400 W. M. DAVIS — OBSERVATIONS IN SOUTH AFRICA 



west and the Cedarbergen on the east, it was noted that the western face 

 of the last named range presented rather well graded slopes, in which 

 narrow obsequent ravines were sharply cut. It did not seem as if the 

 smooth slopes and the sharp cut ravines could be due to a single set of 

 erosive processes acting together in an undisturbed physiographic cycle. 

 Some change seemed to have taken place whereby the ravine streams have 

 been enabled to incise their steep courses beneath the open slopes that 

 they should have formerly occupied on the graded mountain side. Some- 

 thing of the same kind was suggested in the distant view of the north 

 slope of Klein Zwartberg, as is indicated in the sketch, figure 6. Sim- 

 ilar features are described by Schwarz (a, 49), who gives a figure in 

 which the contrast of graded mountain slope and incised ravines is even 

 more distinct than in the cases that I had seen ; but, as in the other exam- 

 ples of relatively recent dissection described by the author cited, this one 

 is ascribed essentially to the effects of stream revival by continental 

 movement. It is quite possible that such may be the case; but it seems 

 to me desirable to maintain an open mind on that point for the present, 

 until it can be more fully determined what share climatic change may 

 have produced here as well as in the production of terraces. 



The Dwyka Formation 



general features 



Extent and stratigraphic relations. — The Dwyka formation, probably 

 of Permian date and of variable thickness up to 1,000 feet or more, is the 

 lowest member of the Karroo system, which covers some 200,000 square 

 miles in the interior of South Africa between latitudes 26 and 33% de- 

 grees south. The area of the Dwyka is shown in figure 1. It reaches the 

 eastern coast at various points in Natal and Cape Colony between latitudes 

 29 and 33% degrees south. It rests for the most part unconformably 

 on a grooved and striated surface of older rocks, but along its southern 

 border it follows conformably a series of sandstones and shales. It con- 

 sists chiefly of an unstratified and consolidated clastic groundmass with 

 sub angular or rounded scraps, stones, and boulders of many kinds, the 

 finer textured stones and boulders being usually well scratched. It is 

 conformably overlaid by a coal-bearing series of shales and sandstones. 

 It lies for the most part essentially undisturbed, but along its southern 

 border it has been strongly folded, along with older and younger strata, in 

 a series of anticlines and s}^nclines, which form the Witteberg ranges de- 

 scribed in a preceding section. The area now occupied by the Dwyka is 



