306 G. L. COLLIE PLATEAU OF BRITISH EAST AFRICA 



the underlying rocks are frequently exposed; everywhere residual boul- 

 ders of lava cover the plains. 



The newer lavas have been poured forth in such abundance that they 

 have destroyed the previous drainage and ini2)osed new conditions. Steep 

 slopes have been made, extending from the Eift out upon the Athi plains 

 to the east. The average gradient of the old lava slopes is three feet to 

 the mile, while that of the new lavas is 75 feet. 



The introduction of these new slopes served to bring in a new drainage 

 scheme. The streams flow down slopes in courses of remarkable parallel- 

 ism, and they are all true consequent streams — consequent upon the 

 slopes determined by the flow of the lava. In their upper courses the 

 streams are actively eroding; they flow in V-shaped valleys that reach 

 depths of 100 feet or more. The sides of the valleys, though steep, rarely 

 show outcrops; there is usually much fine soil, partly residual, partly 

 volcanic dust. Numerous ravines are developed on the valley slopes, and 

 these reach back into the flat, relatively narrow interstream spaces. The 

 country is well dissected, though it has not yet reached maturity. The 

 streams flow across a succession of hard and soft lava beds. In the softer 

 rocks, wdde open, amphitheater-like valleys are formed. The streams 

 tend to be ponded by waste, with the result that small lakes and marshy 

 areas abound. In the harder beds the valleys are narrow, georgelike, and 

 ungraded. Near the Eift the valleys become narrower and deeper, and 

 there is a great development of ravines, which are forcing their way back 

 into the Eift itself, and which some day they are bound to penetrate and 

 to drain. 



On the Nandi side the drainage conditions are somewhat different. 

 The descent from the Eift to A'ictoria Xvanza is about 4,350 feet — an 

 average of over 70 feet to the mile. The rainfall is about 60 inches annu- 

 ally. Under these conditions erosion is a tremendous factor. 



The comparatively narrow divide between the Eift and the Victoria 

 basin is being attacked with vigor, especially from the west. On the 

 western slope at the very outset there are deep, intricate ravines uniting 

 to form permanent streams. Dissection has become mature, though 

 numerous buttelike residuals remain scattered over the face of the coun- 

 try. There is everywhere in this area a tumult of divides and deep val- 

 leys, spurs, ridges, crests, amphitheaters, and pinnacles. The V-shaped 

 valleys are frequently 500 feet in depth. About half way to the lake, in 

 the region about Fort Ternan, the country is more maturely dissected 

 and fewer residuals occur. The valleys are wider and more shallow; this 

 is increasingly the case as the lake is approached. Finally the valley 



