• THE RIFT VALLEY 315 



such an exliibition of viiloanisiii as here. Many of the massive ridges, 

 such as the Eburru Range, are probably lava volcanoes, though their 

 ridgelike form and the absence of cones may indicate fissure-flow genesis 

 for these great masses. 



LAKES AND LAKE BA8IN8 



Three of the basin-like depressions which occur in the valley contain 

 lakes, named respectively Xaivasha, Xakuru, and Elementeita. These 

 lakes have no outlets and all are alkaline, though Naivasha is but slightly 

 so. Two of the lakes occupy basins which lie parallel to the main tec- 

 tonic lines of the valley. Xaivasha is in a round basin, and is less evi- 

 dently of the foregoing type. All of the lakes are shallow, and they are 

 gradually being filled by debris brought in by the intermittent streams 

 that flow into them. Xaivasha is the most shallow of the group, its 

 greatest depth being but 35 feet and its average less than 20 feet. There 

 has been, probably, a period of greater rainfall in the region within recent 

 times and the lakes have covered a much wider extent of , territory than 

 they do now, though they have not stood long enough at any given level 

 to form definite beaches or other shorelines. Near the southern end of 

 Naivasha there is a steep-walled gorge about 125 feet above the present 

 level of the lake. This was doubtless an outlet for the lake at one time, 

 when the level stood at the higher elevation indicated by the surrounding- 

 lacustrine plains. 



DRAINAGE OF THE VALLEY 



The higher portions of the plateau on either side of the Rift receive a 

 much more abundant rainfall than the valley bottom does, and the 

 streams which head up on the heights have sufficient impetus to reach the 

 floor of the valley, though they become intermittent in the arid climate 

 of the valley bottom. Ravines are abundant on the valley sides, and they 

 reach from top to bottom; they even notch the crest at intervals. The 

 upper scarps are well dissected by ravines, but the lower ones are not, for 

 rainfall steadily diminishes from the top of the valley to the bottom. 

 The large ravines descend the higher scarp face and pass across the first 

 platform in narrow V-shaped gorges 100 feet deep or so; on the second 

 scarp and platform they retain the same general form and size; thence 

 they pass down the lower scarp and out on the floor of the valley. At the 

 outset the ravines, as they deploy on the valley bottom, are broad and 

 shallow, but their sides speedily disappear and they merge into low allu- 

 vial fans, and their course is ended on these subaerial deposits. 



The permanent streams are usually small and their valleys do not 

 differ materially from the ravine type. The topography of the valley 



XXII— Bull. Geol. Soc. Am.,.Vol. 23, 1911 



