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The topography of the country being the result of prolonged denudation it is reasonable to infer that the oldest surfaces likely be preserved are portions of some of the platforms of erosion successively established by the wearing down of the land to the sea-level. Relics of these platforms occur both in the Highlands and among the Southern Uplands. Allusion has already been made to the Rat-topped moorlands which in the eastern Grampians reach heights of 3000 to 4000ft above the sea. The most familiar example perhaps is the top of Lochnagar, where, at the level of 3500 ft, the traveller finds himself on a broad undulating moor more than a mile and a half long sloping gently towards Glen Muick and terminating on the north in a range of granite precipices. The top of Ben Macdhui stands upon nearly a square mile of moor exceeding 4000 ft in elevation. These mountains lie within granite areas but not less striking examples may be found among the schist's.The mountains at the head of Glen Clova and Glen Isla, for instance, sweep upwards into a broad moor some 3000 ft above the sea, the more prominent parts of which have received special names - Driesh, Mayar, Tom Buidhe, Tolmount, Cairn na Glasha. It would hardly be an exaggeration to say that there is more level ground on the tops of these mountains than in areas of corresponding size in the valleys below. That these high plateau's are planes of erosion is shown by their independence of geological structure, the upturned edges of the vertical and contorted schist's having been abruptly shorn off and the granite having been wasted and levelled along its exposed surface. Among the Southern Uplands exist traces of a similar tableland of erosion. The top of Broad Law on the confines of Peeblesshire and Selkirkshire, for example, is a level moor comprising between 300 and 800 acres above the contour line of 2500 ft and lying upon the upturned edges of the greatly denuded Silurian grits and shales. An instructive example of the similar destruction of a much younger platform is to be found in the terraced plateau's of Skye, Eigg, Canna, Muck, Mull and Morven, which are portions of what was probably originally a continuous plain of basalt. Though dating back only to older Tertiary time, this plain has been so deeply trenched by the forces of denudation that it has been reduced to mere scattered fragments. Thousands of feet of basalt have been worn away from many parts of its surface; deep and wide valleys have been carved out of it; and so enormously has it been wasted, that it has been almost entirely stripped from wide tracts which it formerly covered and where only scattered outliners remain to prove that it once existed.
It is curious that broad flat topped mountains are chiefly to be found in the eastern Parts of the country. Traced westwards, these forms gradually give place to narrow ridges and crests. No contrast, for instance can be greater than that between the wide elevated moors of the eastern Grampians, and the crested ridges of western Inverness-shire and Argyllshire, Loch Hourn, Glen Nevis, Glencoe or that between the broad uplands of Peeblesshire and the precipitous heights of Galloway. Geological structure alone will not account for these contrasts. Perhaps the cause is to be sought mainly in differences of rainfall. The western mountains, exposed to the fierce lash of the Atlantic rains, sustain the heaviest and most constant precipitation. Their sides are seamed with torrents which tear down the solid rock and sweep its detritus into the glens and sea lochs. The eastern heights, on the other hand, experience a smaller rainfall and consequently a diminished rate of erosion. No doubt, too, the preponderance of rainfall in the west has persisted for an enormous period.  |
