434 THE DWYKA FORMATION 



a certain extent the character of a morainal deposit. 

 It lies upon a well-striated rock surface, and is a hard 

 sandy mud or clay with large blocks and smaller frag- 

 ments of various kinds of rock scattered through it. 



The boulder-beds, etc., of the Vaal-Harts Valley can be 

 regarded as of fluvioglacial origin, while the occasional 

 patches of tillite with a shaly matrix may have been 

 formed in small glacial lakes within the morainal area, 

 i.e., the area which belonged to the land rather than to 

 the water. 



Evidence of the movement of solid ice over a surface 

 of earlier deposited tillite occurs as far south as Eland's 

 Vlei in the Western Karroo. In Natal, 2 north of that 

 latitude, the tillite rests upon a glaciated surface of the 

 Table Mountain series. It seems likely that the tillite 

 to the south of Eland's Vlei also rests upon a glaciated 

 surface of the Bokkeveld or Witteberg beds, but this has 

 not yet been proved. 



The Dwyka series in the south is certainly much 

 thicker on the average than it is north of the Karroo, 

 and a gradual diminution in thickness has been noticed 

 in passing northwards along the western border of that 

 country from Karroo Poort to Calvinia. This is in per- 

 fect concord with the fact that the transgression, or 

 gradual extension of the water area, and consequently 

 of the shore line, took a northerly direction as shown by 

 the increasing gap in the succession below the Dwyka 

 series. There are no representatives of the Lower 

 Dwyka shales in the north, and a considerable thickness 

 of the southern tillite must have been deposited before 

 the northern tillite began to be laid down. The few 



