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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 disintegration of the underlying series (a), 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 endured 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 magnitude of denudation. And this impression will be deepened when we reflect that the bulk of the materials 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.
LAND-FORMS IN REGIONS OF HORIZONTAL STRATA
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 discussed the action of existing agents of change, and saw reason to conclude that the denudation everywhere 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 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 configurations have not been determined solely by the mineralogical composition of the rocks. There is yet another factor to be taken into consideration. The form assumed by a land-surface under denudation depends 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 determining 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 consider 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 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 considerable 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 results 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 (f) 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 relatively recent geological times, many of the most striking surface-features are obviously due to deformation 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 direct result of that action, but even after such irregularities 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 movement necessarily results in the fracturing and wrinkling of the crust, which as it sinks is compelled to occupy