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of recent uplift are characterised by steep profiles, by peaks and knife-edged aretes; the structures are often unstable, and yield readily to the agents of erosion, so that rock-falls and landslips are constantly taking place. In regions of ancient uplift, on the other hand, the profiles are generally softer; peaks and sharp-crested ridges are of less frequent occurrence, weak structures have disappeared, and the degradation of the mountains does not advance so rapidly. The levelling process, however, though slower, is quite apparent. The valleys are widened and deepened, the mountains crumble down, and, should the base-level of erosion be retained, the whole area will eventually be flattened out and resolved into a plain of erosion.

Such then are the several stages through which a region of mountain-uplift must pass. First comes the stage of youth, when the surface configuration corresponds more or less closely with the underground structure. Next succeeds the stage of middle-life, when such coincidence is all but obliterated, when the valleys of youth have been exalted and its mountains have been laid low. Last comes old age and final dissolution, when the whole region has been reduced to its base-level. But the decay of a mountain-chain does not always proceed without interruption. Not infrequently the base-level is disturbed; new horizontal movements of the crust take place, and bulgingup of the region is accompanied by further folding and fracturing of the strata. The mountain-system renews its youth. On the other hand, the old baselevel may be destroyed by subsidence of the crust, and the mountains, partially or wholly drowned, may in time become largely buried under new accumulations of sediment. Re-elevation taking place, erosion recommences, and the degradation of the region is resumed. In the structure of not a few mountain

[graphic]

F1g. 56. Structure Of The Ardennls (after Comet and Briart).

MM, the existing surface; the light-shaded area above this level represents the rock-masses removed by denudation. The Silurian rocks at the base of the section are indicated by thin white lines. Above these, on the left-hand side of the section, between C and M, come Devonian conglomerate, sandstone, shale, and limestone; next in succession follow the Carboniferous strata at and above M; A A, BB, C C, are dislocations.

chains we may read the history of many such vicissitudes.

So completely have some mountains been removed -by denudation, that without some knowledge of geological structure we should never have divined their former existence. An instructive example is furnished by the Carboniferous tracts of Belgium and Northern France. The structure of these regions shows that formerly a considerable range of mountains extended between Boulogne and Aix-la-Chapelle. At or towards the close of Carboniferous times a great earth-movement, acting in a direction from south to north, buckled up the strata, and these, yielding to the pressure, snapped across, and extensive overthrustine followed alone the line referred to, the Carboniferous beds being inverted and overlaid by Devonian strata. The mountains of upheaval which thus came into existence attained a great elevation, the higher parts of the range reaching probably not less than 16,000 or iS.ooo feet. The section (Fig. 56) will show how completely the surface has been remodelled, how mountains of elevation have been replaced by a plain of erosion.

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

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