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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 recognise the fact that shore-lines now and again owe their very existence to organic action.
PLAINS OF ACCUMULATION AND OF EROSION—PLATEAUX OF
OR EROSION VALLEYS—BASINS—COAST-LINES.
WE 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.
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, 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 intluence 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 represent 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