180 THE NORTHERN DWYKA 



or an ice sheet. The shales must have been deposited 

 in quiet water in which the boulders dropped to the 

 bottom from floating ice. 



The northern boulder-beds frequently include bands in 

 which large boulders are very numerous, but in the south 

 such occurrences are rare. In the Tanqua Karroo a 

 fairly constant band of large boulders stretches for many 

 miles north and south of Eland's Vlei. It is about fifteen 

 feet thick and some of the boulders are from three to 

 four feet in diameter, though most of them are less 

 than half this size. Another definite boulder-band has 

 been found in the valley of the Witteberg's River south 

 of Laingsburg, and is shown on Plate XIII. The largest 

 block seen in the photograph is ten feet across. 



Along a line extending from Modder River Station 

 south-westwards to the Orange River the normal type of 

 Dwyka is replaced by soft sandstone, which is calcareous 

 in places and encloses occasional small boulders. 



The tillite of Calvinia is very much like that of Prieska, 

 being fairly soft and sometimes shaly. In the Tanqua 

 Karroo it is slightly harder, while a little north of Karroo 

 Poort it becomes more indurated as the result of earth- 

 movement. 



Throughout the southern outcrops the tillite is a 

 hard blue rock from which the pebbles cannot easily 

 be broken ; when the rock is struck with a hammer the 

 fracture is more likely to pass through a pebble than 

 round it, while the northern rock breaks up readily and 

 the pebbles can easily be removed from the matrix. 



There is a rough cleavage developed in the southern 

 rock, parallel to the strike of the beds, but at various 



