144 THE DESICCATION OF AFRICA. 



at least some parts of their courses. This vigorous energy of 

 their flow has as a result an intense destructive influence on 

 the beds of the rivers, and erosion is more marked as a 

 trenching or downward cutting effect than as a lateral one, 

 such as we see in Europe, where alluvial valleys are the more 

 characteristic. The amount of sand and gravel carried by a 

 river depends on the energy of the stream, which is measured 

 by its velocity; the sand and gravel dragged along by the river 

 acts as a rasp that wears away the rocks, and hence the greater 

 the velocity the greater is the erosion. The greatest velocity, 

 and consequently the greatest amount of sand and boulders 

 borne by the streams is at the sources of the streams where 

 they commence in the hills. This portion of the rivers is, then, 

 enabled to arode its bed more rapidly than the lower ones, 

 and its action is called Headstream erosion. Headstream 

 erosion is the means by which the short, rapid coast streams 

 eat back through the heart of the coastal mountains and tap 

 the waters of the inland system, whose waters are more 

 sluggish. Headstream erosion has tapped the Niger and the 

 Congo, diverting their waters from the Sahara and leaving it 

 dry. Headstream erosion, again, has tapped the Zambesi 

 basin from the east and has diverted its waters, which once 

 flowed south-west through the Kalahari, and has turned it 

 into a desert. 



Africa is such a homogeneous mass; its features are so 

 distinctive, different from those of other lands, yet the same 

 in character throughout the length and breadth of its enormous 

 extent, that an illustration taken from one part is an explana- 

 tion for any other area. The nature and effect of headstream 

 erosion, then, may be illustrated by a perfect small example 

 in Cape Colony and can be apphed to the elucidation of 

 problems in central and northern Africa. This example occurs 

 in the hills south of Grahamstown. A ridge of Carboniferous 

 sandstone forms the edge of a great tableland, 2,500 feet 

 above sea-level; to the south, the coast-shelf is a thousand 

 feet lower, and the ridge forms, as it were, the hardened edge 

 of a step in a gigantic staircase. On the north, a river scoured 

 a valley in which Grahamstown lies; it flowed east by south 

 and eventually entered the Great Fish River. The rivers on 

 the south of the ridge, heading in the thousand-feet cliffs of 

 a former sea-coast, have been working vigorously to breach 

 this rampart, and three gaps have been made in it. The flrst 

 two near Grahamstown, Howieson's Poort and Woest Hill, 

 have simply excavated large basins in the hills and are every 

 year stealing a little more from the northern drainage-area, 

 with its rivers running in beds with only moderate falls. 

 The third poort, however, is the Blaauw Krans, where the river 

 has actually pierced the ridge and captured all the eastward- 

 flowing water. The continuation eastwards is the Cap River, 



