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CHAPTER VI

LAND-FORMS IN REGIONS OF HIGHLY FOLDED AND DISTURBED STRATA {continued)

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.

IN our last chapter we considered the history of a mountain-chain, following that history from the stage of youth to old age and final dissolution. This last we recognised in the plain of erosion. We have next to trace the subsequent history of such a plain. The geological structure of many mountain-chains, as already indicated, reveals the fact that these are often the result of more than one uplift. After having been for long ages subjected to erosion, and even to subsequent subsidence and sedimentation, the same region has again yielded to lateral crush, and new series of folds and thrust-planes have come into existence. But the crust does not always yield in this particular fashion. Not infrequently relief from pressure is obtained by widespread bulging-up of the surface, one or more broad swellings with perhaps corresponding broad depressions appear, instead of an intricate arrangement of more or less closely compressed folds. We may for convenience■ sake speak of the latter as resulting from axial uplift, and of the former as due to regional uplift, even although it be obvious that in most wide regions of uplift there must be an axis or line of maximum movement.

Now it can be shown that one and the same region has not infrequently experienced both kinds of uplift. Axial uplifts have in time been succeeded by regional uplifts; for again and again we encounter ancient

[graphic]

Fig 57. Diagrammatic Section Across A Plateau Of Erosion.

Isoclinal folds.

plains of erosion occurring at various levels above the sea, their geological structure showing clearly that they have replaced old mountains of complicated structure. Such elevated plains may be termed plateaux or table-lands of erosion, to distinguish them from plateaux of accumulation or deposit. The characteristic feature of the latter, it will be remembered, is the general coincidence of the surface with the underground structure, while the former shows no such correspondence. The structure of a tableland of erosion may thus be represented as in Fig 57. Many such table-lands are recognised in Europe, the Highlands and Southern Uplands of Scotland and the Scandinavian plateaux being good examples. Ancient plateaux of the kind are all more or less denuded, trenched, and furrowed by valleys to such an extent that the plateau character is often somewhat obscured. For no sooner is a plain of erosion uplifted than a new cycle of erosion begins. The direction of the drainage is determined, in the first place, by the slope of the ground, and this we can readily understand may be somewhat diversified. The surface may be canted either in one direction only, or in more than one, for the crustal movement is unlikely to be equal in amount throughout the whole region of uplift. Hence, the primeval rivers may all flow in one particular direction, or they may trend to various points of the compass. However that may be, it is certain that in course of time they must gradually deepen their valleys, and the plateau must eventually come to be cut up very much in the same way as a plateau of accumulation. But the mountains of circumdenudation resulting from this process will differ considerably in character from those carved out of horizontal strata. The varying structure of the rocks will necessarily influence erosion, and thus lead to a greater diversity of form. Should the strata be steeply inclined, and this will usually be the case, then it is obvious that the harder masses must come in time to project beyond the more readily reduced: rocks with which they are associated. The general surface of the plateau will thus tend to assume a corduroy configuration, the long ridges coinciding with

the outcrops of the "harder rocks," while the intervening parallel hollows will correspond with the outcrops of the more yielding strata. In short, the land-features evolved by denudation will have a general resemblance to those produced in a region of slightly inclined and gently undulating formations. But owing to the very varied character of the rocks and their more complicated structures, the surfacefeatures of a plateau of erosion will be more pronounced and much more irregular. In such a region the larger rivers, being frequently of primeval origin, will often be found to cut across mountain-ridge after mountain-ridge, and to follow courses more or less transverse to the corduroy surface. Others may keep closely to the outcrops, and run in the direction of the "strike" or trend of the strata, while some may take now one route and now another. The original surface of the plateau will generally be indicated by the direction of the main drainage-lines or principal rivers, while the subsequent slopes due to erosion will usually be manifested by the course of tributary streams. During the progress of denudation, however, many modifications of the drainage will be brought about. Cases of the capture of principal rivers by lateral streams working their way back or across the strike can hardly fail to occur, and these and other changes may render the original drainagelines obscure and hard to trace.

To such an extent have many ancient plateaux of erosion been denuded, so deeply have they been trenched, that their surface has become resolved into a truly mountainous region, wherein all the elevations are mountains of circumdenudation, the tops of which are the only remaining relics of the original plateausurface. Such mountains, owing generally to the durability 'of their rocks and the strength of their structure, are not so readily demolished as the mountains in a range of recent uplift. They may not often emulate these in height and grandeur, their profiles may as a rule be less wild and irregular; but such is not always the case. When a plateau of erosion stands at a great elevation, the mountains carved out of it are apt to rival the boldest and most abrupt of Alpine heights. Such abrupt slopes and the profound valleys that intervene are the result of relatively rapid and powerful vertical erosion. But when a plateau has only a moderate elevation, the configuration of its mountains tends to be less abrupt, and to approximate in character to that attained by a true mountain-chain during the period of its maturity, when all weak structures have been demolished and the surface no longer coincides with the folds of the strata. And this is just what might have been expected, when it is borne in mind that in each case the fundamental geological structure is the same. A mountain-chain is composed mainly of highly flexed and folded rocks. Subjected to erosion, the whole region is remodelled and eventually reduced to a base-level. But the rockstructure remains; the plain of erosion is composed, just as the mountains were, of highly flexed and folded

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