Page:South African Geology - Schwarz - 1912.djvu/46

 the valley, which they score with characteristic grooves, and cause the valley to assume a U-shaped curve quite different from the V-shaped outline of the ordinary river valleys. The boulders themselves are ground flat on one or more sides, according as they have been forced to turn in their bedding of ice, and the surfaces are scored with sets of parallel scratches. Such facetted and ice-scratched boulders are very characteristic, and can never be mistaken for river boulders or boulders of any other origin. Several times in the history of the world glacial conditions have come over parts of the globe which are now warm; after the Coal Period, in South Africa, there was an extensive glaciation, the ice-scored valleys can still be seen in Prieska and the southern Transvaal, and all south of this there is found the Dwyka conglomerate, which is partly a terrestrial, partly a marine boulder clay. In Europe and America the Great Ice Age occurred when man appeared first on earth, and the whole of the country thus glaciated is now covered with a thick covering of boulder clay. There was no corresponding Ice Age at this time in South Africa, but it occurred in New Zealand and in the south of South America.

If there are hard projections in the course of the glacier, these may project through the ice and form a sort of island, which is called a nunatuk. If, however, the ice surmounts the obstruction, it becomes characteristically marked; the side which lies facing the