the retiring ice front.
In North America these rivers are
the Red River of the North in
Minnesota, the Dakotas, and Mani- ia \
toba; and the St. Lawrence River ~-4 erations og
system. To the ice dam which lay eS ?
across the Red: River valley we
owe the fertility of that vast plain
of lake deposits where is to-day the Fia. 353. — Map of Lake Agassiz (after
‘ . : Upham).
most intensive wheat farming of
the northwest (Fig. 352). Lakes Winnipeg, Winnipegoosis, and
Manitoba, and the Lake of the Woods, are all that now remain of
this greatest of the glacial lakes, which in honor of the distinguished
founder of the glacial theory has been called Lake Agassiz (Fig.
353). With their natural outlet blocked by the ice in northern
326 EARTH FEATURES AND THEIR MEANING
Manitoba and Keewatin, the waters of the Red were swollen by
melting from the retiring glacier and spread over a vast area before
finding a southern outlet along the course of the present Lake
Traverse and the valley of the Minnesota River. Along this route
"Oye hoe Ae
eit
Weta th ty td
a.
i
@..,
%,
o Scale of mites - Ff
oy, ~
tg sony esse pity
—~
Fig. 354. — Map of the southern end of the Lake Agassiz basin, showing the position
of some of the beaches and the outlet through the former Warren River (after
Upham).
there flowed a mighty flood which carved out a broad valley many.
times too large for the Minnesota, its present occupant, and this
giant prehistoric river has been called the Warren River (Fig. 354).
GLACIAL LAKES aoe
It is interesting to follow this ancient waterway and to discover
that, like our normal, present-day streams, it was held up in narrows
wherever outcroppings of harder rock had constricted its channel
(Fig. 355). The upper end of the Warren River valley is now
Scale of Ailes
pts — 2
Fig. 355.— Narrows of the Warren River below Big Stone Lake, where it passed
between jaws of hard granite and gneiss (after Upham).
occupied by the long and relatively narrow Lakes Traverse and
Big Stone, each the result of blocking by delta deposits where a
tributary stream has emerged into the valley, but this gigantic
channel continues down to and beyond Minneapolis, occupied as
far as Fort Snelling by the
Minnesota River—a mere Ne Stanthany \
pygmy compared to its prede- SY
cessor. To the earnest student \
of glacial geology there can be ee
fewsightsmoreimpressivethan *7 J ._/¥
are obtained by standing at Sire RA
Fort Snelling, just above the LBi
confluence of the Minnesota ey ij
and the Mississippi rivers, and he é
surveying first the steep and Ame . S
narrow valley of the Missis- W/ Scale (“
sippi above the junction,—a e nN cass
stream fitted to its valley for Ire. 356.— Map of the valley of the Warren
the simple reason that it has ae in the ee
carved it, —and then gazing ike it at. Fort Snelling (after avinnsiil
up and down that broad valley
in which the great Warren River once flowed majestically to the
sea, now the bed of the Minnesota above the Fort and of the Mis-
Sissippi below it (Fig. 356).
328 EARTH FEATURES AND THEIR MEANING
Just as the “ parallel roads ”’ of Glen Roy, roads in name only,
are the beaches of earlier glacial lake stages, so in Lake Agassiz
we have parallel beaches of the barrier type which are often roads
in fact as well as in name, and which mark the stages of successive
lakes within this vast basin. The Herman beach, corresponding
to the highest level of the lake, is thus a sharp topographic bound-
ary between lake deposits and morainal accumulations, and is
yoo” 2k
“ —
yos2 Ve) 23-4 bow yo76_ 10794 a= NLT LK ae:
Fig. 357. — Portion of the Herman quadrangle of Minnesota, showing the position
of the Herman beach on the shore of the former Lake Agassiz. The lake basin is
to the left, and the pitted morainal deposits appear to the right (U.S. G. §.).
further itself a well-marked topographic feature composed of wave-
washed and hence well-drained materials (Fig. 357). Farmers of
the district have been quick to realize that these level and slightly
elevated ridges lack the clay which would render them muddy in
the wet seasons, and are thus ideally adapted for roads. They -
have in many sections been thus used over long stretches and are
known as the “ ridge roads.”’
te
3
+ A Se eee ne _ >
AEN TES SNELL IY hist
bd - ‘ ~ man fa ot ~~
rae, re sone tee eine
GLACIAL LAKES 329
Episodes of the glaclal lake history within the St. Lawrence
valley. — Within this great drainage basin it has apparently
been possible to read the records of each stage in the latest lake
history — complex as this has been. We have only to recall the
lake stages cited from the Scottish glens and remember that each
new stage was begun in a retirement of the glacier front which un-
blocked an outlet of lower level than the last. This sequence
might, however, have been varied by a temporary readvance of the
ice, as indeed once occurred in the Huron-Hrie lobe of the great
North American glacier.
The crescentic lakes of the earlier stages. —So long as the
glacier covered the entire drainage basin of the St. Lawrence
Fia. 358. — The continental glacier of North America in an early stage of its reces-
sion, when it covered the entire St. Lawrence drainage basin. The dashed line
is the approximate position of the divide (based on a map by Goldthwait).
River system, all water was freely drained away by streams which
flowed away from the ice front (Fig. 358). So soon, however,
as at any point the front had retired behind the divide, impound-
ing of the waters must locally have occurred. Lakes of this type
are to-day to be seen in Greenland and in the southern Andes;
and though upon a diminutive scale, some idea of their aspect may
be obtained from the appearance of the Mirjelen Lake of Swit-
zerland, here blocked by a mountain glacier (Fig. 446, p. 411).
330
gy
Nee
ay
0 Scale
Sree ar 100 Mi.
Fig. 359.—Outline map of the
early Lake Maumee, with the
bordering moraine and _ the
water-laid moraine remaining
on the site of the former ice cliff.
EARTH FEATURES AND THEIR MEANING
Within all areas of small relief, such as
the prairie country surrounding the
present Laurentian lakes, the earlier
and smaller stages of such ice-blocked
lakes are generally crescentic in out-
line. This is because a moraine in
most cases forms the land margin of
the lake, and because the ice cliff
upon the opposite border, although
somewhat straightened, as a conse-
quence of wave-cutting and iceberg
formation, still retains the convex
outlines characteristic of ice lobes
(Fig. 359).
Within each of the Great Lake basins a crescentic lake early ap-
peared at that end of the depression which was first uncovered
“Nt
L
@ ee
om. 4
amis a,
-
.
onl
° ele,
a, (Saer™ 3
.
_ whe
Fig. 360.— Map to show the first stages of the ice-dammed lakes within the’
St. Lawrence basin (after Leverett and Taylor).
GLACIAL LAKES ool
by the glacier: Lake Duluth in the Superior basin, Lake Chicago
in the Michigan basin, and Lake Maumee in the Huron-Erie
basin (Fig. 360).
We may now, with profit, trace the successive episodes of the
glacial lake history, considering for the earlier stages those changes
which occurred within the Huron-Erie basin, since, these are in
essential respects like those of the Michigan and Superior basins,
although worked out in greater detail. Lake Chicago must,
however, be brought into consideration, since in all save the earli-
est and the later stages, the waters from the Huron-Erie depression
were discharged through the Grand River into this lake and
thence by the so-called ‘ Chicago outlet” into the Mississippi
(plate 20 A).
The early Lake Maumee. — The area, outline, and outlet of
this lake are indicated upon Fig. 360. Its ancient beaches have
been traced, as well as the water-laid moraine beneath its former
ice cliff; and no observant traveler who should take his way
down the ancient outlet from Fort Wayne, Indiana, past the town
of Huntington, could fail to be impressed by its size, suggesting
as it does the great volume of water which must once have flowed
along it. Now a channel a mile or more in width, its bed for the
twenty-five miles between Fort Wayne and Huntington may be
seen from the tracks of the Wabash Railway as a series of swamps
merely, while at Huntington the Wabash river enters by a young
V-shaped valley at the side, much as the Mississippi emerges into
the old channel of the Warren River at Fort Snelling,. Minnesota
(see p. 327).
The Huron River of southern Michigan, which now discharges
into Lake Erie, then found its lower course blocked by the glacier
and was thus compelled to find a southerly directed channel now
easily followed to the northern horn of the crescent of Lake
Maumee.
The later Lake Maumee. — When the ice lobe had retired its
front sufficiently, an outlet lower than that at Fort Wayne was
uncovered past the city of Imlay, Michigan, into the Grand
River, and thence through Lake Chicago and its outlet into the
‘Mississippi. This old outlet south of Chicago follows the course
of the present Drainage Canal and the line of the Chicago &
Alton Railway. The traveler journeying southward by train from
302 EARTH FEATURES AND THEIR MEANING
Chicago has thus the opportunity of observing first the beaches
of the former lake, and then the several channels which were
joined in the main outlet at the station of Sag (plate 20 A).
In this stage of our history Lake Maumee pushed a shrunk
arm up past the site of Ypsilanti in Michigan (Fig. 361), the well-
marked beach being found on Summit Street opposite the State
Normal College. The Huron River, which in the first lake stage
FIM NE J * aa
SON tae” M7
é.
os 4
oi
Fic. 361.— Outline map of the later Lake Maumee and of its ‘Imlay outlet’ to
Lake Chicago (after Leverett).
had followed the valley now occupied by the Raisin River south-
ward into Indiana, now discharged directly into a bay upon this
arm of Lake Maumee, and so formed a delta at Ann Arbor.
Lakes Arkona and Whittlesey. — The ice front in the Huron-
Erie basin now retired so far that the impounded waters, instead
of following the more direct “ Imlay outlet ”’ to the Grand, passed
at a lower level completely around “ the thumb” of Michigan
into the Saginaw basin. Meanwhile a crescent-shaped lake had
developed in that basin, so that now the waters of the Maumee
basin were joined to those in the Saginaw basin as a common,
lake, just as the lowering of the waters in Glen Roy caused a
union with those of Glen Glaster in the example cited for illus-
GLACIAL LAKES 333
tration. Our records of this third North American lake stage,
referred to as Lake Arkona, are however most imperfect, for the
reason that it was followed by a readvance of the ice front which
ee ie ee,
yes aye ae a)
ip eal: Mae A
Fig. 362.— Outline map of Lakes Whittlesey and Saginaw (after Leverett).
closed the passage around ‘‘ the thumb ”’ and raised the level of
the waters until an outlet was found past the town of Ubly at a
lower level than the “ Imlay outlet.’”’” When the waters of a
a. ; BES
Ke \\ < Oy Ss a
U/ \ SI ee rt
( et AS SEI Gy ——fy
ee. \\ \ NS > 5 . Me, SPY
Soa 3h SS SS \s
ing f. %, S Qe
Looe a i
Fig. 363. — Map of the glacial Lake Warren, the last of the lakes in the Huron-Erie
basin, which discharged through the ‘‘Grand River outlet 3 into the Mississippi
(after Leverett).
lake are thus rising, strong beach formations result, and those of
; ’ this stage, which is known as the Lake Whittlesey stage, are much
i q the strongest that are found within the Huron-Erie basin. Traced
334 EARTH FEATURES AND THEIR MEANING
for some three hundred miles entirely around the southern and
western margins of Lake Erie, this beach is for much of the dis-
tance the famous “ ridge road ” (Fig. 362).
Lake Warren. — As the ice advance which had produced Lake
Whittlesey came to an end, the normal recession was resumed
and a lake once more formed as a body common to the Saginaw
and Erie basins. This lake, known as Lake Warren, extended
a shrunk arm far eastward along the ice front into western New
York, though it was still blocked from entering the great Mo-
hawk valley (Fig. 363).
Lakes Iroquois and Algonquin.— It must be evident that
toward the close of the Lake Warren stage a profound change was
Fig. 364. — Map of the Glacial Lake Algonquin (after Leverett).
imminent — a transfer of the glacial waters from their course to
the Mississippi and the Gulf to the trench which crosses New -
York State and enters the Atlantic. So soon as the ice front. had
retired sufficiently to lay bare the bed of the Mohawk, an outlet
‘was found by this route and its continuation down the Hudson '-
valley to the sea. The Lake Ontario basin now became occupied
by a considerably larger water body known as Lake Iroquois, and
oH
hy
Le
e
j Ng
,
fi
1p
|
fos
py
i Ss
2
al eet
eM
fe Z
ie
be
,
bs
4
oe
+. "
ee
in,
aml
- fi
e
hh
&
*
GLACIAL LAKES 330
the three upper lakes, then joined as Lake Algonquin, discharged
their combined waters into Lake Iroquois at first through a great
channel now strongly marked across Ontario in the course of the
Trent River and Lake Simcoe, the so-called ‘ Trent outlet.”
At this time a smaller Lake Erie probably occupied the basin of
that lake, and later the Trent outlet was abandoned for the Port
Huron outlet (Fig. 364).
The Nipissing Great Lakes. — We have now followed the ice
front step by step in its retreat across the valley of the St. Law-
rence system. The successive unblocking of outlets offers but
one further possibility — the opening of the French River-Nip-
Fia. 365.— Outline map of the Nipissing Great Lakes with their outlet past North
Bay into the Champlain Sea.
issing Lake-Ottawa River, or ‘“‘ North Bay outlet.” Though not
so to-day, the bed of this ancient channel was then much lower
than that of the ‘‘ Mohawk outlet,” and so soon as the glacier
had in its retreat uncovered this northern channel, the waters of
the upper lakes discharged through it past the site of Ottawa
and into an arm of the sea which then occupied the lower St.
Lawrence valley and has been called the Champlain Gulf or Sea
336 EARTH FEATURES AND THEIR MEANING
(Fig. 365). The level of the waters was lowered and the area
of the lakes correspondingly reduced.
The reader who has had no opportunity to observe these an-
cient channels which carried the swollen waters of the former
glacier lakes, will find it interesting to consider that every one of
them has been fixed upon by engineers for improvement as arti-
ficial waterways. Thus we have the Illinois Drainage Canal
and projected ship canal along the ‘ Chicago outlet,” the pro-
jected Mississippi-Lake Erie Canal along the “ Fort Wayne out-
let,’”’ the Grand River canal project to connect Lake Michigan and
Saginaw Bay along the course of the ‘‘ Grand River outlet,” the
Trent Canal along the ‘“‘ Trent outlet,’ the Erie Canal along the
‘“ Mohawk outlet,’’ and, lastly, the proposed Georgian Bay ship
canal to the ocean along the ‘‘ North Bay” or “ Nipissing outlet.’’
Summary of lake stages.— We have omitted in this sum-
mary of late lake history in the Laurentian basin all the less
important lake stages, including some of a transitional nature
which were represented by beaches and outlets easily traced to-
day. This is because it is an outline only which it seems best to
present, and the episodes of this abridged history may be tabu-
lated as follows:
EPISODES OF GLACIAL LAKE HISTORY
MississipP1 DRAINAGE
Lake Maumee (early), Fort Wayne outlet.
Lake Maumee (late), Imlay City outlet.
Lake Arkona, ‘‘thumb”’ outlet.
Lake Whittlesey (with readvanee of glacier), Ubly outlet.
Lake Warren, ‘‘thumb”’ outlet.
ATLANTIC DRAINAGE
Lakes Iroquois and Algonquin (early), Trent and Mohawk outlets.
Lakes Iroquois and Algonquin (late), Port Huron and Mohawk
outlets.
Nipissing Great Lakes, North Bay outlet.
Permanent changes of drainage affected by the glacier. — While
the lake history which we have sketched is made up of episodes
which endured only while the ice front lay between certain: sta-
tions upon its retreat, there were none the less brought about the
5
|
Va
F
1
%
3
RE LAEP ELLE HR
GLACIAL LAKES 337
profoundest of permanent modifications in the drainage of the
region. It is possible to restore upon maps in part only the pre-
glacial drainage of the north central states, but we know at least
that it was as different as may be from that which we find to-day.
The Missouri and the Ohio take their courses to-day along the
margin of the glaciated area as an inheritance from the border
drainage of the ice age. Within the
have in many cases been compelled by
enter upon new courses, or even to travel
in the opposite direction along their
former channels. In districts of con-
siderable relief these diversions have
sometimes caused the streams to plunge
over the walls of deep valleys, and it
may truthfully be said that we owe
much of our most beautiful scenery in
part to the carving and molding of
glaciers, but especially to the cascades
and waterfalls directly due to their in-
terference with drainage.
Many diversions or reversals of former
drainage lines, through the influence of
the continental glacier, are at once sug-
gested by the abnormal stream courses,
which appear upon our maps, and the
correctness of these suggestions may
often be confirmed by very simple ob-
servations made upon the ground.
The map of Fig. 366 shows how differ-
ent was the preglacial drainage of the
that of to-day.
glaciated regions rivers
morainal obstructions to
"Buffalo |
a ee
é
pak
a
a
2
Scale apiles.
—a——
Qo 50
Fig. 366. — Probable preglacial
drainage of the upper Ohio
region (after Chamberlin and
Leverett).
upper Ohio region from
An interesting additional example is furnished by the Still
River which in Connecticut is tributary to the Farmington, and
is no less remarkable for its abnormal northerly course and sluggish
current perpetrated in its name, than for the way in which it is joined
to the Farmington system (Fig. 367 A). A careful study of the
district has shown that the Still River was once a part of the
Naugatuck and flowed southward toward Long Island Sound like
other rivers of the district (Fig. 367 B). It possessed, however,
338 EARTH FEATURES AND THEIR MEANING
an advantage in a narrow belt of softer rock along its course, and
because of this advantage it captured a portion of one of the tribu-
taries to the Farmington (Fig. 367 C). The continental glacier
later covered the region, and on its retreat laid down morainal
obstructions directly across this river and also at the head of the
severed arm of the Farmington tributary (Fig. 367 D). The now
impounded waters found their lowest outlet near Sandy Brook,
and in waterfalls and cascades the now reversed river falls one
Fic. 367. — Diagrams to illustrate the episodes in the recent history of the Still
River tributary to the Farmington in Connecticut. A, present drainage; B, early
stage; C, after capture of a tributary to the Farmington; D, after blocking by
morainal obstructions of the ice age.
hundred feet to the bed of that stream. With the aid of the
excellent topographic maps which are now supplied by a generous
government at a merely nominal price, such bits of recent history
may be read at many places within the glaciated region.
Glacial Lake Ojibway in the Hudson Bay drainage basin.—
When by passing over the “ height of land” in northern Onta-
rio the greatly reduced continental glacier had vacated the basin
of St. Lawrence drainage, it was in a position to impound those
waters which normally drained to Hudson Bay. The lake which
then came into existence has been called Lake Ojibway and was the
latest of the entire series. Though of but recent discovery in
a country till lately a trackless wilderness, its extension seems to
have been that of the clay beds suited for farming. The beaches
and outlets remain to be mapped when the country has been
made more easily accessible.
GLACIAL LAKES 339
READING REFERENCES FOR CHAPTER XXIII
Parallel roads of Glen Roy : —
CHARLES Darwin. Observations on the Parallel Roads of Glen Roy
and of Other Parts of Lochaber in Scotland, with an attempt to prove
that they are of Marine Origin, Phil. Trans., vol. 8, 1839, pp. 39-82.
Louis Acassiz. Geological Sketches, Boston, 1876, vol. 2, pp. 32-76.
T. T. Jamieson. On the Parallel Roads of Glen Roy and their Place in
the History of the Glacial Period, Quart. Jour. Geol. Soc. Lond.,
vol. 19, 1863, pp. 235-259.
Glacial Lake Agassiz : —
Warren Upuam. The Glacial Lake Agassiz. Mon. 25, U.S. Geol. Surv.,
pp. 658, pls. 38.
F. W. Sarpreson. Beginning and Recession of St. Anthony’s Falls,
Bull. Geol. Soc. Am., vol. 19, 1908, pp. 29-36.
Glacial lakes in the St. Lawrence valley : —
CHAMBERLIN and Sauisspury. Geology, vol. 3, pp. 394-405.
Frank Leverett. Outline of the History of the Great Lakes (Presi-
dential Address), 12th Rept. Mich. Acad. Sci., 1910, pp. 19-42. The
Pleistocene Features and Deposits of the Chicago Area. Chicago,
1897, pp. 86, pls. 8 (Chicago Outlet).
H. L. Farrcsiitp. Glacial Lakes in Western New York, Bull. Geol. Soc.
Am., vol. 6, 1895, pp. 353-374, pls. 18-23; Glacial Waters in Central
New York. Bull. 127, N. Y. State Mus., 1909, pp. 66, pls. 42, and
maps in cover.
Early lakes in the Erie basin : —
Frank LEVERETT. On the Correlation of Moraines with Raised Beaches
of Lake Erie, Am. Jour. Sci. (3), vol. 43, 1892, pp. 281-301.
F. B. Taytor. The Great Ice Dams of Lakes Maumee, Whittlesey, and
Warren, Am. Geol., vol. 24, 1899, pp. 6-38, pls. 2-3; Relation of
Lake Whittlesey to the Arkona Beaches, 7th Rept. Mich. Acad. Sci.,
1905, pp. 30-36.
Frank Leverett. The Ann Arbor Folio, Folio No. 155, U.S. Geol. Surv.,
1908, pp. 10-12.
CHAPTER XXIV
THE UPTILT OF THE LAND AT THE CLOSE OF THE
ICE AGE
The response of the earth’s shell to its ice mantle. — There
is now good reason to believe that the earth’s outer shell makes
a response by oscillations of level due to the loading by ice, on the
one hand, and to the removal of this burden upon the other. We
know, at least, that both in northern Europe and in North America
areas which have undergone depression during and elevation after
the ice age, correspond closely to the regions which were ice cov-
ered. Wherever in these regions there was high relief before the
advent of the ice, river valleys were drowned at the land margins
and were also gouged out into troughs through erosion by the
outlet tongues upon the margin of the ice sheet. Such furrowed
and half-submerged valleys have a characteristic U-shaped sec-
tion, so that their walls rise precipitously from the sea. From
their typical occurrence in Scandinavian countries the name fjord
has been applied to them.
It is now no less clear that the removal of the ice blanket broweht
from the earth a relatively quick response in uplift, which began
before the ice front had retired across the present international
boundary of the United States, and ‘that this uplift continued
until the final disappearance of the ice. A far slower elevation of
a somewhat different nature has continued, even to the present
day.
It is obvious that at the time of their formation all shore lines
referable to the work of waves must have been horizontal, and
hence any variations from a perfect level which they reveal to-day
must indicate that a tilting movement of the ground has occurred
since the waters departed from their basins. We have thus.
provided for us in the positions of these ancient water ‘planes,
particularly because of their wide extent, a complete record the
refinement of which is not easily overstated. Interpreting this
. 340
I
Ne
ee <> ae ee 8h = Bt ig de
Dy Pa ea ene ee: he
oe ay ‘ie :
UPTILT OF LAND AT CLOSE OF ICE AGE 341
record, we find that it was the uptilt of the land to the northward
which brought the glacial lake history to an end and inaugurated
the present system of St. Lawrence drainage. The outlet of the
Nipissing Great Lakes is to-day more than a hundred feet above
the level of the outlet at Port’: Huron, where the upper lakes are
now discharging their waters, and this difference in level can
only be ascribed to an upward tilting of the land since the latest
of the glacial lake stages.
The abandoned strands as they appear to-day. — The traveler
by steamer upon the upper lakes, as he comes within view of
each rocky headland, may note
how the profile against the ho-
rizon is notched by a series of
steps or terraces (Fig. 368), TE ee ae
and if he has followed the dis- ! | ore
cussion in previous chapters, ~-22——7 JssP ee
he will suspect that these ter- Fie. 368.— The notched rock headland
races mark thenow abandoned Bove Bal Ne eee
shore lines which have come
to their present position through a series of uplifts of the ground
accompanied by earthquake shocks. As his steamer skirts the
shore he may chance to note a cave within the rock cliff which
represents the now elevated sea-arch of an ancient shore.
Disembarking from the steamer and traveling inland at any
point where the shores are high, the traveler is certain to come
upon still more convincing proofs of the ancient strands; perhaps
in a storm beach of the unmistakable “ shingle,” half buried though
it may be under dunes of newly drifted sand, or possibly at higher
levels the highway has been cut through a shingle barrier as
fresh and unmistakable as though formed upon the present shore.
Sometimes it is the rock cliff and terrace, at other times barrier
ridges of shingle, or, again, it is the sloping cliff and terrace cut
in the drift deposits; but of whatever sort, if studied with proper
regard to the topography of the district, the evidence is clear
and unmistakable.
The records of uplift about Mackinac Island.— Nowhere are
the records of the recent uplift of the lake region more easily read
than about Mackinac Island in the straits connecting Lake Michi-
gan with Lake Huron. Approaching the island by steamer from
342 EARTH FEATURES AND THEIR MEANING
St. Ignace, its profile upon the horizon is worthy of remark (Fig.
369). From a central crest broken by minor irregularities and
bounded on all sides by a cliff, the island profile slopes gently
away toa still lower cliff, below which is another terrace.
| Algonqui
quin Level y; nice;
Tale ot Algonquin Level Nipissing Level
esi es ~ — = NO aoe aes . 3
CS eete Bes ee) ~ —— = Py, coe nee A :
ie oe Sid = age egress: ata ates se
- Cea zs a 7
roe = cone" ye ead 6<‘aae Ome
Fig. 369.— View of Mackinac Island from the direction of St. Ignace. The ir-
regular central portion is the only part of the island that was not submerged in
Lake Algonquin. The terrace at its base is the old shore line of Lake Algon- .
quin, and the lower terrace the strand of Lake Nipissing (after a photograph by |
Taylor). .
When we have reached the island and have climbed to the )
summit, we there find the surface which is characteristic of erosion.
by running water, whereas at lower levels are found the forms
carved or molded by the action of waves. This central “ island,’”
superimposed upon the larger island, is all that rose above Lake
Algonquin, the earliest of the glacial lakes in this northern dis-
trict; and as we look out from the observatory upon the summit,
it is easy to call up a picture of
the country when the lake stood
at the base of this highest cliff.
To the northward one sees the
“‘ Sugar Loaf ”’ rise out of a sea
of foliage, as it formerly did
‘| from the waters of Lake Algon-
a Ba ; ' quin (Fig. 370). It is a huge
Fig. 370.— The “Sugar Loaf,” a stack stack near the former island
near the shore of Lake Algonquin, as
it is seen from the observatory upon shore. If we turn now to the
Mackinac Island (after a photograph southward and direct our gaze
Mee a toward the Fort, we encounter
a veritable succession of beach ridges formed of shingle and ranged~
like a series of waves within the cleared space of the “ Short
Target Range”’ (Fig. 371). These ridges mark each a stage within
Eby
ses, 2
: eS ne a Wwe
i hie Ee *
4s se
UPTILT OF LAND AT CLOSE OF ICE AGE 343
a series of successive uplifts which have brought the island to
its present height.
Fic. 371. — View from the Sera upon Mackinac tied across the ‘Short
Target Range” toward the Fort. Beach ridges appear in succession within the
cleared space (after a photograph by Rossiter).
NS. ab ae Saas
oe ae .¥
Ny: AAT.
Fig. 372.— Notched stack of the Nipissing Great Lakes at St. tenses
(after a photograph by Taylor).
6 oa
344 EARTH FEATURES AND THEIR MEANING
If now we descend from our position and visit the “ battle-
field,’ we find there a great ridge of level crest, behind which
the British force was stationed in its defense of the island in
1812. Near by in the woods is Pulpit Rock, a strikingly perfect
stack of the Nipissing Lake. Across the straits at St. Ignace is an
Fia. 373.—Series of diagrams to
illustrate the evolution of ideas
concerning the uplift of the lake
region since the ice age. A,
simple northerly up-canting
(Gilbert) ; B, northerly acceler-
ation of the up-canting (Spen-
cer and Upham); C, northerly
“feathering out’’ of beaches
(Spencer and Upham) ; D, hinge
line of up-canting found within
the lake region (Leverett); E,
multiple and northwardly mi-
grating hinge lines of up-canting
(Hobbs).
even finer example of the notched
stack (Fig. 372). Other less prom-
inent beaches, but all later than the.
Nipissing Lakes, intervene between
this level and the present shore to
mark the stages in the continued up-
lift of the land.
The present inclinations of the up-
lifted strands. — It is not enough that
we should have recognized the marks
of former shores now at considerable
elevations above the existing lakes;
if we are to know the nature of the
uplift, we must prepare accurate maps
based upon measurements by precise
leveling at many localities. Such
methods are, however, of compara-
tively recent application in this field ;
and, as in the investigation of so many
other problems, the earlier observa-
tions were largely of the nature of
reconnaissances with the elevation of
beaches estimated by comparatively
crude methods only. The evolution
of ideas concerning the uptilt has,
therefore, been a gradual one.
It was early observed that the
beaches corresponding to a given lake
stage were higher to the northward
and northeastward, and the natural
conclusion from this was that the
earth’s crust had here been canted,
like a trap door (Fig. 373, A). As we are to see, this but half-
correct assumption has led to a striking prophecy relating to future
=) Soi aS
SE OR TN
ie
Lal
peepee
a
A
*
4%
},
?
ie
aoe
405
»
A'
2
ee
fae
a.
’
ven
‘lines of uptilt.
UPTILT OF LAND AT CLOSE OF ICE AGE 345
changes within the lake region which we now know to be with-
out warrant in the facts. Later it was learned that the uptilt
of the lake beaches is much accelerated to the northward (Fig.
373, B), and that new beaches make their appearance from be-
neath others as
we proceed in
this direction —
there is a “ feath-
ering out” of
beaches to the
northward (Fig.
373, C).
The hinge
— Still later in
the study of the
region, it was
learned that the
axis or fulcrum Fia. 374.— Map of the Great Lakes region to show iso-
about which th bases and hinge lines of uptilt. a, isobase of the Chicago
: . outlet; 6, main hinge line of the Lake Whittlesey beach
region has been (Leverett) ; b!, hinge line of the Lake Warren beach (Tay-
uptilted, instead lor) ; c,isobase of the Port Huron outlet; d, main hinge
Seay to. the Fs donal hingoliaeadt Algonquin beastion in Doo Cova
southward of the peninsula (Hobbs) ; 1, isobase of the Lake Superior outlet
lake district, as for the Algonquin beaches (Leverett) ; m, isobase of the
had been as- same outlet for the Nipissing beaches (Leverett).
sumed by Gilbert, lay within the region and about halfway up
the basin of Lake Michigan (Fig. 373, D, and Fig. 374). Simi-
larly, in the uptilt which followed the ice retreat in northern
Europe a definite hinge line of movement has been discovered.
Lastly, it has been shown, as a result of the use of precise level-
ing methods, that not one but several hinge lines of movement
lie within the region, and that the separate sections into which
they divide the area are each in turn characterized by increased
up-cant as we proceed to the northward (Fig. 373, H and Fig. 374).
The beaches of Lake Maumee, the earliest of the series of lakes
within the Huron-Erie lobe and within the extreme southern
portion of the Great Lakes area, show only the slightest possible
northerly uptilt, and the well-marked hinge line disclosed in the
346 EARTH FEATURES AND THEIR MEANING
Whittlesey beach is evidence that the elastic recoil, as it were,
from the weight of the mantling glacier did not begin until after
the draining of Lake Whittlesey. The determination by Taylor
that there is a similar initial hinge line in the Warren beach —
that this strand begins its uptilt some fifteen miles farther north-
east than does the Whittlesey beach — is one of the greatest im-
portance in obtaining a correct idea of the recent uplift; for it
re |
<8 ICE
sswes oe MME.
,
& os
dy
NI Pil ICE
fe eee
B
g ee
58 Pat ICE
sf
te
Cc
g
= TCE
We
le aerate
ie oan)
D
gly
|
eae
<5 es ee pa ea _ 16
Bi 1D)
eT a
poeeee
i‘ ¥F
Fig. 375. — Series of idealistic diagrams to indicate the nature of the quick recovery
of the crust by uplift in blocks unloaded of the ice in succession. A further and
slower uptilt, added after the completion of the first movement, is brought out in
the last diagram (b’).
~~
shows that the draining of Lake Whittlesey was followed by |
a period of quick uplift and seismic activity, that the stage of
ont ei mee te Civ, _ a3 “
- — cot ce tt GOL I TT SIT ae F ee ea iy Re a, he saad otieee bg eo ee so
ee on 5 ema? am “a i ts" Pe ta eS Aor ares = es aE im ae os a
ave ™ oe eer, aw ates = ar roe 7 , ow ee ’ . - .
i aE = = _ - “4 bs = _ . ‘s
— re c. ae poe - ses ; - :
eT ee ME EES
“+ . at >
; 7 er wie
UPTILT OF LAND AT CLOSE OF ICE AGE 347
Lake Warren was one of comparative stability of the land,
and, lastly, that the draining of Lake Warren was followed by a
second period of rapid uplift and earthquake disturbance.
The strongly marked hinge lines, additional to the initial one
indicated for the Algonquin beaches in the profiles by Gold-
thwait from the west shore of Lake Michigan, when considered in
the light of this northeasterly migration of the still earlier hinge
line in the southern district, are best explained through the as-
sumption of a succession of quick recoveries of the crust by up-
lift, separated by periods of relative stability, and brought on by
the removal in turn of the ice burden from successive blocks of
the shell which are separated by the several hinge lines (Fig. 375).
The elaborate study of erosion in the outlet of Lake Agassiz
had indicated identical interruptions in the up-canting process
for that basin.
Future consequences of the continued uptilt within the lake
region. — One of the most distinguished of American geologists,
Dr. G. K. Gilbert, in order to determine whether the uptilt revealed
by canted beach lines is still in progress, carried out an elaborate
study upon the gauge records preserved at the various gauging
stations about the Great Lakes. Upon the basis of these studies,
he concluded that the uplift continues, that the axes of equal
uplift (isobases) take their course about fifteen degrees north of
west, so that the lines of greatest uptilt should be perpendicular to
this direction, or fifteen degrees east of north. He further believed
that the basin was undergoing an up-cant in the simple manner of
a trap door, the hinge of which lay to the southward of Chicago,
and the study of the gauge records led him to believe that ‘“ the
rate of change is such that the two ends of a line one hundred miles
long and lying in a south-southwest direction are relatively dis-
placed four tenths of a foot in one hundred years.”
Gilbert’s prophecy of a future outlet of the Great Lakes to
the Mississippi. — The natural rock sill, over which the waters
of Lake Chicago once flowed to the Mississippi, is to-day but
eight feet above the common mean level of Lakes Michigan and
Huron, and if the tilting of the lake region were to continue upon
_ Gilbert’s assumption of a canting plane with the hinge of the
- movement to the south of Chicago, a time must come when the
“Chicago outlet’”’ will again come into use and the lakes once
348 EARTH FEATURES AND THEIR MEANING
more drain to the Mississippi and the Gulf. Upon the basis of
his measurements, Gilbert ventured the prophecy that the first
high-water discharge into the Mississippi should occur in from
five hundred to six hundred years, and for continuous discharge
in fifteen hundred years. In twenty-five hundred years Niagara
Falls should at low water stages be dry from this cause, and in
thirty-five hundred years it should have become extinct.
This prophecy, emanating from a high scientific authority and
relating to changes of such profound economic and commercial
importance, has been often quoted and has taken a firm hold upon
the popular imagination. Obviously, it depends upon the now
exploded theory that the lake basin has been canted as a plane
and that the axis of uptilt lies somewhere to the southward of
the lake region, or, in any event, to the southward of the present
Port Huron outlet. We know to-day that instead of being uni-
formly distributed over the entire lake region, the uptilting goes
on at a much higher rate within the northern .areas, and that
since the early stage of Lake Whittlesey the hinge line of uplift
has been steadily migrating northward with the retreat of the
ice and is now well to the northward of the present outlet. There
is, therefore, no known uptilt of the district which separates
the present from the former Chicago outlet, and there is no ap-
parent natural cause which should result in the reoccupation of
the old outlet to the Mississippi. The prophecy must be regarded
as one that has been outgrown with the progress of science.
Geological evidences of continued uplift.—It has recently
been claimed, on the basis of a reéxamination of Gilbert’s study
of the lake gauge records, that his methods are open to serious
criticism and that in reality the figures afford no evidence of con-
tinued uplift of the region. However this may be, there are not
lacking geological evidences which do not admit of doubt, and
these are in a striking way confirmatory of the latest conclusions
upon the manner of the recent uplift.
If our conclusions have been correct, the several lake basins
should now be behaving in different ways as regards the changes
upon their shores. If it is true that the lines of greatest uptilt
run north-northeasterly, there should be, speaking broadly, a
“spilling over’ of waters upon the south-southwesterly shores
and a laying bare of the north-northeasterly shore terraces of the
RE SE FE
Senate ae teen
UPTILT OF LAND AT CLOSE OF ICE AGE 349
_ basins. This should, however, be true only of basins whose
outlets are to the northeastward of the existing main hinge line
of uptilt. Lake Huron, having its outlet at the southern margin
of its basin, should not have its waters encroaching upon the
southern shore, for the simple reason that any continued uptilt
of the basin can only have the effect of pouring more water through
the outlet. Lake Michigan and Saginaw Bay, which are arms of
the Huron basin, ought, however, to become flooded upon their
southern shores, were it not that the hinge line of uptilt to-day lies
to the northward of the outlet at Port Huron, and, further, that the
two connecting channels still have their beds lower than the sill of
the outlet channel. Now the evidence goes to show that no en-
—croachment of waters is occurring upon the Chicago shore of Lake
Michigan, and although the shores of Saginaw Bay are so exces-
sively flat as to reveal slight changes of level by large migrations
of the strand, yet the ancient meander posts fixed by the early
surveys are still found near the water’s edge.
Drowning of southwestern shores of Lakes Superior and Erie. —
Within the basins occupied by Lakes Superior and Erie, a wholly
different condition is found. In each case the outlet is found
to the northeastward (Fig. 374, p. 345), and the northwesterly trend
of the isobases from these outlets is responsible for a continued
elevation from uptilt of the outlets with reference to the western
and southern shores. In consequence, the waters are encroach-
ing upon these shores, and rivers which there enter the lake are
drowned at their mouths, with the formation of estuaries. Upon
Lake Superior these changes are very marked near Duluth and par-
ticularly in the St. Louis River, within which, since the early treaty -
with the Indians, certain rapids have disappeared and submerged
trunks of trees are now found in the channel of the river. As
far east as Ontonagon essentially the same conditions are found.
Upon the shores within the Porcupine Mountain district, the
waters are clearly rising. Here old cedar trees may be seen, in
some cases dead but still upright and standing in from six to eight
inches of water a number of feet out from the present shore,
while others near the shore, but upon the land and still living, are
washed by the waves, and losing their lower bark in consequence.
An old road along the shore has had to be abandoned because of
the encroaching water.
350 EARTH FEATURES AND THEIR MEANING
Upon the opposite or northeastern shore of the lake, on the
other hand, the land is everywhere rising out of the water, and
the waves are now building storm beaches well out upon the wave-
cut terrace. Here the streams, instead of forming estuaries by
drowning, drop down
in rapids to the level
of the lake.
At the southwest-
ern margin of Lake
Erie there is every-
where evidence of a
rapid encroachment
by the water. Inthe
caves of South Bass
. Island stalactites,
which must obviously
2. EFPIE
Port Clinton
7s
wee Id.
ce ay Dy
om Saale oo
Fic. 376.— Portion of the Inner Sandusky Bay, to have formed above
afford a comparison of the shore line of 1820 with the lake level, are
that of to-day (after Moseley). now permanently sub-
merged. It is, however, about Sandusky Bay upon the south-
west shore that the most striking observations have been made.
Moseley has collected historical records of the killing of forest
trees through a submergence which was the result of an advance
of the water upon the shores. It seems to be proven from his
studies that the water is now risingin Sandusky Bay at a rate of
about 2.14 feet per century. In Fig. 376 there is a comparison
of the shores of the inner bay separated by an interval of about
ninety years.
READING REFERENCES FOR CHAPTER XXIV
Uptilt in basin of Lake Agassiz : —
WarREN Upoam. The Glacial Lake Agassiz, Mon. 25, U.S. Geol. Surv.,
pp. 474-522.
Uptilt in Laurentian Basin : —
G. K. Grupert. Recent Earth Movement in the Great Lakes Region,
18th Ann. Rept. U. S. Geol. Surv., 1898, Pt. ii, pp. 595-647.
J. W. Spencer. Deformation of the Algonquin Beach, ete., Am. Jour.
Sci. (3), vol. 41, 1891, pp. 14-16. -
F. B. Taytor. The Highest Old Shore Line of Mackinac Island, Ad
Jour. Sci. (3), vol. 43, 1892, pp. 210-218.
q \ UPTILT OF LAND AT CLOSE OF ICE AGE 351
A. C. Lawson. Sketch of the Coastal Topography of the North Side of
_ Lake Superior, with reference to the abandoned strands, etec., 20th
Ann. Rept. Geol. and Nat. Hist. Surv. Minn., 1893, pp. 181-289,
pls. 7-12.
J.B. Woopworts. Ancient Water Levels of the Champlain and Hudson
Valleys, Bull. 84, N.Y. State Mus., 1905, pp. 265, pls. 28.
EK. L. Moseitry. Formation of Sandusky Bay and Cedar Point, Proc.
Ohio State Acad. Sci., vol. 4, 1905, Pt. v, pp. 179-238.
F. E. Wrieut. Rept. Geol. Surv. Mich, for 1903, 1905, p. 37.
J. W. GotptHwait. The Abandoned Shore Lines of Eastern Wisconsin,
Bull. 17, Wis. Geol. and Nat. Hist. Surv., 1907, pp. 134, pls. 37; A
Reconstruction of Water Planes of the Extinct Glacial Lakes in the
Lake Michigan Basin, Jour. Geol., vol. 16, 1908, pp. 459-476; Iso-
bases of the Algonquin and Iroquois Beaches and their Significance,
Bull. Geol. Soe. Am., vol. 21, 1910, pp. 227-248, pl. 5; An Instru-
mental Survey of the Shore Lines of the Extinct Lakes Algonquin and
Nipissing in Southwestern Ontario, Mem. 10, Dept. of Mines, Canada,
1910, pp. 57, pls. 4.
Wituiram H. Hosss. The Late Glacial and Post-glacial Uplift of the
Michigan Basin, Pub. 5, Mich. Geol. and Biol. Surv., 1911, pp. 68,
pls. 2.
Lawrence Martin. [Postglacial Modifications in and Around the Great
Lakes], Mon. 52, U. S. Geol. Surv., 1911, pp. 455-459.
Uptilt in northern Europe : —
G. pe Grrr. Quaternary Changes of Level in Scandinavia, Bull. Geol.
Soe. Am., vol. 3, 1892, pp. 65-68, pl. 2. .
-H. Mounrue. Studies in the Late Quaternary History of Southern
Sweden, paper No. 25, Livret Guide, Cong. Géol. Intern., 1910, pp.
96, many plates and maps.
CHAPTER XXV
NIAGARA FALLS A CLOCK OF RECENT GEOLOGICAL
TIME
Features in and about the Niagara gorge.—A striking ex-
ample of those permanent alterations of drainage which have
resulted from the presence of the late continental glacier in North
America is to be found in the Niagara gorge between Lakes Erie
and Ontario. With the aid of borings many of the now buried
channels of the region have been followed out, and in a later para-
graph we shall refer to some of the stronger lines of the earlier
drainage system. Before undertaking the study of Niagara his-
tory, it is essential that one become somewhat familiar with the
present topography in and about the Niagara gorge.
Below the present cataract the river flows through a deep gorge
for about seven miles’ before issuing at the Lewiston Escarpment
(Fig. 381, p. 355). This gorge has been cut in beds of rock sedi-
ments which dip at a gentle angle southward toward Lake Erie.
The capping of the rock series is a compact and relatively resist-
ant limestone which is known as the Niagara limestone, beneath
which there are alternating beds of shale with thinner limestone
and sandstone. The plain formed by the upper surface of the
limestone capping terminates in the Lewiston Escarpment, which
is transverse to the direction of the gorge and seven miles distant
below the Falls. The depth of the gorge varies markedly, the
above-water portion being represented at the upper end by the
height of the cataract, one hundred and sixty-five feet, while at
its lower end near Lewiston it is twice that amount. Halfway
down the gorge a sharp turn is made at an angle of more than
ninety degrees, and the upstream arm is extended to form a
basin which contains the famous whirlpool. This visible exten-
sion of the upper gorge is continued in a buried channel, the St.
Davids Gorge, which extends to the escarpment, broadening as
it does so in the form of a trumpet. The materials which fill ©
this earlier channel are notably coarse glacial deposits (Fig. 389).
352
A CLOCK OF RECENT GEOLOGICAL TIME 353
Directly above the whirlpool the Niagara gorge is first con-
tracted, but almost immediately swells out into the form of a
sausage, which under the name of the Eddy Basin extends to the
constricted channel occupied by the Whirlpool Rapids. This Gorge
of the Whirlpool Rapids extends to and a little above the railroad
bridges, where it again suddenly widens and deepens and with
surprisingly uniform cross section now continues as far as the cat-
aract. This uppermost section is known as the Upper Great
Gorge. About a mile below the whirl-
pool is that remarkable projection into
the gorge from the Canadian wall which
is known as Wintergreen Flats, below
which and nearer the river are Fosters
Flats. Almost throughout its entire
length the Niagara gorge is bordered
on either side by a narrow and gently é
‘incurving terrace eroded below the gen- Fic. 377.— Ideal cross section
eral level of the plain and meeting the of the Niagara gorge to show
gorge in a sharp angle (Fig. 377). She rae ree:
The features immediately about the cataract show that the Falls
are to-day in a condition which, so far as we know, has occurred
but once before in their entire history — the waters of the river
are divided unequally by an island, and for this reason, as we shall
see, the cataract enters over the side wall of the gorge instead of
at its end (Fig. 381), although the turning of the channel from this
cause is combined with a bend of the river.
The drilling of-the gorge. — There appear to be two important
processes which are responsible
for the recession of the Falls,
the rate of which is determined .
<p largely by the resistance of the
“<x limestone capping and the tena-
city of the looser shale beneath
= it. One of the eroding processes
Fia. 378. — View of the bed of the Niagara operates from below and under-
River above the cataract, where water mines the cap until the unsup-
has been drained off in installing a power ported cornice falls in blocks
plant. Some separated blocks of lime-
stone are still in place (after J. W. to the bottom of the Borge ;
Spencer). the other makes its attack di-
2A
354 EARTH FEATURES AND THEIR MEANING
rectly from above, selecting for the purpose the lines of jointing
of the rock which it widens by solution and corrasion until the
included blocks are in so far separated that they are torn out and
go over the brink of the Falls (Fig. 378). This process of over-
head attack in the powerful currents just abové a cataract is even
P
|
< iy Ny =
Ni iin Zi
saa AY Ky --
Sie
beta oy
Fia. 379. — Falls of St. Anthony, looking westward from Hennepin Island in 1851
(after N. H. Winchell, daguerreotype by Hessler of Chicago).
better illustrated by the Falls of St. Anthony near Minneapolis,
which have had a similar history of recession to that of the Niagara
Falls (Fig. 379). ’
The blocks of the capping limestone at Niagara Falls are to
some extent fixed in size by the joint planes present in them, and
as they fall to the bottom of the gorge, they promote or retard the
further recession of the Falls according as they can or cannot be
moved about by the churning currents beneath the cataract. Of
the retarding effect there is an illustration in the accumulation of
the blocks below the American and the intermediate Luna Falls
(plate 23 A), which the weaker currents upon the American side
find too heavy to handle. The Canadian Fall, with its much greater
power, is an example of the promotion of recession through the
churning about of the blocks at the base of the cataract. We have
here to do with a churn drill which bores its way into the bottom
A CLOCK OF RECENT GEOLOGICAL TIME 35)
of the gorge with increasing radius of rotary motion with each in-
crease in volume of the falling water. Under this rotary churning
the soft shales are torn out near the bottom and in suc-
OD | ;
* un | os Nit { 14
95 wy”
4 > bs < p\) Wy NS f
aoe gi Ke -----1- E
: é By te : dl 4 y M ali NT e i :
¢, bi whine v ay =— >
\ ‘ a ty m © | I
= = % a a
pages LESS * 5 | ‘ fh
: yh
ey:
be "
ay
e
| 4%
Fic.” 380.—Ideal section to show
the nature of the drilling process
beneath the cataract. ;
cession the harder layers
above until the capping is
reached (Fig. 380). The con-
‘ditions appear now to be such
that the effective work is
largely concentrated, as it -
usually has been, near the ey
middle of the channel, and |
so the gorge recedes with a
PE es
Sa : =
ry fas
rarvenare re
EEL DIRE LBD
sect Sie
(ae
EO Pic asoe RPTS
: margin of the earlier river SN Means te he
f bed remaining as a terrace on s, a Hee
4 either side and extending to 2 Ht
the former river bank (Fig. va bi
377). o ‘Seale af Miles. 2
As must have been noted, a a eta ee
+s 1G. .— Plan and section of the Niagara
“oats peculiarity of the id aa il gorge, showing how in each section the
tion of the churn drill beneath — depth is proportional to the width, except
the cataract is that the depth in the lowest section where subsequent river
. : ‘action of the normal type has modified the
of the gorge will bear a direct bed of the channel (plan after Taylor and
proportion to its width, and section after Gilbert).
AO NR en mt ey en gen ote pes oo
306 EARTH FEATURES AND THEIR MEANING
if the volume of water has varied during the process of recession,
these changes in volume will be registered in the width and also
in the depth of that section of the gorge which was drilled at the
time — the cross section of the gorge at any place is proportional
to the volume of the water falling in the cataract which produced
it, modified, however, by the competency to handle the joint blocks
of definite size (Fig. 381).
The present rate of recession. — There are various sketches,
more or less accurate, made in the early part of the’ nineteenth
wm Ae eye
"a m7 In Nini =
Fig. 382. — Comparison of a sketch of the Canadian Fall made with the aid of a
camera lucida in 1827 with a photograph taken from the same view point in 1895
(after Gilbert).
century, and from He later period there are daguerreotypes, photo-
graphs, and maps, which refer especially to the Canadian Fall; and
which, taken together; render possible a comparison of the earlier
with the later brinks. By comparing the earliest with the recent
views it is seen at a glance that the Falls are receding, and at a”
quite appreciable rate (Fig. 382). A careful comparison of the
SA RTT I CTE EIT CI tI I RR a i —— z
of SS a er ty - P *
: — = eit ee a a
4
ta
4
13
4
id
iq
_—
.
A CLOCK OF RECENT GEOLOGICAL TIME 301
maps made in 1842, 1875, 1886, 1890, and 1905 of the brink of
the Canadian Fall (Fig. 383) indicates that for the period covered
the rate of recession has been about five feet per year, and similar
studies made of the
American Fall show that ats 7 ae
it has been receding at Pei
the rate of only three V4
inches per year, or one
twentieth the rate of the
recession of the Canadian
Fall.
Future extinction of the
American Fall.—It is_ if GES
because of this many 2 eg ogee:
times more rapid reces-
sion of the Canadian
Fall that the Niagara
cataract, instead of lying
athwart the gorge, enters
it from its. side. The
Canadian Fall is thus in
reality swinging about ae sem ¢ ae
the American, and the WG, (
time can already be ‘
gs roughly estimated when fy. 383. — Map to show the recession of the brink
this more effective drill- of the Canadian Fall, based upon maps of differ-
ing tool will have brought ent dates (after Gilbert).
about a capture, so to speak, of the American Fall through the
cutting off of its water supply. It will then be drained and left
literally “high and dry,” an enduring witness to the geological
effect of an island in making an unequal division of the waters for
the work of two cataracts.
As already pointed out, the inefficiency of the American Fall
. as an eroding agent is amply attested by the wall of blocks
already appearing above the water below it. The tourist who a
thousand years hence pays a visit to the Niagara cataract, pro-
vided the water flow is allowed to remain as it has been, will find
q above this rampart of blocks a bare cliff in part undermined, and
surmounted by a nearly flat table surface which is cut off from the
358 EARTH FEATURES AND THEIR MEANING
existing cataract by a higher section of the gorge (Fig. 384). It
is quite likely that this table will furnish the most satisfactory
viewpoint of the future cataract of that date.
=>
Vr, Cina Fry
2
PRESENT
Fia. 884. — Comparison of the present with the future falls.
The captured Canadian Fall at Wintergreen Flats. — What we
have predicted for the future of the present American Fall will
be the better understood from the study of a monument to ear-
lier capture made long before the upper section of the gorge had
been cut or the whirlpool had come into existence. The tables
were then turned, for it was a fall upon the Canadian side of the
gorge that was captured by one upon the American. The locality
is known as Wintergreen Flats, or sometimes as Fosters Flats;
though the first name properly applies to a higher surface near the
Fig. 385. — Bird’ green Flats,
showing the section of the river bed above the cliff and the blocks of fallen Niagara
limestone strewn over the abandoned channel below (after Gilbert).
A CLOCK OF RECENT GEOLOGICAL TIME 359
brink of the gorge, and Fosters Flats to a lower plain near the level
of the river (see Fig. 381, p. 355). The peculiar topographic fea-
tures at this locality are well brought out in Gilbert’s bird’s-eye
view of the locality (Fig. 385); in fact, in some respects better
than they appear to the tourist upon the ground, for the reason
that the abandoned channel and the Flats on the site of the since
undermined island are both heavily forested and so not easy to
include in a single view.. For one who has studied the existing
cataract this early monument is full of meaning. Standing, as
one may, upon the very brink of the former cataract, it is easy
to call up in imagination the grandeur of the earlier surroundings
and to hear the thunder of the falling water. A particularly vivid
touch is added when, in digging over the sand about the great
blocks of fallen limestone underneath the brink, one comes upon
the shells of an animal still living in the Niagara River, though only
in the continual spray beneath the cataract.
The Whirlpool Basin excavated from the St. Davids Gorge. —
It has already been pointed out that a rock channel now filled with
glacial deposits extends from the Whirlpool Basin to the edge of
the escarpment at St. Davids (Fig. 389, p. 363). In plan this
buried gorge has a trumpet form, being more than two miles wide
at its mouth and narrowing to the width of the upper gorge before
it has reached the Whirlpool. Near the Whirlpool it has been in
part excavated by Bowman Creek, thus revealing walls that are
well glaciated. Different opinions have been expressed concerning
the origin of this channel, one being that it is the course either of
a preglacial river or one incised between consecutive glacial in-
vasions ; and another that it is a cataract gorge drilled out between
glacial invasions after the manner of the later Niagara gorge. In
either case its contours have been much modified by the later
glacier or glaciers, whose work of planing, polishing, and widening
is revealed in the exposed surfaces; and it is not improbable that
a cataract has receded along the course of an earlier river valley.
As we shall see, there are facts which point rather clearly to an
earlier cataract which ended its life immediately above the present
Whirlpool. When the later Niagara cataract had receded to near
_ the upper end of the Cove section, or near the present Whirlpool,
the falling water must have been separated from this older channel
and its filling of till deposits by only a thin wall of rock, and this
360 EARTH FEATURES AND THEIR MEANING
must have been constantly weakened as its thickness was further
reduced.
When this weakened dam at last gave way, it must have pro-
duced a debacle grand in the extreme. It is hardly to be conceived
that the ‘‘ washout ” of the ancient channel to form the Whirl-
pool Basin could have occupied more than a small fraction of a
day, though it is highly probable that the broken rock partition
below the Whirlpool was not immediately removed entire. The
manible-like termination of the Eddy Basin immediately above
the Whirlpool has led Taylor to believe that the cataract quickly
reéstablished itself at this point upon the last site of the extinct
St. Davids cataract. If reduced in power for a short interval, as a
result of the obstructions still remaining in the lately broken dam
below the Whirlpool, the remarkable narrowing of the gorge at
this point would be sufficiently accounted for. |
Being compelled to turn through more than a right angle after
it enters the Whirlpool Basin, the swift current of the Niagara
River is forced to double upon itself against the opposite bank
and dive below the incoming current before emerging into the
Cove section below the Whirlpool (Fig. 386).
In tearing out the loose.deposits which had filled this part of
the buried St. Davids Gorge,
many bowlders of great size
were left which slid down the
slope and in time produced an
armor about the looser deposits
beneath, so as to protect them
and prevent continued excava-
tion. Thus it is found that the
submerged northwestern wall
of the basin is sheathed with —
bowlders large enough to retain
showing the rock side walls like those of their positions and so stop a
the Niagara Gorge, and the drift bank natural process of placer out-
which forms the northwest wall (after washing upon a gigantic scale
Gilbert). (Fig. 386).
The shaping of the Lewiston Escarpment. — To understand
the formation of the Lewiston Escarpment cut in the hard Niagara
limestone, it is necessary to consider the geology of a much larger
A CLOCK OF RECENT GEOLOGICAL TIME 361
area — that of the Great Lakes region as a whole. To the north
of the Lakes in Canada is found a most ancient continent which
was in existence when all the area to the southward lay below the
waters of the ocean. In a period still very many times as long
ago as the events we have under discussion, there were laid down
off the shore of this oldland a series of unconsolidated deposits
which, hardened in the course of time, and elevated, are now repre-
sented by the shales, sandstone, and limestone which we find, one
above the other, in the Niagara gorge in the order in which they
were laid down upon the ocean floor. The formations represented
Fic. 387. — Map* to show the cuestas which have played so important a part in
fixing the boundaries of the Lake basins, and also the principal preglacial rivers
by which they have been trenched (based upon a map by Grabau).
in the gorge are but a part of the entire series, for other higher mem-
bers are represented by rocks about Lake Erie and even farther
tothe southward. ‘These strata, having been formed upon an out-
ward sloping sea floor, had a small initial dip to the southward,
and this has been probably increased by subsequent uptilt, including
the latest which we have so recently had under discussion. At
the present time the beds dip southward by an angle of less than
four degrees, or about thirty-five feet in each mile.
362 EARTH FEATURES AND THEIR MEANING
When the elevation of the land in the vicinity of this shore had
caused a recession of the waters, there was formed a coastal plain
on the borders of the oldland like that which is now found upon
our Atlantic border between the Appalachians and the sea (Fig.
272, p. 246). The rivers from the oldland cut their way in narrow
trenches across the newland, and because of the harder limestone
formations, their tributaries gradually became diverted from their
earlier courses until they entered the trunk stream nearly at right
angles and produced the type of drainage
network which is called ‘trellis drainage.”
===] It is characteristic of this drainage that
exert few tributaries of the second order will
= flow up the natural slope of the beds, but
| on the contrary these natural slopes are
-==4 followed in the softer rock nearly at right
cS =e angles again to the tributaries of the first
order of magnitude (Fig. 387). Thus are |
produced a series of more or less parallel
escarpments formed in the harder rock and
having at their base a lowland which rises
gradually in the direction of the oldland
until a new escarpment is reached in the
next lower of the hard formations. Such
Fia. 388.—Bird’s-eye view ‘ ‘ y s
of the cuestas south of flat-topped uplands in series with inter-
Lakes Ontario and Erie mediate lowlands and separated by sharp
(after Gilbert). escarpments are known as cuestas (see p.
246), and the Lewiston Escarpment limits that formed in Niagara
limestone (Figs. 387 and 388).
Episodes of Niagara’s history and their correlation with those
of the Glacial Lakes. — Of the early episodes of Niagara’s history,
our knowledge is not as perfect as we could desire, but the later
events are fully and trustworthily recorded. The birth of the
Falls is to be dated at the time when the ice front had here first
retired into what is now Canadian territory, thus for the first time
allowing the waters from the Erie basin to discharge over the Lewis-
ton Escarpment into the basin of the newly formed Lake Iroquois
(Fig. 364, p. 334). Since the level of Lake Iroquois was far above .
that of the present Lake Ontario, the new-born cataract was not
the equivalent in height of the escarpment to-day. The Iroquois
A CLOCK OF RECENT GEOLOGICAL TIME 363
waters then bathed all the lower portion of the escarpment, so
that the foot of the Fall was upon the borders of the Lake.
In order to interpret the history of the Niagara gorge, we must
remember that the effective drilling of this gorge was in each stage
dependent mainly upon
the volume of water dis-
charged from Lake Erie,
a large discharge being
recorded by a channel
drilled both wide and
deep, while that pro-
duced by the discharge
of a smaller volume was
correspondingly narrow
and shallow. To-day
the gorges of large cross
section have, moreover,
a relatively placid sur-
face, whereas through the
constricted sections the
water of the river is un-
able to pass without first
raising its level at the
upper end and under the ;
ne dth e d h Fic. 389.— Sketch map of the greater portion of
ead thus proauced rush- the Niagara Gorge to show the changes in cross
ing through under an in- _ section in their relations to Niagara history
creased velocity. The based upon a map by Taylor).
best illustration of such a constricted section is the Gorge of the
Whirlpool Rapids.
Our reading of the history should begin at the site of the present
cataract, since the records of later events are so much the more
complete and legible, and it should ever be our plan to proceed
from the clearly written pages to those half effaced and illegible.
_ As we have learned, the most abrupt change in the cross section
of the gorge is found a little above the railroad bridges, where the
Upper Great Gorge is joined to the Gorge of the Whirlpool
Rapids (Fig. 389). In view of the remarkably uniform cross
section of the Upper Great Gorge, there is no reason to doubt that
it has been drilled throughout under essentially the same volume
losing of Trenr
Outlet ond rhe
opening of ovtler
into Erte basir,,
364 EARTH FEATURES AND THEIR MEANING
of water, and that its lower limit marks the position of the former
cataract when the waters from the upper lakes were transferred
from the ‘‘ North Bay Outlet ” into the present or ‘‘ Port Huron
Outlet ” and Lake Erie. As the upper limit of the Gorge of the
Whirlpool Rapids thus corresponds to the closing of the ‘“ North
Bay Outlet’ and the extinction of the Nipissing Great Lakes,
so its lower limit doubtless corresponds to the opening of that outlet
and the termination of the preceding Algonquin stage; for in the
stage of the Nipissing lakes the water of the upper lakes, as we
have learned, reached the ocean through the northern outlet.
Mr. Frank Taylor, who has given much study to the problem
of Niagaran history, believes that the Middle Great Gorge, com-
prising the Eddy Basin and the Cove section, represents the gorge
drilling which occurred during the later stage of Lake Algonquin
after the “‘ Trent Outlet ’’ had been closed and the waters of the
upper lakes had been turned into the Erie Basin.
Summarizing, then, the episodes of the lake and the gorge TUE
are to be correlated as follows: —
GLaAcIAL LAKE NIAGARA GoRGE |
Drilling of the gorge from the
Lewiston Escarpment to the Cove
section above the Wintergreen Flats.
Drilling of Middle Great Gorge.
Early Lakes Iroquois and Algon-
quin.
Later Lakes Iroquois and Algon-
quin with upper lakes discharging
into Erie basin.
Nipissing Great Lakes with the
upper lake waters diverted from
Lake Erie.
Recent St. Lawrence drainage
since the waters of the upper lakes
Drilling of the narrow Gorge of
the Whirlpool Rapids.
Drilling of Upper Great Gorge to
the present cataract.
were discharged into Lake Erie
through occupation of the Port
Huron Outlet.
Time measures of the Niagara clock. — In primitive civiliza-
tions time has sometimes been measured by the lapse necessary
to accomplish a certain task, such, for example, as walking the
distance between two points; and the natural clock of Niagara ~
has been of this type. But men possess differences in strength
and speed, and the same man is at some times more vigorous than
A CLOCK OF RECENT GEOLOGICAL TIME 365
at others, and so does not work at a uniform rate. The cataract
of Niagara, charged with the pent-up energy of the waters of all
the Great Lakes, can rush its work as it is clearly unable to do at
times when the greater part of this energy has been diverted.
Units of distance measured along the gorge are therefore too un-
reliable for our use, with the unique exception of the stretch from
the railroad bridges to the site of the present cataract, within
which stretch the gorge cross sections are so nearly uniform as to
indicate an approximation to continued application of uniform
energy. This energy we may actually measure in the existing
cataract, and so fix upon a unit of time that can be translated into
years.
In order to secure the normal rate of recession of this Upper
- Great Gorge, we should add to the volume of water in the Canadian
Fall that now passing over the American; and for the reason that
the blocks which fall from the cataract cornice and are the tools
of the drilling instrument approximate to a definite size fixed by
their joint planes, the effect of this added energy it is not easy
to estimate. We may be sure, however, that the drilling action
would be somewhat increased by the junction of the two Falls,
and thus are assured that the average rate of recession within the
Upper Great Gorge has been somewhat in excess of the five feet
per year determined by Gilbert for the present Canadian Fall.
The Upper Great Gorge is about two miles in length, and its begin-
ning may thus be dated near the dawning of the Christian Era.
The Whirlpool Gorge was cut when the ice vacated the North Bay
Outlet in Canada, and still lay as a broad mantle over all north-
eastern Canada. For the earlier gorge and lake stages, the time
estimates are hardly more than guesses, and we need not now con-
cern ourselves with them. .
The horologe of late glacial time in Scandinavia. — A glacial
timepiece of somewhat different construction and of greater refine-
ment has been made use of in Scandinavia to derive the “ geo-
chronology of the last 12,000 years.”’ Instead of retreating over
the land and impounding the drainage as it did so, the latest con-
tinental glacier of Scandinavia ended below sea level, and as it
retired, its great subglacial river laid down a giant esker known as
the Stockholm Os, which was bordered by a delta and fringed on
either side by water-laid moraines of the block type. These re-
366 EARTH FEATURES AND THEIR MEANING .
cessional moraines are upon the average less than 1000 feet apart,
and are believed to have each been formed in a single season. The
delta deposits which surround the esker are of thin-banded clay,
and as an additional uppermost band is found outside every mo-
raine, these bands are also believed to represent each the delta
deposit of a single year. In studies extending over many years,
Baron de Geer, with the aid of a large body of student helpers,
has succeeded in completing a count of moraines and clay layers,
and so in determining the time to be 12,000 years since the ice
front of the latest continental glacier lay across southern Sweden.
The fertility of conception and the thoroughness of execution of
this epoch-making investigation recommend its conclusion to the
scientific reader.
READING REFERENCES FOR CHAPTER XXV
G. K. Gitpert. Niagara Falls and their History, Nat. Geogr. Soc.
Mon., vol. 1, No. 7, 1895, pp. 203-236.
F. B. Taytor. Origin of the Gorge of the Whirlpool Rapids at Niagara,
Bull. Geol. Soc. Am., vol. 9, 1898, pp. 59-84.
A. W. Grasavu. Guide to the Geology and Paleontology of Niagara
Falls and Vicinity, Bull. N. Y. State Mus., vol. 9, No. 45, 1901, pp.
1-85, pls. 1-11.
J. W. Spencer. The Falls of Niagara, ete. Dept. of Mines, Geol. Surv.
Branch, Canada, 1907, pp. 490, pls. 43. |
G. K. Gitpert. Rate of Recession of Niagara Fans, ete. Bull. 306, U.S.
Geol. Surv., 1907, pp. 31, pls. 11.
G. DE GEER. uniariary Sea Bottoms of Western Sweden. Paper 23, |
Livret Guide Cong. Géol. Intern., 1910, pp. 57, pls. 3.
CHAPTER XXVI
LAND SCULPTURE BY MOUNTAIN GLACIERS
Contrasted sculpturing of continental and mountain glaciers. —
In discussing in a previous chapter the rock pavement lately un-
covered by the Greenland glacier, we learned that this surface had
been lowered by the processes of plucking and abrasion, the com-
bined effect of which is always to reduce the irregularities of the
surface, soften its outlines, and from sharply projecting masses to
develop rounded shoulders of rock — roches moutonnées.
Though the same processes act in much the same manner beneath
mountain glaciers, though here upon all parts of the bed, they are,
in the earlier stages at least, subordinated to a third process more
important than the two acting together. Sculpture by mountain
glaciers, instead of reducing surface irregularities and softening
outlines, increases the accent of the relief and produces the most
sharply rugged topography that is known. In nearly all places
where Alpinists resort for difficult rock climbing, mountain gla-
ciers are to be seen, or the evidence for their former presence may
be read in unmistakable characters.
Wind distribution of the snow which falls in mountains. —
Until quite recently students of glaciation have concerned them-
selves but little with the work of the wind in lifting and redis-
tributing the snow after it has fallen. We have already seen that,
for the continental glaciers, wind appears to be the chief trans-
- porting agent, if we except the marginal lobes where glacier flow
assumes large importance. In the case of mountain glaciers, also,
we are to find that for the earlier stages particularly wind is of the
first importance as a redistributing agent. In the higher levels
snow is swept up from the ground by all high winds, and does not
find a resting place until it is dropped beneath an eddy in some
irregularity of the surface; and if the inherited surface be rela-
367
368 EARTH FEATURES AND THEIR MEANING
tively smooth, this will be found in most cases upon the lee of the
mountain crest.
In normal cases at least the inherited irregularities of the higher
zones of mountain upland are the gentle depressions which develop
at the heads of streams. These become, then, the sites of snow-.
drifts that are augmented in size from year to year, though at
first they melt away in the late summer.
The niches which form on snowdrift sites. — Wherever a drift
is formed, a process is set in operation, the effect of which is to
hollow out and lower the ground beneath it, a process which has
been called nivation. The drift shown in Fig. 390 was photo-
graphed in late summer at an elevation of some 9000 feet in the
Yellowstone National Park. The very gently sloping surface
Fig. 390. — Snowdrift hollowing its bed by nivation and building a delta (at the
left). Quadrant Mountain, Yellowstone National Park.
surrounding the drift is covered with grass, but within a zone a
few feet in width on the borders of the drift no grass is growing,
and in its place is found a fine brown soil which is fast becoming
the prey of the moving water derived by melting of the drift.
This is explained by the water permeating the crevices of the rock
and being rent by the nightly freezing. Farther from the drift
the ground is dry, and no such action is possible. With each suc-
ceeding spring the augmented drift as it melts carries all finely
comminuted rock material down slopes beneath the snow to emerge
at the lowest margin and be there deposited in the form of a delta. ~
By the operation of this process of nivation the higher parts of the
LAND SCULPTURE BY MOUNTAIN GLACIERS 369
drift site are lowered as deposition goes on upon the lower. The
combined effect is thus to produce a niche or faintly etched amphi-
theater upon the slope of the mountain (Fig. 391).
ot oh er a ee er ern nal ee wt ~~. a
———
Fic. 391.— Amphitheater formed on a drift site in northern Lapland (after a
photograph by G. von Zahn).
The augmented snowdrift moves down the valley — birth of
the glacier. — In still lower air temperatures the drifts enlarge with
each succeeding year until they endure throughout the summer
season. From this stage on, an increment of snow is left from each
succeeding season. No longer entirely wasted by melting, the
time soon comes when the upper snow layers will by their weight
compress the lower into ice, and the mass will begin to creep down
the slope along the course of the inherited valley. The enlarged
snowdrift which feeds this ice stream is called the névé or firn.
Against the sloping cliff which had been shaped by nivation
at the upper margin of the snowdrift, that snow which is not of
sufficient depth to begin a movement towards the valley separates
from the moving portion, opening as it does so a cleft or crevasse
2B
370 EARTH FEATURES AND THEIR MEANING
parallel to the wall. This crack in the snow is called by its Ger-|
man name Bergschrund or Randspalte, and may perhaps be re-
ferred to as the marginal crevasse
(Fig. 392). |
The excavation of the glacial
amphitheater or cirque.—It has
been found that the marginal cre-
vasse plays a most important réle
in the sculpture of mountains by
glaciers, for the great amphitheater
which is everywhere the collecting
basin for the nourishment of moun-
tain glaciers is not an inherited
feature, but the handiwork of the
ice itself. This was the discovery
of Mr. W.D. Johnson, an American -
topographer and geologist, who, in
order to solve the problem of the
Se SY ' amphitheater allowed himself to be
Fia. 392.— The marginal crevasse or lowered into such a crevasse upon
Bergschrund on the highest margin
of a glacier (after Gilbert). the Mount Lyell glacier of the
Sierra Nevadas in California. 7
Let down a distance of a hundred and fifty feet, he reached the
bottom of the crack, and in a drizzling rain of thaw water stood
upon a floor composed of rock masses in part dislodged from a wall
which extended some twenty feet upwards upon the cliff side of the
crevasse. It was evident that the warm air of the day produced
the thaw water which was constantly dripping and which filled
every crack and cranny of the rock surface. With the sinking of
the sun below the peaks the sudden chill, so characteristic of the
end of the day in high mountains, causes this water to freeze and
thus rend the rock along its planes of jointing. Broad and thin
plates of ice, loosened by melting at the walls, could be extracted
from the crevices of the rock as mute witnesses to the powerful
stresses developed by this most vigorous of weathering processes.
In short, the rock wall above the glacier, which in its initial
stage was the upper wall of the niche hollowed beneath the snow-
drift, is first steepened and later continually both recessed and
deepened by an intensive frost rending which is in operation at
Crag |
He
|
f
{
LAND SCULPTURE BY MOUNTAIN GLACIERS 371
the base of the marginal crevasse. The same process does not go
‘on as rapidly above the surface of the névé for the reason that the
necessary wetting of the rock surface does not there so generally result
from the daily summer thaw.
At the bottom of the marginal SAX Bp Ey» re
crevasse alone is this condition Px 5 CaS =
fully realized. Intensive frost i
action where the rock is wet with Y VW i\N SS pe
thaw water daily is thus a J Nea So
fundamental cause, both of the W—N is (7 Vy me.
hollowing of iftsite Y Ne) | v7) y
g of the early drift site peg? NES
to form the niche, and of the YN BRS Ay) i CS
later enlargement of this niche
into an amphitheater or cirque "
when the drift has been trans- *%9- 393-— Niches and cirques in the same
: vicinity in the Bighorn Mountains of
formed into the névé of a Wyoming. A, A, unmodified valleys;
glacier. Inasmuch as the cre- B, B, niches on drift sites ; C, C, cirques
vasse formswhere thesnowand >" rea =U. ee) a segs
ice pull away from the rock
toward the middle of the depression, the cirque wall in its early
stage has the outline of a semicircle. In the Bighorn Mountains
of Wyoming, all stages, from the unmodified valley heads to the
. full-formed cirque, may be seen near
one another (Fig. 393). It will be
noted that wherever a glacier has
formed, as indicated by the cirque,
there is a series of lakes which have
developed in the valley below (see
p. 412). |
Life history of the cirque. —JIn its
earliest stage the cirque is more or
less uniformly supplied with snow
from all sides, and so it enlarges by
recession in a manner to retain its
early semicircular outline. In a later
Fic. 394. — Subordinate small cir- stage a larger proportion of the snow
ques in the amphitheater on the peaches the cirque at its sides so that
west face of the Wannehorn . A
above the Great Aletsch Glacier its further enlargement causes it to
of Switzerland. broaden and to flatten somewhat that
o Scale. IMiles.
372 EARTH FEATURES AND THEIR MEANING
Fia. 395. —‘‘ Biscuit cutting” effect of glacial sculpture in the Uinta Mountains of
Wyoming (after Atwood).
part of its outline which represents the head of the valley (Fig.
398, p. 364). As the territory of the upland is still further invested
Fig. 396.—Two intersecting inverted
cones representing glacial cirques of dif-
ferent sizes, to show that their intersec-
tion is the are of a hyperbola, the curve
to which the col approximates.
by the cirques, their nourish-
ment becomes still more irreg- —
ular, and the circular outline
gives place to a- scalloped
border, as the amphitheater
becomes differentiated into
subordinate smaller cirques,
each of which corresponds to a
scallop of the outline (Fig. 398
and Fig. 394).
Grooved and fretted - up-
lands. — The partial -invest-
ment by cirques of a mountain
upland yields a type of topog- q
raphy quite unlike that pro-
duced ‘by any other geological —
process. The irregularly con-
nected remnants of the inher- —
ited upland resemble nothing —
so much as a layer of dough ©
from which biscuits have been
cut (Fig. 395). Thesurface as —
a whole, furrowed as it is below —
a
PLATE 18.
4 r r — ot) Pe ee tee i...
Y A
AES &
\ . : ¢
x ; :
ae od
Model of the Malaspina Glacier and the fretted upland above it (after model by
L. Martin).
LAND SCULPTURE BY MOUNTAIN GLACIERS 373
the cirques, may be described as a grooved upland (plate 19 A).
A further continuation of the process removes all traces of the
earlier upland, for the cirques intersect from opposite sides and
thus yield palisades of sharp rock pinnacles which rise on pre-
cipitous walls from a terraced floor. This ultimate product of
cirque sculpture by glaciers is called a fretted upland (plate 18
A and 19 B).
_ The features carved above the glacier. — The ranges of pin-
.nacles carved out by mountain glaciers have become known by
J various names of foreign derivation, such as aréte, grat, aiguille
? -__
Fig. 397.— A col shaped like a hyperbola between Mount Sir Donald and Yogo
Peak in the Selkirks (after a plate by the Keystone Plate Co.).
mountains, “ files of gendarmes,” etc. They may, perhaps, be
best referred to as comb ridges, and according to their position they
are differentiated into main and lateral comb ridges, as will be
clear from the second map of plate 19.
With the gradual invasion of thé upland upon which the cirques
have made their attack, the area from which winds may gather
eS ee ee ee eee
374 EARTH FEATURES AND THEIR MEANING
up the snow is steadily diminished, and hence cirque recession is
correspondingly retarded. Cirques which have approached each
other from opposite sides of the ridge until they have become tan- —
gent at one point may, however, still receive nourishment at the .
sides and so continue to cut down the intervening rock wall to —
form a pass or col. The theoretical curve which results from
this intersection is that
known as the hyperbola,
of which an _ illustration
oe < bop fe is afforded by Fig. 396.
by most of the mountain
passes in glaciated moun-
tain districts, and a par-
ticularly good illustration
is furnished from the
vicinity of Glacier on the
line of the Canadian Pa-
Fic. 398.— Diagrams to illustrate the progres- cific Railway (Fig. 397).
sive investment of an upland by cirques with Upon either side of the
the formation of comb ridges, cols, and horns. eo] the land mass is left
I, early stage, youth; II, intermediate stage ;
III, late stage, maturity.
An approximation to this —
| form is clearly furnished
at U/ a
a more or ‘ise triangular
base (Fig. 398, III) into a sharp horn or tooth. An illustration
of such a horn. is furnished by the Matterhorn in the Swiss Alps,
or by Mount Sir Donald in the Selkirks, though less noteworthy —
examples may be found in every maturely glaciated mountain |
district.
The features shaped beneath the glacier. — Those features ,
which are carved above the glacier — the comb ridge, the col,
and the horn — are all shaped as a result of intensive weathering —
upon the cirque wall. The shaping at lower levels is accomplished |
by processes in operation below the glacier surface, where weather- —
ing is excluded and where plucking and abrasion work together —
to tear away and grind off the rock surface. By their joint action —
the valley is both deepened and widened, directly to the height of | :
the glacier surface, and indirectly through undermining as far up —
as rock extends. Thus the valley is transformed into one of broad —
ae ee
i) ee ee
cee Ee Aone ty =
in high relief, rising from
eu ereeen) Bee
fist 1
Sasa ore
Ph erie wr ie
LAND SCULPTURE BY MOUNTAIN GLACIERS 375
and flat bed and precipitous side walls — the U-shaped section
illustrated by valleys of the Swiss Alps and in fact in all districts
which have been strongly glaciated by mountain glaciers (Fig.
399).
As high up in the valley as it has been occupied by the glacier,
the bed is rounded, smoothed, and polished, and marked by the
characteristic glacial scorings or
strie which point down the val-
ley. Above the level of the gla-
cier’s upper surface, on the other
hand, erosion is accomplished
through undermining or sapping,
a process which always leaves
precipitous slopes of ragged sur-
face made up of the joint planes Fic. 399. — The ier Kern valley
on which the fallen blocks have Z ee ea oy enone
separated from the cliff. Thus
there is found a sharp line which separates the smoothly rounded
< ‘ unlatigny
TN Ls
olla aj i , ee
{ fy
wi rt ef llr, “a y em wr mu ‘th ull
Ms cel Ti ae
Von vt iyi ww" fe
yp tn
oN
aoe SG ae : >
vA oe ; ie oh Caneel « =
aa ll ee zail «that. Mle il ft h
iii fit - a i ae ten Mt
et a
[ me Y
ah aalit |
‘
iis Lt ae
SS x * .
* MQ) 2) X
. \ oS ne " ‘4
Fic. 400. — Glaciated valley wall in the Sierra Nevadas of California, showing the
sharp line which separates the abraded from the undermined rock surface (after
a photograph by Fairbanks).
376 EARTH FEATURES AND THEIR MEANING
rock surface below from the jagged and precipitous one above
(Fig. 400). Inasmuch as this boundary usually separates the
scalable from the inaccessible slopes above, snow is apt to lodge
at this level and make it strikingly apparent.
If uplift of the land occurs while glaciers occupy the valleys of ©
mountains, an increased capacity for deepening the valley is im-
parted to these ice
streams, and We find,
as agrgstlt, a dees
central valley of U
cross section exca-
vated within a rela-
tively broad trough
visible above the
shoulder oneither side
of the later furrow.
Save only for its
characteristic curves,
such a valley bears
close resemblance to
| amature stream val-
ley which has been rejuvenated (see p. 173). The remnants of the
earlier glacier-carved valley are, as’already stated, gently curving
high terraces so common in Switzerland, where they are known as
albs or high mountain meadows. These albs may be seen to special
advantage on the sides of the Chamonix valley (Fig. 401), the
Lauterbrunnen valley, or in fact almost any of. the larger Alpine
valleys.
The cascade stairway in glacier-carved valleys. — If now, instondl
of giving our attention to the cross section, we follow the course
of the valley that has been occupied by a glacier, we find that it
descends by a series of steps or terraces having many backwardly ‘a
directed treads (plate 19), whereas a normal and well-established. 4
river valley has only forward grades. Because of these back-
ward grades the stream waters are impounded, and so lakes —
are found strung along the valley in chains as the larger beads
are found in a rosary, and these are the characteristic rock basin
lakes sometimes referred to as ‘‘ Paternoster Lakes” (see’p. 412
and Fig. 402).
tr
Fig. 401.— View of the Vale of Chamonix from the
séracs of the Glacier des Bossons. The alb of the
opposite side is well brought out.
PLATE 20.
in the Sierra Nevadas of California
(after I. C. Russell).
Map of the surface modeled by mountain glaciers
(GRANT LAKE
LAND SCULPTURE BY MOUNTAIN GLACIERS 377
When the backward grades upon the valley floor are especially
steep, the rock step becomes a rock bar, or Riegel, of which nearly
every Alpine valley has its examples. In a walk from the Grimsel
to Meiringen many such bars are passed. Carrying in suspension
the sharp rock sand from the glacier deposits along its bed, the
Way
Fic. 402.— Map of an area near the continental divide in Colorado, showing an
unglaciated surface to the west of the divide, where the westerly winds have cleared
the ground of snow, and the glacier-carved country to the eastward. Note the
regular forms of the youthful cirque, the glacier stairway, and the rock basin lakes
(U.-8..G. 8.):
‘stream which succeeds to the glacier as it vacates its valley saws
its way through these obstructions with a rapidity that is amazing,
thus producing narrow defiles, of which the Gorge of the Aar near
Meiringen and that of the Gorner near Zermatt are such well-
known examples (Fig. 403).
It is characteristic of rivers that the tributaries cut their val-
leys more rapidly than does the main stream within the neighbor-
‘ing section, though they cannot cut lower than their outlets —
the side streams enter accordantly. This is easily explained be-
cause the grades of the tributary streams are the steeper, and, as
‘we well know, the corrasion of a valley is augmented at a most
378 EARTH FEATURES AND THEIR MEANING
amazing rate for each increase of its grade. No such law controls
the processes of plucking and abrasion by which the glacier lowers
its floor, for these processes
aa appear to depend for their
% efficiency upon the depth of
the ice, and the supply of
cutting tools, quite as much
as upon the grade of the
bed. To apply a homely
illustration, the hollowing
of flagstones upon our walks
is dependent more upon the
number of persons that pass
over them, and upon their
size and the number of pro-
truding nails in their boot
heels, than upon the grades
upon which they are placed.
At all events we find that
the main glacier valleys are
cut deeper than the side
Lg 2% valleys, so that the latter
Fic. 403. —Gorge of the Albula Rivernear become hanging valleys —
Berkum in the Engadine, cut through a rock they enter the main valley,
eohd as river which has succeeded to the not upon ‘ta be d, but some
glacier.
distance above it (Fig. 404).
The U-shaped hanging valleys, like the main valley, are much
too large for the
streams which now fill
them, and these di-
minutive side streams
plunge over the steep
wall of the main valley
in ribbon-like falls so
thin that the wind
turns them aside and
disperses the water in
ce +
the spray of a “ bridal and non-glaciated side valleys tributary to a glaci-
veil.”’ Such falls are ated main valley (after Davis).
= $2 e. ; a » ye .
P]
Pi
t |
:
LAND SCULPTURE BY MOUNTAIN GLACIERS 379
found by the hundred in every glaciated mountain district, impart-
ing to it one of the greatest of its scenic charms.
Co/ VV \\.
Comb
Horn Ridge
Alb
Overdeape:
U-velley
U-valley
Fig. 405. — Character profiles in landscapes sculptured by mountain glaciers.
The character profiles which result from sculpture by mountain
glaciers. — The lines which are repeated in landscapes carved by
mountain glaciers are easy to recognize (Fig. 405). The highest
horizon lines are the outlines of horns which are separated by cols.
Z
Vie
ose = fn : " Lay, me)
Pb Wicked
Fic. 406. — Flat dome shaped under the margin of a Norwegian ice cap with pro-
jecting rock knobs and moraines in foreground.
Minaret-like palisades, or “ files of gendarmes,” often run for long
distances as the characteristic comb ridges. Lower down and
380 EARTH FEATURES AND THEIR MEANING
lacking the lighter background of the sky, we make out with less
distinctness the U-valley, either with or without the albs to show
that the sculpturing process has been interrupted by uplift.
The sculpture accomplished by ice caps. — In the case of ice
caps, the only rock exposed is found in the neighborhood of the
_ me
i
» ———s SSS
Fic. 407. — Two views illustrating successive stages in the shaping of tinds
or ‘‘bee-hive’’ mountains.
margin — the projecting islands known as nunataks. It is es-
sential for the existence of the ice cap that the rock base should
LAND SCULPTURE BY MOUNTAIN GLACIERS 381
have relatively slight irregularities compared to the dimensions of
the cap itself. Except in very high latitudes this base must be
somewhat elevated, for like mountain glaciers ice caps are nour-
ished by the surface air currents, and their snows are deposited
above the snow line.
The Norwegian tind or beehive mountain. — Within temperate
or tropical climes the snow line lies so high that only the loftier
mountains are able to support glaciers. It follows that those
which are formed flow upon relatively high grades with corre-
spondingly high rate of movement and increased cutting power.
Within high latitudes the snow is found nearer the sea level, and
glaciers are for the most part correspondingly sluggish in their
movements as well as less active denuding agents.
To this condition characteristic of high latitude glaciers, there
is added in Norway another in the peculiar shape of the basement
beneath the recent and the still existing glaciers. The plateau of
Norway is intersected by a network of deep and steep walled fjords,
and the glaciers have developed as small ice caps perched upon
veritable pedestals of rock, over the margins of which their out-
let tongues of ice descend on steep slopes into the fjord. The tops
of the pedestals thus come to be shaped by the plucking and abrad-
ing processes into flat domes (Fig. 406), while the knobs of rock,
which as nunataks reach above the surface of the ice, divide the
outflowing ice tongues at the margin of the pedestal. These
tongues being much more active denuding agents, because of their
steep gradients, continually lower their beds, thus transforming
the earlier knobs of rock into high and steep mountains of more or
less circular base. Such ‘‘ beehive’ mountains upon the margins
of the fjords are the characteristic Norwegian tinds (Fig. 407).
READING REFERENCES FOR CHAPTER XXVI
I. C. Russety. Quaternary History of Mono Valley, California, 8th
Ann. Rept. U. S. Geol. Surv., 1889, pp. 329-371, pls. 27-37.
F. E. Marrues. Glacial Sculpture of the Bighorn Mountains, Wy-
oming, 21st Ann. Rept. U. 8. Geol. Surv., 1900, Pt. ii, pp. 179-185,
pl. 23.
W. D. Jonnson. Maturity in Alpine Glacial Erosion, Jour. Geol., vol. 12,
1904, pp. 569-578.
G. K. Gusert. Systematic Asymmetry of Crest Lines in the High
Sierras of California, zbid., pp. 579-588.
382 EARTH FEATURES AND THEIR MEANING
Emm. DE Martronne. Sur la Formation des Cirques, Ann. de Géogr.,
vol. 10, 1901, pp. 10-16.
W.M. Davis. Glacial Erosion in North Wales, Quart. Jour. Geol. Soe.
Lond., vol. 65, 1909, pp. 281-350, pl. 14.
Ep. Brickner. Die Glazialen Ziige im Antlitz der Alpen, Naturw.
Wochenschr., N. F., vol. 8, 1909.
Wituram H. Hosss. Characteristics of Existing Glaciers, pp. 1-96.
CHAPTER XXVII
SUCCESSIVE GLACIER TYPES OF A WANING
GLACIATION
Transition from the ice cap to the mountain glacier. — A study
of existing glaciers leads inevitably to the conclusion that although
subject to short period advances and retreats, yet, broadly speak-
, ~ >
O i aS
N.7 - Bie,
0 T aI L
Q s*
« oN¢s Ja) /, %
Po. ees “ STAGE It 00 ip
AP OLN FF een he me a Qe
‘ ‘2 ; : a
ene a 5 }
. / : x
S § )
’ : Z
\
ed
<
. o wy
Xf . a
fo RS i : iia
; Nan ae /
“reeset” ADADIATING Wee seud "
DENORITIC GLACIERS HIORSE SHOE
GLACIER GLACIERS
Fic. 408.— Schematic diagram to show the relationships of glacier types formed
in succession during a receding hemicycle of glaciation.
ing, glaciers are now gradually wasting away, surrounded by wide
areas upon which are the evidences of their recent occupation.
We are thus living in a receding hemicyele of glaciation.
383
384 EARTH FEATURES AND THEIR MEANING
Many mountain districts which now support small glaciers only,
or none at all, were once nearly or quite submerged beneath snow
and ice. If once covered by an ice carapace or cap, our present
interest in them begins at that stage of the receding hemicycle
when the rock surface has made its teappearance above the surface
of the snow-ice mass. At this stage intensive frostwork, the charac-
teristic high level weathering, begins, and cirques develop above
the scars of those earlier amphitheaters formed in the advancing
hemicycle.
The piedmont glacier. — In this early stage of transition from
the ice cap to the mountain glacier, the ice flows outward to the
mountain front in ill-defined streams divided by the projecting
ridges, and upon reaching the mountain front these streams deploy
upon it so as to coalesce in a great stagnant ice apron whose upper
surface slopes gently forward at an angle of a few degrees at the
most (Fig. 408, stage I). This is the piedmont glacier, a type
Fig. 409.— Map of the Malaspina glacier of Alaska, the best known of one
piedmont glaciers (after Russell).
found to-day in the high latitudes of Alaska and in the southern
Andes (Fig. 409 and pl. 18 B).
During this stage the cirques may be but poorly defined, and
ice flows in both directions from rock divides so that the streams
transect the range, and later, after the glaciers have disappeared, -
may expose a pass smoothed and polished upon its floor and with
GLACIER TYPES OF A WANING GLACIATION 385
strie directed in opposite directions from the highest point. The
pass of the Grimsel in Switzerland furnishes an excellent illustra-
tion of such earlier transection of the range.
The expanded-foot glacier. — As air temperatures continue to be-
come milder, the glacier streams within the mountains are less deep
and hence more clearly
defined, and instead of AMS
coalescing upon the moun- eerie oY 41>
tain foreland, they now Qiea¢s ses
issue from the mountains PN, AM by wg
to form individual aprons rt: WS NY, Yi A
Aare Sa
and are described as ex- “W we sine
panded-foot glaciers (Fig.
408, stage II, and Fig. fe. 0. Map of the Baltoro glacier of the
292, p. 264). ome, a typical glacier of the dendritic
The dendritic glacier.
— Still later in the hemicycle nourishment of the glaciers is di-
minished as depletion from melting increases, so that the glacier
streams no longer reach to the mountain front. Branches con-
tinue to enter the main valley from
the several side valleys like the short
branches of a tall tree, and because of
. this arrangement such a glacier may
be described as a dendritic glacier
(Fig. 408, stage III, and Fig. 410).
Inasmuch as the depletion from
melting increases at a rapid rate in
descending to lower levels, the tribu-
tary glacier valleys ‘‘ hanging ” above
the main valley in the lower stretches
become separated, and may continue
to exist as series of hanging glacierets
upon either side of the main valley be-
low the glacier front (Fig. 408, stage
III, and Fig. 411). It must be clear
oa « 3 as - from this that any attempt to name
Fie, 411.—The Triest glacier, a each separated ice stream without
ical ‘3 MOLE Ue regard to its relationship must lead
which it was lately a tributary. to endless confusion, for glacier size
20
386 EARTH FEATURES AND THEIR MEANING
is in such sensitive adjustment to air temperature that a fall or rise
of a few degrees only in the average annual temperature of the dis-
trict may prove sufficient to fuse many glaciers into one or separate
one ice mass into many smaller ones.
When in high latitudes a dendritic glacier descends in fjords
to below the level of the sea, it is attacked by the water in the same
manner as are the outlets of Greenland glaciers, and is then known
as a ‘‘ tidewater glacier,”
which may thus be a
subtype or variety of the
dendritic glacier (Fig.
412).
The radiating (Alpine)
glacier. — In the pro-
“i gressive wastings of
Fig. 412.— The Harriman fjord glacier of Alaska, dendritic glaciers, there
a tidewater variety of dendritic glacier (after a * hewthat
map by Gannett). comes a time wnen eir
dendritic outlines give
place to radiating ones. Attention has already been called to the
division of the cirque into subordinate basins separated by small
rock arétes and yielding a markedly scalloped border (Fig. 394,
p. 371). When the ice front retires from the main valley into one
- of these mature cirques, the now wasted ice stream is broken up
into subordinate glacierets, each of which occupies one of the
basins within the larger cirque, and these ice streams
flow together to produce a glacier whose compo-
nent elements radiate like the sticks within a lady’s
fan (Fig. 408, stage IV, and Fig. 413).
The horseshoe glacier. — As the glacier draws
near to its final extinction, it is crowded hard
against the wall of the amphitheater in which it
has so long been nourished. Up to this stage it
has offered a swelling front outwardly convex as a of the Rotmoos
direct consequence of the laws controlling its flow. glacier, a radi-
No longer“amply nourished, for the first time its ine _ slacier
fe . mie < of Switzerland
front is hollowed, and it awaits its final dissolu- (after Sonklar).
tion curled up against the cirque wall (Fig. 408,
stage V, and Fig. 414). Practically all the glaciers of the United «
States and southern Canada are of this type. |
es alent
wi;
hee
i
me
e
ae
i.
he
Pog
Be
ie
La
ys ;
it
ie.
GLACIER TYPES OF A WANING GLACIATION 387
The above classification is one depending directly upon glacier
nourishment, and hence also upon size, and upon the stage of the
glacial hemicycle. In order to determine the type of any gla-
cier it is necessary to know the outlines of the mountain valley
-
pee"
% by Ae
eee ey
: \
\
om _
aes teal
AS
ie co
Beate 2
er oe oe oy 2 miles
! | i | sory
Fig. 414. — Outline map of the Asulkan glacier in the Selkirks, a typical horseshoe
glacier.
— its divide — and those of the glacier or glaciers within it. It
is likely that the types of the advancing hemicycle of glaciation
would be much the same, save only for the new-born or nivation
glacier, which would be as different as possible from the horse-
shoe type, to which in size it corresponds. Upon the continent
of Antarctica, where the absence of any general melting of the ice,
even in the summer season and near the sea level, introduces special
conditions, some additional glacier types are found, which, how-
ever, it is not necessary that we consider here.
The inherited-basin glacier. —It may be, however, that gla-
ciers have developed, not upon mountains shaped in a cycle of
_ river erosion, nor yet in succession to an ice cap, as in the nor-
mal cases which we have considered. On the contrary, glaciers
388 EARTH FEATURES AND THEIR MEANING
may develop where basins of one sort or another have been
inherited from the preceding period. In such cases inherited de-
pressions may become more important than the auto-sculpture of
Cees Sr a Series Po =
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Fia. 415.— Outline map of the Illecillewaet glacier, an inherited-basin glacier in
the Selkirks.
the glacier. Glaciers which develop under such conditions may
be described as inherited-basin glaciers. | 3
A partly closed basin between ridges may supply a collecting
ground for snows carried from neighboring slopes by the wind,
GLACIER TYPES OF A WANING GLACIATION 389
and so may yield a broad névé, approaching in size a small ice cap,
yet without developing definite ice streams except upon its border.
Such a glacier is the Illecillewaet glacier of the Selkirks (Fig. 415).
Again in low latitudes the high and pointed volcanic peaks
may push up beyond the snow line into the upper atmosphere,
and so become snow-capped. Definite cirques do not develop well
under these circumstances, and the loose materials of which such
peaks are always composed are attacked in somewhat irregular
fashion from the different sides. This is the case of Mount Ranier
and similar peaks of the Cascade range of North America.
Summary of types of mountain glacier. — In tabular form the
various types of mountain glacier may be arranged as follows: —
MOUNTAIN GLACIERS
Piedmont glacier. Mountain valleys entirely occupied and largely
submerged, with overflow upon the foreland to form a common ice apron
through coalescence of neighboring streams.
Expanded-foot glacier. Valley entirely occupied and an overflow upon
the foreland sufficient to produce individual ice apron.
Dendritic glacier. Valley not completely occupied but with tributary
ice streams ranged along the sides of the main stream, and with hanging
glacierets separated near the glacier foot.
Radiating glacier. Glacier largely ineluded in a cirque with subordi-
- nate glacierets converging below like the sticks in a lady’s fan.
Horseshoe glacier. Small glacier remnants hugging the cirque wall
and having an incurving front. ;
Inherited-basin glacier. Of form dependent on a basin inherited and
not shaped by the glacier itself.
READING REFERENCE FOR CHAPTER XXVII
Wituam H. Hosss. The Cycle of Mountain Glaciation, Geogr. Jour.,
vol. 37, 1910, pp. 268-284.
CHAPTER XXVIII
THE GLACIER’S SURFACE FEATURES AND THE
DEPOSITS UPON ITS BED
The glacier flow. — The downward flow of the ice within a
mountain glacier has been the subject of many investigations and
the topic of many heated discussions since the time when Louis
Agassiz and his companions set a line of stakes across the Aar
/-
\
Fig. 416.— Diagram |
to illustrate the mi-
grations of lines of
stakes crossing a
glacier, due to its
surface movement,
A, original position
of lines; A’, later
positions; aanda’,
original and dis-
torted forms of a
square section of
the glacier surface
near its margin; r,
r’, diagonal cre-
vasses.
glacier and numbered the surface bowlders in
preparation for repeated observations. Their
first observation was that the line of stakes,
which had run straight across the glacier, was
distorted into a curve which was convex down-
stream (Fig. 461, A’), thus showing that the
surface layers have more rapid motion in pro-
portion as they are distant from the side mar-
gins. Summarizing these and later studies, it
may be stated that the glacier increases its rate
of motion from its side margin towards its cen-
ter line, from its bed upwards towards its sur-
face, and below the névé the velocity is greatest
where the fall is greatest and also wherever the
cross section diminishes. In all these particu-
lars, then, the ice of the glacier behaves like a
stream of water. The average rate of flow of
Alpine glaciers varies from a few inches to a few
feet per day, and is greater during the warm
summer season. The Muir glacier of Alaska
has been shown to move at the rate of about
seven feet per day.
In traveling from the névé downward to the
glacier foot, the snow not only changes into.
ice, but it undergoes a granulating process with continued increase
in the size of the nodules until at the foot of the glacier these may
390
THE GLACIER’S SURFACE FEATURES 391
be picked out of the partially melted ice as articulating balls the
size of the fist or larger. Glacier ice has therefore a structure
quite different from that of lake ice, since the latter is developed
in parallel needles perpendicular to the freezing surface.
Crevasses and séracs. — Prominent surface indications of gla-
cier movement are found in the open cracks or crevasses, which
are the marks of its yielding to tensional stresses. Crevasses
are apt to run either directly across the glacier, wherever there is
a steep descent upon its bed, or diagonally, running in from the
margin and directed up-glacier (r, r, r, of Fig. 416), though they
occasionally run longitudinally with the glacier when there is
a rock terrace at the side of the valley beneath the ice. The
diagonal crevasses at the glacier margin are due to the more
sluggish movement where the ice is held back by friction upon the
walls of the valley, as will be clear from Fig. 416. The square a
has by this movement been distorted into the lozenge a’, so that
the line zy has been extended into zv’y’, with the obvious tendency
to open cracks in the direction ss.
Every glacier surface below its névé is marked by steps or
terraces, which are well understood to overlie corresponding steps
of the cascade stairway to be seen in all vacated glacier valleys
(plate 19). The steep risers of these steps are usually marked
by parallel crevasses which cross the glacier. Under the rays
of the sun, which strike them more from one side than from
the other, the slices into which the ice
is divided are transformed into sharp-
ened blades and needles which are
known as séracs (Fig. 401, p. 376, and
Fig. 417).
The numerous crevasses tell us that p46 417.— Transverse crevasses
the ice is many times wrenched apart at the fall below a glacier step
during its journey down the glacier, ‘@nsformed by ae age hs
This has been illustrated by some- sarc ca
what grewsome incidents connected with accidents to Alpinists,
but as they illustrate in some measure both the mode and the rate
of motion of Swiss glaciers, they are worthy of our consideration.
Bodies given up by the Glacier des Bossons. — In the year
1820, during one of the earlier ascents of Mont Blanc, three guides
were buried beneath an avalanche near the Rochers Rouges in
392 EARTH FEATURES AND THEIR MEANING
the névé of the Glacier des Bossons (Fig. 418). In 1858 Dr.
Forbes, who had measured the rate of flow of a number of Alpine
glaciers, predicted that the bodies of the victims of this accident
would be given up by the glacier after being entombed from thirty-
five to forty years. In the year 1861, or forty-one years after the
YEN BS
we Wir
+ Se
> aS
SoS irees
If
ieee Ml LT
Fic. 418.— View of the Glacier des Bossons upon the slopes of Mont Blane show-
ing the position of accidents to Alpinists and the place of reappearance of their
bodies.
}
sos
disaster, the heads of the three guides, separated from their bodies,
with some hands and fragments of clothing, appeared at the foot
of the Glacier des Bossons, and in such a state of preservation that
they were easily recognized by a guide who had known them in
life. Inasmuch as these fragments of the bodies had required
forty-one years to travel in the ice the three thousand meters
which separate the place of the accident from the foot of the
glacier, the rate of movement was twenty centimeters, or eight
inches, per day. |
Various separated parts of the body of Captain Arkwright, who
had been lost in 1866 upon the névé of the same glacier, reap-
peared at its foot after entombment in the ice for a period of thirty-
one years. To-day the time of reappearance of portions of the
bodies of persons lost upon Mont Blanc is rather accurately pre-
dicted, so that friends repair to Chamonix to await the giving up
of its victims by the Glacier des Bossons.
ae a Se ee ee ee Se
m — po
THE GLACIER’S SURFACE FEATURES 393
The moraines. — The horns and comb ridges which rise above
the glacier surface are continually subject to frost’ weathering,
and from time to time drop their separated fragments upon the
glacier. Falling as these do from considerable heights, they reach
the ice under a high velocity, and rebounding, sometimes travel
well out upon its surface before coming to a temporary rest. Upon
a fresh snow surface of the névé their tracks may sometimes be
followed with the eye for considerable distances, and their fall
is a constant menace to Alpine climbers. Below the névé the
larger number of such frag-
ments remain near the
cliff, and the lines of flow
of the ice within the gla-
cier surface are such that
blocks which reach points an)
farther out upon the gla- Fic. 419.— Lines of flow upon the surface of the
cier are later gathered in _Hintereisferner glacier in the Alps (after Hess).
beneath the cliff at the side (Fig. 419). The ridge of angular rock
débris which thus forms at the side of the glacier is called a
lateral moraine (see Fig. 411, p. 385, and Fig. 420).
At the junction of two glacier
y streams, the lateral moraines are joined,
eas and there move out upon the ice sur-
“Joke, face of the resultant glacier as a medial
ees:
: ah moraine. Thus from the number of
medial moraines upon a glacier sur-
ara
esata
ao ine face it is possible to say that the im-
2 aN SNe A portant tributary glaciers number one
A \ if f = more (Fig. 420).
AY
Ty The plucking and abrading processes
e in operation beneath the glacier, quarry
\ the rock upon its bed, and after shap-
SS ing and smoothing the separated rock
Fic. 420.—Lateral and medial fragments, these are incorporated with-
moraines of the Mer de glace inthe lower layers of the ice as engla-
ca ee cial rock débris. In spaces favorable
for its accumulation, a portion of this material, together with much
finer débris and rock flour, is left behind as a ground moraine
upon the bed of the glacier (see Fig. 421).
Ce
394 EARTH FEATURES AND THEIR MEANING
At the foot of the glacier the relatively angular rock débris,
which has been carried upon the surface, and the soled and polished
englacial material from near the bottom, are alike deposited in a
common marginal ridge known as the terminal or end moraine
(plate 21 B). .
Selective melting upon the glacier surface. — The white sur-
face of the glacier generally reflects a large proportion of the sun’s
Fig. 421.—Ideal cross-section of a mountain glacier to show the. position of
moraines and other peculiarities characteristic of the surface of the bed.
rays which reach it, and its more rapid melting is largely accom-
plished through the agency of rock fragments spread upon its
surface. Such fragments, however, promote or retard the melting
process in inverse proportion to their size up to a certain limit,
‘ ANS WS SSRs
. \ \ MY WY QO \ \\
SN
a e Cc
OMY,
IW 2 Layerzwormea by sun.)
Fic. 422.— Fragments of rock of different sizes, to bring out their different
effects upon the melting of the glacier surface.
PLATE 21.
ee Sea
a
i ces
ido. F
¥
A. View of the Harvard Glacier, Alaska, showing the characteristic terraces (after
U. S. Grant).
B. The terminal moraine at the foot of a mountain glacier (after George Kinney).
THE GLACIER’S SURFACE FEATURES 395
and above that size their action is always to protect the glacier
from the sun. This nice adjustment to the size of the rock frag-
ments will be clear from examination of Fig. 422, for rock is a
poor conductor of heat, and in even the longest summer day a
thin outer layer only is appreci-
ably warmed. Large rock blocks,
grouped in the medial and lateral
moraines, hold back the process of
lowering the glacier surface during
the summer, so that late in the
season these moraines stand fifty
feet or more above the glacier as
armored ice ridges.
Isolated and large rock slabs, as
the season advances, may come to
form the capping of an ice pedestal
which they overhang and are known __ the surface of the Great Aletsch
as glacier tables (Fig. 423). Such #12¢#er in 1908.
tables the sun attacks more upon one side than upon the other,
so that the slab inclines more and more to the south and may
eventually slip down until its edges rest against the glacier sur-
- face. Rounded bowlders, which less frequently become perched
upon ice pedestals, may, from a similar process, slide down upon
the southern side and leave a pyramid of ice furrowed upon this
side and known as an ice pyramid.
Fine dirt when scattered over the glacier surface is, on the other
hand, most effective in lowering its level by melting. Use was
made of this knowledge to lower the great drifts of snow which
had to be removed each season during the construction of the
new Bergen railway of southern Norway. Each dirt particle,
being warmed throughout by the sun’s rays, melts its way rapidly
into the glacier surface until the dust well which it has formed is
so deep that the slanting rays of the sun no longer reach it. When
the dirt particles are near together, the thin walls which separate
the dust wells are attacked from the sides in the warm air of sum-
mer days, thus producing from a patch of dirt upon the glacier
surface a bath tub (Fig. 424d). At night the water which fills these
basins is frozen to form a lining of ice needles projecting inward
from the wall, and this, repeated in succeeding nights, may
396 EARTH FEATURES AND THEIR MEANING
entirely close the basin with water ice and produce the familiar
glacier star (Fig. 424 c).
If the dirt upon the glacier surface, instead of being scattered,
is so disposed as to make a patch completely covering the ice to
Gletscherstern
Cc ‘
Fic. 424.—Effects of differential melting and subsequent refreezing upon the
glacier surface. a, dust wells; b, glacier twb produced by melting about a group of
scattered dust particles; c, glacier star produced when the inclosed water of the
glacier well has frozen in successive nights; d, ‘‘ bath tub.’’
the thickness of an inch or more, the effect is altogether different.
Protecting as it now does the ice below, a local ice hillock rises
upon its site as the surrounding surface is lowered, and as this
Fic. 425.— Dirt cone and one with its casing in part removed. Victoria glacier
(after Sherzer).
grows in height its declivities increase and a portion of the dirt
slides down the side. The final product of this shaping-is an
almost perfectly conical ice hill encased in dirt and known as a
THE GLACIER’S SURFACE FEATURES 397
débris, sand, or dirt cone (Fig. 425). The novice in glacier study
is apt to assume that these black cones contain only dirt, but is
rudely awakened to the reality when he attempts to kick them to
pieces. Both glacier tubs and débris cones may assume large
dimensions; as, for example, in Alaska, where they may be properly
described as lakes and hills.
A patch of hard and dense snow which is less easily melted
than that upon which it rests may lead to the formation of snow
cones upon the glacier surface similar in size and shape to the
better known débris cones. Such cones of snow have, with
doubtful propriety, been designated ‘‘ penitents,” for it is pretty
clear that the interesting bowed snow figures, which really re-
semble penitents and which were first described from the southern
Andes under the name of nieves penitentes, are of somewhat dif-
ferent character. |
One further ice feature shaped by differential melting around
rock particles remains to be mentioned. Wherever the seasonal
snowfalls of the névé are exposed in crevasses, they are generally
found to be separated by layers of dirt, and lines of pebbles simi-
larly separate those ice layers which are revealed at the foot
of the glacier. In either case, if the sun’s rays can reach these
layers in an opened crevasse, the half-buried
_ rock fragments are warmed by the sun upon
their exposed surfaces and slowly melt their
way down the ice surface, thus removing from
it a thin layer of snow or ice and causing that
part above the pebble layer to project like
a cornice. This process will go on until the
overhanging cornice protects the pebbles from <
any further warming by the sun, but each gy. 426. —Schematic
lower pebble layer that is reached by the sun _ diagram to show the
will produce an additional cornice, so that ™2mner of formation
ass of glacier cornices.
the original surface may at the bottom have .
been retired by the process a number of inches. These features
are described as glacier cornices (Fig. 426).
Glacier drainage. — Already in the early morning of every
warm summer day, active melting has begun upon the surface of
the Swiss glaciers. Rills of icy water soon make their way along
depressions upon the surface, and are joined to one another so that
398 EARTH FEATURES AND THEIR MEANING
they sometimes form brooks of considerable size (Fig. 427). Such
streams continue their serpentine courses until these are inter-
sected by a crevasse down which the waters plunge in a whirling
vortex which soon develops a vertical shaft of circular section
within the ice. Such shafts with their descending columns of
whirling water are the
well-known’ moulins,
or “mills,” which
may be detected from
a distance by their
gurglingsounds. The
first plunge of the
water may not reach
to the bottom of the
=f SO AN, glacier, in which case
= eee a the stream finds a
i passageway below the
Fig. 427.— Superglacial stream upon the Great surface but above the
Aletsch glacier.
Mit. \ AS
Wf iff] \\\ i
t, i i} .
WANN XK
/ “ ‘
iN WAN
floor until another
crevasse is encountered and a new plunge made, here perhaps to
the bottom. Once upon the valley floor the stream is joined by
others, and pursues its course within an ice tunnel of its own
making (Fig. 421, p. 394) until it issues at the glacier front.
The coarser of the rock débris which was gathered up by the
stream upon the glacier surface is deposited within the tunnel in
imperfect assortment (gravel and sand), while all finer material
and that lifted from the floor (rock flour) is retained in suspension
and gives to the escaping stream its opaque white appearance.
‘This glacier milk may generally be traced far down the valleys or
out upon the foreland, and is often the traveler’s first indication
that a range which he is approaching supports glaciers.
Deposits within the vacated valley. — For every excavation
of the higher portions of the upland through glacial sculpture,
there is a corresponding deposit of the excavated materials in
lower levels. So far as these materials are deposited directly by —
the ice, they form the lateral, medial, ground, and terminal moraines
‘already described. A considerable proportion of them are, how-
ever, deposited by the water outside the terminal moraine; but
a8 with the shrinking glacier the ice front retires in halting move-
~~
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i
ities
THE GLACIER’S SURFACE FEATURES 399
ments over the area earlier ice-covered, the terminal moraines are
ranged along the vacated valley as recessional moraines, each with
a valley train of outwash below. About the apron of the piedmont
glacier, such deposits are particularly heavy (Fig. 428). During
Fic. 428.—Ideal form of the surface left on the site of the apron of a piedmont
glacier. M, moraine; 7’, outwash; C, basin usually occupied by alake; D, drum-
lins (after Penck).
the ‘ice age ”’ the Swiss glaciers extended down the valleys below
the existing ice remnants and spread upon the Swiss foreland as
great piedmont glaciers such as may now be seen in Alaska. To-
day we find there moraines and glacial outwash, a lake in the
middle of the apron site, and sometimes a group of radiating drum-
lins like those found within the ice lobes of the continental glacier
in southern Wisconsin (Fig. 429, and Fig. 344, p. 317).
rs - yi
a eee o6
. 6
Ste 00,0). OO? > 5a eS See
di se Ss, eRe
V, Z
“py fps YU
Mihi ttisiss
Yi Yj:
earlier piedmont glacier of the Upper Rhine. The white area outside the outer-
most moraine is buried in glacial outwash (after Penck and Brickner).
400 EARTH FEATURES AND THEIR MEANING
Behind the recessional moraines within the glaciated valley are
found the valley moraine lakes (Fig. 448, p. 413), in association
with the rock basin lakes due to glacial sculpture (Fig. 447, p. 412).
After the glacier has vacated its valley, the precipitous side walls
become the prey of frostwork and are the scenes of disastrous
avalanches or landslides. Within the cirques, drifts of snow are
nourished long after the ice has disappeared, and as a consequence
the amphitheater walls succumb to the process of solifluxion
(p. 153). .
Diversions and reversals of drainage, which are so characteristic
of the work of continental glaciers, are hardly less common to
glaciated mountain districts. Many of our most beautiful water-
falls have resulted from either the temporary or permanent. ob-
struction of earlier valleys above the falls. The famous Yosemite
Falls offers an interesting illustration of the shifting of an earlier
waterfall, itself no doubt due to ice blocking in a still earlier glacia-
tion (plate 22 B).
' Marks of the earlier occupation of mountains by glaciers. —
It is well that we should now bring together within a small compass
those evidences by which the existence of earlier mountain glaciers
may be proven in any district. These marks are so deeply stamped
upon the landscape that no one need err in their interpretation.
MARKS OF MOUNTAIN GLACIERS
High-level sculpture. The grooved upland with its cirques, or the fretted
upland with its cirques, cols, horns, and comb ridges.
Low-level sculpture. The U-shaped main valley, the hanging side
valleys with their ribbon falls, the glacier staircase with its rock bars and
gorges, the rounded, polished, and striated rock floor.
Deposits. The recessional moraines of till and the valley trains of
sand and gravel, the soled erratic blocks derived always from higher
levels of the valley.
Lakes. The valley moraine lakes and the chains of rock basin lakes.
READING REFERENCES FOR CHAPTER XXVIII
Glacier movement :—
L. Agassiz. Nouvelles Etudes et Expériences sur les Glaciers Actuels,
ete., Paris, 1847, pp. 485-539.
H. Hess. Die Gletscher, Braunschweig, 1904, pp. 115-150.
H.F. Rem. The Mechanics of Glaciers, Jour. Geol., vol. 4, 1896, pp. 912—
928; Glacier Bay and Its Glaciers, 16th Ann. Rept. U. S Geol.
Surv., Pt. i, 1898, pp. 445-448.
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CHAPTER XXIX
A STUDY OF LAKE BASINS
Freshwater and saline lakes. — Lakes require for their exist-
ence a basin within which water may be impounded, and a supply
of water more than sufficient to meet the losses from seepage and
evaporation. If there is a surplus beyond what is needed to meet
these losses, lakes have outlets and remain fresh; their content
of mineral matter is then too slight to be detected by the palate.
If, on the other hand, supply is insufficient for overflow, continued
evaporation results in a concentration ofthe mineral content of
the water, subject as it is to continual augmentation from the in-
flowing streams.
As we have seen, there are in areas of small rainfall special
weathering processes which tend to bring out the salts from the
interior of rock masses, these concentrated salts generally first
- appearing as a surface efflorescence which is ultimately transferred
- through the agency of wind and cloudburst to the characteristi-
cally saline desert lakes.
Lake basins may be formed in many ways. Depressions of
the land surface may result from tectonic movements of the crust ;
they may be formed by excavating processes; but in by far the
greater number of instances they result from the obstruction in
some manner of valleys which were before characterized by uni-
formly forward grades. In relatively few cases loose materials
are heaped up in such a manner as to produce fairly symmetrical
basins.
Newland lakes. — On land recently elevated from the sea,
basins of lakes may be merely the inherited slight irregularities
of the earlier sea floor, in which case they may be assumed to be
largely the result of an irregular distribution of deposits derived
from the land. Lakes of this type are especially well exhibited
in Florida, and are known as newland lakes (Fig. 430). Such
lakes are exceptionally shallow, and are apt to have irregular out-
2D 401
402 EARTH FEATURES AND THEIR MEANING
lines and extremely low banks. Under these circumstances, they
are soon filled with a rank growth of vegetation, so that it is some-
times difficult to properly distinguish lake and marsh.
Mog, ab )
LAN
@
Marsh
meets
ee o oe nat are
Fig. 430.—Map and diagram to bring out the characteristics of newland lakes.
Basin-range lakes. — Newland lakes may be said to have their
origin in an uplift of the land and sea floor near their common
margin. A lake type dependent upon movements of the earth’s crust
*
f oer
tas aks BR ; 2
Fee aha ee
‘a - —— res. Tt 2
alg, =r tre fe
ee acts ‘ ~-L o.
— el PFI 9 pai 8,743 —
ee so ooo eee a a
Ea Ss 2ate aia a == - Yy3 ican
reo — =. a — = pile, G S a
73 ae aay : o, S; _—- -
ps u A % -
~~ oe 4 = Ae 2 i YY, ——_
Sar a LY pea set 7 Rie ihn
SS aes 9 FY RS
a eS : Se ae hee A
— a gy LIP nm =
— a = = re te '
a Se ee = od
et See ee Lo _- OS ~! are
™~ > = » py i — 7S Tea Snes —_ =
SURE SE se en ee ae? esr. ‘ ay A: ~~
ee a a Bee 01) Bs —- “2 —_———
ee TS ae = heyy 7s 3 Sp 5
———a ———__—_ e - ,
t, ——S=- g 7+ ieee soy = a
Ea ee EN eS i
Fig. 431.— View of the Warner Lakes, Oregon (after Russell).
but within interior areas has been described as the basin-range
type and is exemplified by the Warner Lakes of Oregon. _ In this
i
:
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i
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5
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i
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:
7
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; :
A STUDY OF LAKE BASINS 403
district great rectangular blocks of the earth’s crust, which in their
upper portions at least are composed of basaltic lavas, have under-
gone vertical adjustments in level and have been tilted so that
the corresponding corners of neighboring blocks have been given
a similar degree of down-
tilt (Fig. 481). Lakes
formed in this way are
of triangular outline, are
bounded on the two
shorter sides by cliffs,
but have extremely flat
shores on their longest
side. From this shore the
water increases gradually
in depth and attains a
maximum depth at or
near the opposite angle. sts yacteaey, P J Was tee
IG. — Schematic diagrams to illustra e
Such lakes naturally be- characteristics of basin-range lakes.
tray a tendency to appear
in series (Fig. 432), and are unfortunately much too often illus-
trated on a small scale after a shower by the tilted blocks of
imperfectly made cement sidewalks.
Rift-valley lakes. — Another type of lake basin which has its
origin in faulted block movements is known as the rift-valley lake,
Fia. 433. — Schematic diagrams of vide-saiee lakes, and the rift valley of the Jordan
with the Dead Sea and the Sea of Galilee as remnants of a larger lake in which
their basins were included.
and is best exemplified by the great lakes of east Central Africa.
In this type a strip of crust, many times as long as it is wide, has
been relatively sunk between the blocks on either side so as to
produce a deep rift, or what in Germany is known as a Graben
404. EARTH FEATURES AND THEIR MEANING
(trench). Such a basin when occupied by water yields a lake which
is long, straight, deep, and narrow, and is in addition bounded on
the sides by steep rock cliffs. At the ends the
shores are generally by contrast decidedly low.
If the hard rock at the bottom of the lake
could be examined, it would be found to be of
the same type as that exposed near the top of
the side cliffs. The valley of the Jordan in
Palestine is a rift of this character and was at
one time occupied by a long and narrow lake
of which the Dead Sea and the Sea of Galilee
are the existing remnants (Fig. 433).
One of the most striking examples of a rift
valley lake is Lake Tanganyika, while Albert
Nyanza, Nyassa, and
Rudolf in the same
region are similar
(Fig. 434).
Earthquake lakes. —
The complex adjust-
Fia.434.—Mapshow- ments in level of the
a Aly pees Alora surface of the ground
Africa. at the time of sensible
earthquakes are many
of them made apparent in no other way
than by the derangements of the surface
water. This is at such times impounded
either in pools or in broad lakes, which
inasmuch as they date from known earth-
quakes have been called “ earthquake
lakes,’”’ even though in a strict sense any .
lake which has originated in earth move- F1¢ 435.—Earthquake
‘ lakes which were formed
ments might properly be regarded as an in the flood plain of the
earthquake lake. To avoid unnecessary lower Mississippi during
confusion, the term must, however, be re- the earthquake of 1811
stricted to those lakés which are known to)“ j
have been formed at the time of definite earthquakes (Fig. 485).
Reelfoot Lake. in Tennessee, which in late years has acquired” |
undesirable notoriety because of the feuds between the fishermen
oO. Miles. s
A STUDY OF LAKE BASINS 405
of the district and the constituted authorities, is a lake more than
twenty miles across and came into existence during the great
earthquake of the lower Mississippi valley in 1811.
Crater lakes. — The craters of volcanic mountains are natural
basins in which surface waters are certain to be collected, provided
only the supply is sufficient and seepage into the loose materials is
Fic. 436.— View of lake in Poas Crater in Costa Rica, a volcanic crater more
than half a mile across and with walls 800 feet deep. At intervals there is an
ejection of steam mixed with mud and ash after the manner of a geyser (after
H. Pittier).
not excessive. Some craters, still visibly more or less active, are
occupied by lakes (Fig. 436).
In the larger number of cases in which craters become occupied
by lakes, the evidence of continued activity is lacking, and it would
appear in such cases that the lava of the chimney had consolidated
into a volcanic plug, closing the bottom of the crater. Notable
groups of crater lakes are the Caldera of the Roman Campagna
(Fig. 437) and the so-called maare of the Eifel about the Lower
Rhine. Crater lakes are easy to recognize by their circular plan,
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