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Earth sculpture; or, The origin of landfo 




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THE SCIENCE SERIES 



1. The Study of Man. — By A. C. H addon. Illus- 

trated, 8°, $2.00. 

2. The Groundwork of Science. — By St. George 

MlVART. 8°, $1.75. 

3. Rivers of North America. — By Israel C. Rus- 

sell. Illustrated, 8°. 

4. Earth Sculpture. By James Geikie. Illus- 

trated, 8°. 



G. P. PUTNAM'S SONS, New York & London 



XEbe Science Series 

EDITED BY 

Iptofeseor 5, ^cS^een Cattell, O^.U., ipb.D. 

AND 



EARTH SCULPTURE 





fi 


lfe'2V. AalU^Oiy^'/M ^1 



EARTH SCULPTURE 



OR 



THE ORIGIN OF LAND-FORMS 



BY 



JAMES GEIKIE, LL.D., D.C.L., F.R.S., etc. 

MURCHISON PROFESSOR OF GEOLOGY AND MINERALOGY 

IN THE UNIVERSITY OF EDINBURGH ; FORMERLY OF H.M. GEOLOGICAL SURVEY 

AUTHOR OF "the GREAT ICE AGE," "PREHISTORIC EUROPE," ETC. 



ILLUSTRATED 



NEW YORK 

G. P. PUTNAM'S SONS 

LONDON 

JOHN MURRAY 

1898 

s 



Copyright i8g8 

BV 

G. P. PUTNAM'S SONS 



Xibc Iknfctievboctier ipcess, 1Hew ^ovk 



PREFACE 

ALTHOUGH much has been written, especially 
of late years, on the origin of surface-features, 
yet there is no English work to which readers not 
skilled in geology can turn for some general account 
of the whole subject. It is true that all geological 
text-books, and many manuals of geography, devote 
some space to its discussion, while not a few excellent 
treatises deal at large with one or more of its sub- 
divisions. Geological literature is also by no means 
poor in admirable popular monographs descriptive of 
the geology and geography of particular regions, in 
which the origin of their surface-features is more or 
less fully explained. But for those who may be de- 
sirous of acquiring some broad knowledge of the 
results arrived at by geologists as to the development 
of land-forms generally, no introductory treatise is 
available. Possibly, therefore, the present attempt 
to supply a deficiency may not be wholly unaccept- 
able. 

In a work addressed more particularly to non-spec- 
ialists, technical terminology should be employed as 
sparingly as possible, and I have consequently made 
scant use of those neologisms in which, unfortunately, 



-iv PREFACE 

the recent literature of the subject too much abounds. 
Technical words and expressions cannot, however, be 
entirely dispensed with, but those which my readers 
will encounter have, as a rule, been long current, and 
few are likely to be unfamiliar. 

The materials used in the preparation of this book 
are for the most part from the common stock of geo- 
logical knowledge, and it has not been thought neces- 
sary, therefore, to burden the pages with references. 
Those who would pursue the subject further must 
consult the larger text-books of geology in English, 
French, and German, which usually indicate the more 
notable sources of information. The following works 
will also be found very helpful as guides and instruct- 
ors : — 

Sir A. C. Ramsay's Physical Geology and Geography 
of Great Britain. 

Prof. A. H. Green's Physical Geology (chap. xiii.). 

Sir A. Geikie's Scenery and Geology of Scotland. 

Prof. E. Hull's Physical Geology and Geography of 
Ireland. 

Sir J. Lubbock's Scenery of Switzerland and the 
Causes to which it is Due. 

Dr. E. Fraas's Scenerie der Alpen. 

Major J. W. Powell's Canyons of the Colorado. 

MM. De la Noe and Emm. de Margerie, Les 
Formes du Terrain — an admirable and well illustrated 
work, descriptive of the geological origin of land- 
forms. 

Prof. A. Penck's Morphologie der Erdoberfldche — a 



PREFACE V 

masterly review and classification of the surface-feat- 
ures of the earth, with a full discussion of their origin. 
This treatise is particularly rich in references to the 
literature ; the whole history of geological opinion on 
the subject of which it treats may therefore be gath- 
ered from its pages. 

Prof. A. de Lapparent's Legons de Gdographie Phys- 
ique — a most instructive and comprehensive outline 
of geo-morphology. The second half of the work 
deals more particularly with geographical evolution, 
the special treatment of which does not come within 
the limits of my essay. This interesting subject has of 
late years been studied with great assiduity, especially 
by Prof. W. M. Davis and others in North America. 

The maps and sections, and the monographs, me- 
moirs, and reports of our own and other national 
geological surveys are storehouses of information 
and instruction in physiographical geology. Some 
of these works that deal more especially with denuda- 
tion and the relation of surface-features to geological 
structure have indeed become classical. Amongst 
these are Ramsay's notable paper, " On the Denuda- 
tion of South Wales and the Adjacent Counties of 
England " {Memoirs Geological Survey of England, 
vol. i., 1846) ; Heim's Mechanismus der Gebirgsbil- 
dung, etc. (which, although an independent work, was 
yet commenced under the auspices of the Swiss Geo- 
logical Commission) ; Dutton's "Tertiary History of 
the Grand Canon District " {Monograph II. of U. S. 
Geological Survey^. 



1/ 



vi PREFACE 

For the use of several illustrations (Figs. 8, 25, 26, 
75, 78) from Major Powell's Canyons of the Colorado, 
I am indebted to his publishers, Messrs. Flood & 
Vincent. I am under similar obligations to the Coun- 
cil of the Geological Society for a section (Fig. 34) 
borrowed from my brother's paper on the North-west 
Highlands ; to Mr. Stanford for reproductions of il- 
lustrations (Figs. ']'], 83, 87, 88) from my Outlines 
of Geology ; to Herr Tempsky, Vienna, for Figs. 41, 
45, 56, from KirchhofFs Lander kunde des Erdteils 
Europa ; and to my friend, Mr. W. E. Carnegie 
Dickson, for the photographs reproduced on Plates 
I. and II. 

Edinburgh, July i, 1898. 



CONTENTS 

CHAPTER I 

PAGE 

Introductory . . ...... i 

Early views as to origin of Surface-features — Rocks and Rock- 
structures — Architecture of the Earth's Crust — General evidence of 
Rock-removal. 

CHAPTER II -i 

Agents of Denudation ....... i8 

Chemical composition of Rocks — Epigene Agents — Insolation and 
Deflation — Chemical and mechanical action of Rain — Action of 
Frost ; of Plants and Animals ; of underground Water ; of Brooks 
and Rivers — Rate of Denudation — Denudation and Sedimentation 
go hand in hand. 

CHAPTER III 

Land-Forms in Regions of Horizontal Strata 44 

Various factors determining Earth Sculpture — Influence of Geo- 
logical Structure and the Character of Rocks in determining the Con- ^ 
figuration assumed by Horizontal Strata — Plains and Plateaux of 
Accumulation. 

CHAPTER IV 

Land-Forms in Regions of Gently Inclined Strata 73 

Escarpments and Dip-slopes — Dip-valleys and Strike-valleys — 
Forms assumed by a Plateau of Erosion — Various directions of Es- 
carpments — Synclinal Hills and Anticlinal Hollows — Anticlinal 
Hills. 



viii CONTENTS 

CHAPTER V ^ 

PAGE 

Land-Forms in Regions of Highly Folded and Dis- 
turbed Strata ...... -9^ 

Typical Rock-structures in Regions of Mountain-uplift — General 
Structure of Mountains of Upheaval — Primeval Coincidence of Un- 
derground Structure and External Configuration — Relatively weak 
and strong Structures — Stages in the Erosion of a Mountain-chain — 

^ Forms assumed under Denudation — Ultimate face of Mountain-chains. 

CHAPTER VI ■* 
Land-Forms in Regions of Highly Folded and Dis- 
turbed Strata (continued) . . . . . .128 

Structure and Configuration of Plateaux of Erosion — Forms as- 
sumed under Denudation — Mountains of Circumdenudation — His- 
tory of certain Plateaux of Erosion — Southern Uplands and Northern 
Highlands of Scotland — Stages in Erosion of Table-lands. 

CHAPTER VII 

Land-Forms in Regions affected by Normal Faults 

or Vertical Displacements ..... 150 

Normal Faults, general features of — Their connection with Folds 
— Their origin — How they affect the Surface — Faults of the Colorado 
region, and of the Great Basin — Depression of the Dead Sea and the 
Jordan — Lake Depressions of East Africa — Faults of British Coal- 
fields — Bounding faults of Scottish Highlands and Lowlands — Fault- 
bounded Mountains — General conclusions. 

CHAPTER VIII 

Land-Forms due Directly or Indirectly to Igneous 

Action . . 173 

Plutonic and Volcanic Rocks — Deformation of Surface caused by 
Intrusions — Laccoliths of Henry Mountains — Volcanoes, Structure 
and Form of — Mud-cones — ^ Geysers — Fissure-eruptions — Volcanic 
Plateaux — ^Denudation of Volcanoes, etc., and resulting features. 

CHAPTER IxV 

Influence of Rock Character in the determina- 
tion OF Land-forms ....... 195 

1 Joints in Rocks and the part they play in determining Surface- 
features — Texture and Mineralogical composition of Rocks in rela- 
tion to Weathering — Forms assumed by various Rocks. 



CONTENTS ix 

CHAPTER X / 

PAGE 

Land-forms modified by Glacial Action . . . 212 

Geological action of existing Glaciers — Evidence of Erosion — 
Origin of the Ground-moraine : its independence of Surface-moraines 
— Infraglacial smoothing and polishing, crushing, shattering and 
plucking — Geological action of Prehistoric Glaciers — General evi- 
dence supplied by Ancient Glaciers of the Alps. 

CHAPTER XI 
Land-forms modified by Glacial Action {continued^ . 232 

Former Glacial conditions of Northern Europe — Extent of the old 
Inland Ice — Glacial character of Boulder-clay — Central Region of 
Glacial Erosion and Peripheral Area of Glacial Accumulation — Fluvio- 
glacial deposits — Loess, origin of its materials — Glaciation of North 
America — Modifications of Surface produced by Glacial Action. 

CHAPTER XII J 
Land-forms modified by ^olian Action . . . 250 

Insolation and Deflation in the Sahara — Forms assumed by Gran- 
itoid Rocks and Horizontal and Inclined Strata — Reduction of Land- ' 
surface to a Plain — Formation of Basins — Dunes of the Desert — 
Sand-hills of other regions — Transport and Accumulation of Dust — 
Loess, a dust deposit — Lakes and Marshes of the Steppes. 

CHAPTER XIII^ 

Land-forms modified by the Action of Under- 
ground Water ........ 266 

Dissolution of Rocks^Underground Water-action in Calcareous 
lands — Karst-regions of Carinthia and Illyria — Effects of Superficial 
and Subterranean Erosion — Temporary Lakes — Caves in Limestone 
— Caves in and underneath Lava — " Crystal Cellars " — Rock-shelters 
— Sea-caves. 

CHAPTER XIV 
Basins 278 

Basins due to Crustal Deformation — Crater-lakes — Dissolution 
Basins — Lakes formed by Rivers — .(Eolian Basins — Drainage dis- 
turbed by Landslips — Glacial Basins of various kinds ; as in Corries, 
Mountain-valleys, Lowlands, and Plateaux — Ice-barrier Basins — 
Submarine Basins of Glacial Origin. 



X CONTENTS 

CHAPTER XV 

PAGE 

COAST-LINES .... . 315 

Form and general trend of Coast-lines — Smooth or Regular Coasts 
— Influence of Geological Structure on various forms assumed by 
Cliffs — Cliffs cut in Bedded and in Amorphous Rocks — Sea-caves — 
Flat Coast-lines and Coastal Plains — Indented or Irregular Coasts — 
General trends of Coast-lines determined by form of Land-surface — 
Subordinate Influence of Marine Erosion. 

CHAPTER XVI y 

Classification of Land-forms ..... 335 

Plains of Accumulation and of Erosion — Plateaux of Accumulation 
and Erosion — Hills and Mountains : Original or Tectonic, and Sub- r 

sequent or Relict Mountains — Valleys : Original or Tectonic, and 
Subsequent or Erosion Valleys — Basins — Coast-lines. 

chapter xvii 
Conclusion ... ... 364 

The study of the Structure and Formation of Surface-features prac- 
tically involves that of the Evolution of the Land. 

Appendix . ... 373 

Glossary ..... , . 375 

Index . . . . . . . 387 



LIST OF ILLUSTRATIONS 



FIGURE PAGE 

1. Section of Horizontal Strata . . . . 7 

2. Section across an Anticline . . . . . 9 

3. Section across Normal Anticlines and Synclines ... 10 

4. Section across Anticlines and Synclines with Inclined Axes . 10 

5. Section across Faulted or Dislocated Strata ... 11 

6. Section across Unconformable Strata . 41 

7. Section across a series of Alluvial Terraces ... 51 
S. Section and Bird's-eye View of Colorado Plateau (Powell) 54 
9. Diagrammatic Section across Colorado Plateau . . .58 

10. Diagrammatic Section showing Stages of Erosion by a River cutting 

through Horizontal Strata (after Captain Dutton) .... 62 

11. Section across Suderoe (Faroe Islands) on a true scale . 69 

12. Map of an Island composed of Dome-shaped Strata . 74 

13. Section through the Island shown in Fig. 12 74 

14. Section of River-valley . . . . .75 

15. Enlarged section of a portion of the Island shown in Fig. 12 . .77 

16. Diagram Map of Plateau of Erosion . . .78 

17. Section across reduced Plateau of Erosion . . . 79 

18. Longitudinal Section of River Course . 80 

19. Section of Escarpments and Outliers ... 84 

20. Section across the Wealdean Area ( Ramsay) . .84 

21. Section across Permian Volcanic Basin, Ayrshire . . 86 

22. Synclinal Hills and Anticlinal Valleys . . 87 

23. Escarpment Hills and Synclinal Hill . 88 

24. Section across We*.! Lomond Hill and the Ochils 88 

25. Synclinal Valley, West of Green River (Powell) . 89 

26. Anticlinal Ridge, Green River Plains (Powell) . . . 90 

27. Isoclinal Folds . . . 93 

28. Isoclinal Folds . . ... 94 

29. Isoclinal Folds .... .... 94 

30. Overfold passing into Reversed Fault, or Overthrust . 95 



xii LIST OF ILLUSTRATIONS 

FIGURE 

31. Reversed Fault ... . . 

32. Single Thrust-plane . 

33. Section across Coal-basin of Mons (M. Bertrand) 

34. Section from Quinaig to Head of Glenbeg {Geol. Survey) 

35. Synclinal Double-fold 

36. Anticlinal Double-fold . 

37. Diagram of Mountain Flexures . ... 

38. Diagram of Anticlinal Mountains ... 

39. Synclinal Valley shifting toward Anticlinal Axis 

40. Section across the Swiss Alps (A. Heim) . 

41. Summit of Santis, East Side (A. Heim) 

42. Section across the Schortenkopf, Bavarian Alps (E. Fraas) 

43. Section across the Kaisergebirge, Eastern Alps (E. Fraas) 

44. Section across the Val d'Uina (Gilmbel) .... 

45. Sichelkamm of Wallenstadt (Heim) .... 

46. Section across the Northern Limestone Alps (E. Fraas) . 

47. Section across the Diablerets (Renevier) . 

48. Section across Dent de Morcles (Renevier) 

49. Inversion and Overthrust in the Mountains South of the Lake of 

Wallenstadt (E. Fraas, after A. Heim) .... 

50. Symmetrical Flexures of the Jura Mountains .... 

51. Section across Western part of the Jura Mountains (P. Choffat) 

52. Section across part of the Sandstone-zone of the Middle Carpathians 

(Vacek) . 

53. Section across part of the Middle Carpathians (Vacek) 

54. Section across the Appalachian Ridges of Pennsylvania (H. D, 

Rogers) . . . .... 

55. Unsymmetrical Folds, giving rise to Escarpments and Ridges 

56. Structure of the Ardennes (after Cornet and Briart) . 

57. Diagrammatic Section across a Plateau of Erosion . 

58. Section across portion of Southern Uplands, showing Old Red 

stone resting upon Plain of Erosion .... 

59. Section from Glen Lyon to Carn Chois (Geol. Survey) 

60. Section of Normal Fault ... ... 

61. Normal Fault, with High Ground on Downthrow Side . 

62. Normal Fault, with High Ground on Upcast Side . 

63. Faults in Queantoweep Valley, Grand Canon District (Dutton) 

64. Ranges of the Great Basin (Hinman, after Gilbert : length of section 

120 miles) 

65. Section from the Mediterranean across the Mountains of Palestine to 

the Mountains of Moab (after M. Blanckenhorn) . 

66. Section across the Vosges and the Black Forest (after Penck) 



Sand- 



LIST OF ILLUSTRATIONS xiii 

FIGURE PAGE 

67. Section of Coal-measures nearCambusnethan, Lanarkshire, on a true 

scale .... ....... 166 

68. Section on a true scale across " Tynedale Fault," Newcastle Coal-field 168 

69. Section across Great Fault bounding the Highlands near Bimam, 

Perthshire .... . .... 169 

70. Section across Great Fault bounding the Southern Uplands . 170 

71. Diagram Section across Horstgebirge . . . . . .170 

72. Mountain of Granite 175 

73. Plain of Granite overlooked by Mountains of Schists, etc. . . 176 

74. Diagrammatic Section of a Laccolith showing Dome-shaped Eleva- 

tion of Surface above the Intrusive Rock (after G. K. Gilbert) 177 

75. View of Necks — Cores of old Volcanoes (Powell) .... 188 

76. Section of Highly Denuded Volcano, Minto Hill, Roxburgshire . 189 

77. Diagrammatic Section across the Valley of the Tay, near Dundee . 190 

78. View of Mesa Verde and the Sierra el Late, Colorado (Hayden's Re- 

port for 1875) .... . . . 203 

79. Wind Erosion : Table-Mountains, etc., of the Sahara (Mission de 

Chadames) . . ... ... 254 

80. Wind Erosion : Harder Beds amongst inclined Cretaceous Strata, 

Libyan Desert (J. Walther) 254 

8i. Wind Erosion : Stages in the Erosion and Reduction of a Table- 
mountain (J. Walther) 255 

82. Manganese Concretions weathered out of Sandstone, Arabah Mount- 

ains, Sinai Peninsula (J. Walther) . 256 

83. Formation of Sand-dunes . . . ... 259 

84. Advance of Sand-dunes ..... ... 259 

85. Longitudinal Sections of Lake-basins on a true scale . . . 293 

86. Sea-cliff cut in Horizontal Strata . . .... 319 

87. Sea-cliff cut in Strata dipping Inland 320 

88. Sea-cliff cut in Strata dipping Seaward . .... 320 

89. Sea-cliff cut in Beds dipping Seaward ... . 323 

FULL-PAGE PLATES 

Plate I. Joints in Granite, Glen Eunach, Cairngorm (from a photograph 

by W. E. Carnegie Dickson) to face 200 

Plate II. Weathering of Joints in Granite, Cairngorm Mountains (from a 

photograph by W. E. Carnegie Dickson) . . . to face 202 



EARTH SCULPTURE 



CHAPTER I 

IX TROD UCTOR V 

EARLY VIEWS AS TO ORIGIN OF SURFACE-FEATURES ROCKS AND 

ROCK-STRUCTURES ARCHITECTURE OF THE EARTH'S CRUST 

GENERAL EVIDENCE OF ROCK-REMOVAL. 

WHEN geologists began to inquire into the origin 
of surface-features, they were at first led to 
believe that the more striking and prominent of these 
had come into existence under the operation of forces 
which had long ago ceased to affect the earth's crust 
to any marked extent. It is not hard to understand 
how this conception arose. The earlier observers 
could not fail to be impressed by the evidence of 
former crustal disturbances which almost ever^'where 
stared them in the face. Here they saw mountains 
built up of strangely fractured, contorted, and jum- 
bled rock-masses ; there, again, they encountered the 
relics of vast volcanic eruptions in regions now practi- 
cally free from earth-throes of any kind. In one place 
ancient land-surfaces were seen intercalated at inter- 



2 EARTH SCULPTURE 

vals among great successions of marine strata ; in 
other places, limestones, evidently of oceanic origin, 
were found entering into the framework of lofty 
mountains far removed from any sea. It was these 
and similar striking contrasts between the present 
and the past which doubtless induced the belief that 
the earth's crust, after having passed through many 
revolutions more or less catastrophic in character, had 
at last become approximately stable — the occasional 
earthquakes and volcanic disturbances of recent times 
being looked upon as only the final manifestations of 
those forces which in earlier ages had been mainly 
instrumental in producing the varied configuration of 
the land. Mountains and valleys belonged to earth's 
Sturm und Di^ang period. That wild time had 
passed away, and now old age, with its lethargy and 
repose, had supervened. The tumultuous accumula- 
tions of stony clay, blocks and boulders, gravel and 
sand that overspread extensive areas in temperate 
latitudes were believed to be the relics of the last 
great catastrophe which had affected the earth's sur-. 
face. Some notable disturbance of the crust, it was 
thought, had caused the waters of northern seas to 
rush in devastating waves across the land. When 
these diluvial waters finally retired, then the modern 
era began — an era characterised by the more equable 
operation of nature's forces. 

But with increased knowledge these views gradu- 
ally became modified. Eventually, it was recognised 
that no hard-and-fast line separates past and present. 



INTRODUCTORY 3 

The belief in world-wide, or nearly world-wide, cata- 
strophes disappeared. Geologists came to see that the 
fashioning of the earth's surface had been going on 
for a long time, and is still in progress. The law of 
evolution, they have found, holds true for the crust 
of the globe just as it does for the myriad tribes of 
plants and animals that clothe and people it. It is no 
longer doubted that the existing configuration of the 
land has resulted from the action of forces that are 
still in operation, and by observation and reasoning 
the history of the various phases in the evolution of 
surface-features can be unfolded. No doubt the 
evidence is sometimes hard to read in all its details, 
but its general bearing can be readily apprehended. 
The salient facts, the principal data, are conspicuous 
enough, and the mode of their interpretation is in a 
manner self-evident. 

In setting out upon our present inquiry, however, 
it is obvious that we ought, in the first place, to know 
something about rocks and the mode of their arrange- 
ment. We must make some acquaintance with the 
composition and the structure or architecture of the 
earth's crust before we can form any reasonable con- 
clusion as to the origin of its surface-features. Now, 
so far as that crust is accessible to observation, it is 
found to be built up of two kinds of rock, one set be- 
ing of igneous origin, while the other appears to con- 
sist mainly of the products of water action. These 
last are typically represented by such rocks as con- 
glomerate, sandstone, and shale, which are only more 



4 EARTH SCULPTURE 

or less ancient sediments, formed and accumulated in 
the same way as the gravel, sand, and mud of existing 
rivers, lakes, and seas. Another common rock of 
aqueous origin is limestone, of which there are count- 
less varieties — some formed in lakes, like the shell- 
marls of our own day ; others representing the 
calcareous ooze and coral-reefs of ancient seas ; while 
yet others are obviously chemical precipitates from 
water surcharged with carbonate of lime. Now and 
again, also, we meet with rocks of terrestrial origin, 
such, for example, as many beds and seams of peat, 
lignite, and coal, which are simply the vegetable ddbris 
of old land-surfaces. To these land-formed beds we 
may add certain sandstones of wind-blown origin- 
indurated sand-dunes, in short. 

The igneous rocks consist partly of lavas and frag- 
mental materials which have been ejected at the sur- 
face, as in modern volcanoes, and partly of formerly 
molten masses which have cooled and consolidated 
below ground. The former, therefore, are spoken of 
as volcanic, the latter as plutonic or hypogene rocks. 
As it is useful to have some general name for the 
rocks which owe their origin to the action of epigene 
agents {i. e., the atmosphere, terrestrial water, ice, the 
sea, and life), we may term these derivative, since they 
have been built up chiefly out of the relics of pre-ex- 
isting rocks and the debris of plants and animals. By- 
and-by we shall learn that igneous and derivative 
rocks have in certain regions been subjected to many 
remarkable changes, and are in consequence so 



INTRODUCTORY 5 

altered that it is often hard to detect their original 
character. These altered masses form what are called 
the metamorphic rocks. They are typically repre- 
sented by such rocks as gneiss, mica-schist, clay-slate, 
etc. 

The derivative rocks, with which in many regions 
igneous rocks are associated, occupy by far the larger 
portion of the land-surface, entering abundantly into 
the composition of low grounds and mountains alike. 
Most of these derivatives are sedimentary accumula- 
tions, and very many are charged with the remains of 
animals and plants. By noting the order in which 
such stratified deposits occur, and by comparing and 
correlating their fossils, geologists have been able to 
group them into a series of successive systems, the 
oldest being that which occurs at the bottom of the 
series.^ The united thickness of the several systems 
probably exceeds twenty miles, but it must not be 
supposed that all these occur together in any one 
region. Many broad acres of the earth's surface are 
occupied by the rocks belonging to one system only. 
In other countries two or more systems may be present. 
Again, each individual system is of very variable 
thickness — swelling out here, thinning off there : in 
some lands being represented by strata many thou- 
sands of feet in thickness, in others dwindling down 
to a few yards. In short, we may picture to ourselves 
each system as consisting of a series of larger and 
smaller lenticular sheets, irregularly distributed over 

' See Appendix for Table of Geological Systems. 



6 EARTH SCULPTURE 

the earth's surface. The various systems thus fre- 
quently overlap, the younger stealing over the surface 
of the older so as often to bury these out of sight. 

The metamorphic rocks do not appear at the sur- 
face over such extensive areas as those just referred 
to. Nevertheless, they are widely distributed, and 
now and again overspread continuously vast regions. 
The enormous tract that extends from the Great Lakes 
of North America to the shores of the Arctic Ocean 
is almost entirely occupied by them. Another im- 
mense area of crystalline schistose rocks is met with 
in Brazil. The Highlands of Scotland, the Scandi- 
navian Peninsula, and North Finland are in like man- 
ner largely composed of them, and the same is the 
case with many parts of Africa, Asia, and Australia. 
It is further noteworthy that similar rocks form the 
backbones of most of the great mountain chains of 
the globe. As already indicated, metamorphic rocks 
are of various origin, some of them being primarily 
of igneous and others of aqueous formation. Those 
which form the nuclei of the youngest mountain 
chains are sometimes of relatively recent age, while 
those occupying such broad tracts as Brazil, the 
Canadian uplands, etc., are of vast antiquity. Crys- 
talline schistose rocks, with associated granites and 
other igneous rocks, seem everywhere to underlie 
the sedimentary fossiliferous formations. Very often 
the latter are separated by a broadly marked line of 
demarcation from the schists, granites, etc., upon which 
they repose. In other cases the sedimentary rocks 



IN TROD UCTOR Y 7 

become gradually altered as they are traced down- 
wards, until eventually they themselves assume the 
aspect of crystalline schists, penetrated here and there 
by granitoid igneous rocks. 

The origin of those ancient crystalline schists has 
been much discussed, but does not concern us here. 
Some geologists have maintained that the rocks in 
question represent the original cooled crust of the 
globe, while the majority consider them to be all 
metamorphic. It is enough for our present pur- 
pose to know that a pavement of such rocks appears 
everywhere to underlie the sedimentary fossiliferous 
formations. 



Fig. I. Section of Horizontal Strata. 

The upper continuous line, A-B^ = surface of ground ; the lower continuous line, C-D, = sea- 
level; /, limestone ; «, sandstones and shales. 

The great bulk of the derivative rocks being of 
sedimentary origin, it is obvious that they must have 
been at the time of their formation spread out in ap- 
proximately horizontal layers upon the beds of ancient 
lakes and seas. This we are justified in believing 
by what we know of the accumulation of similar 
sediments in our own day. The wide flats of our river- 
valleys, the broad plains that occupy the sites of silted- 
up lakes, the extensive deltas of such rivers as the 
Nile, the Po, the Amazon, the Mississippi, the narrow 



8 EARTH SCULPTURE 

or wide belts of low-lying land which within a recent 
period have been gained from the sea, are all made up 
of various kinds of sediment arranged in gently in- 
clined or approximately horizontal layers. Now, over 
considerable areas of the earth's surface the derivative 
rocks show the same horizontal arrangement, a struct- 
ure which is obviously original. And this is frequently 
the case with younger and older sedimentary strata 
alike. Here, for example (Fig. i), is a section across 
a country, the superficial rock-masses of which are 
horizontally arranged. 

The upper line of the section {A-E) represents, of 
course, the surface of the ground, while the lower we 
shall take to be the level of the sea. The section 
thus shows the geological structure or arrangement 
of the rocks from the surface down to the level of the 
sea. The strata represented consist of a great series 
of sandstones and shales with one prominent bed of 
limestone (/) at the top. In this case we cannot 
doubt that the horizontal bedding is original — that 
the strata were accumulated one above the other in 
the same order as we see them. 

Although such horizontal arrangements are of com- 
mon enough occurrence, and now and again charac- 
terise the sedimentary systems over wide areas, yet, 
as a general rule, strata tend to be inclined. In many 
regions the inclination, or dip, as it is termed, is some- 
times very high — not seldom indeed the beds are seen 
standing on end, like rows of books in a library. This 
last appearance of extreme disturbance is not confined 



INTRODUCTORY 9 

to the strata of any system ; nevertheless, it is more 
characteristic of the older than the younger systems. 
In the sequel we shall have to study these and other 
rock-structures more particularly, but for the present 
we need not do more than make some general 
acquaintance with them. 

A very common arrangement is shown in the next 
diagram (Fig. 2). Here the strata are arranged in 
the form of a truncated arch, or anticline. At X the 



•^.■ 




Y 

Fig. 2. Section Across an Anticline. 

The upper continuous line, A-B^ = surface of ground ; tlie lower continuous line, f -i>, = sea- 
level; J^-y^ = vertical axis. 

beds are approximately horizontal, but from this point 
they dip on the right towards jB, and on the left in 
the direction of A. Note further that the angle of 
inclination is the same on each side of the anticline ; 
in other words, the anticlinal axis {X- V) is vertical. 
From A to B the distance we shall suppose is six 
miles. 

The succeeding section (Fig. 3) we shall take to 
be of equal length. Here we have a succession of 
anticlines, or saddle-backs, separated one from another 
by troughs, or synclines, as they are termed. In other 



lo EARTH SCULPTURE 

words, the strata are undulating. From these sections 
we learn that folds or undulations vary considerably 
in width. In the region represented by Fig. 2 we 
have an area six miles in breadth, consisting of a 
thick series of strata disposed in the form of one sin- 
gle arch or anticline ; while in Fig. 3, representing 



*..E 




Fig. 3. Section across Symmetrical Anticlines and Synclines. 

Upper continuous line, A-B^ = surface of ground ; lower continuous line, C~D^ = sea-level ; 
a a, anticlines ; j j, synclines ; a jr, J ;r, axes of folds. 

an equal area, the strata are folded into a series of 
several anticlines and synclines. In both regions the 
anticlines are symmetrical ; that is to say, their axes 
{a X, s x) are vertical. 

But folds or undulations may follow each other 
much more rapidly than is shown in the preceding 
section. In countries built up of steeply inclined 



■ ^ 




Fig. 4. Section across Unsymmetrical Anticlines and Synclines. 

Upper continuous line, ^ -.5, = surface of ground; lower continuous line, C-X>, = sea-level; 
a x^s J', axes of folds. 

rocks, the undulations of the strata are more abrupt, 
and the axes of the folds are frequently inclined. In 



IN TROD UCTORY 1 1 

Fig. 4, for example, most of the anticlines and syn- 
clines lean over to one side, and this to such a de- 
gree, that here and there upper beds are doubled un- 
der older beds of the same series of strata ; in other 
words, the order of succession appears to be inverted. 
From the fact that strata are generally inclined 
from the horizontal, and frequently curved and folded, 
it is obvious that they have been subjected to the 
action of some great disturbing force, for folding and 




Fig. 5. Section across Faulty or Dislocated Strata. 

./, normal fault, incUoed in the direction of downthrow. 



contortion may affect masses of strata many thousands 
of feet in thickness. Another evident mark of dis- 
turbance is furnished by the presence of dislocations, 
or faults, as they are technically termed, along the 
line of which the rocks have been shifted for, it may 
be, hundreds and sometimes even for thousands of 
feet. One of the simplest kind of faults is shown in 



12 EARTH SCULPTURE 

the preceding illustration (Fig. 5). Here, as in 
preceding figures, the upper line (A-B) represents 
the surface of the ground. At f the strata are tra- 
versed by a fault, which has caused a vertical displace- 
ment of the beds to the extent of, say, 500 feet, for it 
is obvious that the coal and fireclay (8, 9), and the 
strata amongst which they. lie on the left-hand side, 
were formerly continuous with the corresponding 
beds on the other side of the fault. 

From the facts now briefly set forth we may draw 
certain conclusions. In the first place, the extensive 
geographical range of the derivative rocks, most of 
which are of marine origin, must convince us that the 
greater portion of our continental areas has been un- 
der water. It is not to be understood, however, that 
all the land-surfaces occupied by sedimentary strata 
have been submerged at one and the same time. On 
the contrary, the several geological systems have been 
accumulated at widely different periods. This is a 
point, however, to which we shall return : for the 
present, we need only keep in view the prominent 
fact that the existing land-surfaces of the globe are 
composed most frequently of marine strata. There 
are apparently only two ways in which this phenom- 
enon can be accounted for, and these explanations 
come to much the same thing. Either the general 
level of the ocean has fallen, or wide areas of the sea- 
floor have been pushed up from below and converted 
into dry land. Both changes appear to have taken 
place. The bed of the sea has sunk from time to 



INTRODUCTORY 13 

time to greater and greater depths, and has thus 
tended to draw the water away from the surface of 
what are now continental areas. But if the earth's 
crust under the ocean has subsided, it has also been 
elevated within what are now dry lands again and 
again. The folds and corrugations of the strata, and 
the numerous dislocations by which rocks of all kinds 
are traversed, clearly demonstrate that movements of 
the solid crust have taken place. Such crustal dis- 
turbances are probably in chief measure due to the 
fact that the earth is a cooling body. As the solid 
crust sinks down upon the cooling and contracting nu- 
cleus, it must occupy less superficial space. Hence its 
rocky framework becomes subjected to enormous tan- 
gential squeezing and compression to which it yields 
by bending and folding, by fracture and displacement. 
Obviously, then, the mysterious subterranean forces 
must have played an important part in the formation 
of earth-features. Disturbed rocks are of more fre- 
quent occurrence than strata which have retained 
their original horizontality. It is no wonder, there- 
fore, that for a long time the general configuration of 
the land was believed to have been impressed upon it 
by plutonic agency. Indeed, in the case of certain 
mountain chains, we cannot fail to see that the larger 
features of such regions often correspond to a con- 
siderable extent with the main flexures and displace- 
ments of the underlying rocks. In many elevated 
tracts, however, composed of highly disturbed and 
contorted strata, no such coincidence of surface-feat- 



14 EARTH SCULPTURE 

ure and underground structure can be traced. The 
mountain ridges do not correspond to great swellings 
of the crust ; the valleys neither lie in trough-shaped 
strata, nor do they coincide with gaping fractures. 
Again, many considerable mountains are built up of 
rocks which are not convoluted at all, but arranged in 
horizontal beds. More than this, many plateaux and 
even lowlands are composed of as highly flexed and con- 
torted strata as are to be met with in any mountainous 
country. Evidently, therefore, crustal movement is not 
the only factor in the production of surface-features. 

The sections already given will serve to illustrate 
the general fact that underground structure and su- 
perficial configuration do not necessarily correspond. 
Thus in Fig. i we have a series of pyramidal mount- 
ains developed in horizontal strata. The slope of 
the surface, therefore, frequently bears no relation to 
the "He" of the beds below. This is further illus- 
trated in the succeeding figures, where we find de- 
pressions at the surface, while the rocks immediately 
underneath show an anticlinal arrangement ; and, con- 
versely, where the strata are trough-shaped the sur- 
face-feature is not a depression but an elevation. 

In the case of the horizontal strata shown in Fig. 
I we have no difficulty in perceiving that the present 
surface is not that of original deposition. It is impos- 
sible that sedimentary deposits could have been piled 
up in the shape of great pyramids : obviously the beds 
were formerly continuous, as shown by the dotted lines. 
Clearly some " monstrous cantles " have been cut out 



IN TROD UCTOR Y 1 5 

and removed. And the same is necessarily true of 
the folded strata. In each case (Figs. 2, 3, 4) masses 
of strata have disappeared ; the tops or backs of the 
anticlinal arches have been more or less deeply incised, 
and the material carried away. In subsequent pages 
it will be shown that the thickness of rocks thus re- 
moved can be proved to amount in many cases to 
thousands of feet. 

Not less striking is the evidence of rock-removal 
furnished by the phenomena of faults. At the sur- 
face there may be no inequality of level corresponding 
to that seen below (see Fig. 5). Obviously, how- 
ever, a considerable thickness of rock has vanished. 
Were the missing continuations of the strata to be 
replaced upon the high side of the fault, they would 
occupy the space contained within the dotted lines 
above the present surface A—B. Such dislocations 
often interrupt the continuity of the strata in our coal- 
fields. In such regions we may traverse level or 
gently undulating tracts, and be quite unconscious of 
the fact that geologically we have several times leaped 
up or jumped down hundreds of feet in a single step. 
Nay, some rivers flow across dislocations by which 
the strata have been shifted up or down for thousands 
of yards, and in some places we may sit upon rocks 
which are geologically more than a thousand fathoms 
below or above those on which we rest our feet. 
Faults, then, afford clear evidence of the wholesale 
removal of rocks from the surface of the land. 

Such proofs of rock-removal can be appreciated by 



i6 EARTH SCULPTURE 

anyone, and will come frequently before us in the 
discussion that follows. There is another kind of 
evidence, however, leading to the same general con- 
clusion, which may be briefly touched upon at this 
stage of our inquiry. In this and other countries 
there are enormous masses of rock, often widely ex- 
tended, which have cooled and consolidated from a 
state of igneous fusion. Some of these, it is well 
known, have flowed out as lavas at the surface, while 
others were never so erupted, but have solidified 
at greater or less depths below ground. Among the 
latter is granite, a rock believed to be of deep-seated 
origin. Its plutonic character is evinced not less by 
its composition and structure than by its relation to 
the rock-masses that surround it. Every mass of 
granite, then, has cooled and consolidated, probably 
very slowly, and certainly at a less or greater depth 
in the earth's crust. When this rock is met with over 
a wide area at the actual surface, therefore, — forming, 
it may be, great mountains or rolling and broken low- 
lands, — we know that in such regions thick masses of 
formerly overlying rocks have been removed. The 
granite appears at the surface simply because the 
covering of rocks underneath which it cooled and 
solidified has been subsequently carried away. 

The occurrence at the surface of crystalline schists 
and other metamorphic rocks has a similar significance. 
Although the processes by which rocks become so 
highly altered are still more or less obscure, yet there 
can be no doubt that the metamorphism had taken 



INTRODUCTORY 17 

place when the rocks affected were more or less 
deeply buried in the crust. 

While we may safely infer, from the general phe- 
nomena of geological structure, that earth-movements 
have shared in the production of surface-features, we 
must be convinced, at the same time, that some other 
factor has aided in the work of shaping out our lands. 
Earth-movements quite account for the folding and 
fracturing of strata, for the uplifting of great mount- 
ain masses, but they cannot have caused the general 
loss which these masses have sustained. We may 
conceive it possible that subterranean action may 
now and again have resulted in wide-spread shattering 
of rocks at the surface, but such action could not have 
caused the broken material to disappear. Further, 
when we bear in mind that the thickness of rock 
removed from the surface of the land is sometimes to 
be measured by many thousands of feet, or even yards, 
we see at once that subterranean action cannot have 
been directly implicated in the spoliation of the land. 
How, then, have anticlines been truncated ? What 
power has removed the strata from the high side of a 
fault ? What, in a word, has produced that trunca- 
tion and discontinuity of beds which is so common a 
feature of derivative rocks all the world over ? And 
how shall we account for the presence at the sur- 
face of deep-seated plutonic rocks and metamorphic 
masses ? When we have satisfactorily answered 
such questions we shall have solved the problem of 
the origin of surface-features. 



CHAPTER II 

AGENTS OF DENUDATION 

CHEMICAL COMPOSITION OF ROCKS — EPICENE AGENTS — INSOLA- 
TION AND DEFLATION CHEMICAL AND MECHANICAL ACTION 

OF RAIN ACTION OF FROST ; OF PLANTS AND ANIMALS ; OF 

UNDERGROUND WATER ; OF BROOKS AND RIVERS RATE OF 

DENUDATION DENUDATION AND SEDIMENTATION GO HAND 

IN HAND. 

TH E present, geologists tell us, contains the key to 
the past. If we wish to find out how rocks have 
been removed, and what has since become of them, 
we must observe what is taking place under the 
influence of existing agents of change. How, then, 
are rocks being affected at present ? We do not pro- 
ceed far in our investigation before we discover that 
they are everywhere becoming disintegrated. In one 
place they are breaking up into angular fragments ; 
in another, crumbling down into grit, sand, or clay. 
Brooks and rivers and the waves upon our coasts are 
constantly undermining them ; everywhere, in short, 
rocks are being assaulted and reduced. But in order 
to bring this fact more forcibly before the reader, it 
will be well to sketch, as briefly as may be, the general 
character of the warfare which is being waged against 



AGENTS OF DENUDATION 19 

rocks over all the land-surface, and to note the various 
results that flow from this incessant energy of the 
epigene or superficial agents of change. 

As these agents are often associated in their work, 
it is sometimes hard, or even impossible, to say which 
has played the most effective part in the demolition 
of rocks. Nevertheless, it will conduce to clearness 
if we endeavour to consider the operation of each by 
itself, so far, at least, as that is possible. Before 
doing so, however, we must glance for a moment at 
the general characters of rocks. We have already 
taken note of the fact that rocks are of various origin 
— igneous, derivative, and metamorphic. It is now 
necessary to consider their composition and structure, 
for, according as these differ, rocks are variously 
affected by epigene agents, some yielding rapidly, 
others being more resistant. We need not go into 
detail. Their composition and structure may be de- 
scribed in the most general terms. For our purpose 
it will suffice to group them roughly under these four 
heads : Felspathic, Argillaceous, Silicious, and Cal- 
careous rocks. This is very far from being an 
exhaustive classification, but under these groups may 
be included all the rocks that enter most largely into 
the formation of the earth's crust. 

I. Felspathic Rocks. These rocks contain as their 
dominant constituent the mineral, or, rather, the 
family of minerals, known under the name of felspar. 
The group includes nearly all the igneous and most 
of the metamorphic rocks. The derivative rocks that 



20 EARTH SCULPTURE 

come under the same head are of relatively small 
importance. The minerals entering most abundantly 
into the composition of the felspathic rocks are the 
felspars (aluminous silicates of potash, soda, and lime), 
various ferro - magnesian silicates, such as 7nica, py- 
roxene, hornblende, and olivine (aluminous silicates 
of magnesia, lime, iron-oxides, etc.), and quartz (silica, 
silicic acid). The crystalline igneous rocks occur 
either in more or less regular beds (lavas), interstrati- 
fied with derivative rocks, or they penetrate these in 
the form of irregular veins, dykes, sheets, or large 
amorphous masses. The lava-form rocks are ofter\^ 
associated with beds of volcanic debris (tuff, etc.). 
Some igneous rocks are smoothly compact in texture, 
such as obsidian and pitchstone, which are simply 
varieties of volcanic glass ; others, such as basalt, 
consist partly of glass and partly of crystalline in- 
gredients, and vary in texture from compact to coarse- 
grained ; yet others are built up wholly of crystalline 
substances, and may be fine-grained or very coarsely 
granular, as granite. The crystalline schists are 
equally variable as regards texture. They differ, 
however, from the igneous rocks in structure. While 
the latter are confusedly crystalline, the schists show 
a kind of streaky structure or pseudo-lamination, 
their constituent minerals being arranged in rudely 
alternate lenticular layers. 

Igneous rocks and schists are traversed by cracks 
and fissures which usually ramify irregularly in all 
directions. In many bedded igneous rocks (lavas). 



AGENTS OF DENUDATION 21 

however, these cracks, or "joints," as they are termed, 
are somewhat more regular, being, as a rule, disposed 
at approximately right angles to the planes of bed- 
ding. In certain fine-grained rocks, such as basalt, 
the jointing is often very regular, giving rise to a 
prismatic columnar structure, as in the basalts of Staffa 
and the Giant's Causeway. The main fact, however, 
with which we are at present concerned is simply this : 
that all crystalline, igneous, and schistose rocks are 
traversed by cracks and fissures of one sort or another. 
It is further to be noted that these rocks, in common 
with rocks of all kinds, are more or less porous, and 
therefore liable to be permeated, however slowly, by 
percolating water. 

2. Argillaceous Rocks. These rocks are composed 
chiefly of clay, but other ingredients are usually pre- 
sent. Some are soft, such as ordinary brick-clay ; 
others are of firmer consistency, and frequently show 
a fine fissile structure, as in common argillaceous 
shale ; yet others are hard, tough rocks, some of 
which are capable of being cleaved into thin plates, 
as roofing-slate. 

3. Silicious Rocks. These might be described in 
general terms 'as gravel-and-sand rocks. The most 
abundant and widely distributed rocks of this class 
are the sandstones — composed generally of grains of 
quartz (silica) cemented together by carbonate of 
lime, by iron-oxide, or other substance. Cementing 
material, however, is not always present, some sand- 
stones having been solidified by pressure alone. The 



22 EARTH SCULPTURE 

gravel-rocks, or conglomerates, usually consist of 
rounded fragments of quartz or some hard siJicious 
rock. But to this there are exceptions, the stones in 
some conglomerates consisting of calcareous or of 
felspathic rocks or of a mixture of many different 
kinds. A silicious sandstone which has been more or 
less metamorphosed is termed quartz-rock. 

4. Calcareous Rocks. Under this head are grouped 
limestones of every kind. They vary in character 
from soft earthy marls and chalks to hard, granular, 
crystalline limestones and saccharoid marbles. Some 
are nearly pure carbonate of lime ; others contain 
larger or smaller percentages of quartz, clay, iron- 
oxide, and other impurities. 

The Argillaceous, Silicious, and Calcareous groups 
comprise the great bulk of the derivative rocks as 
well as a few metamorphic rocks. They are all origin- 
ally of aqueous or sedimentary origin, and generally 
occur, therefore, in beds or strata. Like the igneous 
rocks, they are more or less porous, although some — 
especially the clay-rocks — are much less permeable 
than others. In addition to the planes of lamination 
and stratification, which characterise most derivative 
rocks, there are other natural division-planes or joints 
which cut across the strata in directions more or less 
perpendicular to the bedding. More irregular usually 
are the joints which intersect hard slates and quartz- 
rock, these being divided generally much in the same 
way as schists and amorphous masses of crystalline 
igneous rock. 



AGENTS OF DENUDATION 23 

There are not a few kinds of rock other than those 
now referred to, but they may be neglected as, from 
our present point of view, of relatively little import- 
ance. Amongst them are rock-salt, gypsum, coal 
and lignite, ironstones, and other ores. All these, 
doubtless, are very notable and valuable, but they are 
neither so abundant nor so widely distributed as the 
above-described groups ; in short, they occupy a very 
subordinate place in the architecture of the earth's 
crust. 

We have now to consider how the superficial or 
epigene agents attack and reduce rocks. And first, 
we may note that rocks at the surface are everywhere 
subject to changes of temperature — warmed by day 
and during summer, cooled at night and during win- 
ter. Thus they alternately expand and contract, and 
this tends to disintegration, for the materials of which 
they are composed often yield unequally to strain or 
tension. This is particularly the case with many crys- 
talline felspathic rocks, such as coarse-grained granite, 
gneiss, and mica-schist — built up, as these are, of min- 
erals that differ in colour, density, and expansibility. 
Even when a rock is homogeneous in composition, it 
is obvious that the heating and cooling of the surface 
must give rise to strain and tension. In countries 
where there is no great diurnal range of temperature, 
as in our own latitudes, any rock-changes due to this 
cause alone are hardly noticeable, since they are 
masked or obscured by the action of other and more 
potent agents. But in the rocky deserts of tropical 



24 EARTH SCULPTURE 

and sub-tropical regions, bare of verdure and practi- 
cally rainless, the effects produced by alternate heat- 
ing and cooling are very marked. The rocks are 
cracked and shattered to a depth of several inches ; 
the surfaces peel off, and are rapidly disintegrated 
and pulverised. Wind then catches up the loose ma- 
terial and sweeps it away, leaving fresh surfaces ex- 
posed to the destructive action of insolation. More 
than this, the grit, sand, and dust carried off by the 
wind are used as a sand-blast to attack and erode the 
rocks against which they strike. In this manner cliffs 
and projecting rocks are undermined, and masses give 
way and fall to the ground, where, subject to the same 
grinding action, especially towards the base, they 
eventually assume the appearance of irregular blocks 
supported upon pedestals. Mushroom-shaped rocks 
and hills of this kind are common in all desiccated 
rocky regions. 

The transporting action of the wind, or " deflation," 
as it is termed, goes on without ceasing day and night 
and during all seasons ; and the result is seen in the 
deeply eroded rocks, enormous masses of which, it 
can be shown, have been thus gradually removed. 
The evidence of denudation is conspicuous, but its 
products have for the most part been carried away. 
In some places, as Professor Walther remarks of the 
Libyan Desert, are great walls of granite rising to 
heights of 6000 feet, but showing no slopes of de- 
bris below, as would infallibly be present under tem- 
perate conditions of climate. In other places, again, 



AGENTS OF DENUDATION 25 

are deeply excavated w^dies containing no beds of 
gravel, grit, and sand, such as would not fail to show 
themselves had the depressions in question been 
formed by water-action alone. Everywhere, deep, 
cave-like hollows have been worn out in the rocks, 
and yet these hold no sediment or detritus, but are 
swept bare. The wind tends, in short, to transport 
all loose material from the scene of its origin to the 
borders of the desert. 

In latitudes like our own, insolation doubtless shares 
in the disintegration of rocks, but the most conspicu- 
ous agent employed in that work is rain. Rain is not 
chemically pure, but always contains some proportion 
of oxygen and carbonic acid absorbed from the atmo- 
sphere ; and after it reaches the ground organic acids 
are derived by it from the decaying vegetable and 
animal matter with which soils are more or less im- 
pregnated. Armed with such chemical agents, it 
attacks the various minerals of which rocks are com- 
posed, and thus, sooner or later, these minerals break 
up. The felspars and their ferro-magnesian associ- 
ates, for example, are decomposed — the carbonic acid 
of the rain-water uniting with the alkalies and alkaline 
earths of those minerals to form carbonates, which 
are carried away in solution. The silica set free by 
this operation is also to some extent removed, while 
the insoluble silicate of alumina, or clay, remains be- 
hind. Such insoluble materials are frequently stained 
yellow-brown or red, owing to the press«fe of iron- 
oxides. In this way felspathic rocks gradually crum- 



26 EARTH SCULPTURE 

ble down. Thus, granite, gneiss, basalt, and other 
rocks largely composed of felspar, usually show a 
weathered crust, which, according to the nature of 
the rock and the length of time its surface has been 
exposed, may vary from less than an inch up to many 
feet, or even yards, in thickness. Some granites, for 
example, are reduced to a kind of gritty clay which 
may be dug with a spade. 

Argillaceous and silicious rocks are not so readily 
affected by the chemical action of rain. Not infre- 
quently, however, when the grains of a sandstone are 
cemented together by some soluble substance, such as 
carbonate of lime, the rock will yield more or less read- 
ily to the solvent action of the water. All calcareous 
rocks, in short, tend to fall an easy prey. If they 
contain few or no impurities, they " weather " with 
little or no crust ; the rock is simply dissolved. Lime- 
stones, however, are seldom quite so pure as this, but 
are usually impregnated in a greater or less degree 
with quartz, clay, or other substance, which after 
the carbonate of lime has been removed remains 
behind to form a crust. The red and brownish 
earths and clays that so frequently overlie calcar- 
eous rocks, such as chalk and limestone, are simply 
the insoluble residue of masses of rock, the soluble 
portions of which have been dissolved and carried 
away by surface-water. 

In all regions where rain falls, the result of this 
chemical action is conspicuous ; soluble rocks are 
everywhere dissolving, while partially soluble rocks 



AGENTS OF DENUDATION 27 

are becoming rotten and disintegrated. In limestone 
areas it can be shown that sometimes hundreds of 
feet of rock have thus been gradually and silently re- 
moved from the surface of the land. And the great 
depth now and again attained by rotted rock testifies 
likewise to the destructive action of rain-water perco- 
lating from the surface. This is particularly notice- 
able in warm-temperate, sub-tropical, and tropical 
latitudes, where felspathic rocks are decomposed not 
infrequently to depths of a hundred feet and more. 
In temperate and northern regions, the amount of 
rotted rock is rarely so great. The thicker rock- 
crusts of southern latitudes are supposed to be due 
to the larger supplies of organic acids derived from 
the more abundant vegetation. To some extent this 
is probably true. But there is another reason for the 
relatively meagre development of rotted rock in tem- 
perate and northern regions generally. Those re- 
gions, as we shall learn later on, have recently been 
subjected to glacial conditions. Broad areas of tem- 
perate Europe and North America have been scraped 
bare by ice-sheets, resembling those of Greenland and 
the Antarctic Circle. In more southern latitudes, 
the rotted rocks have escaped such abrasion and 
denudation, and hence it is not strange that we should 
find them attaining so great a thickness. The decom- 
posed rock-material encountered in the northern parts 
of Europe and America has been formed for the most 
part only since the disappearance of glacial condi- 
tions, while in southern regions rock-decay has gone 



2 8 EARTH SCULPTURE 

on without interruption ever since those lands came 
into existence. 

The disintegrating action of rain in temperate 
and high latitudes is greatly aided by frost, and the 
same is the case in the elevated tracts of more south- 
ern latitudes. Rain renders the superficial portions 
of rock more porous, and thus enables frost to act 
more effectually ; while frost, by widening pores and 
fissures, affords readier ingress to meteoric water. 
Water freezing in soils and subsoils and in the inter- 
stitial pores and minute fissures of rocks forces the 
grains and particles asunder, and when thaw en- 
sues the loosened material is ready to be carried away 
by rain or melting snow and subsequently, it may be, 
by wind. The same process takes place on a larger 
scale in the prizing open of joints and the rending 
asunder of rocks and rock-masses. Hence in Arctic 
regions and at high levels in temperate and southern 
latitudes the wholesale shattering of rocks has pro- 
duced immense accumulations of angular ddbris. To 
such an extent has this action taken place, that in 
some countries the rocks are more or less completely 
buried in their own ruins. By-and-by so great do 
these accumulations become that frost is unable to 
get at the living rock. The loose fragments, how- 
ever, under which it lies concealed, are themselves 
shattered, crumbled, and pulverised, until they are in 
a condition to be swept away by wind or melting 
snow. By this means the solid rock again comes 
within reach of the action of frost, and so the work of 



AGENTS OF DENUDATION 29 

disruption and disintegration continues. The great 
heaps or " screes " of rock-rubbish which cloak the 
summits and slopes of our mountains, and gather 
thickly along the base of precipice and cliff, have 
been dislodged by frost and rolled down from above, 
their progress downward being often aided by tor- 
rential rains, melting snow, and the alternate freezing 
and thawing of the saturated ddbris itself. 

Some reference has already been made to the indi- 
rect action of plants in the disintegration of rocks. 
The various humus acids, as we have seen, are power- 
ful agents of chemical change. Without their aid 
rain-water would be a less effective worker. The 
living plants themselves, however, attack rocks, and 
by means of the acids in their roots dissolve out the 
mineral matters required by the organisms. Further, 
their roots penetrate the natural division-planes of 
rocks and wedge these asunder ; and thus, by allow- 
ing freer percolation of water, they prepare the way 
for more rapid disintegration. Nor can we neglect 
the action of tunnelling and burrowing animals, some 
of which aid considerably in the work of destruction. 
There can be no doubt, for example, that worms, as 
Darwin has shown, play an important part in the form- 
ation of soil, which is simply rotted rock plus organic 
matter. 

We see, then, that the disintegration and decomposi- 
tion of rocks is a process everywhere being carried 
on — from the crests of the mountains down to the 
sea, and in every latitude under the sun. No exposed 



30 EARTH SCULPTURE 

rock-surface escapes attack. In parched deserts as in 
well-watered regions, in the dreary barrens of the far 
north as in the sunny lands of the south, at lofty ele- 
vations as in low-lying plains, the work of rock-waste 
never ceases. Here it is insolation that is the most 
potent agent of destruction ; there it is rain aided by 
humus and carbonic acids ; or rain and frost combine 
their forces to shatter and pulverise the rocks. In 
latitudes where frost acts energetically, the most con- 
spicuous proofs of rock-waste are the sheets and heaps 
of ddbris that are ever travelling down mountain- 
slopes, or gathering at the base of cliff and precipice. 
In lower latitudes the most impressive evidence of 
disintegration is the great thickness attained by rotted 
rock in positions where it is not liable to be readily 
swept away by running water. 

Hitherto we have been considering the superficial 
parts of rock, as these are affected by weathering. 
We are not to suppose, however, that the alteration 
of a rock ceases immediately underneath its crust. 
Rotted rock is not the only evidence of decay. In 
the case of felspathic rocks, it is found that some of 
the constituent minerals, more especially the felspars, 
usually show traces of decomposition at depths of 
many feet or even yards below the weathered super- 
ficial portions. It is hard, indeed, to get a specimen 
of any such rock from the bottom of our deepest quar- 
ries which is perfectly fresh. Water soaks through 
interstitial fissures and pores, and finds its way by 
joints and other division-planes, so that chemical ac- 



AGENTS OF DENUDATION 31 

tion, with resultant rock-decay, is carried on at the 
greatest depths to which water can penetrate. This 
underground water eventually comes to the surface 
again through similar joints, etc., opening upwards, 
and thus forms natural springs. All these springs 
contain mineral matter, derived from the chemical 
decomposition and solution of rock-constituents. 
Many, indeed, are so highly impregnated, that as soon 
as they are exposed to evaporation they begin to de- 
posit some of their mineral matter. Thus vast quan- 
tities of rock-material are brought up from the bowels 
of the earth. To such an extent is this the case in 
certain regions, that the ground is undermined and 
the surface not infrequently subsides. In countries 
where calcareous rocks largely predominate, acidulated 
water filtering down from the surface through fis- 
sures and other division-planes has often licked out a 
complicated series of tortuous tunnels and galleries. 
So far has this process been carried on in some re- 
gions that the whole rainfall finds its way into subter- 
ranean courses, and the entire drainage of the land is 
conducted underground. The dimensions attained 
by many well-known limestone caverns, and the great 
width and depth of the channels through which sub- 
terranean rivers reach the sea, help us to appreciate 
the amount of rock-material which underground water 
is capable of removing. When we add to this all the 
mineral matter leacl^ed out at the surface and carried 
away by streams and rivers, it is obvious that in 
course of time the land cannot fail to have been con- 



32 EARTH SCULPTURE 

siderably modified by chemical action alone. In point 
of fact, it can be shown that from the surface of cer- 
tain regions hundreds of feet of various calcareous 
rocks have thus been gradually removed ; while in 
other cases the contour of the ground has been nota- 
bly affected by the collapse of underground channels 
and chambers. But if the results of the chemical 
action of meteoric water be most evident in places 
where calcareous rocks predominate, yet the thick- 
ness attained in other countries by the crusts of less 
soluble rocks shows plainly enough that the whole 
land-surface of the globe is affected by the same 
action. 

We may now consider the mechanical action of 
terrestrial water, by means of which the more or less 
insoluble residue of disintegrated rock is removed. 
Weathered rock is generally very porous, and is thus 
readily pulverised by frost. Some crusts crumble 
away as they are formed, while others adhere more 
persistently. On slopes and in mountain-regions 
generally, decomposed and disintegrated materials 
are seldom allowed to remain long in situ — rain and 
melting snow soon sweep away the finer portions. 
Great thicknesses of rotted rock are, therefore, some- 
what exceptional in such places. Where, on the other 
hand, the land-surface is plain-like, or gently undu- 
lating, and the drainage sluggish, weathered materials 
are not so readily removed. Nevertheless, under the 
influence of rain alone, or of rain and melting snow, 
the products of rock-waste are everywhere travelling, 



AGENTS OF DENUDATION 33 

slowly or more rapidly, according to circumstances, 
from higher to lower levels. In temperate latitudes, 
where the rainfall is distributed over the year, this 
transference of material is not so conspicuous as in 
countries where the rainfall is crowded into a short 
season. Even in our own country, however, one may 
observe how in gently undulating tracts rain washes 
the finer particles down the slopes and spreads them 
over the hollows. After exceptionally heavy or long- 
continued rain this process becomes intensified — fine 
mud, silt, sand, and grit are swept into the brooks 
and streams, and the swollen rivers run discoloured to 
the sea. ' Similar floods often result from the, melting 
of snow in spring. During such floods our rivers are 
generally more turbid than when they are swollen 
merely by heavy or continuous rain. When thaw en- 
sues weathered rock-surfaces crumble down, while 
superficial accumulations of disintegrated materials 
become more or less saturated by melting snow. To 
such a degree is this soaking sometimes carried, that 
the whole surface of sloping fields may be set in mo- 
tion. The soils creep, slide, and occasionally flow. 
Not infrequently also the subsoils and disintegrated 
rock-surfaces on steep inclinations collapse and slide 
into the valleys. Everyone, in short, is familiar with 
the fact that flooded rivers are invariably muddy, and 
that the mud or silt which discolours them has been 
abstracted from the land. 

In temperate lands of small extent like England the 
rivers are under ordinary conditions somewhat clear. 



34 EARTH SCULPTURE 

But in continental tracts the larger rivers are always 
more or less turbid. This is due to many causes. 
Some rivers, for example, head in glaciers, and are 
thus clouded at their very origin. Others, again, 
cross several degrees of latitude, and traverse differ- 
ent climatic regions. Hence it will rarely happen 
that snow is not melting or rain falling in some part 
of a great drainage-area. Many rivers, again, after 
escaping from the mountains, flow through countries 
the superficial formations of which are readily under- 
mined and washed away, and thus the main stream 
and its affluents become clouded with sediment. It is 
in tropical and subtropical latitudes, of course, that 
the most destructive effects of rain are witnessed. 
During the wet season the rivers of such regions dis- 
charge enormous volumes of mud-laden water. 

We may conclude, then, that under the influence of 
atmospheric agents rocks are everywhere decomposed 
and disintegrated ; and, further, that there is a uni- 
versal transference from higher to lower levels of the 
materials thus set free. Now and again, it is true, 
there may be long pauses in the journey — the materi- 
als may linger in hollows and depressions. Eventu- 
ally, however, they are again put in motion, and by 
direct or circuitous route, as the case may be, find 
their way into the rivers, and finally come to rest in 
the ocean. The river-systems of the world, then, are 
the lines along which the waste products of the land 
are carried seawards. But rivers are much more than 
mere transporters of sediment. Just as in desert 



AGENTS OF DENUDATION 35 

lands wind employs disintegrated rock-material as a 
sand-blast, so rivers use their stones, grit, and sand 
as tools with which to rasp, file, and undermine the 
rocks over which they flow. In this way their chan- 
nels are gradually deepened and widened. Some of 
the transported material is held in solution, part is 
carried in mechanical suspension, and another portion 
is pushed and rolled forward on the bed. It is the 
solid ingredients, of course, that act as eroding agents. 
While much of the finer sediment finds its way into 
the drainage-system by the agency of rain and melt- 
ing snow, the coarser materials are derived chiefly 
from the destruction of the rocks that underlie or 
overhang the course of a river and its feeders. In 
temperate and northern latitudes natural springs and 
frost are responsible for much of the rock ddbris which 
cumbers the beds of streams, but much also is dis- 
lodged by the undermining action of the water itself. 
Rock-fragments when first introduced are more or less 
angular, but as they travel down stream they often 
break up into smaller pieces along natural cracks or 
joints, and the sharp corners and edges of these get 
worn away by mutual attrition, and by rasping on the 
rocky bed. In this manner the several portions gradu- 
ally become smoothed and rounded — -the process of 
abrasion resulting necessarily in the production of 
grit, sand, silt, etc. Thus in a typical river-course, 
consisting of mountain-track, valley-track, and plain- 
track, we note a progressive change in the character 
of the sediments as the river is followed from its 



36 EARTH SCULPTURE 

source to the sea. In the mountain-track, where the 
course is steep and usually in a rocky channel, angular 
and subangular fragments abound, and the detri- 
tus generally is coarse. In the valley-track, the inclin- 
ation of which is gentle, well-rounded gravel, with 
grit and sand, predominate, the latter becoming more 
plentiful as the plain-track is approached. In the 
plain-track the prevailing sediments are fine sand 
and silt. 

The amount of material removed by a river de- 
pends on the volume of the water, the velocity of the 
current, and the geological character of the drainage- 
area. Thus, the larger the river, other things being 
equal, the greater the burden of sediment. Again, 
a rapid current transports material more effectively 
than a gentler stream, while rivers that flow through 
lands whose rocks are readily eroded carry more 
sediment than rivers of equal volume and velocity 
traversing regions of more resistant rocks. Should a 
lake interrupt the current of a river, all the gravel, 
sand, and mud may be intercepted, and the stream 
will then issue clear and pellucid at the lower end of 
the lake, as the Rhone does at Geneva. The lake, in 
short, acts as a settling reservoir. By and by, how- 
ever, the lacustrine hollow becomes silted up and con- 
verted into an alluvial flat, through which the silt-laden 
water winds its way towards the ocean. Reaching 
that bourn, the current of the river is arrested, and 
its sediment thrown down. Should no strong tidal 
current sweep the coast, removing sediment as it ar- 



AGENTS OF DENUDATION 37 

rives, the sea becomes silted up in the same way as 
the lake, and in time a delta is formed. The growth 
of the latter necessarily depends partly on the activ- 
ity of the river and partly upon the depth of the 
estuary and the action of waves and tidal currents. 
But if nothing interrupted the growth of a delta — 
were all the materials brought down by a river to ac- 
cumulate at its mouth — it is obvious that the rate of 
increase of a delta would enable us to form an esti- 
mate of the rate at which the drainage-area of the 
river was being eroded. It is certain, however, that 
such conditions never obtain. Even in the quietest 
estuaries much of the sediment is carried away by the 
sea. The rate of delta-growth must be exceeded by 
that of fluviatile transport. 

Geologists, however, have adopted another method 
of estimating the loss sustained by the land. They 
can measure the amount of material held in solution, 
and of solid matter carried in suspension and rolled 
forward on the bed of a river. As might have been 
expected, the amount varies with the season of the 
year in each individual river, while different rivers 
yield very different results. But even in the case of 
the least active streams the transported material is 
much more considerable than might have been sup- 
posed. Hence one need not wonder that in spite of 
obstacles the deltas of many rivers advance seawards 
more or less rapidly. The delta of the Rhone, for ex- 
ample, pushes forward at the rate of about 50 feet 
annually, while that of the Po increases by more than 



38 EARTH SCULPTURE 

70 yards, and that of the Mississippi by 80 to 100 
yards in the same time. 

It is sufficiently obvious that the material carried 
seawards by rivers must afford some indication of the 
rate at which the surface of the land is being lowered 
by subaerial action. Having ascertained the annual 
amount discharged by any individual river, we learn, 
at the same time, to what extent the drainage-area of 
that river is being denuded. In the case of the Mis- 
sissippi, for example, it has been calculated that the 
amount of sediment removed is equal to a lowering 
of the whole drainage-area by g ^^ „ th of a foot. In 
other words, could we gather up all the material 
discharged in one year, and distribute it equally over 
the wide regions drained by that river and its tributa- 
ries, we should raise the land-surface by g ^^ ^ th of a 
foot. That does not seem to be much, but at this rate 
of erosion one foot of rock will be removed from the 
Mississippi basin in 6000 years ; and the Mississippi is 
not so active a worker as many other rivers. An aver- 
age of many estimates of the similar work performed 
by rivers in all quarters of the globe shows that the 
rate at which drainage-areas generally are being low- 
ered is one yard in 8000 to 1 1,000 years. It must not 
be supposed that this erosion is equal throughout any 
drainage-area. As a rule, denudation will take place 
most rapidly over the more steeply inclined portions 
of the ground. On mountain declivities and hill 
slopes rock-disintegration and the removal of waste 
products will proceed more actively than upon low 



AGENTS OF DENUDATION 39 

grounds and plains. The work of erosion will be 
carried on most effectively in the torrential tracts of 
streams and rivers. Indeed, we may say that it is in 
valleys generally that we may expect to find the most 
cogent evidence of erosion now in action. 

A little consideration will show that the estimates 
just referred to do not tell us all the truth concerning 
denudation. They show us only the amount of waste 
material which is swept into the sea. They afford no 
indication of the actual amount of rock-disintegration 
and erosion. Rock-rubbish gathers far more rapidly 
in mountain-regions than it can be removed by run- 
ning water. Indeed, over a whole land-surface rocks 
are disintegrated and ddbris accumulates from year to 
year. Nor is the amount of material brought down 
by a river to its mouth an index even to the activity 
of the river itself as a denuding and transporting 
agent. Enormous volumes of detritus are deposited 
in valleys or come to rest in lakes and inland seas. 

Hitherto we have been treating of the work done 
by the atmosphere and running water. Some refer- 
ence has also been made to frost as a potent disin- 
tegrator of rocks. But we have still to consider the 
action of glaciers in modifying the surfaces over which 
they flow. It can be shown that valleys have been 
widened and deepened, and broad areas more or less 
remodelled, by flowing ice, so that glaciers must not 
be ignored in any general account of denuding agents. 
It will be more convenient, however, to leave them 
for the present ; for however interesting and import- 



40 EARTH SCULPTURE 

ant their action may be, it is yet of minor consequence 
so far as the origin of surface-features as a whole is 
concerned. For similar reasons we may delay the con- 
sideration of marine erosion. The action of the sea 
upon the land is necessarily confined to a narrow belt, 
whereas that of the subaerial agents affects the whole 
surface of the land. 

We may take it that the denudation of the surface, 
rendered everywhere so conspicuous by the discon- 
tinuity of strata, has been effected mainly by the at- 
mosphere and running water. Other agents have, 
no doubt, played a part, but those just referred to 
must be credited with the chief share in the work of 
erosion. Such is the general conclusion to which we 
are led by the study of causes now in action. And 
observation and reflection combine to assure us that 
subaerial erosion has been equally effective during 
the formation of all the derivative rocks which enter 
so largely into the framework of the earth's crust. 
For these rocks are for the most part of sedimentary 
origin — they tell us of ancient lakes, estuaries, and 
seas. All their materials have been derived from the 
degradation of old land-surfaces, partly no doubt by 
the sea, but in chief measure by subaerial agents. 
And the great thickness and extent attained by many 
of the geological systems enable us to form some idea 
of what is meant by denudation. What, for instance, 
shall we say of a system composed essentially of sed- 
imentary strata reaching a thickness of several thou- 
sand feet, and occupying an area of many thousand 



AGEXTS OF DENUDATIOX 41 

square miles ? Obviously, the materials of such a 
system have been derived from the waste of ancient 
lands. Mountain-masses must have been disinte- 
grated, and removed in the form of sediment, and 
gradually piled up, layer upon layer, on the floor of 
the sea. Every bed of sedimentary rock, in short, is 
evidence of denudation. 

Further, it has been ascertained that in the build- 
ing up of the various great geological systems the 
same materials have been used over and over again. 
Sediments accumulated upon the sea-bottom have 
subsequently, owing to crustal movements, entered 



Fig. 6. Section across Unconformable Strata. 

a 3, beds of sandstone, shale, etc. \ b b^ conglomerates and sandstone resting discordantly 
or unconfonnably upon a a; u u, line of unconformity'. 

into the formation of a new land-surface, and there- 
after, attacked by the epigene agents of change, have 
again been swept down to sea as gravel, sand, and 
mud. The history of such changes is easily read in 
the rock-structure known as iinconformity . In the 
accompanying section (Fig. 6), for example, two sets 
of strata are shown — the upper {b) resting discord- 
antly or unconformably upon the lower (a). The 
lower series of sandstones and shales is charged with 
the remains of marine and brackish-water organisms 



42 EARTH SCULPTURE 

and of land-plants. The overlying strata {S) are like- 
wise of aqueous origin, and consist chiefly of con- 
glomerates and sandstones below, and of somewhat 
finer-grained sedimentary beds above. Like the older 
series (a), they likewise contain marine and brackish- 
water fossils. The beds {a) introduce us to an estu- 
ary, or shallow bay of the sea, into which sediment is 
carried from some adjacent land. The whole series 
has evidently been deposited in water of no great 
depth, as is shown by the character of the rocks and 
their fossil contents. And as the strata attain a 
thickness of more than 2000 feet, we must infer that 
during their accumulation the sea-floor was slowly 
subsiding, the rate of sedimentation probably keep- 
ing pace with the subsidence. In other words, the 
bed of the sea appears to have been silted up as fast 
as it sank, so that relatively shallow-water conditions 
persisted during the deposition of the land-derived 
sediments. Then a time came when the sea-floor 
ceased to sink and another movement of the crust 
took place, which resulted in the folding of the sedi- 
mentary strata and the conversion of the sea-bottom 
into dry land. The folded rocks were now subjected 
during some prolonged period to the action of the 
various subaerial agents of erosion, whereby the whole 
land-surface was eventually denuded and planed down. 
When the work of erosion had been so far completed, 
the entire region again subsided, and formed the bed 
of a shallow sea. Under these conditions the drowned 
land-surface became overspread in time with new ac- 



AGENTS OF DENUDATION 43 

cumulations of sediment, derived from the degradation 
of adjacent areas that still continued above sea-level. 
The strata {B) are in point of fact largely composed of 
materials derived from the breaking up and disinte- 
gration of the underlying series («), just as the latter 
have themselves been derived from the demolition of 
pre-existing rock-masses. After the formation of the 
upper series {B) the region was re-elevated, and once 
more formed a land-surface, which has doubtless en- 
dured for a long period, seeing that much erosion 
has taken place, the horizontal beds having been 
greatly denuded, trenched, and furrowed, so that at 
the bottom of deep valleys the underlying older series 
has been laid bare and eaten into by running water. 

Such is the kind of tale which one may read almost 
everywhere. The very existence of sedimentary strata 
implies denudation of land-areas — denudation and 
sedimentation go hand in hand. When we bear in 
mind that the average thickness of the sedimentary 
rocks which overspread so large an area of the dry 
lands of the globe cannot be less than 8000 or 10,000 
feet, we cannot fail to be impressed with the magni- 
tude of denudation. And this impression will be 
deepened when we reflect that the bulk of the mate- 
rials entering into .the composition of the derivative 
rocks has been used over and over again. The mere 
thickness of existing sedimentary strata, therefore, is 
very far indeed from being an index to the amount 
of erosion which has been effected since the deposition 
of the oldest aqueous strata. 



^ 



CHAPTER III 

LAND-FORMS IN REGIONS OF HORIZONTAL 
STRA TA 

VARIOUS FACTORS DETERMINING EARTH SCULPTURE INFLUENCE 

OF GEOLOGICAL STRUCTURE AND THE CHARACTER OF ROCKS IN 
DETERMINING THE CONFIGURATION ASSUMED BY HORIZONTAL 
STRATA PLAINS AND PLATEAUX OF ACCUMULATION. 

HITHERTO we have been considering erosion 
from one point of view only. We glanced first 
at the general evidence of denudation as furnished by 
the abrupt truncation and discontinuity of strata, and 
by the appearance at the surface of rocks which could 
never have originated in that position. Then we dis- 
cussed the action of existing agents of change, and 
saw reason to conclude that the denudation every- 
where conspicuous must be the result of that action. 
Some reference has also been made to the fact that 
rocks are of various composition and consistency, and 
therefore tend to yield and crumble away unequally. 
It follows from this that denudation will be retarded 
or hastened according as the rocks succumb slowly or 
more rapidly to the action of eroding agents. Given 
an elevated plane-surface of some extent, composed 

44 



LAND-FORMS IN HORIZONTAL STRATA 45 

of rocks of different degrees of durability, and it is 
obvious that such a surface must in time become 
irregularly worn away. The readily eroded rocks 
will disappear most rapidly, and thus by and by the 
plane-surface will be more or less profoundly modified 
and come to assume a diversified configuration. The 
relatively hard and resisting rocks will determine the 
position of the high grounds, while the low grounds 
will practically coincide with the areas occupied by 
the more yielding rock-masses. 

This we shall find holds true to a large extent of 
all land-surfaces. Nevertheless, existing configura- 
tions have not been determined solely by the min- 
eralogical composition of the rocks. There is yet 
another factor to be taken into consideration. The 
form assumed by a land-surface under denudation de- 
pends not only /on the composition of rocks^ but very 
largely on the(^mode of their arrangement.) Certain 
rock-structures, as we shall learn, favour denudation, 
while others are more resisting. So dominant, indeed, 
has been the influence of geological structure in de- 
termining the results worked -out by erosion, that 
without a knowledge of the structure of a country we 
can form no reliable opinion as to the origin of its 
surface-features. 

But even this is not all. We have likewise to con- 
sider the geological history of the land with a view to 
ascertain what appearance it presented when rains and 
rivers were just beginning the work of erosion. For 
it is obvious that the direction of the drainage must 



46 EARTH SCULPTURE 

have been determined in the first place by the original 
inclination of the surface. 

Once more, we know that existing land-surfaces 
have often been disturbed by subterranean action, 
and that such action has not infrequently led to con- 
siderable modification of drainage-systems. It is 
remarkable, however, how persistent are great rivers 
in maintaining their direction. ' When it has been 
once fairly established, a large river may outlive many 
revolutions of the surface. River-valleys are not 
seldom older than the mountain-ridges which they 
sometimes traverse ; or, to put it in another way, new 
^mountains may come into existence without deflecting 
the rivers across whose valleys they may seem at one 
time to have extended — for the rivers have simply 
sawed their way through the ridges as these were 
being gradually developed. 

The history of the denudation of a land-surface is 
in truth often highly complicated and hard to read. 
Many factors have aided in determining the final re- 
sults of erosion, and it is not always possible to assign 
to each its proper share in the work. But we may 
truly say that the sculpture of the land — the form it 
has assumed under denudation — has been determined 
mainly by these three factors : {a) the original slope 
of the surface ; {b) the geological structure of the 
ground ; and (c) the character of the rocks. 

Both hypogene and epigene agents, therefore, have 
been concerned in the evolution of land-forms. In 
regions much disturbed by subterranean action within 



LAND-FORMS IN HORIZONTAL STRATA 47 

relatively recent geological times, many of the most 
striking surface-features are obviously due to deforma- 
tion and dislocation of the crust. All such features, 
however, sooner or later become modified by epigene 
action, and thus it has come to pass that in countries 
which have existed as dry land for vast periods of time, 
undisturbed in' the later stages of their history by 
crustal movement, the surface-features are such as 
only epigene action can account for. Original 
irregularities of the ground, the result of hypogene 
action, have been obliterated and replaced by an 
outline wholly due to denudation. 

The existence of fractured and folded strata enables 
us vividly to realise the fact that hypogene action has 
played a prominent part in the evolution of land-forms. 
Not only are many inequalities of the surface the di- 
rect result of that action, but even after such irregu- 
larities have been removed, the various positions 
assumed by the flexed and fractured rocks have 
largely determined the configuration subsequently 
worked out by the epigene agents of change. Thus 
both directly and indirectly crustal movements have 
had a large share in the production of surface-features. 
It is not necessary for our purpose to inquire into the 
causes of such movements. In the opinion of most 
geologists they are due to the secular' cooling of the 
earth. As the nucleus cools it contracts, and the 
already cooled crust sinks down upon it. This- move- 
ment necessarily results in the fracturing and wrinkling 
of the crust, which as it sinks is compelled to occupy 



48 EARTH SCULPTURE 

a smaller superficial area. The deformation brought 
about in this way varies in extent. In some places 
the general subsidence of the crust has not been 
marked by much disturbance of the rocks ; the orig- 
inal horizontality of the strata has been largely pre- 
served. In other regions the reverse is the case, the 
strata having been everywhere folded and fractured ; 
and between these two extremes are many gradations. 
The various structures assumed by disturbed rock- 
masses show that crustal movements are of two kinds, 
horizontal and vertical. Folding and its accompany- 
ing phenomena are obviously the result of tangential 
pressure. Sometimes the strata are so folded as to 
present the appearance of a series of broad, gentle 
undulations. At other times the folds are pressed 
closely together and bent over to one side in the 
direction of crustal movement. In certain regions so 
great has been the horizontal thrust, that masses of 
rock, thousands of feet in thickness, have sheared 
under the pressure and travelled forwards for miles, 
older rocks being pushed forward bodily over younger 
masses. But besides such horizontal movements there 
are vertical movements of the crust, typically repre- 
sented by the dislocations known as norrnal faults. 
Normal faults are more or less vertical displacements, 
often of small amount, but not infrequently very 
great. Many are vast rents traversing the crust in 
some determinate direction, the rocks on one side of 
the fault having subsided for hundreds or even for 
thousands of feet. We may reserve for the present. 



LAND-FORMS IN HORIZONTAL STRATA 4Q 

however, any further discussion of the rock-structures 
that result from hypogene action. All that we need 
at present bear in mind is the general fact that the 
crust of the earth is subject to deformation. 

We now proceed to inquire more particularly into 
the influence of geological structure and the character 
of rocks upon the development of land-forms. We 
shall therefore consider first the form assumed by 
lands built up of approximately horizontal strata. 
This is the simplest kind of geological structure : the 
tale it tells is not hard to read. We can follow it 
from first to last in all its details. But if we succeed 
in grasping what is meant by the denudation of hori- 
zontal strata, we shall have little difficulty in explaining 
the origin of surface-features in regions the geological 
structure of which is much more complicated. 

As common examples of horizontally bedded strata 
we may take the alluvial deposits that mark the sites 
of vanished lakes ; the terraces of gravel, sand, and 
silt that occur in river-valleys ; deltas, and raised 
beaches. Fluvio-marine deposits and raised beaches 
of recent age generally form low plains rising but a 
few feet or yards above sea-level. Their inclination 
is seawards, usually at so low an angle that they often 
appear to the eye level, or approximately so. This 
gently sloping surface is an original configuration, for 
it^corresponds with the structure of the various under- 
lying deposits, the general inclination or dip of which 
is in the same direction as the surface. When that 
surface is approximately level denudation necessarily 



so EARTH SCULPTURE 

proceeds very slowly, although in time the action of 
rain alone will suffice to lower the general level. But 
however much raised beaches and deltas of recent age 
may have been modified superficially by subaerial 
denudation, we must admit that their most character- 
istic features are original, and due to the mode of 
their formation. 

The same holds true to a large extent of recent 
lacustrine and fluviatile deposits. The wide flats that 
tell us where lakes formerly existed, and the broad 
alluvial tracts through which streams and rivers 
meander, are, like deltas and raised beaches, plains of 
accumulation. It goes without saying, however, that 
many of these plains are more or less eroded, and 
have acquired an undulating, furrowed, and irregular 
surface. Some alluvial tracts, indeed, have been so 
cut up by rain and running water that, in the rough, 
rolling ground over which he toils, the traveller may 
find it hard to recognise the characteristic features of 
a plain. 

In a broad river-basin alluvial terraces and plains 
usually occur at various heights, marking successive 
levels at which the river and its tributaries have flowed 
while deepening their courses. The lowest terraces 
and flood-plains are, of course, the youngest, and show, 
therefore, least trace of subaerial erosion. As we re- 
cede from these modern alluvia and rise to higher 
levels, the terraces and plains become more and more 
denuded. The highest-lying river-accumulations, in- 
deed, may be so much eaten into and washed down 



LAND-FORMS IN HORIZONTAL STRATA 51 

that only scattered patches may remain, and few or no 
traces of the original flat surface can then be recog- 
nised. Thus fluviatile terraces and recent alluvia all 
tend to become modified superficially, while at the 
same time they are undermined and cut into by 
streams and rivers. 

The plains of accumulation at present referred to 
belong to a recent geological age, and consist for the 
most part of incoherent deposits, such as gravel, sand, 
clay, silt, loam, and so forth. And it is worthy of 
note that the nature of the deposits has to some ex- 




FiG. 7. Section across a Series of Alluvial Terraces. 

r, solid rocks ; i, oldest terrace ; i^, second terrace ; 3, third and youngest terrace ; 4, river and 
recent alluvial plain. 

tent influenced the denudation of the ground. Thus 
terraces and plains composed mainly of gravel tend 
to retain their original level surface, while similar 
flats of clay and loam of the same age as the gravel 
have frequently been furrowed and channelled to such 
an extent that the originally level surface has largely, 
or even entirely, disappeared. The reason is obvious, 
for clay and loam are somewhat impervious, while 
gravel is highly porous. Consequently rain falling on 
the surface of the latter is rapidly absorbed, and little 
or no superficial flow is possible. But in the case of 
the more impervious deposits rain is absorbed very 



52 EARTH SCULPTURE 

sparingly, and naturally tends to produce inequalities 
as it seeks its way over the gently inclined surface. 

The origin and present aspect of such recent plains 
of accumulation are so obvious and so readily accounted 
for, that it is hardly necessary to do more than cite a 
few examples. Amongst the most notable are the 
great deltas of such rivers as the Mississippi, the 
Amazon, the Rhone, the Po, the Danube, the Rhine, 
the Niger, the Ganges, etc., and the broad flats and 
terraces which occur within the drainage-areas of the 
same rivers. The vast plains of the Aralo-Caspian 
area, and the far-extended tundras of Northern Siberia, 
are likewise examples of plains of accumulation, all 
of which belong to recent geological times. How- 
ever much some of these plains may have been 
furrowed and trenched by running water, we yet have 
no difificulty in recognising that the general form of 
the surface is due to sedimentation. The deposits of 
which they are built up have been laid down in 
approximately horizontal or gently inclined layers, and 
the even or level surface is thus simply an expression 
of the arrangement of the bedding. In a word, the 
geological structure has determined the configuration 
of the surface. 

But it is needless to say that horizontal strata are 
not confined to low levels, nor do they always consist 
of unconsolidated materials, like gravel, sand, and 
clay. Horizontal strata of such rocks as sandstone, 
shale, limestone, basalt, etc., enter largely into the 
composition of certain lofty plateaux and mountain- 



LAND-FORMS IN HORIZONTAL STRATA 53 

regions. And they belong, moreover, to very differ- 
ent geological periods, some being of comparatively 
recent formation, while others date back to ages 
incalculably remote. 

One of the most interesting and instructive regions 
of the kind is the remarkable plateau of the Grand 
Canon district of Arizona and Utah. This plateau 
occupies an area of between 13,000 and 16,000 
square miles, and is traversed by the Colorado River 
of the West, which follows a tortuous course tow- 
ards west-south-west through a succession of pro- 
found ravines or canons. The strata visible at the 
surface are approximately horizontal, and attain a 
thickness of many thousand feet. It may be said, 
therefore, that the prevalent plain-like character of 
the surface is an expression of the underground struct- 
ure — that, in short, the Grand Canon district is a 
plateau of accumulation. This, in a broad sense, is 
doubtless true ; but when we come to examine the 
configuration and structure of the district more closely, 
we find reason to conclude that the original surface 
has been greatly modified by denudation. We learn, 
moreover, that the strata are not quite horizontal. 
The inclination is certainly gentle, but a slope of only 
one degree, if continued for a few miles, will result 
in a fall of several hundred feet. If a surface be in- 
clined at an angle of one degree, then for every 
eleven miles of distance it will lose 1000 feet of ele- 
vation. Now, in the Grand Canon district the gen- 
eral inclination of the strata is towards north and 



EARTH SCULPTURE 



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O 
Q 
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U 

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O 



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north-east, while the slope 
of the surface is in the 
opposite direction. Thus 
it comes to pass that strata 
which lie open to the day 
upon the south-west mar- 
gin of the plateau gradu- 
ally descend towards north 
and north-east, until, in a 
distance of 120 miles or 
thereabouts, they lie bur- 
ied at a depth of several 
thousand feet. It is not 
quite true, therefore, that 
in the Grand Canon dis- 
trict the form of the 
ground is an exact expres- 
sion of the underground 
structure. On the con- 
trary, the average slope 
of the surface is against 
and not with the average 
dip of the strata. Never- 
theless , it cannot be 
doubted that the general 
configuration of the re- 
gion — its plateau-charac- 
ter — has, in the first place, 
been determined by the 
approximately horizontal 



LAND-FORMS IN HORIZONTAL STRATA 



55 



disposition of the strata, and that it may be rightly 
termed a plateau of accumulation. A glance at the 
geological history of the district will show how far the 
plateau-character is original, and to what extent and 
by what means it has been subsequently modified. 

Reference has been made to the fact that the rocks 
composing the plateau are chiefly of aqueous origin, 
and approximately horizontal. 

Here and there in the bottoms of deep cafions we 
get peeps at another set of rocks that form the 
pavement upon which the horizontal strata repose. 
With the history of these older underlying rocks we 
need not concern ourselves further than to note 
that they are of Pre-Cambrian and early Palaeozoic 
age. It is with the superincumbent masses that we 
have to deal. Those attain a vast thickness, and 
range in age from Carboniferous down to Eocene 
times. At the beginning of the Carboniferous Period 
the district formed a portion of the sea-floor, and 
similar marine conditions obtained during the deposi- 
tion of all the succeeding systems of strata down to 
the close of Cretaceous times. Throughout all that 
long succession of ages the sea would appear never to 
have been deep, although during the early part of the 
Carboniferous Period it was probably deeper than in 
subsequent times. When we consider that the marine 
sediments reach a united thickness of over 15,000 feet 
it may at first sight appear impossible that so thick a 
mass of materials could accumulate in a shallow sea. 
The explanation, however, is simple enough — sub- 



56 EARTH SCULPTURE 

sidence kept pace with sedimentation. Slowly and 
gradually the bed of the sea went down — slowly and 
gradually it was silted up by sediments derived from 
the adjacent land. 

At last, towards the close of Cretaceous times, cer- 
tain new crustal movements began — elevation ensued, 
and the sea finally retired from the district. An ex- 
tensive lake now occupied the site of the plateau- 
country for a prolonged period, during which sediments 
were washed down as before from the neighbouring 
uplands, and gathered over the level surface of the 
Cretaceous marine strata until they had reached a 
thickness of 5000 feet or more. As these deposits 
appear likewise to have been laid down chiefly in 
shallow water, it may be inferred that the slow subsid- 
ence of the area which accompanied the accumula- 
tion of the underlying marine strata was repeated 
during the lacustrine period. 

The whole region, it will be understood, had been 
elevated at the close of Cretaceous times ; but the 
movement was differential, the greatest rise having 
been experienced by the uplands surrounding the la- 
custrine basin. Eventually the river, escaping over 
the lower lip of that basin, deepened the outlet and 
succeeded in draining the lake, which was then re- 
placed by an alluvial plain. At this stage the nearly 
level surface of the drained lake-bed sloped gently 
from east-north-east to west-south-west, and thus de- 
termined the direction of the primeval Colorado 
River and its larger tributaries, which headed then 



LAND-FORMS TN HORIZONTAL STRATA 57 

as now in the high lands overlooking the basin. 
When these waters first began to wander across the 
alluvial plain, the slope of the surface and the inclina- 
tion of the underlying sedimentary strata doubtless 
coincided. But these conditions were ere long dis- 
turbed by successive movements of elevation, and the 
prevalent horizontality of the strata was modified. 
Here and there the beds were bent or flexed, and 
traversed by great fractures along which the strata 
became vertically displaced for thousands of feet. 
Yet, strange to say, none of these earth-movements 
succeeded in deflecting the main drainage of the dis- 
trict. The Colorado and its chief affluents continued 
to flow in the courses they had attained at the final 
disappearance of the great lake. It is clear, there- 
fore, that the bending and dislocation of the strata 
must have proceeded very slowly, for the rivers were 
able to cut their way across both flexures and faults 
as fast as these showed at the surface. 

Before the great lake had vanished some portions 
of the older marine strata had been elevated, and 
formed part of the land surrounding the basin. Here 
they were for a long period exposed to the erosive 
action of epigene agents, and must have suffered 
much loss. But all such denudation sinks into insig- 
nificance when we consider the magnitude of the 
erosion which has taken place since the great lake 
dried up. Fortunately, owing to the simple geologi- 
cal structure of the Grand Canon district, the amount 
of that erosion can be readily estimated. According 



EARTH SCULPTURE 



«■ *- ■ 



■0 



«', 






to Captain Dutton, the average 
thickness of strata removed from 
an area of 13,000 to 15,000 square 
miles cannot have been under 10,- 
000 feet. This may seem a startHng 
conclusion, but it is based on evi- 
dence which cannot be gainsaid. 
Throughout the major portion 
of the plateau-country horizontal 
Carboniferous strata occupy the 
surface. As these are followed 
northward they gradually dip in 
that direction under younger strata 
(Permian, Mesozoic, and Cainozoic 
rocks), until they are buried at last 
to a depth of 10,000 feet and more. 
Now Captain Dutton has shown 
that this vast thickness of over- 
lying strata formerly extended 
throughout the whole Grand 
Canon district. This is proved by 
the fact that many outliers or relics 
of the rocks in question still re- 
main, scattered at intervals over 
the broad surface of the Car- 
boniferous strata. They form 
conspicuous table-shaped and pyr- 
amidal hills, rising more or less 
abruptly above the great Carbon- 
iferous platform. The accompany- 
ing diagram shows the general 



LAND-FORMS IN HORIZONTAL STRATA 59 

relations of those isolated "buttes" and "mesas," as 
they are termed, to the underlying Carboniferous 
rocks and the strata at Z", of which they are detached 
outliers. The dotted line {a-b^ indicates the level 
originally attained by the plateau. All the rock that 
formerly existed between a-b and the surface of the 
Carboniferous strata (C) has been denuded away. 

How has this enormous erosion been effected, and 
what are the more prominent features of the denuded 
area ? A low-lying plain of accumulation, such as a 
delta, cannot experience much erosion ; the surface 
is approximately level, or has only a very gentle in- 
clination, and any water flowing over it must be 
sluggish and ineffective. But conceive such a plain 
upheaved for several hundred feet, and it is obvious 
that the fall of the river to the sea will then be in- 
creased and its erosive action greatly augmented. It 
will therefore proceed to dig a deeper and wider 
course for itself. Now let us suppose that an ele- 
vated plain is traversed not by one main river only, 
but by numerous affluents, each with its quota of 
tributary streams. The running waters will continue 
to deepen their channels until the gradient by the pro- 
cess is gradually reduced to a minimum and vertical 
erosion ceases. The main river will be the first to 
attain this base-level — a level not much above that of 
the sea. The plain-track will gradually extend from 
the sea inland until the same low gradient is attained 
throughout the whole course of the river. In time 
all the affluents with their tributaries will arrive at the 
same stage. 



6o EARTH SCULPTURE 

But rivers do not only cut vertically ; they also un- 
dermine their banks and cliffs, and thus erode hori- 
zontally ; hence it follows that the valleys will be 
widened as well as deepened. The widening process 
may be greatly aided by the action of wind, rain, 
springs, and frost. Not infrequently, indeed, these 
agents play as important a part as the streams them- 
selves. Under the conditions now described an ele- 
vated plain will in course of time be cut up into more 
or less numerous segments, the upper surfaces of 
which will represent the original level of the land ; 
where the interval between two valleys is wide we 
shall have a broad, flat-topped segment ; where the 
interval is short the segment will be correspondingly 
restricted in size. In a word, the segments will vary 
in extent according to the multiplicity and intricacy 
of the valley-system. 

A word now as to the form of the slopes and cliffs 
bounding the valleys. We are dealing, it will be re- 
membered, with an elevated plain of accumulation. 
The horizontal strata, we shall suppose, are more or 
less indurated beds of conglomerate, sandstone, shale, 
and limestone. All rocks, as we have seen, are 
traversed by natural division-planes ox joints, and these 
in the case of stratified rocks consist of two sets 
intersecting each other and the planes of bedding at 
approximately right angles. Horizontal strata are in 
this way divided up into rudely cuboidal, quadrangu- 
lar, or rectangular blocks. Joints are, of course, lines 
of weakness along which, when rocks are undermined, 



LAND-FORMS IN HORIZONTAL STRATA 6i 

they tend to give way. Thus when horizontal strata 
are cut into by rivers and undermined they break off 
at the joints, and vertical cliffs result. It does not 
often happen, however, that in a considerable series 
of strata all the beds are of quite the same character. 
Frequently some are relatively harder and unyielding, 
while others are softer and more readily reduced. 
Let us suppose that the uppermost bed cut into by 
the river is somewhat hard and difficult to grind 
through. In time the water saws its way down into 
the succeeding stratum, which we shall take to be a 
soft or easily eroded shale. In the overlying hard 
rock the river has been able to cut merely a narrow 
steep-sided trench. The shale, however, offers much 
less resistance to the vertical and lateral action of the 
water, and is thus rapidly intersected and washed 
away from underneath the superincumbent harder 
stratum. The latter, losing its support, then yields 
along its joint-planes, and a larger or smaller slice is 
detached from the wall of the cliff and falls in ruins. 
In this way the cliffs gradually retire as they are un- 
dermined — in a word, the ravine is not only deepened 
but widened. 

Much of the rock ddbris dislodged from the cliffs 
falls into the river, and is gradually broken up and 
carried away ; but some comes to rest at the base, 
forming a talus, and thus retards the denudation of 
the shale. To the action of the river we must add 
that of other epigene agents, such as wind, rain, 
springs, and frost, under the influence of which the 



62 



EARTH SCULPTURE 



shale weathers away more rapidly than the overlying 
rock, and eventually forms a sloping stage upon 
which the ddbris derived from the receding cliffs 
continues to accumulate. Meanwhile, however, the 
river digs down through the shale and encounters, we 
shall suppose, another thick stratum of hard rock. 
Lateral erosion by the running water is now reduced 
to a minimum ; slowly the current saws its way down 




Fig. io. Diagrammatic Section Showing Stages of Erosion by a 
River Cutting through Horizontal Strata. (After Captain Button.) 

A, relatively hard rocks ; o-, relatively soft strata \ r r, river at successive stages as valley 
is deepened and widened. 



vertically, just as it did in the uppermost unyielding 
bed, until it again reaches a second layer of shale. 
The undermining action is now repeated, and a sec- 
ond line of rock-wall begins to retreat in the same 
manner as the first. And so the process goes on with 
all the succeeding strata through which the river cuts, 
until it finally attains a minimum gradient and ceases 
to erode. But note that, while the deepening of the 
ravine proceeds, the cliffs never cease to retire. Each 



LAND-FORMS IN HORIZONTAL STRATA 63 

individual layer of softer rock continues to waste 
away more rapidly than the harder bed above it. 
Thus eventually a river-valley appears bounded, not 
by vertical cliffs, but rather by a succession of hori- 
zontal tiers of precipitous faces, corresponding to 
the outcrops of the several strata of harder rock — 
separated the one from the other by the longer or 
shorter slopes yielded by the shales. 

Finally, we may further note that the recession of 
the cliffs will be much influenced by the rate at which 
their basal portions are undermined. Each slice re- 
moved from a steep rock-face narrows the width and 
increases the inclination of the sloping stage above. 
Hence, as Captain Dutton has clearly shown in his 
admirable description of the Colorado Canons, the de- 
scent of ddbris from each stage is facilitated, while the 
weathering of the soft rocks and the undermining of 
the overlying harder beds are accelerated. Thus, 
curiously enough, as the same author remarks, the 
state of affairs at the bottom influences the rate of 
recession at the summit. 

When a river has reached its base-level and ceases 
to erode, the valley-slopes and cliffs, nevertheless, 
under the influence of weathering, continue to retire. 
The ddbris showered down from above now tends to 
accumulate below, and thus affords protection to the 
rocks against which it is banked. And the talus thus 
formed continues to rise higher and higher. The ex- 
posed strata above, however, having no such protec- 
tion, weather as befofe, each rock-tier retreating, but 



64 EARTH SCULPTURE 

at a gradually diminishing rate. What form the 
ground will ultimately assu*me will largely depend 
upon climatic conditions. If the climate be moist and 
frost be active in winter, the sharp edges of the rock- 
tiers will, be bevelled off, and the sloping surfaces will 
become heavily laden with dibris and disintegrated 
rock-material, the further degradation and removal of 
which will be retarded by the growth of vegetation. 
Thus, in time, the sharp angles will tend to disappear, 
and a somewhat undulating slope will replace the 
more strongly marked features which the. same rocks 
would have yielded under arid conditions. 

Let us now recall what was said as to the cutting 
up of our elevated plain into a multiplicity of flat- 
topped segments, and we shall see reason to conclude 
that these segments must be bounded by steep faces, 
the aspect of which will vary according to the nature 
of the strata and the character of the climate. If the 
climate be arid, and the strata consist of alternate 
hard- and soft beds of variable thickness, the bound- 
ing walls of the segments may in some places be ap- 
proximately vertical, or they may show a succession 
of short cliffs with intermediate sloping stages. If, 
on the other hand, the climate be moist, those features 
will be more or less softened and modified. In the 
former case step-like profiles will abound ; in the latter 
the ground will likewise ascend in stages, but these 
will be less accentuated, and may even be in large 
part replaced by continuous slopes. Again, each flat- 
topped segment of the denuded area, eaten into on all 



LAND-FORMS IN HORIZONTAL STRATA 65 

sides, will continually contract, the bounding cliffs 
and slopes retiring step by step until they eventually 
meet atop. The flat summit now disappears, and is 
replaced by a sharp crest, ridge, peak, or rounded 
top, as the case may be. Each diminishing segment, 
in short, ultimately acquires a more or less strongly 
pronounced pyramidal form. This, however, is not 
the final stage. Denudation continues — pyramidal 
hills, dome-shaped heights, and crested ridges gradu- 
ally crumble down, until at last all abrupt and pro- 
minent irregularities of surface disappear, and the once 
elevated plain returns to its former state, that of a 
gently undulating or approximately flat stretch of 
low-lying land. The cycle of erosion is completed. 

Thus in the erosion of a plateau of horizontal strata 
we recognise the following stages : — (i) The excava- 
tion of deep trenches by streams and rivers ; (2) the 
gradual sapping and undermining of cliffs, etc., the 
widening of valleys, and the consequent cutting up of 
the plateau into a multitude of flat-topped blocks or 
segments ; (3) the progressive contraction of the seg- 
ments, and their conversion into pyramidal or round- 
topped hills and crested ridges ; and (4) the continued 
reduction and lowering of the hills and final resolution 
of the plateau into a plain. 

This plain, in the hypothetical case we have been 
considering, is supposed to be at a level very little 
above that of the sea. But the minimum level to 
which a region tends to be reduced need not be at 
such a low elevation. The streams and rivers dis- 



66 EARTH SCULPTURE 

charging into a great lake or inland sea cannot erode 
their valleys below the level of the quiet water which 
is the receptacle of their sediment. That surface 
becomes for them a base-level of erosion, and all 
their energies are employed in the task of reducing 
to that level the land over which they flow. Soon or 
late, however, the outlet of the lake will be deepened, 
the surface of the latter will fall, and the base-level 
will, of course, be lowered at the same time. But 
should a slow movement of elevation affect the lower 
end of the great lake, and thus, by counterbalancing 
the work of river erosion at its outlet, maintain the 
surface at approximately the same level for a pro- 
longed period of time, then denudation may eventually 
succeed in reducing to that base-level all the lands 
that drain into the lake. The lake might be entirely 
silted up, but so long as the movement of elevation 
persisted, and the river (at the former outlet of the 
lake) continued to saw its way down as rapidly as the 
ground was .upheaved, the old base-level of erosion 
would be maintained. 

We may now return to the Grand Canon district 
and the question of its erosion. During the progress 
of the great denudation the interior spaces of the dis- 
trict, according to Captain Dutton, " occupied for a 
time the relation of an approximate base-level of 
erosion." The whole region has been greatly ele- 
vated, but this upheaval was not effected all at one 
time. On the contrary, in place of one single con- 
tinuous movement a succession of uplifts has taken 



LAND-FORMS IN HORIZONTAL STRATA 67 

place, each separated from the other by a period of 
repose. It was during one of these prolonged pauses 
that enormous sheets of strata, averaging some 10,000 
feet in thickness, were gradually broken up and re- 
moved from the surface of the Carboniferous rocks, 
while the latter themselves were planed down to a flat 
expanse. This Carboniferous platform served for a 
long time as a base-level of erosion. The horizontal 
masses under which it lay buried were first deeply 
incised by the Colorado River and its affluents and 
their countless tributaries. The strata thus became 
broken up into innumerable separate blocks or seg- 
ments, which, little by little, were reduced in size and 
most of them eventually demolished. But before the 
last remaining " buttes " and " mesas " could be re- 
moved a great change supervened. A general up- 
heaval of the entire area for several\ thousand feet 
took place, and the base-level to which the district 
had been so largely reduced was destroyed. The 
gradients of all the rivers now increased, and the 
velocity of the currents was correspondingly aug- 
mented, with the result that the erosion of ravines 
and canons recommenced. 

It is beyond the purpose of these pagc^ to trace 
further the history of the Grand Canon district. But 
those who wish to have an adequate conception of 
what is meant by river erosion would do well to con- 
sult Captain Dutton's work. From it they will learn 
how the Colorado River has, within a very recent 
geological period, dug out a valley " more than 200 



68 EARTH SCULPTURE 

miles long, from 5 to 1 2 miles wide, and from 5000 to 
6000 feet deep." From our present point of view the 
chief lesson which we derive from a study of the 
Grand Canon district is simply this : that horizontally 
arranged strata tend under the action of epigene 
agents to form flat-topped mesas and pyramidal hills 
and mountains. The contours of those prominent 
features and the detailed sculpturing of cliffs and 
rock-terraces will depend largely upon the character 
of the strata out of which the hills and mountains are 
carved, and also to a great extent upon the climate. 
In a dry elevated tract like that of the Canon district 
the influence exerted by the petrological character of 
the strata in determining the detailed features of the 
ground is everywhere conspicuous. In other regions 
where moister climatic conditions prevail this influ- 
ence, although never absent, is yet not so strongly 
marked. 

In the foregoing discussion the configuration as- 
sumed by horizontal strata has been dealt with in 
such detail that it is not necessary to cite more than 
a few other examples to show that wherever the same 
geological structure occurs denudation has resulted 
in the production of similar land-forms. 

The lonely group of the Faroe Islands, lying about 
half-way between Scotland and Iceland, are the relics 
of what at one time must have been a considerable 
plateau. They extend over an area about seventy 
miles in length from north to south, and nearly fifty 
miles in width from east to west. The original 



LAND-FORMS IN HORIZONTAL STRATA 69 



plateau could not have been less than 
3500 square miles in extent. But as 
the islands have everywhere experienced 
excessive marine erosion, it is certain 
that the plateau out of which they have 
been carved formerly occupied a much 
wider area. The geological structure 
of the islands is very simple. They are 
built up of a great succession of basalts 
with thin intervening layers of tuff 
(volcanic dust, etc.) arranged in ap- 
proximately horizontal strata. The 
islands are for the most part high and 
steep, many of them being mere mount- 
ain-ridges that sink abruptly on one or 
both sides into the sea. The larger 
ones show more diversity of surface, 
but possess very little level land. All 
have a mo;untainous character, and, 
owing to the similarity of the rocks and 
their arrangement, exhibit little variety 
of feature. They form as. a rule strag- 
gling, irregular, flat-topped masses, and 
sharper ridges, that are notched or 
broken here and there into a series of 
isolated peaks and truncated pyramids. 
Sometimes the mountains rise in gentle 
acclivities, but more generally they show 
steep and abrupt slopes, which in several 
instances have inclinations of 25° to 



1^ 






i 



«3 



70 EARTH SCULPTURE 

27° or even 30°. In many places they are yet steeper, 
their upper portions especially becoming quite pre- 
cipitous. They everywhere exhibit a well-marked 
terraced character ; precipices and long walls of bare 
rock rise one above another, like the tiers of some 
Cyclopean masonry, and are separated usually by 
short intervening slopes, sparsely clothed with grass 
and moss, or sprinkled with tumbled rock-rubbish. 
The coasts are usually precipitous, many of the islands 
having only a few places where a landing can be 
effected. Not a few are girt by cliffs, ranging in 
height from 200 or 300 feet up to 1000 feet, and even 
in some places exceeding 2000 feet. The best-defined 
valleys are broad in proportion to their length. Fol- 
lowed up from the head of a sea-loch, they rise some- 
times with a gentle slope until in the distance of two 
or three miles they terminate in a broad amphitheatre- 
like cirque. In many cases, however, they ascend to 
the water-parting in successive broad steps or terraces. 
Each terrace is cirque-shaped, and framed in by a 
wall of rock, the upper surface of which stretches 
back to form the next cirque-like terrace, and so on 
in succession until the series abruptly terminates at 
the base, it may be, of some precipitous mountain. 
Occasionally the neck between two valleys running 
in opposite directions is so low and flat that it is with 
difficulty that the actual water-parting can be fixed. 
In such cases we have a well-defined hollow, bounded 
by precipitous, terraced hill-slopes, crossing an island 
from shore to shore. Were the land to be slightly 



LAJSID-FORMS IN HORIZONTAL STRATA 71 

depressed such hollows would form sounds separating 
adjacent islands, while the valleys that head in cirques 
would form sea-lochs. There can be no doubt, in- 
deed, that the existing fiords of the Faroes simply 
occupy the lower reaches of land-valleys, and that the 
sounds separating the various islands from each other 
in like manner indicate the sites of long hollows of 
the character just described. In a word, the islands 
are the relics of a plateau of comparatively recent 
geological age, for the rocks date no further back 
than Oligocene times. All the land-features are the 
result of subaerial erosion guided and determined by 
the petrological character and horizontal arrangement 
of the strata. The precipitous cliffs of the coast-line 
owe their origin, of course, to the undermining action 
of the sea, the rocks ever and anon giving way along 
the well-marked vertical joint-planes. 

In Great Britain horizontal strata occupy no broad 
areas. But wherever they put in an appearance 
they yield the same surface-features. Thus in the 
north-west Highlands we have the striking pyrami- 
dal mountains of Canisp, Suilven, and Coulmore, 
carved out of horizontal red sandstones of Pre-Cam- 
brian age. In Caithness, again, we have the peaked 
and truncated pyramids of Morven, Maiden Pap, and 
Smean, hewn out of approximately horizontal Old Red 
Sandstone strata. Ingleborough is another good 
example of a pyramidal mountain having a similar 
geological structure. Many illustrations are likewise 
furnished by the horizontal strata of other lands.. 



72 EARTH SCULPTURE 

Thus pyramidal and more or less abrupt hills, the 
precipitous sides of which are defined by vertical joints, 
are common in the horizontally bedded " Quadersand- 
stein " of Saxon Switzerland. So again in the region 
of the Dolomites, whenever the strata are horizontal 
the mountains carved out of them tend to assume 
pyramidal forms. In a word, we may say that all the 
world over the same geological structure gives rise to 
the same land-forms. 

River-courses hewn in horizontal strata will vary 
somewhat in form according to the nature of the 
rocks and the character of the climate. In regions 
built up of relatively unyielding rocks, or of alterna- 
tions of these and less resisting beds, the valleys tend 
to be trench-like, and the mountain-slopes are more 
or less abrupt. But under the influence of rain, 
springs, and frost these harsh features are toned 
down, river-cliffs are benched back, and abrupt ac- 
clivities are replaced by gentler slopes. Should the 
strata consist of soft materials throughout, there will 
be a general absence of harsh features ; round-topped 
hills and moderate valley-slopes will characterise the 
land. Nevertheless, whether the strata be " hard " or 
"soft," thick-bedded or thin-bedded, or show alterna- 
tions of many different kinds, and whether the climate 
be arid or humid, equable or the reverse — tropical, 
temperate, or arctic — the same general type of surface- 
features can always be recognised. 



CHAPTER IV 

LAND-FORMS IN REGIONS OF GENTL Y INCLINED 

STRA TA 

ESCARPMENTS AND DIP-SLOPES DIP-VALLEYS AND STRIKE-VAL- 
LEYS FORMS ASSUMED BY A PLATEAU OF EROSION VARIOUS 

DIRECTIONS OF ESCARPMENTS SYNCLINAL HILLS AND ANTI- 
CLINAL HOLLOWS — ANTICLINAL HILLS. 

THE most characteristic land-forms met with in 
regions where the strata are inchned in some 
general direction are escarpments and dip-slopes, the 
former coinciding with the outcrops, and the latter 
with the inclination or dip of the strata. In such 
regions some streams and rivers not infrequently 
flow in the direction of dip, and thus cut across the 
escarpments, while others may traverse the land along 
the base of the escarpments. 

The origin of these phenomena is not hard to trace. 
Let us suppose that some wide tract of horizontal 
strata has been elevated along an axis so as to form 
a considerable island. If the movement of elevation 
were slowly effected the sea would doubtless modify 
the land-surface as it arose, but for simplicity's sake 
we shall ignore such action, and suppose that the 
new-born land exists as an elongated island, the sur- 

73 



74 EARTH SCULPTURE 

face sloping away at a low angle on either side of a 
somewhat flattened axis. (Fig. 12.) At first, then, 
the surface coincides with the underground structure 
— a dome-shaped land formed of dome-shaped strata. 
(Fig. 13.) It is obvious that the drainage will be in 




Fig. 12. Map of an Island Composed of Dome-Shaped Strata. 

The strata are inclined in the direction of the arrows. 

the direction of the dip of the strata — all the main 
rivers will take the quickest route to the sea. But as 
we cannot suppose that the surface of the new-made 
land would be without some irregularities, the streams 
and rivers would not actually follow straight courses. 



Fig. 13. Section through the Island Shown in Fig. 12. 

Slopes of surface coincide with arrangement of strata. 

On the contrary, it could not but happen that one 
stream would eventually join another, and in this way 
many might become tributaries of one or more large 
rivers. Thus we should have certain courses cut in 
the general direction of the dip, while others joining 
these would in some places go with the inclination of 



LAND-FORMS IN GENTL Y INCLINED STRA TA 75 

the strata, and in other places would traverse that at 
various angles. The strata consist, we shall suppose, 
of " hard " and " soft " rocks — limestones, sandstones, 
shales, etc., and they are well jointed at right angles 
to the planes of bedding. Thus, while the strata dip 
seaward, one set of joints is inclined at a high angle 
in the opposite direction — the other set cutting the 
strata in the direction of the dip. Now so long as 
the streams follow the dip it is obvious that they will 
tend to form trench-like valleys — the rocks will be 
undermined and give way along vertical joint-planes. 




Fig. 14. Section of River- Valley. 

The valley coincides in direction with the " strike " of the strata, i. £■., it trends at right angles 
to the dip or inclination ; d^ cliff determined by joint ; s j, springs ; r, river. 

We need not for the present consider the modifica- 
tions arising from the varying character of the rocks. 
It is enough to remember that since they yield along 
the joint-planes, they tend to produce vertical or 
steeply inclined walls in the same manner as if they 
were horizontally bedded. But when the course of a 
stream is more or less at right angles to the dip of 
the strata, the valley it forms will not have the same 
trench-like aspect. On one side of such a valley the 
strata dip away from the stream, and when under- 
mined they yield along the joints which incline inland. 



76 EARTH SCULPTURE 

A cliff thus determined is not so liable to be broken 
down by the action of springs and frost. Under- 
ground water tends to move away down the dip- 
planes, so that no springs come out on the face of 
the cliff d (Fig. 14), which is only renewed from time 
to time by the undermining action of the river and 
the consequent collapse of the rock along a steeply 
inclined joint. On the opposite side of the valley 
the conditions are different. There the dip is towards 
the river — a weak structure, for the strata are easily 
undermined and sapped by springs, coming out along 
the planes of bedding {s, s). Hence they readily give 
way, their debris sliding and rolling towards the river. 
Thus valleys that coincide in direction with the out- 
crop of the strata will usually show a somewhat pre- 
cipitous cliff on one side and a more or less gentle 
slope on the other. 

We shall not follow the subsequent history of the 
erosion of our island in any detail. It is obvious, 
however, that it must pass through the same stages 
of erosion as any similar area of horizontally bedded 
rocks. The rivers and their multitudinous feeders 
will deepen and widen their valleys until the ground 
is cut up into a more or less numerous series of seg- 
ments or blocks. But these will differ in form from 
those which are carved out of horizontal strata. In- 
stead of flat-topped mesas and buttes and pyra- 
midal-shaped hills, we shall have a series of heights 
presenting escarpments towards the watershed and 
long slopes in the opposite direction. (Fig. 15.) 



LAND-FORMS IN GENTLY INCLINED STRATA 77 

« 

Eventually these will largely disappear, and the whole 
region will be resolved into a gently undulating plain 
of erosion. 

Now let us suppose that this plain is upheaved and 
converted into a plateau, the surface of which has a 
very gentle inclination in the same general direction 
as the dip. (See Fig. 16, p. 78.) The section at the 
side of the map shows the geological structure. Here 
obviously the surface-slope is not so great as the 



Fig. 15. Enlarged Section of a Portion of the Island Shown 
IN Fig. 12. 

Upper dotted line shows original surface ; e e^ outcrops of *' hard " beds forming escarpments. 

inclination of the underlying strata ; the plateau is 
therefore a plateau of erosion. 

The map represents the course of a main stream 
with its tributaries. The trend of the drainage will 
naturally be in the same direction as the dip, and the 
rivers must therefore traverse the outcrops of the 
strata. Were the surface of the plateau quite even 
the waters would, of course, descend by a direct route 
to the sea. For various reasons, however, it is very 
unlikely that such should be the case. The strata 
had no doubt been planed down to a base-level, but 
some inequalities would still exist — the outcrops of 
the most durable rocks would here and there project, 



78 



EARTH SCULPTURE 



however slightly, above the general surface. We 
may suppose, for example, that the outcrops of the 
limestones, {e e) would form low ridges, rising, it 
might be, only a few feet or yards. Such slight 
inequalities would suffice, however, to divert the 
waters to right or left. The rivers and streams being 




Fig. 1 6. DIAGRAM Map of Plateau of Erosion. 

e e, low ridges formed by outcrops of limestone, which are seen in section at the side. 

turned in this manner out of their direct course would 
be compelled to flow along the outcrops until depres- 
sions in the ridges allowed them to resume their 
original direction. 

After such a drainage-system had been well estab- 
lished, and the whole surface of the land had been 
subjected to the action of the various epigene agents 



LAND-FORMS IN GENTL Y INCLINED STRA TA 79 

of change for some protracted period of time, the 
inequaUties of surface would become greatly accent- 
uated. The regions occupied by " softer " rocks 
would be generally lowered, so that the outcrops of 
the harder beds would stand up more and more promi- 
nently. These, however, would not remain unchanged. 
On the contrary, each bed of hard rock, constantly 
undermined by the wearing away of the softer under- 
lying strata, would continue to recede at its outcrop. 
This retreat would be most marked in places where 
the rivers flowed along the base of the escarpments. 
But even where rivers were absent the escarpments 



Fig. 17. Section across Reduced Plateau of Erosion. 

The upper dotted line represents original surface of plateau as shown in Fig. i6. 

would still mark the outcrops of the harder beds. 
These, no doubt, might not be so prominent as the 
others, and would not retreat so rapidly, but they 
would nevertheless come to form striking features in 
the landscape. In a word, the region would eventu- 
ally be traversed from left to right by pronounced 
lines of escarpment rising to many hundreds of feet 
above the low grounds at their base, and falling away 
in a long gentle slope in the direction of the dip. 
When these land-forms were fully developed a section 
across the reduced plateau would show the structure 
seen in Fig. 17. 



8o 



EARTH SCULPTURE 



In the case we have been considering the surface 
of the plateau of erosion is inclined in the same direc- 
tion as the dip of the strata. Consequently all the 
escarpments face the water-parting of the region, and 
all the dip-slopes sink towards the sea. But the sur- 
face of such a plateau may be inclined against the 
direction of the dip ; the outcrops, instead of facing 
the water-parting, may look seawards. Nevertheless, 
should hard beds be intercalated amongst more yield- 
ing strata, escarpments are certain to make their 
appearance under the influence of denudation, and 




Fig. i8. Longitudinal Section of River-Course. 

River flowing from a lo b ; /,, outcrop of hard stratum ; ;r ;rx, shales ; ■w\ position of water- 

faU when nver-bed has been eroded to the level /' ; «,», position of waterfall when 

river-bed has been eroded to the level /*, 

may become quite as prominent as in the case we 
have just been considering. Nor will the character 
of river-valleys excavated in the direction of the 
" strike " of the strata differ ; cliffs will tend as before 
to be developed on one side, and gentle slopes on the 
other. But in the river-courses that traverse the 
strike more or less at right angles we shall meet with 
certain marked contrasts. In regions where the 
rivers flow in the same direction as the dip of gently 
inclined strata, waterfalls are not readily formed. 
When the outcrop of a relatively hard bed is en- 
countered the overlying softer rocks may be rapidly 



LAND-FORMS IN GENTLY INCLINED STRATA 8i 

washed away, and the surface of the underlying hard 
bed be exposed. At most, however, this simply gives 
rise to a rapid, which can only approach the character 
of a waterfall when the strata are inclined at a high 
angle. But when the strata dip up-stream the condi- 
tions are reversed. The outcrop of every hard ledge 
then gives rise to a cascade, and should the hard rocks 
attain a considerable thickness a notable waterfall may 
be produced. In the diagram annexed (Fig. i8) the 
upper line shows the course of a river {a-B) flowing 
across a series of strata inclined at a low angle up- 
stream. At h we see the outcrop of a bed of hard 
sandstone or other relatively durable stratum, under- 
laid and overlaid by soft shales. It is obvious that 
the river cannot lower the surface of the overlying 
soft shales {s^) much below the outcrop of the hard 
stratum. So long as that endures the beds at i-" are 
safe. It is otherwise, 'however, with the underlying 
shales {s). These are more or less rapidly eroded, 
and in the process of their removal the superjacent 
hard stratum is undermined, and from time to time 
gives way along its joint-planes. In this manner the 
waterfall (w') gradually retreats further up the valley 
(w^), and a gorge comes into existence. 

Thus in the river-courses of a plateau of erosion, 
composed of gently inclined strata with an up-stream 
dip, waterfalls tend to be developed at the outcrops 
of intercalated hard beds. But, as erosion proceeds, 
these waterfalls retreat up the valley, and so are 
gradually replaced by gorges. 



82 EARTH SCULPTURE 

Now it may be said generally that in all regions 
composed of gently inclined strata, amongst which 
relatively hard beds are intercalated, escarpments and 
dip-slopes are developed by denudation. When the 
dip of the strata is persistent over a wide extent of 
country, we shall have more or less prominent escarp- 
ments traversing such a region continuously for 
miles. The escarpments will obviously vary in char- 
acter with the angle of dip and the nature and thick- 
ness of the rocks. If the hard bed or beds be of no 
great thickness and the dip high, the resulting escarp- 
ment and slope will constitute a somewhat narrow 
ridge ; but if the thickness of the hard beds be very 
considerable and the dip gentle, the escarpment may 
assume the form of a belt of plateau or a range of high 
ground, having a more or less diversified surface. 
England supplies some excellent examples of the 
kind. The general inclination of the strata between 
the borders of Wales and the North Sea is easterly, 
at a low angle ; consequently, as we walk in that 
direction we cross the outcrops of several great geo- 
logical systems. These are built up of sedimentary 
rocks, some of which are relatively soft and yielding, 
such as clay and shale, while others are harder and 
generally more porous, such as limestone, chalk, etc. 
Hence in time the latter have come to form a series 
of more or less prominent escarpments or belts of 
high ground, separated by broad tracts of gently 
undulating low ground. Starting from the foot of 
the Malvern Hills, and proceeding in an easterly 



LAND-FORMS, IN GENTL Y INCLINED STRA TA 83 

direction, we first traverse low-lying plains of sand- 
stone and argillaceous beds, until on the other side of 
the Severn we reach the Cotswolds, a belt of high 
ground over 1000 feet in height, and reaching in 
places a width of 30 miles. The rocks of which these 
hills are composed consist principally of limestones, 
which, as they dip gently eastwards, are succeeded by 
a series of argillaceous beds, forming again a region 
of undulating plains. Traversing these plains in 
the direction of dip, we eventually encounter another 
broad belt of high ground — the escarpment of the 
Chalk. This escarpment in its turn is succeeded by 
a low-lying region composed chiefly of relatively soft 
argillaceous beds and other non-indurated strata. 

A glance at any geological map of the country will 
show that all the prominent hills and high grounds 
of central and south-eastern England are developed 
along the outcrops of the Jurassic limestones and the 
Chalk, and thus have a general northerly or north- 
easterly trend. We cannot doubt that the present 
irregularities of the surface are the result of long-con- 
tinued epigene action, guided by the character of the 
rocks and the geological structure of the ground. 
The yielding strata have been worn away more rap- 
idly than the harder rocks, while the escarpments 
formed by the latter have slowly retreated as denuda- 
tion proceeded. This is sufficiently evidenced by the 
fact that detached outliers of the more durable beds 
are met with lying beyond the general outcrop of the 
series. Thus in Fig. 19 the outliers of Chalk (i, 2) 



84 EARTH SCULPTURE 

were obviously at one time connected with the main 
mass C — the dotted Hne representing the conditions of 
surface that formerly obtained. In a word, the de- 
tached masses have been left behind during the retreat 
of the escarpment to its present position. The course 
of the River Thames, whose head-waters rise on the 
east side of the Cotswold Hills, was doubtless deter- 



FiG. ig. Section of Escarpments and Outliers. 

mined by the inclination of the original surface of the 
ground. It will be observed that, like the streams 
represented in Fig. i6, this river flows across the out- 
crops of the Jurassic and Cretaceous strata, cutting 
through the Chalk escarpment between Wallingford 
and Reading. 

Although wide regions may be built up of strata 



Fig. 20. Section across the Wealden Area. (Ramsay.) 
a. Upper Cretaceous strata ; I, Lower Greensand, etc.; c, Weald clay; d, Hastings sands, etc. 

dipping continuously in one direction, yet it is more 
usual to find the direction of dip changing. Such 
changes may occur at wide intervals, or they may suc- 
ceed each other within narrower limits. Sometimes 
we may have the beds of a broad area arranged in 



LAND-FORMS IN GENTL Y INCLINED STRA TA 85 

one single anticline or syncline as the case may be. 
In other places the undulations of the strata may be 
numerous. Many examples of, such structures might 
be cited from the rocks of Great Britain. Restrict- 
ing attention for the present to gently inclined and 
undulating strata, we encounter a fine illustration 
of a broad anticline in the Chalk Downs and the 
Weald. (Fig. 20.) The latter might be described as 
a wide amphitheatre, open to the sea on the east, but 
surrounded in all other directions by bold bluff-like 
hills. Here the configuration has had precisely the 
same origin as the escarpments of the Midlands. The 
North and South Downs coincide with the outcrops 
of the Chalk, while the enclosed low grounds have 
been excavated out of underlying argillaceous and 
other unconsolidated strata. The Chalk, one cannot 
doubt, originally extended over the whole of the 
Wealden area, as shown by the dotted lines in Fig. 
20. That high ground formerly existed within this 
area is clearly indicated by the fact that the escarp- 
ment of the Downs has been sawn across by streams 
flowing out from the heart of the Weald. Obviously 
when these streams first began to flow, the water- 
parting in the axis of the Weald must have been at a 
higher level than the present summit of the Downs. 
The whole surface has been lowered by epigene ac- 
tion — the less readily reduced rocks and rock-struct- 
ures forming as usual the most prominent features in 
the landscape. 

The denudation of a broad anticline composed of 



86 EARTH SCULPTURE 

harder rocks intercalated among a series of more 
yielding strata results, as in the Wealden area, in the 
formation of lines of escarpment facing each other. 
In the case of a denuded syncline of similar strata 
escarpments are likewise developed, but their faces 
are now turned in opposite directions. Fig. 2 1 shows 
the geological structure of a portion of Ayrshire. 
Here we have a series of hard volcanic rocks (t'^), old 
lavas, in fact, intercalated between underlying and 
overlying sedimentary strata — chiefly sandstones and 
shales. The result is the same as in all the cases 
already considered — the more durable rocks crop out 




Fig. 21. Section across Permian Volcanic Basin, Ayrshire. 

f, Carboniferous strata ; z', volcanic rocks ; /, Permian sandstones. 

Strongly and form escarpments, but these look away 
from and not towards each other. 

In regions which have experienced much denuda- 
tion, gently inclined strata, when arranged in a series 
of anticlines and synclines, not infrequently give rise 
to an undulating surface. But this surface does not 
coincide with the deformations of the rocks below. 
In point of fact, anticlines are not infrequently repre- 
sented at the surface by depressions, and synclines by 
elevations. These phenomena are best developed 
when beds or masses of durable nature are intercalated 
in a series of more yielding rocks. In the accom- 



LAND-FORMS IN GENTL Y JNCLINED STRA TA 87 

panying section (Fig. 22) it will be observed that syn- 
clines coincide with hills, and anticlines with valleys. 
This configuration has been determined by the geo- 
logical structure. In each hill we have practically two 
escarpments placed back to back. The beds h h are 
relatively harder than others in the series. Had no 
such beds occurred the synclines would probably not 
have been so strongly emphasised by elevations. But 
the presence of one or more hard beds in series of un- 
dulating and relatively soft strata does not necessarily 
give rise to synclinal hills. The hard beds in such a 
series would no doubt in time crop out at the surface 



•. s> 




Fig. 22. Synclinal Hills and Anticlinal Valleys. 

s Sy synclines ; a a^ anticlines \ h k^ relatively hard beds. 

and project above the base-level of the district ; but 
if in the synclinal troughs they descended below that 
level, they could have no influence upon the surface. 
Thus in the section (Fig. 23) a relatively hard bed 
crops out and forms escarpments at e e, but it descends 
below the base-level, b b, in the two synclinal troughs 
(j-i s^), which remain unaffected by it. In the third 
trough (^^), however, it remains above the base-level, 
protecting the underlying softer beds, and thus forming 
a hill. 



88 EARTH SCULPTURE 

When a series of undulating strata contains no 
intercalated hard beds, but is of much the same 
consistency throughout, the synclines still offer the 
stoutest resistance to denudation, anticlines being 
relatively weak structures. In the former the strata 
are not liable to be undermined and displaced by the 



1^^li^^:Z^^Tr7rr^^S..it-rf^77^ 



Fig. 23. Escarpment Hills and Syncllnal Hill. 

e e, hard bed ; j^ s^ j^, synclinal troughs ; 6 d, base-level. 

action of springs. In the latter, however, the strata 
hang away from the axis, and water percolating 
through them, and coming out along the bedding- 
planes, tends to their demolition. But this is a mat- 
ter which will be considered more fully when we come 




Fig. 24. Section across West Lomond Hill and the Ochils. 

tf, igneous rocks ; ^, red sandstones, etc. ; c, basalt. 

to consider the surface-forms yielded by steeply in- 
clined and highly folded strata. 

In regions long exposed to denudation all weakly 
built hills tend to disappear. Hence in such countries 
anticlinal hills are of very rare occurrence. Now and 
again they do occur, but only when they happen to be 
composed of more durable rocks than those which 



LAND-FORMS IN GENTL Y INCLINED STRA TA 89 

repose upon their flanks. The Ochils of Kinross 
afford us a good example. (Fig. 24.) Here we 
have an underlying series of hard igneous rocks, «, 
folded along an axis from which they dip away on 
both sides below overlying sheets of red sandstone. 
These red sandstones almost certainly at one time 
extended across the anticline, which has thus been 




Fig. 25. Synclinal Valley West of Green River. (Powell.) 

much denuded. But, owing to the greater durability 
of the igneous rocks, the anticline, of which they 
form the axis, continues to show as a prominent 
elevation. 

Hitherto we have been considering the surface- 
forms assumed by gently folded strata in regions 



9° 



EARTH SCULPTURE 



which have been subjected for a more or less pro- 
longed period to subaerial denudation. In areas 
where deformation of the strata has been effected 
within geologically recent times, not infrequently 
some coincidence may be observed between the un- 
dulations at the surface and the underground struct- 




FiG. 26. Anticlinal Ridge, Green River Plains. (I'owell.) 

ure. The Colorado district we have described as a 
region of practically horizontal strata. Here and there, 
however, the rocks are more or less folded, and when 
such is the case they often give rise to corresponding 
folds at the surface. In the region traversed by 
Green River, for example, the horizontal strata occa- 



LAND-FORMS IN GENTL V INCLINED STRA TA 9 1 

sionally show anticlines and synclines, as in the follow- 
ing sketches from Major Powell's description of the 
Canon country, where the synclinally arranged beds 
in Fig. 25 form a valley, while the anticlinal strata in 
Fig. 26 appear as a swelling ridge. 

Such coincidence of underground structure and 
superficial configuration, however, is not always to be 
traced even in so young a land as the Canon district, 
while, as already remarked, it is of very uncommon 
occurrence in lands of high geological antiquity. 



CHAPTER V 

LAND-FORMS IN REGIONS OF HIGHLY FOLDED 
AND DISTURBED STRATA 

TYPICAL ROCK-STRUCTURES IN REGIONS OF MOUNTAIN-UPLIFT 

GENERAL STRUCTURE OF MOUNTAINS OF UPHEAVAL PRIMEVAL 

COINCIDENCE OF UNDERGROUND STRUCTURE AND EXTERNAL 
CONFIGURATION RELATIVELY WEAK AND STRONG STRUCT- 
URES — STAGES IN THE EROSION OF A MOUNTAIN-CHAIN 

FORMS ASSUMED UNDER DENUDATION ULTIMATE FATE OF 

MOUNTAIN-CHAINS. 

WE have now to study the various land-forms 
that characterise regions where highly folded 
strata occur. Deformation of the crust has taken 
place in all ages of the world's history. In some 
countries rock-plication and folding date back to the 
earliest period of which geologists have any certain 
knowledge. In other places the deformations belong 
to relatively recent times. Again, we find evidence 
to show that certain areas have experienced such 
changes at many successive periods. As might have 
been expected, the oldest rock-folds have suffered 
excessive erosion, while the youngest have experienced 
less. We are thus able to study in different countries 
the successive phases through which a region of highly 

92 



LAND-FORMS IN HIGHLY FOLDED STRATA 



93 



disturbed strata must necessarily pass. We see it in 
its youth in such mountains as the Alps, the Hima- 
layas, the Cordilleras, and in its old age in the 
Appalachians and the mountains of Scandinavia and 
Britain. 

Let us now briefly consider some of the typical 
kinds of structure presented by the more steeply in- 
clined strata. In regions of moderately inclined rocks 
the folds, as we have seen, are symmetrical anticlines 
and synclines. the axes of which are vertical, the beds 




Fig. 27. Isoclinal Folds. 

Axes moderately inclined from the vertical. 

dipping away from or towards the axes at approxi- 
mately equal angles. (See Fig. 22, p. 87.) Folds of this 
kind, however, are not restricted to areas of moderately 
inclined strata ; they are met with also in regions where 
the rocks as a rule dip steeply. But in such regions 
the anticlines and synclines are usually more or less 
unsymmetrical — their axes are inclined. In Fig. 27 
we have represented a series of moderately inclined 
folds. In Fig. 28 the inclination of the axes is still 
greater. As the folds in these two diagrams all lean 
in one direction, they are said to be isoclinal. Very 
frequently the inclination of the axes increases to such 
a degree that one fold may come to lie almost hori- 



94 EARTH SCULPTURE 

zontally upon another, as in Fig. 29. But when the 
axes are so highly inclined as that the folds usually 




Fig. 28. Isoclinal Folds. 

Axes much inclined. 



tend to become disrupted. All folds are the result 
of horizontal push or tangential pressure, and when 
this is very great they may yield by shearing, and 




Fig. 29. Isoclinal Folds. 

Axes horizontal = overfolds. 



one limb be thrust forward over the other, producing 
what is known as a reversed fault. (Figs. 30, 31.) 

So overpowering has been the horizontal move- 
ment in some cases that masses of rock thousands of 



LAND-FORMS IN HIGHL V FOLDED STRA TA 



95 



feet in thickness have been buckled up and sheared, 
or, simply yielding to pressure, have sheared without 
folding, and been thrust forward for miles along a 




Fig. 30. OvERFOLD Passing into Reversed Fault or Overthrust. 

gently inclined or even an approximately horizontal 
plane. These great reversed faults are termed over- 
thrusts or thrust-planes. Sometimes such thrust- 




FiG. 31. Reversed Fault. 

planes occur singly (Figs. 32, 33), at other times the 
rocks have yielded again and again, great sheets hav- 
ing been sliced off successively and driven forward 
one upon the other. (Fig. 34. ) 

Another structure encountered in regions of much 




^ ^A^'■T^ >A^;-/^ 



Fig. 32. Single Thrust-Plane. 



96 



EARTH SCULPTURE 



disturbed strata is the synclinal double-fold, shown in 
the annexed diagram. (Fig. 35.) In this case two 
anticHnal folds approach each other from different 
directions, the syncHnal depression between the 
approximating anticlines being occupied by highly 
convoluted strata. 

The converse of this structure is the anticlinal dotible- 
fold as shown in Fig. 36. Here two synclinal folds 

Til. i <t*. 




Fig. 33. Section across Coal-Basin of Mons. (M. Bertrand.) 

Z>^ D"^^ Lower and Upper Devonian ; CI, Carboniferous Limestone ; Cr, Cretaceous ; T, 

Overfold and thrust-plane. Devonian and Carboniferous strata turned upside 

down above the thrust-plane. 

approach each other, while in the intervening space 
the strata are arched into a great anticline. The beds 
within the anticline, it will be observed, are much 
compressed below, while they open out above. This 
is known as fan-shaped structure. 

Reverse faults and thrust-planes have been referred 
to, but it must be noted that normal faults also now 
and again occur in complicated regions. The former, 
as we have seen, are the result of horizontal, the latter 
of vertical movements of the crust. Reversed faults, 
therefore, are almost entirely restricted to regions 



Hf^ 






c^ 



1-1 
O 



a 

H 

o 

H 
5 



r^ S 



o fe 

1*1 m 

z s 

S ^ 

'-' s 

tAi g 



B " 




98 EARTH SCULPTURE 

where the rocks are more or less steeply inclined and 
contorted. Normal faults, on the other hand, occur 
under all conditions of rock-structure — traversing 
alike horizontally arranged strata and inclined and 
folded beds of every kind. 

So much, then, for the general types of structure 
met with among highly folded strata. So far as our 
present knowledge goes, complex folding, such as is 




Fig. 36. Anticlinal Double-fold. 

seen in true mountains of uplift, has resulted from 
horizontal movement in one direction. This is shown 
by the manner in which most of the more closely 
compressed and steeper folds of a mountain-chain 
tend to lean over one way. Under the influence of 
an irresistible horizontal thrust the strata find relief 
by folding, and the crust bulges upwards, the flexured 
rocks naturally bending over in the direction of least 
resistance. The resulting structure may be shown 
diagrammatically as in Fig. 37. In this diagram only 



LAND-FORMS IN HIGHL V FOLDED S TEA TA 99 

folds are represented ; in many cases, however, the 
rocks are not merely flexed, folded, and contorted, 
but dislocated and displaced. Frequently, indeed, 
they have yielded to the intense pressure by shearing, 
and slice after slice, hundreds or even thousands of 
feet in thickness, has been pushed forward and piled 
one on top of the other. Although the closer folds 
tend as a rule to lean over in the direction of crustal 
movement, yet occasionally they are inclined in the 
opposite direction, thus giving rise to the well-known 




Fig 37. Diagram of Moqntain Flexures. 

The arrow shows the direction of thrust. 

fan-Structure seen in the anticlinal double-fold, Fig. 
36. Now and again, too, the folds may open out, 
and so form symmetrical flexures with vertical axes, 
or normal anticlines and synclines. The cause of 
such variations in the folding of the strata is an in- 
teresting question, but does not concern us here. 

When a tract of highly disturbed rocks has been ex- 
posed to erosion for a very prolonged period, it is 
usually hopeless to attempt to reconstruct the original 
configuration of the ground, save in a very general 
way. The primeval land-forms that may have re- 
sulted from crustal deformation have been entirely 
remodelled or removed by denudation. But there 



loo EARTH SCULPTURE 

are many regions where similar extensive deformation 
has taken place at a relatively recent geological date, 
and where, therefore, time has not sufficed for the 
obliteration of all surface-features due to crustal dis- 
turbance. In the younger mountain-chains of the 
world, underground structure and orographical fea- 
tures to a certain extent coincide. The study of 
these mountains, therefore, enables us to realise the 
conditions that formerly obtained in tracts of highly 
complicated structure, from which, under long-con- 
tinued erosion, all tra,ce of the original configuration 
of the ground has vanished. Not only so, but the 
havoc wrought by epigene action upon even the 
youngest of our mountains shows us how and by 
what means the complicated mountain-chains of 
earlier days have gradually been reduced. For, just 
as lands built up of horizontal and gently inclined 
strata have experienced various degrees of erosion, 
thus enabling us to trace the successive stages through 
which such lands must pass, so regions of highly com- 
plex structure present us with various phases of denud- 
ation. And thus, by comparing one tract with 
another, we may spell out the whole story ; and in 
the degraded relics of former mountain-systems we 
read the fate that must eventually overtake the proud- 
est elevations of the present. 

The study of the land-forms assumed by highly 
flexured strata should naturally begin with the exam- 
ination of some young mountain-chain. But even 
the youngest of such mountains has already under- 



LAND-FORMS IN HIGHL Y FOLDED STRA TA lo i 

gone much erosion, and its structure is often ex- 
tremely complicated. To examine any one system in 
detail, and to follow the whole process of its denuda- 
tion, would be a laborious work, far beyond the limits 
of our present inquiry. All that we desire is to ascer- 
tain if we can how far geological structure and oro- 
graphical configuration coincide during the period of 
a mountain's infancy and early youth, and by what 
means its original form becomes modified and event- 
ually remodelled. For this purpose we may profitably 
begin our study by considering first some hypothetical 
case. \\^ shall suppose, then, that under tangential 
pressure the horizontal strata of some region have 
bulged up and become folded along a given line or 
zone. Under such conditions great faults and thrust- 
planes would be likely enough to occur ; but for the 
sake of simplicity we shall ignore these, and fix our 
attention only on the flexing and folding. We shall 
suppose further that our mountain-chain is the result 
of one prolonged continuous earth-movement. How, 
then, will the elevation of the strata affect the sur- 
face ? Will the complex folding of the rocks give 
rise to similar intricate deformations of the surface ? 
This does not necessarily follow, for, were the move- 
ment of elevation very slow and protracted, the grad- 
ually rising surface might be so continually reduced 
by denudation that underground structure and exter- 
nal form would rarely or never correspond. But, on 
the other hand, were the rate of elevation in excess 
of the rate of erosion, the larger folds of the strata 



I02 EARTH SCULPTURE 

might be expected to give rise to similar undulations 
at the surface. It is very doubtful, however, whether 
the latter would ever be as strongly pronounced as 
the former ; for at great depths the folds would be 
pressed closely together, while they would naturally 
tend to open out upwards into broader undulations. 
Hence, deeply buried rock-masses might be intensely 
flexed and folded, while the surface might show only 
a more or less pronounced bulging. The infant 
mountain might appear as merely one single long 
swell or undulation, with smooth slopes, declining at 
no great angle to the low grounds. Or there might 
be a series of two or more such undulations. The 
study of existing mountain-chains, however, leads to 
the belief that in some cases at least very considera- 
ble deformation of the surface has accompanied 
mountain-making, all the larger folds of the strata 
being probably at first represented above ground by 
corresponding ridges and depressions. 

We do not know whether the elevation of a moun- 
tain-chain was ever suddenly effected. So far as we 
can judge from the evidence supplied by geological 
structure, it would seem as if the horizontal move- 
ments of the crust had been gradual and protracted, 
and often interrupted by long pauses. There is little 
reason to doubt, however, that during the growth of 
a mountain-chain sudden snapping of rocks under 
pressure must have occurred frequently enough, and 
that earthquakes of greater or less intensity must 
have accompanied the upheaval. If such has been 



LAND-FORMS IN HIGHL Y FOLDED STRA TA 103 

the case, it would follow that the surface might be 
very considerably affected — rocks might be shattered 
and weakly constructed ridges shaken down — so that 
the anticlinal ridges of a mountain-chain might well 
have presented, even in the days of its infancy, a 
broken and ruptured surface. 

But; to return to our hypothetical mountain-chain, 
we shall suppose this consists of a series of parallel 
ridges which attain their greatest elevation along a 
line or axis not far removed from the thrust-side of 
the chain. From this axis the ridges decline gradually 
in importance in the direction of earth-movement, 
and eventually die out in a series of gentle undula- 
tions. Each of the ridges, we shall suppose, coin- 
cides with an anticline, and each of the intervening 
hollows with a syncline. In a word, we shall take 
the surface to be a more or less exact expression of 
the geological structure, the undulations of the ground, 
however, being less pronounced than those of the 
strata at considerable depths. The diagram (Fig. 37, 
page 99), will represent a section across such a chain. 
It will be observed that all faults and possible intru- 
sions of igneous rock are neglected. 

In any series of stratified rocks some are sure to 
be more porous than others, while all will be traversed 
by joints or cracks approximately at right angles to 
the bedding-places. This, then, we shall take to be 
the case with the rocks of which our young mountain- 
chain is composed ; and we shall suppose that the 
parallel ridges extend in a linear direction for many 



104 EARTH SCULPTURE 

miles, gradually declining in elevation towards both 
ends of the chain. With these conditions of surface, 
it is obvious that drainage will take place in the di- 
rection of the great longitudinal valleys or synclinal 
troughs, while a set of transverse streams will flow 
down the slopes of the anticlinal ridges. Many of 
these will thus become tributary to the rivers making 
their way along the axial hollows. All the rivers in 
course of time must cut into the rocks, but it is obvi- 
ous that the transverse streams will be of a torrential 
character, and will tend therefore to carve out nar- 
rower, deeper, and straighter channels than the larger 
rivers can excavate in the less inclined, broad axial 
depressions. Immense quantities of rock-material 
will be swept down from the anticlinal ridges to 
accumulate in heaps and sheets in the synclinal 
troughs, or to be swept away more readily, according 
as the gradients of the latter are gentle or steep. 
Erosion, in short, will be carried on most actively 
upon the anticlinal mountains. This would naturally 
follow, whatever the character of the geological struc- 
ture might be, for the erosive action of running water 
increases with the gradient. 

But in all cases denudation is hastened or retarded 
according as the rock-structure is weak or strong. If, 
therefore, the mountains of our hypothetical chain be 
more weakly built than the parallel synclinal troughs, 
the former will tend to be reduced more rapidly than 
the latter. This can be shown diagrammatically as 
in Fig. 38, p. 105. Here we have a section across two 



LAND-FORMS IN HIGHLY FOLDED STRATA 105 



anticlinal mountains and a synclinal valley. The strata 
consist of a series of more or less porous sandstones 
separated by intervening layers of impermeable clay- 
rocks. Moreover, they are jointed, and the joints 
traversing the anticlines tend to open out upwards, 
while the reverse is the case with those cutting the 
synclines. Some of these joints may be shrinkage- 
cracks which came into existence during the slow con- 
solidation of the strata, perhaps long before the latter 
were flexed and folded. But a large proportion no 
doubt would be produced while the rocks were being 
bent and doubled up. In whatever way formed, 
joints are readily permeated by meteoric water, which 
finds its way down from the surface and soaks into 




Fig 38. Diagram of Anticlinal Mountains 

Pervious strata (stippled) and impervious layers (thin lines) ; jj\ joints, cutting strata at right 
angles ; z/, valley ; j j, springs coming out at junction of pervious and impervious beds. 

the porous strata below. Constantly augmented from 
above, the water thus imbibed is forced to percolate 
through the porous beds in the direction of the dip. 
Hence wherever these beds are truncated (as in the 
valley) the water comes out at the surface as natural 
springs. Thus in the illustration springs appear at 
s s, where permeable sandstones are underlaid by im- 



io6 



EARTH SCULPTURE 



permeable clay-rocks. The effect of these springs is 
not hard to understand. They tend to undermine 
the sandstones, and as the dip of the strata is towards 
the valley, rock-falls and landslips must continue to 
take place until the anticline is reduced. Anticlinal 
mountains separated by a synclinal trough are thus 
in a state of unstable equilibrium. Sapped and un- 
dermined by rain, frost, and springs, their existence 




Fit! 39. Synclinal Valley Shifting towards Anticlinal Axis. 

a, synclinal valley ; d^ anticline ; z', valley, gradually widened in the direction of the arrow. 



cannot be prolonged. On the other hand, the strata 
in the synclinal trough, although consisting of the 
same materials, will be relatively more durable. Their 
arrangement favours their preservation ; they are not 
sapped and undermined as in an anticline, but are 
reduced chiefly by the vertical erosion of the rivers 
that traverse them. 

The anticlines of our mountain-chain are thus not 
only deeply incised by transverse streams and torrents, 



LAND-FORMS IN HIGHLY FOLDED STRATA 107 

but they are liable all along their flanks to the under- 
mining action of the longitudinal rivers and their allies, 
— rain, frost, and springs. Quite undisturbed by 
earthquakes, their destruction by epigene action is, 
nevertheless, assured. But if the young mountain- 
chain be liable, as all such mountains are, to earth- 
quake-shocks, the demolition of the already weakened 
anticlines will often be greatly accelerated. 

Unsymmetrical anticlines are not less liable to de- 
struction than those we have just been considering. 
Indeed, their arrangement must lead sometimes to 
the gradual shifting of a longitudinal river from a 
synclinal to an anticlinal axis. Thus a river occupy- 
ing the syncline a (Fig 39), and eventually cutting 
more or less deeply into the underlying strata, will 
tend to work its way towards the axis of the anticline 
d. For it will be observed that the beds of that anti- 
cline dip into the valley, while those on the other side 
dip away from it. The latter, therefore, is a strong 
structure, and the valley-cliffs will recede relatively 
slowly in that direction, while rock-falls and landslips 
will prevail on the side of d. The valley, therefore, 
will be widened most readily towards d ; and, the like 
conditions obtaining in all the longitudinal valleys of 
a chain, the time will come when every similarly con- 
structed anticlinal ridge may be reduced. 

Many other modifications of the drainage of a 
mountain-chain will be brought about by the action 
of the streams and rivers. Thus a transverse stream, 
which as a rule works more energetically than a longi- 



io8 EARTH SCULPTURE 

tudinal river, may now and again succeed in cutting 
its way back across an anticline so as to tap some ad- 
jacent synclinal trough. If the bottom of this trough 
should chance to be at a higher level than that of the 
hollow into which the transverse stream makes its 
way, the river of the invaded syncline may be cap- 
tured by the stream. Thus we should have the phe- 
nomenon of a longitudinal river changing its course 
and becoming transverse. 

The chief point, however, which we have at present 
to bear in mind is simply this : that anticlinal struc- 
tures are weak and tend to be reduced ; while synclinal 
arrangements are relatively strong, and consequently 
more persistent. We should expect to find, therefore, 
in all mountains of upheaval, exposed for any time to 
denudation, that synclinally arranged strata will not 
infrequently appear in a tolerable state of preserva- 
tion ; while anticlinal beds will often be deeply eroded. 
Let us, then, turn our attention to the structures met 
with in such a region as the Alps, and see how far 
they bear out these elementary conclusions. 

That great chain is a typical example of what are 
known as mountains of elevation. It consists essen- 
tially of a succession of anticlines and synclines, 
chiefly unsymmetrical. The strata are not only folded 
and often exceedingly contorted, but the structure is 
still further complicated by vast thrust-planes and 
normal faults. Moreover, the chain is the result, not 
of one, but of many successive earth-movements. But 
the chief movement — that, namely, to which the 



LAND-FORMS IN HIGHLY FOLDED STRATA 109 

mountains owe most of their present elevation — took 
place at a relatively late geological period. Many of 
the folded and fractured rocks, indeed, are of no 
greater antiquity than the soft clays and sands over 
which London is built. And yet, although the chain 
belongs to so late a date, its rocks everywhere bear 
witness to great erosion. Enormous masses of ma- 
terial have been gradually removed, and the original 
surface, due to folding and displacement, has been 
more or less profoundly modified. 

The sketch-section across the Swiss Alps (Fig. 40, 
p. 1 10) gives the general arrangement of the strata, 
and enables us in some faint measure to appreciate 
the degree of denudation which has already been ex- 
perienced by these relatively young mountains. 
Grant, if you will, that the folding of the strata may 
have resulted in a kind of chaos at the surface — that 
the ground along the axes of anticlinal arches may 
have been ruptured, and the rocks everywhere tum- 
bled in confusion — yet we have still to account for 
the wholesale removal of the abundant ddbris — the 
shattered reefs and dislodged mountain-masses. We 
cannot, in short, escape from the conclusion that an 
enormous amount of denudation has taken place. 
So profoundly has the original configuration been 
modified, that it is only when the mountains are 
viewed in the broadest way that any coincidence be- 
tween underground structure and surface-features 
can be observed. Even where anticlines still form 
hills and mountains it is obvious that they have yet 



^ X 






LAND-FORMS IN HIGHL Y FOLDED STRA TA in 

suffered extensive degradation. (See Fig. 41.) Not 
infrequently, indeed, they are more or less deeply 




Fig. 41. Summit of Santis, East Side (A.. Heim). 

Anticlinal mouiUEun. 

trenched — valleys running along their axes, an ap- 
pearance well shown in Fig. 42. Synclinal hollows 



^<:liorf4i>.ko|i/ 




Fig. 42. Section across the Schortenkopf, Bavarian' Alps 
(E. Fraas). 
Anticlinal valley in calcareous rocks and shales (Triassic.) 



EARTH SCULPTURE 



now and again coincide with depressions at the sur- 
face, as in Fig. 43 ; but they just as often, or even 



ni-nHr K»i3«' 



Voritr ~XA,st.1 




Fig. 43. Section across the Kaisergebirge, Eastern Alps (E. Fraas). 

Synclinal valley in calcareous rocks and shales CTriassic). 




Fig. 44. Section across the Val d'Ui.n'A (Gumbel). 

Triassic strata resting on crystalline schists. 




Fig. 45. Sichelkamm of Wallenstadt (Heim). 

Sickle-shaped overfold. 



LAND-FORMS IN HIGHL Y FOLDED STRA TA 113 

more frequently, form elevations, as in Figs. 44, 45. 
In every case, however, the evidence of denudation 




Fig. 46. Section across the Northern Limestone Alps (E. Fraas). 

/, Crystalline schists ; ^, Permian ; j, Bunter ; ^, Muschellcalk ; 5, Limestone (Wetterstein- 
kalk) ; 6, Dolomite ; 7, Jurassic and Cretaceous. 

is conspicuous. Nor is this less clearly seen in the 
more complicated structures of the Alps. In the fol- 




Fig. 47. Section across the Diablerets (Kenevier). 
Tertiary strata showing a succession of overfolds. 

lowing section, for example (Fig. 46), we have a series 
of various calcareous strata and underlying schists 
compressed into folds and dislocated, the tops of the 



114 



EARTH SCULPTURE 



anticlines having in each case been removed. Take 
again the section of the Diablerets (Fig. 47), in which 
the Tertiary strata are doubled back upon themselves 




Fig. 48. Section across Dent de Morcles (Renevier). 

I Schistose rocks, etc, ; 2, Carboniferous strata ; 3. Jurassic strata ; ^, Cretaceous strata ; 5, 
Tertiary strata ; ^, ^, t^", Cretaceous and Tertiary rocks inverted ; T', thrust-plane. 

in a series of sharp overturned flexures. A similar, 
but somewhat more complicated, structure appears in 
the Dent de Morcles (Fig. 48), where the remarkable 




Fig. 49. Inversion and Overthrust in the Mountains South of the 
Lake of Wallenstadt (E. Fraas, after A. Heim). 

f, Schistose rocks ; /, Permian ; wj, hj\ Jurassic ; c, Cretaceous ; c, Eocene. The Permian 
strata (^f) are turned upside-down and thrust upward over the contorted Eocene (f). 

overturn flexure rests upon a thrust-plane. Here, 
again, the strata, it will be observed, are doubled back 
upon themselves, or turned upside-down. Obviously 
these mountains are monuments of excessive erosion. 



LAND-FORMS IN HIGHLY FOLDED STRATA 115 

Similar evidence of vast rock-removal is furnished by 
the remarkable double-folds and overthrusts in the 
mountains of the Cantons Glarus and St. Gall, as 
described by Heim and others (See Fig. 49.) 

Similar conclusions may be drawn from the appear- 
ances presented by every kind of rock-structure 
throughout the whole extent of the Alps. 

In the Jura mountains the rock-foldings are some- 
times symmetrical, and anticlines and synclines now 
and again coincide with hills and valleys respectively, 
as in Fig. 50. 

It will be observed, however, that the synclinal 
strata have suffered less erosion than the intervening 




Fig. 50. Symmetrical Flexures of the Jura Mountains. 

Anticlinal mountains and synclinal valleys. 

anticlinal strata. In the western part of the same 
range of mountains the folds are less symmetrical, 
but they yield the same evidence of denudation. The 
accompanying section (Fig. 51, p. 116) shows, indeed, 
that the saddlebacks have not only been considerably 
reduced, but are even beginning to develop into val- 
leys ; while the synclines, on the other hand, have 
experienced less erosion, those with approximately 
vertical axes appearing as dominant heights. 

Excellent examples of the same phenomena are 
furnished by the Carpathians — a mountain-chain also 



J5 



.t\ 



< 

S V 

O X 

IS 5 

Pi .a 

w 5 

g I 

o ^ 

CA S 



14 







a -2 



a 

h 

c 
m 
z; 
o 

S) 

K 
Z 

o 









Ii6 



LAND-FORMS IN HIGHL Y FOLDED STRA TA 



117 



of relatively recent age. Fig. 52 ^ 
(p. 116) exhibits the structure of ^ 
a part of the chain in which the 
folds are unsymmetrical. Here 
it will be observed that the tops of 
the anticlines have in every case 
been greatly reduced ; but the 
synclines, owing to the isoclinal ^ 
arrangements of the strata, do -^ 
not tend to develop into hills. ,i 
In point of fact, unsymmetrically 
folded strata behave very much 
in the same way as beds having 
a persistent dip in one direction. 
When the anticlines have been 
truncated the strata appear at 
the surface as a series of isoclinal 
beds, some of which are rela- 
tively more resistant than others. 
In time, therefore, these harder 
beds crop out as well-marked 
ridges or escarpments, according 
as the angle of dip is high or rela- 
tively low. But no sooner do 
the axes of the folds approach the 
vertical, and the flexures become 
symmetrical, than the superior 
strength of the synclinal structure ^ 
at once asserts itself. This is well 
illustrated by Fig. 53, where we have a series 






z 
< 

< 

a, 
■< 
U 

l-l 

Q 
Q 



< 



2 .S 



of syn- 



• EARTH SCULPTURE 

clinal troughs forming conspicuous mount- 
ains, while the intermediate anticlines cor- 
respond for the most part with valleys 
and depressions. 

If it be true, therefore, that the denuda- 
A tion of young mountains, such as the Alps 
G and the Carpathians, has been guided and 
5- determined to a large extent by geological 
g structure, we ought to meet with still 
> stronger evidence of a like kind in mount- 
g ^ ain-ranges of greater antiquity. The 
^ iS mountain-systems we have been consider- 
o ^ ing are of Csenozoic age ; they are among 
o I the latest great upheavals of the world. 
S ^ We see in the Appalachian Chain of North 
< g America a very much older system, for it 
" s came into existence about the close of 
t I Palaeozoic times. Being of such enormous 
"^ I antiquity, the Appalachians ought to give 
H m evidence of correspondingly great denuda- 
o tion. All the weak geological structures 
'fj < should have collapsed and disappeared 
y 2 ages ago ; the heights ought not to coin- 
N u cide with anticlines. The accompanying 
I . section across a portion of the chain in 
'f, "? Pennsylvania shows that this has actually 
^ ^ happened, symmetrical synclines having as 
usual developed into hills, while anticlines 
have been degraded. 

Similar evidence might be adduced from 



O 



LAND-FORMS IN HIGHLY FOLDED STRATA 119 

many other regions, but enough has been advanced 
to show that in the process of erosion and denudation 
of mountains of upHft, anticlines, as compared with 
synclines, are essentially weak structures. When the 
flexures are symmetrical the synclines tend to be 
carved into hills, but when the axes are inclined the 
strata often give rise to a series of prominent escarp- 
ments or to a succession of ridges with intervening 
hollows, the escarpments and ridges corresponding to 
the outcrops of the more resistant rocks. (Fig. 55.) 
Comparing mountain-chain with mountain-chain, 
we find, as might have been expected, that the oldest 
mountains, if they are the least prominent, are at the 
same time the most stable. They have endured so 
long that much of their primeval elevation has been 
lost ; the weakly built structures have been demolished, 
and only the stronger now remain. Great rock-falls 
and landslips are therefore seldom heard of among 
such mountains. It is quite otherwise with the 
younger uplifts of the globe. The valleys of the 
Alps, the Caucasus, the Himalayas, the Cordilleras, 
and other chains of relatively recent age are cumbered 
with chaotic heaps of fallen rock-masses. From time 
to time peaks and whole mountain-sides collapse and 
slide into the valleys ; and this rapid degradation will 
continue until every weak structure has been removed. 
The hills and mountains of our own country have long 
since passed through this phase of unstable equilib- 
rium. In the younger mountain-chains of the globe 
underground structure and superficial configuration 



I20 EARTH SCULPTURE 

Still to a certain extent coincide, but in the more 
ancient and therefore more highly denuded mountain- 
systems such coincidence is of very rare occurrence. 
Anticlinal mountains built up of porous and relatively 
impermeable strata are restricted to regions of recent 
uplift, and have no long life before them. 

We have seen that in the case of plains and plateaux 
of accumulation the original surface of the ground is 
an expression of the geological structure, the general 
direction of their drainage-systems being determined 



Fig. 55. U.NSYMMETRicAi. Folds, Giving Rise to Escarpments and Ridges. 

h h, hard beds ; s s, soft beds. 

by the average inclination of the strata. The same 
is no doubt to a large extent true of regions of mount- 
ainous uplift ; the shape of the surface and the 
direction of the streams and rivers must at first have 
been determined by the arrangement or architecture 
of the rocks. But while it is comparatively easy to 
realise the conditions that obtained in a plateau-coun- 
try during the early stages of its existence, it is very 
much harder to picture to ourselves the general aspect 
which a mountain-chain must have presented at the 
time of its upheaval. We are justified by the evidence 



LAND-FORMS IN HIGHLY FOLDED STRATA 121 

in believing that the larger inequalities of the surface 
must often have coincided with corresponding flexures 
and other deformations of the strata. But we need 
not suppose that all the convolutions, fractures, and 
displacements now laid bare in precipice and gorge 
actually appeared as such at the surface. Laboratory- 
experiments have shown that a great deal of flexing, 
folding, contortion, and displacement may take place 
underground, while the surface simply swells up or 
bulges. And that may quite well have been the case 
with many mountain-chains. Yet we cannot ignore 
the possibility or probability that folding and displace- 
ment of strata may sometimes have resulted in whole- 
sale rupture and confusion at the surface. We need 
not wonder, therefore, if we sometimes find it hard to 
account for certain vagaries in the drainage-systems 
of mountain-chains. Even the youngest of these 
chains has experienced so much denudation, that it 
is often impossible to realise the surface-conditions 
which may have determined the initial directions of 
the rivers. The longitudinal watercourses doubtless 
follow the axial arrangement of the strata, some of 
them occupying structural hollows (synclines), while 
others run along the backs of anticlines, or follow the 
outcrops of relatively softer rocks. The origin of 
certain transverse river-courses is harder to under- 
stand. Some of these may cut across a succession of 
great ridges ; they break through the mountains in 
such a way as to suggest that they are perhaps follow- 
ing a line of fracture. Most commonly, however, this 



122 EARTH SCULPTURE 

is certainly not the case. Sometimes it can be shown, 
as already indicated, that a transverse stream has 
simply eaten its way back into the heart of the 
mountain-ridge, which it has eventually breached or 
" g^PPsd'" ^"^d so worn down as to encroach upon the 
drainage-area of some adjacent longitudinal valley. 
Transverse streams working back in this Vv^ay have 
not infrequently captured longitudinal rivers, which 
thus appear to mysteriously forsake their own valley 
in order to break through a mountain-ridge. Perhaps 
most of the sudden changes in direction of Alpine 
rivers are illustrations of this system of capture. It 
is possible, however, as some geologists have sup- 
posed, that certain transverse river-courses may have 
been determined by the presence of a series of minor 
crustal folds, arranged at right angles to the main or 
longitudinal flexures of a mountain-chain. But we 
know so little of the actual conditions of surface that 
obtained when such a chain was being upheaved, that 
we must often be content to remain in ignorance of 
the causes that may have led to the sudden deflection 
of a river across a mountain-ridge. When we bear in 
mind, however, that the present lines of drainage can 
agree only in a general way with those that came into 
existence at the birth of a chain — that many anticlinal 
arches, now laid bare and deeply eroded, may never 
have shown at the original surface — it is not hard to 
understand how certain transverse river-courses may 
have come to intersect a succession of ridges. In 
many cases such courses may really indicate the 



LAND-FORMS IN HIGHLY FOLDED STRATA 123 

primeval inclination of the ground, the rivers having 
cut their way at first without any reference to deeply 
buried structures, which were only to be exposed 
later on during the general process of denudation. 

Although we may vainly endeavour to trace the 
history of all the river-courses of a mountain-chain, 
we need be in no doubt as to the ultimate fate of the 
mountains themselves. It is more difficult certainly 
to discover the various stages in the erosion of a 
mountain-system than in that of a plateau of accu- 
mulation ; but we are assured that all elevated lands, 
whatsoever their origin, tend to be lowered to their 
base-level. Should that base-level be steadfastly 
maintained, mountains and plateaux alike must event- 
ually be reduced to the condition of plains of erosion. 
But the modifications of the surface of a mountain- 
region developed during the process of erosion are 
infinitely complex. This is due partly to the very 
varied composition of the rocks, and partly to the 
complicated geological structure. 

The surface-features of a denuded plateau of accu- 
mulation have a general sameness ; there is little 
variety in the form of the hills and mountains — all 
are more or less pyramidal. In regions of gently 
inclined and undulating strata the features due to 
erosion are more diversified, and this diversity be- 
comes greater as the dips of the strata increase and 
change rapidly in direction. The foothills that flank 
the base of so many mountains of uplift are com- 
posed very often of symmetrically folded strata, but as 



124 EARTH SCULPTURE 

we pass inwards to the main chain the folds become 
steeper and unsymmetrical, and the structure is 
rendered still more complex by vast overthrusts 
and shearing-planes. As the structural complexity 
increases, and the rocks are thrown and twisted into 
every possible position, the surface-features are con- 
stantly changing, so as to show, often within narrow 
limits, every variety of cliff and ridge and peak. We 
see then that it is geological structure chiefly that 
determines the form of the ground ; and since the 
inclination, the folding, and the shearing of rocks 
must be attributed to crustal movement, it is clear 
that hypogene action has played a most important 
part in the formation of mountains. We may say with 
truth that all true mountain-ranges owe their origin 
to deformation of the crust. But the shape which 
they ultimately assume is solely the result of erosion. 
It is hypogene action which provides the rough blocks ; 
it is by epigene action that these are subsequently 
carved and chiselled, the forms of the sculptured 
masses being determined by the nature and structure 
of their materials. In regions of recent uplift, the pro- 
cess of sculpturing, although considerably advanced, 
has not yet sufficed to obliterate the original or 
primeval shape of all the masses. But in elevated 
tracts of great antiquity the land-blocks have been 
entirely remodelled. In the general lowering of the 
surface by denudation, mountain-masses have been 
removed, and what were formerly depressed areas 
now often appear as dominant elevations. Mountains 



LAND-FORMS IN HIGHLY FOLDED STRATA 125 

of recent uplift are characterised by steep profiles, 
by peaks and knife-edged ardtes ; the structures are 
often unstable, and yield readily to the agents of 
erosion, so that rock-falls and landslips are constantly 
taking place. In regions of ancient uplift, on the 
other hand, the profiles are generally softer ; peaks 
and sharp-crested ridges are of less frequent occur- 
rence, weak structures have disappeared, and the 
degradation of the mountains does not advance so 
rapidly. The levelling process, however, though 
slower, is quite apparent. The valleys are widened 
and deepened, the mountains crumble down, and, 
should the base-level of erosion be retained, the 
whole area will eventually be flattened out and 
resolved into a plain of erosion. 

Such then are the several stages through which a 
region of mountain-uplift must pass. First comes the 
stage of youth, when the surface configuration corre- 
sponds more or less closely with the underground 
structure. Next succeeds the stage of middle-life, 
when such coincidence is all but obliterated, when the 
valleys of youth have been exalted and its mountains 
have been laid low. Last comes old age and final 
dissolution, when the whole region has been reduced 
to its base-level. But the decay of a mountain-chain 
does not always proceed without interruption. Not 
infrequently the base-level is disturbed ; new hori- 
zontal movements of the crust take place, and bulging- 
up of the region is accompanied by further folding 
and fracturing of the strata. The mountain-system 



126 



EARTH SCULPTURE 



renews its youth. On the other hand, the old base- 
level may be destroyed by subsidence of the crust, 
and the mountains, partially or wholly drowned, may 
in time become largely buried under new accumula- 
tions of sediment. Re-elevation taking place, erosion 
recommences, and the degradation of the region is 
resumed. In the structure of not a few mountain- 




FiG. 56. Structure of the Ardennes (after Cornet and Briart). 

^jT/, the existing surface; the light-shaded area above this level represents the rock-masses 
removed by denudation. The Silurian rocltsat the base of the section are indicated by thin 
white lines. Above these, on the left-hand side of the section, between C and M^ come 
Devonian conglomerate, sandstone, shale, and limestone ; next in succession follow the 
Carboniferous strata at and above M ; A A, B B, C C, are dislocations. 

chains we may read the history of many such vicissi- 
tudes. 

So completely have some mountains been removed 



LAND- FORMS IN HIGHL Y FOLDED S TRA TA 127 

by denudation, that without some knowledge of geo- 
logical structure we should never have divined their 
former existence. An instructive example is furn- 
ished by the Carboniferous tracts of Belgium and 
Northern France. The structure of these regions 
shows that formerly a considerable range of moun- 
tains extended between Boulogne and Aix-la-Cha- 
pelle. At or towards the close of Carboniferous times 
a great earth-movement, acting in a direction from 
south to north, buckled up the strata, and these, 
yielding to the pressure, snapped across, and exten- 
sive overthrusting followed along the line referred to, 
the Carboniferous beds being inverted and overlaid 
by Devonian strata. The mountains of upheaval 
which thus came into existence attained a great 
elevation, the higher parts of the range reaching 
probably not less than 16,000 or 18,000 feet. The 
section (Fig. 56) will show how completely the sur- 
face has been remodelled, how mountains of elevation 
have been replaced by a plain of erosion. 



CHAPTER VI 

LAND-FORMS IN REGIONS OF HIGHLY FOLDED 
AND DISTURBED STRATA {continued) 

STRUCTURE AND CONFIGURATION OF PLATEAUX OF EROSION 

FORMS ASSUMED UNDER DENUDATION MOUNTAINS OF CIR- 

CUMDENUDATION HISTORY OF CERTAIN PLATEAUX OF ERO- 
SION SOUTHERN UPLANDS AND NORTHERN HIGHLANDS OF 

SCOTLAND — STAGES IN EROSION OF TABLE-LANDS. 

IN our last chapter we considered the history of a 
mountain-chain, following that history from the 
stage of youth to old age and final dissolution. This 
last we recognised in the plain of erosion. We have 
next to trace the subsequent history of such a plain. 
The geological structure of many mountain-chains, as 
already indicated, reveals the fact that these are often 
the result of more than one uplift. After having been 
for long ages subjected to erosion, and even to sub- 
sequent subsidence and sedimentation, the same region 
has again yielded to lateral crush, and new series of 
folds and thrust-planes have come into existence. But 
the crust does not always yield in this particular 
fashion. Not infrequently relief from pressure is ob- 
tained by widespread bulging-up of the surface, one 
or more broad swellings with perhaps corresponding 

128 



LAND-FORMS IN HIGHLY FOLDED STRATA 129 



broad depressions appear, instead of an intricate ar- 
rangement of more or less closely compressed folds. 
We may for convenience' sake speak of the latter as 
resulting from axial uplift, and of the former as due 
to regional uplift, even although it be obvious that in 
most wide regions of uplift there must be an axis or 
line of maximum movement. 

Now it can be shown that one and the same region 
has not infrequently experienced both kinds of uplift. 
Axial uplifts have in time been succeeded by regional 
uplifts ; for again and again we encounter ancient 




Fig 57. D1AGKA.MMATIC Section across a Plateau of Erosion. 

Isoclinal folds. 

plains of erosion occurring at various levels above 
the sea, their geological structure showing clearly 
that they have replaced old mountains of complicated 
structure. Such elevated plains may be termed plat- 
eaux or table-lands of erosion, to distinguish them 
from plateaux of accumulation or deposit. The 
characteristic feature of the latter, it will be remem- 
bered, is the general coincidence of the surface with 
the underground structure, while the former shows 
no such correspondence. The structure of a table- 
land of erosion may thus be represented as in Fig 57. 
Many such table-lands are recognised in Europe, 
the Highlands and Southern Uplands of Scotland 



13° EARTH SCULPTURE 

and the Scandinavian plateaux being good examples. 
Ancient plateaux of the kind are all more or less de- 
nuded, trenched, and furrowed by valleys to such an 
extent that the plateau character is often somewhat 
obscured. For no sooner is a plain of erosion up- 
lifted than a new cycle of erosion begins. The di- 
rection of the drainage is determined, in the first 
place, by the slope of the ground, and this we can 
readily understand may be somewhat diversified. The 
surface may be canted either in one direction only, 
or in more than one, for the crustal movement is un- 
likely to be equal in amount throughout the whole 
region of uplift. Hence, the primeval rivers may all 
flow in one particular direction, or they may trend to 
various points of the compass. However that may 
be, it is certain that in course of time they must 
gradually deepen their valleys, and the plateau must 
eventually come to be cut up very much in the same 
way as a plateau of accumulation. But the mountains 
of circumdenudation resulting from this process will 
differ considerably in character from those carved out 
of horizontal strata. The varying structure of the 
rocks will necessarily influence erosion, and thus lead 
to a greater diversity of form. Should the strata be 
steeply inclined, and this will usually be the case, then 
it is obvious that the harder masses must come in 
time to project beyond the more readily reduced 
rocks with which they are associated. The general 
surface of the plateau will thus tend to assume a cor- 
duroy configuration, the long ridges coinciding with 



LAND-FORMS IN HIGHLY FOLDED STRATA 131 

the outcrops of the "harder rocks," while the inter- 
vening parallel hollows will correspond with the out- 
crops of the more yielding strata. In short, the 
land-features evolved by denudation will have a gen- 
eral resemblance to those produced in a region of 
slightly inclined and gently undulating formations. 
But owing to the very varied character of the rocks 
and their more complicated structures, the surface- 
features of a plateau of erosion will be more pro- 
nounced and much more irregular. In such a region 
the larger rivers, being frequently of primeval origin, 
will often be found to cut across mountain-ridge after 
mountain-ridge, and to follow courses more or less 
transverse to the corduroy surface. Others may keep 
closely to the outcrops, and run in the direction of 
the "strike" or trend of the strata, while some may 
take now one route and now another. The original 
surface of the plateau will generally be indicated by 
the direction of the main drainage-lines or principal 
rivers, while the subsequent slopes due to erosion will 
usually be manifested by the course of tributary 
streams. During the progress of denudation, how- 
ever, many modifications of the drainage will be 
brought about. Cases of the capture of principal 
rivers by lateral streams working their way back or 
across the strike can hardly fail to occur, and these 
and other changes may render the original drainage- 
lines obscure and hard to trace. 

To such an extent have many ancient plateaux of 
erosion been denuded, so deeply have they been 



132 EARTH SCULPTURE 

trenched, that their surface has become resolved into 
a truly mountainous region, wherein all the elevations 
are mountains of circumdenudation, the tops of which 
are the only remaining relics of the original plateau- 
surface. Such mountains, owing generally to the 
durability of their rocks and the strength of their 
structure, are not so readily demolished as the moun- 
tains in a range of recent uplift. They may not often 
emulate these in height and grandeur, their profiles 
may as a rule be less wild and irregular ; but such is 
not always the case. When a plateau of erosion 
stands at a great elevation, the mountains carved out 
of it are apt to rival the boldest and most abrupt of 
Alpine heights. Such abrupt slopes and the profound 
valleys that intervene are the result of relatively rapid 
and powerful vertical erosion. But when a plateau 
has only a moderate elevation, the configuration of its 
mountains tends to be less abrupt, and to approximate 
in character to that attained by a true mountain-chain 
during the period of its maturity, when all weak 
structures have been demolished and the surface no 
longer coincides with the folds of the strata. And 
this is just what might have been expected, when it 
is borne in mind that in each case the fundamental 
geological structure is the same. A mountain-chain 
is composed mainly of highly flexed and folded rocks. 
Subjected to erosion, the whole region is remodelled 
and eventually reduced to a base-level. But the rock- 
structure remains ; the plain of erosion is composed, 
just as the mountains were, of highly flexed and folded 



LAND-FORMS IN HIGHLY FOLDED STRA TA 133 

rocks. When that plain is upHfted en masse to form 
a plateau it is obvious that epigene action must tend 
to evolve out of the plateau mountains and ridges 
which, in their form and alignment, will closely re- 
semble those that existed over the same area before 
the old plain of erosion had come into existence. The 
rocks and rock-arrangements, being the same in both 
cases, must under denudation tend to produce a sim- 
ilar configuration. No doubt there might be certain 
contrasts, but these would not be due so much to 
geological structure as to changes in the character of 
the rocks. The planing away of great mountain- 
masses might well expose quite a different series of 
rocks, and these, when the region was again uplifted 
and carved into hill and valley, would doubtless 
weather differently from the rock-masses under which 
they formerly lay buried. But the general geological 
structure remaining the same, mountains and ridges 
would necessarily be developed along the old lines. 

We may now consider the structure of certain plat- 
eaux of erosion which there is every reason for be- 
lieving existed at one time as plains — plains which 
had previously replaced mountain-systems. A good 
example is ready to our hand in the Southern Uplands 
of Scotland — that belt of high ground which is drained 
by the Clyde, the Doon, and other streams flowing 
north-west, and by the Cree, the Dee, the Nith, and 
the Annan flowing south-east. The north-east sec- 
tion of the region is traversed by the Tweed, with an 
easterly to north-easterly course ; while the extreme 



134 EARTH SCULPTURE 

south-west portion is watered by the Stinchar, flowing 
in a south-west direction. The whole area drained 
by those rivers and streams might be described as a 
broad undulating plateau, furrowed and trenched by 
narrower and wider valleys. The mountains are some- 
what tame and monotonous — flat-topped elevations 
with broad, rounded shoulders and smooth grassy 
slopes. The rocks composing the region consist for 
the most part of greywackes and shales, the former 
being usually hard greyish-blue rocks arranged in 
beds of variable thickness. They are' much more 
abundantly developed than the shales which are asso- 
ciated with them, although now and again the latter 
attain considerable importance. The strata usually 
dip at high angles, often approaching the vertical, and, 
the same beds coming again and again to the surface, 
it is obvious that we are dealing here with a vast suc- 
cession of steeply inclined and closely pressed anti- 
clinal and synclinal folds. In many natural exposures, 
as on the coast and in the valleys, the intensely folded 
character of the strata is clearly revealed. Obviously 
the strata have been squeezed together, and affected 
in precisely the same way as the rocks of the Alps. 
Frequently, indeed, we find that overthrusting has 
taken place, the rocks having yielded to tangential 
pressure by shearing. The general trend or " strike " 
of the strata is from south-west to north-east, while 
the dip is sometimes north-west, sometimes south-east, 
changing now and again very rapidly, at other times 
remaining constant for long distances. In the former 



LAND-FORMS IN HIGHLY FOLDED STRATA 135 

case the folds are not infrequently approximately 
symmetrical ; in the latter they are necessarily un- 
symmetrical. In a word, the geological structure is 
that which characterises all mountains of elevation 
like the Alps. Nor can we reasonably doubt that 
when the folding and fracturing took place the crust 
bulged up and a series of superficial ridges and hol- 
lows — a true mountain-chain — came into existence. 
That was a very long time ago, however, for the up- 
lift dates back towards the close of Silurian times. 
Then followed a protracted period of denudation, 
during which our mountains of folded rocks must 
have passed through the various stages of adolescence, 
maturity, and old age. Much of the region was re- 
duced to the condition of a low plain, diversified in 
part by swelling hills of less and greater height All 
this work had been accomplished, and the degraded 
hills were continuing to crumble away, when the 
whole region was once more uplifted, and so converted 
into a table-land or plateau with an undulating surface. 
This movement of elevation had been completed, 
and renewed erosion had furrowed and trenched the 
plateau to some extent, before the beginning of Old 
Red Sandstone times, for the lowest or bottom beds 
of the Old Red Sandstone series here and there oc- 
cupy valleys carved out of the underlying Silurian 
greywacke and shale. To what extent the plateau 
was submerged during the Old Red Sandstone period 
we cannot tell. Probably the submergence was great- 
est over the north-east portion of the region, for it is 



EARTH SCULPTURE 



B ■;.£ 



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in that quarter that we meet with 
the most extensive and continu- 
ous accumulations of Old Red 
Sandstone rocks. Be that as it 
may, we know that some time 
before the succeeding Carbon- 
iferous period re-elevation en- 
sued and a new cycle of erosion 
was inaugurated, during which 
the Old Red Sandstone rocks 
and the underlying Silurian 
strata were more or less pro- 
foundly denuded. Thereafter 
followed an epoch of renewed 
subsidence on a more extensive 
scale, when much of the plateau 
was drowned in the Carbonifer- 
ous sea, and marine sediments 
of that age were distributed over 
areas which had probably never 
been overflowed by the waters 
of Old Red Sandstone times. 
Judging from the present dis- 
tribution of the Carboniferous 
strata, it seems likely that the 
plateau was, as before, more 
deeply submerged towards north- 
east and south-east than in other 
directions. So far as we can tell, 
the region has never since been 



LAND-FORMS IN HIGHLY FOLDED STRATA 137 

depressed below the sea, but in succeeding Permian 
and Triassic times long stretches of inland lakes or 
seas penetrated into the heart of the plateau, occupy- 
ing hollows which were certainly in existence during 
the preceding Carboniferous period. 

Such, without going into details, is a general out- 
line of the chief changes which have taken place in 
the Southern Uplands of Scotland. A plateau which 
came into existence towards the end of the Silurian 
period might well be expected to show a highly de- 
nuded aspect. It is true that during Old Red Sand- 
stone and Carboniferous times it was considerably 
depressed, and so escaped much erosion, but in the 
intervals separating those stages denudation must 
have been in active progress, as it has continued to 
be since the final disappearance of marine conditions. 
No doubt much rock has been removed from the 
whole surface of the region in question. Not only 
have wide and deep valleys been excavated, but the 
broad-backed hills and mountains can hardly fail to 
have been greatly reduced in height. It is still pos- 
sible, however, to trace the general configuration of 
the original surface. The average slope of the plateau 
appears to have been towards the south-east. This is 
indicated by the direction of the principal rivers — the 
Annan, the Nith, the Ken, and the Cree. It is fur- 
ther shown by the distribution of the Old Red Sand- 
stone and later geological formations. Thus strata 
of Old Red Standstone and Carboniferous age oc- 
cupy the Merse and the lower reaches of Teviotdale, 



138 EARTH SCULPTURE 

and extend up the valleys of the Whiteadder and the 
Leader into the heart of the Silurian uplands. In 
like manner Permian sandstones are well developed 
in the ancient hollows of Annandale and Nithsdale. 
Along the northern borders of the Southern Uplands 
we meet with similar evidence to show that even as 
early as the Old Red Sandstone period the ancient 
plateau along what is now its northern margin was 
penetrated by valleys that drained towards the north. 
But the main water-parting then, as now, lay not far 
south of this northern margin ^ ; in other words, the 
surface of the ancient plateau, a few miles back from 
its northern boundary, sloped persistently towards 
the south-east. Now the strike or general trend of 
the strata throughout the whole of these Uplands is 
south-west and north-east. We cannot doubt, there- 
fore, that when the ancient plain of erosion was up- 
lifted, and so became a plateau, the surface would be 
marked by many more or less well-defined ridges and 
hollows, probably none very prominent, but all hav- 
ing a north-east and south-west trend. The average 
slope of the surface being towards south-east, the 

' Many modifications of the drainage have been effected which cannot be re- 
ferred to here. It may be pointed out, however, that the head-waters of the 
Nith flow towards the north until they reach the broad Nithsdale, whence the 
drainage is directed south-east, so that Nithsdale may be said to cut right across 
the Uplands from north-west to south-east. This is probably a case of capture, 
the Nith, working back, having gradually invaded the northern drainage-area 
and captured such streams as the Afton and the Connel. The Clyde and the 
Doon are the only rivers of any size which have preserved tlieir north-westerly 
course, and the head-waters of the former have just escaped capture by the 
Tweed. 



LAND-FORMS IN HIGHLY FOLDED STRATA 139 

flow of the principal rivers would follow that direction, 
they would cut their channels across the outcrops of 
the strata. But the "corduroy" character of the 
plateau would now and again lead to occasional de- 
flections, while some streams and rivers would be 
conducted for long distances parallel to the strike of 
the strata. In a word, two sets of principal valleys 
would tend to be formed, namely, transverse and longi- 
tudinal valleys. Examples of the former have already 
been cited, such as the Cree, the Ken, and the Nith, 
and amongst the better-known longitudinal valleys 
may be mentioned those of the Teviot, the Ettrick, 
and the Yarrow. But a glance at any good map of 
the region will show that all the more important 
streams have a tendency to flow either in a transverse 
or a longitudinal direction, while many run now in one 
of these directions and now in the other. 

The Southern Uplands thus prove to be merely a 
highly eroded plateau. Their geological structure 
shows that towards the close of Silurian times the 
greywackes and shales were buckled up, folded, and 
faulted, and doubtless appeared at first as a range of 
true mountains of elevation. Thereafter followed a 
prolonged period of erosion, interrupted, it is true, at 
successive stages by partial submergence, but result- 
ing finally in the demolition of the old mountains of 
elevation and the conversion of the tract into a plain 
of erosion. Then came a final regional uplift, when 
that plain was converted into a plateau, which still 
exists, but in a highly denuded and eroded condition. 



I40 EARTH SCULPTURE 

The Northern Highlands of Scotland might be 
cited as another plateau of erosion with a somewhat 
similar geological history. There, as in the south, 
there is evidence to show that vast earth-movements 
resulted, towards the close of Silurian times, in the 
formation of great mountains of elevation. The 
thrust-planes visible in the north-west part of that 
region are on a much more extensive scale than those 
met with in the Southern Uplands. Probably the 
mountains of elevation which appeared over the site 
of the present Highlands were loftier and bolder than 
the pre-Devonian heights of Southern Scotland. 
They may quite possibly have rivalled the Alps in 
grandeur, for the folding and general disturbance of 
the rocks are quite as remarkable as the confusion 
seen in the mountains of Switzerland. We may well 
believe that when the Highland mountains first up- 
rose, their external form and internal structure would 
more or less closely coincide. No sooner had they 
come into existence, however, than the usual cycle of 
erosion would commence, and it is certain that after 
a prolonged interval they were to a large extent re- 
duced to their base-level — much of the formerly ele- 
vated area acquiring the character of a plain of erosion. 
Subsidence next ensued, and that plain became grad- 
ually overspread with sediment, several thousand feet 
of Old Red Sandstone strata being deposited on the 
planed and abraded surface of the ancient rocks. At 
a subsequent date the whole region was uplifted and 
converted into dry land, forming a plateau country, 



LAND-FORMS IN HIGHLY FOLDED STRATA 141 

which, so far as we know, has never since been com- 
pletely submerged, although it may well have ex- 
perienced many oscillations of level. 

It is out of that ancient plateau that the Highland 
mountains have been carved. The original surface- 
slope is, as usual in such cases, indicated partly by the 
direction of the principal drainage-lines and partly by 
the summits of the mountains, which decline in eleva- 
tion as they are followed outwards in the direction of 
the chief lines of drainage. Again, the main water- 
partings separating the more extensive drainage-areas 
of the country mark out in like manner the dominant 
portions of the same old plateau-land. The water- 
parting of the North-west Highlands runs nearly 
north and south, keeping quite close to the western 
shore, so that nearly all the drainage of that region 
flows inland. The average inclination of that section 
of the Highlands is therefore easterly, towards Glen- 
more and the Moray Firth. In the region east of 
Glenmore the land slopes in the directions followed 
by the rivers Spey, Dee, and Tay. These two regions 
— the North-west and the South-east Highlands — 
are separated by the remarkable depression of Glen- 
more, running through Lochs Linnhe, Lochy, and 
Ness, and the further extension of which towards 
north-east is indicated by the straight coast-line of the 
Moray Firth as far as Tarbat Ness. This long de- 
pression marks a line of fracture and displacement of 
very great geological antiquity. The old plateau of 
the Highland area was fissured and split in two, that 



142 EARTH SCULPTURE 

portion which lay to the north-west sinking along the 
line of fissure to a great but unascertained depth.^ 
Thus the waters that flowed down the slopes of the 
north-west portion of the fractured plateau were 
dammed by the long wall of rock that rose upon the 
south-east side of the fissure, and compelled to flow 
off to north-east and south-west along the line of dis- 
placement. The erosion thus induced sufficed in 
course of time to hollow out Glenmore and all the 
mountain-valleys that open upon it from the west. 

The dominant portion of the ancient plateau east 
of the great fault is approximately indicated by a line 
drawn from Ben Nevis through the Cairngorm and 
Ben Muich Dhui Mountains to Kinnaird Point. 
North of that line the drainage is towards the Moray 
Firth ; east of it the rivers discharge to the North 
Sea ; while an irregular winding line, drawn from Ben 
Nevis eastward through the Moor of Rannoch, and 
southward to Ben Lomond, forms the water-parting 
between the North Sea and the Atlantic, and probably 
marks approximately another dominant area of the 
fractured table-land. 

The geological structure of the Highlands agrees 
so far with that of the Southern Uplands, that the 
dominant "strike" of the strata is south-west and 
north-east. This, therefore, is the trend of the flexures 
and folds and of all the larger normal faults and great 

' It is probable that movements have taken place again and again at different 
periods along this line of weakness, and these movements may not always have 
been in one direction. 



LAND-FORMS IN HIGHLY FOLDED STRATA 143 

thrust-planes. Now such a structure would naturally 
determine the disposition of the surface-features 
worked out by erosion. Before the beginning of the 
Old Red Sandstone period, the pre-existing mount- 
ains of uplift had been largely degraded to a base- 
level. Much of the region, in other words, had been 
converted into a plain of erosion, which subsequently 
became depressed and buried under thick accumula- 
tions of sediment, derived in chief part from the de- 
nudation of such parts of the Highland area as still 
remained in the condition of dry land. After the 
deposition of the Old Red Sandstone the whole region 
was elevated en masse, and converted into a plateau 
or table-land. The surface of that plateau would 
doubtless be somewhat undulating and diversified. 
Probably the " stumps " of the highly denuded mount- 
ains, which had supplied materials for the formation 
of the Old Red Sandstone, still formed dominant 
areas. But wide regions had been planed down, and 
these would be marked by a kind of "corduroy" 
structure — parallel lines of escarpment and ridges 
with intervening hollows, corresponding to the suc- 
cessive outcrops of " harder " and "softer" rocks. 
The regions overspread by the Old Red Sandstone, 
on the other hand, would be approximately level, 
sloping gently, however, towards the north, north- 
east, and south-east. We may, therefore, conceive 
the surface of the ancient Highland Plateau to have 
been from the first more irregular than that of the 
Southern Table-land. The primeval rivers would 



144 EARTH SCULPTURE 

doubtless follow the average slopes of the plateau, 
and would thus sometimes cross the outcrops at all 
angles, and sometimes flow in the direction of the 
strike for longer or shorter distances. The great de- 
pression on the line of the Caledonian Canal, although 
partially filled with the sediments of Old Red Sand- 
stone times, probably still formed a well-marked 
feature at the surface of the plateau when this was 
first uplifted. And the same may well have been the 
case with many other lines of fracture. In short, 
although the average slope of the ground determined 
the general direction of the drainage, the corrugated 
and often much diversified surface of the plateau 
must have led to endless deflection of the water-flow. 
Again, as erosion proceeded, and the valleys were cut 
deeper and deeper, many modifications of the drainage 
would naturally arise, cases of the " capture " of one 
stream by another having been of common occurrence. 
It is not, however, with the history of such changes 
that we have to do, but rather with the character of 
the existing valleys and mountains which have been 
carved and chiselled out of the ancient plateau. Of 
the valleys it may be said in general terms that they 
are all valleys of erosion. Many have been hollowed 
out along the outcrops, and are thus longitudinal, while 
others have been cut out across the " strike," and to 
this extent are transverse. Some of the former are 
of primeval antiquity : they correspond in direction 
not only with the strike of the strata, but with what 
seems to have been the original slope of the plateau. 



LAND-FORMS IN HIGHLY FOLDED STRATA 145 

the valley of the Spey being the most conspicuous ex- 
ample. The transverse valleys, represented typically 
by Glen Garry and the valley of the Tay, are obvi- 
ously also of great age, since they in like manner in- 
dicate the general slope of the plateau in the regions 
where they occur. A large proportion of the longi- 
tudinal valleys that drain into these transverse valleys 
are in all probability of subsequent origin, although 
some of them may have been outlined at as early a 
date as the latter. Although none of the longitudinal 
valleys can be described as synclinal, they may all 
nevertheless be termed structural, inasmuch as they 
coincide with the strike of the rocks. So likewise we 
may term Glenmore a structural hollow, since it occurs 
along a line of fracture ; and the same is the case with 
Glen Docherty and Loch Maree. These lines of 
fractures no doubt showed at the surface of the plateau 
when it was first uplifted, and so determined the di- 
rection of drainage and erosion. But all the valleys 
as we now see them are valleys of erosion, their di- 
rection having been determined sometimes by the 
average slope of the plateau, sometimes by the geo- 
logical structure. 

The mountains of the Highlands are likewise monu- 
ments of erosion, owing their existence as such some- 
times to the relative durability of their materials, 
sometimes to their geological structure, or to both 
causes combined. They are all, without exception, 
subsequent or relict mountains. Thus, in the follow- 
ing section from Glen Lyon to Carn Chois we see 



146 EARTH SCULPTURE 

that the present configuration of the surface does not 
coincide with the complicated underground structure. 
It is the same, indeed, throughout all the Highland 
area. Take a section across any portion of that 
region, and you shall find that the more continuous 
" ranges " are developed along the outcrops — they are, 
in short, escarpment mountains. So great has been 
the erosion, however, within such " ranges," that their 
alignment usually becomes obscured, and we are con- 
fronted by confused groups of mountains, drained by 
streams flowing in every possible direction. " Any 

C*xr^ CKaiS 



Fig. 59. Section from Glen Lyon to Carn Chois. {Geol. Survey.) 

7«, mica-schist, etc. ; /, limestone; gr, greywacke, etc. ; J, amphibolite schist ; g, granite ; d^ 

diorite ; _/", fault. 

wide tract of the Highlands," as we have elsewhere 
remarked, "when viewed from a commanding posi- 
tion, looks like a tumbled ocean, in which the waves 
appear to be moving in all directions. One is also 
impressed with the fact that the undulations of the 
surface, however interrupted they may be, are broad ; 
the mountains, however much they may vary in their 
configuration according to the character of the rocks, 
are massive and generally round-shouldered, and often 
somewhat flat-topped ; while there is no great dis- 
parity of height amongst the dominant points of any 
individual group. Let us take, for example, the knot 



LAND-FORMS IN HIGHLY FOLDED STRATA 147 

of mountains between Loch Maree and Loch Tor- 
ridon. There we have a cluster of eight mountain- 
masses, the summits of which do not differ much in 
elevation. Thus in Llathach two points reach 3358 
feet and 3486 feet ; in Beinn AUigin there are also 
two points reaching 3021 feet and 3232 feet respect- 
ively ; in Beinn Dearg we have a height of 2995 feet ; 
in Beinn Eighe are three dominant points, 3188 feet, 
3217 feet, and 3309 feet. The four masses to the 
north are somewhat lower, their elevations being 2860 
feet, 2370 feet, and 2892 feet. The mountains of 
Lochaber and the Monadhliath Mountains exhibit 
similar relationships ; and the same holds good 
with all the mountain-groups of the Highlands. One 
cannot doubt that such relationship is the result 
of denudation. The mountains are monuments of 
erosion ; they are the wreck of an old table-land, 
the upper surface and original height of which are 
approximately indicated by the summits of the vari- 
ous mountain-masses and the direction of the princi- 
pal rivers. If we in imagination fill up the valleys 
with the rock-material which formerly occupied their 
place, we shall in some measure restore the general 
aspect of the Highland area before its mount- 
ains began to be shaped out by Nature's saws and 
chisels." 

A table-land of erosion, long exposed to denuda- 
tion, must obviously pass through the same phases 
as a plateau of accumulation. The elevated plain of 
complicated geological structure is first traversed by 



148 EARTH SCULPTURE 

rivers, the courses of which are determined by the 
average slope of the land. As valleys are deepened 
and widened, and the whole surface comes under the 
influence of the epigene agents, new tributary streams 
continue from time to tifne to make their appearance, 
and eventually a perfect network of drainage-lines is 
established. Wherever the rocks yield most readily 
to erosion hollows are formed, and many of these 
will necessarily coincide with the outcrop or strike 
of the strata. Longitudinal valleys thus tend to be 
developed. As denudation proceeds, the capture 
of streams by rivers and of rivers by streams often 
takes place, and the hydrographic system becomes 
more or less modified, but the general direction 
of the chief lines of drainage remains unchanged. 
Eventually transverse rivers are found cutting across 
mountain-ridge after mountain-ridge, the latter hav- 
ing only been developed after the rivers had come 
into existence. With the deepening and widening 
of the main valleys, and the continual multiplica- 
tion of subsidiary hollows by springs, torrents, and 
streams, the whole plateau eventually becomes cut 
up into irregular segments of every shape, form, and 
size — a rolling mountain-land. Waterfalls, rapids, 
and other irregularities have now disappeared from 
the courses of the older rivers and streams, except, 
it may be, towards their heads, where more or less 
numerous feeders are busy cutting their way back 
into the mountains. Should the base-level be main- 
tained, the process of denudation must continue until 



LAND-FORMS IN HIGHLY FOLDED STRATA 149 

the rolling mountain-land is finally reduced and re- 
solved once more into a plain of erosion. 

It is seldom, however, that a cycle of erosion is 
allowed to pass through all its stages. The study of 
man)' ancient plateaux has shown that the base-level 
is not infrequently disturbed — sometimes by eleva- 
tion, at other times by depression. Long before the 
eroded plateau has been completely reduced, subsid- 
ence may ensue, and the drowned land may then 
become buried under vast accumulations of marine 
sediments. Should the region be once more up- 
heaved and converted into dry land, streams and 
rivers will again come into existence, and flow in 
directions determined by the slopes of the surface. 
Thus ere long another hydrographic system will be 
developed which may differ entirely from its prede- 
cessor, both as regards direction and arrangement. 
As the rivers cut their way down through the super- 
imposed marine strata they will eventually reach the 
buried land-surface, across which they will run with- 
out any reference to the former configuration. Should 
the base-level remain unchanged, a time will come 
when the overlying marine strata will be entirely 
removed, but the direction and general arrangement 
of the river-system acquired when the land was new- 
born will be maintained. Thus the direction of many 
transverse rivers, which in ancient plateau-lands are 
found cutting across mountains of every shape and 
disposition, have not infrequently been determined 
by the surface-slope of overlying masses, almost every 
vestige of which has since disappeared. 



CHAPTER VII 

LAND-FORMS IN REGIONS AFFECTED BY 
NORMAL FAULTS OR VERTICAL DISPLACEMENTS 

NORMAL FAULTS, GENERAL FEATURES OF THEIR CONNECTION 

WITH FOLDS THEIR ORIGIN HOW THEY AFFECT THE SUR- 
FACE FAULTS OF THE COLORADO REGION, AND OF THE GREAT 

BASIN DEPRESSION OF THE DEAD SEA AND THE JORDAN 

LAKE-DEPRESSIONS OF EAST AFRICA FAULTS OF BRITISH 

COAL-FIELDS BOUNDING FAULTS OF SCOTTISH HIGHLANDS 

AND LOWLANDS FAULT-BOUNDED MOUNTAINS GENERAL 

CONCLUSIONS. 

IN Chapter III. a short account was given of 
the dislocations or fractures by which rocks are 
frequently traversed. These, as we saw, are of two 
kinds — normal faidts and reversed faulis or over- 
thrusts. The latter have been sufficiently referred 
to in connection with the appearances presented by 
highly flexured strata, amongst which, indeed, they 
are most usually encountered. Normal faults of vari- 
ous importance may likewise often be seen travers- 
ing areas of disturbed and contorted rocks. When 
such is the case, however, the larger of these faults 
not infrequently prove to be of later date than the 
flexures and thrust-planes. The latter are the result 

150 



I 

VERTICAL DISPLACEMENTS 151 

of former horizontal movements of the crust ; the 
normal faults, on the other hand, are vertical dis- 
placements due to later movements of direct subsid- 
ence. It will be understood, therefore, that reversed 
faults or overthrusts are practically confined to regions 
of highly flexed and contorted strata, while normal 
faults traverse every kind of geological structure. The 
latter, however, are certainly best displayed in areas 
of horizontal and moderately inclined strata, while 
they often form lines of separation between these and 
contiguous areas of highly disturbed rock-masses. 

The amount of downthrow of normal faults is very 
variable. Sometimes it does not exceed a few feet 
or yards, in other cases it may reach thousands of 
feet, so that strata of vastly different ages may be 
brought into juxtaposition. The smaller faults usu- 
ally extend for very short distances, while the larger 
ones may continue for hundreds or even thousands 
of miles. The course of great faults is usually 
approximately straight, but not infrequently it is 
curved. Very often they are accompanied by a series 
of smaller parallel dislocations ; and now and again, 
in place of one great fault, with accompanying minor 
dislocations, we may find a series of more or less 
closely set parallel minor faults. When the down- 
throw of all these minor faults is in one and the same 
direction, the result is practically the same as if there 
had been only one major dislocation with a large 
downthrow. Another fact may be noted : faults, 
especially large ones, often split up, as it were, into 



152 EARTH SCULPTURE 

two or more. A major fault may begin as a mere 
crack or fracture, with little or no accompanying 
rock-displacement. But as it continues the amount 
of downthrow gradually increases until a maximum is 
reached, after which the displacement usually de- 
creases until finally the fault dies out. In not a few 
cases, however, the degree of downthrow varies very 
irregularly. 

Frequently faults are intimately connected with 
folds and flexures. This is shown at once by the 
fact that large dislocations very often trend in the 
same direction as the strike of the strata. Now and 
again, indeed, when a large fault can be followed to 
the end, it is found gradually to die out in a fold or 
flexure. In other words, what is a fault in one place 
is represented elsewhere by a flexure. It is not hard 
to see how that should be. Strain or tension must 
obviously be set up along the margin of a sinking 
area. If, for example, subsidence should take place 
within an area of horizontal strata, the horizontal 
position of the rocks along the margin of the sinking 
area will be interfered with. The pull or drag of the 
descending mass will cause the strata of the adjacent 
relatively stable area either to bend over or snap 
across. Should the movement be slow and pro- 
tracted, the rocks will probably at first yield by 
bending ; but as the movement continues they will 
eventually give way, and a fold will thus be replaced 
by a fracture. Towards either end of such a fault, 
therefore, we should expect it to die out into a siinple 



VERTICAL DISPLACEMENTS 153 

flexure or monoclinal fold. Probably most normal 
faults are in this way preceded by folding, except in 
cases where they have been more or less suddenly 
produced. 

Although normal faults may be looked upon as the 
result of direct subsidence, it is obvious that in some 
cases they may well have resulted from movements 
of elevation. During the slow uplifting of a broad 
plateau strain and tension will come into play along 
the margin of the rising area. Folds will thus be 
formed, and these will be replaced eventually by frac- 
tures and displacements. The resulting structure 




i 

Fig. 60. Section of Normal Fault. 

will thus be practically the same as if the folding and 
faulting had been produced by a movement of subsid- 
ence. Thus in Fig. 60 the fault f might have been 
caused either by the direct subsidence of the strata 
at X or by the elevation of the strata at a. 

There is reason to believe that some large faults 
have resulted from crustal movements continued 
through long periods of time. The rock-displace- 
ments may have been very slowly and gradually ef- 
fected, or the movement may have been more rapid, 
but interrupted again and again by longer or shorter 
pauses. Or, again, the rate of movement may have 



154 EARTH SCULPTURE 

varied from time to time, and occasionally it may even 
have been sudden and catastrophic. But such evi- 
dence as we have would lead us to infer that vertical 
displacements, whether the result of downward or of 
upward movements, have not been more rapidly ef- 
fected than horizontal deformations. No doubt a 
sudden dislocation of the crust of large extent would 
show directly at the surface. But somewhat similar 
results would follow if the dislocation, without being 
quite sudden, were yet to be developed more rapidly 
than the rate of superficial erosion and denudation. 
Cases of the kind are well known, and to some of 
these reference will presently be made. It is with 
faulted rocks, however, as with folded mountains : 
when movement has ceased the inequalities caused at 
the earth's surface tend to be reduced and greatly 
modified. The epigene forces are untiring in their 
action, so that in course of time areas of direct sub- 
sidence tend to become filled up and the surrounding 
high-lying tracts to be worn down. To such an ex- 
tent has this taken place, that in the case of certain 
great faults of high geological antiquity no inequality 
at the surface indicates their presence, and it is only 
by studying the geological structure that we are able 
to ascertain that such dislocations exist. 

Bearing In mind the activity of the denuding agents, 
we might expect that normal faults of geologically re- 
cent date should show most prominently at the surface. 
And this to a large extent is doubtless true. Never- 
theless, as we shall learn by-and-by, there are certain 



VERTICAL DISPLACEMENTS 155 

faults of prodigious antiquity which still cause very 
marked inequalities at the surface. These often form 
the boundaries between highlands and lowlands. In 
such cases, however, the disparity of level is due not 
so much to vertical displacement, as to the fact that 
the lowlands are usually composed of less enduring 
materials than those which enter into the framework 
of the adjacent highlands. When a fault of great 
age traverses strata of much the same consistency 
(say sandstones and shales), the rocks on either side 
of the dislocation, we find, have been planed down to 




i 

Fig. 5i. Normal Fault, with High Ground on Downthrow Side. 

the same level. Thus in the low-lying coal-fields of 
Scotland the gently undulating surface gives no in- 
dication of the presence of the numerous dislocations 
which have been detected underground. Downthrows 
of hundreds of feet give rise to no superficial inequal- 
ities. It is only when one of these faults has brought 
relatively hard and soft rocks into juxtaposition that 
a marked surface-feature results. And in this case 
the hard rock invariably rises above the level of the 
soft rock, no matter on which side of the dislocation 
it happens to lie. Thus in Fig. 61 the hard rock a 
forms an eminence, although it is on the downthrow 
side of the fault, simply because it has withstood denud- 



iS6 EARTH SCULPTURE 

ation more effectually than the soft rock {f). In Fig. 62, 
again, it is obvious that the high ground at x owes its 
origin to the presence of the relatively hard rock (Ji). 
To this matter, however, we shall return in the sequel. 
Meanwhile we must consider, first, the appearances 
presented in regions where vertical movements of the 
crust have taken place within relatively recent times. 
The Colorado Plateau affords some excellent 
examples of simple folds and normal faults of com- 
paratively recent age. These have often profoundly 
affected the surface, lines of cliffs and bold escarp- 
ments rising along the high side of each dislocation. 




Fig. 62. Normal Fault, with High Ground on Upcast Side. 

The plateau, in short, has been split across by well- 
marked normal faults, some of which can be followed 
for hundreds of miles. Yet the strata on both sides 
of such dislocations are of much the same character 
and consistency. Here, then, it might be supposed 
that the fracturing and displacement had been sud- 
denly effected. There is striking evidence, however, 
to show that such has not been the case. Although 
some of the faults referred to have a downthrow of 
several thousand feet, yet they have had no effect in 
disturbing the course of the Colorado River, which tra- 
verses the faulted region. The same, as we have seen, 



VERTICAL DISPLACEMENTS iS7 

holds true with regard to the flexures of that area. 
It is obvious, in a word, that the process of flexuring 
and faulting has proceeded so slowly that the river 
has been able to saw its way across the inequalities 
as fast as these appeared. But while the rate of 
river erosion has equalled that of crustal movement, 
the denudation of the plateau outside of the river- 
courses has not. Deformation and dislocation of the 
plateau have thus given rise to marked surface-feat- 
ures. Yet even in the case of these relatively young 
faults we find that the features determined by them 
have been very considerably modified by denudation. 
In the following section, for example, we see three 



Fig. 63. Faults in Queantoweep Valley, Grand CaSon District. 

(Dutton.) 

faults of 1300 feet, 300 feet, and 800 feet displacement 
respectively traversing the same series of strata, and 
yet giving rise to marked inequalities at the surface. 
The dotted lines, however, show to what an extent 
these features have been modified by denudation. 
There is an obvious tendency of the escarpments and 
cliffs to become benched back as they retreat, so that 
they do not show the abrupt character which they 
would have possessed had no superficial waste accom- 



158 EARTH SCULP TURE 

panied and succeeded the crustal movements. (See 
Fig. 63.) 

In the Great Basin that extends between the bold 
escarpment of the Sierra Nevada, on the one hand, and 
the Wahsatch Mountains on the other, we encounter 
another series of large faults, which have deter- 
mined the leading features of the region. It would 
appear that the area of the Great Basin formerly 
attained a considerably greater elevation than at pre- 
sent. Towards the close of Tertiary times the whole 
of this area, including the adjacent Sierra Nevada 
and the Wahsatch Mountains, was upheaved in the 
form of a broad arch. The crust thus subject to ten- 
sion yielded by cracking across, and a system of long 
parallel north and south fissures was formed. In 
other words, the broad arch was split into a series of 
oblong blocks many miles in extent. When the 
movement of elevation ceased and subsidence en- 
sued, the shattered crust settled down unequally 
between the Sierra Nevada in the west and the Wah- 
satch Mountains in the east. The amount of dis- 
placement along the margins of the Great Basin is 
very great ; the fault at the base of the Sierra, for 
example, is estimated to be not less than 15,000 feet, 
while that which severs the Basin from the Wahsatch 
Mountains is also very great. The numerous parallel 
ranges that diversify the surface of the Great Basin 
itself are simply oblong crust-blocks, brought into 
position by normal faults. Being of so recent an age, 
they have suffered comparatively little modification. 



VERTICAL DISPLACEMENTS 



159 



Nevertheless, they do not fall to 
show the tool-marks of epigene 
action — everywhere escarpments 
are retreating, and one can see 
that already vast masses of rock 
have been removed from the sur- 
face. The accompanying dia- 
gram (Fig. 64) will serve to give 
a general idea of the geological 
structure of the Basin ranges. 
There is no reason to believe 
that the crustal movements above 
referred to were sudden or cata- 
strophic in character. Probably 
they were no more rapid than 
those which have affected the 
plateau of the Colorado. 

We are not without evidence 
of similar recent dislocations in 
the Old World, and there as 
elsewhere they give rise to more 
or less pronounced surface-feat- 
ures. One of the most interest- 
ing examples is seen in the great 
depression that extends north- 
wards from the Gulf of Akabah 
by the Wady el Arabah, the 
Dead Sea, the valley of the Jor- 
dan, and Lake Tiberias. This 
long hollow would appear to 




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i6o EARTH SCULPTURE 

have come into existence at or about the close of 
Tertiary times. It is everywhere bounded by normal 
faults or by steep monoclinal folds, the one kind of 
structure passing into the other. Before this depres- 
sion came into existence the region it now traverses 
appears to have been a broad continuous plateau, 
built up of ancient crystalline and Palaeozoic rocks 
below, and approximately horizontal strata of Meso- 
zoic age above. At what particular date this plateau 
of accumulation first appeared, and how long it re- 
mained undisturbed, we cannot tell. Possibly the 
movement of subsidence to which the Dead Sea owes 
its origin may have coincided with the upheaval that 
resulted in the formation of the plateau. However 
that may have been, the latter was eventually tra- 
versed by a series of monoclinal folds and parallel 
faults, and between these the great depression of the 
Jordan came into existence. The Mesozoic strata of 
the plateau retain their approximately horizontal po- 
sition close up to the depression along its eastern 
margin, while the descent from the west is much less 
abrupt. But this is only broadly true. When the 
region is more closely investigated, the relatively gen- 
tle dip of the strata along the west side of the depres- 
sion is found to be interrupted again and again by 
more or less sharp monoclinal folds and by normal 
faults, the presence of which is betrayed at the sur- 
face by corresponding sudden changes in the form of 
the ground. In other words, the descent from the 
plateau on the west is often by a series of broader 



VERTICAL DISPLACEMENTS 



i6i 



and narrower terraces and escarp- 
ments, running parallel with the 
trend of the great hollow. The 
western margin of the Dead Sea, 
for example, is determined by a 
vertical displacement, similar in 
character to, but not so extensive 
as, that which bounds it on the 
east. The section (Fig. 65) will 
serve to illustrate the geological 
structures referred to. 

The flexures and faults of this 
interesting region do not date 
beyond the close of the Tertiary 
period, and consequently there 
has not been sufficient time to 
allow of a complete modification 
of the surface by epigene action. 
The most conspicuous features 
of the district are determined 
by folds and fractures — under- 
ground structure and surface- 
configuration to a large extent 
coincide. But everywhere also 
we observe the evidence of ero- 
sion and denudation. Great 
sheets of rock have been grad- 
ually removed from the surface, 
which is seamed and scarred by 
innumerable ravines and water- 



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\J' 



2 ^ 



i62 EARTH SCULPTURE 

courses, many of these being now dry and deserted. 
According to Professor Suess, the Jordan depres- 
sion continues north between the Lebanon and the 
Anti-Lebanon, through the valley of the Nahr el Asi 
(the Orontes) to near Antioch. The same geologist 
is further of opinion that the great trough of the Red 
Sea and most of the lacustrine hollows of East Africa 
are in like manner due to direct subsidence of the 
crust, the probability being that they and the Jordan 
depression all belong to one and the same system of 
crustal deformation. It is noteworthy that the de- 
pressed areas of Africa lie in zones or belts having an 
approximately meridional direction, that they are not 
margined or surrounded by mountain-ranges, but are 
sunk in broad plateaux, and, moreover, are accom- 
panied by abundant evidence of volcanic action. The 
troughs are mostly broad, and vary much and con- 
stantly in height above the sea, so that they are obvi- 
ously not the result of erosion. In many places they 
are flanked on both sides by abrupt declivities com- 
parable in character to those that overlook the Dead 
Sea. In some cases, however, steep bluffs and cliffs 
are confined to one side of a depression only. In 
short, we have in East Africa the same phenomena 
which confront us in Palestine. The earth's crust in 
all those regions has evidently yielded to strain or 
tension by snapping across and subsiding. In place 
of one simple normal fault, however, we see a com- 
plex system of parallel dislocations and flexures, the 
folded and shattered rocks having settled down un- 



VERTICAL DISPLACEMENTS 163 

equally, while molten matter and loose ejecta issued 
here and there in less or greater abundance along 
the chief lines of rock-disturbance. 

Similar greological structures on a smaller scale 
may be seen nearer home, and are well exemplified 
in the res^ion of the Vossfes and the Black Forest. 
These opposing mountains are the counterparts of 
each other, being built up of the same rocks, arranged 
in very much the same way. The basement rocks 
are granite and crystalline schistose rocks, which are 
overlaid by a series of Mesozoic strata. In the 
\"osges the dip of these strata is westerly, while the 
corresponding rocks in the Black Forest are inclined 
towards the east. Between the two ranges, as every- 
one knows, lies the basin of the upper Rhine, a basin 
which, like that of the Jordan, has been determined 
by a number of parallel normal faults. The Meso- 
zoic strata in the region surrounding the two ranges 
attain a thickness of at least 5000 feet, and there can 
be no doubt that these originally extended from west to 
east across what is now the basin of the Rhine. This 
is shown by the simple fact that the strata in question 
occupy that basin. (See Fig. 66, p. 164.) Doubtless 
the Mesozoic rocks were originally deposited in ap- 
proximately horizontal positions. Subsequently the 
sea retreated from the area, and a wide land-surface — 
probably an elevated plain or plateau — occupied its 
place. Eventually, in early Tertiary times, the region 
was subjected to crustal movements, and traversed 
from south to north by a series of dislocations, with 



164 



EARTH SCULPTURE 




•O 4) 



en . 



2 M 



downthrows in opposite di- 
rections. As a result of these 
displacements the Rhenish 
basin came into existence, 
while the rock-masses along 
its margins were pushed up 
to form the ranges of the 
Vosges and the Black 
Forest. The crustal move- 
ments referred to appear to 
•- \ have been continued down to 
i. s post-Tertiary times, and 
have probably not yet 
ceased, the frequent earth- 
quakes experienced in the 
neighbourhood of Darm- 
stadt being perhaps an 
of progressive 
subsidence along lines of 
dislocation. It is interest- 
1 00- ing to note that these crustal 
movements have been ac- 
companied from time to 
time by volcanic action. 
The well-known Kaiserstuhl 
near Freiburg, for example, 
is the skeleton of what must 
have been a very consider- 
able volcano. 

The evidence that subsid- 



en ^ c 



w I *^. indication 



o g-2 



VERTICAL DISPLACEMENTS 165 

ence in the Rhenish basin has continued into the 
post-Tertiary period is so striking that it may be 
briefly referred to here. Deep borings have shown 
that the Pleistocene deposits in the valley of the 
Rhine in Hesse occupy a profound hollow, surrounded 
on all sides by older rocks, the bottom of the basin 
being 270 feet deeper than the lowest part of its rim 
at Bingen. These deposits, however, are not lacus- 
trine, but fluviatile. Hence we must infer that fluv- 
iatile deposition has kept pace with the crustal 
movement. As the bottom of the Rhine valley has 
slowly subsided, the river has flowed on without 
interruption, continuously filling up the gradually 
deepening basin with its sediment. This is only 
another example of the fact that movements of the 
crust, whether of elevation or depression, have often 
proceeded so slowly that they have been unable to 
modify the direction of streams and rivers. 

While we recognise the influence of earth-move- 
ments in determining the form of the surface in the 
region under review, it is obvious that much rock- 
material has been removed. The presence of the 
Mesozoic strata in the basin of the Rhine shows that 
these must formerly have extended continuously over 
the adjacent tracts. Yet they have since been largely 
denuded away from the higher parts of the Vosges 
and the Black Forest, so that the underlying crystal- 
line rocks have been laid bare, and now appear at the 
surface over considerable areas. 

When we turn our attention to regions of highly 



i66 EARTH SCULPTURE 

dislocated rocks, where the crustal displacements are 
of much greater antiquity than those we have just 
been considering, the surface-features, we find, have 
often been so modified by denudation that the posi- 
tion and even the very existence of normal faults can 
be determined only by close observation. In other 
cases, however, they give rise to marked features at 
the surface. 

The following section across a portion of the Lan- 
arkshire coal-field is drawn upon a true scale. The 
section traverses several normal faults, the largest 




Fig. 67. Section of Coal-Measures (on a True Scale) near 
Cambusnethan, Lanarkshire. 

being a displacement of 350 feet, yet there is no feat- 
ure at the surface to indicate its presence. 

It is only by studying the geological structure that 
the existence of such dislocations can be discovered. 
The strata of the region in question are of much the 
same consistency throughout, and have therefore 
yielded equally to the various agents of erosion. 
Thus all inequalities of surface which may originally 
have resulted from faulting have been smoothed out. 
It is doubtful, however, whether such relatively small 
faults ever did show at the surface. The amount of 
displacement effected by them usually diminishes up- 
wards, so that the highest coal-seams are hardly dis- 



VERTICAL DISPLACEMENTS 167 

located to such an extent as those which occur at 
lower levels. Many small faults, indeed, die out up- 
wards altogether. And when we remember that the 
rocks now exposed at the surface were formerly 
covered by enormous sheets of strata which have 
since been removed by denudation, it is not hard 
to believe that even some of the larger faults of 
our coal-fields may actually have died out before 
the original surface of the Carboniferous strata was 
reached. 

Some normal faults, however, are so very extens- 
ive — the amount of displacement is so very great — 
that we must believe they did reach the earth's surface 
at the time of their formation. Yet where these 
faults traverse strata having much the same charac- 
ter, they produce no inequalities of level at the sur- 
face. A good example is the Tynedale fault of the 
Newcastle coal-field, which has a downthrow in some 
places of 1200 feet, and yet its existence is not be- 
trayed by the configuration of the ground. (See 
Fig. 68, p. 168.) 

Great normal faults, however, usually do show more 
or less conspicuously at the surface. This is due to 
the fact that by their means areas of soft and hard 
rock are often brought into juxtaposition. Many ex- 
amples might be cited from Great Britain. Thus 
in Scotland the Central Lowlands, consisting largely 
of relatively soft rocks, have been brought against 
the harder rocks of the Highlands on the one hand, 
and those of the Southern Uplands on the other. A 



i68 EARTH SCULPTURE 

line drawn from Stonehaven in a south-west direction 
to the Clyde near Helensburgh is at once the geo- 
logical and geographical boundary of Highlands and 
Lowlands, while a similar line extending from Dun- 
bar to the coast of Ayrshire near Girvan forms the 
corresponding boundary of the Lowlands and the 

Owse Btirn. 




Fig. 68. Section on a True Scale across " Tynedale Fault," 
Newcastle Coal-Field. 

Southern Uplands. The lines in question are great 
dislocations, having in places downthrows of 5000 to 
6000 feet. But there can be no doubt that the in- 
equalities at the surface are due not so much to the 
amount of vertical displacement as to the different 
character of the rocks on opposite sides of the faults. 
This is well shown by the fact that the disparity of 
level along a line of dislocation varies with the char- 



VERTICAL DISPLACEMENTS 169 

acter of the rocks which are brought into juxtaposi- 
tion. Thus, when soft sandstone, as in Strathmore, 
abut against hard crystalline rocks, the latter rise 
more or less abruptly above the former — the line 
of demarcation between Highlands and Lowlands is 

SE. 







Fig. 69. Section across Great Fault Bounding the Highlands 
near birnam, perthshire. 

A, *' hard " grits and shales ; j, relatively " soft " sandstones, etc. Demarcation between 
Highlands and Lowlands well marked. 

Strongly pronounced. But when, as between the val- 
leys of the Earn and the Teith, the hard igneous 
rocks of the Lowlands are brought against the crys- 
talline schists of the Highlands, the geographical 
boundary of the two regions is not nearly so well 
marked — the Highland mountains seem to merge 
gradually into the Lowland hills. And the same 
phenomena are conspicuously displayed along the 
margin of the Lowlands and the Southern Uplands. 
In a word, it is obvious that while the position of the 
boundaries that separate the Lowlands from the 
mountain-areas to north and south has been deter- 
mined by normal faults, the existing configuration is 
the result of long-continued and profound denudation. 
The accompanying sketch sections (Figs. 69, 70) 
will serve to illustrate the foregoing remarks. 



170 



EARTH SCULPTURE 



Normal faults, as we have seen, have often deter- 
mined the boundaries between lowlands and high- 
lands. Not infrequently, indeed, it can be shown 



A 



S£ 




Fig. 70. 



Section across Great Fault Bounding the 
Southern Uplands. 



A, " hard " greywack^s, etc.; z", " hard " igneous rocks and overlying conglomerate c. 
Demarcation between Uplands and Lowlands not well marked. 

that the dominance of certain mountains is due rather 
to the sinking down of adjacent low-lying tracts than 
to bulging up of the crust within the mountain-areas 




Fig. 71. Diagram Section across Horstgebirge. 

«, granite, gneiss, etc., forming the " Horst" ; (5, stratified rocks of relatively late age, resting 

upon a, dropped down along lines of dislocation yy,- e;, outlier of b, showing that 

the strata b were formerly continuous between A and B. 

themselves. Such mountains are, of course, bounded 
by faults, and are known to German geologists as 
Horste or Rumpfgebirgc, the Harz being a good ex- 
ample. The Horste of Middle Europe are composed 
for the most part of crystalline schists and Palaeozoic 
rocks, more or less highly flexed and disturbed. The 



VERTICAL DISPLACEMENTS 171 

mountains usually rise somewhat suddenly above the 
surface of the relatively undisturbed and approxi- 
m.ately horizontal Mesozoic strata of the adjacent low 
grounds, and for a long time it was supposed that 
these strata in the immediate vicinity of the Horste 
were littoral deposits. Such, however, is not the case. 
They are of relatively deep-water origin, and, before 
faulting supervened, may have covered much of the 
.high lands which now overlook them. It is obvious, 
in short, that the Horste represent portions of the crust 
which have maintained their position ; they are mount- 
ains which testify to a former higher crustal level ; 
the surrounding tracts have broken away from them, 
and dropped to a lower position. 

Probably enough has now been advanced to show 
that normal faults have had no inconsiderable share 
in determining surface-features. This, as might have 
been expected, is most conspicuous in regions of re- 
cent crustal deformation and fracture, where epigene 
action has not had time to effect much modification. 
In cases of very ancient fracture and displacement, 
however, the surface-features, as we have seen, are 
very greatly modified, and if well-marked disparity of 
level is still often met with along lines of dislocation, 
this is mainly due to the fact that rocks of unequal 
endurance have been brought into juxtaposition. In 
a case of very considerable displacement it will usu- 
ally happen, indeed, that crystalline schists, plutonic 
rocks, or hard Palaeozoic strata will occur upon the 
high side and relatively softer strata on the low side 



172 EARTH SCULP TURE 

of the fault. However prolonged and intense epigene 
action may have been, such a fault will nevertheless 
cause a marked feature at the surface, so long as the 
general surface of the land remains considerably above 
the base-level. But when the latter is approached 
denudation will eventually cease on the low side of 
the fault, while material will continue to be removed 
from the high side, and the disparity between the two 
will thus tend gradually to disappear. In short, the 
irregularities of surface determined by the presence 
of faults pass through the same cycle of changes as 
all other kinds of geological structure. Should the 
base-level remain undisturbed epigene action must 
eventually reduce every inequality, no matter what its 
origin may have been. Again, were such a reduced 
land-surface to be re-elevated and converted into a 
plateau, the lines of dislocation that happened to 
separate areas of hard rock from regions of soft rock 
would once more determine the boundaries between 
high and low ground. The surface of the soft rocks 
would be lowered most readily, while the more durable 
hard rocks would come to form elevations. In a word, 
the features that obtained before the land was reduced 
to base-level would, under the influence of denudation, 
tend to re-appear. 



CHAPTER VIII 

LAND-FORMS DUE DIRECTLY OR INDIRECTLY 
TO IGNEOUS ACTION 

PLUTONIC AND VOLCANIC ROCKS — DEFORMATION OF SURFACE 

CAUSED BY INTRUSIONS LACCOLITHS OF HENRY MOUNTAINS 

VOLCANOES, STRUCTURE AND FORM OF MUD-CONES GEY- 
SERS — FISSURE-ERUPTIONS VOLCANIC PLATEAUX DENUD • 

ATION OF VOLCANOES, ETC., AND RESULTING FEATURES. 

IN preceding pages we have had frequent occasion 
to refer to igneous rocks. These, as we have 
seen, may be broadly grouped under two heads — Plu- 
tonic rocks and Volcanic rocks. The former have 
cooled and solidified at a less or greater depth below 
the surface ; the latter, on the other hand, have been 
extruded at or near the surface. No hard and fast 
line, however, can be drawn between these two groups. 
All plutonic rocks are indeed intrusive — they have 
solidified below ground ; but the same is true of the 
sheets and dikes which traverse a volcano, and which, 
along with the bedded lavas and tuffs they traverse, 
are properly described as of volcanic origin. It will 
be understood, then, that the term plutonic is restricted 
to intrusive rocks which have consolidated at rela- 
tively great depths, while the term volcanic includes 

173 



174 EARTH SCULPTURE 

all igneous rocks which enter or have entered into 
the formation of a volcano, or which have evidently- 
proceeded from any focus or foci of eruption. 

It is needless to say that we can know nothing by 
direct observation of the conditions and phenomena 
which attend the intrusion of deep-seated plutonic 
rocks. But so many of these have been laid bare by 
denudation, their composition and their relation to sur- 
rounding rock-masses have been so carefully studied, 
that geologists have learned much concerning igneous 
action of which but for denudation they must have 
remained largely ignorant. They have ascertained, 
for example, that such lavas as rhyolite, andesite, and 
basalt have their deep-seated equivalents in the plu- 
tonic granites, syenites, and gabbros. That is to say, 
we know that the same molten mass solidifies at great 
depths as granite or other wholly crystalline rock, and 
at the surface as rhyolite or other semi-crystalline lava. 
In short, plutonic rocks and their volcanic equivalents 
have practically the same chemical composition. An 
acid lava comes from an acid magma, a basic lava from 
a basic magma. Hence it is inferred that many plu- 
tonic rocks now exposed by denudation may have been 
the deep-seated sources from which ancient lavas have 
proceeded. On the other hand, there is reason to 
believe that many plutonic masses may never have 
had any such volcanic connections. 

But whether or no a given plutonic mass be the 
deep-seated source of. some long-vanished volcano or 
volcanoes does not concern us here. We have sim- 



LAND-FORMS DUE TO IGNEOUS ACTION 175 

ply to recognise the fact that its exposure at the 
surface is the direct result of profound denudation. 
Whether its intrusion had any effect in deforming the 
surface we cannot tell. Probably, in cases where 
none of the material was extruded to the surface by 
contemporaneous volcanic action, there may have 
been some bulging up of the ground. Deformation 
of the crust, in short, may quite well have accom- 
panied the subterranean movements of great masses 
of molten matter. But so long a time has elapsed 
since the granites and other highly crystalline plutonic 
rocks were intruded — so enormous has been the thick- 
ness of rock removed from above them — that such 
intrusion cannot be said to have had any direct effect 
in the production of existing surface-features. It is 
quite true that many hills and mountains are com- 
posed largely or even exclusively of plutonic rocks ; 



Fig. 72. Mountain of Granite. 

g, granite sending veins into schists, etc., (j). Tlie schists have been more readily 
lowered by erosion than the granite. 

but that is simply owing to the fact that these rocks 
are usually more durable than the rocks through 
which they rise. When, as not .infrequently happens, 
plutonic masses are of less durable consistency and 



176 EARTH SCULPTURE 

construction than the rocks that surround them, the 
latter invariably dominate and overlook the former. 
Thus while granite often forms prominent mountains 
(Fig. 72, p. 175), not infrequently it is found occupy- 
ing low tracts flanked by mountains of schist, slate, 
or other rock. (Fig. 73. ) 




Fig. 73. Plain of Granite Overlooked by Mountains of Schists, etc. 

gy granite ; j, schists, etc. The granite has been more readily lowered by erosion 
than the surrounding schists. 

We must conclude, then, that whatever effect may 
have been produced at the surface by the intrusion of 
the more ancient plutonic rocks of England and other 
countries, such superficial effects, if any, have long 
since disappeared. The present configuration of the 
ground occupied by such rocks is wholly the result of 
epigene action. But when we consider the phenomena 
of more recent intrusions of igneous rock, we find 
reason to conclude that these have not only had a 
direct effect at the surface, but that this effect has 
not yet in all cases been removed by denudation. 
The ground has bulged up, and the swelling of the 
surface is still conspicuous. Among the most re- 



LAND-FORMS DUE TO IGNEOUS ACTION 177 

markable examples known are the laccoliths or lac- 
colites (stone cisterns) of the Henry Mountains 
(southern Utah), which have been described by Mr. 
Gilbert. In that region molten rock, instead of 
ascending to the surface and building up mountains 
by successive eruptions, has stopped at a lower hori- 
zon, insinuated itself between the strata, and opened 
for itself a chamber by lifting all the superior beds. 
(See Fig. 74.) Proceeding from a laccolith are in- 




FiG. 74. Diagrammatic Section of a Laccolith Showing Dome-Shaped 
Elevation of Surface above the Intrusive Rock. (After G. K. Gilbert.) 

P^ pipe or conduit ; sh^ sheet ; d d, dilces. 

trusions of the same kind of igneous rock (trachyte), 
some of which {sheets) have squeezed themselves be- 
tween adjacent beds, while others (dikes) traverse the 
strata at less or greater angles. These remarkable 
rocks have been intruded in a great series of strata 
ranging in age from Carboniferous to Cretaceous, 
amongst which they are irregularly distributed, some 



178 EARTH SCULPTURE 

appearing in the Carboniferous, some in the Jura- 
Trias, and others in the Cretaceous. From the low- 
est to the highest laccohth the range is not less than 
4000 feet, those which are above not infrequently 
overlapping those which lie below. " Their horizon- 
tal distribution is as irregular as the arrangement of 
volcanic vents. They occur in clusters, and each 
cluster is marked by a mountain. In Mount Ellen 
there are perhaps thirty laccolites ; in Mount Holmes 
there are two ; and in Mount Ellsworth one. Mount 
Pennell and Mount Hillers have each one large and 
several small ones." The highest of these mountains 
attains an elevation of over 11,000 feet, rising some 
5000 feet above the plateau at its base. The strata 
of which that plateau is built up are approximately 
horizontal, and appear at one time to have been cov- 
ered by some thousands of feet of Tertiary deposits, 
the nearest remains of which occur at a distance of 
thirty miles from the Henry Mountains. Mr. Gilbert 
is of opinion that the laccolites were most probably 
intruded after the deposition of the Tertiary strata, 
and before their subsequent removal by erosion. 

The whole structure of the Henry Mountains shows 
that the actual surface was affected by those intru- 
sions, the horizontal strata being arched upwards so 
as to form dome-shaped elevations, rising prominently 
above the general level of the plateau. The laccoliths 
are all of considerable size, the smallest measuring 
more than half a mile, and the largest about four 
miles in diameter. The mountains formed by them 



LAND-FORMS DUE TO IGNEOUS ACTION 179 

consist of a group of five individuals separated by low 
passes, but having no definite range or trend. The 
subsequent erosion of these mountains, Mr. Gilbert 
remarks, has given the utmost variety of exposure to 
the laccoliths. In some places these are not yet un- 
covered, and we see only the arching strata which 
overlie them, the strata being cut across by only a 
few dikes or traversed by a network of dikes and 
sheets. In other places denudation has partly bared 
the laccoliths or even completely exposed them, so 
that their original form can be seen. In yet other 
places the bared laccolith itself has been attacked by 
the elements, and its original form more or less 
changed. It is even quite possible that occasionally 
laccoliths may have been entirely demolished, and 
that some of the truncated dikes now visible at the 
surface may mark the old fissures or conduits through 
which such vanished laccoliths were injected. 

From the evidence just referred to, it is obvious 
that intrusions of igneous rock, if of sufficient thick- 
ness, are capable of warping the surface, and of form- 
ing more or less considerable elevations. But as 
erosion tends to reduce all such upheavals more or 
less rapidly, it is only those of relatively recent age 
that can retain any trace of their original configura- 
tion. All masses of intrusive rock of great geological 
antiquity, which now form hills and mountains, do so 
in virtue of their greater resistance to the action of 
epigene agents. They may have arched up the rocks 
underneath which they formerly lay buried, and so 



1 8o EAR TH SCULP TURE 

produced more or less prominent elevations at the 
surface, but such primeval land-forms have been en- 
tirely removed — the features now visible are the 
direct result of erosion and denudation. 

Of true volcanic rocks it is not necessary to say 
much. Their eruption at and near the surface gives 
rise to hills and mountains of accumulation, the gen- 
eral aspect and structure of which are sufficiently fa- 
miliar. The typical volcano is a truncated cone, built 
up usually of successive lava-flows and sheets of loose 
ejecta. At the summit is the central cup, or crater, 
marking the site of the vertical funnel, or throat, 
through which the various volcanic products find 
passage to the surface. These are naturally arranged 
round the focus of eruption in a series of irregular 
sheets, beds, and heaps, which dip outwards in all 
■directions. It is this disposition of the materials 
which gives its characteristic form to a volcano. The 
upper part of the cone inclines at an angle of 30° to 
35°, but this steep slope gradually decreases until 
towards the base the inclination may not exceed 3° 
or 5". In a typical volcano, therefore, the internal 
geological structure and the external configuration 
coincide — the mountain with its graceful outline is 
the direct result of subterranean action. It is obvi- 
ous, however, that the quaquaversal arrangement of 
the lavas and tuffs is a weak structure. Many cones, 
it is true, are braced and strengthened by dikes and 
other protrusions of molten rock, which consolidate in 
the cracks and fissures that often traverse a volcanic 



LAND-FORMS DUE TO IGNEOUS ACTION i8i 

mountain in all directions. But, although such in- 
trusions may delay, they cannot prevent the ultimate 
degradation of a volcano which has ceased to be 
active. 

Active and dormant or recently extinct volcanoes 
differ in form, to some extent, according to the pre- 
valent character of their constituent rocks, and the 
manner in which these have been heaped up. Some 
cones consist of cinders, or other fragmental ejecta, 
with which no lava may be associated. Not infre- 
quently, again, such cones have given vent to one 
or more lava-flows. From small cinder-cones, show- 
ing a single couUe, to great volcanoes built up of a 
multitudinous succession of lavas and sheets of frag- 
mental materials, there are all gradations. The 
smaller cones are often the products of a single 
eruption ; while the larger cones owe their origin 
to many successive eruptions, between some of which 
there may have been prolonged periods of apparently 
complete repose. The beautiful symmetry of the 
typical cone is often disturbed. This is due some- 
times to the shifting of the central focus of eruption ; 
sometimes to the escape of lava and ejecta from 
lateral fissures opening on the slopes of the mountain. 
Not infrequently, also, the symmetry of a growing 
cone is liable to modification by the action of the 
prevalent wind, the loose ejecta during an eruption 
falling in greatest bulk to leeward. 

Tuff-cones and cinder-cones range in importance 
from mere inconsiderable hills to mountains approach- 



i82 EARTH SCULPTURE 

ing or exceeding looo feet in height. In the typical 
cinder-cone the crater is small in proportion to the 
size of the volcano ; it is simply an inconsiderable 
depression at the summit of the cone. Occasionally, 
however, we meet with large crateral hollows, mostly 
now occupied by lakes ringed round by merely an 
insignificant ridge of fragmental materials. Some- 
times, indeed, such large hollows show no enveloping 
ring whatsoever. Extensive craters of this kind are 
believed to be the result of explosive eruptions, and 
it is quite possible, or even probable, that their width 
has been considerably increased by subsequent cav- 
ing in of the ground. Cinder-cones and tuff-cones 
vary in form according to the character of their con- 
stituent materials. When coarse slags and scoriae 
or pumice predominate, the sides of the cone may 
have an inclination of 35", or even of 40°. When 
the materials are not quite so coarse, the angle of 
slope is not so great ; it diminishes, in short, as the 
ejecta become more finely divided, so as sometimes 
not to exceed 15°. 

Just as there are cones composed chiefly or exclus- 
ively of fragmental materials, so there are volcanoes 
built up of one or of many successive lava-flows, with 
which loose ejecta may be very sparingly associated, 
or even sometimes absent altogether. Lava-cones 
likewise vary in shape and size according to the 
nature of their component rocks. Some form abrupt 
hills of no great height ; while others are depressed 
cones, attaining a great elevation and sloping at a 



LAND-FORMS DUE TO IGNEOUS ACTION 183 

very small angle, so as to occupy wide tracts. The 
abrupt cones consist chiefly of the more viscous lavas 
which have coagulated immediately round the focus 
of eruption. The depressed cones, on the other 
hand, are built up of the more liquid lavas, which 
flow out rapidly, and reach relatively greater distances 
from the focus of eruption. Not infrequently the 
cones formed by the outwelling of very viscid lava 
show no crater — the lava coagulates around and 
above the vent. In other cases the top of the abrupt 
dome-shaped cone is blown out- by escaping gases, 
and a crater-shajDed hollow is thus formed. The 
volcanoes of the Hawaiian Islands present the grand- 
est examples pf the eruption of liquid lavas. Hawaii 
itself is made up of five volcanic mountains, ranging 
in height from some 4000 feet up to nearly 14,000 
feet. All these are depressed cones. Mauna Loa 
(13,675 feet), for example, has a broad, flattened 
summit, sunk in which is the great cauldron-like 
crater, some 3^ miles in length by \\ in width, and 
800 feet deep. From the lip of this crater the mount- 
ain slopes outwards at an angle of 3°, which gradu- 
ally increases to 7'^, the diameter of the mountain 
at its base being not less than 30-40 miles. 

But composite cones, built up of lava and loose 
ejecta, are of far more common occurrence than 
cones composed of lava alone. To this class belong 
most of the better-known volcanic mountains. Their 
general characters have already been outlined in the 
short description we have given of a typical volcano. 



i84 EARTH SCULPTURE 

It remains to be noted that many composite vol- 
canoes show a cone-in-cone structure. During some 
paroxysmal eruption the upper portion of a volcano 
may be destroyed— shattered and blown into frag- 
ments. Or, as a result of long-continued activity, 
the mountain becomes partially eviscerated, and the 
upper part of the cone eventually caves in, and a vast 
cauldron is formed, after which a protracted period 
of repose may ensue. When the volcanic forces 
again come into action a younger cone, or it may 
be several such cones, gradually grow up within the 
walls of the old crater. The younger cones may 
rise in the middle of the great hollow, or they may 
be eccentric, as in the case of Vesuvius, which has 
grown up upon the rim of the large crater of Monte 
Somma. 

Of comparatively little importance from our pre- 
sent point of view are mud-volcanoes. Some of these 
owe their origin to the escape of steam and hot 
water through disintegrated and decomposed volcanic 
materials, either tuff or lava, or both. They are 
usually of inconsiderable size, many being mere 
monticles, while others may exceed loo feet in height. 
They show craters atop, and have the general form 
of tuff-cones. Their origin is obvious. The mud is 
simply flicked out as it bubbles and sputters, and the 
material thus accumulates round the margins of the 
cauldron, until a cone is gradually built up. Other 
so-called mud-volcanoes have really no connection 
with true volcanic action, but owe their origin to the 



LAND-FORMS DUE TO IGNEOUS ACTION 185 

continuous or spasmodic escape of various gases, 
such as marsh-gas, carbonic acid, sulphuretted hydro- 
gen, etc. The mud of which they are chiefly com- 
posed is saline, and usually cold. Now and again, 
however, stones and dibris may be ejected. These 
" volcanoes " (variously known as salses, air-volcanoes, 
and maccalubas) usually form groups of conical hill- 
ocks like miniature volcanic cones. Here also may 
be noted, in passing, the sinter-cones formed by those 
eruptive fountains of hot water and steam which are 
known under the general term of geysers. When 
the geyser erupts on level, or approximately level, 
ground, the sinter tends to assume a dome-shape ; 
when, on the other hand, the springs escape upon a 
slope, the silicious deposits are not infrequently ar- 
ranged in successive terraces. 

All the volcanic eruptions to which we have been 
referring have proceeded from isolated foci. Some 
volcanoes are quite solitary, others occur in irregular 
groups, while yet others appear at intervals along a 
given line. These last are obviously connected with 
great rectilinear or curved dislocations of the earth's 
crust ; not a few of the former, however, apparently 
indicate the sites of funnels or pipes which have been 
simply blasted out by the escape of elastic vapours. 
There is yet another class of volcanic eruptions which 
have played a prominent part in geological history, 
although they are not now so common. These are the 
fissure or massive eruptions, of which the best ex- 
amples at the present time are furnished by Iceland. 



i86 EARTH SCULPTURE 

Lavas, usually of the more liquid kind, well out some- 
times simultaneously from more or less numerous 
vents situated upon lines of fracture, or from the 
lips of the fissures themselves. Usually such floods 
and deluges of lava are not accompanied by the dis- 
charge of any fragmental materials. Sheet after 
sheet of molten rock has been discharged in this 
manner so as to completely bury former land-surfaces, 
filling up valleys, submerging hills, and eventually 
building up great plains and plateaux of accumula- 
tion. The basalt-plains of Western North America, 
which occupy a larger area than France and Great 
Britain, are the products of such massive eruptions, 
the lavas reaching an average thickness of 2000 feet. 
The older basalts of Iceland, the Faroe Islands, the 
Inner Hebrides, and Antrim are the relics of similar 
vast fissure eruptions. And of like origin are the 
basaltic plateaux of Abyssinia and the Deccan in 
India. The volcanic phenomena of the Hawaiian 
Islands have also much in common with fissure or 
massive eruptions. 

The forms assumed by the materials accumulated 
at the surface by subterranean action are all more or 
less distinctive and characteristic. Hills, mountains, 
plains, and plateaux, which owe their origin directly 
to volcanic activity, agree in this respect, that their 
internal structure and external form coincide. Even 
the most perfectly preserved examples of volcanic ac- 
cumulation, however, are seldom without some trace 
of the modifying influence of epigene action. The 



LAND-FORMS DUE TO IGNEOUS ACTION 187 

shape of a volcanic cone, for example, during its 
period of growth is subject to modification. Wind 
affects the distribution of loose ejecta, while rain and 
torrents sweep down materials, and gullies and ravines 
furrow the slopes of the mountain. The ravages 
thus caused continue to be repaired from time to 
time so long as the volcano remains active. But 
when its fires die out and the mountain is given over 
to the undisputed power of the epigene agents, the 
work of degradation and decay proceeds apace. The 
rate of this inevitable destruction is influenced by 
many circumstances — by the nature and structure of 
the materials, for example, and the character of the 
climate. Thus, cones built up of loose scoriae are 
likely to endure for a longer time than cones com- 
posed of fine tuff and hardened mud. Rain falling 
upon the former is simply absorbed, and consequently 
no torrents scour and eat their way into the flanks of 
the cones, while tuff- and mud-cones are more or less 
rapidly washed down and degraded. Again, a com- 
posite volcanic mountain of complicated structure, 
the product of several closely associated vents, but- 
tressed and braced by great pipes of crystalline rock 
and an abundant series of larger and smaller dikes, 
is better able to withstand the assaults of epigene 
agents than a cone of simpler build. Sooner or later, 
however, even the strongest volcanic mountain must 
succumb. Constantly eaten into, sapped, and under- 
mined, it will eventually be levelled. 

In regions of extinct volcanoes we may study every 



EARTH SCULPTURE 



Stage in the process of demolition. Isolated cones 
and groups of cones crumble away, until all the lavas 
and tuffs ejected from the old vents may have disap- 
peared, and the only evidence of former volcanic 
action that may remain are the basal portions of the 
dikes that proceeded from the foci, and the solid 
cores with which the latter were finally plugged up. 
(See Fig. 75.) As these cores usually consist of more 




Fig. 75. View of Necks = Corf.s of Old Volcanoes. (PoH-ell.) 

durable materials than the rocks they pierce, they 
tend to form somewhat abrupt conical hills. It goes 
without saying that such extreme cases of denudation 
are met with only in regions where volcanic action 
has for a long time been extinct. Excellent exam- 



LAND-FORMS DUE TO IGNEOUS ACTION 189 

pies on a relatively small scale are furnished by the 
so-called "Necks" of Scotland, of which the accom- 
panying section (Fig. 76) shows the general phe- 
nomena. Similar structures occur in many parts of 
Europe and North America. 

Mtnfo Hill 




S H 

Fig. 76. Section of highly Denuded Volcano. Minto Hill, 
Roxburghshire. 

N^ throat or neck of volcano plugged up with ejectamenta, angular and suhangular stones, 
grit, dust, etc. ; .b", Silurian rocks ; /?, Old Red Sandstone strata. 

Frequently the products of great volcanic eruptions 
of vast geological antiquity have been largely pre- 
served, owing to their subsequent burial under sedi- 
mentary accumulations. Many of the hill-ranges of 
Central Scotland, for example, are built up of lavas 
and tuffs. These are the relics of volcanoes which 
came into existence in Paleeozoic times, and after 
erupting molten and fragmental materials for longer 
or shorter periods, eventually died out, becoming sub- 
merged and covered with sedimentary accumulations 
to depths of several thousand feet. Subsequent ele- 
vation of the region brought these sediments under 
the operation of the agents of erosion, and in time 
great thicknesses were removed, so that ultimately 
the ancient volcanic rocks were again laid bare and 
in their turn exposed to denudation. But if the lat- 



EARTH SCULPTURE 



I 

i 












ii I'Vi 






-(^ 



(i> 






M 



^\ 



Q 2 



>■ 3 

< is 

H .5 

H - 



X 






r^ o 



ter now form hills, it is simply be- 
cause they consist for the most part 
of more durable rock than the form- 
ations amongst which they lie. 
It is needless to say that all trace 
of their original configuration has 
disappeared. Indeed that had al- 
ready vanished before the extinct 
volcanoes became entombed. Now 
and again the sites of the old foci 
of eruption seem to be indicated by 
bosses and dikes of intrusive rock, 
but the general form and aspect of 
the hills are solely the results of 
erosion, determined and guided by 
geological structure and the nature 
and character of the old volcanic 
materials. They are true hills of 
circumdenudation. (See Fig. "/y.) 
The massive or fissure eruptions 
of former times have in like man- 
ner been largely modified by subse- 
quent epigene action. Although 
some of these belong to a compar- 
atively recent geological period, 
they have yet been so carved and cut 
up, that their original plateau- 
character has become obscured or 
even lost. Yet there can be no 
doubt that they formerly existed 



LAND-FORMS DUE TO IGNEOUS ACTION 191 

as broad plains and plateaux, occupying many thou- 
sands of square miles. The older hills of Iceland, all 
the Faroe Islands, and the basalt hills of the Inner 
Hebrides and Antrim are the relics of vast plateaux, 
which were all probably at one time connected. The 
general aspect of the hills carved out of such plateaux 
is well illustrated by the Faroe Islands, to which some 
reference has been made in Chapter III. 

It is believed, as already mentioned, that massive 
eruptions have proceeded rather from systems of fis- 
sures than from separate and individual foci, after the 
manner of most modern volcanoes of the cone and 
crater type. During the eruption of the plateau-basalts 
of Antrim and the Inner Hebrides, molten rock 
underlay not only those regions, but wide areas be- 
yond, in the north of Ireland and through out cen- 
tral and southern Scotland and the north of Eng- 
land. All these areas are traversed by dikes of 
basalt, which become more and more abundant as 
they are followed towards the regions occupied by 
the basalt-flows. It is from these dikes that the 
latter appear to have proceeded. From the dikes 
that are now seen striking across Scotland and the 
north of England probably no outflow of lava took 
place ; the fissures up through which the molten rock 
came did not in those regions reach the surface. 
They are now exposed simply owing to denudation. 
Not a few dikes indeed still lie concealed. In the coal- 
fields these are found cutting across the lower seams, 
but wedging out before the upper seams are reached. 



192 EARTH SCULPTURE 

The larger dikes in central Scotland often form 
conspicuous objects in a landscape. Owing to the 
superior durability of the basalt, they rise above the 
surface of the sedimentary rocks they traverse, and 
may occasionally be followed for miles, running as 
they do like great walls or prominent ridges across 
dale and hill. As examples may be cited two large 
parallel dikes which may be traced for many miles 
from Friarton Hill, near Perth, in a westerly direc- 
tion. Near Dupplin, the more northerly of the two 
gives rise to a long prominent bank, which is fol- 
lowed for some miles by an old Roman road. In 
the neighbourhood of Crieff both dikes are equally 
conspicuous, rising as bold wall-like ridges, the more 
prominent of the two forming the steep crag upon 
which Drummond Castle is perched. When dikes 
cut through rocks as durable as themselves they 
cease to produce any marked feature at the surface. 
On the other hand, when the rocks traversed by 
them are the most resistant, the presence of the dikes 
is indicated by long trenches or hollows at the surface. 
Nothing could be so impressive and suggestive of the 
potency of long-continued erosion than the cropping 
out of these remarkable dikes. Their intrusion 
appears to have taken place in Tertiary times, and 
the great majority of those which occur in the main- 
land of Britain never actually communicated with 
the surface at the time of their formation. They 
cooled and consolidated below ground, yet we now 
see them laid bare not only in the low grounds and 



LAND-FORMS DUE TO IGNEOUS ACTION 193 

in valleys, but upon hill-slopes and hill-tops. Obvi- 
ously hundreds of feet of rock have been removed 
from the whole land-surface since those dikes were 
injected. 

In fine, then, we conclude that many most con- 
spicuous and characteristic features of the land owe 
their origin to igneous action. In some places the 
intrusion of masses of molten rock has produced 
more or less prominent swelling and bulging at the 
surface, while the outpouring of volcanic materials 
has resulted in the formation of hills and mountains, 
and of plains and plateaux of accumulation. Ere 
long, however, all such land-forms become modified 
by epigene action, and more or less completely 
changed. Intrusive masses formerly deeply buried 
are eventually exposed, and, owing to the more rapid 
removal of the rocks through which they rise, may 
come to form mountains of circumdenudation, while 
these in their turn tend to be reduced to a base-level. 
Volcanoes, in like manner, are broken down and 
crumble away, until it may be the only relics that 
remain are plugged-up vents, and the dikes proceed- 
ing from them, every fragment of the cones having 
vanished. Or the lavas of former times, having 
been interbedded with and deeply buried under strata 
of aqueous formation, may, owing to their superior 
durability, come to form escarpment-hills and mount- 
ains, when the strata originally deposited above 
them have been removed by denudation. So again 
volcanic plateaux are dug into by erosion, and pass 



194 EARTH SCULPTURE 

through a well-marked cycle of changes. The plat- 
eaux are broken up into groups of pyramidal mount- 
ains, and these in time are reduced, and may even 
be entirely replaced by plains of erosion. Thus in 
lands which have been for long periods of time ex- 
posed to erosion, although evidence of former igneous 
action may abound, and irruptive and eruptive rocks 
may enter prominently into the formation of the 
more striking surface-features, the shape of the latter 
we see is entirely the result of denudation and erosion. 
If the igneous rocks now form hills and mountains, 
it is because of their superior durability. Intrusive 
and effusive rocks alike appear at the surface, and 
the forms they assume depend chiefly upon the geo- 
logical structure and mineralogical character of the 
masses. 



CHAPTER IX 

INFLUENCE OF ROCK CHARACTER IN THE 
DETERMINATION OF LAND-FORMS. 

JOINTS IN ROCKS AND THE PART THEY PLAY IN DETERMINING 
SURFACE-FEATURES TEXTURE AND MINERALOGICAL COM- 
POSITION OF ROCKS IN RELATION TO WEATHERING FORMS 

ASSUMED BY VARIOUS ROCKS. 

THE origin of surface-features, as we have now 
learned, is frequently complex. Only in very 
few cases can we assert that any prominent feature 
is the direct result of crustal movement alone. In 
time all features due to plutonic or subterranean 
action become more or less modified. We are justi- 
fied in maintaining that the great mountain-chains 
of the globe owe their origin indeed to folding and 
fracturing of the crust ; but even the youngest of 
these has yet been so profoundly modified by epigene 
action, that the external configuration no longer 
coincides, save in a general way, with the internal 
geological structure. Each chain as a whole owes 
its existence to crustal deformation, but the individual 
mountains of which it consists are largely monuments 
of erosion. And so of land-surfaces generally we 
may say that their more prominent features are the 

195 



196 EARTH SCULPTURE 

result of denudation, guided and controlled by geo- 
logical structure. We cannot study the configuration 
of the land, however, without perceiving that the 
relative durability of rocks has also had some share 
in determining the form of the surface. In regions 
composed largely of "soft" rocks we may note a 
general absence of abrupt and broken outlines ; the 
surface even when hilly is usually rounded and gently 
undulating. It is otherwise when "hard" rocks pre- 
dominate, the features assumed by these tending to 
be less smooth and flowing. The surface becomes 
more diversified still, however, when both soft and 
hard rocks occur together. In a word, hard rocks at 
all elevations offer most resistance, while soft rocks 
more readily succumb to epigene action. We thus 
arrive at the general conclusion that the form assumed 
by the land under long-continued erosion and denud- 
ation is determined directly by the character of the 
rocks and the mode of their arrangement, and in- 
directly, of course, by igneous action and crustal 
movements, to which the most striking and conspicu- 
ous geological structures are due. 

These general conclusions have now been suffi- 
ciently illustrated, and we may next consider certain 
surface-features a little more closely. Rocks, as we 
have seen, consist roughly of two great classes — those 
which occur in more or less distinct beds or strata, and 
those which show no such arrangement, but appear as 
amorphous masses. The former class is typically 
represented by sandstones, shales, and limestones, the 



INFLUENCE OF ROCK CHARACTER 197 

latter by granite, syenite, and other eruptive rocks. 
Most of the bedded rocks are fragmental or clastic ; 
but crystalline rocks, such as the various lavas, not in- 
frequently assume bedded forms. With few excep- 
tions all great amorphous rock-masses are crystalline. 
There is yet another important group of crystalline 
rocks — the schists — which to some extent simulate 
the characteristic structures of clastic rocks. Thus 
they often show a kind of bedding, and their foliation 
mimics, as it were, the lamination of shaly strata. 
The foliation and bedding, however, are commonly 
more or less puckered and contorted. 

Now all rocks are traversed by natural division- 
planes or joints, and these, in the case of well-bedded 
strata, are usually disposed at approximately right 
angles to the planes of bedding. Thus, as we have 
seen, beds of sandstone, etc., are divided up into 
somewhat quadrangular or cuboidal blocks. Old 
lava-flows, in like manner, often show at least two 
similar sets of vertical joints, and not infrequently 
these are cut by a third set, disposed at approxi- 
mately right angles to the others. Not a few bedded 
igneous rocks and intrusive " sheets," again, assume 
a more or less columnar aspect, owing to the sym- 
metrical arrangement of the joints. In amorphous 
masses of crystalline rocks, on the other hand, uni- 
form jointing as a rule is absent. Their division- 
planes run in various directions, and are often 
extremely irregular. In some places they may be 
very closely set, in other places they are far apart. 



198 EARTH SCULPTURE 

Thus while bedded strata of all kinds, breaking up 
along the joints, tend to give rise to rectangular feat- 
ures at the surface, amorphous crystalline rocks, 
quarried by epigene action, generally yield irregular 
contours. And the same is the case with the crystal- 
line schists, the jointing of which is as a rule capri- 
cious and uncertain. 

It is obvious, therefore, that surface-features must 
be greatly influenced by the character of rock-joints. 
Apart altogether from other geological structures, 
joints must largely determine the physiography of 
the surface. To such an extent is this the case, that 
it is generally easy to tell at a glance whether any 
particular mountain is composed of amorphous crys- 
talline rocks, of schists, or of regularly bedded strata. 
Mountains carved out of horizontal strata tend, as we 
have seen, to assume pyramidal forms, while in the 
case of inclined beds erosion and denudation result 
in the formation of escarpments and dip-slopes. This, 
however, only holds true when relatively hard beds 
are intercalated among a series of softer strata. 
Should the rocks throughout be of much the same 
consistency no escarpments will be developed, but the 
whole will wear away equally, and so give rise to a 
gently undulating surface. Usually, however, a thick 
series of strata will be found to comprise rocks of 
various degrees of durability ; and in general, there- 
fore, bedded rocks, whether horizontal or inclined, 
tend to yield rectangular outlines. But when the dip 
greatly increases, and the strata are more or less vio- 



INFLUENCE OF ROCK CHARACTER 199 

lently contorted, the beds are often crushed and con- 
fusedly shattered or jointed, while at the same time 
the rocks themselves may become metamorphosed, 
and eventually pass into the condition of schists. 
Rectangular outlines are thus gradually replaced by 
the jagged, rough, and abrupt configuration which is 
so characteristic of slaty and schistose or foliated 
rocks. 

Amongst the crystalline schists rectangular out- 
lines are not common. Now and again, however, 
when different kinds of schists rapidly alternate in 
successive sheets or beds, some will almost certainly 
weather more rapidly than others. The outcrops of 
the less yielding rocks will thus tend to project ; but 
as jointing is usually irregular and confused, such out- 
crops seldom show rectangular outlines. Exception- 
ally, well-marked escarpments may be met with, but 
the general high dip and contorted character of the 
rocks forbid such formations. When steep wall-like 
outcrops of schists occur, they have very often been 
determined by the presence of normal faults or of 
thrust-planes. In short, while the foliation and 
pseudo-bedding of schistose rocks now and again 
give rise to surface-features which are more charac- 
teristic of truly bedded strata, yet such features are 
apt to be strongly modified by the vagaries of the 
jointing. 

In amorphous crystalline masses, which show 
neither bedding nor foliation, the character of the 
joints usually varies with the nature of the rock. In 



200 EARTH SCULPTURE 

granite, for example, there are usually three sets of 
joints, one of which traverses the rock in an approxi- 
mately horizontal direction, or may have a dip now 
in one direction, now in another. The vertical joints 
often cut each other at right angles, but not infre- 
quently they meet at more or less acute angles. In 
addition to these main joints, however, there are often 
others. Sometimes the joints are wide apart, and 
they then enclose large rectangular or rhomboidal 
blocks. At other times they are set so closely 
together that the rock when exposed breaks up into 
a mass of angular ddbris. As the character of the 
jointing varies in this way within narrow limits, the 
rock tends to assume broken interrupted contours. 
On the other hand, when the disposition of the joint- 
planes is more regular and better defined, the hori- 
zontal joints maintaining their direction for some 
distance, granite not infrequently breaks up as if it 
were a bedded igneous rock. A mountain-wall so 
constructed rises in a series of gigantic steps, like 
tiers of cyclopean masonry, interrupted by entering 
and re-entering angles. Where the " horizontal " 
joints are much inclined a corresponding change in 
the direction of the main rock-ridges and reefs may 
be observed. Not infrequently, however, the hori- 
zontal jointing is obscure and ill-defined or even 
wanting, and the chief contours of the surface are 
then determined by the vertical joints alone. Under 
such conditions the mountain-slope shows irregular 
vertical or steeply inclined walls, ridges, and but- 



Plate 




Joints in granite, Glen Eunach, Cairngorm. 



INFLUENCE OF ROCK CHARACTER 201 

tresses, which often run into each other as they are 
followed upwards, and may eventually taper off to a 
point. (See Plate I.) 

The influence of joints, however, is apt to be 
greatly obscured by the manner in which rocks them- 
selves disintegrate and crumble down. The sharply 
angular rock-faces defined by joints are slowly or 
more rapidly eaten into by epigene action, and the 
rock exfoliates or crumbles down irregularly accord- 
ing to its character. Indeed, this rotting action has 
often proceeded very far before the joint-faces are laid 
bare. When a mass of rock, losing its support, falls 
away, the new surface exposed has already become to 
a larger or smaller extent disintegrated and decom- 
posed, so that frost and rain are enabled rapidly to 
reduce and modify it. Hence the sharp irregular 
outlines which joints naturally tend to produce are, 
in the case of such rocks as granite, generally rounded 
off. Basalt-rocks in like manner often weather readily 
and become decomposed and disintegrated along 
planes of jointing, and thus give rise to a somewhat 
rounded and lumpy configuration. But there is often 
much diversity of surface displayed by one and the 
same rock-mass, the basalt in some places weathering 
rapidly into rounded forms, while in other places, 
especially where the rock is fine-grained and compact, 
the sharp angles of the jointing are better preserved. 
(See Plate II.) 

The usually finer-grained rhyolites, trachytes, ande- 
sites, and phonolites are not as a rule so readily dis- 



202 EARTH SCULPTURE 

integrated as normal granites and basalts. Their 
joints, moreover, are not only less uniform, but fre- 
quently very abundant and closely set. Such rocks, 
therefore, are readily broken up. Mountains carved 
out of them usually show sharp crests and peaks, 
while their slopes are hidden under curtains of angular 
debris, through which ever and anon are protruded 
reefs, ridges, buttresses, and bastions of such portions 
of the rock-mass as are less profusely jointed. (See 
Fig. 78, p. 203.) 

In short, we may say that every well-marked rock- 
type breaks up and weathers in its own way, so that 
under the influence of denudation each assumes a 
particular character. We see this even in the case of 
well-bedded aqueous rocks. Planes of bedding and 
jointing no doubt are the lines of weakness along 
which rocks most readily yield, but each individual 
rock-species weathers after its own fashion — the dif- 
ferent kinds of shale, sandstone, conglomerate, and 
limestone are decomposed, disintegrated, and crum- 
bled down at different rates, and each in a special 
way, according to its mineralogical compc sition and 
state of aggregation. Thus, although a region built 
up of bedded aqueous rocks may show the same 
general configuration throughout — horizontal strata 
giving rise to pyramidal-shaped hills and mountains, 
while inclined strata of variable consistency present 
us usually with a series of escarpments and dip-slopes 
— yet with all this sameness the details of rock-sculpt- 
uring may be singularly varied. And the same is 



Plate II. 




\\'eathering of joints in granite, Cairngorm Mountains. 



204 EARTH SCULPTURE 

true of the crystalline schists. Mountains composed 
of such rocks have much the same general configura- 
tion. But when viewed in detail they show with 
every change in the character of the rock some corre- 
sponding change in the aspect of the surface. Again, 
in the case of granite, gabbro, and other massive 
igneous rocks, all these doubtless break up and pro- 
duce characteristic configurations. But in each indi- 
vidual case we may note many details of sculpturing 
which are not the result of jointing, but of variations 
in the texture, and even in the mineralogical compo- 
sition of the rock. We may note further that one 
and the same kind of rock does not necessarily always 
present quite the same aspect under weathering and 
erosion. Much will depend on the character of the 
climate, on the elevation of the region in which it 
occurs, and on the nature of the surface, whether, for 
example, that be steeply or gently inclined. 

The characteristic forms assumed by rocks are, of 
course, best seen in places where these are well ex- 
posed. In low-lying tracts the rock-surface is usually 
more or less concealed beneath alluvial deposits and 
other superficial accumulations of epigene action. It 
is in river-ravines and along the sea-coast, or better 
still amongst the mountains, that rock-weathering 
must be studied. Even at the higher levels, however, 
the rocks are often largely concealed under their own 
ruins. Sheets and cones of debris extend downwards 
from the base of every projecting cliff and buttress. 
Hence in the case of mountains carved out of bedded 



INFLUENCE OF ROCK CHARACTER 205 

rocks, the rectangular outlines tend to become ob- 
scured, ' projecting rock-ledges gradually disappear 
under piles of ddbris, and a smooth slope may replace 
in whole or in part the rectangular corbel-steps of the 
typical pyramid, while steep escarpments may be 
smoothed off to more or less gentle inclines. In the 
case of mountains composed of schistose rocks the 
general steep inclination and contorted character of 
the bedding and the varied character of the rocks 
themselves favour the preservation of abrupt and 
irregular slopes. There is a general absence of 
horizontal or gently inclined platforms upon which 
debris may come to rest. The great mass of the 
material loosened and detached by weathering rolls 
and shoots downwards to the screes accumulating at 
the base of the mountains. These, as denudation 
advances, are of course continually extending up- 
wards. But the characteristic configuration of the 
rocks above the scree-line is maintained, and not 
obscured, as so frequently happens in the case of 
horizontal or gently inclined strata. Amorphous 
igneous masses break up in so diverse a manner, that 
mountains composed of such often show much variety 
of feature. The upper limits of the scree-line are 
very tortuous, here sweeping up almost to the very 
crest of a mountain, there hugging the base of gaunt 
cliffs and precipices. Or, when horizontal jointing is 
well defined, we may have a succession of abrupt ledges 
breaking the continuity of a scree-slope. When, on the 
other hand, vertical joints are most pronounced bare 



2o6 EARTH SCULPTURE 

rock-walls and steep ridges rise more or less abruptly 
above the limits of the depressed scree-line below. 

In regions subject to well-marked dry and rainy 
periods even low grounds and plateaux not infre- 
quently show much bare rock. This is due to the 
fact that disintegrated rock-material tends to be 
swept rapidly downwards by heavy torrential rains. 
Should the land be well clothed with vegetation, the 
reduction of the surface is much retarded. The rocks 
may become rotted to great depths, as in Brazil, but 
the decomposed material remains in situ. Where veg- 
etable life in such latitudes is less prolific the surface 
becomes scorched and dried, and disintegrated rock- 
material is readily removed when the rainy season 
comes round. Under these conditions the surface- 
features, due to epigene action, are usually strongly 
pronounced. A plateau of granitoid rock, for ex- 
ample, owing to inequalities of structure, texture, 
and composition, often yields a highly diversified sur- 
face ; rounded blocks and boulders of all shapes and 
sizes appear scattered broadcast, while sporadic 
masses, stacks, cones, tors, crags, and peaks, and ir- 
regular winding gullies and depressions, are every- 
where encountered. But the same phenomena, if 
somewhat less prominently developed, are seen again 
and again in temperate latitudes. The " tors " of 
Cornwall are in their way as striking as the kopjes 
of Mashonaland. Many other kinds of rock, after 
long exposure to the weather, present similar fantastic 
outlines. The " Quadersandstein " of Saxon Switzer- 



INFLUENCE OF ROCK CHARACTER 207 

land, for example, which over considerable areas lies 
in approximately horizontal strata, has suffered great 
erosion, the characteristic features of the region being 
conical hills or pyramids and broad bastions, along 
the flanks of which the naked rock appears. Thus 
exposed to weathering, the sandstones yield along the 
vertical joint-planes and fall away somewhat unequally, 
and so stacks and columns eventually become separ- 
ated from the main rock-masses, and often weather 
into odd and picturesque forms. 

The surface-features assumed by limestone are very 
characteristic, and these, as in the case of all stratified 
rocks, are determined by bedding and jointing. But 
the soluble character of limestone causes it to weather 
in a manner peculiar to itself. Bare surfaces are eaten 
into, and become irregularly honeycombed and fur- 
rowed — the rock, in short, is corroded by the chemical 
action of rain. Should the ground be steeply inclined, 
the surface of the limestone shows numerous more or 
less parallel gutters and trenches, separated by narrow 
ridges which are frequently sharp and knife-edged. 
Upon gentler slopes the gutters are less regular, and 
the ridges are often somewhat rounded ; the whole sur- 
face, indeed, may be rudely mammillated, and tra- 
versed or interrupted by abrupt furrows and smoother 
depressions. These appearances are most marked 
when the limestone is pure ; when it contains much 
insoluble matter the characteristic ridges and trenches, 
rounded humps and hollows, are seldom well devel- 
.oped. It is needless to add that endless modifications 



2o8 EAR TH SCULP TURE 

of the surface-forms referred to result from the char- 
acter of the bedding and jointing, the latter having 
often determined the direction of the gutters and fur- 
rows. The appearances now described (the Kar- 
renfelder of German writers) are not confined to 
any particular level, but occur at all levels, being 
most pronounced, however, on high plateaux and in 
mountain-regions where there is little or no vegetable 
covering. Excellent examples are met with in the 
calcareous tracts of the Alps, in the Jura, in the plat- 
eaux of the Cevennes, in the Pyrenees, at Gibraltar, 
and many other places in Europe. 

Owing to its solubility, limestone is not only cor- 
roded at the actual surface, but joints and fissures are 
widened by the same solvent action, and thus, in time, 
underground channels are licked out, and streams 
and rivers are gradually conducted into subterranean 
courses. These now become widened and deepened, 
not only by chemical solution, but by the mechanical 
action of running water. Thus, in limestone regions, 
the whole drainage may be directed underground. 
Considerable streams and rivers plunge suddenly into 
the depths, and after a longer or shorter course may 
reappear at the surface, or they may flow on until 
they make their final escape on the floor of the sea. 
The surface of a limestone country is often drilled by 
more or less vertical holes and pipes of variable width, 
which communicate directly with subterranean streams 
and rivers. These pipes are, no doubt, in many cases, 
licked out by meteoric water, but not infrequently 



INFLUENCE OF ROCK CHARACTER 209 

they are caused by the collapse of the undermined 
rocks. Owing to various causes, engulfed streams 
now and again abandon their courses, and work their 
way to lower levels, and in course of time such aban- 
doned channels may become disclosed by the falling- 
in of the roof, or by the more gradual denudation and 
truncation of the rock by surface-action. Hence, in 
regions built up of calcareous rocks, caves are of com- 
mon occurrence, many of them being of large dimen- 
sions, and often branching in all directions. 

Caves and other hollows are not infrequently worked 
out by weathering in many other kinds of rocks, but in 
no case do they attain the size of those which we so 
commonly encounter in areas occupied by limestone, 
as will be shown in a succeeding chapter. 

We need not, however, enter into further detail as 
regards the characteristic weathering of particular 
rocks. It is enough for our purpose to recognise the 
fact that composition and texture play no unimport- 
ant part in determining the aspect assumed by rocks 
under denudation. In preceding pages we have dis- 
cussed the origin of the salient features of a land- 
surface. Looked at broadly, it is obvious that the 
more elevated and more depressed areas owe their 
existence primarily to movements of the earth's crust. 
Thus all the great mountain-tracts and plateaux of 
Europe may be looked upon as regions of relative 
uplift, while the broad low grounds above which they 
rise may be described in general terms as regions of 
relative depression. In a word, the larger features of 



2IO EARTH SCULPTURE 

the land have been blocked out by subterranean 
action, they are the result of crustal deformation. 
Viewed from a nearer standpoint, however, we recog- 
nise that every feature due to deformation has been 
more or less profoundly modified by denudation, 
guided and determined by the geological structure 
and relative durability of the rocks. Approaching 
still nearer, we see how each particular kind of rock 
wears away in some particular and characteristic fash- 
ion, so that surface-features vary infinitely in detail 
quite independently of the geological structure. Thus 
the part played by subterranean action is merely to 
provide the rough block which the epigene agents 
subsequently sculpture into shape. With few excep- 
tions, the land-features that now meet our eye are the 
direct result of erosion and accumulation, the modify- 
ing influence of which is always more or less conspic- 
uous even in cases of recent crustal deformation. 

Now if it be true that the character of a land-sur- 
face is determined by geological structure and the 
nature of the rocks, we should expect to meet with 
very considerable diversity of configuration in regions 
built up of many varieties of rock arranged in many 
different ways. And such undoubtedly is the case ; 
but it is less true of temperate and northern regions 
than of more southerly latitudes. Not that the influ- 
ence of rock-structure is ever quite lost even in the 
former, but it is often obscured. In the contours 
of the higher Alps, for example, it is conspicuous 
enough, but the lower mountain-slopes not infre- 



INFLUENCE OF ROCK CHARACTER 211 

quently fail to show it, or show it much less plainly. 
Further north, as in our country and in Scandinavia, 
undulating and flowing configurations prevail amongst 
the mountains. Broken and serrated outlines are sel- 
dom seen, and usually only at the higher elevations. 
Mountains built up of bedded rocks, of schists, of mas- 
sive igneous rocks, are not so strongly differentiated 
as similar mountain-masses are in more southern 
lands. It is only when they are looked at more closely 
that the influence of geological structure and petro- 
graphical character becomes apparent. Everywhere, 
however, we find that this influence has been more 
or less interfered with ; mountains which, under the 
ordinary action of the atmosphere, must have assumed 
serrated crests and peaks, appear instead with rounded, 
smoothed, and softened outlines; projecting buttresses, 
reefs, and ridges have lost much of their angularity, 
and escarpments likewise are frequently bevelled off. 
These remarkable modifications of the surface are 
due to glaciation. There is no reason to doubt that 
before the advent of the Ice Age rock character and 
geological structure were as strongly expressed in 
the configuration of our hills and valleys as they are 
now in regions which have never experienced glacia- 
tion. Indeed, so long a time has elapsed since the 
disappearance of our ice-fields and glaciers, that the 
smoothed and rounded surfaces are again breaking 
up, and the more irregular and angular contours and 
outlines which obtained in preglacial ages are thus in 
process of gradual restoration. 



CHAPTER X 

LAND-FORMS MODIFIED BY GLACIAL ACTION 

GEOLOGICAL ACTION OF EXISTING GLACIERS — EVIDENCE OF ERO- 
SION ORIGIN OF THE GROUND-MORAINE : ITS INDEPENDENCE 

OF SURFACE-MORAINES — INFRAGLACIAL SMOOTHING AND POL- 
ISHING, CRUSHING, SHATTERING, AND PLUCKING — GEOLOGI- 
CAL ACTION OF PREHISTORIC GLACIERS GENERAL EVIDENCE 

SUPPLIED BY ANCIENT GLACIERS OF THE ALPS. 

AT the close of the last chapter reference was made 
to the fact that the surface-features of certain 
regions have been modified by subsequent glacial 
action. This action, as we have indicated, tends to 
efface or obscure the characteristic forms assumed by 
rock-masses under the influence of weathering. In 
other words, ice is an eroding agent, but it works in 
a different way from the ordinary epigene agents. 
While the latter tend to produce manifold irregulari- 
ties of the surface, and to develop angular outlines 
for the most part, the former tends, on the other hand, 
to smooth away inequalities and to replace angular 
outlines with rounded contours. It is demonstrable, 
therefore, that ice is an eroding agent, but some geo- 
logists have doubted whether it is very effective, and 
are of the opinion that the utmost it can do is to 



GLACIAL ACTION 213 

smooth and abrade to a very limited extent. As it is 
important, from our present point of view, that we 
should clearly understand this question of glacial 
erosion, we may consider the evidence in some little 
detail. 

For this purpose we may approach the subject much 
in the same way as a geologist would do were he en- 
deavouring to prove for the first time that rivers are 
potent agents of erosion. Doubtless, in such a case, 
his first care would be to describe the work done by 
existing rivers ; thereafter he would depict the char- 
acter and attempt to set forth the precise origin of 
alluvial terraces, plains, and deltas ; and, finally, he 
would adduce evidence to prove that all such forma- 
tions are products of erosion, and that by the gradual 
removal of such products valleys have been originated 
or deepened. In like manner we shall consider first 
the character of existing glacial action ; then we shall 
inquire into the nature and origin of ancient glacial 
accumulations ; and finally we shall show how these 
last are evidence of extensive glacial erosion, and 
how, by their removal, valleys have been widened 
and deepened, and rock-basins of particular kinds 
have been formed. 

I. The geological action of existing glaciers. — The 
most obvious work performed by an Alpine glacier 
is that of transport and accumulation. The wreck of 
the adjacent mountains, strewn upon its surface, is 
continually carried forward, and eventually heaped 
up in the form of terminal moraines. The infragla- 



2 1 4 EAR TH SCULP TURE 

cial dibris extruded at the lower end of the ice-flow 
bears, usually, a very small proportion to the supply 
of rock-rubbish travelling at the surface. This, how- 
ever, is not invariably the case, even in the Alps. 
Not infrequently small " summit glaciers," lying upon 
mountain-slopes, bear no superficial detritus, while 
infraglacial ddbris, nevertheless, is constantly being 
extruded at their lower ends. Thus the small Stampfl- 
kees Glacier (Zillerthal), overlooked by hardly any 
exposed rock-surfaces, and consequently carrying lit- 
tle or no superficial rock-rubbish, yet exhibits at its 
terminal front a bottom- or ground-moraine some ten 
or fifteen feet thick. But that which is the exception 
in Alpine lands is the rule in Arctic regions. The 
tongues of ice protruding from the vast mer de glace 
of Greenland are almost entirely free from the super- 
ficial ddbris^ and yet they eject ground-moraine in 
abundance. The same, as we shall see presently, is 
the case with most of the Norwegian glaciers. It is 
obvious, therefore, that the relative importance of 
ground-moraine, as a product of glacial action, is really 
greater than a glance at the phenomena of any ordin- 
ary Alpine glacier would at first lead one to suppose. 
The general nature of Alpine ground-moraine is 
well known. It consists simply of an aggregate of 
rock-fragments, grit, sand, and mud or clay, often 
frozen or pressed together, and so included in the 
lower or basal portion of the glacier. Many of the 
stones are subangular and blunted, and striated, 
smoothed, or polished on one or more sides. No 



GLACIAL ACTION 215 

one doubts that this material has travelled under- 
neath, and partly enclosed in the ice-flow, and that 
the rock-surface over which it has been carried is 
abraded, smoothed, and polished by its filing action. 
Everyone, in short, admits that some degree of ero- 
sion is the result of glacial action. Were that action 
entirely confined to mere abrasion and smoothing of 
rock-surfaces, it yet could hardly be considered insig- 
nificant. The fine powder or flour of rock which 
renders all glacial rivers turbid, shows that glacial 
grinding is really of great importance. It has been 
computed, for example, that the river extending from 
Aar Glacier carries away daily 280 tons of solid mat- 
ter in suspension. Again, the Justedal Glacier, drain- 
ing an ice-field 820 square miles in extent, discharges 
in a summer day 1968 tons of sediment. This is in 
excess of the average daily discharge during the year, 
which Helland estimates at 180 million kilogrammes. 
To this should be added the mineral matter carried in 
solution, amounting to 13 million kilogrammes, so 
that solid and dissolved materials taken together come 
up to 189,950 tons. This would form a mass equal 
to 90,252 cubic yards. According to the same geolo- 
gist, the VatnajokuU (Iceland), draining an ice-field 
ten times larger than that of the Justedal, discharges 
annually 14,763,000 tons of sediment — an amount 
equal to 7,194,000 cubic yards of rock. Thus, even 
if a glacier does no more than abrade and smooth its 
bed, the amount of rock ground into powder is neither 
insignificant nor unimportant. 



2i6 EARTH SCULPTURE 

But is this all the erosion that a glacier accom- 
plishes ? What about the ddbris of its ground-moraine 
— whence, is that derived ? Professor Heim and 
others maintain that in the case of a large number of 
glaciers (Alps, Himalaya, New Zealand) infraglacial 
detritus comes chiefly from superficial sources. Over- 
lying morainic rubbish, it is supposed, finds its way 
through crevasses to the bottom of the ice. Now 
there can be no doubt that surface-moraines are 
frequently engulfed in crevasses ; but then the rock- 
rubbish engulfed in this way sooner or later reap- 
pears at the surface of the glacier further down the 
valley. Obviously in such cases the dibris does not 
descend to the bottom of the glacier, but is simply 
engorged at some distance from the surface, and again 
becomes exposed, owing to the curving upwards of 
the lines or planes of flow and the ablation of the 
surface. If crevasses penetrated the whole thickness 
of a glacier, doubtless ddbris plunging into them 
might well reach the bottom of the ice, and be in- 
cluded as ground-moraine. But the plasticity of ice 
necessarily limits the depths to which a crevasse can 
extend. The larger glaciers, according to Heim, are 
never penetrated to the bottom by crevasses, which 
when not kept open and deepened by ablation do not 
exceed a depth of 100-150 metres. Superficially 
carried rock-rubbish, therefore, can reach the bottom 
of a moderately thick glacier only along the margin, 
where the crevasses open to the rock-head. Here 
and there, perhaps, debris may occasionally descend 



GLACIAL ACTION 217 

by nioiilins ; but as a rule the bed of such a glacier 
can receive only a very meagre supply of rock-frag- 
ments from above. And if this be the case with the 
relatively small glaciers of the Alps, it must be the 
same in a more marked degree with those of high 
northern and Arctic lands. 

Reference has already been made to the fact that 
even in the Alps certain summit-glaciers are so placed 
that no ddbris is showered upon them, and yet these 
glaciers extrude more or less conspicuous ground- 
moraines. In a word, the existence of ground- 
moraines does not depend upon the presence of 
superficial moraines. The latter are not infrequently 
wanting ; the former, on the contrary, never are. 
This is well seen in the case of the Norwegian gla- 
ciers, which, as compared with those of the Alps, 
might be described as almost devoid of surface-fsf^^rw. 
Nevertheless, ground-moraines are always in evidence, 
appearing not only under the tongue-like glaciers 
which protrude from the plateau ice-fields, but at the 
base of the more or less steep walls in which those 
ice-fields usually terminate. 

The great development of superficial moraines in 
the Alps as contrasted with their meagre appearance 
in Scandinavia is easily explained. In the former 
region we have a complicated series of mountain- 
groups and chains, the crests of which overlook pro- 
found cirque-like depressions. It is in these broad 
and deep troughs and basins that snows accumulate 
to form the reservoirs from which glaciers flow. 



2i8 EARTH SCULPTURE 

Even at its very source, therefore, an Alpine glacier 
has roQkrddbris supplied to it from above, and as it 
passes down its mountain-valley frost and avalanches 
keep up a constant bombardment, so that the farther 
it flows the greater becomes the amount of detritus 
eventually piled up in its terminal moraines. Nor- 
way, on the other hand, is a lofty plateau, more or 
less deeply trenched by fiords and valleys. The 
snows, therefore, accumulate upon a wide and rela- 
tively flat or undulating surface, not dominated by 
peaks or ridges. In the central part of a Norwegian 
snow-field the surface is more or less continuous, and 
seldom interrupted by crevasses. Now and again, 
however, these are encountered, and their walls show 
stratified ndvd above graduating downwards into com- 
pact ice. Towards the margin of such an ice-field 
crevasses become more frequent, and in these snow 
and ndvd are seen gradually thinning-off as the term- 
inal wall is approached, until at last the blue ice is 
wholly exposed. In short, the Scandinavian plateau 
supports true ice-sheets, comparable in all respects, 
save as regards their extent, to the great " inland ice " 
of Greenland. In places, longer or shorter tongues 
of ice project from the ice-sheet into valleys ; in other 
places, where no valleys are present, the sheet simply 
terminates in a continuous ice-wall. 

Such being the character of the Scandinavian ice- 
fields, we need not wonder at the absence of superfi- 
cial moraines. No mountains overlook the plateaux; 
it is only when the ice creeps outwards into valleys 



GLACIAL ACTION 219 

that it is liable to have rock-rubbish dumped on its 
surface. Moreover, the course of such valley-glaciers 
is so short as a rule, and their rate of flow so com- 
paratively rapid, that conspicuous lateral moraines 
cannot be accumulated. It is further noteworthy 
that Norwegian glaciers do not form prominent ter- 
minal moraines, and these are composed chiefly of 
water-worn gravel and blunted and subangular stones. 
Sharply angular blocks and fragments do not predom- 
inate as in the end-moraines of Alpine glaciers. In 
a word, the Norwegian terminal moraines appear to 
consist mainly of infraglacial and fluvio-glacial detritus, 
which the ice builds up into low mounds and ridges. 
But if superficial moraines are sparingly devel- 
oped, the same is not the case with ground-moraines. 
These are seen not only under the glacier-tongues in 
valleys, but they are conspicuous likewise under the 
bordering ice-walls of the plateau -sheets. Every- 
where, also, from the margin of these sheets, as from 
the valley-glaciers, flow streams and torrents of turbid 
water. 

The phenomena exhibited by the Scandinavian ice- 
fields are exemplified on a much larger scale in Green- 
land. There, as in Norway, superficial moraines are 
entirely wanting, except where the ice-sheet protrudes 
long tongues into mountain-valleys and fiords. Where 
the ice-sheet terminates upon land ground-moraines 
are conspicuous. Nansen, for example, tells us that 
at Austmannatjern, where he left the inland ice after 
his famous traverse, enormous accumulations of mo- 



220 EARTH SCULPTURE 

raine were seen. These were of true infraglacial ori- 
gin, consisting largely of blunted and striated stones, 
which could only have been transported by the ice as 
ground-moraine. No Nunatakkr occurred within 
the mer de glace near this place, and not a vestige of 
surface-moraine was visible. Dr. Hoist, Dr. Dry- 
galski, and others have referred to the appearance of 
ground-moraines in Greenland, and the phenomena 
in question have also been described by Professor 
Chamberlin. The latter shows that the tongues of 
ice proceeding from the local ice-caps and from the 
great inland ice are crowded towards their base with 
ground-moraine, the lower strata of the ice for a 
thickness of 50 to 70 feet above the bottom showing 
layers and irregular sheets of clay, mud, sand, stones, 
and boulders, all of which are of infraglacial origin, 
while the upper and much thicker mass of ice is free 
from such inclusions. It is not necessary to enter 
into greater detail, but it may be added that in 
Greenland as in Norway turbid water escapes in large 
volume from the " inland ice." 

Reflecting upon the facts thus briefly recapitulated, 
we must conclude that glaciers are powerful agents of 
erosion. Not only do they grind, smooth, and polish 
rock-surfaces, as everyone admits, but they also quarry 
their beds. The stones and boulders of the ground- 
moraines are derived directly from below by the ice 
itself. In the case of Alpine glaciers, no doubt debris 
may occasionally be introduced to the ground-moraine 
from above ; but this descent of superficial detritus 



GLACIAL ACTION 221 

cannot take place in the plateau-sheets of Scandi- 
navia, nor in the local ice-caps of the great " inland 
ice " of Greenland. In some way or other, rocks under- 
lying a glacier are liable to disruption and displace- 
ment ; and such, we cannot doubt, is the chief source 
of the stones and grit and clay of ground-moraines 
generally. There is direct evidence, indeed, to show 
that glaciers not only abrade and smooth, but rupture 
the rocks over which they flow. Professor Heim 
refers to an observation of Von Escher on the Zmutt 
Glacier, underneath which were seen projecting reefs 
of schist glaciated atop, which had been fractured and 
sundered by the glacier. Again, Professor Simony 
has described the appearance presented on the bed of 
one of the Dachstein Glaciers (Karls-Eisfeld) during 
the temporary retreat of the ice. What struck him 
most was not so much the smoothed and polished 
surfaces as the broken and disrupted masses, the shat- 
tering being most marked in places where the rock- 
ledges faced the direction of the ice-flow. The 
prevailing character of the erosion. Professor Simony 
remarks, is that of a continuous rock-shattering. On 
the north side of the glacier, where the surface had 
become depressed for 40 to 60 feet, the exposed rocks 
showed polishing in only a few places, glacial pressure 
having resulted rather in a wholesale superficial shat- 
tering, and in the production of a rubble of angular 
fragments. 

Similar phenomena have been observed by MM. 
Penck, Bruckner, and Baltzer at the Uebergossenen 



22 2 EARTH SCULPTURE 

Aim. During the past thirty years this glacier has 
retreated for two or three hundred yards. Its de- 
serted bed is traversed by a belt of hornblende- 
slate, which, like the adjacent rock-masses, is well 
glaciated and sprinkled with large striated blocks of 
gneiss. In some places, however, the hornblende- 
slate, after having been smoothed and polished, has 
been broken up, and debris, consisting of smaller and 
larger fragments and blocks, polished on one side only, 
are found incorporated in ground-moraine a little fur- 
ther down. This is a clear case of infraglacial quar- 
rying. Another good opportunity of studying the 
results of modern glacial action has been afforded by 
the retreat of the Lower Grindelwald Glacier. The 
lowering of its surface has exposed two rock-terraces. 
One of these is well glaciated, showing roches mou- 
tonndes with conspicuous Stoss and Lee-Seiten. Be- 
tween the mammillated rocks stretch several shallow 
rock-basins, some of them being filled with water. 
One of these, according to Professor Penck, measured 
26 feet in breadth, 42 feet in length, and 3-|- feet in 
depth, and was smooth and ice-worn from end to end. 
Both terraces are trenched by the deep gully of the 
Lutschine, the upper portions of the rocky walls being 
conspicuously striated and fluted, while here and there 
they present the shattered surfaces which are equally 
characteristic of glacial action. 

Professor Bruckner has in like manner described 
the broken and ruptured rocks and smoothed surfaces 
which appear side by side upon the bed of a glacier. 



GLACIAL ACTION 223 

Thus at the Mazellferner he saw resting upon the 
jagged projecting out-crops of certain rocks a block, 
many cubic metres in size, enclosed in ground-moraine, 
along with which it had travelled over the cracked 
and shattered rock-ledges. The ground-moraine was 
squeezed in between the disjointed masses. In 
another place, where the bed-rock was well smoothed 
and striated, he observed an irregular rough cavity or 
hollow, from which a slab of rock had evidently been 
extracted. In the recently deserted beds of the 
Obersalzbachkees (Hohe Tauern) and the Hornkees 
(Zillerthal) he noticed that the rocks were jointed in 
a direction approximately parallel to their upper sur- 
face — a structure which has favoured their rupture 
and displacement. Here and there, in the midst of a 
well smoothed area, rough cavities indicated whence 
slabs had been removed ; and now and again the de- 
tached fragments themselves were detected. Many 
such loose slabs were observed by the same geologist 
on the bed of the Stampflkees. On one side they ex- 
hibited the parallel striation characteristic of rock 
which has been glaciated in situ, while the other sides 
were rough and irregular, and showed no trace of 
abrasion. That fragments of this character are not 
more frequently extruded at the lower end of a glacier 
is readily understood when we remember that they 
could not travel far below ice without losing their 
rough surfaces, and becoming more or less glaciated 
all over. 

Professor Chamberlin has recorded the occurrence 



2 24 EARTH SCULPTURE 

of similar phenomena in connection with some of 
the large tongues of ice which are protruded from the 
great " inland ice " of Greenland. He says : " The 
rubbing of the glacier (Bowdoin Glacier) against 
the shoulders of rock projecting from the side of the 
valley gave opportunity for observing some of the 
special phenomena of such situations. At one point 
the process of ' plucking ' was well indicated (though 
not actually observed) on the lee-slope of a spur of 
gneissoid rock. Blocks ranging up to three or four 
feet in width and length, and one or two feet in thick- 
ness had been detached in considerable numbers. 
The process involved much breaking and bruising 
with relatively little wear. Corners and angles were 
broken off, and heavy bruise marks were observed 
both on the blocks and on the sides and edges of the 
cavities from which they had been removed. At 
some points considerable crushed rock was observed. 
On the other hand, systematic grooves and striae 
were not abundant nor pronounced. The dynamic 
impression given was that of a forceful tearing out of 
blocks by the action of a relatively rigid agency, 
which did not press the blocks hard upon the lee- 
slope after their removal." 

It is clear, then, that under existing glaciers and 
ice-fields rocks are sometimes smoothed and polished, 
sometimes crushed and shattered. The pressure of 
the ice tends to disrupt rock-masses, which yield or 
resist according to their character and structure, and 
fragments detkched must often serve as wedges to 



GLACIAL ACTION 225 

dislocate and detach others. Nor can it be doubted 
that the rocky bed of a glacier is also attacked by 
frost. The constant outflow of water shows that in- 
fraglacial melting goes on all the year round. The 
temperature at the bottom of the ice oscillates about 
the freezing-point, and as a glacier flows on its way 
thawing and freezing must be continually taking place. 
In this way joints are no doubt opened, rock-masses 
loosened, and larger and smaller fragments become 
more readily plucked and dragged out of place. 

We cannot, therefore, hesitate to conclude that ice 
in motion, whether in the form of glaciers or of ice- 
caps, is a powerful agent of erosion. It not only 
abrades and smooths, but breaks up and quarries the 
rocks over which it flows, and the debris thus obtained 
constitutes the true ground-moraine. 

2. Geological action of prehistoric glaciers. Ge- 
ologists rightly insist upon the potency of river-ero- 
sion. The study of modern denudation has quite 
convinced them that valleys can be and have been 
excavated by running water. In proof of this they 
point not only to the present action of rivers — to the 
rate of transport of sediment — but to the immense 
accumulations formed by river-action in prehistoric 
times. The broad alluvial plains of river-valleys, the 
great deltas which encroach upon the sea, the wide 
stretches of flat lands occupying the sites of silted-up 
lakes, are all cited as evidence of the potency of run- 
ning water as a producer and transporter of sediment. 
So in like manner the glacialist appeals to far-ex- 



2 26 EARTH SCULPTURE 

tended accumulations of ground-moraines as proof 
of the efficiency of flowing ice as an agent of erosion 
and transport. 

The study of modern glacial action is carried on 
under certain obvious disadvantages. The bed of a 
glacier is concealed from our view. Now and again 
we may get a peep under the ice ; or, better still,- we 
may have the opportunity of examining the ground 
from which a glacier has temporarily retired. But 
the portions of a glacier's bed thus at times exposed 
are not those where erosive action is most intense. 
A glacier thins away towards its extremity, and the 
rate of motion at the same time diminishes, so that 
pressure and erosion must decrease with the attenua- 
tion of the ice. To such an extent is this the case, 
that the snout of a glacier deploying upon a rela- 
tively flat surface often rests upon its terminal mo- 
raines, or even overrides the fluvio-glacial gravels 
spread out in front of it. Such facts have led some 
observers to conclude that glaciers do not erode at 
all, and did the facts referred to stand alone there 
would be some justification for that conclusion. It 
should be remembered, however, that were observers 
of river-action to confine attention to the broad plain- 
track — to the region known as the "base-level of 
erosion " — they would no doubt readily come to the 
conclusion that running water transports and deposits 
sediment ; but, by following the process of reasoning 
just alluded to, they might also infer that rivers are 
incapable of erosion. Were the beds of existing 



GLACIAL ACTION 227 

glaciers as open to investigation as the channels of 
rivers, we should probably hear little about the feeble 
erosive action of ice. But although we cannot make 
direct observations underneath the central and thicker 
portions of a glacier, we can yet examine great val- 
leys and broad lowland regions which have been 
formerly subjected to intense glaciation. And the 
evidence of effective glacial erosion there displayed 
is too clear to be wholly misunderstood. Let us then 
consider the general results which have been obtained 
by the careful investigation of certain well known 
glaciated regions — the Alpine lands of Central Europe. 
At the climax of the Glacial Period the snow-line 
in the Alps appears to have been upon an average 
some 4700 feet lower than now. Viewed from the 
north, the mountains must at that time have pre- 
sented the appearance of a great ice-field, broken 
here and there by Nunatakkr — the protruding peaks 
of the dominant elevations of the secondary ranges, 
and bounded on the south by the snow-clad ridges of 
the Central Chain. In a word, so thick was the ice 
in the valleys that as the glaciers made their way to 
the low grounds they frequently coalesced or became 
confluent across intervening mountain-ridges. Under 
such conditions it is obvious that the formation and 
accumulation of superficial moraines must have been 
relatively limited. The area buried under ndvd and 
ice was greatly in excess of that which remained 
uncovered. If it be true, therefore, that ground- 
moraines consist chiefly of xookrclibris derived from 



228 EARTH SCULPTURE 

superficial sources, those of the Glacial Period should 
be of little importance. The very reverse, however, 
is the case. The ground-moraines assume an enorm- 
ous development, their dimensions being in direct 
proportion to the size of the ice-flows. The larger the 
body of ice, the greater the mass of ground-moraine. 
It must be admitted, therefore, that the materials 
of the old ground-moraine cannot have been derived 
from superficial sources. Some have suggested, how- 
ever, that the accumulations in question consist to a 
large extent of the products of weathering, of torren- 
tial and fluviatile action, which had gathered over the 
mountain-slopes and in the valleys before the advent 
of the Glacial Period. There is no reason to believe, 
however, that rock-rubbish throughout the Alpine 
lands attained a greater development at the beginning 
of the Ice Age than it does now. The old snow-fields 
and glaciers doubtless gradually extended as the tem- 
perature fell. As the depression of the snow-line 
continued, rock-rubbish would accumulate abundantly, 
just as at present, in every valley occupied by a gla- 
cier. For a long time, too, superficial moraines would 
assume a relatively great importance, so that large 
terminal moraines would mark every pause in the 
progress of the ice-flows. But as the glaciers thick- 
ened in the valleys, and more and more bare rock dis- 
appeared below the ice, the supply of detritus from 
above would become gradually limited, until in many 
places, as in the region of the secondary ranges, it 
practically ceased altogether. Were a glacial period 



GLACIAL ACTION 229 

to supervene at present, each individual glacier would 
begin to advance, and as it progressed the zone of 
most active rock-shattering by frost would descend 
with it to lower and lower levels. But at each step 
in its advance the glacier would encounter no greater 
accumulations of rock-rubbish than had all along 
gathered in its neighbourhood. In short, as Dr. Bohm 
remarks, weathering would proceed no more rapidly 
in front of one of the enormous glaciers of the Ice 
Age than it does now in the vicinity of existing gla- 
ciers. "When the Inn Glacier," he says, "had ad- 
vanced as far as Innsbruck, it would enter no zone of 
more active rock-shattering than is met with to-day in 
front of the glaciers of the Oetzthal." It is obvious, 
therefore, that if the glaciers of the Ice Age derived 
their subglacial detritus either from above or from 
frost-riven debris and superficial deposits lying in their 
path, their ground-moraines could not at any one 
place have attained a greater thickness than those of 
existing Alpine glaciers ; and yet, as is well known, 
the old ground-moraines reach an astonishing thick- 
ness, their bulk being in direct proportion to the size 
of the former ice-flows. 

One may readily exaggerate the importance of the 
rock-rubbish which is almost everywhere conspicuous 
in the Alps. The enormous screes of angular blocks 
and ddbris which shoot down from cliff and buttress 
contain prodigious quantities of materials. Here, we 
are apt to think, is sufficient loose material where- 
with to form ground-moraines as thick and extensive 



2 30 EARTH SCULPTURE 

as those of the Glacial Period. But is this actually 
the case ? If all the ddbris in question could be lifted 
and equally distributed over the Alpine lands it would 
certainly not sufifice to raise the general surface of 
those lands by more than a few feet or yards. The 
old morainic accumulations, on the other hand, could 
they be replaced, would add considerably to the av- 
erage height of the surface. Professor Penck has 
shown, for example, that the morainic accumulations of 
the Isar Glacier average a thickness of 20 metres, and 
cover an area of some 1800 square kilometres. They 
have been derived from an area 2800 square kilometres 
in extent. Could they be restored, therefore, they 
would raise the general surface by about 13 metres. 
In other words, an area of 108 1 square miles has been 
lowered by some 41 feet. In Dr. Penck's estimate only 
the morainic matter has been considered, the equally 
great mass of fluvio-glacial gravels (consisting almost 
exclusively of remodified infraglacial detritus) has 
been entirely neglected. Further, we must remember 
that during the formation of the moraines and fluvio- 
glacial gravel, enormous quantities of the fine flour 
of rocks — the result of glacial grinding — must have 
been carried away in suspension, and deposited in 
regions far beyond the glaciated areas. 

Such considerations as these show that the old 
morainic accumulations cannot consist merely of the 
superficial rock-rubbish which the old glaciers found 
ready to hand, and swept out as they advanced. All 
such loose accumulations, after excessive glacial con- 



GLACIAL ACTION 231 

ditions had supervened, must erelong have become 
exhausted, and can form only a small proportion of 
the ancient ground-moraines. Whence, then, was the 
great bulk of the material derived ? Surely from 
infraglacial sources, as the direct result of glacial 
erosion. The immense ice-flows of the Glacial Period 
must at an early stage have completed the removal 
of preglacial detritus — none of that detritus can now 
linger underneath any existing glacier, either in the 
Alps or in Norway. Yet, as we have seen, ground- 
moraines are forming at present in both regions. In 
the Alps, according to Professor Heim and others, the 
ground-moraines are fed from the surface, but this 
can be true to only a very limited extent. The pla- 
teau ice-sheets of Norway carry no superficial detritus, 
and their ground-moraines are, therefore, supposed 
by some to represent the rock-rubbish which gathered 
over the Scandinavian heights in preglacial times ! 
A vast ice-sheet, as we know, overflowed those re- 
gions during the Glacial Period, and buried the low 
grounds to great depths under the detritus which 
it carried outwards from the mountains, and yet we 
are to believe that much loose rock-rubbish of pre- 
glacial age still remains to be removed from the con- 
tinuously ice-covered plateaux of Norway ! Must we 
likewise believe that the " inland ice " of Greenland, 
which has probably persisted since Pliocene times, 
has not yet succeeded in removing the products of sub- 
aerial weathering, which came into existence before 
glacial conditions had supervened in Arctic regions ? 



CHAPTER XI 

LAND-FORMS MODIFIED BY GLACIAL ACTION 
{Continued) 

FORMER GLACIAL CONDITIONS OF NORTHERN EUROPE EXTENT 

OF THE OLD INLAND ICE GENERAL CHARACTER OF BOULDER- 
CLAY CENTRAL REGION OF GLACIAL EROSION AND PERIPH- 
ERAL AREA OF GLACIAL ACCUMULATION FLUVIO-GLACIAL 

DEPOSITS— LOESS, ORIGIN OF ITS MATERIALS — GLACIATION OF 

NORTH AMERICA MODIFICATIONS OF SURFACE PRODUCED BY 

GLACIAL ACTION. 

IF a study of the glacial and fluvio-glacial deposits 
of the Alpine lands leaves us in no doubt as 
to the efficiency of glacial erosion, an investigation 
of the similar accumulations of Northern Europe 
and North America is even more convincing. The 
boulder-clays of those wide regions are true ground- 
moraines, recalling in every particular the ground- 
moraines of the Alpine lands. At the climax of the 
Glacial Period a great ice-sheet covered all Northern 
and North-western Europe, extending east from the 
British area to the Timan mountains, and south to 
the German ranges. The ice-sheet thus occupied an 
area of 2,500,000 square miles or thereabout in extent. 
Above the surface of this inland ice peered some of 

232 



GLACIAL ACTION 233 

the loftier mountain-tops of Scandinavia, and a few 
Nunatakkr in the British Islands. In the low grounds 
of Scotland the sheet could hardly have averaged less 
than 2500 to 3000 feet in thickness. In some of the 
Norwegian fiords it exceeded 5500 feet. Taking the 
elevation of the ice-shed in Scandinavia as 7000 feet, 
and the height reached by the ice-front upon the 
northern slopes of the mountains of Germany as 1350 
feet, we get a thickness for the ice-sheet in South 
Sweden of 2900 feet, of 2500 feet in Denmark, and 
of 1300 feet or thereabout in the neighbourhood of 
Berlin. 

It is obviously impossible that the ground-moraines 
of an ice-sheet of such dimensions could have been 
derived or even supplemented to any extent from 
superficial sources. The boulder-clays are the direct 
products of glacial erosion. They consist essentially 
of unweathered material. Boulders, smaller stones, 
grit, sand, and the finer-grained rock-meal or flour are 
all alike fresh ; they have not been altered chemically 
as they would have been had they come from super- 
ficial sources. They could not have been derived 
from above, and they cannot represent the weathered 
rook-dibris of preglacial times. 

The external configuration assumed by boulder- 
clay seems likewise to point to the infraglacial origin 
of the deposit. In relatively narrow mountain-valleys 
it forms broad terraces or platforms — now trenched 
and furrowed by streams and rivers. In broad low- 
land tracts, as in Tweeddale and Nithsdale, it is ar- 



234 EARTH SCULPTURE 

ranged in parallel banks, mounds, and ridges, the 
longer axes of which coincide with the trend of glaci- 
ation. Over wide plains, on the other hand, it rises 
and falls in long, gentle swellings. This varying con- 
figuration is undoubtedly original — it is not the result 
of subsequent subaerial erosion. In mountain-valleys 
the ice-flow, subject to no deflection, must have pro- 
ceeded continuously in one direction, and its ground- 
moraine, we may suppose, would thus tend to accrete 
more or less regularly. In the broader lowland tracts, 
however, as in the lower reaches of Nithsdale, Teviot- 
dale, and Tweeddale, the same uniformity of condi- 
tions did not exist. Each of these broad depressions 
was occupied by mers de glace, formed by the conflu- 
ence of ice-flows streaming out from various ice-sheds. 
Under such conditions the movement of the united 
currents could not be so equable, and in consequence 
of variations in the pressure of the ice, and in the 
lines of most rapid motion, the ground-moraine would 
tend to heap up in banks or ridges, the longer axes 
of which would necessarily coincide with the direc- 
tion of ice-flow.' 

' The "dnimlins'' and "drums "of Ireland and Scotland appear to be repre- 
sented in Sweden by certain banks of boulder-clay, which are described by De 
deer as a novel kind of radical moraines. He recognises their strong resem- 
blance to the drumlins of New England (Geol. Foren. Fork., 1895, p. 212). 
Drumlins occur in the Island of Rugen, but they would seem to be rare in North 
Germany. Recently Dr. K. Keilhack has observed them in Neumark(ya/«-^., 
d. konigl. preuss. geol. Landesanslalt fur 1893, 1895, p. 190). They have 
been recognised also in the low grounds of Switzerland by Dr. Friih {Jahres- 
bericht d. Si. Gallischen Naturwissensch. Ges., 1894-95). It is probable, how- 
ever, that the lenticular mounds and banks of till known under the name of 
drumlins have not all been formed in the same way. Thus the short lenticular 



GLACIAL ACTION 235 

Once more, over the peripheral areas of the inland 
ice, as in the great plains of Germany, the influence 
exerted by the confluence of ice-flows just referred to 
would no longer be felt, at least to the same extent. 
When the ice had fairly escaped from uplands and 
hilly ground all minor movements would merge in 
one continuous broad outflow, the ground-moraine, 
as a result, being spread out more or less uniformly. 

Looked at broadly. Northern Europe displays a 
central region of glacial erosion and a peripheral area 
of glacial accumulation. In the former, as in the 
Scandinavian peninsula, Finland, and the more ele- 
vated portions of the British Islands, bare rock is 
conspicuous over wide districts, while glacial accumu- 
lations, confined for the most part to hollows and 
depressions, attain as a rule no great thickness. Out- 
side of such areas of special erosion, on the other 
hand, as in the low grounds of England and the plains 
of Northern Europe, naked rock appears only at 
intervals, while morainic materials and fluvio-glacial 
deposits reach their greatest development. 

Under the ice-sheet rock-grinding and rock-shatter- 
ing were carried on side by side. No doubt the 
boulder-clays frequently rest upon a smoothed and 

drumlins of South Galloway appear to owe their origin to glacial erosion. 
They are the relics of the sheet of boulder-clay which accumulated under the 
last general mer de glace that overwhelmed Scotland. At » later stage the 
Southern Uplands supported local ice-sheets and large glaciers which, flowing 
out upon the adj.icent low grounds, ploughed into and greatly denuded the old 
boulder-clay. The drumlins of this region are, in short, simply roches 
motttonne'cs, composed sometimes entirely of boulder-clay, at other times partly 
of boulder-clay and partly of solid rock. 



236 EARTH SCULPTURE 

Striated surface, but just as frequently the ground- 
rock is shattered, crushed, and jumbled, and the 
debris mixed up with the overlying till. Such phe- 
nomena are not confined to any particular area. 
Examples of finely smoothed and of jumbled rock- 
surfaces may often be seen in one and the same 
quarry or other opening. The latter, however, are 
best developed in places where the ground-rock 
tended to yield most readily to the pressure of ice. 
Massive crystalline rocks are perhaps oftener smoothed 
than shattered below till ; but again and again their 
jointed structure has led to their ready disruption, 
boulder-clay has been squeezed into their crevices, 
and numerous blocks, some of large size, have been 
torn out and enclosed in the till. The result of this 
infraglacial disruption, however, is better seen in the 
case of bedded rocks, especially when the dip of the 
strata has happened to coincide with the direction of 
ice-flow. In such cases the boulder-clay has often 
been forced in between the bedding-planes, and broad 
ledges and reefs of rock have been wedged up and 
forced out of place. Not only so, but in the case of 
chalk and certain Tertiary formations, the pressure 
of the ice-sheet has not infrequently squeezed the 
rocks into folds and flexures of such a character that 
the disturbance and contortion have sometimes been 
attributed to subterranean action. Superficial curv- 
ing, flexing, and displacement of the kind referred to 
are met with both in high and low-lying regions ; but 
as the more yielding strata are best developed within 



GLACIAL ACTION 237 

the latter, it is there that we meet with the most 
striking evidence of infraglacial disruption and quar- 
rying. 

From the various facts above referred to we are 
justified in concluding that glacier-ice is a most effect- 
ive agent of erosion. It not only abrades, rubs, 
smooths, and polishes, but crushes, folds, disrupts, 
and displaces rock-masses, the amount of disturbance 
being in proportion to the resisting power of the 
rocks and the pressure exerted by the ice. Other 
things being equal, more crushing and displacement 
will be effected under a massive ice-sheet than under 
a small valley-glacier. It is obvious, therefore, that 
during the prolonged existence of an ice-sheet, trans- 
port and accumulation must result in very consider- 
able modifications of the surface. The central area 
of dispersion becomes gradually lowered by the ab- 
straction o{ xo<:k.-debris which is carried forward and 
accumulated over the peripheral area occupied by the 
mer de glace. Hence it is that in the former region 
ground-moraines are seldom very thick, and usually 
consist of local materials. As they are followed out- 
wards, however, they gradually attain a greater depth, 
and are more widely spread, the local materials 
becoming more and more mixed with far-travelled 
detritus, until eventually the latter begins to predom- 
inate. The depth attained by the ground-moraines 
in the plains of Europe is often great, individual 
sheets of boulder-clay often exceeding one hundred 
feet in thickness. 



238 EARTH SCULPTURE 

Such boulder-clays, however, are not the only evid- 
ence of glacial erosion. With them are frequently 
associated beds of gravel and sand and laminated clay, 
consisting exclusively of erratic materials. These 
are admittedly the products of infraglacial water- 
action ; the materials have been derived principally, 
if not exclusively, from the washing and sifting of 
infra- and intra-glacial detritus. Extensive beds of 
such aqueous accumulations underlie the ground- 
moraines in some places, and in other places separate 
one mass of ground-moraine from another. Great 
mounds, banks, and sheets of the same character, 
which obviously are similar in origin to the fluvio- 
glacial detritus of the Alpine Vorldnder, fringe the 
margins of the ground-moraines, and sweep over 
wide areas in North Germany and Russia. All these, 
therefore, must be taken account of if we would form 
an adequate conception of the amount of erosion 
effected by the mers de glace of the Ice Age. 

The diluvial deposits of North Germany necessarily 
vary in thickness. Sometimes they are only a few 
feet, at other times they exceed 200 yards. Dr. 
Wahnschaffe has collected the results of numerous 
borings made in those regions, from which we learn 
that in East Prussia they range in thickness from 20 
feet up to 490 feet, in West Prussia from 20 feet to 
360 feet, in Posen from 35 feet to 240 feet, in Bran- 
denburg from 30 feet to 670 feet, in Mecklenburg 
from 6 feet to 430 feet ; in the province of Saxony a 
depth of 400 feet has been noted. Mr. Amund Hel- 



GLACIAL ACTION 239 

land, after conferring with geologists to whom the 
diluvial accumulations of the great plains are familiar, 
comes to the conclusion that the deposits probably 
attain an average thickness of 150 feet. The ma- 
terials being partly of local and partly of foreign ori- 
gin, he deducts the former (estimated at 50 feet), 
and thus obtains a thickness of 100 feet for the 
detritus derived from Sweden and Finland, and spread 
over the low grounds of North Germany, etc. Ac- 
cording to this geologist, the glaciated areas of 
Sweden and Finland, which supplied the detritus, are 
some 800,000 square kilometres in extent (497,120 
square miles), while the area in Russia and North 
Germany over which Swedish and Finnish erratic 
materials are spread is estimated at 2,040,000 square 
kilometres (1,267,656 square miles). Were those 
materials therefore transferred to the lands from which 
they have been derived, they would raise the general 
surface by 255 feet. This estimate, it need hardly be 
said, is a mere rough approximation, and is probably ex- 
cessive. But even if it be supposed that Helland has 
exaggerated both the amount of foreign erratic ma- 
terials and the extent of the area over which it is dis- 
tributed, we shall still be compelled to admit that the 
surface of Scandinavia must have been greatly modi- 
fied by glacial erosion. If we deduct two-thirds from 
Helland's result we have still left sufficient material 
to raise the general surface of Finland and Sweden 
by 85 feet. 

In the following chapter reference is made to the 



240 EARTH SCULPTURE 

loss as being primarily a flood-loam of glacial times. 
Much of that occurring in the river-valleys of Central 
Europe has, no doubt, been derived from the Alpine 
lands ; but the vast accumulations of loss in Southern 
and South-eastern Russia doubtless owe their origin 
chiefly to the flood-waters escaping from the margins 
of the old " inland ice." All these deposits, as we shall 
see, have been more or less rearranged and modified 
by subaerial action, but the materials themselves 
would seem to have resulted, in largest measure at 
least, from the washing and weathering of glacial ac- 
cumulations. In short, they are additional evidence 
of the effective erosive action of flowing ice. 

The researches of geologists in North America are 
on all fours with those carried on in Europe. They 
tell precisely the same tale. The American boulder- 
clays, fluvio-glacial gravels, and loss present us with 
similar phenomena. As in Europe so in North 
America, broken and ruptured rocks are of common 
occurrence under the overlying ground-moraines. 
The ice-sheet, as Dana remarks, " carried debris for 
the most part, not from the slopes and summits of 
emerged ridges, but from those underneath it. . . . 
It obtained its load by abrading, ploughing, crushing, 
and tearing from those underlying slopes and sum- 
mits. . . . The ice-mass was a coarse tool ; but 
through the facility with which it broke and adapted 
itself to uneven surfaces, it was well fitted for all 
kinds of shoving, tearing, and abrading work. More- 
over it was a tool urged on by enormous pressure. 



GLACIAL ACTION 241 

A thickness of 1000 feet corresponds to at least 50,000 
pounds to the square foot. The ice that was forced 
into the openings and crevices in the rocks had 
thereby enormous power in breaking down ledges, 
prizing off boulders, and in abrading and corroding." 

3. Modifications of the surface produced by glacial 
action. Having now learned that glacier-ice is a most 
effective eroding agent, we have next to consider the 
modifications of the land-surface brought about by 
glacial action. Looked at broadly, as we have seen, 
each glaciated region shows a central area of erosion 
and a peripheral area of accumulation. Not that 
erosion and accumulation are confined in this way 
each to a separate tract, but simply that in the central 
area erosion is in excess of accumulation, while in the 
surrounding region the reverse is the case. It will 
conduce to clearness, therefore, if we consider first the 
characteristic features which are the direct result of 
glacial erosion. Thereafter we shall glance at the 
aspect presented by a land more or less covered with 
glacial and fluvio-glacial detritus. 

Unquestionably the most notable features of a 
well glaciated country is its rounded and flowing 
configuration, a configuration which is always most 
striking when viewed in the direction of glaciation. 
Tors, peaks, buttresses, and ridges have been smoothed 
down, escarpments bevelled off, and asperities in 
general softened. This is the direct result of glacial 
abrasion, but accumulation also has helped in the 
production of a flowing contour, for many of the 

i6 



242 EARTH SCULPTURE 

dimples and smooth depressions upon hill-tops and 
hill-slopes are more or less due to glacial deposition. 
While projecting rock-masses have been abraded and 
removed, irregular hollows, gullies, ravines, and other 
rough depressions have often been partially or com- 
pletely obliterated by the deposition in them of 
morainic materials, abrasion and accumulation to- 
gether having thus resulted in the production of a 
more or less undulating surface. In the phenomena 
of " crag and tail " we see another effect of the same 
twofold action. Isolated stacks and bastions of rock, 
which faced the direction of ice-flow, have been 
rounded and bevelled-off, and frequently a hollow 
dug out in front, while morainic ddbris has been 
heaped up behind to form the so-called " tail " of the 
hill. There are endless modifications of this structure. 
Thus in many hilly tracts which have been completely 
overwhelmed by an ice-flow we may often trace series 
of parallel ridges and intervening hollows of various 
width, height, and depth, which obviously extend in 
the direction of former glaciation. These are the 
result partly of erosion and partly of accumulation. 
The hollows show where the rock has most readily 
yielded to glacial erosion, while the ridges consist of 
irregular-shaped masses and ledges of more durable 
rocks, and of morainic material which has gathered 
in their rear. Into these and other details of glacial 
action, however, it is not necessary to go. For our 
purpose it is enough to recognise the general fact 
that glaciation tends to obscure and obliterate the 



GLACIAL ACTION 243 

features which result from the action of the ordinary- 
agents of erosion and denudation. Hence all well- 
glaciated areas show a somewhat monotonous outline 
— round-backed rocks, smoothed and undulating hill- 
slopes and hill-tops, — in a word, undulating contours 
are everywhere conspicuous. 

The effect produced by glacial action is perhaps 
most strikingly displayed in regions the more elevated 
portions of which have risen above the surface of the 
ice, and so escaped abrasion. In the great valleys of 
the Alps, for example, how strongly contrasted are 
the glaciated and non-glaciated areas ! In the Upper 
Engadine the valley slopes up to a height of 2000 
feet or thereabout are conspicuously abraded, while 
above that level all is harsh and rugged. It is the 
same in our own islands, as, for example, in the Outer 
Hebrides, where the whole area is smoothed and 
rounded up to a height of 1500 or 1600 feet, above 
which level the rocks present quite a different aspect. 

But glacier-ice does not only abrade and bevel-off 
prominent rock-ledges, peaks, tors, bastions, and but- 
tresses, it also excavates hollows, which may vary in 
extent from a few feet or yards in depth and width to 
great depressions measuring many fathoms deep and 
not a few miles in extent. Here, however, we come 
upon the vexed question of the origin of rock-basins, 
the consideration of which may be conveniently de- 
ferred for the present. 

The transfer of detritus from the area of dominant 
glacial erosion, and its distribution over the peripheral 



244 EARTH SCULPTURE 

area of dominant accumulation, has very considerably 
modified the aspect of the land. Could we remove 
all glacial deposits from our own broad lowland val- 
leys, it Is certain that the sea would In many places 
penetrate far inland. On the continent the Baltic 
would overflow wide tracts in the plains of Northern 
Germany, for the bottom of the deposits of that re- 
gion descends frequently below the level of the sea. 
And similar changes would be brought about were the 
glacial accumulations of North America to disappear 
— the sea would encroach upon the land. Very con- 
siderable modifications were likewise effected in the 
drainage-systems of extensive regions. In Europe 
and North America alike, the irregular deposition and 
distribution of glacial and fluvlo-glaclal accumulations 
have often led to remarkable changes in the directions 
followed by the streams and rivers, which reappeared 
as the great mers de glace melted away. Throughout 
the peripheral areas of dominant deposition preglacial 
courses and channels were largely filled up with de- 
tritus, and not Infrequently had become In this way 
obliterated, so that the streams and rivers of post- 
glacial times were often deflected and compelled to 
erode new channels. 

It is not with such changes, however, that we are 
at present concerned, but rather with the various 
forms assumed by glacial accumulations. Ground- 
moraines, as we have already seen, present certain 
typical configurations. And the same is true of lat- 
eral and terminal moraines, and of fluvlo-glacial de- 



GLACIAL ACTION 245 

posits. In areas of dominant glacial accumulation, 
as in Schleswig-Holstein and North Germany, the 
ground-moraines often occupy the surface over exten- 
sive regions, and form wide plains with a softly undu- 
lating surface. The ground rises and falls gently in 
long, broad swellings and depressions, which do not 
seem to follow any particular direction. In other re- 
gions, as in the Lothians and elsewhere in our own 
lowlands, the undulations of the boulder-clay not in- 
frequently show a rudely parallel arrangement. Ever 
and anon, however, all traces of definite orientation 
disappear, and the ground then simply rises and falls 
irregularly as in the plains of North Germany. But 
in some of the broader dales of Scotland the config- 
uration of the boulder-clay becomes strongly defined, 
the accumulation being arranged in a well marked 
series of long parallel banks known as " drums " or 
" sowbacks." Elsewhere, again, as in Galloway and 
in many parts of Ireland, the ground-moraines often 
assume the form of short or more or less abrupt len- 
ticular hills, or "drumlins," as they are termed. 

Another set of characteristic glacial land-forms are 
the eskers, or osar. These are somewhat abrupt 
banks and ridges of gravel and sand, which are be- 
lieved to have been formed in tunnels underneath the 
great mers de glace. They are well seen in certain 
tracts of our own islands, but reach their greatest de- 
velopment in Sweden, where they traverse the land as 
great embankments, rising to a height of 50 or 100 
feet above the general level, and following a sinuous 



246 EAR TH SCULP TURE 

or river-like course for distances of sometimes 150 
miles or more. 

Other hillocks and hills of glacial origin are lateral 
and terminal moraines. The former are practically 
confined to mountain-valleys, while the latter are met 
with, not only in mountain-valleys, but in lowlands 
often far removed from any elevated region. In 
mountain-valleys such moraines consist chiefly of an- 
gular ro<:k-ddbris, but in low grounds opposite the 
mouths of mountain-valleys they are usually com- 
posed more largely of ground-moraine, together with 
gravel and sand and a certain admixture of angular 
debris and blocks, sometimes the one and sometimes 
the other kind of material predominating. In Eu- 
rope, the most remarkable terminal moraines are 
those which denote the limits reached by the glaciers 
and ice-sheets of the Glacial Period. They are strongly 
developed in the Vorldnder of the Alps, in Southern 
Scandinavia, Schleswig-Holstein, North Germany, 
and Finland ; and on a smaller scale they abound in 
our own islands. Looked at broadly, such moraines 
occur as more or less abrupt mounds and crescent- 
like or undulating ridges. Opposite the mouths of 
important mountain-valleys they are often disposed 
in concentric series, one or more dominant lines of 
banks and ridges with many subordinate hummocks, 
heaps, and irregular low mounds lying behind and 
between them. Not infrequently they present a most 
tumultuous appearance — cones, mounds, banks, and 
ridges confusedly heaped together, and thus enclosing 



GLACIAL ACTION 247 

multitudinous hollows and depressions of all shapes 
and sizes, many of which contain lakes or pools, while 
others are occupied by bogs or simply clothed with 
grass and herbage. The hillocks and ridges vary 
much in height and size, among the most conspicuous 
being those of Piedmont and Lombardy, where they 
occasionally attain the exceptional elevation of more 
than a thousand feet above the adjacent low grounds. 
More usually in the Alpine Vorldnder they do not ex- 
ceed two or three hundred feet. The terminal mor- 
aines of the great Baltic Glacier in Finland, North 
Germany, Denmark, and Southern Sweden present 
much the same appearance as those of the Alpine 
Vorldnder. The most conspicuous are those which 
mark the extreme limits reached by that great ice- 
stream. These, rise more or less abruptly above the 
level of the broad plains of gravel, sand, and boulder- 
clay which sweep outwards from their base into the 
low ground of North Germany and Poland. The land 
lying between those external ridges and the shores of 
the Baltic forms a typical paysage moramique — wide 
plains traversed now and again by winding irregular 
ridges of gravel and sand, and more or less abundantly 
sprinkled with mounds and banks of similar materials. 
Here and there these hillocks crowd more closely to- 
gether, giving rise to a tumultuously undulating sur- 
face ; while in other places they are drawn out in 
curving lines and belts, or bands. Throughout the 
whole area shallow lakes and lakelets, bogs, and mo- 
rasses are abundantly developed. The surface of the 



248 EARTH SCULPTURE 

flat lands lying within this great morainic tract is 
usually formed superficially of fluvio-glacial deposits, 
and the same is the case generally with the low grounds 
immediately outside of the paysage morainique. 

To sum up the general results of glacial action, we 
may say that this action is entirely mechanical. Un- 
der the influence of ordinary weathering each par- 
ticular kind of rock tends to assume a more or less 
characteristic outline. With glaciation, however, this 
is not the case. All rocks subjected to glacial action 
become abraded after one and the same fashion. 
The tendency of that action is to reduce asperities, to 
smooth and flatten the surface. But glacial action has 
usually been arrested long before its work has been 
completed. It is only here and there that projecting 
rocks have been ground away and reduced to a plain 
surface. In most cases they are simply rounded off, 
and so rocky hill-slopes tend to assume mammiform 
outlines. Some rocks are, of course, more readily re- 
duced than others ; but whether the rocks be hard or 
soft, they all acquire the same undulating configura- 
tion. In regions of dominant glacial erosion the 
rounded and undulating surface is often in part due 
to glacial accumulation, the abrupt depressions of the 
ground being not infrequently filled up and replaced 
by smoothly outlined hollows. 

Where the region of glacial erosion merges into 
that of glacial deposition, it is often hard to say 
whether morainic matter or solid rock enters more 
largely into the formation of the banks and hillocks 



GLACIAL ACTION 249 

that extend outwards from the base of the mountain- 
area. Eventually, however, we pass into the region 
of dominant accumulation — the region of ground- 
moraines and eskers, of terminal moraines, lakes, and 
fluvio-glacial plains. 



CHAPTER XII 

LAND-FORMS MODIFIED BY y£OLIAN ACTION' 

INSOLATION AND DEFLATION IN THE SAHARA FORMS ASSUMED 

BY GRANITOID ROCKS AND HORIZONTAL AND INCLINED 
STRATA REDUCTION OF LAND-SURFACE TO A PLAIN FORM- 
ATION OF BASINS DUNES OF THE DESERT SAND-HILLS OF 

OTHER REGIONS — TRANSPORT AND ACCUMULATION OF DUST 

LOESS, A DUST DEPOSIT LAKES AND MARSHES OF THE 

STEPPES. 

AT the outset of our inquiry into the origin of sur- 
face features, we briefly considered the general 
nature of the work done by the principal epigene 
agents. We saw that these agents are often so 
closely associated in their operations that their in- 
dividual share in the final result can hardly be de- 
termined. In our country, for example, erosion is 
effected by the combined action of the atmosphere, 
of frost, and of rain and running water. There are 
many regions, however, in which one or other of 
these agents is by far the more conspicuous worker. 
Thus, at lofty elevations in temperate regions, and 
throughout the higher latitudes, the most potent 
causes of rock-disintegration and removal are frost, 
snow, and ice. In warm-temperate, subtropical, and 

250 



^OLIAN ACTION 251 

tropical lands, on the other hand, it is usually the 
chemical and mechanical action of the rain and run- 
ning water which impresses the observer, while in 
rainless and desiccated regions insolation and defla- 
tion play the most important rdle. It is in the latter, 
therefore, that the erosive action of wind is best 
displayed. Not that this action is confined to such 
areas, for it may be observed almost everywhere, and 
more particularly in mountain-regions. Outside of 
deserts, however, the wind acts chiefly as a transporter 
of rock-material. In all latitudes incoherent deposits 
of sand, exposed and dried, come under its power, 
and tend to be piled up in heaps and ridges or spread 
out in sheets. In this way certain more or less 
prominent land-features owe their origin directly to 
wind ; and as we have devoted some space to the 
consideration of the action of ice as a special agent 
of erosion and accumulation, we shall now take a 
rapid glance at the more notable surface-features that 
result from the destructive and reproductive action of 
the atmosphere. 

In desiccated regions rock-disintegration and the 
transport and accumulation of superficial materials 
are mainly the work of insolation and deflation — rain 
and running water necessarily play a very subordinate 
part. This is certainly the case in the Sahara — the 
most extensive tract of desert in the Old World. 
This vast region stretches across Africa from the At- 
lantic coast to the valley of the Nile, and from the 
northern borders of the Soudan to the Atlas Mount- 



252 EARTH SCULPTURE 

ains and the Mediterranean, an area equal in size to 
two-thirds of Europe. The surface of the Sahara is 
sufficiently diversified, and is not, as popularly sup- 
posed, entirely covered with blowing sand. Dunes, 
no doubt, spread over enormous territories, but wide 
tracts and broad basins of loam and clay, with saline 
lakes and marshes, likewise present themselves, whilst 
elsewhere rocky and stony plateaux, and even lofty 
mountains, occupy extensive areas. Ever and anon, 
moreover, verdant oases appear, and these are so 
numerous that they must altogether form no incon- 
siderable portion of the whole Sahara. The entire 
area might be described as an old plateau of accumu- 
lation, built up, as it appears to be for the most part, 
of horizontally or gently inclined strata. Probably its 
mean altitude is not less than 2000 feet, only a small 
portion lying to the south of Algeria being below the 
level of the Mediterranean. The rocky areas of the 
region are broken up into a succession of narrower 
and broader terraces or plateaux — now in many places 
traversed by dry, winding gullies, ravines, valleys, and 
other abandoned watercourses, or largely replaced by 
groups of bare pyramidal hills, buttes, mesas, and ir- 
regular rock-masses. Over wide areas blowing sands 
are absent, while elsewhere they are heaped up and 
spread out to such an extent that the rocky frame- 
work of the country becomes entirely concealed. 

Wind erosion is naturally best studied in the bare 
portions of the desert. Under the influence of inso- 
lation the rocks crumble down, and the disintegrated 



^OLIAN ACTION 253 

material is swept onward by the wind. Hard, com- 
pact stones acquire a polish like that given by a lapid- 
ary's wheel, while rocks of unequal consistency yield 
irregularly, the softer portions being removed and 
the harder parts left standing in relief. Where the 
surface of the land is very uneven the air-currents 
streaming between opposing heights have ground out 
deep hollows and gullies. In like manner curious 
niches, cirques, and amphitheatres have been excava- 
ted in the walls of the dry wadies. Everywhere, in- 
deed, the rocks are abraded, fretted, honeycombed, 
and undermined. Undermining is, in truth, one of 
the most notable stages in the general reduction of 
the surface. The bulk of the sand driven forward 
by the wind rises only a few feet above the surface, 
hence cliffs and stacks wear away rapidly below until 
the overhanging mass collapses and topples down, 
whereupon the same action is repeated upon the fallen 
ddbris. Hence isolated rock-masses often take peculiar 
mushroom-shapes. 

Among the most fantastic forms assumed under the 
action of the wind are those met with among granites 
and granitoid rocks. Often rising boldly above the 
general level, they show no trace of talus or ddbris, 
but are swept bare to the base, and to the fanciful 
Arab they often simulate the appearance of ele- 
phants, apes, camels, panthers, and the like. In 
Europe granite hills and mountains frequently show 
rounded summits, and are usually well mantled with 
talus. In the desert, on the other hand, they are 



254 



EARTH SCULPTURE 



much more rugged and abrupt, their precipitous 
flanks bare of ddbris, and their serrated crests and 
peaks recalHng, according to Walther, the bold and 
abrupt dolomite mountains of South Tyrol. The 
horizontally arranged strata of the desert assume very 




Fig. 79. Wind Erosion: Table Mou.vtains, etc., of the Sahara 
(Mission de Chadames). 

different forms, and have been carved into tabular, 
conical, and pyramidal hills, with a general resem- 
blance to the buttes, mesas, and pyramids of the 
Colorado region. (Fig. 79.) When the strata are 




Fig. 80. Wind Erosion : Harder Beds amongst Inclined Cretaceous 
Strata. Libyan Desert. (J. Walther.) 

inclined the outcrops of the harder beds project, and 
we have in like manner a reproduction of the escarp- 
ments and dip-slopes which the same geological struct- 
ure gives rise to in well watered lands. (Fig. 80.) 



^OLIAN ACTION 



25s 



The projecting ledges and escarpments, however, are 
always honeycombed and dressed in a different way, 
betokening everywhere the characteristic action of 
the wind. 

The final result of wind erosion is the reduction of 
inequalities and the production of a plain-like surface. 




E 





IV 




Fig. 8i. Wind Erosion: Stages in the Erosion and Reduction of a 
Table-Mountain. (J. Walther.) 

In the Eastern Sahara wide areas of rocky land have 
been thus levelled. (Fig. 81.) Such areas are usually 
more or less abundantly besprinkled and paved with 
angular stones, usually dark brown or black, and so 
highly polished that they glance and glitter in the sun. 



2s6 



EARTH SCULPTURE 



It is obvious that such stones are derivative ; they are 
the relics of massive beds of sandstone, through which 
they were formerly distributed, and which have since 
been gradually disintegrated and removed. In some 
places, indeed, massive inclusions of the kind (man- 
ganese concretions), of all shapes and sizes, project 
from the surface of the sandstone in which they are 
still partly embedded. On the lee side of such con- 
cretions the sandstone has been sheltered from the 
attack of the wind, while it has been planed away in 




f*¥^^^ 



Fig. 82. Manganese Concretions weathered out of Sandstone ; 
Arabah Mountains, Sinai Peninsula. (J. Walther.) 

front. No stone withstands the action of the wind 
so well as the hard flints, jaspers, and silicious con- 
cretions, which are so commonly met with in the sedi- 
mentary strata of the Libyan desert. When the latter 
have become disintegrated and gradually removed by 
the wind, the hard nodules and concretions remain, 
and thus the broad plains are covered over with sheets 
of "gravel" and shingle. The Sserlr, according to 
Walther, are nothing more than rocky lands levelled 
by wind-erosion ; the more yielding materials have 



^OLIAN ACTION 257 

been swept away, while the hard inclusions left behind 
are now concentrated at the surface. 

Another result of deflation may be referred to. 
Now and again in wind-swept plains and plateaux 
the rocks, according to their nature, are variously 
affected. Some are disintegrated and rotted more 
readily than others. These, therefore, tend to be 
more rapidly reduced below the general level, and 
shallow basins are thus formed which are sometimes 
occupied by water for shorter or longer intervals. 
Such is probably the origin of the Caldeiraos of 
Bahia, where the general configuration of the surface 
has some resemblance to that of an ice-worn region — 
gently rolling ground, namely, showing innumerable 
shallow depressions winding amongst multitudinous 
bare-backed, dome-shaped rocks. 

The disintegrated material removed from a rocky 
desert is eventually spread out and piled up in sheets 
and heaps of sand, which travel onwards in the direc- 
tion of the prevalent wind. In the Eastern Sahara 
bare rocky plateaux prevail, and sand-wastes are 
usually of inconsiderable extent. In the Western 
Sahara, on the other hand, the whole area is more 
or less smothered in sand. There vast stretches of 
dunes move with the trade-winds. Advancing to 
the south-west, they reach the banks of the Niger 
and the Senegal, and are here and there forcing these 
rivers southward. Again, passing to the west, they 
touch the Atlantic coast between Cape Bojador and 
Cape Blanco, and stream out to sea so as to form 



258 EARTH SCULPTURE 

a belt of sand-banks extending several miles from 
the shore. For long ages, therefore, a great current 
of sand has been constantly flowing out of the desert. 

The dunes of a desert appear to move more readily 
than those of maritime regions. Possibly this may 
be due to the better rolled character of the constitu- 
ent grains, to the drier condition of the sand, to the 
want of any binding materials, and the absence of 
a fixed nucleus, such as is so commonly acquired for 
the formation of coastal dunes. In the central por- 
tions of a desert they are generally arranged in series 
of long parallel undulations, that extend in a direc- 
tion at right angles to that of the prevalent wind. 
Elsewhere they may be more irregular in their 
grouping and arrangement, individual sand hills not 
infrequently assuming a crescentic or sickle-like 
shape. They vary much in height, not often exceed- 
ing 250 feet, although occasionally reaching 500 or 
even 600 feet. 

It need hardly be said that dunes are not restricted 
to desert regions. Wherever incoherent deposits 
are dried and exposed to the air, they are liable to 
drift with the wind. Hence blowing sands are well 
developed upon certain sea-coasts and lake shores, 
and in the broad, flat valleys of many large rivers. 
If the surface over which sand is blown be level and 
free from obstructions, the sand does not necessarily 
accumulate in heaps and banks, but is often spread 
out in successive horizontal layers, forming a sand- 
plain. But wherever obstructions intervene, such as 



^OLIAN ACTION 



259 



prominent rocks, trees, bushes, or what not, these 
give rise to inequalities in the distribution of the 
sand. A steep talus of grains gathers in the sheltered 
lee, while a more gently sloping bank gradually rises 
on the windward side of the obstruction, until this is 




Fig. 83. Formation of Sand-dunes. 

o, obstacle intercepting sand ; w, windward side; /, leeside; fj, sea-level. 

eventually overtopped and buried. (See Fig. 83.) In 
this way a dune is formed, and continues to increase 
until it reaches its maximum height, determined by 
the strength of the wind and the supply of the 
materials, and probably in some measure also by the 



Cfiu.ci- i«<»Jt»-fr*^ 



., au. 



> /riy 



JkS w* *U M*K'> 

Fig. 84. Advance of Sand-Dunes, 

Illustrated by the burial of a church, and its subsequent reappearance, in the neighbourhood 
of the Kurisches Hafl. (G. Berendt.) 

size of the sand-grains. As the sand continually 
travels up the gentler windward slope, and comes 
to rest on the steeper leeward slope, it follows that 
a dune itself must constantly, if slowly, move for- 
wards. Thus in time the nucleus that gave origin 



2 6o EARTH SCULPTURE 

to such a sand-hill may become again exposed. (See 
Fig. 84, p. 259.) 

Coastal sand-hills, like those of inland regions, are 
frequently arranged in successive parallel ridges or 
undulations. These, however, are often interrupted 
by transverse hollows, and the dunes frequently run 
into one another irregularly. I n other places little or no 
parallel arrangement can be traced, the hills and hum- 
mocks showing a tumultuous and tumbled surface of 
winding and straggling ridges, of isolated banks and 
knolls, and confused groups of mounds and hillocks, 
the hollows amongst which form a perfect labyrinth. 
Should grasses or other vegetation clothe the dunes, 
these become fixed, but in the absence of any plant- 
growth the surface of the sand-hills is kept in constant 
motion by the wind. 

In the hollows amongst sand-dunes marshes, pools, 
and lakes now and again appear. In some parts of 
the Sahara, for example, long straggling basins of 
groundwater extend between the sand-ridges. Again, 
the advance of sand-dunes from a coast has often ob- 
structed the natural drainage, and formed swamps 
and lakes of larger or smaller extent. The lagoons, 
which in many places are separated from the sea, 
have frequently been cut off from the outside ocean 
by the combined action of the waves and the wind in 
raising up sand-banks and -dunes. 

In desert regions the bulk of the sand driven for- 
ward by the wind rises to no great height above the 
surface ; its abrading and scouring action is largely 



yEOLIAN ACTION 261 

confined to the basal portions of the rocks against 
which it is borne. But the finer-grained matter — the 
powdery dust — is often swept upwards to great 
heights, and may be transported for hundreds or 
even thousands of miles from the place of its origin. 
As might have been expected, however, it is over the 
region immediately surrounding a desiccated area 
that the dust chiefly falls. In time such regions be- 
come more or less thickly mantled with this dust, 
which usually yields a fertile soil. After long ages of 
accumulation the whole surface of the dust-covered 
tracts becomes greatly modified. Inequalities are 
smoothed over, and everywhere softly flowing feat- 
ures are produced. As no hard-and-fast line separates 
an area of wind-erosion from one of dust-accumula- 
tion, sand and dust become commingled along the 
borders of the two regions, or there is a gradual transi- 
tion from the one kind of material to the other. The 
fertility of the Nile Valley is rightly attributed to the 
fine silt and loam of the annual floods, but desert- 
dust has also added its share to the soil of Egypt. 
Similarly it is believed that dust has played an im- 
portant part in the formation of the fine porous soils 
of many other lands. According to Baron Richtho- 
fen, the vast loss accumulations of China are true 
dust-deposits. Loss is a fine-grained, homogeneous 
calcareous and sandy loam, penetrated vertically by 
numerous root-like pores and tubes, which have the 
same effect on the deposits as joints in rocks — they 
allow the loss to cleave in a vertical direction. When 



262 EARTH SCULPTURE 

it is intersected, therefore, by streams and rivers it 
forms bold bluffs and cliffs. It usually contains land- 
shells, and now and again the bones of land animals. 
Fresh-water shells rarely occur, while marine organ- 
isms are wholly wanting. In Northern China this 
remarkable accumulation covers vast areas, and at- 
tains in places a thickness of 1500 feet or even of 
2000 feet. The regions occupied by it have the 
aspect of extensive plains, which look as if they 
might be traversed with ease in any direction. They 
are abundantly intersected, however, by deep valleys 
and precipitous rock-like gullies and ravines, in the 
vertical walls of which the natives have excavated 
their dwelling-places. Richthofen believes that this 
great deposit has been gradually accumulated by the 
winds flowing outwards from the desiccated regions 
of Central Asia. Vast quantities of fine sand and 
dust are there swept up during storms and scattered 
far and wide, and in this manner adjoining territories, 
such as the grassy steppes, are ever and anon receiv- 
ing increments to their soil. The finely sifted mate- 
rial thus obtained is highly fertile and favours the 
growth of the grasses, so that every fresh deposit of 
dust tends to become fixed, and the steppe-formation 
continues to increase in thickness. It is this con- 
tinued growth of vegetation, keeping pace, as it were, 
with the periodical accumulation of soil, which is sup- 
posed to produce the porous capillary structure re- 
ferred to above as the cause of the vertical cleavage 
of the loss. 



^OLIAN ACTION 263 

Loss occurs in many other countries, but it no- 
where attains so vast a development as in China. In 
Europe we meet with it in the valley of the Rhine 
and in the low grounds traversed by the Danube, 
where, although it forms no enormous plains like 
those of Northern China, it nevertheless mantles the 
ground so as in some measure to conceal the older 
features of the land. The extensive sheets of black 
earth which cover the surface of the great plains of 
Southern Russia are also a variety of loss. The 
origin of the European deposits has been much dis- 
cussed by geologists, but it seems to be now the gen- 
eral opinion that the materials of the loss were, in the 
first place, introduced into the low grounds chiefly by 
the flooded rivers and inun3ai;ions of the Ice Age. 
Muddy water escaping from the glaciers of the Alps 
and other mountains, and from the terminal front of 
the great "inland ice" of Northern Europe, doubtless 
drowned wide areas, while torrents derived from the 
melting snows of extraglacial tracts must likewise 
have swept down large quantities of fine-grained sedi- 
ment. Thus, long after the periodical inundations of 
glacial times had diminished in extent and finally 
ceased, the lower reaches of the great valleys and the 
broad plains, formerly subject to floods, must have 
been more or less sheeted with sandy loams. We 
know now that Tundra- and Steppe-conditions have 
succeeded in Central Europe. Already towards the 
close of glacial times a well marked Tundra-fauna 
had spread south to the Alps and west into France 



264 EARTH SCULPTURE 

and England. At that^period the climatic conditions 
were probably such as are now experienced in North- 
ern Siberia. Eventually, however, these conditions 
gradually gave way, — the Tundra-fauna began to re- 
treat, until by and by it was supplemented by a no 
less characteristic Steppe-fauna, the range of which 
seems to have been as extensive as that of the former. 
The Arctic lemming, Arctic fox, reindeer, musk-ox, 
and glutton of the Tundras were now replaced by the 
jerboa, pouched marmot, tailless hare, little hamster 
rat, and other forms, the common denizens to-day of 
the Steppes of Eastern Russia and Western Siberia. 
It is certain, then, that a dry Steppe-climate has pre- 
vailed at no distant date, geologically speaking, 
throughout Central and Western Europe. Thus we 
may be sure that dust-storms must formerly have 
been as common in France and Belgium and the re- 
gions lying to the east as they are now in Russian 
and Asiatic Steppes. It was during the prevalence 
of such climatic conditions, as geologists think, that 
the wide-spread flood-loams of the Glacial Period were 
so largely re-assorted and remodified by deflation, and 
the lossic accumulations assumed their present aspect 
and distribution. 

Mention has been made of the fact that marshes 
and lakes occur now and again in the hollows amongst 
sand-dunes. They are met with likewise amongst 
dust-deposits. Thus pools and large and small sheets 
of water sometimes dapple the surface or extend over 
broad areas of the wind-swept Steppes. Such basins, 



^OLIAN ACTION 265 

doubtless, are partly due to the unequal distribution or 
heaping-up of fine sand and dust. In some cases, 
however, they seem to have been caused by the un- 
equal removal of superficial materials. 

In fine, then, we conclude that wind-erosion is most 
effective in dry, desert regions. Its influence is, no 
doubt, world-wide ; but as an active agent in levelling 
the land — in cutting, carving, undermining, and re- 
moving rock — wind plays the dominant part in de- 
siccated lands. We note, further, that the forms 
assumed by rocks subject to wind-erosion are largely 
determined by geological structure and the nature of 
the rocks themselves, just as in temperate latitudes 
feeble structures and relatively soft rocks are the first 
to yield. Lastly, we recognise that certain wind- 
blown accumulations have a world-wide distribution, 
and occur under all conditions of climate. Sand-dunes 
may be met with wherever incoherent deposits of 
sufficiently fine grain are exposed to the action of 
the wind. Dust, on the other hand, is pre-eminently 
a product of relatively dry regions and of deserts — 
wherever, indeed, the land is naked or only partially 
clothed with vegetation, dust is formed, and may be 
swept up and transported by the wind. 



CHAPTER XIII 

LAND-FORMS MODIFIED BY THE ACTION OF 
UNDERGROUND WATER 

DISSOLUTION OF ROCKS UNDERGROUND WATER-ACTION IN CAL- 
CAREOUS LANDS KARST-REGIONS OF CARINTHIA AND ILLYRIA 

EFFECTS OF SUPERFICIAL AND SUBTERRANEAN EROSION 

TEMPORARY LAKES CAVES IN LIMESTONE CAVES IN AND 

UNDERNEATH LAVA "CRYSTAL CELLARS" ROCK-SHELTERS 

SEA-CAVES. 

IN Chapter VII. it was pointed out that subterranean 
action had played a most important part in the 
production of certain surface-features. In particular 
it was shown that depression of the surface has fre- 
quently taken place as a result of that action. We 
have now to consider another kind of action altogether, 
which, although by no means so important as that 
just referred to, nevertheless now and again causes 
the surface in certain regions to subside. Rocks, as 
we have seen, are very variously acted upon by water 
— a few are readily soluble, but the great majority 
are not. The most important of the soluble rocks 
are rock salt, gypsum, and limestones of every kind. 
These are all more or less easily removed by meteoric 
water. Rock salt is so very soluble that it is seldom 

266 



ACTION OF UNDERGROUND WATER 267 

or never found cropping out at the surface ; any sur- 
face-exposure in temperate lands would rapidly disap- 
pea;r. It is only in dry and rainless tracts, therefore, 
that rock salt can exist as a superficial accumulation. 
Gypsum is more readily dissolved than limestone, but 
both rocks become eaten into at the surface, and, ac- 
cording to circumstances, are more or less rapidly 
washed away. This process of dissolution, it is need- 
less to add, is not confined to the surface. Meteoric 
water penetrates the ground, and circulates through 
the crust to considerable depths. After pursuing a 
shorter or longer course, it reappears at the surface 
as springs, the waters of which are more or less 
abundantly charged with dissolved mineral matter, 
according to the nature of the rocks through which it 
has passed. In this way enormous quantities of sol- 
uble materials are brought up from below ; in short, 
wholesale chemical erosion goes on underground. 
It follows that in regions where soluble rocks enter 
largely into the framework of the land the surface 
must in time subside slowly or suddenly. The copious 
outpouring of brine-springs gradually reduces beds 
and sheets of rock salt, and the overlying strata sink 
down and thus produce depression at the surface. 
And the same result is brought about by the dissolu- 
tion of gypsum, limestone, and dolomite. Sometimes 
the surface slowly subsides, but now and again it col- 
lapses suddenly, producing earthquakes, accompanied 
by much fracturing and shifting of the rocks. Thus 
it is believed that the earthquakes which disturbed 



268 EARTH SCULPTURE 

the Visp-Thal in Valais during the summer and au- 
tumn of 1855 were the result of the caving-in of the 
rocks consequent on the dissolution and removal of 
gypsum, for the springs of that district bring to the 
surface annually over 200 cubic metres of the mineral 
in solution. Similarly, it can hardly be doubted that 
many of the larger and deeper depressions of the sur- 
face which appear in regions of calcareous rock are 
the result of sudden collapse due to the removal of 
material by underground water. 

As rock salt and gypsum do not enter largely into 
the composition of the crust, they are less important 
from our point of view than limestones. The latter 
not only attain in many cases a much greater thick- 
ness, but they are far more widely distributed, and 
extend over much broader areas of the earth's sur- 
face. It is in regions of calcareous rocks, therefore, 
where underground water plays the most prominent 
rdle, and where its action in modifying surface-features 
is best displayed. In a former chapter reference has 
been made to the fact that in countries occupied by 
limestone, the drainage is often largely or even wholly 
conducted underground. The rocks are so penetrated 
in all directions by rifts, clefts, and tunnels, that the 
water which falls at the surface very soon disappears. 
Concerning the origin of these subterranean spaces 
there is not much difference of opinion. Geologists 
recognise that they have been worked out by 
chemical and mechanical water-erosion. But while 
some have maintained that the underground water 



ACTION OF UNDERGROUND WATER 269 

has licked and worn out a passage for itself chiefly 
along the normal divisions of the rocks — their joints 
and bedding-planes — others have held that the main 
lines of underground drainage have been determined 
by faults or dislocations. Both views are doubtless 
true : some caves and underground tunnels appear to 
have no connection with faults ; others, on the con- 
trary, follow these, although many of the channels 
connected with them have been worked out along 
joints and bedding-planes. 

Underground water usually follows a zigzag and 
irregular course — now plunging downwards at high 
angles, or even vertically, through relatively con- 
stricted clefts and fissures ; now winding through ap- 
proximately horizontal tunnels, or forming lake-like 
expansions in broad and lofty halls and chambers ; 
now dividing into more or less numerous torrents and 
streams, which zigzag downwards to lower and lower 
levels. In time many changes are effected. Here 
and there passages are blocked with sediment or by 
falls from the roof, and become partially or wholly 
abandoned, the water, dammed back, rising and mak- 
ing its escape by other clefts and hollows. Thus 
eventually the limestone becomes traversed in all 
directions by a perfect net-work of intercrossing chan- 
nels — winding and angulate, low and lofty, broad and 
narrow — many of which become abandoned by the 
water as it works its way to lower and lower lev- 
els. To what depth from the surface considerable 
tunnels can be excavated by chemical and mechanical 



270 EARTH SCULPTURE 

erosion we cannot tell. It is obvious, however, that 
a limit must be reached when the pressure of the 
superincumbent and surrounding rocks becomes so 
great that no vacant spaces can exist. Water de- 
scending from the surface must thus eventually be 
forced by hydrostatic pressure to rise again and 
escape at lower levels than its source. Large under- 
ground channels, therefore, probably descend to no 
great depth from the surface, and their size is natur- 
ally limited by the structure of the rock in which they 
are excavated. Where this is much jointed and fis- 
sured it is obvious that the span of a cavern cannot 
be great ; the disjointed rocks, losing support, tend to 
collapse. The widest underground chambers do not 
exceed lOO yards in width. 

In course of time the whole surface of a country is 
gradually lowered by denudation. This change goes 
on most rapidly no doubt in regions where the super- 
ficial rocks are more or less impermeable. But lands 
composed chiefly of limestone do not escape— corro- 
sion, especially, proceeds more or less rapidly. Ever 
and anon, too, the surface sinks slowly or suddenly as 
the case may be, consequent on the withdrawal of 
rock-material from below. The peculiar deformations 
caused by such changes are among the most charac- 
teristic features of limestone regions. Typical regions 
of the kind show no regular river-systems ; brooks 
and rivulets are wanting. Water sinks at once into 
the ground by pipes and swallow-holes, clefts and 
fissures. In the lower-lying parts of such lands now 



ACTION OF UNDERGROUND WATER 271 

and again rivers suddenly emerge at the surface, and 
after usually a short course may again disappear be- 
low ground. In the rainy season water often rises 
through the apertures by which the surface is more 
or less abundantly pierced, and dry valleys and wide 
basin-shaped depressions become flooded. Of course 
when the supply fails the water again returns to the 
depths from which it was discharged. 

In the karst-regions of Carinthia and Illyria these 
phenomena are very well displayed. The funnel- 
shaped depressions communicating with underground 
galleries, which with us are termed swallow-holes, are 
known in Carinthia as dolinas. These vary in width 
and depth from a few yards up to half a mile in width, 
and from 100 to 200 yards and more in depth. Most 
of them, however, are small — 40 or 50 yards across, 
and about 30 yards or so in depth. Their bottom is 
somewhat flat, and often covered with loam or clay. 
The larger ones are relatively shallower in proportion 
to their width than the others. Not less character- 
istic features of the karst-lands are the so-called blind- 
valleys and dry-valleys. Through the former a river 
flows to disappear into a tunnel at the closed or blind 
end. The dry-valleys have no river ; the bottom is 
usually irregular and often pitted with dolinas. Be- 
sides these land-forms, geographers recognise another 
kind of depression, the so-called "kettle-valleys," 
which are trough-like or dish-shaped basins of vari- 
able extent, some of them having an area of several 
hundred square miles. Not infrequently the smaller 



272 EARTH SCULPTURE 

ones run in parallel zones following the direction of 
the strike of the strata. All these surface-features are 
for the most part the result of underground erosion. 
Some of the dolinas may have been eroded by water 
descending through fissures from the surface ; but 
probably the greater number, and certainly all the 
larger ones, have been caused by the caving-in of 
underground tunnels. So, again, the blind-valleys and 
dry-valleys appear in most cases to form part of the 
subterranean drainage-system, now exposed by col- 
lapse of roof and the general degradation of the sur- 
face. The natural bridges or arches which are seen 
often enough in such regions are simply the relics of 
old underground tunnels and waterways, the ruins of 
which often cumber the depressions of the surface. 
It is hardly worth while adding that the numerous 
limestone caverns in which geologists have hunted 
so successfully for remains of primeval man and his 
associates are merely the abandoned courses of an- 
cient underground streams and rivers. Almost ev- 
erywhere, indeed, in great limestone-regions one may 
trace at the surface evidence of the effects produced 
by subterranean erosion. The trough-shaped basins 
(kettle-valleys) referred to above seem to owe their 
origin in the first place to determinate fissures. 
These are widened by the action of the surface-water 
as it passes underground, and the depression at the 
surface increases as the rock becomes undermined, 
collapse taking place from time to time. If the col- 
lapse be recent the bottom of the kettle-valley is 



ACTION OF UNDERGROUND WATER 273 

Strewn with broken rock-dSris. Not a few kettle- 
valleys in limestone-plateaux, however, may have been 
partially excavated by superficial water-action before 
the system of underground drainage was established, 
but by the action of the latter they have since been 
more or less modified. It may be taken as generally 
true that most of the depressions or basins, great and 
small, which are so characteristic of karst-lands, are 
either largely or wholly due to the corrosive and 
erosive action of underground water. 

Lakes, as we have seen, often appear periodically 
in these regions. Some are very regular in their 
coming and going, others only show at intervals after 
unusually heavy rain or long-continued wet weather. 
One of the best-known examples is the Lake of Jes- 
sero, or Zirknitz, in Carniola, which appears now and 
then in the broad valley of the Planina. This river, 
after flowing underground for a long distance, returns 
to the surface, and shortly afterwards winds through 
a wide plain encircled by high cliffs of limestone. 
The plain is pierced by hundreds of dolinas, from 
which, after excessive or continuous rain, the water 
wells and rushes until the whole wide area is trans- 
formed into a lake. The extent and depth and the 
duration of this temporary lake vary ; and the inter- 
vals between its successive appearances are likewise 
inconstant ; sometimes only a year, or two or three 
years may elapse, but intervals of ten and even of 
thirty years have been experienced. Not a few de- 
pressions in the surface of calcareous tracts may be 



2 74 EARTH SCULPTURE 

rendered impermeable by the accumulation in them of 
loam and clay, and these may then be occupied by 
permanent lakes. 

The influence of subterranean water is not, of 
course, confined to regions of soluble rocks. Where- 
ever water circulates in the crust rocks are attacked, 
and their constituents become liable to chemical 
change. In this manner immense quantities of min- 
eral matter are brought up from below, some of it to 
be thrown down at the surface, where in time it may 
form massive accumulations. The mechanical action 
of subterranean water is also recognised almost every- 
where, and more particularly in places where the geo- 
logical structure is weak, where rocks are in a state 
of unstable equilibrium. But the effect of under- 
ground water in bringing about rock-falls and land- 
slips in such regions has already been sufficiently 
discussed. 

Although caverns naturally occur most numerously 
and attain the largest size in the more readily soluble 
rocks, they are also met with in many other kinds. 
They appear, for example, not infrequently in lava. 
Some of the smaller of these are merely large blisters 
or bubbles, formed by the segregation of the absorbed 
water-vapour while the lava was in a semi-fluid condi- 
tion. The more extensive lava-caves have a different 
origin. While lava is flowing it necessarily cools rap- 
idly at the surface, and in this way becomes crusted 
over. If the crust thus formed be of sufficient thick- 
ness and strength, it remains steadfast, forming a kind 



ACTION OF UNDERGROUND WATER 275 

of tunnel, out of which the still liquid lava issues. Such 
lava-caves are of common occurrence in Hawaii, 
Mexico, California, the Canary Islands, Iceland, etc. 
Some are only a few feet in height and breadth, others 
may be 20 to 30 feet broad, 6 to 10 feet in height, and 
many yards in length. In certain volcanic regions 
lava-caves obtained much larger dimensions, but there 
is reason to believe that these have been modified by 
subsequent erosion. One in Hawaii has a width at 
the entrance of 130 feet, a height of 20 feet, and a 
length of 260 feet. Another (the Raniaka Cave) is 
1200 feet long. Water flowing in cavities under the 
lava-coulees of Auvergne (as in the neighbourhood 
of Clermont) has cut out courses in the subjacent 
granite, and issues at the lower ends of the lava- 
streams through natural arcades. And many similar 
examples of subterranean tunnels and caves might be 
cited from other regions, where the erosion has been 
effected chiefly by the mechanical action of water 
upon relatively insoluble rocks. 

Mention may also be made of the great cavities 
which occasionally occur in faults. The spaces be- 
tween the two walls of a fault or dislocation are 
usually filled up either with rocV-dSris, or subse- 
quently infiltrated mineral matter, or with both. 
Now and again, however, the filling-up is only partial, 
and chambers of some size remain. These are often 
lined with finely crystallised minerals, and form what 
are known in Switzerland as " crystal-cellars." 

Of caves solely due to erosion it is not necessary to 



2 76 EARTH SCULPTURE 

say much. Shallow caves (rock-shelters) are fre- 
quently met with in river-valleys, where one can see 
that they owe their origin to the under-cutting action 
of the water. More extensive are the caves often 
excavated by the sea. These necessarily vary in 
appearance with the character of the rocks in which 
they are excavated. The presence of a cave indicates 
some weak structure — some rock or rock-arrangement 
which has offered less resistance to the attack of waves 
and breakers. Vertical dikes of basalt, for example, 
are often so abundantly jointed, that they are broken 
up and removed more readily than the rocks they 
traverse, although the latter may consist of "softer" 
material, such as sandstone. The highly jointed 
basalt, notwithstanding its superior hardness, is easily 
shattered. The mere force of the waves combined 
with hydraulic pressure in some joints, and the com- 
pression and expansion of air in others, suffices to 
rupture and burst the weak structure, and with each 
drop of the wave large and small fragments may 
sometimes be seen falling from the roof and sides of 
the cave. The cave thus increases in height as the 
sea works its way inland, until not infrequently it 
communicates with the surface by a "blow-hole," 
through which in storms not only spray but spouts of 
water, and even gravel and larger stones, are ejected. 
Similar caves are frequently formed in well jointed 
sandstones and in many other kinds of rock. They are 
very common, for instance, in Orkney and Shetland, 
and they are well known also in Cornwall and the 



ACTION OF UNDERGROUND WATER 277 

West of Ireland. In time the whole roof of such 
caves may give way, and the latter then appear as 
narrow ravine-like or gorge-like inlets. This can 
happen only when the land-surface does not rise to 
'any great height above the sea. When the rocks 
above a sea-cave are too strongly built or too thick 
to permit of a downfall of the roof, the cave may at- 
tain very considerable dimensions. But as all rocks 
are traversed by lines of weakness, a limit must be 
reached beyond which caves cannot be widened. By 
and by the rocks will cease to be self-supporting, and 
collapse must take place. 

Caves of marine origin are seldom met with far 
removed from existing coast-lines. They are natur- 
ally confined to the latter, and to those lines of old 
sea-level known generally as " raised beaches." Their 
position at the base of old sea-cliffs renders them liable 
to early obliteration, for they tend to be obscured by, 
and eventually to be concealed underneath, a talus of 
dibris. They are not singular, however, in that re- 
spect, for many of the most interesting and important 
of the limestone caverns of Western Europe have 
been hidden in the same way, their discovery having 
been the result either of mere accident or of patient 
scientific research. 



CHAPTER XIV 

£ASINS 

BASINS DUE TO CRUSTAL DEFORMATION CRATER-LAKES DIS- 
SOLUTION BASINS LAKES FORMED BY RIVERS jEOLIAN BASINS 

DRAINAGE DISTURBED BY LANDSLIPS GLACIAL BASINS OF 

VARIOUS KINDS, AS IN CORRIES, MOUNTAIN- VALLEYS, LOW- 
LANDS, AND PLATEAUX ICE-BARRIER BASINS — SUBMARINE 

BASINS OF GLACIAL ORIGIN. 

ALL the varied topographical features of the land 
owe their origin either to subterranean or to 
superficial agents, or to both. This is true of eleva- 
tions and depressions alike. It would seem possible, 
therefore, to classify hollows according to the mode 
of their formation. Not a few, however, are of com- 
plex origin, having resulted partly from hypogene 
and partly from epigene action. Indeed, we might 
group all basins roughly in two divisions, according 
as they owe their origin more or less directly to 
crustal deformation and fracture, or to the action of 
surface-agents. Epigene action, however, is so mani- 
fold and diverse — the agents of erosion, of transport, 
and accumulation act in so many different ways — that 
a more detailed grouping is desirable. Any classifi- 
cation adopted must be more or less arbitrary and in- 

278 



£ASINS 279 

complete, but it will serve our purpose to group 
basins as follows : — 

1. Tectonic basins. 

2. Volcanic " 

3. Dissolution " 

4. Alluvial 

5. ^olian " 

6. Rock-fall 

7. Glacial 

I. Tectonic Basins. These owe their origin di- 
rectly to deformation of the earth's crust, whether 
the result of warping or of fracture, or both. In this 
class are included many inland seas, and most of the 
larger lakes of the globe. The Aralo-Caspian de- 
pression, with its numerous sheets of water and de- 
siccated basins, the Dead Sea, Issyk-Kul, the lakes of 
Equatorial Africa, the Great Salt Lake of Utah, and 
very many others are true tectonic basins. A large 
number of such basins occur in relatively dry and 
rainless regions. On the other hand, many are met 
with in temperate regions. The great fresh-water 
lakes of North America and Europe (Superior, 
Huron, Michigan, Ladoga, Onega, etc.) occupy tec- 
tonic basins. These lakes, it will be noted, are 
confined to the glaciated areas of the two continents, 
and their character as tectonic basins has been modi- 
fied and obscured by glacial erosion and accumula- 
tion. There seems no reason to doubt, however, that 
the depressions are the result of crustal deformation. 
Tectonic basins are usually somewhat flat-bottomed or 



2 8o EARTH SCULPTURE 

gently undulating. Occasionally they are traversed 
by narrow winding hollows, which have been traced 
for longer or shorter distances. These have fre- 
quently the character of river-ravines and valleys, and 
are suggestive, therefore, of a former land-surface 
which has become depressed. Similar indications of 
depression are afforded by the highly indented coast- 
lines of some of the larger lakes of this class, the 
long inlets and projecting headlands recalling the ap- 
pearances presented by the fiord-coasts of Norway 
and Scotland. 

The crustal deformation may consist of simple 
subsidence^ — a wide area of relatively flat or gently 
undulating land sinking below the level of adjacent 
tracts ; or the subsidence may be the effect of dis- 
location and displacement. Again, basins have come 
into existence between contiguous high grounds un- 
dergoing elevation. Once more, the formation of 
an anticline across the drainage-area of a lowland 
region might bring extensive lakes into existence. 
Similarly it is conceivable that lakes might be formed 
in mountain-valleys by the swelling up of the crust at 
the base of the mountains, or by the formation of new 
flexures in the mountains themselves, having a direc- 
tion transverse to the valleys. We cannot, however, 
point to any particular valley-basin formed in this way. 
Earth-movements of this kind would seem to take 
place very, slowly, so slowly, as a rule, that rivers are 
able to saw across the obstructions as fast as they 
rise. 



£ASINS 281 

2. Volcanic Basins. The lakes of this class form a 
well marked group, many of them occupying the sites 
of extinct volcanoes. Not a few, therefore, occur in 
the cup-shaped depressions of volcanic cones. Others, 
again, may not be walled round by volcanic ejecta, 
but occupy explosion-craters — the more or less deep 
concavities produced by paroxysmal outbursts. No 
hard-and-fast line, however, can be drawn between 
these two varieties of crater-lake. Some explosion- 
craters are encircled by ridges of ejecta, while the 
cup-shaped depressions of certain volcanic cones are 
of such a depth that, were the cones themselves to be 
removed, a considerable concavity would still remain. 
Amongst well known crater-lakes are the Maars of 
the Eifel, some of which are 70 feet br less in depth, 
while others are not much below 200 feet. Of the 
same character are the crater-lakes of Auvergne, 
which vary in depth from less than 100 to 350 feet ; 
and the similar lakes of Central Italy, one of which. 
Lake Bracciano, is said to be 950 feet deep. In all 
the great volcanic regions of the globe, indeed, lakes 
of this character are recognised. Other volcanic 
lakes have had a different origin. Sometimes lava, 
at other times fragmental ejecta, or streams of tu- 
faceous mud and debris have entered valleys and 
obstructed the drainage. The Lac d'Aydat of Au- 
vergne, for example, is confined by a barrier of lava, 
and the same is the case with the large Yellowstone 
Lake. So, again, the enormous torrents of mud and 
ddbris which poured down to the low grounds during 



282 EARTH SCULPTURE 

the great eruption of Bandaisan in 1888 gave rise to 
four volcanic barrier-lakes. After volcanoes have 
erupted for a prolonged time the ground often be- 
comes depressed, and large and small subsidences of 
the surface are not infrequently the result of the 
earthquakes that accompany volcanic action. 

3. Dissolution Basins. In regions of soluble rocks, 
as we have seen, many inequalities of the surface 
are brought about by the chemical and mechanical 
action of underground water. Most frequently the 
depressions produced by the collapse of subterranean 
galleries and caves contain no water. Sometimes, 
however, as Professor Penck has pointed out, warp- 
ing of the crust has brought the corroded and tun- 
nelled limestone rocks under the influence of the 
subterranean water-level, so that sink-holes and other 
superficial depressions have become more or less 
deeply filled. Again, should tectonic movements 
carry down a honeycombed calcareous region so that 
its basal portions sink below the sea-level, the mete- 
oric water descending from the surface will be dammed 
back in sinks and other hollows. The water-surface 
of wells in such districts is known to rise and fall with 
the tide. From various causes, also, the underground 
outlets of dolinas, etc., become closed with accumula- 
tions of insoluble earthy materials, and the bottoms 
of other depressions are rendered impermeable by 
similar deposits washed into them by rain- or snow- 
water. Similar changes have been brought about by 
glacial action, the outlets for the escape of under- 



BASINS 283 

ground water having been closed by morainic debris. 
For these and other reasons lakes are by no means 
always wanting in regions of highly honeycombed and 
tunnelled calcareous rocks. 

Soluble rocks deeply covered with strata of more 
durable character do not escape corrosion, but are 
gradually removed by underground water, and thus 
bring about slow subsidence or sudden collapse of the 
surface, and the shallow basins formed in this way 
may become filled with water. 

4. Alluvial Basins. The broad alluvial flats of 
rivers often show slight depressions caused by irregu- 
lar accumulation. These during floods may become 
lakes, and endure for a longer or shorter time. Again, 
rivers tend to change their courses, and their deserted 
loops often persist as lakes. In rainless regions the 
rivers flow with a gradually lessening volume, until 
eventually they may dry up. It is obvious that the 
sediment transported by such rivers must gradually 
raise the level of their lower courses, and in time pro- 
duce shallow basins. In the dried-up courses them- 
selves pools and " creeks " not infrequently occupy 
the deeper hollows, and are probably maintained by 
water coming from underground sources. Once 
more, in well watered regions rivers now and again 
form lakes. A main stream, for example, by carrying 
down large quantities of detritus, tends to raise the 
surface of its bed above that of its tributaries, in the 
lower reaches of which lakes thus come into exist- 
ence. In like manner tributary streams occasionally 



284 EARTH SCULPTURE 

throw more detritus into the main valley than the 
river in the latter can at once dispose of. Partial 
dams are thus produced, and large valley-lakes form 
above the obstructions, of which the Silser See and 
Silvaplana See in Upper Engadine are examples. 

5. y^olian Basins. Another class of basins owe 
their origin to the action of the wind. Some are 
erosion-basins caused by the removal of loose, weath- 
ered rock-material. Professor Pumpelly seems to have 
been the first to recognise basins of this kind, which 
were observed by him in Mongolia. They have since 
been encountered in many other regions, as in Bahia, 
in Central Asia, and' elsewhere. Sometimes these 
basins form temporary lakes, at other times the water 
remains more or less persistently. Some interest- 
ing examples have been described by Mr. G. K. 
Gilbert as occurring in Arkansas and elsewhere in the 
Great Plains of North America. Basins of this kind 
are naturally confined to relatively dry regions — to 
regions where the rocks and soils are not sufficiently 
protected by vegetation. Reference may also be 
made to the temporary or more persistent lakes 
which owe their origin to the unequal distribution of 
wind-blown accumulations, some account of which 
has already been given. 

6. Rock-Fall Basins. Rock-falls and landslips not 
infrequently disturb local drainage, and may cause 
lakes to appear. Many small lakes of this class oc- 
cur in the Alps and other mountain regions where the 
geological structures are weak and liable to collapse. 



BASINS 285 

7. Glacial Basins. The basins coming under this 
head are essentially of two kinds. Some are hollows 
of excavation, others owe their origin to the unequal 
heaping up of glacial and fluvio-glacial deposits. It 
is not always possible, however, to distinguish sharply 
between the two. In many cases excavation and 
accumulation have alike been concerned in their 
formation. The glacial origin of both is at once 
suggested by the fact that they are confined to 
regions which yield other and independent evidence 
of former glacial action. We note further that their 
presence has no immediate or direct connection with 
the character of the rocks or with the geological 
structure of the tracts in which they lie. They occur 
in crystalline, igneous, and schistose rocks, and in 
sedimentary strata of all kinds and of all degrees 
of induration — conglomerate, sandstone, greywacke, 
clay-slate, shale, limestone, gravel, etc. They are 
not restricted to areas of folded, contorted, and 
fractured rocks, but appear with all their character- 
istic features equally well developed in places where 
the strata are gently undulating and approximately 
horizontal. 

The formerly glaciated areas of the earth's surface 
are pre-eminently the lake-lands of the world. We 
have only to look at a series of good maps to see 
that this is the case. Taking Europe as an example, 
we find that very few lakes occur in regions over 
which ice-sheets and glaciers have not at one time 
extended, the most notable of those lakes being the 



286 EARTH SCULPTURE 

volcanic basins of Auvergne, the Eifel, and Central 
Italy. What non-glaciated region of our continent 
can show a lake-dappled surface like Finland ? 
Where in extraglacial tracts can we find anything to 
compare with the pay sage morainique of North Ger- 
many and Russia ? Precisely the same phenomena 
confront us in North America. How abundantly are 
lakes distributed over all the vast tract formerly oc- 
cupied by the great inland ice ! South of the glacial 
boundaries they are practically unknown. 

We note further that the vertical distribution of 
the class of lakes now under consideration is not less 
suggestive of their origin. Cirque-lakes and other 
high-level lakes are not confined to any one region, 
they occur in mountain-tracts all the world over, 
wherever these have formerly nourished glaciers. 
Low-lying valley-lakes like those of the Alps have, 
on the other hand, a much more restricted distribu- 
tion. They abound in the mountains of temperate 
latitudes, where great valley-glaciers formerly existed, 
but they are looked for in vain in the mountains of 
the warmer zones, the lower reaches of whose valleys 
have never been glaciated. Again, in the northern 
tracts of Europe and North America glacial basins 
are not even confined to mountain-valleys, but occur 
more or less abundantly over the lowlands that sweep 
out from the mountains. In a word, there is a close 
connection between glaciation and the development 
of lake-basins. 

Basins of glacial origin naturally vary much in char- 



BASINS 287 

acter, according to their position and the particular 
mode of their formation. Some, as mentioned above, 
are rock-basins, others are barrier-basins, and many 
are partly both. It must be added that not a few 
lakes met with in glaciated regions are not of glacial 
origin. This is particularly the case in mountain-val- 
leys, where barrier-basins have often been formed by 
rock-falls and fluviatile action. Glacial basins may 
be roughly grouped as follows : — 

1. Cirque or Corrie basins. 

2. Mountain-valley basins. 

3. Lowland and Plateau basins. 

4. Ice-barrier basins. 

5. Submarine basins. 

I. Cirque or Corrie basins are confined to mount- 
ain regions. Frequently they appear as niche-like 
indentations on mountain-slopes at high elevations 
above the bottoms of the adjacent valleys. At other 
times they are set farther back from the brow of a 
valley, forming cup-shaped depressions in the flanks 
of the higher crests and ridges. When such is the 
case the water escaping from them may flow for a 
longer or shorter distance before it reaches the ter- 
minal shoulder of a mountain to plunge downwards to 
the valley below. In detail, cirques vary in character 
with the nature of the rocks and their geological 
structure. Many have a crater-like appearance, some 
of the wider ones resembling the section of a steep- 
sided amphitheatre, while the narrower ones show 



288 EARTH SCULPTURE 

more abrupt slopes. Although now and again the 
converging slopes may be relatively smooth and not 
so steep, yet as a rule they are rugged and precipit- 
ous, showing bare, gaunt walls of rock, trenched and 
furrowed by torrent action and shattered by frost. In 
regions which have formerly supported glaciers cirques 
are more or less flat-bottomed, or saucer-shaped, and 
consequently many are occupied by lakes. It is 
worthy of note that such corrie-lakes, or tarns, are 
usually deeper in proportion to their extent than the 
large valley-lakes of lower levels. Many corrie-lakes 
rest in true rock-basins ; others seem to be wholly 
dammed by moraines ; while yet others are partly 
rock-basins, partly barrier-basins. Not a few have 
been drained by the water escaping from them cut- 
ting back its channel. Others, again, would seem to 
be filled up by rock-falls and the detritus and ddbris 
shot down from the surrounding heights. Many 
cirques, on the other hand, have never contained 
lakes, their flat bottoms sloping gently, but continu- 
ously, outwards. That cirque-basins have been for- 
merly occupied by glaciers is shown by the presence of 
moraines and the frequent appearance of roches mou- 
tonndes and striae, the direction of which indicates an 
outflow of ice from the depressions. These marks of 
glacial action are confined to the bottom of a cirque ; 
the precipitous rock-walls show none. 

The question of the origin of cirque-lakes has some- 
times been obscured by confounding the origin of the 
cirques with that of the basins which occupy their 



BASINS 289 

bottoms. While some geologists have attributed both 
to the action of glacier-ice, by others they are believed 
to be the result of aqueous erosion. The cirques 
themselves are doubtless in many cases the work of 
converging torrents, aided by frost. Very frequently, 
however, frost, rather than running water, has been 
the chief eroding agent, as may be seen in Norway, 
where, in immediate proximity to the ;?/z//-line,. cirques 
are now being formed. The basin at the bottom of 
a cirque, however, is the work neither of running 
water nor of frost alone, but has been ground out by 
glacier-ice. In the Highlands and the Southern Up- 
lands of Scotland the head-waters of streams and riv- 
ers often proceed from cirque-basins, especially in the 
more elevated districts. Many of the smaller feeders, 
however, come from cirques which have no basin, and 
this is particularly the case in the less elevated portions 
of the mountain regions. The origin of the latter is ob- 
vious ; we see them being formed at present. Springs, 
summer torrents, snow-water, and frost- — all play their 
parts. The converging mountain-slopes direct the 
drainage to one point, the result being the formation of 
a more or less abrupt funnel-shaped depression resem- 
bling the section of an inverted hollow cone. The form- 
ation of a basin at the apex of this inverted cone by 
aqueous action is impossible. The torrent escaping 
from the cirque simply digs its channel deeper, cuts 
its way back, and by its undermining action tends to 
increase the slope of the surrounding walls. Add to 
this the action of frost in splitting up the rocks and 



290 EARTH SCULPTURE 

detaching larger and smaller masses, and one can 
readily understand how a cirque must increase in ex- 
tent. Cirques of this character occur under all con- 
ditions of climate and in every mountain region of 
suitable structure, in temperate, subtropical, and trop- 
ical zones alike.^ But the flat-bottomed cirque is 
restricted to regions which are now, or have recently 
been, subjected to glaciation. Cirque-basins are 
familiar features in the Alpine lands of temperate 
latitudes, and they are met with likewise, but only 
at lofty elevations, in the warmer zones. When a 
mountain area was subjected to glaciation, the cirques, 
which occurred in immediate proximity to the snow- 
line, would form admirable reservoirs for the accumul- 
ation of snow and ndvd, and the formation of " summit 
glaciers." The shape of a cirque would greatly favour 
glacial erosion by enabling the ice to concentrate its 
grinding and disrupting action upon the point tow- 
ards which the mountain-slopes converged. Hence, 
in time, the bottom of such a cirque could not fail to 
be ground out, and the basin thus formed, owing 
to the conditions that so specially favoured erosion, 

_' Although the trae cirque usually presents the appearance of a niche-like 
indentation in a mountain-slope, not a few valleys terminate upwards in great 
amphitheatre-like cirques, the walls of which are often very steep. Such 
cirque-valleys appear now and again in our European mountains. As examples, 
may be cited the great cirque of Gavarni in the Pyrenees, the valleys of the 
Hallstadter See and the Konigs See, and of the Trenta and the Wochein 
in the Alps, and the great cirque-valleys of Norway, such as that near Lunde 
(Jostedalsbrae), the precipitous encircling walls of which rise more than 3000 
feet above the bottom of the valley. Glen Eunach (Cairngorm Mountains) is a 
good example of a Scottish valley with a cirque-shaped head. Such great 
cirque-valleys often contain lakes. 



BASINS 



291 



would tend to be relatively deeper than the rock- 
basins excavated in a broad mountain-valley. 

The vertical distribution of corrie-basins in any 
given tract of mountains shows that they are closely 
related to former snow-lines. They occur in belts, or 
zones, and are not irregularly scattered over a whole 
region. Amongst the Scottish mountains two such 
zones can be recognised. In the lower part of these 
the corrie-basins range from 1 500 feet to 2400 feet or 
thereabouts ; in the upper they occur between 2400 
feet and 3400 feet. Consequently, the two zones are 
met with together only among the most elevated 
mountain-groups. In the mountains of Middle Ger- 
many the zone of cirque-basins lies between 3000 feet 
and 3500 feet above sea-level ; and Professor Partsch 
has pointed out the significant fact that the cirques, 
as we follow them from west to east, rise to higher 
and higher levels, showing, as he says, that the snow- 
line of glacial times gradually ascended as it passed 
eastward into the interior of the continent. Simi- 
larly in the Alps and the Pyrenees, cirque-basins oc- 
cur in definite zones, and form harmonious systems 
in the several mountain-groups, each zone marking 
out a former snow- or ne've-leveU 

' Professor Penck gives the following table to show the relative heights at- 
tained by mountain-lakes — the zones of greatest development of high-level 
lakes. He includes in this table not only cirque-lakes, but many small barrier- 
lakes : 

Norway 1000-1600 metres. 

Hohe Tatra . ... 1500-2100 " 

Eastern Alps (Central Zone) . . 1700-2800 " 

Graubunden Alps .... 2000-2700 " 



292 



EARTH SCULPTURE 



It is interesting further to note that in North and 
Middle Europe the cirque-basins affect chiefly the 
mountain-slopes that face the north and north-east. 
Thus of 78 in the uplands of Norway, according to 
Helland, 50 face the north, while 19 open towards 
the east. So, again, Partsch states that of 35 in the 
mountains of Middle Germany 19 look north and 
north-east, 1 3 east and south-east, and only 3 face the 
south and west. This distribution, as Penck remarks, 
is quite in keeping with existing conditions, for at 
present most snow accumulates on northern and east- 
ern exposures. On southern exposures it quickly 
melts, while from the western declivities of the 
mountains it is blown away by the prevailing west 
winds. 

2. Mountain-Valley Basins. This class includes all 
lakes of glacial origin occurring in mountain-valleys 
or closely connected with these. In some regions 
they are seen only at the very heads of the valleys, 
which may be cirque-shaped or not ; elsewhere they 
appear towards the lower ends of the valleys, from 
which they now and again extend into the low 
grounds ; or they may occur outside of the mountains 
altogether, opposite the mouths of great mountain- 



Transylvanian Alps . 


1900-2100 metres 


Pyrenees 


1800-2400 


Sierra Nevada (Granada) 


. 2900-3200 ' ' 


Himalaya 


. 4000-5000 " 


Sierra Nevada (S. Marta) 


. 3900-4000 ' ' 


Andes of Peru 


. 4300-4600 " 


Andes of Chili 


. 1700-3000 " 


New Zealand Alps . 


600-1200 " 



BASINS 



293 



valleys. Many of these 
are rock-basins, others are 
barrier-basins, that is, the 
water has been 



im- 



pounded by the unequal 
deposition of glacial and 
fluvio-glacial detritus. 
The large majority, how- 
ever, partake of both char- 
acters ; the lakes occupy 
rock-basins, the lower 
ends of which have been 
heightened by morainic 
and fluviatile accumula- 
tions. Many of the lakes 
in question attain a great 
depth. Amongst the 
lakes of the Alps, for ex- 
ample, we find depths 
of 469 feet (Zurich), 826 
feet (Constance), 1013 feet 
(Geneva), 1135 feet (Gar- 
da), 1 34 1 feet (Como), 
2800 feet (Maggiore). 
Similar relatively deep 
lakes occur in Scotland. 
Loch Lomond, for in- 
stance, has an extreme 
depth of 630 feet, and 
Loch Ness of 780 feet. 



n 



m 



M S. 

^ 4! 

:^ JS 

S = 

a 2 

u ■S 

S S 

O & 



2 



294 EARTH SCULPTURE 

The mean depth of such lakes often approaches, and 
occasionally even exceeds, half of the extreme depth. 
But when we take Into account the superficial area 
of the lakes, it becomes obvious that the basins they 
fill are mere shallow pans or troughs. The depth of 
Lake Como, for example, is only 130th part of its 
length ; while the Lake of Geneva and Lake Garda 
are respectively 230 and 280 times longer than they 
are deep. Again, the length of Loch Ness is 136 
times, and that of Loch Lomond 1 76 times greater 
than the depth. 

The valley-basins of the Alps and other elevated 
regions of Europe are of relatively recent age. Not 
one is certainly known to be of older date than the 
Glacial Period. Further, they all lie within tracts 
which have been more or less severely ice-worn. 
Add to this the suggestive fact that they are distri- 
buted without any reference to the geological struct- 
ure of the regions in which they appear. The late 
Sir A. C. Ramsay was the first to show that such 
basins had been excavated by glaciers. In the case 
of a glacier, as we have seen, erosion is carried on 
throughout the whole extent of its bed. It is obvious, 
however, that rock-grinding and rock-rupturing will 
proceed most actively under the thickest mass of the 
glacier, and the position of this thickest part will de- 
pend on the character of the valley and the number 
and size of the tributary glaciers. After the glacier 
has attained its maximum depth and speed its thick- 
ness progressively diminishes, and its rate of motion 



BASINS 29s 

at the same time gradually decreases as it flows on its 
way. Under these conditions a shallow trough must 
eventually be eroded in the bottom of the valley, the 
depth and extent of which will have a definite relation 
to the importance of the glacier. Towards the ter- 
minal part of the ice-flow erosion ceases, while accu- 
mulation there reaches its maximum, morainic debris 
and fluvio-glacial detritus being dumped upon and 
spread over the valley-bottom, the surface of which 
may thus be considerably raised. Hence, partly by 
erosion under the glacier, and partly by accumulation 
in the valley at and below its terminal front, a trough 
or basin is formed. On the disappearance of the 
glacier, a valley-lake comes into existence, the river 
escaping from which may by and by work its way 
down through the morainic and fluvio-glacial deposits, 
and thus gradually lower the level of the lake, until the| 
rock-head is reached, after which the lowering of the 
level becomes a much slower process. 

Valley-basins of the kind described occur, like 
icirque-basins, indeterminate zones. Just as the latter 
indicate former ndvd-Wn&s, so the former mark out 
the limits reached by valley-glaciers. In the loftier 
mountain tracts of temperate and northern regions, 
two or mofe zones of cirque-basins are found rising 
one above the other, each zone representing a former 
position of the n^v^-Ym&. In like manner we have 
in the same regions corresponding zones of valley- 
basins, each of which marks a distinct stage of former 
glaciation. The basins in the lower reaches of the 



296 EAR TH SC ULP TURE 

valleys and at the base of the mountains belong to 
the period of maximum glaciation, when the snow- 
line descended to its lowest level ; while the basins 
at or near the heads of the valleys are products of 
later epochs, when the snow-line had retreated to 
greater altitudes. 

The valley-basins of a great mountain-range are 
typically developed where the valleys open freely 
upon the low grounds, for under such conditions the 
old glaciers, meeting with no obstructions, could 
readily creep outwards from their mountain-fastnesses 
and deploy upon the Vorland. 

Thus, in the case of the Alps, no barrier obstructed 
the outflow of the glaciers into Piedmont and Lom- 
bardy, and similar conditions obtained along the 
north front of the mountains east of the valley of the 
Aar. It is in those regions, therefore, that the lower 
valley-basins are best developed. The enormous sea 
of ice that flowed down the Rhone Valley, on the 
other hand, was dammed back by the opposing range 
of the Jura, and deflected to right and left. Hence 
the basins excavated by that great glacier differ to 
some extent from the typical valley-basins described 
above. Round the lower ends of the latter terminal 
moraines are usually more or less well developed. 
We look in vain, however, for such moraines circling 
round the lower ends of the Lake of Geneva or Lake 
Neuchatel and the smaller lakes in its neighbour- 
hood. The Neuchatel basin has not been excavated 
by an ordinary valley-glacier in the usual way ; it did 



BASINS 



297 



not come into existence under the lower reaches of 
such a glacier. Its position at the base of the Jura, 
and the direction of glaciation in its neighbourhood, 
show that it is a true defiection-basin. When a 
glacier is obstructed and turned aside from the path 
it would follow did no such obstacle intervene, the 
ice heaps up, and its erosive action, therefore, be- 
comes intensified, so that a basin is eventually hol- 
lowed out in front of the opposing barrier. The 
basin occupied by the Lake of Geneva is of a more 
complex structure. The upper portion of the lake, 
which formerly extended up the valley of the Rhone 
as far as Bex, is comparable to one of the lakes of 
Lombardy ; it is a mountain-valley basin. The north- 
ern half, however, is a deflection-basin, which, like 
the basin of Neucha,tel, owes its origin to the erosion 
induced by the barrier of the Jura, which caused a 
great heaping-up of ice between those mountains and 
the Alps. 

Most of the rock-basins of the Alps have been 
more or less modified by fluviatile action. The levels 
of many lakes have in this way been raised, and the 
true character of their basins obscured. Were all 
the morainic and fluviatile accumulations in their 
neighbourhood to be removed, the area of some of 
the lakes would be considerably reduced.^ On the 

' It has been estimated that the surface of Lake Constance would fall 200 
feet, and its area be reduced by a third, were the deposits which partially dam 
it up to be removed. So, in like manner, could we conjure away the superficial 
accumulations in the plains of Lombardy below Como, that lake would lose 
nearly 500 feet of its depth, and about half of its area. 



298 EARTH SCULPTURE 

Other hand, not a few were formerly more extensive 
than they are now. Streams and rivers are gradually 
pushing their deltas forward into the upper reaches 
of a lake ; and the same process takes place in other 
parts of the same basin opposite the mouths of lateral 
streams and torrents, so that in not a few cases lakes 
have been divided into two or more. Again, very 
many lakes have been entirely silted up. 

We have spoken of the rock-basins which are so 
commonly encountered towards the lower and upper 
ends of mountain-valleys. It must not be supposed 
that glacially eroded basins occur nowhere else in 
mountain-valleys. Those referred to may, indeed, be 
taken as the normal types of valley-basins ; each has 
been excavated under the lower reaches of a glacier, 
the lateral and terminal moraines and fluvio-glacial 
gravels of which usually appear in their immediate 
neighbourhood. Rock-basins, however, have been 
eroded elsewhere in the bed of a glacier, as in the 
case of the deflection-basins already described. These, 
as we have seen, owe their origin to the increased 
erosion caused by notable obstructions in the path of 
an ice-flow. It not infrequently happens that a 
mountain-valley becomes constricted owing to the 
mutual approach of its flanks ; the valley-bottom ex- 
pands and contracts as the opposing mountain-slopes 
recede or advance. When a valley of this character 
is occupied by a glacier it is obvious that each con- 
striction must form an obstacle in its path, with the 
result that under the heaped-up ice erosion will be 



BASINS 299 

intensified on the bed of the valley above the con- 
striction, and a shallow basin will be ground out. On 
the disappearance of the glacier a lake will necessarily 
appear, and many such lakes occur in highly glaciated 
mountain tracts ; frequently, however, lakes of this 
kind become silted up, and their former presence is 
then only indicated by flat sheets of alluvium. Again, 
it is well known that valley-basins of the normal type 
often show irregular depths, and it is not always easy 
to say how these have originated. Sometimes they 
are the result of valley constriction, sometimes of 
sudden changes in the direction of the valley, which 
have caused the ice to erode more energetically on 
one side than the other, for the line of most rapid 
motion in a glacier, as in a river, will shift from 
the centre to the side, or from side to side, with 
the windings of its course. Again, inequalities in the 
floor of a rock-basin may sometimes be due to the 
unequal resistance of the rocks. Nor must we forget 
that during its final melting a glacier might dump 
debris in a very confused fashion over its bed, while 
the subsequent deposition of alluvial matter swept 
into the lake at many different points by streams and 
torrents would similarly tend to produce inequalities. 
But all valley-lakes, it must be remembered, are 
not rock-basins. On the contrary, not a few Alpine 
lakes, and many which occur in similar positions in 
the mountains of other lands, are true barrier-basins, 
dammed up wholly by morainic or by fluvio-glacial 
detritus, or by both. Again, numerous small lakes 



300 EAR TH SC ULP TURE 

and pools occur in the cup-shaped and irregular de- 
pressions of the paysage morainique at the base of a 
mountain region. The moraines of this region mark 
the limits reached by the larger valley-glaciers. One 
of the most typical localities for the development of 
small morainic lakes of the kind referred to is the 
dreary district of the Dombes, in the valley of the 
Rhone. There, however, many of the pools are of 
artificial origin, and used as fishponds by the inhabi- 
tants. But it is the morainic character of the ground 
that makes this possible. 

Thus the paths of the old valley-glaciers are fre- 
quently marked by the appearance of glacial lakes, 
large and small, and variously formed. Great valley- 
basins may be restricted to the mountains, or may 
extend for some distance into the Vorldnder, or may 
occur wholly outside of the mountains. Most of 
these are rock-basins, but their depth has often 
been increased by accumulations of superficial ma- 
terials. Other valley-basins are essentially barrier- 
lakes. Lastly, beyond the lowest valley-basins, 
generally well out upon the low grounds, we encoun- 
ter the numerous pools and lakelets of the paysage 
morainiqtte. 

3. Plateati and Lowland Basins. The glacial basins 
we have hitherto been considering are products of 
the action of individual glaciers, small or great as the 
case may have been. They occur, therefore, either 
within mountain-valleys, or in their proximity. But 
over the wide tracts formerly invaded by the " inland 



BASINS 301 

ice" of Northern Europe glacial lakes are not con- 
fined to mountain-valleys and the adjacent Vorldnder, 
but are scattered broadcast over plateaux and low- 
lands. In those regions two areas of special lake- 
development may be recognised : (i) An area in 
which glacial erosion has been in excess of glacial 
accumulation ; and (2) an area in which, conversely, 
accumulation has been in excess of erosion. In the 
former tracts roches moutonnies abound ; the surface 
is thus often rapidly undulating. Low-lying, round- 
backed rocks extend on every side, while here and 
there the general monotony of the landscape is 
partly relieved by bare hills and now and again by 
bald mountain-heights, all scraped, bared, worn, and 
abraded by severe glacial action. In the countless 
dimples and irregular hollows of the surface lakes of 
all shapes and dimensions make their appearance, 
and the presence of innumerable bogs and marshes 
show further how many shallow sheets of water have 
been gradually obliterated. The most notable region 
of the kind in Europe is Finland, a land of lakes. 
But excellent examples occur in our own islands, 
such as the Outer Hebrides and the low-lying, rocky 
coast-lands of the tract lying between Loch Ewe and 
Loch Laxford. In North America the particular 
lake-lands of which we now speak are practically con- 
fined to and nearly co-extensive with the Dominion 
of Canada. 

Of the basins developed in those regions some 
have been excavated, while others are barrier-basins. 



302 EARTH SCULPTURE 

The distribution of the former cannot always be satis- 
factorily explained. We may suppose that under a 
general ice-sheet some rocks would yield more readily 
than others. Some geologists are of opinion that 
certain rock-basins may be of preglacial origin, and 
that all the ice did was to plough out the alluvia with 
which such basins had been filled. The hollows 
themselves, they think, may have been caused by the 
weathering and rotting of rock, and the subsequent 
removal of the disintegrated materials by wind or 
other superficial agency. According to others the 
depressions may be tectonic basins filled up in pre- 
glacial times and only re-excavated by glacial action. 
Some of the larger lakes, such as Lakes Lodoga, 
Onega, and others in Northern Europe, and the Great 
Lakes of North America, almost certainly occupy 
tectonic basins, modified no doubt by considerable 
glacial erosion and accumulation. But the far more, 
numerous small rock-basins of the regions now under 
review are unquestionably hollows of erosion. Some 
appear to have been ground out in places where the 
rocks offered less resistance to erosion, but probably 
the position of a larger number has been determined 
by the form of the ground. This is seen in the fre- 
quent appearance of rock-basins in places where the 
glacial current suffered constriction or obstruction. 
Thus in broken, hilly ground the thickness of ice 
and the rate of flow would vary from place to place, and 
unequal erosion of its bed would follow as a natural 
course. Not infrequently prominent obstructions 



BASINS 



3°3 



rose in its path, and in front of these deflection- 
basins were eroded, which usually extend in a direc- 
tion at right angles to the trend of the ice- flow. If 
the obstruction were an isolated hill or mountain the 
hollow often assumed a horse-shoe shape, encircling 
the base of the hill. Much morainic ddbris was usu- 
ally accumulated in the rear by such an obstruction, 
so as to form a long, sloping " tail." Again, valleys 
which have chanced to coincide in direction with the 
ice-flow not infrequently show a succession of two 
or more constriction-basins. In flat lands of tolera- 
bly even surface, however, deflection- and constric- 
tion-basins are wanting, the great majority of the 
lakes being drawn out in the direction of ice-flow. 
Although, owing to the presence of glacial and other 
superficial accumulations, we cannot always be sure 
whether such lakes rest wholly in rock-basins or not, 
there can be no doubt that they owe their origin to 
glacial action, partly to erosion and partly to accu- 
mulation. In the low grounds of Lewis (Outer He- 
brides) the multitudinous lakes almost invariably tend 
to assume a linear direction, and by far the larger 
number are arranged along one or other of two lines, 
which strike as nearly as maybe N.W. and S.E., and 
N.E. and S.W. respectively. Not infrequently one 
and the same lake shows both lines of direction, one 
portion of the water trending at right angles to the 
other. Nearly all the longest and most considerable 
lakes range from S.E. to N.W. This is the direction 
of glaciation, and the lakes having this particular 



304 EARTH SCULPTURE 

trend rest sometimes in true rock-basins, sometimes 
in hollows between parallel banks formed wholly of 
glacial deposits, or partly of these and solid rock. 
The north-east and south-west lakes, on the other 
hand, are drawn out more or less at right angles to 
the path of the old ice-flow. They follow precisely the 
line of "strike " (or general direction of the outcrop- 
ping ledges or reefs of gneiss) ; when this direction 
changes there is a corresponding change in the trend 
of the lakes. Thus in places where the strike is east 
and west we have east and west lakes, which wheel 
round to south-west as soon as the strike shifts to 
that direction. In preglacial times the low-lying 
tracts of Lewis were in many places traversed by a 
series of rough ridges and interrupted escarpments, 
with intervening hollows corresponding to outcrops 
of the harder and the less resisting beds of gneiss. 
The dip of the rocks being generally south-east, the 
escarpments naturally faced the north-west. The 
inland ice, which subsequently overflowed this region 
from south-east to north-west, then advanced against 
the dip-slopes of the gneissose rocks, which were 
ground bare, while bottom-moraine was here and there 
deposited in front of the cliffs, knolls, and rocky ledges 
and ridges formed by the outcrops of the harder 
beds. Hence, when the ice finally disappeared, the 
hollows lying between parallel rock-ridges and escarp- 
ments were unequally coated with bottom-moraine, 
and an abundant series of longer and shorter troughs 
were thus prepared for the reception of water. The 



BASINS 305 

north-east and south-west lakes are consequently bar- 
rier-lakes, dammed up wholly by boulder-clay or with 
rock and boulder-clay together. The manner in which 
the two groups of lakes now described frequently 
unite offers no difficulty. In many places the old 
strike-ridges have been cut across by the ice at right 
angles, and a new system of ridges and hollows has 
resulted. And it is not surprising, therefore, to find 
that not only lakes but also streams exhibit both 
directions, now trending north-west and south-east, 
and then turning sharply off at right angles to the 
course previously followed. 

When we leave the highly abraded and ice-worn 
regions of roches moutonndes — the lands of multitu- 
dinous lakes and lakelets — we eventually enter upon 
tracts over which glacial accumulation has been in 
excess, of erosion. Here lakes become much less 
numerous, and are met with only at intervals. Most 
of them extend over shallow depressions in the surface 
of the old ground-moraines, but a few occupy rock- 
hollows ground out in front of prominent obstruc- 
tions. Lakes of the former kind were formerly much 
more plentiful, but owing to their limited depths 
many have been silted up, and are now replaced by 
alluvial flats. The deeper deflection-basins, on the 
other hand, have been more persistent as lakes, but 
they are comparatively few in number. Passing still 
farther outwards, and leaving behind the gently un- 
dulating and rolling plains, throughout which ground- 
moraine forms the dominant deposit at the surface, 



3o6 EARTH SCULPTURE 

we reach at last the paysage inorainique, with its tu- 
multuous hills, knolls, ridges, and embankments, and 
find ourselves once more in a region of lakes, or 
rather of lakelets, pools, and marshes. Among the 
most conspicuous examples of such a region is the 
paysage morainique of the last great Baltic glacier, 
extending from west to east through East Holstein, 
Mecklenburg-Strelitz, Uckermark, Neumark, South- 
ern Pomerania, and the higher parts of West and 
East Prussia. Another well known region of similar 
character is the corresponding lake-dappled paysage 
morainique of North America, which embraces such 
vast tracts in the Northern States of the Union. 

4. Ice-Bai''rier Basins. In existing glacier-regions 
ice-dammed lakes now and again appear. Of these 
the Marjelen See on the Aletsch glacier may be taken 
as an example. Their origin is simple enough. 
When a glacier advances across the mouth of a trib- 
utary valley, the stream flowing in the latter is 
dammed back, and a lake comes into existence. In 
the Alps lakes of this kind have formed from time to 
time, the sudden bursting of the' ice-dams occasionally 
causing enormous devastation. In our own and other 
formerly glaciated countries the relics of such lakes 
— some of which must have persisted for long periods 
— are of not infrequent occurrence. The well known 
" Parallel-Roads " of Glen Roy are simply the beaches 
of an ice-barrier lake. 

5. Submarine Basins. Here we are not con- 
cerned with the large and small basins that mark the 



BASIJVS 



307 



floor of the great oceanic troughs, all of which are 
doubtless tectonic. The hollows to which we would 
now refer are certain relatively smaller basins occur- 
ring in immediate proximity to the shores of recently 
depressed lands. The regions in which they appear, 
although submerged, form, nevertheless, a continua- 
tion of the continental plateau. The true border of 
the European continent, for example, extends in the 
north-west as far seaward as the loo-fathom line at 
least, and there is good ground for believing that 
within geologically recent times a large part, if not 
the whole, of that now depressed region existed as 
dry land. The sea-lochs of Scotland and the fiords 
of Norway simply occupy old mountain-valleys, while 
the numerous islets lying off those coasts and the 
British Islands themselves were all at one time con- 
nected and joined to the mainland of Europe. The 
basins to which we now call attention form two more 
or less well marked groups. One of these is practi- 
cally confined to the fiords, the other is developed 
chiefly in front of islands that face the fiords. 

Although it is not possible to go into much detail, 
it is nevertheless necessary to indicate the character- 
istic features of a typical fiord region. Norway, as 
we have already learned, is an ancient plateau, deeply 
incised and cut up, as it were, into irregular segments. 
These segments vary much in extent and form — some- 
times the surface of the fjeld is flat and undulating, 
elsewhere it is scarped and worn into irregular groups 
and masses of variously shaped mountains and ridges 



3o8 EARTH SCULPTURE 

without any determinate arrangement. The oro- 
graphy is everywhere in strong contrast to that of 
the Alps, with their extended parallel chains and 
longitudinal valleys. Not less strong is the contrast 
between the fiord-valleys of Norway and the valleys 
of the Alpine chain. The latter in cross-section are 
commonly V-shaped, while the former are U-shaped. 
Again, fiord - valleys have relatively few lateral 
branches, the opposite being the case with the great 
valleys of the Alps, which are joined by numerous 
tributaries. Were the Alpine lands to be so sub- 
merged as to convert such valleys as the Rhone or 
the Inn into arms of the sea, it is obvious that numer- 
ous broad and long inlets would ramify right and 
left from these arms into the mountains. The fiord- 
valleys of Norway do not branch after that fashion ; 
the hydrographic system of the country, as Professor 
Richter well observes, is imperfectly developed. The 
principal channels of erosion are the deep, trench- 
like fiord-valleys, the tributaries which reach these 
from the fjelds or plateaux being relatively insignifi- 
cant. The main stream, flowing through a deep 
mountain-valley, has cut its way down to the level of 
the sea, which it enters at the head of a fiord. Below 
this point, however, few or no side valleys, as a rule, 
break the continuity of the fiord-walls. Numerous 
tributary waters, some of which are hardly less im- 
portant than the head-stream, do indeed pour into the 
fiord, but they have not yet eroded for themselves 
deep trenches. After winding through the plateau- 



BASINS 309 

land in broad and shallow valleys their relatively- 
gentle course is suddenly interrupted, and they at 
once cascade down the precipitous rock-walls to the 
sea. The side valleys that open upon a fiord are 
thus truncated by the steep mountain-wall as abruptly, 
Dr. Richter remarks, as if they had been cut across 
with a knife. 

Mountain-valleys of the V-shaped Alpine type are 
not wanting in the fjeld, but as they are followed in- 
land towards the low water-partings of the plateau 
they soon lose their character and acquire softer 
features. The valleys of the fjeld-lands are for the 
most part broad and open, many lakes being scat- 
tered along the courses of the streams. We are here 
dealing, in fact, with a plateau lake-land, a region in 
which glacial erosion has been in excess of accumula- 
tion. It is through this gently undulating, highly ice- 
worn plateau-land, with its shallow valleys, that the 
profound, chasm-like fiord-valleys have been cut to 
depths of 3000 to 6500 feet. That these enormous 
gorges are the work of erosion is not doubted by 
geologists, but the problem of their origin is never- 
theless complex. Much has been written upon the 
subject, but no one has given a more lucid description 
of the actual facts, or a more intelligible explanation 
of their meaning, than Professor Richter, and him, 
therefore, we shall follow. 

If we admit that a fiord is simply a partially drowned 
land-valley, and that the profound hollow in which it 
lies has been eroded by river action, how is it that the 



3IO EARTH SCULPTURE 

side streams have succeeded in doing so little work? 
Why should the erosion of the main or fiord-valleys 
be so immeasurably in advance of that of the lateral 
valleys ? Obviously there must have been a time 
when the process of valley formation proceeded more 
rapidly along the lines of the present fiords and their 
head-valleys than in the side valleys which open upon 
these from the fjelds. At that time the work of rain 
and running water could not have been carried on 
equally over the whole land, otherwise we should find 
now a completely developed hydrographic system — 
not a plateau intersected by profound chasms, but an 
undulating mountain-land with its regular valleys. 
Nor can we believe that the present distinctive feat- 
ures of fjeld and fiord originated contemporaneously 
under a general ice-sheet. The wild rock-walls of the 
fiords, mostly ice-worn though they be, are not glacial 
features. Ice does not carve out canons. According 
to Dr. Richter, the remarkable contrast between the 
deep valleys of the fiords and the shallow side valleys 
that open upon them from the fjelds — the profound 
erosion in the former, and the arrest of erosion on the 
plateau — admits of only one explanation. While 
rivers and rapid ice-streams, flowing in previously ex- 
cavated valleys, were actively engaged in deepening 
these, the adjacent fjelds were buried under sheets of 
ndvd. At the time the fiords assumed their present 
characteristic features, the snow-line must have been 
depressed below its existing level, and large glaciers, 
preceded by torrential rivers, must eventually have 



BASINS 311 

flowed down the fiords to the submarine bars that 
now appear at or near their entrances. Such condi- 
tions obtained during certain stages of the Glacial 
Period, both before and after the epoch of maximum 
glaciation. While the fiords were being deepened, 
first by rivers and thereafter by large glaciers, the 
fjelds were undergoing effective glacial denudation, 
so that in time their configuration became greatly 
modified. The mountain-ridges with their regular 
hydrographic system, as developed in preglacial 
times, were by and by broken up and replaced by the 
undulating rocky and lake-dappled plateaux which we 
now see. In short, while rivers and glaciers were 
deepening the great valleys and making their walls 
steeper, the intervening mountain-heights were grad- 
ually being reduced and levelled by denudation. Un- 
derneath the firn and ice of the plateau the erosion of 
deep gullies was at a standstill. It was somewhat 
otherwise in the Alps, where the hydrographic system, 
perfectly regular in preglacial times, was only slightly 
modified by subsequent glacial action. Yet even there 
erosion proceeded most rapidly along the chief lines 
of ice-flow. Were the great rock-basins of the prin- 
cipal Alpine valleys pumped dry we should find the 
mouths or openings of the side valleys abruptly trun- 
cated, and their waters cascading suddenly into the 
ice-deepened main valleys. For, as Dr. Wallace has 
shown, it is the present \ak&-surface, not the lake- 
bottom, that represents approximately the level of the 
preglacial valley. In a word, erosion proceeded most 



312 EARTH SCULPTURE 

actively in the main valleys, the bottoms of which 
have been lowered for several hundred feet below the 
bottoms of the side valleys. Precisely the same phe- 
nomena are repeated in Scotland. Were all the 
water to disappear from the Highland lakes and sea- 
lochs, we should find waterfalls and cascades at the 
mouth of every lateral stream and torrent. 

But another marked character of the fiords has yet 
to be mentioned. They are always deeper than the 
sea immediately outside, usually very much deeper. 
Some fiords show only one basin-shaped depression, 
while others may contain a succession of troughs. 
Frequently these basins are confined to the fiord, 
but in many cases they extend for less or greater 
distances beyond the entrance. In their form and 
disposition they are comparable to the great valley- 
basins of the Alps and similarly glaciated mountain 
tracts, and there can be little doubt that they have 
had a like origin. Were Scotland to be elevated so 
far as to run the sea out of her fiords, the latter 
would appear as mountain-valleys, each with one or 
more considerable lakes, in this and other respects 
exactly resembling the Highland glens that drain 
eastward into Loch Ness and the Moray Firth. The 
rock-basins in those glens, like the corresponding 
basins of the Alpine valleys, have often been modi- 
fied by the accumulation of morainic ddbris and river- 
detritus at their lower ends. Many Highland lakes, 
in short, are deeper than they would be were all the 
superficial deposits in the glens to be removed. We 



BASINS 313 

may well believe that the same is most likely to be 
true of the fiord-basins — the lips of the basins may 
in many cases be buried to some depth under mo- 
rainic debris and more recent marine deposits. But 
that they are true rock-basins is shown by the fact 
that in not a few cases the sea-floor at the entrance 
is awash, ice-worn rocks every here and there rising 
to the surface and forming low islets and skerries. 
The fiord-basins in the depressed mountain-valleys 
of Scotland and Norway have obviously been ground 
out by large glaciers in the same way as the valley- 
basins of the Alpine lands. There are many other 
regions which show highly indented coasts, with long 
inlets stretching far inland, but these do not always 
contain basins. The latter only appear in places 
where large glaciers have formerly, existed. Thus 
there are no fiord-basins in the Rias of Northern 
Spain, nor in the inlets of the Istrian and Dalmatian 
coasts, nor in the highly indented coast-lands of Aus- 
tralia and South-east China. But basins are always 
present in the ice-worn sounds of New Zealand, and 
in the true fiords of the higher latitudes of America. 
In a word, fiords are merely the drowned valleys of 
severely glaciated mountain tracts. A very slight de- 
pression of the land or rise of the sea-level would 
convert Loch Maree and Loch Lomond, and the 
great Alpine valleys that open upon the plains of the 
Po, into typical fiords. 

Islands, as everyone knows, are scattered more or 
less abundantly along a fiord-indented coast. Dur- 



314 EARTH SCULPTURE 

ing the stage of maximum glaciation the glaciers, 
advancing beyond the fiords, coalesced in many cases 
to form a general ice-sheet which overflowed those 
islands in whole or in part. It is obvious that the 
steeper islands — those which rose more or less ab- 
ruptly above the general level of the sea-floor — must 
have formed obstacles to the outflow of the mer de 
glace. Some of these mountainous islets were com- 
pletely drowned in ice, while the tops of others soared 
above the level of the ice-sheet as Nunatakkr, only 
their less elevated portions being overwhelmed. On 
the sea-floor, in front of such islands we usually en- 
counter more or less well marked depressions or 
basins, some of which attain a great depth. These 
are well indicated by the Admiralty's charts of our 
Scottish seas. We cannot, of course, tell whether 
those basins are wholly excavated in rock, or whether 
they may not owe some of their depth to unequal 
accumulation of glacial and marine deposits. But 
their form and disposition and the whole configura- 
tion of the sea-floor so exactly recall the aspect of 
the ice-worn low grounds of the Outer Hebrides, 
the rocky coast-lands of North-west Scotland, and the 
fjelds of Norway, that we can hardly doubt that the 
bottom of the Minch and adjacent areas owes its 
characteristic features to glaciation — that the deep 
troughs hugging the shores of the rocky islands that 
face the mainland are deflection-basins, ground out 
by the great mer de glace on its passage into the 
Atlantic. 



CHAPTER XV 

COAST-LINES 

FORM AND GENERAL TREND OF COAST-LINES SMOOTH OR REG- 
ULAR COASTS INFLUENCE OF GEOLOGICAL STRUCTURE ON 

VARIOUS FORMS ASSUMED BY CLIFFS CLIFFS CUT IN BEDDED 

AND IN AMORPHOUS ROCKS SEA-CAVES — FLAT COAST-LINES 

AND COASTAL PLAINS — INDENTED OR IRREGULAR COASTS 

GENERAL TRENDS OF COAST-LINES DETERMINED BY FORM 

OF LAND-SURFACE SUBORDINATE INFLUENCE OF MARINE 

EROSION. 

THE coast-lines of the globe — the varied forms 
they assume and the directions they follow — 
are an interesting study. Wandering alongshore 
and observing the effects of wave-action, we are soon 
convinced that here, as in landward areas, hard rocks 
and strong structures tend to resist erosion, while 
soft rocks and weak structures more readily succumb. 
When we so frequently find the former projecting 
seawards in capes and headlands, while the latter are 
often cut back in bays and inlets, it might almost 
seem as if both the shape and the direction of coast- 
lines had been determined solely by marine action. 
But this cannot be altogether true. If bays and all 
other inlets and arms of the sea were the result of 

315 



3i6 EARTH SCULPTURE 

marine erosion alone, the most highly indented coasts 
should also be the oldest. If not, then they should 
occupy positions peculiarly exposed to the battering 
and undermining of waves and breakers, or they 
should be excavated in the softest and most yielding 
rocks. The very opposite of all this, however, is 
the case. Not only are highly indented coast-lines 
of relatively recent age, but they frequently consist 
of the hardest kinds of rock, and they are, moreover,, 
not subject to wave-action in any greater degree than 
coasts which are smooth and regular. If indenta- 
tions were always due to marine erosion, the sea 
should be still eating its way into the land at the 
head of most fiords, estuaries, and other inlets. In- 
stead of advancing in such places, however, it is more 
frequently receding. Rivers entering the heads of 
estuaries and sea-lochs gradually push their deltas 
outwards. Not only so, but in long, narrow inlets 
and fiords waves and breakers do very little work — 
they are practically powerless. Since such inlets, 
therefore, are neither extended nor widened by the 
sea, they cannot owe their origin to its action. How- 
ever potent an agent of erosion it may be, we cannot 
credit it with the formation of the numerous deep 
indentations of such a coast as that of Norway. In 
point of fact, the general tendency of marine erosion 
is to reduce irregularities — to cut back headlands, to 
silt up intervening bays, and to stretch banks and 
ridges across the mouths of estuaries and other nota- 
ble indentations of the land, so as eventually to shut 



COAST-LINES 317 

these off more or less completely. Hence all coasts 
which can be shown to be of relatively great age have 
a gently sinuous or profusely curved outline. Con- 
versely, as we have indicated, highly indented coasts 
are of recent origin — the sea has not yet had time 
to reduce their irregularities. 

We must distinguish between the form and the 
general trend of a coast-line. The varying shape of 
cliff and low shore is no doubt largely determined by 
the manner in which the rocks yield to the sea, but 
the general direction followed by a coast obviously 
depends on the form of the land. If the latter be 
mountainous, with great valleys opening on the sea, 
the coast-line will usually be more or less deeply 
indented. If, on the other hand, it be a low-lying, 
gently undulating land, there will be a general ab- 
sence of deep and long inlets, although broad and 
shallow bays may be numerous. Such a land may be 
margined by steep cliffs or it may be bordered by low 
plains, or by both. In short, however much the sea 
may modify the form of its coasts, it is evident that 
it has had but a small share in determining their 
direction. The latter obviously depends on the 
position of the sea-level and the shape of the land. 
Hence a very slight elevation or depression of the 
land would in many cases completely change the 
direction of the coast-lines. An elevation of 300 feet, 
for example, would lay dry the bed of the North Sea 
and the English Channel, while an elevation of 600 
feet would not only join the British Islands to the 



3 1 8 EAR TH SCULP TURE 

Continent, but cause the shores of Europe to advance 
some 50 or 60 miles beyond the Outer Hebrides and 
Ireland. 

We shall first, therefore, treat of the various forms 
assumed by coast-lines, and thereafter the causes 
which have determined their general trends will be 
more particularly considered. When we run our eye 
over a map of the world we are struck by the fact 
that in some places the coasts are relatively smooth 
and unbroken, while in other regions they are more 
or less deeply indented. We have thus at least two 
principal types, which we may classify as {a) smooth 
or regular coasts, and {F) indented or irregular coasts. 

Smooth or Regular Coasts. These may be high 
and steep, or low and gently shelving, the one kind 
often alternating with the other. Their chief charac- 
teristic is the absence of prominent inlets. A steep, 
regular coast, as shown upon a small-scale map, has a 
softly undulating or sinuous course, or presents a suc- 
cession of smaller and larger curves. It need hardly 
be said that when such a long line of cliffs is examined 
in detail, many minor irregularities make their ap- 
pearance. In some places the cliffs project boldly 
beyond the average coast-line to form headlands, 
elsewhere they curve backwards, or their continuity 
may be interrupted by more or less numerous creeks, 
gullies, and small inlets, which could only be repre- 
sented upon a map of a very large scale. The cliffs, 
moreover, may vary in form at relatively short inter- 
vals, or they may preserve great uniformity of char- 



COAST-LINES 



319 



acter for long stretches. All such inequalities and 
differences are due to the nature of the rocks and the 
mode of their arrangement. Bedded rocks, for ex- 
ample, owing to the regularity of their joints, tend to 
form cliffs with even faces. If the strata be horizon- 
tal, it is obvious that the cliffs must be vertical, or 
nearly so, since the rocks naturally yield along their 
approximately vertical division-planes. When a slice 
has been detached from the cliff, the new surface ex- 
posed is an even wall of rock. But as the beds 
entering into the formation of such a cliff are likely 
to yield unequally to weathering, the smooth wall of 
rock sooner or later becomes etched and furrowed. 
(Fig. 86.) Now and again, however, owing to the 













I 






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c 


n 


I 






1 


''A- 




(■ 




i 






.r 


. 








' 


fc 






- 


'■■ 


i 


\ 


\ 


'1 


:t 


• 


• 


\ 




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_'' 


V 


■'1 


~ 


-/ 






^^ — — — ===^ 

_ . -. — i . . ■ ■ 




, 


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Fig. 86. Sea-Cliff Cut in Horizontal Strata. 
Jj\ joints. 

nature of the rocks, or to the rapid retreat of the 
cliffs, weathering has not sufficient time to effect any 
marked modification of the surface. When the strata, 
instead of being horizontal, are inclined, and the dip 
is inland, or away from the coast, the joint-planes 
necessarily have an inclination towards the sea, and 
the cliffs naturally slope in the same direction. (Fig. 
87, p. 320.) On the other hand, should the strata 



32° 



EARTH SCULPTURE 



dip seaward, cliffs hewn out of them have a tendency 
to overhang, because the division-planes along which 




Fig. 87. Sea-Cliff Cut in Strata Dipping Inland. 
jj, joints. 

the rocks yield are now inclined away from the shore. 
(Fig. 88.) Cliffs having this structure are in a state 
of unstable equilibrium — the truncated beds being 
apt to slide forward — so that actually overhanging 




Fig. 88. Sea-Cliff Cut in Strata Dipping Seaward. 
j j, joints. 

cliffs of this kind are not often met with. Not infre- 
quently, indeed, when strata dip seaward at a relatively 
low angle they form natural breakwaters, and the 
waves do not succeed in cutting out a cliff. 

In all cases, when the strike of the strata coincides 



COAST-LINES 321 

approximately with the trend of a coast-line — the dip 
being either seaward or landward — the forms assumed 
by cliffs are largely determined by the position of the 
strike-joints. The regularity of a line of cliffs is 
likewise greatly controlled by the position of the dip- 
joints, which, it will be remembered, cut the strike- 
joints at approximately right angles. If the former 
be somewhat wide apart, and not strongly pronounced 
or discontinuous, the sea-wall may run continuously 
for miles without any marked interruptions. On the 
other hand, should the dip-joints be in places more 
numerous and closely set, they will form lines of 
weakness, and thus allow the waves to sap and notch 
the cliff, so that all such cliffs tend to assume rect- 
angular outlines, the faces of the sea-wall and the in- 
dentations that break its continuity being determined 
by the double set of joints. And the same holds 
true in the case of horizontal strata. 

It goes without saying that the cliffs of a regular 
coast are evidence of marine erosion. The sea acts 
like a great horizontal saw, forming rock-shelves and 
terraces that increase in width as the cliffs are under- 
mined and cut back. So effectually has the work 
been done in many cases that at high tide these 
terraces of erosion are completely covered. Fre- 
quently, however, islets, stacks, and low reefs and 
skerries appear — fragments of land which owe their 
preservation to the superior hardness of the rocks at 
the sea-level, or to some peculiarity of structure, such 
as the paucity or absence of joints. Lofty stacks are 



322 EARTH SCULPTURE 

perhaps most commonly met with in the case of 
horizontal or approximately horizontal strata, or of 
gently inclined beds, when the strike coincides with 
the general trend of a sea-wall. But smaller stacks, 
reefs, and skerries are usually most abundant when the 
coast-line cuts across the strike, and the truncated 
rocks differ much as regards durability. Such a 
coast-line is usually very ragged or frayed out. The 
cliffs are often approximately vertical, but usually 
show many narrow and broader indentations, while 
long parallel ranges of reefs, skerries, stacks, and 
islets diversify the surface of the terrace of erosion. 

Of the various forms presented by the projecting 
bastions and towers of a line of cliffs, and by the 
islets and stacks of the sea-shelf, it is not necessary to 
say more than that these necessarily vary with the 
nature of the rocks and the geological structure. In 
the case of horizontal strata they all have a tendency 
to assume pyramidal or conical shapes, and similar 
forms are usually seen in the cliffs of massive struct- 
ureless accumulations like boulder-clay. Stacks built 
up of inclined strata are usually less regular in form. 
With a low dip the truncated beds are necessarily 
unstable, and the tendency to collapse is greater than 
it is in the case of a horizontal arrangement. But 
with a high dip the structure becomes more resisting, 
especially if the beds be thick and massive. When 
the strata are folded we not infrequently find that 
projecting headlands, islets, and stacks coincide with 
synclinal arrangements. In short, it may be said 



COAST-LINES 323 

generally that the geological structures which best 
withstand the action of the eroding agents in mount- 
ainous and inland regions are just those which offer 
the most resistance to the assaults of waves and 
breakers. Finally, it must be borne in mind that 
the action of the sea in the reduction of a steep coast- 
line is always more or less aided and modified by 
other epigene agents. Were it not for the action of 
springs and frost coast-cliffs would often be steeper 
and more abrupt than they generally are, the tendency 
being for cliffs of all kinds of structure to become 
benched backwards. Overhanging and absolutely 
vertical rock-walls are by no means so common as 
one might suppose ; however steep a cliff may be, it 
usually has an inclination seawards. The accompany- 




FiG. 89. Sea-Cliff Cut in Beds Dipping Seaward. 

a (T, cliff-face detemuned by master-joint ; cliff may yield along several joints in succession, as 

at h—b. 

ing diagram, representing strata dipping seawards, 
shows how a cliff may be overhanging or not accord- 
ing as the beds yield in a wholesale fashion along one 
joint-plane, or bed by bed along different joint-planes. 
The cliff-face a — a coincides with a master-joint. It 
is obvious, however, that yielding may take place ir- 



324 EARTH SCULPTURE 

regularly along different joints, and we may have the 
overhanging cliff benched back and replaced by the 
sloping face b — b. 

Massive crystalline igneous rocks yield forms of cliff 
that offer strong contrasts to cliffs excavated in bedded 
strata. Owing to inequalities in their composition, 
texture, and structure, and to the frequent irregularity 
of their joints, they are prone to assume particularly 
rugged, broken, and bizarre forms, amongst which we 
may look in vain for any trace of the rectangular 
outlines so commonly present in the case of bedded 
rocks. The faces of the cliffs are very rarely approxi- 
mately even, but vary indefinitely, the harder and 
more sparingly jointed portions projecting, it may be, 
to form buttresses and bastions, while the softer and 
more shattered portions are eaten away and replaced 
by coves and gullies. Now and again, however, when 
the joints are more regular, as in the columnar struct- 
ure of many basalts, etc., and the approximately rect- 
angular joints of certain granites, mural cliffs may 
appear. The crystalline schists, again, exhibit every 
variety of feature. But inasmuch as their bedding is 
usually more or less highly inclined or contorted, and 
their jointing is irregular, they do not often show the 
rectangular forms that are characteristic of cliffs hewn 
out of sedimentary strata. Their coast-lines are usu- 
ally as steep and rugged as those of massive crystal- 
line rocks, but they present greater variety of forms, 
the alternation of different kinds of schist and the 
highly inclined, curved, or contorted bedding, and ir- 



COAST-LINES 325 

regular joints often giving rise to most complex and 
peculiar features. Rugged stacks and skerries are 
very commonly present when either massive crystal- 
line rocks or schists form the coast-line. 

Of the formation of caves by marine action we have 
already spoken. Caves are not confined to any one 
kind of rock or rock-structure, and naturally vary in 
form and extent with the character and the arrange- 
ment of the masses in which they are excavated. 
When the rocks at the base of a sea-cliff are of un- 
equal durability the undermining action of the waves 
and breakers must result either in the formation of 
caves or in the irregular retreat of the sea-wall. Much 
will depend on the character of the rocks above the 
reach of the tide. Should these be massive and not 
traversed by many joints, the conditions will be fa- 
vourable for the formation of large caves. It is obvi- 
ous, however, that if well marked joints be plentifully 
present the rocks cannot be undermined to any extent 
before collapse takes place. 

We may now very shortly consider the appearances 
presented by flat or gently shelving, regular coast- 
lines. As a rule these are softly sinuous, showing a 
succession of broad, evenly curved bays separated 
usually by low capes and headlands. Shores of this 
character are often bordered by banks of beach- 
gravels and sand-dunes, behind which not infrequently 
appear salt-water or brackish-water lagoons. In the 
absence of the latter we may have a coastal plain 
traversed by parallel series of old beach-gravels and 



326 EARTH SCULPTURE 

sand-dunes. Such coastal plains obviously owe their 
origin to the action of streams and rivers, and are 
typically represented by those great deltas which we 
have referred to in an earlier chapter as examples of 
plains of accumulation. But the material carried by 
rivers to the sea does not always accumulate opposite 
their mouths. Tidal currents often prevent the rapid 
growth of deltas by sweeping much of the material 
away and depositing it alongshore, so as to form 
gradually a far - extended coastal - plain. The low 
plains that fringe the Atlantic shores of the Southern 
States of North America consist in this way of the 
sediment brought down by numerous streams and 
rivers, collected and redistributed by the sea. In- 
deed, of coastal-plains generally it may be said that 
they are either directly or indirectly of fluviatile ori- 
gin. The delta of a great river is the direct product 
of river-action. Immense quantities of alluvial mat- 
ter, however, are swept down to sea, and accumulate 
upon the bottom at no great distance from the shore. 
Should a negative movement of sea-level take place, 
a narrower or broader belt of sea-floor then becomes 
dry land, the new coastal plain having been built up 
chiefly of sediment washed down by streams and 
rivers. Coastal plains are thus not infrequently the re- 
sult of crustal movements. As showing the depend- 
ence of coastal plains upon the activity of rivers. 
Professor Penck has pointed out that such plains are 
invariably absent from coasts to which no considerable 
streams and rivers descend. 



COAST-LINES 327 

In fine, as regards regular coast-lines, we see that 
they are not fixed, but oscillating, retreating in some 
places, advancing elsewhere. Cliffs, stacks, and sker- 
ries show us where the land is losing, and coastal 
plains where it is gaining. Much sediment washed 
down from the land comes to rest in quiet bays, 
and these in time tend to be filled up. We note also 
how detritus derived from cliffs and rocky headlands 
is apt to be swept by tidal currents into the same 
quiet receptacles. Thus, while cliffs retreat, the flat 
shores of adjacent bays often advance, until a definite 
relation between the steep and low coasts has been 
established. When at last the coast-line presents, in 
the words of Reclus, " a series of regular and rhyth- 
mical curves," it may become relatively stable. But 
by the continuous descent of sediment from the land 
and its accumulation along low shores, and by the 
gradual retreat of cliffs elsewhere, complete stability 
is impossible. 

Indented or h-regular Coasts. When we consider 
the surface of the earth's crust as a whole we recog- 
nise two great areas, an oceanic depressed region and 
a continental elevated region, or, shortly, an oceanic 
basin and a continental plateau. The larger land- 
masses are all situated upon, but are nowhere co-ex- 
tensive with, this plateau, considerable portions of 
which are under the sea-level. In regions where 
existing coast-lines approach the margin of the conti- 
nental plateau, they are apt to run for long distances 
in one determinate direction, and, whether the coastal 



328 EARTH SCULPTURE 

land be high or not, to show a gentle sinuosity. 
Their course is seldom interrupted by bold headlands 
or peninsulas, or by long intruding inlets, while fring- 
ing or marginal islands rarely occur. Where, on the 
other hand, the coast-line retires to a great distance 
from the edge of the oceanic basin, its continuity is 
constantly interrupted, and fringing islands usually 
abound. Thus the coast-lines of West Africa owe 
their freedom from deep indentations, their con- 
tinuous direction, and general absence of fringing 
islands, to their approximate coincidence with the 
steep boundary -slopes of the continental plateau. 
Conversely, the irregularities characteristic of the 
coast-lines of North-west Europe, and the corre- 
sponding latitudes of North America, are determined 
by the superficial configuration of the same pla- 
teau, which in those regions is relatively more de- 
pressed. In a word, coast-lines are profusely indented 
or not according as they recede from or approach the 
edge of the continental plateau. Hence all highly 
indented coast-lines are evidence that the land is sink- 
ing, or has recently sunk, the directions of the coast- 
line depending on the form or configuration of the 
submerged land. If the region be devoid of river- 
valleys, as most desert areas are, the coast-line will 
show no prominent indentations. If, on the other 
hand, it be well watered and mountainous, its shores 
will be interrupted by more or less numerous narrow 
inlets running often far into the land, while peninsulas 
and fringing islands will probably abound. The fiord- 



COAST-LINES 329 

coasts of the higher latitudes of both hemispheres are 
typical examples of the kind. Indeed, we may say 
that irregular coasts are dominant in the higher lati- 
tudes, while smooth coasts are more characteristic of 
lower latitudes. Irregular coast-lines, however, are 
by no means restricted to high latitudes, but are met 
with in every zone. They abound in the Mediterra- 
nean : the whole east coast of Asia is more or less 
deeply indented and margined by islands, large and 
small ; Australia, Madagascar, Brazil, the Isthmus of 
Panama, and many other tropical and subtropical 
lands, show in places more or less deeply indented 
coast-lines. So widely distributed, in short, are such 
coast-lines that the present would appear to be a 
period rather of depression than of elevation. It is 
true that in the fiord-coasts we usually meet with evi- 
dence to show that the land has recently risen, but 
much greater uplift would be required to restore 
those regions to their former level. 

Indented or irregular coasts are thus not the result 
of marine erosion. The fiords of high latitudes and 
the narrow inlets of non-glaciated lands are simply 
submerged land-valleys ; the intricate coast-lines of 
such regions have been determined by preceding 
subaerial denudation. The general trend or direction 
of the coasts everywhere, therefore, is the result of 
crustal movements, the actual form or character of 
the coast-line, its regularity or irregularity, depending 
very largely on its position with reference to the true 
margin of the great continental plateau. In all 



330 EARTH SCULPTURE 

regions where the marginal areas of that plateau are 
depressed we find a highly indented seaboard and 
numerous fringing islands. Such is the case, as al- 
ready remarked, in the northern latitudes of North 
America and Europe, and the phenomena there are 
repeated in the corresponding latitudes of South 
America. Again, the manifold irregularities of the 
coasts of South-eastern Asia, and the multitude of 
islands between that continent and Australia and 
New Zealand, are all evidence that the surface of the 
continental plateau in those regions is extensively 
invaded by the sea. On the other hand, where ex- 
isting coasts approach the margin of the plateau, 
they are, upon the whole, more regular, showing few 
or no important indentations or fringing islands. 
The actual margin, however — the zone where conti- 
nental plateau and oceanic basin meet — is somewhat 
unstable and liable to movements of elevation and 
depression. Where the latter kind of movement has 
recently occurred, therefore, inlets and gulfs make their 
appearance, as at Rio Janeiro, on the coast of Brazil. 
Movements in the opposite direction, however, by 
laying bare the crustal shelf of marine erosion and sedi- 
mentation, only produce a flat and regular shore-line. 
In fine, then, when we consider the geographical 
development of our lands and their coast-lines, we 
must admit that crustal movements have played a 
most important rdle. But the inequalities of surface 
resulting from such movements are universally modi- 
fied by denudation and sedimentation. Table-lands 



COAST-LINES 331 

and mountains are gradually demolished, and the 
basins and depressions in the surface of the great 
continental plateau become slowly filled with their 
detritus. Thus inland seas and lakes tend to vanish, 
inlets and estuaries are silted up, and the land in 
places advances seaward. To the action of rain and 
rivers that of the sea is added, so that by the com- 
bined operation of all epigene agents the irregularities 
of coast-lines tend to become reduced. This is best 
seen in regions where the seas are comparatively shal- 
low — where the coast-lines are withdrawn for some 
considerable distance from the edge of the great 
oceanic depression. In such shallow seas sedimenta- 
tion and erosion proceed apace. But when the coast- 
lines are not far removed from that depression, they 
are necessarily washed by deeper waters, and become 
modified chiefly by erosion. 

" Should they preserve that position for a prolonged period of 
time, they will eventually be cut back by the sea. In this way 
a shelf or terrace will be formed, narrow in some places, broader 
in others, according to the resistance offered by the varying 
character of the rocks. But no inlets or fiords can result from 
such action. At most the harder and less readily demolished 
rocks will form headlands, while shallow bays will be scooped 
out of the more yielding masses. In short, between the narrower 
and broader parts of the eroded shelf or terrace a certain pro- 
portion will tend to be preserved. As the shelf is widened sedi- 
mentation will become more and more effective, and in places 
may come to protect the land from further encroachment by the 
sea. All long-established coast-lines thus acquire a character- 
istically sinuous form." 

" To sum up, then," as we have elsewhere remarked, " the 



332 EARTH SCULPTURE 

chief agents concerned in the development of coast-lines are 
crustal movements, sedimentation, and marine erosion. All the 
main trends are the result of elevation and depression. Consid- 
erable geographical changes, however, have been brought about 
by the silting-up of those shallow and sheltered seas which in 
certain regions overflow wide areas of the continental plateau. 
Throughout all the ages, indeed, epigene agents have striven to 
reduce the superficial inequalities of that plateau by levelling 
heights and filling up depressions, and thus, as it were, flattening 
out the land-surface and causing it to extend. The erosive ac- 
tion of the sea, from our present point of view, is of compara- 
tively little importance. It merely adds a few finishing touches 
to the work performed by the other agents of change." 

But if it be true that all the main trends of our coast- 
lines are the result of crustal movements, it is no less 
true that many of the indentations that break the 
continuity of an otherwise regular coast-line are often 
due to the same cause. The general trend of the 
coast-line of South America, for example, from Per- 
nambuco to the mouth of the River Plate, coincides 
with the direction of the continental plateau, and may 
be said, therefore, to have been determined by crustal 
movements. The shores, however, have been greatly 
modified by sedimentation, and to a less extent by 
erosion, while the numerous indentations and islets at 
and near Rio Janeiro are evidence of recent depres- 
sion. In a word, it holds true for all the coast-lines 
of the globe that not only their general direction, but 
their more or less numerous indentations, are the 
result of crustal movements. Estuaries, fiords, and 
inlets generally are merely the seaward prolongations 



COAST-LINES 333 

of valleys and other hollows of the land. The indent- 
ations due to marine erosion are relatively so insig- 
nificant, that they can be rarely expressed upon a map 
of small scale. It is the form of the land that deter- 
mines the character of a coast-line. An indented 
coast-line is the result of depression ; a smooth, flat 
shore with no indentations is more usually, although 
not always, due to elevation or sedimentation. But a 
featureless desert-land, smoothed out by seolian ero- 
sion and accumulation, would necessarily be bounded 
by an even coast-line, whether that coast-line were 
the result of upheaval or depression. Finally, the 
coast-lines of regions which have remained for a long 
time undisturbed by crustal movements tend, as we 
have seen, to assume a special form. Erosion and 
sedimentation in this case combine to produce " a 
series of regular and rhythmical curves." 

We have made no reference to the interesting fact 
that plants and animals play a certain part in the 
formation of coast-lines in some regions. This is 
only conspicuous, however, in tropical and subtropi- 
cal latitudes. The mangrove-tree, for example, which 
flourishes along the margins of low, shelving shores, 
forms dense belts of jungle, which continue to extend 
seaward until the depth becomes too great. Some 
of these jungles attain a width of ten or even of 
twenty miles, and are in places rapidly extending. 
Professor Shaler is inclined to think that on the coast 
of Florida the trees may advance over the sea-floor 
at the rate of twenty to thirty feet in a century. 



334 EARTH SCULPTURE 

The closely set roots and rootlets bring about the 
deposition of sediment, and flotsam and jetsam of all 
kinds become entangled, so that eventually a low 
mole is formed along the swampy shore, which bars 
the escape of rain-water towards the sea, and thus 
marshes capable of supporting fresh-water plants and 
various bushes and trees come into existence. 

In other warm seas coral plays a not unimportant 
part in the formation of new lands. Fringing-reefs, 
barrier-reefs, and atolls are of great interest from 
many points of view, but into the history of their 
formation we need not enter. It is enough to recog- 
nise the fact that shore-lines now and again owe their 
very existence to organic action. 



CHAPTER XVI 

CLASSIFICA TION OF LAND-FORMS 

PLAINS OF ACCUMULATION AND OF EROSION — PLATEAUX OF 

ACCUMULATION AND OF EROSION HILLS AND MOUNTAINS ; 

ORIGINAL OR TECTONIC, AND SUBSEQUENT OR RELICT MOUNT- 
AINS VALLEYS ; ORIGINAL OR TECTONIC, AND SUBSEQUENT 

OR EROSION VALLEYS BASINS COAST-LINES. 

WE have now passed in rapid review the more 
salient and notable features of the land-sur- 
face, and have discussed the several causes of their 
origin. The present chapter may therefore be de- 
voted to the classification of those features, and will 
serve as a general summary of the results arrived at. 

The leading features to be recognised are plains, 
plateaux, hills and mountains, valleys, basins, and 
other hollows and depressions of the surface, and, 
lastly, coast-lines. 

I. Plains. These are areas of approximately flat 
or gently undulating land. It is needless to say, 
however, that plains almost invariably have a general 
slope in one or more directions. This, however, is 
so gentle, as a rule, that it is hardly perceptible. 
They are confined to lowlands ; but now and again, 

335 



336 EARTH SCULPTURE 

in the case of very extensive areas, the surface of a 
plain rises inland so imperceptibly that it may attain 
an elevation eventually of several thousand feet. 
This, however, is exceptional. Elevated fiat lands 
are usually termed plateaux. Two kinds of plains 
are recognised, m'xz., plains of accumulation zx\A plains 
of erosion. A plain of accumulation is built up of 
approximately horizontal deposits, so that the external 
surface is a more or less exact expression of the 
internal geological structure. All such plains tend to 
become modified by epigene action. If the plain be 
at or near a base-level of erosion, rain and running 
water have little effect upon it, but under certain 
conditions the surface may be considerably modified 
by the action of the wind. If the plain be traversed 
by a great river, or margined by the sea or by an 
extensive lake, sand-dunes may invade it more or 
less abundantly. Many coastal plains, indeed, have 
been formed partly by aqueous sedimentation and 
partly by the activity of the wind blowing sand 
before it from the exposed beaches. The higher 
a plain rises above its base-level the more it is sub- 
jected to aqueous erosion, and the more irregular 
and undulating does its surface become, the nature 
of the materials of which it is composed having no 
small influence in determining the character and ex- 
tent of the denudation. Other things being equal, 
a plain consisting chiefly of impervious argillaceous 
deposits is more readily washed down than one built 
up largely of sand, shingle, gravel, and other more 



CLASSIFICATION OF LAND-FORMS 337 

or less porous materials. Many plains of accumu- 
lation are among the richest and most fertile tracts 
in the world, while others (and these are usually the 
most extensive) are relatively infertile, not a few 
being more or less destitute of vegetable covering. 
Among European plains of accumulation may be 
mentioned the French Landes, the far-extending flats 
of the Low Countries, and the grassy Steppes of Hun- 
gary and Russia. The arid wastes of the Aralo- 
Caspian depression and the broad Tundras of Siberia, 
the Prairies of North America, and the Llanos and 
Pampas of South America, are all more or less plains 
of accumulation — their approximately flat or gently 
undulating surface is due directly either to aqueous 
sedimentation or to wind-action, or to both. 

Not infrequently, however, the superficial accumu- 
lations of such tracts are of no great thickness, but 
spread over and conceal old plains of erosion. A 
plain of erosion is distinguished by the fact that its 
surface does not necessarily coincide with the under- 
ground structure. It is only when the plain has 
resulted from the levelling of a series of horizontal 
strata that external form and internal structure can 
agree. In the great majority of cases no such coin- 
cidence occurs. The plains in question may consist 
either of horizontal or slightly inclined and gently 
undulating, or highly folded and contorted, strata, or 
they may be composed largely or wholly of igneous 
or of schistose rocks. They are the base-levels to 
which old land-surfaces have been reduced ; they re- 



338 EARTH SCULPTURE 

present the final stage of a cycle of erosion. Occur- 
ring as they usually do in lowlands, they are liable to 
become covered with alluvial and other deposits, and 
thus at the surface often show as plains of accumula- 
tion. Now and again they have been submerged 
and more or less deeply buried under marine sedi- 
ments, and thus when re-elevated the new-born lands 
present the appearance of plains of accumulation. 
Probably the great majority of the latter are merely 
superimposed on pre-existing plains of erosion. The 
wide low-lying tracts through which the larger rivers 
of the globe reach the sea are often plains of erosion 
more or less covered or concealed under alluvial 
deposits. 

2. Plateaux or Table-Lands. No hard-and-fast line 
can be drawn between plains and plateaux. The 
term plateau, however, is usually applied to any flat 
land of considerable elevation which is separated 
from lowlands by somewhat steep slopes. When' a 
plateau is built up of horizontal beds it is described 
as a plateau of accumulation — external form and inter- 
nal structure coinciding. When such is not the case, 
when the arrangement of the rocks and the general 
shape of the surface do not agree, we have what is 
termed 2l plateau of erosion. In a word, plateaux are 
simply elevated plains. But, standing as they do at 
a higher level, they are necessarily subject to more 
active and intense erosion, and, according to their 
age, are correspondingly more deeply incised and 
abraded. Plateaux of all kinds eventually become 



CLASSIFICA TION OF LAND-FORMS 339 

cut up into segments, and these progressively diminish 
in extent as erosion proceeds. Every table-land, in 
short, tends to acquire an irregular mountainous as- 
pect. As examples of highly eroded plateaux of 
accumulation may be cited the Plateau of the Colo- 
rado, the Uplands of Abyssinia, and the Deccan of 
India. Plateaux of erosion, as might have been 
expected, are far more common, many excellent ex- 
amples occurring in our own continent, such as the 
highly denuded plateaux of Scandinavia and Scotland 
and the plateau of Central France. 

3. Hills and Mountains. Just as we cannot sepa- 
rate plains from plateaux by any hard-and-fast line, so 
we find it impossible to distinguish clearly between 
hills and mountains. In general we may say that the- 
term hill is properly restricted to more or less abrupt 
elevations of less than 1000 ft., all the altitudes ex- 
ceeding this being mountains. The terms, however, 
are loosely used, for in very lofty mountain regions 
eminences considerably above 1000 ft. are spoken of 
as hills, while in low-lying tracts heights of only a few 
hundred feet not infrequently become dignified with 
the name of mountains. It is obvious, in short, that 
just as plains merge into plateaux, so there must be a 
gradual transition from hills into mountains. For 
purposes of classification, therefore, it is not neces- 
sary to distinguish between the latter, and we shall 
treat of them both under the common head of mount- 
ains. From our present point of view, then, a 
mountain is simply a more or less abrupt elevation. 



340 EARTH SCULPTURE 

or somewhat sudden increase in the slope of a land- 
surface, and may be of any height from less than one 
hundred feet upwards. It may also be of any extent, 
and either isolated or more or less closely associated 
with other elevations, forming regular or irregular 
groups or definite ranges. Notwithstanding the 
great differences of elevation, of form, and of ar- 
rangement of hills and mountains, it is obvious that 
all these fall naturally into two divisions, namely, (a) 
elevations which have been formed as such either by 
epigene or by hypogene action, and {6) elevations 
which have been carved out of pre-existing rock- 
masses by epigene action alone. To avoid periphrasis, 
we shall speak of these two kinds of elevations as 
original or tectonic mountains, and subsequent or relict 
mountains, respectively. 

{a) Original or Tectonic Mountains. Under this 
head come many of the most insignificant as well as 
the majority of the greater elevations of the globe. 
Some of these have been piled or heaped up at the 
surface — they have grown into heights by gradual 
accumulation, and may therefore be termed accumu- 
lation-mountains. This group naturally includes all 
volcanic cones and hills, geyser mounds, mud-volca- 
noes, etc. Many of these, no doubt, are mere monti- 
cles and hillocks, but all alike owe their origin to the 
extrusion of materials from below and the accumula- 
tion of these at the surface. Of much less import- 
ance are the eminences formed by the direct action 
of epigene agents, hardly any of which ever reach the 



CLASSIFICA TION OF LAND-FORMS 341 

height and dimensions that are usually associated 
with the term mountain. Nevertheless, they form 
not infrequently conspicuous land-features, and can- 
not be ignored in our classification of land-forms. 
Among them are included morainic and fluvio-glacial 
hills and ridges of every kind, sand-dunes, etc. 

But by far the most important tectonic mountains 
are those which have resulted from the flexuring and 
fracturing of the earth's crust, — deformation-mount- 
ains, as they may be termed. All the great mount- 
ain-ranges of the globe come under this group. 
The majority of these owe their origin essentially to 
tangential pushing and crushing ; they consist for the 
most part of flexed and contorted rocks. Now and 
again, however, we meet with mountain-ranges the 
rocks of which may show no conspicuous folds and 
flexures. Ranges of this kind have been determined 
by series of great parallel fractures and dislocations of 
the crust ; the ranges are, in short, vast rectangular 
blocks of strata which may not otherwise be much 
disturbed. The Alps, the Himalayas, and the Cor- 
dilleras of America are typical examples of deforma- 
tion-mountains composed of highly folded rocks. 
Dislocations are, of course common enough among 
such chains and ranges, but their distinguishing char- 
acter is the folding and contortion — hence they are 
termed folded or flexured mountains. The faulted 
ranges of the Great Basin (North America) are nota- 
ble examples of the other kind of deformation-mount- 
ains. In these ranges the strata are sometimes 



342 EARTH SCULPTURE 

horizontal, or approximately so, but are more usually 
inclined. Folding and flexing may be absent, or only 
partially and locally developed. The characteristic 
features of such ranges are the great faults that 
bound them, and hence they may be spoken of as 
dislocation-mountains. In the same category would 
come the Horste of German geologists. These are 
mountains bounded by dislocations — they project 
above the general level because the rocks surround- 
ing them have been dropped down by faulting. Un- 
der the head of deformation-mountains we may also 
include those gibbosities, or prominent swellings of 
the surface, caused by the intrusion below of masses 
of molten matter. They are typically represented 
by the Henry Mountains of Utah and the Elk 
Mountains of Colorado, and may be termed lacco- 
lith-mountains. 

Of course, all deformation-mountains are more or 
less denuded, some of them to such an extent that 
their original configuration can only be guessed at. 
But since they owe their elevation above adjacent 
lowlands to crustal movements, they are entitled to 
be classed as tectonic mountains. 

ip) Subsequent or Relict Mountains. Mountains 
belonging to this great class frequently form irregular 
groups, — there is often an absence pf arrangement in 
separate parallel or interosculating ridges and ranges 
such as characterises tectonic mountains. This ab- 
sence of alignment or orientation, however, is by no 
means general, and is most characteristic of relict 



CLASSIFICA TION OF LAND-FORMS 343 

mountains which have been carved out of horizontal 
and gently undulating strata, the strike of which is 
constantly changing. When the strike runs persist- 
ently for long distances in one direction, the mount- 
ains in such a region now and again form more or less 
parallel ranges, having the same trend as the strike. 

The direction and to a large extent the shape or 
form of relict mountains are thus mainly determined 
by the geological structure. They are the more salient 
portions of plateaux which are in process of being re- 
duced to some base-level of erosion. Plateaux of 
accumulation are eventually cut up into segments, 
which, progressively diminishing in extent and height, 
form irregular groups of tabular and pyramidal hills 
and mountains. Hills and mountains hewn out of 
plateaux of erosion, on the other hand, not infrequently 
simulate the arrangements that are most characteristic 
of deformation-mountains. Should the strata consist 
of a thick series of relatively soft rocks, with here and 
there interbedded rocks of a less yielding kind, all 
dipping at a moderate angle in one direction, the out- 
crops of the harder rocks eventually come to project 
prominently. We thus have long lines or ranges of 
escarpments, separated from each other by parallel 
hollows. When the strata dip at a high angle, how- 
ever, the outcrops of the harder rocks often form 
series of narrower and broader ridges, rather than 
well-marked escarpments and dip-slopes, but the ridges 
continue to be separated by strike-valleys. Even 
when the rocks of a plateau are highly contorted and 



344 EARTH SCULPTURE 

schistose, they nevertheless sometimes tend to be 
carved into ridges and ranges, marking the outcrops 
of the less readily reduced masses. More frequently, 
however, owing to the direction given to the drainage 
by the original slopes of the surface, or to the uni- 
form character of the rocks, or, it may be, to complex 
geological structure, all trace of any definite linear 
arrangement disappears — parallel ranges and interven- 
ing hollows are replaced by amorphous groups of 
heights and irregularly diverging or radiating valleys. 
This is frequently due to the presence of great masses 
of plutonic rocks, such as granite. Igneous intrusions 
of one kind or another, indeed, often play a not un- 
important rdle in giving variety to the surface of such 
regions. Lastly, we may note that when the flexured 
rocks of a plateau are arranged in symmetrical folds, 
the synclines, by offering greater resistance to denuda- 
tion than the adjacent anticlines, tend to be developed 
into synclinal mountains. 

As examples of tabular and pyramidal relict mount- 
ains we may cite the Red Sandstone Hills of Suther- 
land, Ingleborough in Yorkshire, the picturesque and 
often fantastic hills of Saxon Switzerland, the basalt- 
heights of the Faroe Islands and Iceland, and the 
buttes and mesas of the Colorado Plateau. Through- 
out the Lowlands of Scotland we meet with diversified 
features, all the elevations being of subsequent form- 
ation, or the result of denudation. The Lowlands 
are, in short, a plain of erosion, the surface of which 
has been greatly modified by epigene action. The 



CLASSIFICA TION OF LAND-FORMS 345 

more prominent knolls, hills, heights, and ranges of 
all kinds mark the outcrops of the relatively hard 
rocks, which in most cases are of igneous origin. 
Many of the isolated knolls and abrupt eminences 
are the necks of ancient volcanoes, and these are 
usually scattered irregularly without reference to the 
dip of the surrounding strata. Most of the bolder 
crags and escarpments, however, are formed by the 
outcrops of sheets and beds of basalt, etc. As the 
dip is continually changing, such escarpments face 
almost every point of the compass. When the strike 
is more persistent the outcrops of volcanic and intrus- 
ive rocks often form considerable ranges, such, for 
example, as the Ochils, the Sidlaws, the Pentlands, 
the Bathgate Hills, the Campsie Hills, and others. 
All these heights might be termed escarpment-hills. 
So again the outcrops of the calcareous Mesozoic 
strata of England form still more persistent ranges 
of escarpment-hills, traversing the country from 
N.N.E. to S.S.W. The Moors and Wolds of York- 
shire, the Cotswolds, the Chiltern Hills, and the 
Downs are examples. In all these cases the dip of 
the strata is moderate. In highly eroded regions of 
steeply inclined strata the surface-features are some- 
times regular, showing a succession of parallel mount- 
ain-ranges with intervening hollows. Sometimes, 
however, they are more or less irregular, the hills 
and mountains being grouped together without any 
trace of linear arrangement. The Highlands of 
Scotland to some extent illustrate the former class 



346 EARTH SCULPTURE 

of relict mountains, the general trend of the ranges 
and intervening depressions of certain areas being 
S.W. and N.E. In the Southern Uplands the same 
linear arrangement is occasionally apparent, but 
hardly so marked as in some parts of the Highlands. 
The difference is probably in chief measure due to 
the fact that throughout the Southern Uplands the 
rocks show little variety, while in the Highlands the 
reverse is the case, zones and belts of very different 
kinds of rock alternating. 

The forms assumed by the relict mountains of a 
highly denuded plateau of erosion do not necessarily 
differ from those of similarly constructed tectonic 
mountains. The folded mountains of a region of 
uplift, after long-continued denudation, eventually be- 
come greatly modified, the dominant elevations no 
longer coinciding with anticlinal axes, but with the 
outcrops of the more resisting rock-masses, and now 
and again with synclinal axes. Such highly modified 
tectonic mountains, from a certain point of view, 
might be described as mountains of circumdenudation, 
but it is better to distinguish them. They should be 
recognised as tectonic mountains through all the 
various stages of erosion, until they are reduced to 
their base-level. Should such a plain of erosion be- 
come a plateau, the mountains eventually carved out 
of it might well repeat the forms and the arrange- 
ments of the antecedent tectonic mountains, but they 
would be true relict mountains — the dominant portions 
of a highly degraded plateau. 



CLASSJFICA TION OF LAND-FORMS 347 

4. Valleys. The term valley has various significa- 
tions. Usually we mean by it the hollow through 
which a stream or river flows. But some valleys 
contain no streams ; they are mere elongated depres- 
sions — either narrow or broad, shallow or deep. 
Naturally, however, all depressions in the surface of 
a land which is not rainless tend to be filled or 
traversed by running water. By far the great ma- 
jority of valleys — using the word in its widest mean- 
ing — are either the direct result of erosion, or have 
been greatly modified by it. Nevertheless, not a few 
valleys owe their origin to other causes. In short, 
we can recognise at least two kinds of valleys, viz., 
{a) valleys which have been formed either by hypo- 
gene action or by epigene action other than that of 
running water ; and {E) valleys which are true hollows 
of erosion. These we shall briefly describe as original 
or tectonic valleys, and subsequent or erosion valleys. 

(a) Original or Tectonic Valleys. Of these we 
distinguish two kinds — valleys which owe their origin 
to the irregular accumulation or heaping up of ma- 
terials at the surface, and valleys which are the 
result of crustal deformation. The former class, or 
constructional valleys as they may be termed, are of 
comparatively little importance. They occur some- 
times in volcanic regions as depressions in the surface 
of the various volcanic accumulations, or as hollows 
separating adjacent cones, sheets of lava, or heaps of 
ejecta. Similarly the depression lying between lines 
and ranges of dunes and moraines may be termed 



348 EARTH SCULPTURE 

constructional valleys. Sometimes such valleys trend 
for miles in one and the same direction ; more usually, 
perhaps, they are winding, short, and interrupted. In 
a word, any hollows at the surface produced by the 
irregular distribution of materials, whether by volcanic 
action or by epigene action of any kind, we should 
class as constructional valleys. 

Of much more importance are deformation-valleys. 
Theoretically we may group these as (i) dislocation- 
valleys and (2) synclinal valleys. But not infrequently 
a deformation-valley has been determined partly by 
fracture and partly by flexure, such as the valley of 
the Jordan. Dislocation-valleys may extend for long 
distances between parallel faults, or they may follow 
the line of one great dislocation alone. Valleys of 
this kind are approximately straight or gently curved, 
and are of not infrequent occurrence. The valley of 
Glen App in Ayrshire and the great hollow traversed 
by the Caledonian Canal are good examples. The 
valley of the Rhine between the Vosges and the 
Black Forest is another. Synclinal valleys, as might 
have been expected, are best developed in mountains 
of recent uplift, where the surface-features not in- 
frequently coincide more or less closely with the 
underground rock-structure. Such valleys naturally 
trend in the same general direction as the mountains 
amongst which they occur. 

Original or tectonic valleys of all kinds are, of 
course, liable to modification by erosion. Many con- 
structional valleys, it is true, are dry, and in the 



CLASSIFICATION OF LAND-FORMS 349 

absence of running water may remain for long periods 
comparatively unchanged. But wherever rain falls 
and water flows we look for evidence of erosion. 
Hence, even the most recently formed dislocation 
and synclinal valleys show traces of modification. As 
regards the older dislocation-valleys, so great has 
been the amount of subsequent erosion that the val- 
leys as we now see them have obviously been ex- 
cavated by epigene action. They are simply hollows 
which have been worked out along lines of weakness. 
As such dislocations go down to great but unknown 
depths, they necessarily affect a vast thickness of 
rock. However much, therefore, these rocks may be 
denuded, the fracture remains as a line of weakness, 
and determines the direction of erosion. The surface 
may have been planed down again and again to a 
base-level, but with each re-elevation a valley tends to 
reappear in the same place. Synclinal valleys, on 
the other hand, are far less persistent. When we 
find a river flowing continuously along the bottom of 
a synclinal hollow, we may usually feel assured that 
the hollow is of relatively recent geological age. 
To this, however, there are occasional exceptions. 

{S) Subsequent or Erosion Valleys. If it be some- 
times hard or even impossible to draw a clear line be- 
tween original and relict mountains, it is just as 
difficult to separate tectonic from subsequent val- 
leys. No doubt it is easy enough to distinguish be- 
tween a young anticlinal mountain and any relict 
mountain carved out of a plateau. But even the 



350 EARTH SCULPTURE 

youngest deformation-mountains have sometimes 
been so denuded that they might be classed as relict 
mountains. It is the same with valleys. Dislocation- 
valleys no doubt tend to endure ; they occupy more 
or less permanent lines of weakness. Synclinal val- 
leys, on the other hand, soon become modified. The 
mountains on either side are weakly built, and are 
thus prone to collapse, while the intervening synclinal 
structure offers stronger resistance. The rivers no 
doubt flow at first along structural hollows or syn- 
clinal troughs, but in time the lines of drainage tend 
to become modified ; a river shifts its course as the 
anticlinal mountains are reduced, and the syncline 
ere long ceases to form a valley. It is not surprising, 
therefore, to find that the strike-valleys of a recent 
mountain-uplift often do not coincide with synclinal 
troughs, but are true valleys of erosion. It is just in 
such regions, however, where tectonic valleys are of 
most frequent occurrence. We can have but little 
doubt that all the longitudinal rivers of a recent 
mountain-chain flowed at first in true structural or 
tectonic hollows. Possibly also the transverse valleys 
of such a chain may sometimes have been determined 
by minor folds and fractures. In the main, however, 
they are the result of erosion. 

In ancient plateaux of erosion, composed of highly 
flexed and faulted strata, we not infrequently en- 
counter surface-features which recall those of recent 
mountain-chains. Such plateaux often assume a 
mountainous aspect, and the mountains sometimes 



CLASSIFICATION OF LAND-FORMS 351 

exhibit a more or less well-marked series of long par- 
allel ranges with intervening longitudinal depressions. 
Transverse valleys also can be recognised, but these 
present certain marked contrasts to the transverse 
valleys of a recent mountain-chain. The latter are 
generally arranged at approximately right angles to 
the longitudinal valleys, and are consequently, upon 
the whole, of less importance than these.' In a de- 
nuded plateau of erosion, however, the transverse 
valleys radiate in different directions from the more 
elevated portions of the plateau, cutting persistently 
across the parallel ranges and longitudinal depressions 
at all angles, and forming the highways of the more 
important rivers. It is only occasionally that the 
larger rivers flow in the direction of the strike. In 
short, it becomes obvious that the trend of the larger 
rivers in an ancient plateau of erosion has been de- 
termined by the original slopes of the surface, and 
has only an accidental connection with particular geo- 
logical structures. In the gradual development of 
mountain and valley, however, the varying resistance 
offered by the different kinds of rocks and rock- 
arrangements has naturally come into play. Hence 
we find mountain-ranges tend to be developed along 

' This, however, is only true in a general way, and is most conspicuously the 
case when a mountain-chain is relatively broad. In the Alps, for example, most 
of the larger and longer valleys are longitudinal. In mountain-chains of in- 
considerable width the longitudinal valleys are less broad and more frequently 
interrupted, and the transverse valleys become relatively more important. 
Frequently, indeed, the rivers flowing from the dominant crests of such chains 
follow at first a somewhat zigzag course — now running in longitudinal hollows, 
now crossing the strike — until eventually they become wholly transverse. 



352 EARTH SCULPTURE 

the outcrops of the harder or more durable rocks. 
And thus it is obvious that the intervening parallel 
depressions or longitudinal valleys must as a rule be 
of later origin than the main lines of drainage which 
traverse the strike at all angles. In short, when the 
surface-features of such a denuded plateau are com- 
pared with the aspect presented by the folded mount- 
ains of a recently elevated chain, we find that the 
contrasts are much more striking than the resem- 
blances. In the former the main lines of drainage 
are independent of the geological structure, the larger 
and more prominent valleys radiating in many differ- 
ent directions, and thus traversing the strike at all 
angles. If they sometimes follow the strike it is only 
when that has happened to coincide in direction with 
the slope of the ancient plateau. In general, how- 
ever, the strike-valleys of such a region have been 
worked out by lateral streams. In the case of a 
broad mountain-chain of recent uplift, on the other 
hand, the main lines of drainage — the longer and 
broader valleys — follow the strike, while the narrower 
and shorter transverse valleys open into these pri- 
marily at approximately right angles. In time, how- 
ever, many modifications necessarily occur, and the 
same streams and rivers are found flowing now in 
one direction, and now in another, sometimes follow- 
ing, and at other times crossing, the strike. In a 
word, the valleys that traverse an old plateau are 
wholly the work of erosion, the direction of the prin- 
cipal drainage-lines having been determined by the 



CLASSIFICATION OF LAND-FORMS 353 

surface-slopes of the original plane of denudation, and 
not by the geological structure. The principal valleys 
of a young mountain-chain, on the other hand, coin- 
cided at first with great structural hollows ; and not 
a few still follow the lines of synclinal troughs and 
longitudinal fractures and dislocations. 

If it be true that the valleys of a plateau are the 
work of erosion, this is not less true of the river-val- 
leys of lowland regions. The main direction of the 
drainage in such regions has doubtless been deter- 
mined by the average slope of the original surface, 
and has no necessary connection with the geological 
structure of the underlying rocks. But however in- 
dependent of the general rock-arrangement the aver- 
age direction of the rivers may be, it is obvious that 
their courses have often been profoundly modified by 
the nature and structure of the materials through 
which these courses have been cut. Not only are 
they liable to frequent deflection, but the form and 
character of the valley constantly change as different 
kinds of rock and rock-structures are traversed. A 
river cutting through horizontal strata or igneous 
rocks with well-marked vertical jointing, is usually 
flanked by approximately vertical cliffs. But as the 
valley is widened the cliffs tend to become benched 
backwards or even to be replaced by slopes. So, 
again, courses cut in the direction of the dip, or 
against the dip, may be bounded on either side by 
steep cliffs, which, under the influence of epigene 
action, often become resolved into slopes as the val- 



354 EARTH SCULPTURE 

ley is widened. In all those cases the valley-cliffs 
and slopes on the one side have the same general as- 
pect as those on the other. But when a river cuts its 
way along the strike of moderately inclined strata its 
course assumes a different form. On the one side 
cliffs, and on the other, where the rocks dip into the 
valley, slopes tend to be developed. Again, as a 
river in its journey across a wide tract will necessarily 
traverse rocks and rock-structures of very different 
degrees of durability, its valley will widen or contract 
according as the rocks are more or less readily eroded. 
In one place the river meanders through a plain bor- 
dered by gentle slopes, in another place it hurries 
through a narrow and sometimes approximately 
straight or gently winding gorge, the latter often in- 
dicating the site of former cascades, waterfalls, and 
rapids. In a word, every change in the form and 
character of a valley of erosion is determined by the 
nature of the rocks and rock-arrangements with which 
the river and its assistant agents have to deal. 

Waterfalls frequently mark the outcrops of relat- 
ively hard rock-masses. The Falls of Niagara, for 
example, owe their origin to the intercalation of a 
bed of hard limestone amongst more yielding strata, 
which have a gentle dip upstream. By the constant 
wash of the water the soft shales underlying the lime- 
stone are gradually removed, and the overlying mass, 
losing its support, breaks away from time to time 
along its joint-planes. In this manner the Falls have 
slowly retreated from Queenstown, and the gorge of 



CLASSIFICATION OF LAND-FORMS 355 

Niagara has been formed. The Falls of Clyde are 
due to a precisely similar geological structure, and 
many ravines and gorges in the valleys of our low- 
lands have originated in the same way as the gorge 
of Niagara. 

The occurrence of great waterfalls in a long-estab- 
lished hydrographic system is somewhat anomalous, 
and leads at once to the suspicion that the drainage- 
system has been interfered with. Waterfalls cannot 
be of any great age. Sooner or later they must be 
cut back and replaced by ravines or gorges. Their 
presence, therefore, shows either that the valleys in 
which they occur are throughout of recent age, and 
that the rivers have not yet had time to reduce such 
irregularities, or that the drainage-system, if long 
established, has since been disturbed by some other 
agent than running water. In deformation-mount- 
ains of recent age we naturally expect to meet with 
cascades and waterfalls, for the streams and rivers of 
such a region are relatively young. They have only, 
as it were, commenced the work of erosion. But 
plains and plateaux of erosion which have existed for 
ages as dry land, and in which a complete hydro- 
graphic system has been long established, should 
show no great waterfalls. Yet we find cascades and 
waterfalls more or less abundantly developed in all 
the plains and plateaux of Northern Europe and the 
corresponding latitudes of North America; and most 
of these- lands are of very great antiquity, their main 
lines of drainage having been established for a long 



356 EARTH SCULPTURE 

time. Obviously the hydrographic systems have 
been disturbed, and the disturbing element has been 
glacial action. During the Ice Age the long-estab- 
lished preglacial contours were greatly modified. 
Frequently, indeed, the minor valleys in plateaux and 
plains were completely obliterated, while even the 
main valleys were often choked with ddbris. When 
glacial conditions passed away, and streams and rivers 
again flowed over the land, they could not always 
follow the old lines of drainage continuously, but 
were again and again compelled to leave those and 
to cut out new courses in whole or in part. Hence 
the frequent occurrence of cascades and waterfalls in 
formerly glaciated lands. 

Another cause for the existence of waterfalls in 
long-established hydrographic systems must be sought 
for in crustal disturbances. In general, deformations 
of the crust would seem to have been very gradually 
brought about, so gradually, indeed, that they have 
often had little or no influence upon the courses of 
great rivers. Anticlines slowly developing across a 
river-valley have been sawn through by the river as 
fast as they arose. Dislocations, in like manner, 
would seem to have been very slowly developed. 
Frequently these have traversed a river-valley with- 
out in any way disturbing the drainage, the rate of 
erosion having been equal to that of the displace- 
ment. On the other hand, we know that faulting 
or dislocation may sometimes be rather suddenly 
effected. Thus, a large fault crossing a river-valley 



CLASSIFICATION OF LAND-FORMS 357 

and having its downthrow in the direction in which 
the river is flowing would certainly produce a water- 
fall. Such, indeed, would appear to be the origin of 
the great falls of the Zambesi. 

A volume might be written on the many appear- 
ances presented by subsequent or erosion valleys, but 
it is beyond our purpose to enter into further details. 
It is enough to recognise the fact that the great ma- 
jority of river- valleys have been excavated by the 
rivers themselves. Even the most recent tectonic 
valleys have often been profoundly modified by sub- 
sequent erosion. In all regions, whatever their char- 
acter may be, whether plateaux and plains of erosion 
or accumulation, or true mountains of elevation, the 
streams and rivers are constantly striving to reduce 
the land to their base-level. The main directions or 
lines of erosion are early established ; but in the course 
of time many modifications arise, owing to the work 
of the streams and rivers and of epigene agents gen- 
erally. At first, it may be, the rivers descend by a 
succession of steps or by alternate steep and more 
gentle declivities. Cascades, waterfalls, and rapids, 
and here and there barrier-lakes, may abound. But 
eventually the irregularities are removed and a true 
curve of erosion is produced. Each river has then its 
relatively short torrent-track, and its longer valley- 
and plain-tracks. As erosion proceeds the plain-track 
continues to encroach inland upon the valley-track, 
while the latter eats back into the torrent-track. At 
the same time the entire surface of the land is being 



35 8 EARTH SCULPTURE 

continuously reduced, until at last hills and mount- 
ains gradually disappear, and the whole region is re- 
placed by a plain. The cycle of erosion, however, is 
not often allowed to proceed without interruption. 
Sometimes an upward movement increases the gradi- 
ents, and so in time the revived rivers deepen their 
courses, and " valley within valley " appears. Or the 
whole region may become subject to glaciation, during 
which the preglacial drainage-system may be consid- 
erably modified by erosion here and accumulation 
there. When at last the ice-covering vanishes, lakes, 
rapids, cascades, and waterfalls diversify the water- 
courses. But the removal of these features is only a 
matter of time. By and by all the direct effects of 
glaciation must disappear. Again, long before a 
cycle of aqueous erosion is completed the land may be 
submerged and more or less deeply covered under 
new accumulations. Should re-elevation eventually 
ensue, a new hydrographic system will then come 
into existence, but this may not coincide in any part 
with that of the old buried land-surface. 

In regions where soluble rocks, such as limestone, 
abound, the hydrographic system usually presents 
strong contrasts to those we have just been consider- 
ing. Much of the rainfall finds its way below ground, 
where a complex series of channels is gradually licked 
out, until eventually the whole drainage may become 
subterranean. Usually, however, the drainage is 
partly superficial and partly underground, the rivers 
flowing for longer or shorter distances in ordinary 



CLASSIFICATION OF LAND-FORMS 359 

valleys of erosion, until they suddenly plunge below. 
Sometimes they emerge from their subterranean 
courses again and again ; at other times they never 
reappear at the surface, but discharge their waters on 
the sea-floor. Owing to the frequent collapse of 
tunnels and caverns, the surface of a calcareous region 
is apt to show many irregular depressions, and the 
superficial hydrographic system is necessarily very 
imperfectly developed. 

5. Basins. The large and small depressions of the 
surface, like other superficial features, have been 
formed in various ways. Some are the result of hypo- 
gene action, others owe their origin to epigene . ac- 
tion, while yet others are due to both. Not a few, for 
example, are hollows caused by deformation of the 
crust, and may be termed tectonic basins. Some, 
again, occupy the site of extinct volcanoes, or are due 
in one way or another to volcanic action ; they are 
our volcanic basins. Depressions caused by the re- 
moval of soluble materials from below, as in lime- 
stone countries, may be called dissolution basins ; 
while the terms alluvial and ceolian may well be ap- 
plied to all basins which are the result of fluviatile 
and seolian action respectively. Landslips, etc., by 
obstructing drainage form a series of rock-fall basins, 
while glacial action is responsible for a large class of 
basins, some of which are rock-basins, others barrier- 
basins, and yet others partake of the character of 
both. All these are termed glacial basins. 

Unless they are very capacious and extensive. 



36o EARTH SCULPTURE 

basins soon become obliterated. Erosion and sedi- 
mentation are too active to permit of their prolonged 
duration. Exceptionally, however, tectonic basins 
may long outlive the land-surface upon which they 
first appeared. If the deformation of the crust to 
which they owe their origin be continued, erosion 
and sedimentation may be unable to obliterate them. 
Should the bed of a great lake subside at approxi- 
mately the same rate as alluvial matter accumulates 
upon it, while at the same time the effluent river can- 
not succeed in draining the lake dry, it is obvious 
that the latter may endure for a very long time. 
Sediments reaching a thickness of many thousands 
of feet might come to be deposited in such a lake, 
although the water itself had never been more than 
a few hundred feet in depth. The lake would form 
the base-level for all the surrounding region, the sur- 
face of which, perhaps mountainous to begin with, 
would be gradually lowered, and might pass through 
a complete cycle of erosion before the lake ceased 
to exist. In a word, a great lake or inland sea may 
become the burial-place of the high grounds that sur- 
round it, for it bears the same relation to these as an 
ocean to a continent. 

The great majority of lakes, however, do not oc- 
cupy tectonic basins, and must sooner or later disap- 
pear. Even tectonic basins, the beds of which have 
ceased to subside, must eventually be obliterated. 
As a matter of fact, none of the existing lakes of 
the world can be shown to be of great geological 



CLASSIFICATION OF LAND-FORMS 361 

antiquity. All alike, large and small, are of recent 
age. As regards their geographical distribution, it 
is singular and suggestive that they appear most 
abundantly in glaciated lands, in mountains, plateaux, 
and lowlands alike. None of these can be shown to 
have existed before the Glacial Period, and, with few 
exceptions, all must be attributed to the direct and 
indirect action of flowing ice. The preglacial hydro- 
graphic systems have been disturbed mainly by 
glacial erosion and accumulation. Many of the larger 
basins, however, such as those of Lakes Ladoga and 
Onega in Europe, and Lakes Superior, Michigan, 
and others in North America, are probably to a large 
extent tectonic, and due to warping or deformation 
of the crust. Not a few of the smaller lakes, again, 
occupy hollows caused by the irregular accumulation 
of fluviatile sediments, or by the blocking of streams, 
etc., by rock-falls and landslips, while here and there 
they rest in depressions produced by the dissolution 
and removal of soluble materials. Outside of the 
glaciated areas comparatively few lakes of any kind 
exist, and the most important of these occupy tec- 
tonic and volcanic basins. 

6. Coast-Lines. Two types of coast may be distin- 
guished, namely, regular or smooth, and irregular 
or indented. The former may be high and steep or 
gently shelving, and when expressed upon a map 
show a softly undulating or sinuous course. The 
shape assumed by the coasts themselves is naturally 
determined by the nature of the rock-masses and 



362 EARTH SCULPTURE 

their geological structure, and the manner in which 
they succumb before the action of waves and breakers. 
The coastal configuration is likewise influenced in 
many places by accumulation, for the coast-line is not 
fixed, but continually oscillates, retreating in some 
places, advancing elsewhere. ' Irregular or indented 
coast-lines are typically represented by such regions 
as Norway. Here the continuity of the coast-line is 
repeatedly interrupted by long inlets, while a -multi- 
tude of islands fringe the land. Obviously, the trend 
of such a coast-line is determined by the configura- 
tion of the land ; the long inlets and fiords are merely 
the submerged lower reaches of mountain-valleys. 
All highly indented coasts, indeed, are evidence that 
the land is either sinking now or has recently sunk. 

In general, it may be said that the average trend 
and configuration of the coast-lines of the globe are 
determined by the position of the continents in rela- 
tion to the great oceanic depression. The former 
are nowhere co-extensive with what is known as the 
continental plateau, considerable areas of which are 
below the sea-level. When the coast-lines approach 
the margin of that plateau, they generally continue 
for long distances in one particular direction, are 
rarely much indented, and show few or no fringing 
islands. Conversely, when they recede from the 
edge of the plateau, their trend becomes irregular, 
following now one direction, now another, numerous 
inlets appear, and marginal islands usually abound. 
Indented or irregular coasts are not the result of 



CLASSIFICATION OF LAND-FORMS 363 

marine erosion. Fiords, rias, and other indentations 
are simply submerged valleys. The intricate coast- 
lines of North-west Europe, of Greece and other 
parts of the Mediterranean lands, of Alaska, and 
many other regions have been determined by anteced- 
ent subaerial erosion. 



CHAPTER XVII 

CONCLUSION 

THE STUDY OF THE STRUCTURE AND FORMATION OF SURFACE- 
FEATURES PRACTICALLY INVOLVES THAT OF THE EVOLUTION 
OF THE LAND. 

IN the preceding chapters we have been inquiring 
into the origin of surface-features, and have come 
to the general conclusion that these cannot be ac- 
counted for without some knowledge of geological 
structure. We have learned that the crust of the 
earth has experienced many changes — rocks have 
been tilted, compressed, folded, fractured, and dis- 
placed. In some places elevation, in other places 
depression, has taken place, or both kinds of move- 
ment have affected the same area at different times. 
The crust has further been disturbed in many regions 
by vast intrusions of molten matter ; while frequently 
volcanic action has cumbered the surface with lava 
and fragmental ejecta. It might seem, therefore, as 
if the varied configuration of our lands — mountain 
and valley, height and hollow — might be largely if 
not exclusively due to subterranean action. But the 
study of geological structure has shown us that enorm- 

364 



CONCL USION 365 

ous masses of material have been removed from the 
land-surface, and that however much that surface 
may have been influenced by crustal disturbance, yet 
its varied features, as a rule, owe their origin directly 
to denudation. Great mountain-chains have, indeed, 
been upheaved from time to time, fractures and dis- 
placements have again and again taken place ; but 
even the youngest mountains have been so modified 
by the various epigene agents of change that fre- 
quently their original configuration has been almost 
completely destroyed. Earth sculpture, in a word, 
is everywhere conspicuous, and in regions which 
have remained for long ages undisturbed by subter- 
ranean action the latter has had only an indirect 
influence in determining the form of the surface. 
All the great ranges of tectonic mountains are of 
relatively recent age. Time has not yet sufficed for 
their complete reduction. On the other hand, the 
mountains that were upheaved in the earlier stages 
of the world's history have been either completely 
remodelled or entirely demolished. If elevations 
still often mark the sites of the chains and ranges of 
Palaeozoic times, their internal geological structure yet 
shows that they are no longer tectonic but relict 
mountains. In short, we see that epigene agents 
are constantly endeavouring to remove the irregular- 
ities which result from crustal disturbance. Eleva- 
tions are gradually lowered, and sunken areas filled 
up. But the process of levelling the land is not in- 
frequently interrupted by renewed crustal movements. 



366 EARTH SCULPTURE 

No sooner, however, do fresh elevations appear than 

the cycle of erosion begins again. 

" The hills are shadows, and they flow 
From form to form, and nothing stands." 

Although, as a rule, it is not hard to prove that 
certain surface-features owe their origin to erosion, it 
is often very difficult, or even impossible, to follow 
out the whole process — to trace the various stages in 
the evolution of surface-features. Pyramidal mount- 
ains composed of horizontally arranged beds are 
obviously relict mountains ; they have been carved 
out of horizontal strata. That much anyone can see, 
and for the student of physical geography it is 
enough, perhaps, to be able to distinguish such 
mountains from those of a different build. But a 
geologist cannot be content with this : he will en- 
deavour to trace out the whole history of the process. 
He will ascertain, if he can, the age of the strata, and 
the conditions under which they were accumulated, 
and subsequently elevated and eroded. It is the 
story of the evolution or development of the land and 
its surface-features that he will strive to unfold. In 
some cases the evidence is so simple, full, and clear, 
that its meaning can hardly escape him. More fre- 
quently, however, it is complicated, incomplete, and 
hard to read. We may have no doubt whatever that 
the various surface-features of the region we are ex- 
amining owe their origin to denudation ; but we shall 
often experience great difficulty in discovering the 
successive stages through which the land must have 



CONCLUSION 367 

passed before it assumed its present configuration. 
In this volume we have confined attention very much 
to the simple part of the subject, and have tried to 
show what kinds of features are due to hypogene and 
epigene action respectively. Incidentally, however, 
reference has been made to the successive geological 
changes which have preceded and led up to existing 
conditions. It is almost impossible, indeed, to con- 
sider the formation of surface-features without at the 
same time inquiring into their geological history. 
And not infrequently we find that the configuration 
of a land is the outcome of a highly involved series of 
changes. To understand the distribution of its hills 
and valleys, its plains and plateaux, and the whole 
adjustment of its hydrographic system, we may have 
to work our way back to a most remote geological 
period. But if it be true that the present cannot be 
understood without a knowledge of the past, it is no 
less true that physical conditions which have long 
passed away can often be realised in the existing 
arrangement of surface-features. This is no more than 
might have been expected ; for if, as we all believe, 
there has been a continuity in geological history, the 
germ of the present must be found in the past, just as 
the past must be revealed in the present, if only we 
have skill to read the record. Evolution, in a word, 
is not less true of the land and its features than of 
the multitudinous tribes of plants and animals that 
clothe and people it. 

This fascinating branch of geology has been fol- 



368 EARTH SCULPTURE 

lowed with much assiduity by many workers in many 
lands. But it is still in its infancy, and much remains 
to be accomplished. We have all learned the lesson 
of denudation. We know that rivers have excavated 
valleys, that the whole land-surface is being gradually 
lowered by the activity of the epigene agents. But 
comparatively few have set themselves the task of 
working out in all its details the history or evolution 
of the varied configuration of particular areas. Yet 
who can look at the map of a well-watered region, a 
land of mountain and glen, of rolling lowlands and 
countless valleys, without a wish to trace out the devel- 
opment of its numberless heights and hollows ? What 
a world of interest must often be concentrated in the 
history of a single river and its affluents ! At what 
time and under what conditions did it first begin to 
flow ? How was its course and those of its tributa- 
ries determined ? Has the hydrographic system ever 
been disturbed ? and if so, to what extent and in what 
manner has it been modified? These and many 
similar questions will come before the investigator, 
and in searching for answers he must often unfold a 
strange and almost romantic history. \ 

Naturally investigation of the kind leads up to the 
larger inquiry — When and how has the land itself 
been developed ? It is matter of common knowledge 
that the lands within a common area are of very 
different age. Some have only recently appeared ; 
others are of vast antiquity. And the older ones can 
always be recognised by the extent to which earth- 



CONCLUSION 369 

sculpture has been carried on. It is obvious, there- 
fore, that a knowledge of the features produced by 
erosion, apart from other geological evidence, must 
often help us to determine the relative antiquity of 
land-surfaces. We do not doubt that when the his- 
tory of the hydrographic systems of the continents 
has been better worked out, when the evolution of 
surface-features has been more closely followed, our 
knowledge of land-development will acquire a pre- 
cision to which it cannot at present lay claim. Geolo- 
gists will then also be better prepared to attack and 
perhaps to solve the largest problem of all — the 
origin of our continental areas and oceanic basins. 
Not that we can expect or desire that students of 
nature should refrain from theorising and speculating 
in that direction until the fuller knowledge we de- 
siderate has been acquired. Theory must often be 
in advance of the evidence. It may be that we are 
already in possession of the truth — that the con- 
tinental plateau and the oceanic depression, as many 
maintain, are primeval wrinkles of the crust. At 
present, however, this view can only be considered 
probable, or, as some would say, possible — a brilliant 
suggestion which seems to explain much that is 
otherwise unintelligible. 

Another question that will obtrude itself when we 
are investigating the origin of surface-features is that 
of time. Surely a very long period would be re- 
quired for the completion of a cycle of erosion, for 
the upheaval of a great mountain-chain and its subse- 



370 EARTH SCULPTURE 

quent resolution to a plain of erosion, for the cutting 
up of a lofty plateau into hill and valley, and its final 
complete degradation. We find it difficult to conceive 
the lapse of time involved in the process, and the 
difficulty is increased when we remember that cycles 
of erosion have frequently been interrupted by long 
pauses, during which the regions involved have been 
submerged, and not only protected from denudation, 
but more or less deeply buried under new accumula- 
tions. Yes, assuredly, we must admit that many long 
ages have passed since the process of land-sculpture 
began. But physicists tell us that we can no longer 
draw unlimited drafts upon the Bank of Time. We 
have no immeasurable and countless aeons to fall back 
upon. Moreover, various estimates of the rate at 
which denudation is now being carried on, based as 
these are on the amount of materials carried seawards 
by rivers, have demonstrated that the demand for 
unlimited time is not justified. Even under existing 
moderate climatic conditions our own land is being 
levelled at a rate that will ensure its ultimate degrad- 
ation within a period not so infinitely remote as 
geologists formerly supposed. In short, the cumula- 
tive effect of small changes is much greater than was 
at first realised. Further, their study of the past has 
taught geologists that the climate of the world has 
changed from time to time. And if so, then the 
rate of denudation must likewise have varied. In 
our own temperate lands we see how slowly erosion 
is effected — our streams and rivers are but seldom 
clouded with much sediment. Even after the lapse 



CONCLUSION 371 

of many years their courses remain apparently un- 
modified. In less temperate lands, however, erosion 
often proceeds apace ; watercourses are deepened 
and widened in an incredibly short time. During 
a tropical storm of rain as much erosion of soil and 
rock and transport of material are effected within a 
limited drainage-area as would tax a British river 
with all its tributaries to accomplish in a year or a 
number of years. Now these islands of ours have 
experienced many vicissitudes — tropical, subtropical, 
and arctic conditions have formerly obtained here — 
and we need not doubt, therefore, that the present 
rate of denudation has often been exceeded in the 
past. When streams and rivers began their work of 
erosion in the British area, it is probable that the 
climatic conditions were more favourable for that 
work than is now the case. In a word, although the 
work performed by geological agents of change has 
been the same in kind, it has necessarily varied in 
degree from time to time. The present rate of ero- 
sion in Britain, therefore, can be no infallible index 
to that of the past. But however rapidly denudation 
may have proceeded in former ages, the shaping out 
of our hills and valleys, even under the most favour- 
able conditions, must have been a slow process. 
Nevertheless recent investigations leave little room 
for doubting that the time required for the evolution 
of all the multitudinous forms assumed by the land 
has been exaggerated. The tale told by our relict 
mountains and erosion valleys does not support the 
claim for unnumbered millions of years. 



APPENDIX 

TABLE OF GEOLOGICAL SYSTEMS, AND THEIR PRINCIPAL 

SUBDIVISIONS. 



QUATERNARY OR ( Recent. 
POST-TERTIARY } Pleistocene. 



TERTIARY or 
CAINOZOIC 



SECONDARY OR 
MESOZOIC . 



Pliocene. 

Miocene. 

Oligocene. 

Eocene. 

Cretaceous. 

Danian (not represented in England). 

Senonian (Upper Chalk with Flints). 

Turonian (Middle Chalk). 

Cenomanian (Lower Chalk and Upper Greensand). 

Albian (Gault). 

■ ■ ) /Lower Greensand and Wealden 
beds). 



Urgonian . 
Neocomian 



Jurassic. 

Purbeckian . 
Portlandian . 
Kimeridgian 
Corallian 
Oxfordian . 
Bathonian 



White Jura or Malm of Germany. 



Brown Jura or Dog- 
ger of Germany. 



Bajocian (Inferior Oolite) 
Toarcian (Upper Lias) . 
Liasian (Middle and Lower Lias 
in part) .... 
Sinemurian (Lower Lias in part) 
Hettangian (Infra-Lias) 

Triassic. 

Rhaetic. 

Keuper. 

Mubchelkalk (not represented in England), 

Bunter. 



Black Jura or 

Lias of 

Germany. 



373 



374 



APPENDIX 



PRIMARY OR 
PALEOZOIC 



Permian. 

Zechstein (Magnesian Limestone and Marl Slate). 
Rothliegendes (Red Sandstones, Conglomerates, 
and Breccias). 
Carboniferous. 
Coal Measures. 
Millstone Grit. 

Carboniferous Limestone Series. 
Devonian and Old Red Sandstone. 
Upper. 
Devonian . \ Middle. 
Lower. 



Old Red 

Sandstone. 



Upper. 
Lower. 



Silurian. 

Upper. 

Lower. 
Cambrian. 

Upper. 

Middle. 

Lower. 
Pre-Cambrian or Archsan. 



[Note. — The names of the subdivisions of the various systems given in this 
table are those generally accepted. Many, it will be seen, are of English 
origin ; others are foreign. Beside some of the latter the English equivalents 
(which are still current) are placed within parenthesis. A few German equiva- 
lents are given because reference is made to them in the text.] 



GLOSSARY 

Abrasion : the operation o( wearing away by aqueous or glacial action. 

Acid igneous rocks : rocks which contain a large percentage of silica to a 
small percentage of bases. 

Agglomerate : volcanic fragmental rock, consisting of large angular, sub- 
angular, and roughly rounded blocks, confusedly huddled together. 

Alluvium : a deposit resulting from the action of rivers or of tidal currents. 

Amygdaloidal (Gr. amygdalon, an almond ; eidos, an appearance) ; applied to 
igneous rocks containing vesicular cavities which have become filled, or 
partially filled, with subsequently introduced minerals. The cavities are 
frequently almond-shaped ; the mineral kernels are termed amygdules. 

Anticline (Gr. and, against ; klino, I lean) : a geological structure in which 
strata are inclined in opposite directions from a common axis ; /. e. , in a 
roof-like form. When its axis is vertical, an anticline is symmetrical ; in 
an unsymmetricel anticline the axis is inclined. 

Archaean : synonymous with Pre-Cambrian. See Table of Geological Systems. 

Arenaceous : applied to strata which are largely or wholly composed of sand. 

Argillaceous : applied to rocks composed of clay, or in which a notable pro- 
portion of clay is present. 

Ash, volcanic : the finest-grained materials ejected during volcanic eruptions. 

Basalt : a dark, hemicrystalline, basic igneous rock. 

Base-level of Erosion : that level to which all lands tend to be reduced by 

denudation. A land base-levelled is usually very slightly above the sea-level, 

and shows a gently undulating or approximately flat surface. 
Basic igneous rocks : rocks which contain a large percentage of bases to a 

low percentage of silicic acid. 
Beaches, raised : former sea-margins ; sometimes appear as terraces of gravel, 

sand, etc., sometimes as shelves cut in solid rock ; occur at all levels, from 

a few feet up to several hundred yards above the sea. 

375 



376 GLOSSARY 

Biotite {Biot, French physicist) : a black or dark-green mica ; occurs as a con- 
stituent of many crystalline igneous and schistose rocks. 

Bombs, volcanic : clots of molten lava shot into the air from a volcano ; hav- 
ing a rotatory motion, they acquire circular or elliptical forms, and are often 
vesicular internally, or hollow. 

Bosses : large amorphous masses of crystalline igneous rock which have cooled 
and consolidated at some depth from the surface, and are now exposed by 
denudation. 

Boulder-day : typically, an unstratified clay more or less abundantly charged 
with angular and subangular stones of all shapes and sizes up to large 
blocks ; the bottom or ground-moraine of prehistoric glaciers and ice-sheets. 

Bunter (Ger. bunt, variegated) : one of the subdivisions of the Triassic system ; 
the sandstones of the Bunter are often spotted or mottled. 

Buttes (Fr.) and mesas (Sp.) : names given, in the Territories of the United 
States, to conspicuous and more or less isolated hills and mountains. 
Buttis are usually craggy, precipitous, and irregular in outline ; mesas are 
flat-topped or tabular. 

Cainozoic (Gr. kainos, recent ; zoe, life). See Table of Geological Systems. 
Calciferous : applied to strata which contain carbonate of lime as a binding or 

cementing material ; or to strata among which numerous beds of limestone, 

or other calcareous rocks, occur. 
Calc-sinter (Ger. kalk (calx), lime ; sinter, a stalactite) : a deposit from water 

holding carbonate of lime in solution. 
Cambrian (Cambria or Wales) : name given by Professor Sedgwick to one of 

the Palaeozoic systems which was first carefully studied in Wales. 
Carboniferous : name given to the great coal-bearing system of the Palseozoic 

rocks. 
Chalybeate (L. chalybs, steel) : applied to water impregnated with oxide of 

iron. 
Chlorite (L. chloritis) : a greenish mineral present in some schistose rocks ; often 

occurs in igneous rocks as a product of alteration. 
Clastic (Gr. klastos, broken): applied to rocks composed of fragmental materials. 
Clinkers (Dut. klinker, that which sounds) : the cindery-like masses forming 

the crust of some kinds of lava. 
Concretion : a body formed by irregular aggregation or accretion of mineral 

matter, very often round a nucleus ; may be spherical, elliptical, or quite ir- 
regular and amorphous. Concretionary , formed of or containing concretions. 
Coulee (F.) ; a stream of lava, whether flowing or become solid. 



GLOSSARY 377 

Crag-and-tail : a hill or crag showing an abrupt and often precipitous face on 
one side, and sloping away gradually to the low ground in the opposite 
direction. 

Cretaceous : name given to the great chalk-bearing system of the Mesozoic 
strata. 

Crust of the Earth : the outer portion of the earth which is accessible to 
geological investigation. 

Curve of Erosion : A typical river has its steep mountain-track, its moderate 
valley-track, and its gentle plain-track. In the case of young rivers, the 
change from the one track to the other is often abrupt. In older river- 
courses, such irregularities tend to be more and more reduced — the transi- 
tion from the one track to the other becomes gradual — until eventually the 
course may be represented by a single curve, flattening out as it descends 
from source to mouth. This is the curve of erosion. 



Debacle (F.) : a tumultuous rush of water, sweeping forward rock debris, etc. 

Deflation : the denuding and transporting action of the wind. 

Degradation : the wasting or wearing down of the land by epigene agents. 

Denudation : the laying bare of underlying rocks by the removal of superficial 
matter ; the process by which the earth's surface is broken up and the ma- 
terials carried away. 

Derivative rocks : rocks which have been formed out of the materials of pre- 
existing minerals, rocks, and organic remains. 

Detritus : any accumulation of materials formed by the breaking-up and wear- 
ing-away of minerals and rocks. 

Devonian : name given to one of the Palaeozoic systems ; it is well developed in 
Devonshire. 

Diluvium : name given to all coarse superficial accumulations which were for- 
merly supposed to have resulted from a general deluge ; now employed as a 
general term for all the glacial and fluvio-glacial deposits of the Ice Age. 

Diorite (Gr. dioros, a boundary between) : a crystalline igneous rock, belonging 
to a group intermediate in composition between the basic and acid groups. 

Dogger : one of the subdivisions of the Jurassic system in Germany, etc. 

Dolerite (Gr. doleros, deceptive) : a crystalline basic igneous rock. 

Dolina (It.) : name given to the funnel-shaped cavities which communicate 
with the underground drainage-system in limestone regions. Similar cavi- 
ties are known in this country as sinks and swallow-holes. 



378 GLOSSARY 

Dolomite (Dolomieu, the French geologist) : carbonate of calcium and magne- 
sium ; occurs as a crystallised mineral, and also as a grahular crystalline 
rock (magnesian limestone). 

Drum, Drumlin (Ir. and Gael, druman, the back, a ridge) : a ridge or bank of 
boulder-clay alone, or of "rock "and boulder-clay. Ridges of this kind 
often occur numerously. There seem to be two varieties — (a) long parallel 
ridges or banks, and [b) short lenticular hillocks ; the former usually consist 
of glacial accumulations alone ; the latter not infrequently contain a core or 
nucleus of solid rock, or they may show solid rock at one end and glacial 
materials at the other. 

Dyas (LL. the number two) : name sometimes applied to the Permian system 
with reference to its subdivision into two principal groups. 

Eocene (Gr. eos^ dawn ; kainos^ recent) : see Table of Geological Systems. 

Epigene (Gr. epi, upon ; gennao, I produce) : applied to the action of all the 
geological agents of change operating at or upon the earth's surface ; also 
to all accumulations formed by the action of those agents. 

Erratics : boulders and fragments of rock which have been transported, gener- 
ally by the agency of glaciers or floating ice, and are therefore foreign to 
the places in which they occur. 

Eruptive rocks : massive igneous rocks generally ; properly only those which 
have heen extruded at the surface are truly eruptive ; molten masses which 
have been intruded in the crust, and therefore below the surface, are ir- 
ruptive. 

Eskers (Ir. eiscir, a ridge) : ridges of gravel and sand which appear to have 
been formed in tunnels underneath the great glaciers and ice-sheets of 
former times ; same as the Swedish osar. 



Felspars (Ger. feld, a field ; spath, spar) : a group of minerals, common con- 
stituents of many igneous and schistose rocks. 

Fire-clay : properly a clay suitable for the manufacture of fire-bricks ; in geo- 
logy, is applied to the argillaceous layer underlying most coal-seams, which 
consists generally of some kind of clay, but is not always suitable for fire- 
bricks. 

Fluvio-glacial : applied to sedimentary deposits resulting from the action of 
streams and rivers escaping from a glacier or an ice-sheet. 

Foliated rocks : another name for schist and schistose rocks. See Schist. 

Formation: a series of rocks having some character in common, whether of 
origin, age, or composition ; often applied to a group of strata containing a 



GLOSSARY 379 

well-marked and distinctive assemblage of fossils — a group of subordinate 
importance to a system. 
Fragmental rocks : see Clastic and Derivative. 

Gabbro (It.) : a coarsely crystalline basic igneous rock. 

Geanticline (Gr. ge, the earth ; and F. anticline) : a broad or regional arching 
or bending up of the crust — thus, a geanticline may be composed of strata 
showing all kinds of geological structure. It is simply a bulging or swelling 
up of the crust which affects a wide region. Geosyncline is just the oppo- 
site : it is a wide or broad region of depression, i. e., a sinking of the earth's 
crust as a whole. 

Geysers (Icel.) : eruptive fountains of hot water and steam. 

Giants' kettles : large pot-holes often observed in the deserted beds of old 
glaciers ; they are believed to have been drilled by water descending from 
the surface of the glaciers and setting stones and boulders in rapid rota- 
tion. 

Glacial Period : the deposits of the Ice Age referred to in the text belong for 
the most part to the Pleistocene system. Cold climatic conditions, however, 
had set in before the close of the Pliocene, and were continued into the Re- 
cent period — the last of our snow-fields and glaciers having vanished during 
the formation of some of the youngest raised beaches — a time when Neo- 
lithic man lived in Britain. 

Gneiss (Ger.); one of the more coarsely crystalline schistose rocks. 

Granite (It. granito) : one of the deep-seated plutonic crystalline igneous rocks. 
Granitoid, having the structure of granite. 

Greywacke (Ger. grauwack^) : a sedimentary rock, somewhat metamorphosed; 
common in the Palaeozoic systems. 

Grit : generally a coarse-grained arenaceous rock ; the harder kinds are used 
for grindstones. 

Ground-moraine : the rock-rubbish formed by the grinding action of glaciers 
and ice-sheets. 

Gypsum (Gr. gypsos, chalk) : a crystalline mineral composed of sulphate of 
lime. 

Hade : a miner's term for the inclination or deviation of a lode or fault from 

the vertical. 
Haematite (Gr. haimatites, blood-like) : a mineral compound of oxide of 

iron, which yields a blood-red streak when scratched. 
Holocrystalline (Gr. holos, whole ; F. crystalline) : applied to igneous rocks 

composed entirely of crystalline ingredients, as granite. 



38o GLOSSARY 

Hornblende (Ger. horn, horn ; blenden, to dazzle) : a mineral constituent of 
many crystalline igneous and schistose rocks. 

Horste : name given by German geologists to isolated mountains severed by 
dislocations from rock-masses vrith which they were formerly continuous, 
but which have since subsided to a lower level. Kumpfgebirge (lit., rump- 
mountains) is another name for this type of mountain. 

Humous acids : general name for the various acids met with in the humus or 
vegetable mould, and which are derived from the decomposition of organic 
matter. 

Hypogene (Gr. hypo, under ; gennao, I produce) : applied to geological action 
under the earth's surface, and to the products of that action ; opposed to 
Epigene {q. v.). 

Infraglacial : applied to deposits formed and accumulated underneath, or in 

the bottom parts of, glaciers and ice-sheets ; and to the geological action 

of the ice upon rocks over which it flows. 
In situ : in its original situation ; applied to minerals, fossils, and rocks which 

occupy their natural place or position. 
Insolation : the geological action of the sun's heat upon rocks at the surface. 
Intraglacial : applied to rock-fragments embedded in the central and upper 

portions of glaciers and ice-sheets. 
Intrusive rocks : molten rocks which have been injected among pre-existing 

rock-masses. 
Inversion : a geological structure in which strata have been so folded as to be 

turned upside down. 
Isoclinal (Gr. isos, equal ; klino, to lean) : applied to strata folded in a series 

of unsymmetrical anticlines and synclines whose axes all incline in one and 

the same direction. 

Joints : natural division-planes which intersect bedded and amorphous rocks 
of all kinds. In bedded rocks two sets of joints are usually recognisable 
{master-joints), which cut each other at approximately right angles. In 
crystalline igneous and schistose rocks the joints as a rule are somewhat 
irregular ; but to this there are exceptions— as in certain granites, basalts, 
etc. — many of the fine-grained igneous rocks showing prismatic jointing or 
columnar structure. 

Jurassic (from "Jura Mountains) : one of the Mesozoic systems. 

Karnes : ridges and mounds of gravel and sand generally, but now and again 
of rude rock-rubbish. They are of glacial and fluvio-glacial origin, having 
been accumulated, in many cases, along the terminal margins of large 
glaciers and ice-sheets. 



GLOSSARY 381 

Kaolin (Chin, kaoling) : a fine clay resulting from the chemical decomposition 
of felspar. 

Keuper (Ger.) : one of the subdivisions of the Triassic system. 

Laccolith (Gr. lakkos, a cistern ; lithos, stone) : name given to intrusive rocks 
which, when rising from below, have spread out laterally, so as to form 
lenticular masses, thereby lifting the rocks above them so as to form dome- 
shaped swellings at the surface. 

Lapilli (L.) : small stones ejected from volcanoes in eruption. 

Lee-seite (Ger.) : the side of a hill or prominent rock in a glaciated region 
which has been sheltered or protected by its position from the abrading 
action of the ice-flow. The opposite side, exposed to that action, and 
therefore ' ' glaciated, "is termed the Stoss-seite. 

Lias : one of the subdivisions of the Jurassic system. 

Lignite : brown coal, not so highly mineralised as common coal. 

Maars : name given in the Eifel to crater-lakes. 

Macalubas : mud-volcanoes, so called from the well-known Macalube, near 

Girgenti, in Sicily. 
Magma : the molten or plastic material which, when cooled and solidified, 

forms crystalline, hemicrystalline, or glassy igneous rocks. 
Malm : one of the subdivisions of the Jurassic system in Germany, etc. 
Master-joint : see Joints. 
Mesa : see Buttes. 

Mesozoic (Gr. mesos, middle ; zoe, life) : see Table of Geological Systems. 
Metamorphic (Gr. meta, expressing change ; niorphe, form) : applied to rocks 

which have been more or less completely changed in form and structure — 

their constituent materials having been rearranged. 
Mica : a group of minerals, common constituents of many igneous and schistose 

rocks. 
Millstone Grit : one of the subdivisions of the Carboniferous system. 
Miocene (Gr. meion, less ; kainos, recent) : one of the Cainozoic systems. 
Monocline (Gr. monos, single ; klino, to lean) : the simplest kind of fold ; an 

abrupt increase of dip in gently inclined or approximately horizontal strata, 

followed by an equally abrupt return to the original position. 
Moulin (F. , a mill) ; an approximately vertical cavity or shaft worked out in 

a glacier by water descending from the surface through a crevasse. See 

Giants' kettles. 



382 GLOSS AR V 

Necks : plugged-up pipes of volcanic eruption ; the throats of old volcanoes 

which have been laid bare by denudation. 
N6ve (F.) : granular snow ; the condition assumed by snow on its passage into 

glacier-ice. 

Obsidian : a volcanic glassy rock. 

Old Red Sandstone : see Table of Geological Systems. 

Oligocene (Gr. oHgos, little ; kainos, recent) : one of the Cainozoic systems. 

Olivine : a greenish mineral ; a common constituent of many basic igneous 

rocks. 
Oolite (Gr. oSn, an egg ; lithos, stone) : a granular limeLtone, common in tf 

Jurassic system, which on this account used to be known as the OBlitic 

formation. 

Osar (Swedish) : see Eskers. 

Outlier : a detached mass of rock resting upon and surrounded on all sides by 
older rocks. 

Overfold : an overturned or inverted fold ; the axis so inclined that one limb of 
the fold is doubled back under the other. When the axis becomes hori- 
zontal, or nearly so, the fold is recumbent. 

Overthrust : a faulted overfold ; the fold has been dislocated, and one limb. 
pushed over the other along a thrust-plane. 

Palseozoic (Gr. palaios, ancient ; zoe, life) : see Table of Geological Systems. 

Parallel roads : old lake-beaches, seen in Glen Roy (Scottish Highlands) and 
other valleys in its neighbourhood. 

Paysage morainique : a region abundantly covered with terminal moraines. 

Perched blocks : boulders transported by glacier-ice and stranded in pro- 
minent positions. 

Petrography (Gr. peiros, a rock ; grapho, to describe) : the study of rocks — 
Petrology and Lithology. 

Phonolite (Clinkstone) (Gr. phone, sound ; Hthos, stone) : a volcanic crystalline 
rock ; when fresh and compact it has a metallic ring or clink under the 
hammer. 

Pleistocene (Gr. pleisios, most ; kainos, recent) : one of the Post-Tertiary 
systems. 

Pliocene {Gr. pleion, more ; kainos, recent) : one of the Cainozoic systems. 

Plutonic (Pluto, the god of the infernal regions) : applied to deep-seated 
igneous action ; also to deep-seated igneous rocks — those which have 
cooled and consolidated at some depth from the surface. 



GLOSSAR y 383 

Post-Tertiary or Quaternary : the youngest group of systems. See Table. 
Pre-Cambrian or Archaean : the oldest S5'stem of rocks. 
Pumice : any froth-like, foam-like, spongy, porous, or cellular lava. 
Pyroxene (Gr. pur, fire ; xtnos, a guest) : a family of minerals, common con- 
stituents of many crystalline igneous, and of some schistose rocks. 

Quadersandstein : name given in Saxony, Bohemia, and Silesia to the Cre- 
taceous system ; so called because the sandstone of which it is chiefly 
composed is traversed by abundant well-marked vertical joints, that cause 
the rock to weather into square, tabular, and pyramidal hills, and pillar- 
like masses. 

Quaquaversal (L. quaqua, wheresoever ; versus, turned) : applied to strata 
which dip outwards in all directions from a common centre ; dome-shaped 
strata. 

Quartz (Ger.) : common form of native silica ; the most common of all rock- 
forming minerals. 

Quaternary : alternative name for Post-Tertiary. 

Raised beaches : see Beaches. 

Recent period : the latest of the geological systems ; passes gradually into the 
present or existing condition of the earth. 

Reversed faults : in these the hade or inclination of the fault is in the direc- 
tion of upthrow — lower rocks having been pushed over higher rocks. See 
Overthrust and Thrust-plane. 

Revived rivers : when the rivers of a region have succeeded in cutting their 
channels down to the base-level, they have a slight fall and flow sluggishly. 
Should the whole region then be elevated, » hile the direction of its slopes 
remains unchanged, the erosive energy of the rivers is renewed, and they 
are said, therefore, to be revived. 

Rhxtic (from the Rhsetian Alps) : one of the subdivisions of the Triassic system. 

Rhyolite (Gr. rheo, to flow ; lilhos, stone) : an acid volcanic rock. 

Roches moutonn^es : rocks rounded like the back of a sheep ; name given to 
rocks which have been abraded, rounded, and smoothed by glacial action. 

Rothliegendes (Ger.) : one of the subdivisions of the Permian system. 

Rurapfgebirge (Ger.): same as Horste (q. v.). 

Salses (Fr.) : another name for mud-volcanoes or Macalubas (q. v.). 

Schist (Gr. schistos, easily split) : a crystalline rock in which the constituent 

minerals are arranged in rudely alternate parallel layers or folia ; a foliated 

rock. 



334 GLOSS A Ji V 

Scoris (Gr. skoria, dross) : loose fragments of slaggy, cindery lava. 

Screes (Icel. skritha, fallen rocks on a hillside) : <t Westmoreland term for the 
sheets of loose angular stones which gather upon hillsides and at the base of 
cliflfs, etc. 

Shearing : the yielding of a rock to compression, strain, and tension during 
crustal movements, whereby the solid mass is compelled to flow, so that a 
kind of fluxion-structure is developed in it ; frequently under such condi- 
tions dislocation takes place — the rock gives way and one mass is pushed 
over another. 

Sheet : molten matter intruded between bedded rocks. 

Stalactites (Gr. stalaktos, dropping) : the icicle-like pendants hanging from the 
roofs of limestone caves, formed by the drip of water holding carbonate of 
lime in solution. 

Stalag^mites (Or. stalagmos, a dropping) : the calcareous deposit formed upon 
the floor of a cavern by the drip of water from the roof. 

Stoss-seite : see Lee-seite. 

Stri£, g^lacial : scratches, furrows, etc., engraved upon rock-surfaces by glacial 
action. 

Strike : the general direction or run of the outcrops of strata. 

Swallow-holes : see Dolina. 

Syenite (from Syene, Egypt) : a holocrystalline igneous rock of deep-seated 
origin. 

Syncline (Gr. syn, together ; klino, I lean) : a basin or trough-shaped arrange- 
ment of strata ; the strata dip from opposite directions inwards to one 
common axis. When the axis is vertical the syncline is symmetrical ; when 
inclined, unsymvietrical. 

Systems : the larger divisions of strata included under the Palaeozoic, Mesozoic, 
Cainozoic, and Quaternary groups. 

Terrigenous : applied to marine accumulations the materials of which have 
been derived from land ; opposed to abysmal, applied to marine deposits the 
constituents of which have not been so derived. 

Thrust-plane : a Reversed fault {q. v.), the hade or inclination of which 
approaches horizontality ; a common structure in regions of highly flexed 
rocks. 

Till : another name (Scottish) for Boulder-clay {q. v.). 

Tors : the peculiar and often fantastic prominences met with in regions of 
granite which have been long exposed to weathering, as on Dartmoor. The 
kopjes of Mashonaland are an example of the same phenomenon. 



GLOSSARY 385 

Trachyte (Gr. trackys, rough) : a hemicrystalline volcanic rock. 

Travertine : another name for Calc-sinter (g. v.). 

Triassic (Gr. irias, three) : one of the Mesozoic systems. 

Tufa, or calcareous tufa: same as Calc-sinter, Travertine (q. v.). 

Tuff: a volcanic fragmental rock ; usually applied to the finer-grained ejecta of 
volcanic eruptions ; may consist almost entirely of lapilli (g. v.) or of the 
finest sand and dust, or of a mixture of coarse and fine ingredients. 

Unconformable : not conforming in position, or not having the same inclination 
or dip with underlying rocks ; applied to strata which rest upon an eroded 
surface of older rocks ; unconformity or unconformability, the condition of 
not being conformable. 

Underday : the bed upon which a coal-seam rests. 

Uniclinal (L. unus, one ; Gr. klino, to lean) : applied to a series of strata dip- 
ping in one and the same direction. 

Upthrow, upcast : that side of a fault on which the strata lie at a higher level 
than their continuations on the other side of the fault. Normal faults are 
usually described as downthrows ; reversed faults as upthrows. 

'Wady (Ar.) : a ravine or watercourse, dry except in the rainy season. Some 
wadies are perennially dry. 

Weathering: applied to the decomposition, disintegration, and breaking up 
of the superficial parts of rocks under the general action of changes of tem- 
perature, and of wind, rain, frost, etc. 

Zeolites (Gr. zeo, I boil ; lit/ws, stone) : a group of minerals, so called because 
they bubble up in the blowpipe flame ; often met with filling up vesicular 
cavities, etc., in igneous rocks. 



INDEX 



Aar Glacier, 215 

Abrasion by ice, 21b, 241, 248 

Abyssinia, plateaux of, 186, 339 

Accumulation-mountains, 340 

Achumore, 97 

yEolian action, 24, 250 

— basins, 257, 260, 2S4 
African coasts, 328 

— lakes, 162, 279 
Afton Water, 138 
Air volcanoes, 185 
Aix-la-Chapelle, 127 
Akabah, gulf, 159 
Aletsch Glacier. 306 
Alluvial basins, 283 

— terraces, 7, 49, 50 

Alpine glaciers, work done by, 213, 
217 

— lands, glacial phenomena of, 227, 

246, 247 
Alps, the, 93, 109, iig, 208, 214, 216, 
217, 231, 284, 291, 293, 296, 312, 

351 
Amazon, delta, 52 

— river, 7 
Andes, cirques, 292 
Andesite, 174, 201 

Animals, geological action of, 29 
Annan Water, 133 
Anliclinal double-fold, 96 

— hills and mountains, 88,91, 104, HI 

— valleys, 10, 85, 86, 112, 116, 117 
Anticlines, symmetrical, 85, 86, 88, 

90, 105, H2, 115, 117, iig 

— unsyrametrical, 10, 93, 94, 99, 107, 

116, 120 
Antilebanon, 162 
Antrim, basalts, 186, 191 
Appalachian Mountains, 93, 118 



Aqueous rocks, 3, 4, 22 
Arabah Mountains, 256 

— Wady, 159 

Aralo-Caspian depression, 52, 279, 

337 
Ardennes Mountains, 127 
Argillaceous rocks, 21 
Arizona, 53 

Arkansas, aeolian basins of, 284 
Auvergne, caves, 275 

— lakes, 281 
Axial uplift, 129 

Bahia, 257, 284 

Baltic Glacier, 247, 306 

— paysage morainique, 247 
Baltzer, Prof., 221 
Bandaisan, 282 

Barrier lakes, 281, 293, 298, 305 
Basalt, 20, 21, 174 

— caves, 276 

— plains and plateaux, 186 

— sea-cliffs, 324 

— veeathering, 26, 201 

Base-level of erosion, 59, 63, 66, 87, 

140, 143, 149, 226, 343, 360 
Basins, origin and classification, 278, 

279, 359 
Bathgate Hills, 345 
Bavarian Alps, 112 
Beach gravels, 325 
Belgium, carboniferous districts, 127 
Ben AUigin, 147 

— Dearg, 147 

— Eighe, 147 

— Lomond, 142 

— Muich Dhui, 142 

— Nevis, 142 

— Uidhe, 97 



387 



388 



INDEX 



Berendt, Prof. G., 260 

Berlin, 233 

Bertrand, Prof. M., 95 

Bex, 297 

Bingen, 165 

Birnam, i6g 

Black earth, 263 

— Forest, 163 
Blind valleys, 271 
Bohm, Dr., 229 

Bottom moraine, see Ground-moraines 
Boulder-clay, composition of, 233 

— configuration of, 233 

— marine erosion of, 319 
Boulogne, 127 
Bowdoin Glacier, 224 
Bracciano, lake, 281 
Brandenburg, 238 
Brazil, coasts, 329, 330 

— schistose rocks, 6 

— weathered rocks, 205 
Briart, M., 127 
Brick-clay, 21 

British mountains, 93 
Buttes, 59, 344, 376 

Cairngorm Mountains, 290 
Caithness pyramidal hills, 71 
Calcareous rocks, 208 
Caldeiraos, 257 
Caledonian Canal, 144 
Californian lava-caves, 275 
Cambusnethan, 167 
Canada, schistose rocks, 6 

— lakes, 301 

Canary Islands, lava-caves, 275 

Canisp, 71 

Canons of Colorado, 53, 66 

Cape Blanco, 257 

Cape Bojador, 257 

" Capture" by streams, 108, 122, 131, 

138, 144, 148 
Carinthia, Karst-regions, 271 
Carn Chois, 146 
Carpathian Mountains, 115 
Cascades, see Waterfalls 
Caucasus, 119 
Caverns, 31, 209, 269, 272-277, 282, 

325 
Cevennes, 208 
Chalk, 22 

— escarpments, 83, 84, 345 
Chamberlin, Prof., 220, 223 



Changes of sea-level, 12, 13 
Chemical action of rain, 25 

— of underground water, 30, 267 
Chilian Andes, 292 

Chiltern Hills, 345 
China, dust deposits, 261 
Choflat, P., 116 
Cinder cones, 181, 182 
Circumdenudation mountains, 132, 

MS, 147, 193, 204, 346 
Cirque basins, 287 

— lakes, 286 

— valleys, 70, 290 
Classification of land-forms, 335 
Clermont, 275 

Cliffs, river-, 6t, 68, 72, 76, 353 

— sea-, 71, 319 

— undercut by wind-action, 24 
Climate, influence of, on denudation,- 

64, 72, 370 
Coal, 4, 23 
Coastal plains, 326 
Coast-lines, general trend, 317, 32S, 

361 
Coasts, indented or irregular, 327 

— smooth or regular, 325 
Colorado Plateau, 344 

— faults of, 156 

— river, 53, 57, 67, 156 
Como, lake, 293, 298 
Concretions, 256 

Cone-in-cone structure of volcanoes, 

183 
Conglomerate, 3, 22 
Connel Water, 138 
Constance, lake, 293, 298 
Constriction-basins, 299, 302 
Constructional valleys, 347 
Continental plateaux, 339 
Coral reefs, 334 
Cordilleras, 93, 119 
Cornet, M., 127 
Cornwall, sea-caves, 276 
Corrie, see Cirque. 
Cotswold Hills, 83, 345 
Coulmore, 71 
Crag-and-tail, 242 
Crater lakes, 281 
Cree, river, 133 
Crevasses in glaciers, 216, 218 
Crieff, 192 
Crnstal deformation, 13, 47, 48, 179, 

209, 280, 330 



INDEX 



389 



Crustal movements, influence of, on 
land-surface, 17, 47,99, 157, 158, 
159, 162, 164 

Crystal cellars, 275 

Crystalline schists, origin of, 7 

Curve of erosion, 357, 377 

Cycle of erosion, 65, 125, 140, 148, 
172, 338 

— interrupted, 125, 135, 149 



Dachstein glaciers, 221 
Dana, Prof., 240 
Danube, river, 52, 263 
Darmstadt, 164 
Dead Sea, 159, 162. 279 
Deccan Plateau, 186, 339 
Decomposition of rocks, 25-30 
Dee, river, 133, 141 
Deflation, 24, 250 
Deflection-basins, 297, 303, 314 
Deformation, crustal, 13, 47, 48, 179, 
209, 280, 330 

— mountains, 341 

— valleys, 350 

De Geer, ^aron, 234 
Deltas, 49, 52 

— growth of, 37 

— structure of, 49 

Denmark, thickness of ice-sheet, 233 
Denudation, agents of, 19 

— estimates of rate of, 38, 370 

— evidence of, 12, 13 

— in limestone regions, 270 

— land-forms assumed under, de- 

pendent on various factors, 45 
Depressed areas, 159, 162 
Derivative rocks, 5, 6, 12 
Deserts, 251 

— regular coasts of, 328, 333 
Diablerets, 113 

Dikes, 20, 173, 176, 180, 191, 276 
Diluvial doctrine, 2 
Dip, 8 

— slopes, 73, 77, 254 
Discontinuity of strata, evidence of 

denudation, 14 
Disintegration of rocks, 23-25, 199 
Dislocation mountains, 342 
Dislocations, see Faults. 
Dissolution basins, 282 
Dolinas, 271, 273 
Dolomite mountains, 72 



Dombes, paysage morainique, 300 
Dome-shaped hills, 6 
Dome-shaped strata, erosion of, 74 
Doon, river, 133, 138 
Double-folds, 96, 115 
Downs, 85, 345 
Downthrow side of faults, 155 
Drainage, modifications of, by glacial 

action, 355 
— underground, 31, 268 
Drumlins, 234, 245, 378 
Drummond Castle, 192 
Drums, 234, 245, 378 
Drygalski, Dr., 220 
Dry valleys, 252, 271 
Dunes, 258, 259 
Dust of deserts, 260 
Dutton, Capt., 58, 62, 63, 66, 67, 

158 
Dykes, see Dikes. 



Early views as to origin of land-forms, I 

Earn, valley, 169 

Earthquakes, 164, 267 

East African lakes, 162 

Eifel, 281 

Elevation mountains, 102 

Elk Mountains, 342 

Engadine, 243, 2S4 

English Channel, 317 

Epigene agents, 4, 23 

— general results of their action, 332 

— influence of, in land-sculpture, 46 
Erosion of anticlines, 105 

— of arid regions, 206 

— ■ of calcareous regions, 268 

— of Grand Canon district, 57 

— of horizontal strata, 49 

— of inclined strata, 75 

— of mountains of uplift, 125 

— of volcanic accumulations, 187 

— factors determining results of, 45 
— ■ fluviatile, 35 

— glacial, 215, 287, 293, 300, 311 

— marine, 316 

— rate of, 38, 370 

— valleys of, 347 

— various processes of, 23 
Escarpment mountains, 146 
Escarpments, 73, 76, 79, 82, 84, 88, 

120, 254, 304,343 
Escher, Von, 221 



39° 



INDEX 



Eskers, 245, 249, 378 
Ettrick, river, 13Q 
European ice-sheet, 232 
Evolution of land-forms, 3, 365 

Factors of erosion , 46 
Falls of Clyde, 355 

— Niagara, 254 
Fan-shaped structure, 96 
Faroe Islands, 68, 186, 344 

Faults, bounding Scottish Lowlands, 
167, 168 

— cavities in, 276 

— coal-fields, 95, 127, 155, 166, 167 

— Colorado region, 156 

— connection of volcanoes with, 185 

— East African lakes, 162 

■ — evidence of rock-removal, 15, 158 

— Great Basin, 157 

— influence on surface, 150 

— Jordan Valley, 159 

— normal, 12, 48, 98, 150 

— related to flexures, 152 

— reversed, 94 

■ — Rhine Valley, 163 

Fauna of steppes and tundras, 263 

Felspars, 20, 25, 378 

Felspathic rocks, 20 

Finland, 6 

— glacial erosion, 235, 239 

— lakes, 286, 301 

— moraines, 246 
Fiord basins, 307 
— ■ coasts, 328, 329 
Fissure eruptions, 185, 189 
Fjelds, Norwegian, 308 
Flexures, mountain, 99 

— symmetrical, 118, 119 

— unsymmetrical, 116 
Floods, river, 33 

Fluvio-glacial deposits, 226, 237, 249, 

263 
Fluvio-marine deposits, 49 
Folded mountains, lOi, 341 

— strata, g 

Folding, cause of, 13, 48, 236 
Folds, disrupted, 94 

— influence of, on surface, loi 

— isoclinal, 93, 94 

— symmetrical and unsymmetrical, 

116 

— varieties of, in deeply inclined 

strata, 93 



Fox, Arctic, 264 

Fraas, Dr. E., 112-114 

Fragmental igneous rocks, 4, 179, 

182, 187, 189 
Freiburg, 164 
Frost, action of, 28 
Friih, Dr., 234 



Gabbro, 174 
Galloway, drumlins, 234 
Ganges, river, 52 
Garda, lake, 293 
Gavarni, cirque, 290 
Geneva, lake, 36, 293, 296 
Geological structure, influence of, in 
denudation, 45, 48, 86, 124, 209, 

319 
Germany, cirques, 291 

— glacial deposits, 235, 238, 244-247 

— pay sage morainique and lakes, 286- 
Geysers, 185 

Giant's Causeway, 21 
Gibraltar, 208 

Gilbert, G. K., 159, 176, 284 
Girvan, i58 

Glacial accumulations, 225, 233, 243, 
248, 30T, 305 

— action, land-forms modified by, 

211, 212, 241, 248 

— basins, 285 

— erosion, 215, 220, 224, 235-240, 

248, 287, 292, 298, 303 

— rivers, 215 
Glaciers, Alpine, 213 

— geological action of, 213, 220 

— Norwegian, 214, 217 
Glarus, Canton, 115 
Glassy rocks, 19 
Glasven, 97 
Glenbeg, 97 

Glen Docherty, 145 

— Eunach, 290 

— Garry, 145 

— Lyon, 146 

— Roy, 306 
Glenmore, 141, 142 
Glutton, 264 
Gneiss, 23, 26 
Gorges, origin of, 81 
Grabiinden Alps, 2gi 
Grand Caiion district, 53, 66 
Granite, aeolian erosion of, 252 



INDEX 



391 



Granite, joints in, 200 

— lava-form equivalent of, 174 

— mountains, 175 

— plains, 175 

— presence of, at surface, evidence 

of denudation, 16, 174 

— weathering of, 26, 20 1 
Granitoid rocks, weathering of, 206 
Gravel-and-sand rocks, 21 

Grgat Basin ranges, 15^, 341 
Greenland, ice of, 214, 220, 224 
(ireen River, 8g, 90 
Grindelwald Glacier, 222 
Ground-moraines, 214 

— Alpine, 228 

— source, 220, 228, 233 
— ■ superficial form, 244 
Gumbel, Dr., 113 
Gypsum, 23, 267, 268 



Hallstadter See, 2go 
Hawaii, 183, 275 
Hebrides, Inner, 186, igl 

— Outer, 243, 301, 303 

Helm, Prof. A., no, 114, 116, 216, 

221, 231 
Helensburgh, 168 
Helland, A., 215, 238, 292 
Henry Mountains, 177, 342 
Hesse. 165 
Highlands (Scotland), corries, 289 

— geological structure, 140 

— hydrographic system, 141 

— lake basins, 289, 2gr, 293, 301 
— ■ plateau of erosion, 140 

— pyramidal mountains, 71 

— relict mountains, 145 

— thrust-planes, 97 
Hills, 339 ; see Mountains. 
Himalaya, 93, iig, 216, 2g2 
Hinman, R. , 159 

Hohe Tatra, 291 

Hohe Tauem, 223 

Hoist, Dr., 220 

Holstein, 306 

Horizontal strata, 8, 49, 52, 59, 319 

Hornblende, 20 

Hornkees Glacier, 223 

Horste, 170, 342, 380 

Huron, lake, 279 

Hypogene action, 47 

— rocks, 4 



Ice Age, modification of pre-glacial 

drainage-systems during, 355 
Ice-barrier basins, 306 
Iceland, 185, 215, 275, 344 
Ice-sheet of Europe, 232 
Igneous action, land-forms due to, 

173, 193 

— rocks, 4, 19, 26, 197, 200, 324 
Illyria, 271 

Infraglacial accumulations, 213, 220, 

227, 238, 380 
Ingleborough, 344 
Inland ice of Northern Europe, 232, 

300 
Inn Glacier, 229 
Innsbruck, 229 
Insolation, 23, 250 
Insoluble residue of calcareous rocks, 

26 
Infraglacial detritus, 238, 380 
Inversion, 11, 114 
Ireland, sea-caves, 276, 277 
Ironstone, 23 
Isar Glacier, 230 

Islands, fringing or marginal, 328 
Isoclinal folds, 93, 94, 116, 129 
Issyk-Kul, 279 
Italy, volcanic lakes, 281 

Jerboa, 264 
Jessero, lake, 273 
Joints in rocks, 21, 22 

— influence of, in erosion, 60, 72, 

105, 197, 319, 320 
Jordan Valley, 159 
Jostedalsbrae, 290 
Jura Mountains, 115, 208, 297 
Jurassic escarpments, 83 
Justedal Glacier, 215 

Kaisergebirge, 112 
Kaiserstuhl, 164 
Karls-Eisfeld, 221 
Karrenfelder, 208 
Karst regions, 271 
Keilhack, Dr., 234 
Ken, river, 137 
Kettle valleys, 271, 272 
Kinnaird Point, 142 
Konigs See, 290 
Kopjes, 206 
Kurisches Ha£f, 260 



392 



INDEX 



Laccoliths, 176, 342 

Lac d'Aydat, 281 

Lacustrine deposits, 49 

Ladoga, lake, 279, 302, 361 

Lake-basins, irregular depths, 299 

Lake-lands, 301 

Lakes as settling reservoirs, 36 

— formed by river action, 283 

— in cirques, 287 

— in deserts, 257, 260 

— in glaciated lands, 285 

— in limestone regions, 272, 282 

— in moraines, 300 

— in mountain valleys, 292, 299 

— in Scottish Highlands, 312 

— in steppes, 264 

— in tectonic basins, 279 

— in volcanic regions, 281 

— silted up, 7 

— temporary, 273 

— vertical distribution of high-level, 

291 
Lanarkshire, faults in coal-fields of, 

166 
Landes, French, 337 
Land-forms due to glacial action, 241 
Lava, 4, 20 

— caves in and underneath, 274 

— cones of, 182 

— plutonic equivalents of, 174 
Leader, river, 138 
Lebanon, 162 

Lee-seite, 222, 381 

Lemming, 264 

Lewis, 303 

Libyan Desert, 24, 254, 256, 

Lignite, 4, 23 

Limestone, 22 

— Alps, 113 

— underground drainage in, 268 

— weathering of, 207 
Llanos, 337 
Llathach, 147 
Lochaber Mountains, 147 
Loch Ewe, 301 

— Laxford, 301 

— Lochy, 141 

— Lomond, 293, 313 

— Maree, 145, 146, 313 

— Ness, 293, 312 

— Torridon, 147 
Loss, 240, 261 

Lombardy, moraines of, 247, 296 



Longitudinal valleys, 76, 80, 104, 122, 
139, 145, 148, 350 ; see Strike- 
valleys. 

Lothians (Scotland), 245 

Lowland basins, 300 

Lowlands (Scotland), land-forms in, 
344 



Maars, 281 
Macalabas, 185 
Madagascar, 329 
Marjelen See, 306 
Maggiore, lake, 293 
Maiden Pap, 71 
Malvern Hills, 82, 
Mangrove Swamps, 333 
Marble, 22 
Marl, 22 
Marmots, 264 
Mashonaland, 206 
Massive eruptions, 185 
Mauna Loa, 183 
Mazellferner Glacier, 223 
Mecklenburg, 238, 306 
Merse, 137 

Mesas, 59, 67, 344, 376 
Metamorphic rocks, 5 

— presence at surface proves denuda- 

tion, 16 
Mica, 20 

— schist, 23 
Michigan, lake, 279, 361 
Midlands, escarpments of, 84 
Minerals, common rock-forming, 20 
Minto Hill, 190 

Mississippi, river, 7, 38, 52 
Moab, mountains of, 161 
Monadhliath Mountains, 147 
Mongolia, seolian basins, 284 
Monoclinal folds, 54 
Mons, coal-basin, 95 
Monte Somma, 184 
Moor of Rannoch, 142 
Moors, Yorkshire, 345 
Moraines, lateral, 246 

— superficial, 213, 214, 216 

— terminal, 219, 246, 249, 300, 

301 
Morainic lakes, 300 
Moray Firth, 141, 312 
Morven, 71 
Moulins, 217, 381 



INDEX 



393 



Mount Ellen, 178 

— Ellsworth, 178 

— Hillers, 178 

— Holms, 178 

— Pennell, 178 
Mountains, accumulation, 186 

— anticlinal, erosion of, 104 

— circumdenudation, 58, 65, 67, 6g- 

72, 76, 79, 83, 86, 88, 132, 145, 
147, 193, 204, 343. 346 

— classification of, 339 

— contrast between young and old, 

100, 125 

— demolition of, 123, 125 

— escarpment, 146 

— subsequent or relict, 145 

— upheaval, formation of, loi 

— various ages, 92, 93 

— young, relatively unstable, 119 
Mountain-track of rivers, 35, 377 
Mountain-valley basins, 292 
Mud volcanoes, 185 
Mushroom-shaped rocks, 24, 253 
Musk-ox, 264 

Nahr el Asi, 162 
Necks, 189, 191, 345 
Ness, loch, 141 
Neuchatel, lake, 296 
Neumark, 306 
Neve, 227, 290, 310, 382 

— line, 289, 2gi 
Newcastle coal-field, 167 
New Zealand, 216, 292, 330 
Niagara, 354 

Niger, river, 52, 257 

Nile, river, 7 

Nith, river, 133 

Nithsdale, 233 

North America, glacial deposits, 244 

— ice-sheet, 232, 240 

— lakes, 279, 301 

— paysage morainique, 306 
North Sea, 317 
Norway, cirques, 292 

— cirque-valleys, 290 

— fiords, 233, 307 

— glaciers, 214, 217 
Nunatakkr, 220, 227, 233, 314 



Oases, 252 

Obersalzbachkees Glacier, 223 



Obsidian, 20 

Oceanic basin, 328 

Ochil Hills, 88, 345 

Oetzthal, 229 

Old Red Sandstone mountains, 71 

Olivine, 20 

Onega, lake, 279, 302, 361 

Original mountains, 340 

— valleys, 347 

Orkney, sea-caves, 276 

Orontes, river, 162 

Osar, 245 

Outer Hebrides, 243 

Outliers, 84, 382 

Overfolds, 94, 113, 114 

Overthrusts, 94, 115 



Palestine, mountains, 161 

Pampas, 337 

Parallel roads, 306 

Partsch, Prof., 291, 292 

Paysage morainique, 247, 286, 300, 

306 
Peat, 4 
Penck, Prof., 38, 164, 221, 222, 230, 

282, 291, 326 
Pennsylvania, 118 
Pentland Hills, 345 
Permian Basin, Ayrshire, 85 
Pernambuce, 332 
Perth, 192 
Peruvian Andes, 292 
Phonolito, 201 

Piedmont, moraines, 247, 296 
Pitchstone, 20 
Plains, classification of, 335 

— of accumulation, 49, 186, 326, 335 

— of erosion, 127, 128, 136, 142, 186, 

337 
Plain-track of rivers, 35, 377 
Planina, river, 273 
Plants, geological action, 29 
Plateau, basins, 300 

— continental, 327 

— Norwegian, 308 

— Scottish Highlands, 142 

— Southern Uplands, Scotland, 133 
Plateaux, accumulation, 52, 60, 65, 

186, 338, 343 

— classification of, 338 

— denudation of, 60, 65, 131, 132 

137, 141, 147 



394 



INDEX 



Plateaux, direction of drainage in, 
130, 141, 148, 351 

— erosion, 77, 78, I2g 

— surface inclined against dip, 80, 

— surface inclined in direction of dip, 

77 
Plate, river, 332 
Plutonic rocks, 4, 173 

— lava-form equivalents of, 173 

— presence at surface implies denuda- 

tion, 16, 174 
Poland, moraines, 247 
Pomerania, moraines, 306 
Po, river, 7, 37, 52 
Posen, 238 

Powell, Major, 54, 89, 91, 189 
Prairies, 337 

Pre-Cambrian sandstone mountains, 71 
Prehistoric glaciers, 225 
Prussia, glacial deposits, 238, 306 
Pumpelly, Prof., 2B4 
Pyramidal hills and mountains, 58, 65, 

68-72, 194, 344 
Pyrenees, 290, 292 
Pyroxene, 20, 383 



Quadersandstein, 72, 206, 383 
Quarrying, infraglacial, 221 
Quartz, 20 
Quartz-rock, 22 
Queantoweep Valley, 158 
Quinaig, 97 



Rain, action, 25, 32 
Raised beaches, 4g, 277 
Ramsay, Sir A. C, 85, 294 
Raniaka Cave, 275 
Rapids, 81 

Rat, little hamster, 264 
Reclus, E.; 327 
Red Sea, 162 

Regional elevation, 128, 129 
Regular coasts, 318 
Reindeer, 264 
Relict mountains, 342 
Kenevier, Prof., 113, 114 
Reversed faults, 94 
Rhine Valley, 163, 263 
Rhone, delta, 37, 52 

— river, 36 

— valley, 296, 300 



Rias, 313 

Richter, Prof., 308-310 

Richthofen, Baron, 261 

Rio Janeiro, 330 

River cliffs, recession of, 61, 63 

Rivers, change of course, 108 

— direction of, not influenced by faults 

and flexures, 57, 156, 165 

— erosion by, 59 

— flowing in direction of dip, 77, 80 

— flowing in direction of strike, 75 

— geological action of, 34 

— longitudinal, 107 

— older than mountains they traverse, 

46, 57 

— original course, determined by sur- 

face-slope, 56, 74, 77, 104, 120, 

131, 137, 351, 353 

— terraces of, 7, 51 

— underground, 268 

— valleys, eroded by, 350, 357 
Roches moutonne'es, ill, 234, 288, 301, 

383 
Rock basins, 222, 288, 293 
Rock-fall basins, 284 
Rock-falls, 119 

Rock-flexures, infraglacial, 236 
Rock-flour, 215 
Rock-forming minerals, 20 
Rock-removal, evidence of, 13 
Rock-salt, 23, 266 
Rock-shattering, infraglacial, 221 
Rock-shelters, 276 
Rocks, chemical composition of, 20 

— classes of, 3, 4, 19 

— comparative resistance of, to denu- 

dation, 44, 77, 78 

— disintegration of, 23-26, 198 

— porosity of, 21 

— shattered by frost, 29 
Rodgers, Prof., 118 
Rotted rock, 27 
Rubers Law, 136 
Rugen, 234 

Rule Water, 136 
Rumpfgebirge, 170, 380 
Russia, black earth, 263 

— ground-moraines, 238 

— lakes, 286 



Saddlebacks, see Anticlines. 
Sahara, 251 



INDEX 



395 



St. Gall, canton, 115 

Salses, 185 

Salt Lake, Utah, 279 

Sand-blast, natural, 24 

Sand, blowing, 253 

Sand hills, 259, 325 ; see Dunes. 

Sandstone, 4, 21 

Sand wastes, 257 

Santa Marta (Sierra Nevada), 292 

Saxon mountains, 344 

— Switzerland, 72, 206 
Scandinavia, glacial erosion, 235 

— glaciers, 217 

— ice-sheet, 232, 233 

— moraines, 246 

— mountains, 93 

— plateau, 130 
Schists, 6, 20 

— jointing in, 197, 199 

— marine erosion of, 324 

— presence at surface implies denu- 

dation, 16 

— weathering of, 204, 205 
Schleswig-Holstein, 245, 246 
Schortenkopf, 112 

Scoriae, 182, 384 

Scotland, corries and cirque valleys, 
290 

— thickness of ice-sheet, 233 
Scottish Highlands, 6, 141 
Screes, 29, 205, 229, 384 
Sea, caves, 276 

— cliffs, 317 

— floor, subsidence of, 12 

— geological action, 317 

— lochs, 307 
Sedimentary deposits, 4, 6 

— rocks, 22 

— strata, average thickness of, 43 
Sediment of glacial rivers, 214 
Senegal, river, 257 

Severn, river, 83 
Shale, 3, 21 

Shearing of rocks, 48, 95, 99 
Sheets, intrusive, 20, 173, 177 
Shell marl, 4 
Siberia, 52, 264 
Sidlaw Hills, 345 
Sierra el Late. 203 

Sierra Nevada (Santa Marta), 157, 
292 

— (Spain), 292 
Silicious rocks, 21 



Silser See, 284 

Silvaplana See, 284 

Simony, Prof., 22i 

Sinai Peninsula, 256 

Sink-holes, 282 

Slags, 182 

Smean, 71 

Smooth coasts, 318 

Snow, action of melting, 32 

Snow-line in Alps during glacial pe- 
riod, 227 

Soils, waste of, 34 

Somma, Monte, 184 

Sounds of Faroe Islands, 71 

Southern Uplands (Scotland), 129, 
133, 289 

Sowbacks, 245 

Spain, Has of, 313 

Spey, river, 141, 145 

Springs, influence of, in valley-erosion, 
76 

— mineral, 267, 274 

— natural, 31, 105 
Sserir, 256 

Staff a, 21 

Stampflkees Glacier, 214, 223 

Steppes, 263, 337 

— fauna of, 264 
Stinchar, river, 134 
Stonehaven, 168 
Stoss-seite, 222 

Strata, discontinuity of, evidence of 
erosion, 15 

— gently-inclined, denudation of, 73, 

319 

— highly-folded, denudation of, 92, 

322 

— horizontal, denudation of, 49, 319 
Striae, glacial, 288 

Striated stones, 214 

Strike-basins, 304 

Strike-valleys, 76, 80, 131, 352 ; see 

Longitudinal valleys. 
Submarine basins, 306 
Subsequent mountains, 342 

— valleys, 347 
Suess, Prof., 162 
Suilven, 71 

Summit glaciers, 214, 217, 290 
Superior, lake, 279, 361 
Sutherland, mountains, 344 
Swallow-holes, 208 
Sweden, glaciated areas, 239 



395 



INDEX 



Sweden, osar, 245 

Syenite, 174 

Synclinal double-fold, g6 

— hills and mountains, 10, 86-88, 

344 

— valleys, 89, 104-107 
Synclines, symmetrical, 10, 86-8g, 

105, 112, 115, 117, 118 

— unsymmetrical, 10, 42, 93-96, 99, 

107, no, 112-114 
Systems, geological, 5 

— united thickness of, 5 

Table-lands, 338 

— mountains, 254, 344 
Tailless hair, 264 
Tarbat Ness, 141 
Tarns, 288 

Tay, river, 141, 145 

— valley, igo 
Tectonic basins, 279, 360 

— mountains, 340 

— valleys, 347 
Teith Valley, 169 
Terraced mountains, 70 
Terraces, alluvial, 51 

— marine erosion, 321, 331 
Teviotdale, 234 

— river, 136, 139 
Thames, river, 84 

Thickness of sedimentary strata, 43 

Thrust-planes, 95, 114 

Tiberias, lake, 159 

Timan Mountains, 232 

Torrents, action of, 289 

Tors of Cornwall, 206, 384 

Trachyte, 201 

Transport of vifeathered materials, 

34 
Transverse streams, 104, 107, I2i, 
131, 139, 148, 149 

— valleys, 350 

Transylvanian Alps, cirques of, 292 
Trenta, cirque-valley, 290 

Tuff, 20 

— cones, 181 
Tundras, 52, 263, 337 
Tweeddale, 233 
Tweed, river, 133, 138 
Tynedale fault, 167 

Uckermark, moraines of, 306 



Uebergossenen Aim, 221 

Unconformity, 42, 385 

Underground water, action of, 30, 

266 
Uniclinal orographic blocks, 159 
Upcast side of faults, 155 
Uplift, mountains of, loi 
— regional and axial, 129 
Utah, 53, 177, 279 



Vacek, Dr., 116, 117 
Valais, 268 
Val d'Uina, 112 
Valley-track of rivers, 35, 377 
Valleys, Alpine, 308 

— classification of, 347 

— constructional, 347 

— deformation, 348 

— dislocation, 159, 162 

— erosion, 145, 349 

— in gently-inclined strata, 75 

— in highly-folded strata, 104 

— in horizontal strata, 60 

— longitudinal, 70, 131, 139, 144 

— older than mountains they traverse, 

46, 57 

— submerged, 329 

— subsequent, 349 

— synclinal, 104, 105, 121 

— transverse, 104, 122, 131, 139, 144 

— U-shaped and V-shaped, 308 

— variations in form of, 353 
Vatnajokull, 215 

Veins; 20 
Vesuvius, 184 
Vispthal, 267 
Volcanic basins, 281 

— rocks, 4, 173 
Volcanoes, 180 

— demolition of, 187 
Vorlander, Alpine, 238, 246, 296 



Wadies, 25, 159 

Wahnschaffe, Dr., 238 

Wahsatch Mountains, 157 

Wallace, Dr., 311 

Wallenstadt, mountains, 114 

Walther, Prof., 24, 254, 256 

Water, chemical action on rocks. 25, 

30 
— mechanical action, 33 



INDEX 



397 



Waterfalls, 80, 355 
Wealden, anticline, 85 
Weathering of rocks, 26, igg 
West Lomond Hill, 88 
Whiteadder, river, 138 
Wind, geological action of, 24, 

265 
Wocheinerthal, 2go 
Wolds, Yorkshire, 345 



252, 



Yarrow, river, 139 
Yellowstone Lake, 281 

Zambesi Falls, 357 
Zillerthal, 214, 223 
Zirknitz, 273 
Zmutt Glacier, 221 
Zones of cirques, 291 
Zurich, lake, 293 



The Science Series 



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