The Orange River Grownd Moraine. 115 
favour of Green’s idea* that the Kimberley shales rest upon the 
denuded surfaces of Dwyka conglomerate and Ecca beds. There 
appears to be only one important unconformity in these districts, 
namely, that at the base of the conglomerate, but still, as will 
appear later, if our view of the evidence is correct, the shales over- 
lying the conglomerate must also be to some extent unconformable, 
however slight the gap may be. 
About two miles from Prieska village the main road to the west 
crosses a dry ravine cut by storm-water running off the Doorn Bergen 
to the Orange River. A few yards below the point where the road 
crosses there is a cliff about 25 feet high on the western side of the 
ravine, and lower down there is a similar cliff on the eastern side. 
(Plate X.). The lower 20 feet of the sections show a blue mudstone, 
generally rather friable, but sometimes as hard as the matrix of the 
well-known Dwyka conglomerate of the southern Karroo. The mud- 
stone contains numerous small fragments of various rocks and 
minerals, but on the whole it is a fine-grained hard mud with 
pebbles and boulders of various sizes (up to 24 feet in diameter) 
imbedded in it. The inclusions are scattered through the rock 
without any definite arrangement in beds—in fact, in these two 
sections the rock shows no signs of bedding (Plates XI. and XII). 
In shape the pebbles are angular, sub-angular, and rounded, the 
edges of the larger boulders are always more or less rounded. A 
large proportion of the pebbles and boulders are scratched, and 
flattened on one or more sides. They differ entirely from ordinary 
water-worn pebbles, and bear the closest resemblance to boulders 
found in the glacial tills and moraines, which are forming at the 
present day. The rocks which form the pebbles are very various 
Perhaps the most numerous in this locality are quartzites and hard 
jaspery rocks, but granite, gneiss, amygdaloidal melaphyre, diabase 
and dolomitic limestone are also abundantly represented. These 
are all known to exist im sitw in the Prieska district west of the 
Doorn Bergen, but the pebbles in this section have very probably 
come from the country north of the Orange River, where, from 
Stow’s account + and map, we know that they occupy large areas. 
An important point concerning the boulders of the conglomerate in 
the whole district is that they do not include any fragments of the 
post-Karroo dolerite, which is so frequently met with in the form of 
dykes and sheets in the districts south of the Orange River. 
In the upper 5 feet or so of the sections the rock becomes 
softer and yellow in colour owing to the oxidation of the iron com- 
pounds in the originally blue clay. 
* Q. J. G.S., xliv., 1888, p. 262. TQ) J. iGws.; max. 1874, 
