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thrust-planes. Now such a structure would naturally determine the disposition of the surface-features worked out by erosion. Before the beginning of the Old Red Sandstone period, the pre-existing mountains of uplift had been largely degraded to a baselevel. Much of the region, in other words, had been converted into a plain of erosion, which subsequently became depressed and buried under thick accumulations of sediment, derived in chief part from the denudation of such parts of the Highland area as still remained in the condition of dry land. After the deposition of the Old Red Sandstone the whole region was elevated en masse, and converted into a plateau or table-land. The surface of that plateau would doubtless be somewhat undulating and diversified. Probably the " stumps " of the highly denuded mountains, which had supplied materials for the formation of the Old Red Sandstone, still formed dominant areas. But wide regions had been planed down, and these would be marked by a kind of "corduroy" structure—parallel lines of escarpment and ridges with intervening hollows, corresponding to the successive outcrops of "harder" and "softer" rocks. The regions overspread by the Old Red Sandstone, on the other hand, would be approximately level, sloping gently, however, towards the north, northeast, and south-east. We may, therefore, conceive the surface of the ancient Highland Plateau to have been from the first more irregular than that of the Southern Table-land. The primeval rivers would doubtless follow the average slopes of the plateau, and would thus sometimes cross the outcrops at all angles, and sometimes flow in the direction of the strike for longer or shorter distances. The great depression on the line of the Caledonian Canal, although partially filled with the sediments of Old Red Sandstone times, probably still formed a well-marked feature at the surface of the plateau when this was first uplifted. And the same may well have been the case with many other lines of fracture. In short, although the average slope of the ground determined the general direction of the drainage, the corrugated and often much diversified surface of the plateau must have led to endless deflection of the water-flow. Again, as erosion proceeded, and the valleys were cut deeper and deeper, many modifications of the drainage would naturally arise, cases of the " capture" of one stream by another having been of common occurrence. It is not, however, with the history of such changes that we have to do, but rather with the character of the existing valleys and mountains which have been carved and chiselled out of the ancient plateau. Of the valleys it may be said in general terms that they are all valleys of erosion. Many have been hollowed out along the outcrops, and are thus longitudinal, while others have been cut out across the "strike," and to this extent are transverse. Some of the former are of primeval antiquity: they correspond in direction not only with the strike of the strata, but with what seems to have been the original slope of the plateau, the valley of the Spey being the most conspicuous example. The transverse valleys, represented typically by Glen Garry and the valley of the Tay, are obviously also of great age, since they in like manner indicate the general slope of the plateau in the regions where they occur. A large proportion of the longitudinal valleys that drain into these transverse valleys are in all probability of subsequent origin, although some of them may have been outlined at as early a date as the latter. Although none of the longitudinal valleys can be described as synclinal, they may all nevertheless be termed structural, inasmuch as they coincide with the strike of the rocks. So likewise we may term Glenmore a structural hollow, since it occurs along a line of fracture ; and the same is the case with Glen Docherty and Loch Maree. These lines of fractures no doubt showed at the surface of the plateau when it was first uplifted, and so determined the direction of drainage and erosion. But all the valleys as we now see them are valleys of erosion, their direction having been determined sometimes by the average slope of the plateau, sometimes by the geological structure.
The mountains of the Highlands are likewise monuments of erosion, owing their existence as such sometimes to the relative durability of their materials, sometimes to their geological structure, or to both causes combined. They are all, without exception, subsequent or relict mountains. Thus, in the following section from Glen Lyon to Cam Chois we see that the present configuration of the surface does not coincide with the complicated underground structure. It is the same, indeed, throughout all the Highland area. Take a section across any portion of that region, and you shall find that the more continuous "ranges" are developed along the outcrops—they are,. in short, escarpment mountains. So great has been the erosion, however, within such "ranges," that their alignment usually becomes obscured, and we are confronted by confused groups of mountains, drained by streams flowing in every possible direction. "Any
F1g. 59. Sect1on From Glen Lyon To Carn Cho1s. (Geol. Survey.)
m, mica-schist, etc.; /, limestone; gr, greywacke, etc. ; J, amphibolite schist; g, granite ; d,
dioritc; /". fault.
wide tract of the Highlands," as we have elsewhere remarked, "when viewed from a commanding position, looks like a tumbled ocean, in which the waves appear to be moving in all directions. One is also impressed with the fact that the undulations of the surface, however interrupted they may be, are broad; the mountains, however much they may vary in their configuration according to the character of the rocks, are massive and generally round-shouldered, and often somewhat flat-topped; while there is no great disparity of height amongst the dominant points of any individual group. Let us take, for example, the knot of mountains between Loch Maree and Loch Torridon. There we have a cluster of eight mountainmasses, the summits of which do not differ much in elevation. Thus in Llathach two points reach 3358 feet and 3486 feet; in Beinn Alligin there are also two points reaching 3021 feet and 3232 feet respectively ; in Beinn Dearg we have a height of 2995 feet; in Beinn Eighe are three dominant points, 3188 feet, 3217 feet, and 3309 feet. The four masses to the north are somewhat lower, their elevations Being 2860 feet, 2370 feet, and 2892 feet. The mountains of Lochaber and the Monadhliath Mountains exhibit similar relationships; and the same holds good with all the mountain-groups of the Highlands. One cannot doubt that such relationship is the result of denudation. The mountains are monuments of erosion; they are the wreck of an old table-land, the upper surface and original height of which are approximately indicated by the summits of the various mountain-masses and the direction of the principal rivers. If we in imagination fill up the valleys with the rock-material which formerly occupied their place, we shall in some measure restore the general aspect of the Highland area before its mountains began to be shaped out by Nature■s saws and chisels."