358 Transactions of the Boyal Society of South Africa. 
ultimately has succeeded in cutting its channel to a depth in places of 
400 feet, a small tributary which may possibly have joined it near 
Klipdam was only faced with shales and dolerite. It was therefore 
able to cut its way backwards, at the same time shifting its course 
laterally towards the north-west and finally forming the Harts Valley. 
This river, although possessing only a fraction of the flow of the 
Vaal Eiver, has been able to re-excavate a section of the pre-Karroo 
Kaap Valley, capturing during its progress the drainage which the Vaal 
would have received from the north and north-west. The removal of 
material, principally shale, has been so extensive that the Harts River 
in the lower part of its course now meanders in a broad valley, whereas 
the Vaal is devoid of loops with the exception of the curve known as the 
"Bend," which is due to the excavation of a basin filled with Dwyka 
tillite and shales. The most interesting feature concerning the Harts 
River, and one that is so dependent on the nature and history of the 
palaeozoic land surface is the fact that over a great distance its channel 
possesses a far lower gradient than that of the Vaal River. 
That a tributary should flow at a lower level than the main stream for 
such a distance is almost unique, and upon this peculiarity a great 
irrigation scheme has been based.''' It has been proposed, in fact, to 
divert a portion of the flow of the Vaal River just above Warrenton and 
lead it up the Harts Valley as far at least as Taungs. As a matter of 
fact, the altitude of the Vaal River is 3,880 feet above sea-level, while that 
of Taungs Station is 3,590, and Brussel's siding only 3,700 — that is, the 
water could be taken to within a few miles of Vryburg, 70 miles away to 
the north. 
The Kaap retains its peneplainic features to a marvellous degree ; this 
is apparently due to two causes. Firstly, the Dwyka tillite and shale 
which was banked up against the limestone escarpment has protected the 
face from active erosion up to a very late stage. Secondly, owing to 
the jointed nature of the limestone the rainfall is conducted underground, 
as is proved by the numerous springs along the base of the escarpment. 
The surface has also been protected to a certain extent by a deposit of 
calcareous tufa. South of Griqua Town, however, the Sand River has 
been very active in removing the Dwyka where the peneplain is continued 
over this formation. 
Lastly, we come to the consideration of the Brak River and its various 
tributaries which drain the area north of the main watershed in De Aar, 
Richmond, Victoria West, and Britstown. Over the greater part of this 
tract of typical " Karroo " country the rivers flow along channels but 
slightly incised in the broad flat plains, but here and there they have cut 
narrow gorges through dolerite ridges and occasionally through miniature 
* Report of the Director of Irrigation, &c., Parliamentary paper G41, 1906, p. 57. 
