82 THE BLACK REEF SERIES 



The Black Eeef beds are found wherever the strata 

 lying between the great Campbell Band limestone for- 

 mation and the older rocks are exposed. In the country 

 north of the Mashowing, and also on the Transvaal 

 border near Mafeking, the series is only some thirty feet 

 thick, but in the Vryburg district and in Griqualand 

 West it is from 150 to 300 feet thick, and in Prieska it is 

 probably thicker still. 



Along the Mashowing Valley and to the east as far as 

 Vryburg the series usually makes a prominent escarp- 

 ment, and even where the rocks are covered with sand 

 a distinct rise in the ground marks their position. From 

 Vryburg southwards the Black Eeef series only appears 

 occasionally from beneath the Karroo formation in the 

 Harts-Vaal Valley, and in many places there is evidence 

 that denudation in this area removed the beds over wide 

 areas in Dwyka or Pre-Dwyka times, for the Karroo 

 formation often rests directly on the Ventersdorp or 

 older rocks. In this valley the formation includes a 

 good deal of shale. In Prieska again the Black Reef 

 series is frequently exposed for considerable distances in 

 the highly disturbed belt along the south-western flank 

 of the hills made of the Griqua Town beds. 



B. THE CAMPBELL BAND SERIES. 1 



This group of rocks, one of the most important in the 

 north of the Colony owing to the large area it covers, is 

 named from the great escarpment which stretches south 



1 For descriptions of this series in Cape Colony see Stow, Q. J. G. S. t 

 xxx., 612 ; G. C., iv., 78 ; x., 151, 249 ; xi., 22, 107 ; xii., 60, 144, 176. K. 

 B. Young, T. G. S. S. A., ix., 57. 



