452 TERTIARY EROSION 



represented by the Kaap Plateau and the flat-topped 

 chains of the Doornberg, Ezel Band, Langberg, etc. 

 The lower limit of its altitude here is 4,000 feet. 



By a recurrence of the upward movement of the 

 continent the Orange and Vaal Kivers have trenched 

 deeply into this peneplain to a depth usually of 800 to 

 1,000 feet, in the Stormberg 1,500 feet. In the west 

 the rivers have cut through the Karroo deposits which 

 rest upon the Pre-Dwyka floor, and have laid bare the 

 old drainage system which was in existence in that part 

 of the Colony in Pre-Dwyka times. 



The present drainage system has been greatly modi- 

 fied by its superposition upon that of Pre-Dwyka times, 

 and the numerous terraces at various levels throughout 

 the area prove the varying rates of downward erosion 

 caused by barriers of harder rock. It seems very pro- 

 bable that the last uplift whereby the present features 

 of the Karroo were determined was either of late or 

 post-Cretaceous age. 



The wide flats of the Karroo in the north of Cape 

 Colony and in the Orange River Colony indicate a mature 

 stage of erosion ; the country having been cut nearly to 

 a flat above which rise hills and ridges of more resistant 

 rock generally produced by intrusions of dolerite. It 

 seems not unlikely that for a certain period during this 

 process of degradation of the continent arid or semi-arid 

 conditions prevailed in the northern part of Cape 

 Colony and in the Kalahari. 1 



Returning now to the southern rivers, which we cie- 



1 Davis, Geol. Spc. America, xvii., p. 377, 1906. Du Toit, Tr. Roy. 

 Soc. S. Africa, vol. i., pt. 2, 1909. Passarge, Die Kalahari, ch. xxxv. 



