Page:South African Geology - Schwarz - 1912.djvu/43

 The waters originally flowed from this ridge symmetrically on either side, over a featureless plain. As the land rose the straight courses of the rivers were preserved, and though, by the carving away of softer rock, mountains rose as if to bar the way, the rivers were able to cut through them at a sufficient rate, and they were, therefore, not diverted. In the east, however, a great chain of volcanoes broke out, which piled lavas across the courses of the southerly flowing streams, forcing their waters to flow back and over the main watershed. Those streams are now collected into one main channel, the Orange River, and this breaks across the main watershed at Aliwal North, the only breach in the watershed throughout its entire length.

The rivers always carry a certain amount of gravel and sand with them, which becomes deposited where the river current is slack. In the head reaches of the rivers, where they flow down the mountain-side, blocks of all sizes and shapes fall into the river course, and are moved according to the volume and rate of flow of the stream. The largest are soon left behind, but the residue is carried down stream and dragged along the river bottom. In the process the larger boulders become rounded by grinding against each other, and the bits ground off go to swell the mass of sand; the boulders also grind against the bottom of the river and erode it, producing more sand. The finest particles and the mud are carried in suspension, and are usually carried right out to sea; but the sand and gravel fill the beds, and are shifted downstream at every flood, or may be banked up on the side of the stream and form alluvial flats in the rock channel.

Glaciers. — In cold climates, and in hot ones where the