310 G. L. COLLIE PLATEAU OF BRITISH EAST AFRICA 



In spite of different rock formations and of differing climatic condi- 

 tions^ the whole plateau is quite uniformly graded from ocean to summit 

 near Victoria Kyanza. It is difficult to explain this rather uniform 

 gradation in a satisfactory way, for it looks as if some one factor con- 

 trolled the whole peneplanation, and that factor was the ocean. It is not 

 easy to understand how the ocean, as baselevel, could control peneplana- 

 tion on the high plateau, where arid conditions prevail, where deflation 

 is important, and other factors are operative which are not controlled 

 primarily by the sea. 



The best explanation to be given is as follows : The ocean as a base- 

 level does control the outer belt of the plateau in proximity to it. This 

 belt in turn does exercise some influence on the semi-arid belt which 

 adjoins it to the west. The gradation of the outer belt is wholly carried 

 on by normal erosion, and the amount of this gradation determines how 

 much material may be carried out from the semi-arid belt at any given 

 time and especially in those infrequent years when streams are able to 

 leave the dry region and reach the sea because of hea^'}^ rainfall in the 

 interior. There is always the possibility that material may be carried 

 seaward from the interior. The amount that is to be carried in the 

 favorable years will depend quite largely on the grades that exist on. the 

 outer humid belt down which the detritus from the semi-arid region must 

 pass. In this way the outer humid belt has some control over the de- 

 velopment of the wide, semi-arid belt of the interior. In turn the semi- 

 arid belt controls the rate and kind of degradation on the wet belt of the 

 high plateau to the west. There is abundant precipitation on this high 

 plateau, and it is being degraded by the numerous streams which cross it. 



These streams flow out on the dry belt of the plateau, where they 

 wither and deposit their load as a result. A grade is consequently de- 

 veloped between the wet belt and the dry belt, a grade that can not be 

 changed materially except through the cooperation of the semi-arid part 

 of the plateau. In other words, the wet belt of the high plateau can not 

 be degraded any faster than the semi-arid belt permits. There tends 

 thus to be adjustment between all parts of the plateau in spite of climatic 

 and other differences. 



In the semi-arid belt there are numerous residuals, chiefly of gneiss. 

 Rainfall is fairly constant and abundant on the summits of the higher 

 monadnocks, and they are being lowered by normal erosion down to the 

 level where the arid conditions set in. When the normal erosion ceases 

 or is largely checked, then the residuals are lowered no faster than the 

 general region, and they thus tend to be preserved indefinitely. 



The second or lower cycle of erosion on the plateau is being developed 



