Page:South African Geology - Schwarz - 1912.djvu/160

 the inland lake, and on melting dropped the boulders held frozen in the ice to the bottom, where they became embedded in the mud accumulating there. The shore line ran through Prieska and the Southern Transvaal, and the actual valleys down which the glaciers flowed can still be traced and the ice-scratched pavements and roches moutonnées can be seen. North of the shore line the glaciers, which, indeed, seem to have united at one period into a continuous ice sheet, left great accumulations of morainic material when the ice finally melted, so that all Prieska and the Southern Transvaal are covered with a terrestrial till or boulder clay. South of the old shore line the Dwyka Series begins with normal shales deposited on the Witteberg, which then became more and more conglomeratic as the icebergs discharged their burdens of rubble on to the sea floor; the Dwyka Conglomerate on the south thus is a subaqueous till or boulder clay. The actual transition from terrestrial to sub-aqueous till may be seen in the Tanqua valley, on the west of the Karroo, where a glacier from the land ploughed over the mud and boulders formed beneath the water and caused a glaciated pavement to form on the actual till. The rocks included in the Dwyka Conglomerate as boulders are derived from all the older formations exposed in Prieska, Bechuanaland, and the Transvaal; among others one may find granite and gneiss, diabase and acid lavas, sandstones from the Kheis, Black Reef, and Waterberg Series, dolomite and jaspers from the Pretoria Series. The matrix of the Dwyka Conglomerate in the north is a bluish-grey mud which changes to yellow where the surface water has acted upon it, so that we have the blue and yellow ground very similar in appearance to