Page:South African Geology - Schwarz - 1912.djvu/41

 it to the depression, where, in the course of years, it accumulates. Two interesting pans of this nature occur near Port Elizabeth: one at almost sea level at Bethelsdorp, and the other in the raised beaches, at an elevation of about 300 ft. The Pretoria salt pan owes its salt to the ordinary rock weathering, but the origin of the depression is probably due to volcanic action.

The mouths of rivers where they debouch on the sea are accompanied by certain characteristic features. In lands where the topography is mature, and the sea floor slopes gently downwards, the mud and silt collect about the mouth of the river and cause the formation of a delta. A good example is the Nile delta: the sand has collected to such an extent that the land has advanced considerably into the Mediterranean, and the river finds its outlet through numerous channels in the delta. In South Africa both conditions for the formation of deltas are absent. In many cases, as in the Great Fish River, enormous masses of mud are discharged during a flood, enough to colour the sea for miles; but the steep coast is swept with swift currents which carry the sand away, and deposit it all along the sea bottom. In the Orange River the channel loses itself among the sand dunes, and rarely opens on to the sea directly; but again the formation of a true delta is prevented by the current that washes up from the Antarctic. The opposite extreme to a delta is a drowned river valley: the rock channel of the river has been cut with more or less steep sides, according to the time the river has been employed in fashioning its valley, and then, on the land sinking, the sea invades the land and flows up the channel to a considerable distance. Large numbers of South African rivers are of this type; at the Kowie River, in Bathurst,