THE GEOLOGICAL HISTORY OF THE COLONY 433 



beginning of the change in geographical conditions 

 which eventually brought about the cutting off of the 

 Colonial area from the ocean in the middle of the 

 Bokkeveld period. 



The shore line at the commencement of the Dwyka 

 period lay in an approximately east and west direction 

 through the neighbourhood of Karroo Poort, and the 

 shales and muds which were deposited near it are very 

 like the more argillaceous sediments of the Witteberg 

 series ; they contain none of the fossil plants found in 

 the latter, but a few plants of a similar nature to some 

 of those found in the Ecca beds have been obtained 

 from them. This shore line appears to have gradually 

 crept northwards, but it did not gain much upon the 

 land area to the north before the conditions set in that 

 caused a general glaciation of that land. 



There is clear evidence in Griqualand West, Hope- 

 town, and Prieska that a long valley led south- south- 

 west wards east of the escarpment of the Kaap in Dwyka 

 times, and that it received tributaries from the country 

 to the east and west ; this valley was probably in exist- 

 ence during the deposition of part of the Cape system. 



We have seen in a previous chapter that there can be 

 no doubt of the fact that South Africa north of the 

 th rty-third parallel was in part, at least, covered with 

 snow and ice, and that the Dwyka tillite is made of 

 the mud, sand, pebbles and boulders derived from the 

 glaciated country. 



In the northern parts of the Colony, as well as in the 

 eastern districts of the Transvaal and western portion 



of the Orange River Colony, the Dwyka tillite has to 



28 



