Page:South African Geology - Schwarz - 1912.djvu/81

 the floor consists of stratified rocks the moving ice often strips off slabs, which then go to swell the mass of morainic material. The erosive action of glaciers is small, and is quite overshadowed by the weathering due to the ice lying in contact with the rocks; the rocks become shattered by being kept wet constantly, and the moisture alternately being thawed and frozen in the crevices.

There is another form of erosion common in South Africa, due to the sliding of gravel down the hills. On the top of the slopes the bare rock is exposed, and fragments are continually being broken off by expansion and contraction due to alternating heat and cold. The blocks fall on the soil and gradually sink through it; for when soil becomes wet it swells, and does so round the blocks, because where the blocks rest the rain cannot penetrate. Eventually, a short way from the crest of the hill, the soil will be found to be underlain by a continuous layer of angular blocks, and as the soil moves downwards at every rain it carries with it the gravel beneath, so that the soil acts much as a glacier does, and the gravel beneath represents the ground moraine. Sub-soil gravel on a hillside will perform all the peculiar actions which ground moraines will do: it will ride over obstructions, producing roches moutonnées, pluck off slabs, will fill in depressions, and finally at the bottom, where the river washes away the soil, will leave a mass of gravel very similar to a terminal moraine. In very humid regions, as on the flanks of the snow-capped mountains and on the fringe of the Arctic and Antarctic, such as in the islands of Kerguelen and the Falkland Islands, soil-flow is very marked; it has been called soli-fluction.

Denudation. — These two processes, weathering and