Page:South African Geology - Schwarz - 1912.djvu/39

 fires; the soil becomes dried and exposed, vegetation no longer binds it together, and the first heavy rain causes one of these disastrous avalanches.

Where the country is covered with sand the water simply sinks in, and does not run away by any channel visible on the surface; a large portion of it evaporates and leaves a crust of calcareous tufa beneath the sand. Such a condition is found in Bechuanaland and the Kalahari, and to a large extent in the north-west of the Cape Colony, south of the Orange River.

South African rivers are to a large extent intermittent rivers; even the great Zambesi and Orange Rivers are extremely variable in their flow, though they never actually dry up. The smaller rivers frequently dry up entirely, and when the rain comes, the whole run-off is carried impetuously away to the sea. Supposing, now, that the water which falls in the storm is not enough to fill the whole length of the river channel: where the storm breaks there will be an excess; the river will overflow its banks, and cause a wide temporary vley to form. As storms break more or less in the same place season after season, the temporary vley becomes marked by a bare stretch of silt-covered ground. Such widenings of the river channels are called vloers. The storm passes away, the river ceases to flow, but certain pools remain where the river bed is deeper; these are called kolks.

Pans. — If now the storms do not yield enough rain to flow past the vloer for many years in succession, the lower part of the river may become obliterated and a pan results. This of course can only happen in semi-desert regions, where the crumbling of the rocks of the surface goes on vigorously, and the particles are blown