Page:South African Geology - Schwarz - 1912.djvu/80

 and impinges on the outstanding rocks, and the softer parts are in time worn away. In granite, for instance, the felspars, being softer than the quartz, under the influence of wind erosion will wear away, and the granite will become pitted. In the deserts this kind of erosion is intense.

River erosion is accomplished by the sand and gravel dragged along by the current: the action is that of a continuous application of sandpaper to the sides and floor of the valley. Marine erosion is similarly accomplished by the waves dragging backwards and forwards the rubble fallen from the cliffs.

A peculiar form of erosion occurs when a stone becomes loosened in a rock in the river bed: the current is not enough to sweep it away, and the stone therefore oscillates and gradually grinds its hollow larger. Other stones may be dropped into the hole, and the more there are the more vigorous does the churning and boring action become, until quite deep cylindrical holes result. These are called pot holes, and are sometimes several feet deep. They must be distinguished from the swallow holes, due to solution of limestone along crevices.

Ice erosion is accomplished by the boulders that are frozen in the bottom and sides of glaciers, which, as they move downhill, press against the bottom and sides of the valley like chisels held in a vice. But as the ice melts readily under pressure, the boulders continually shift their position and expose new surfaces to the floor over which they are moving. Grooves are thus cut by these boulders in the floor; they are not continuous, but peter out after a short distance, and also show peculiar chatter marks, like when a cold chisel is held loosely in the hand and made to move over a surface of metal. Where