314 O. L. OOLLTE — PLATEAU OF BRITISH EAST AFRICA 



the scarps to such a degree that they are often obscure. At the same time 

 the platforms have been aggraded by detritus brought down from the 

 heights above. The scarps thus tend to blend more or less with the plat- 

 forms, and almost continuous slopes from top to bottom result. 



Only the lower scarp, which lies below the region of greatest rainfall, 

 is clearly defined. In the neighborhood of Lake Xakuru it is a steep, 

 definite clifP, which forms the west bank of the lake. 



THE VALLEY FLOOR 



The Eift Valley is an area of accumulation ; no streams drain it out- 

 wardly and no debris passes from it. All of the streams entering it from 

 the encompassing escarpments bring in large quantities of detritus, and 

 this is gradually spread out over the valley bottom, chiefly in the form 

 of fans. Volcanic dust and coarser pyroclastic material have drifted 

 across the valley and collected in many depressions. The valley floor 

 thus tends to become debris-laden ; it is a great plain of aggradation, 

 interrupted by numerous volcanic cones and lava masses. 



There are a number of depressions in the floor of the valley that are 

 generally of diastrophic origin; in some cases the lower portions of these 

 basins contain lakes. Volcanoes are numerous, especially to one side or 

 the other of the valley near the escarpments. Lava flows are abundant in 

 all portions. Some of the cones have large craters; thus Suswa has a 

 crater nearly 4 miles across and the crater of Longonot is over 2 miles 

 across. The lava flows may be narrow coulees or broad sheets; they 

 belong to two distinct periods, as noted in the case of the plateau lavas. 

 Kopjes are found separated from the parent sheet; in most cases these 

 are due to faulting, though erosion has played some part in the separa- 

 tion. 



The volcano of Longonot is typical of the Eift Valley cones. Its base 

 is made up of massive lava-sheets arranged in rude terraces. This basal 

 portion is deeply weathered and represents the earlier phase of vulcan- 

 ism; then follow the less eroded younger lavas. The upper part of the 

 volcano is a great ash and tuff cone, well dissected by deep ravines, which 

 Extend from top to bottom ; parasitic cones modify the main cone to some 

 extent. The last phase of vulcanism in the valley was explosive in type, 

 and there are innumerable ash and tuff cones on the valley floor, most of 

 them relatively small in size. 



In the central portion of the valley, especially near Eburru, there are 

 numerous steam jets, hot springs, gas vents, and such evidences of recent 

 volcanic activity. In the region between Gilgil and Elementeita the 

 recent lavas have flooded the country ; nowliere else in the region is there 



