Page:South African Geology - Schwarz - 1912.djvu/36

 rivers disappear, the rivers cut sideways and eventually level the whole land once more to a featureless plain; such a surface is called a peneplain, where the whole country is said to be base-levelled. Instances of peneplains are everywhere in evidence in the coastal regions of South Africa, some of the best being found in Oudtshoorn, Uniondale, and round Grahamstown. The flats are still covered with river material, which often has become hardened. As the continent rose subsequently to the peneplanation, the rivers began to cut downwards once again, and sawed out valleys on the plains, so that one sees the original peneplains high up above the river valleys and the edges abruptly cut off at the commencement of the slope. The peculiar table-topped hills covered with hardened river sand are very characteristic of South African coastal landscape; this must not be confused with the table-topped hills in the Karroo, where the cause of the tops being level is quite different.

If the ground over which a river runs is soft, it will tend to wear it away rapidly, and if it is hard it will cut through it slowly; hence, if there is a bar of hard rock in the midst of soft ground the river receives a check. Two cases are presented: in the first the bar is too hard for the river to remove, hence the bar will act as a temporary base level and the river cannot cut below this in its reaches above; the result is that the country above the rock lip is peneplained, and where the water pours over the bar a waterfall is produced. Admirable examples of this occur in South Africa in the Victoria Falls, where the rock bar is a sheet of basalt, and at the Augrabies Falls, where the Orange River pours over a bar of granite. The flat country of the north-west of Cape Colony and the Orange Free State owe their features