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they are caused by the collapse of the undermined rocks. Owing to various causes, engulfed streams now and again abandon their courses, arid work their way to lower levels, and in course of time such abandoned channels may become disclosed by the fallingin 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 common occurrence, many of them being of large dimensions, 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 unimportant part in determining the aspect assumed by rocks under denudation. In preceding pages we have discussed the origin of the salient features of a landsurface. 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
the land have been blocked out by subterranean action, they are the result of crustal deformation. Viewed from a nearer standpoint, however, we recognise 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 fashion, 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 exceptions, the land-features that now meet our eye are the direct result of erosion and accumulation, the modifying influence of which is always more or less conspicuous even in cases of recent crustal deformation.
Now if it be true that the character of a land-surface 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 influence 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 infrequently 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 seldom seen, and usually only at the higher elevations. Mountains built up of bedded rocks, of schists, of massive 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 petrographical 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 glaciation. 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.
LAND-FORMS MODIFIED BY GLACIAL ACTION
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.
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 irregularities 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 geologists have doubted whether it is very effective, and are of the opinion that the utmost it can do is to 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 endeavouring 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 character 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 formations 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.
1. 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