Earth sculpture; or, The origin of landforms

Front matter

BOUGHT WITH THE INCOME FROM THE

SAGE ENDOWMENT FUND

THE GIFT OF

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 subdivisions. 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 desirous 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 unacceptable.

In a work addressed more particularly to non-specialists, 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 geological knowledge, and it has not been thought necessary, 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 instructors : —

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-features 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 gathered from its pages.

Prof. A. de Lapparent's Legons de Gdographie Physique — 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, memoirs, 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 denudation and the relation of surface-features to geological structure have indeed become classical. Amongst these are Ramsay's notable paper, " On the Denudation of South Wales and the Adjacent Counties of England " {Memoirs Geological Survey of England, vol. i., 1846) ; Heim's Mechanismus der Gebirgsbildung, etc. (which, although an independent work, was yet commenced under the auspices of the Swiss Geological 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 Council 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 illustrations (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.

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 Geological 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 Escarpments — Synclinal Hills and Anticlinal Hollows — Anticlinal Hills.

viii CONTENTS

Chapter V ^ Page

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

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 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 Disturbed Strata (continued) . . . . . .128

Structure and Configuration of Plateaux of Erosion — Forms assumed under Denudation — Mountains of Circumdenudation — History 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 determination OF Land-forms ....... 195

1 Joints in Rocks and the part they play in determining Surface-features — Texture and Mineralogical composition of Rocks in relation 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 evidence 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 Granitoid 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 Underground 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 disturbed 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

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

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 practically involves that of the Evolution of the Land.

Appendix . ... 373

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,

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

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

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

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-

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

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

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 jumbled rock-masses ; there, again, they encountered the relics of vast volcanic eruptions in regions now practically free from earth-throes of any kind. In one place ancient land-surfaces were seen intercalated at intervals 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 accumulations 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 gradually became modified. Eventually, it was recognised that no hard-and-fast line separates past and present.

The belief in world-wide, or nearly world-wide, catastrophes 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 arrangement. We must make some acquaintance with the composition and the structure or architecture of the earth's crust before we can form any reasonable conclusion 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 being of igneous origin, while the other appears to consist mainly of the products of water action. These last are typically represented by such rocks as conglomerate, sandstone, and shale, which are only more 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 countless varieties — some formed in lakes, like the shellmarls 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 originindurated sand-dunes, in short.

The igneous rocks consist partly of lavas and fragmental materials which have been ejected at the surface, 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-existing 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 altered that it is often hard to detect their original character. These altered masses form what are called the metamorphic rocks. They are typically represented 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 accumulations, 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 thousands 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.

the earth's surface. The various systems thus frequently 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 surface 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 immense area of crystalline schistose rocks is met with in Brazil. The Highlands of Scotland, the Scandinavian Peninsula, and North Finland are in like manner 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. Crystalline 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 become gradually altered as they are traced downwards, 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 purpose 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 approximately 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 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 inclined or approximately horizontal layers. Now, over considerable areas of the earth's surface the derivative rocks show the same horizontal arrangement, a structure 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 common enough occurrence, and now and again characterise 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 sometimes 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 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 single arch or anticline ; while in Fig. 3, representing

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

Fig. 4, for example, most of the anticlines and synclines lean over to one side, and this to such a degree, that here and there upper beds are doubled under 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 disturbance 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 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 traversed by a fault, which has caused a vertical displacement 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.