CHAPTER XVI

CLASSIFICATION OF LAND-FORMS

PLAINS OF

OF

ACCUMULATION AND OF EROSION-PLATEAUX ACCUMULATION AND OF EROSION-HILLS AND MOUNTAINS; ORIGINAL OR TECTONIC, AND SUBSEQUENT OR RELICT MOUNTAINS VALLEYS; ORIGINAL OR TECTONIC, AND SUBSEQUENT OR EROSION VALLEYS-BASINS-COAST-LINES.

WE

E have now passed in rapid review the more salient and notable features of the land-surface, and have discussed the several causes of their origin. The present chapter may therefore be devoted 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.

1. Plains. These are areas of approximately flat or gently undulating land. 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,

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 flat lands are usually termed plateaux. Two kinds of plains. are recognised, viz., plains of accumulation and 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 subjected 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 extent 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

or less porous materials. Many plains of accumulation 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 Hungary and Russia. The arid wastes of the AraloCaspian 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 accumulations 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 underground 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 coincidence 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

present the final stage of a cycle of erosion. Occurring 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 accumulation. Now and again they have been submerged and more or less deeply buried under marine sediments, 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 internal 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 a 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

cut up into segments, and these progressively diminish in extent as erosion proceeds. Every table-land, in short, tends to acquire an irregular mountainous aspect. As examples of highly eroded plateaux of accumulation may be cited the Plateau of the Colorado, the Uplands of Abyssinia, and the Deccan of India. Plateaux of erosion, as might have been expected, are far more common, many excellent examples 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 separate 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 exceeding 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 necessary 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,