Page:South African Geology - Schwarz - 1912.djvu/48

 somewhat of the back of a wig formed of long rolls of horsehair. The roches moutonnées give the direction from whence the glacier has flowed, and are important, as the ordinary ice scratches on the glaciated pavements simply give the direction without indicating which way the ice moved.

Under the glaciers there are often sub-glacial streams, and the sand carried by them forms, when the ice melts, long, winding ridges; the moraines also often break through the tops of the tunnels of the sub-glacial streams, when boulders are added to the sand. Such ridges are called kames and eskers. There is an interesting case in Bechuanaland of a somewhat similar train of boulders called Blink-klip breccia; this is rock that has tumbled into an underground river in limestone by the breaking of the roof.

Lakes. — On a high coast it frequently happens that enormous accumulations of sand occur, and between the firm land and the dunes there is a depression. The Knysna lakes are a good illustration of this kind; they are partially salt, from communication with the sea, and partly fresh, from drainage from the land. Even when the water of the lakes is quite salt, almost fresh water can be obtained by sinking wells in the sand round the edge, a feature also found in the Trona lakes, in the Lake Chad region.

Lakes may also be formed by volcanoes, the chimneys of which serve as hollows for the reception of rainwater; in other countries, where the volcanoes have long been extinct, the actual craters become filled with water from subterranean springs. The top of the peak of Tristan da Cunha is occupied by a crater lake. Again, portions of the earth may subside between faults, and leave a