Page:South African Geology - Schwarz - 1912.djvu/35

 by them in the first flat surface, so that a number of winding gorges will result, and each river course will be separated from the next by a stretch of flat ground. Such is the case in South Africa, although most of the plateaux owe their origin to other causes than those supposed in this hypothetical case we are following. Successive rises in the continent will give successively greater fall to the rivers, and, if they are rapid enough, the rivers will only cut downwards, and the gorge-like nature of the river valleys will be perpetuated. If, however, the continent remains stationary after a succession of rises, the rivers will cut downwards to base level, and then, as their power of cutting downwards has reached a limit, further action will be directed towards cutting sideways; the banks will be undercut at the outer edges of the curves, and silt and mud will be deposited on the insides of the curves, so that eventually there will result a broad valley bordered with flats covered with all the debris washed down the river, which is called, collectively, alluvium. As time goes on, the steep sides of the rock banks of the river crumble from the attacks of weathering, and rains wash down the sides, carrying all loose material into the bottom of the valley. Eventually this process goes on till the slopes leading into the valley are cut back till they meet the slopes of the adjoining river, and the watershed between the two rivers is no longer a flat stretch but a crest; the topography of the country is said to have become mature. Tested by this criterion South Africa would be said to be a young country with immature topography, whereas Europe would be an old country.

The extreme case of river action is where the continent remains stationary so long that the crests between the