The Evolution of the Biver System of Griqiialand West. 355 
foothills and despite the facts that the hard strata forming them are highly 
tilted and disturbed. 
From the positions of its gravel terraces the Vaal Eiver seems to have 
occupied a course probably not very far removed from its present one, but 
from the occurrence of fragments of crocidolite and jasper in the gravels 
near Klipdam — it is most likely that the Harts Eiver was not then in 
existence. If we therefore reconstruct in imagination this surface we 
find it extending away from the foot of the basalt terraces of Basutoland, 
north-westwards to beyond Prieska, and northwards over the Orange Eiver 
Colony and the south-western corner of the Transvaal. 
Between Aliwal North and Prieska the fall of the surface will have 
been from the 6,000 to the 4,000-foot contour, or at the rate of 9 or 10 
feet per mile — just about twice the gradient of the Orange Eiver in this 
section. Should this value be thought too much, it may be stated that in 
post-Karroo times the southern and south-eastern portions of the Colony 
appear to have been in a more unstable condition than the interior, and 
that there may have been a tilting of the surface of planation due to a 
greater uplift in the Drakensberg region. A comparison with the Kaap 
Plateau, however, brings out the fact, that in the latter the gradient 
possesses a somewhat greater value. 
In attempting to determine the geological date of this uplift, it seems 
most likely that the movement was closely connected with that by which 
the outliers of Uitenhage (Lower Cretaceous) beds were faulted down in 
the folded belt of the south of the Colony, for in every case the upthrow is 
on the north or inland side. Again in late Cretaceous times there was a 
gental crustal flexuring which affected the belt of high ground (Drakens- 
bergen) running almost parallel to the Indian Ocean — a feature which 
has been ably discussed by Professer Penck.* 
On this assumption the cutting of the Kaap-Stormberg peneplain can 
be ascribed to the close of the Cretaceous epoch while the entrenchment 
of the river valleys and the development of the present surface features 
may have been produced entirely within Tertiary and post-Tertiary times. 
In this connection, therefore, it may be noted that in comparing the 
amount of the denudation in the Karroo with that in Arizona Professor 
Davis f has pointed out that in the former the amount of erosion appears 
to be less than would be expected if it had taken place during much of 
Cretaceous as well as the whole of Tertiary times. 
* A. Penck, " Sitz. Kgl. Preuss. Akad. d. Wissen.," p. 230, 1908. 
t W. M. Davis, Bull. Amer. Geol. Soc, vol. 17, p. 444, 1906. 
