THE DWYKA FORMATION 403 



no indication of its attitude. The rock frequently weathered into sharp 

 pinnacles or spikes, as in plate 50, figure 1, from 1 to 3 feet high, the 

 result of flaking and splintering along surfaces that we afterward found 

 to stand usually about at right angles to the tillite sheets. We made no 

 attempt to measure the thickness of the Dwyka here, as its structural rela- 

 tions were too obscure, nor could we at this place safely make out its 

 structural relations to the underlying or overlying formations. 



The result of this first day's observations was to convince us that all 

 marine, lacustrine, fluviatile, eolian, and volcanic processes must be ex- 

 cluded from any share in making so much of the Dwyka as we had seen, 

 and to prepare us to accept the explanation given for it by the majority 

 of South African geologists, if its outcrops elsewhere should bear equally 

 unmistakable signs of glacial action. 



The Dwyka ridges near Laingsburg. — A short run by train on the morn- 

 ing of August 21 carried us from Matjesfontein about 20 miles eastward 

 to Laingsburg, where we drove out 5 miles southwestward in two-wheels r \ 

 Cape carts to the dry ravine of Witteberg river (see figure 2), a branch of 

 Buff els river, which here flows southward, transversely to the general 

 trend of the Karroo ridges and valleys. The upper Witteberg branches 

 have eroded valleys from 300 to 500 feet deep beneath the planation sur- 

 face that is strewn with cobbles of Witteberg sandstone, as has already 

 been described on an earlier page. The north-south section of the 

 Dwyka at this locality is shown in figure 7, from which it appears that 

 the Dwyka, along with the underlying Witteberg and the overlying 

 Ecca formations, has been strongly tilted and eroded. The Dwyka stands 

 nearly vertical, with steep dip to the north. The lower 1,000 feet, for the 

 most part unstratified, carry pebbles, cobbles, and boulders; the upper 

 700 feet of the formation, as it has been defined by Eogers and others, is 

 made up of ordinary stratified sandstones and shales. This upper division 

 showed no peculiar features ; it might have been well associated, as far as 

 we could see, with the overlying Ecca, from which it was arbitrarily but 

 conveniently divided by a persistent stratum of dark shales which weather 

 whitish, with an efflorescence of gypsum (shown by an undulating line in 

 figure 7 ) , so as to be traceable as a gray band on the ridge slopes for miles 

 around. Our attention was particularly directed to the lower 1,000 feet 

 of the Dwyka, where six members are to be noted, the first, third, and 

 fifth being worn down in longitudinal east-west valleys, the second, 

 fourth, and sixth standing up as ridges 200 or more feet in height. The 

 ridge-making members are each 150 or 200 feet thick; the valley-making 

 members share the rest of the total (see figure 7). 



