The process by which the ancient plateau has been trenched into valleys and confluent ridges is best displayed among the higher mountains, where erosion proceeds at an accelerated pace. The long screes or talus-slopes at the foot of every crag and cliff bear witness to the continual waste. The headwaters of a river cut into the slopes of the parent hill. Each valley is consequently length ened at the expense of the mountain from which it descends.
Where a number of small torrents converge in a steep mountain recess, they cut out a crescent-shaped hollow or half-cauldron, which in the Scottish Highlands is known as a corrie. Usually the upper part of a corrie is formed by a crescent of naked rock, from which long trails of debris descend to the bottom of the hollow. Every distinct variety of rock has its own type of corrie, the peculiarities being marked both in the details of the upper cliffs and crags, and in the amount, form and colour of the screes. The Scottish corries have been occupied by glaciers. Hence their bot toms are generally ice-worn or strewn over with moraine stuff.
Often a small tarn fills up the bottom, ponded back by a moraine. It is in such localities that we can best observe the evidences of the glaciers that once overspread the country. Among these high grounds also the gradual narrowing of ridges into sharp, narrow, knife-edged crests and the lowering of these into cols or passes can be well seen. The stages in this demolition are clearest where the underlying rock is of granite or similarly tough material, which at the same time is apt to be split and splintered by means of its numerous transverse joints. The granite mountains of Arran furnish excellent illustrations.
Where a rock yields to weather with considerable uniformity in all directions it is likely to assume conical forms in the progress of denudation. Sometimes this uniformity is attained by a gen eral disintegration of the rock into fine debris, which rolls down the slopes in long screes. In other cases it is secured by the inter section of joints, whereby a rock, in itself hard and durable, is divided into small angular blocks, which are separated by weather ing and slide down the declivities. In many instances the begin ning of the formation of a cone may be detected on ridges which have been deeply trenched by valleys. The mountain Schiehallion (3,547 ft.) is an instance of a cone not yet freed from its parent ridge. A further stage in denudation results in isolated groups of
cones completely separated from the rest of the rocks among which they once lay buried. Such groups may be carved out of a con tinuous band of rock extending into the regions beyond. The Paps of Jura, for instance, rise out of a long belt of quartzite which stretches through the islands of Islay, Jura and Scarba. In many cases, however, the groups point to the existence of some boss of rock of greater durability than those in the immediate neigh bourhood, as in the Coolins (Cuchullins), and Red hills of Skye and the group of granite cones of Ben Loyal, Sutherland. The roost impressive form of solitary cone is that wherein, after vast denudation, a thick overlying formation has been reduced to a sin gle outlier, such as Morven in Caithness, the two Bens Griam in Sutherland, and the pyramids of red sandstone on the western margin of the shires of Sutherland and Ross and Cromarty. While, in Scotland, the dislocation of rocks has generally prevented the formation of continuous escarpments, there are instances of these in the wide basalt plateaux of the inner Hebrides, where lava has been poured out in nearly horizontal sheets, with occasional layers of tuff or other softer rock between them.
Erosion.—Platforms of erosion, successively established by the wearing down of the land to sea-level, occur both in the Highlands and among the southern Uplands. Allusion has been made to the flat-topped moorlands which, in the eastern Grampi ans, reach heights of 3,00o to 4,000ft. above the sea. The sum mits of Lochnagar and Ben Macdhui may be taken as examples. These mountains lie within granite areas; but not less striking examples may be found among the schists. That these high plateaux are planes of erosion is shown by their independence of geological structure, the upturned edges of the vertical and con torted schises having been shorn off and the granite wasted and levelled along its exposed surface. An example of the similar destruction of a much younger platform is to be found in the terraced plateaux 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 denudation that it has been reduced to scattered fragments.