304 G. L. COLLIE PLATEAU OF BRITISH EAST AFRICA 



of the country. The moiiadnocks of high altitude are deeply eroded; 

 their sides are gashed by ravines which reach from summit to base. The 

 greater precipitation at the summits is the reason for this condition. 



In the Voi-Makindu region erosion has gone on to such an extent that 

 the monadnocks form barely 10 per cent of the total area. In the region 

 of Kiu they are much more numerous and closer together, and here fully 

 one-half of the region remains comparatively unattacked. 



THE RELIEF 



At the eastern border the gneiss plateau is 1,100 feet above the sea; 

 near Kapiti plains, where the lava fioAvs begin to cover the gneiss, the 

 elevation is 5,300 feet; the average rise is 21 feet to the mile. For tlie 

 first 50 miles, until Yoi is reached, the average rise is only about 15 feet 

 to the mile. From Yoi to Tsavo there is a drop of 300 feet, Tsavo being 

 located in a depression probably of tectonic origin. From Tsavo to 

 Makindu the rise is 24 feet to the mile ; from Makindu to Kiu, the latter 

 in the more uneroded portion, the average gradient is 30 feet; from Kiu 

 to Machakos Eoad the average rise is 100 feet to the mile. 



As has been indicated, the gradient of the plateau is greater than 

 ought to be expected from normal erosive processes. It suggests again 

 that tectonic movements have helped to make this relatively large gra- 

 dient. In general the gneiss plateau has a thick soil, alluvial in the 

 shallow valleys and residual on the divides. The country is open and 

 parklike; trees and bushes of semi-arid types abound in most localities. 

 Recent lava flows sometimes cover the gneiss locally; they smooth off 

 even the faint relief of the gneiss, as about Simba, where tliin flows 

 spread out from small volcanoes and cover the country to quite an extent. 

 On the other hand, near Kenani the Ndunga and Yatta flows of volcanic 

 origin enhance the relief of the region. These hills have a sharp rec- 

 tangular outline, which stands out in marked contrast to the flowing 

 outlines of the gneiss hills. The Xdunga hills form a remarkable mesa 

 170 miles long and but 4 miles wide; the lava which formed the flow 

 came from a fissure of the same length, according to those who have 

 examined the region. 



The Lava Province 



EXTENT 



This great region extends from the Kapiti plains to Yictoria Nyanza, 

 a width, measured directly, of 180 miles. The lava belt is of unknown 

 length, but extends across British East Africa far north into Abyssinia 



