THE RTFT VALLEY ?>1'^> 



approaches of the valley. The flows apparently have come from fissures 

 which followed approximately the present course of the valley — fissures 

 that in many cases may later have become the fault-planes of the great 

 rift. 



ESCARPMENTS AND PLATFORMS 



The walls of the valley consist of a series of cliffs and platforms ; they 

 represent, respectively, fault-scarps and fault-block surfaces. The strik- 

 ing feature of the topography is its terraced character, exhibited on a 

 titanic scale. The number of scarps vary. Sometimes there are but two ; 

 again there may be three, four, or even more ; the normal number in this 

 region is three. The number of scarps is determined by the development 

 of subsidiary fault-planes, which branch out from the main fault and 

 produce a larger or lesser number of fault-blocks, as the case may be. 

 Where the railroad descends into the valley there are three scarps and 

 two platforms. The total depth of the valley here is about 1,300 feet, 

 divided between the three scarps as follows: Upper scarp, 350 feet; 

 middle scarp, 700 feet; lower scarp, 250 feet. The height of any given 

 scarp is not uniform, for the fault-blocks are often tilted, the throw at 

 one end being greater than at the other; or there may be unequal sub- 

 sidence in different portions of the same Block. The slopes of the scarps 

 are rarely very steep or precipitous ; they usually permit the accumulation 

 of soil. Some slopes are too steep to allow such accumulation, though 

 talus frequently gathers at the base of such cliffs and it may reach far 

 up the scarp face. 



The soil supports a dense tropical forest on the upper heights, but 

 vegetation becomes scanty toward the bottom of the valley, where arid 

 conditions prevail. 



The fault-planes are not straight, as a rule, but sinuous. The wall of 

 the Rift Valley curves back and forth; the minor faults which produce 

 tlie smaller blocks are generally curved, also. This curving produces 

 blocks somewhat semi-lunar in shape, with their convexity toward the 

 valley, as a rule. The blocks are widest in the middle and taper off at 

 each end; generally the smaller blocks do not exceed a length of 2 or 3 

 miles. Frequently the blocks pitch away from the valley, but often they 

 pitch toward it, or to the north or south. The eastern or Kikuyu wall 

 is the steeper, and it has the more rugged and angular topography. The 

 reason is that the rainfall is less on that side and there is far less accumu- 

 lation of tuffs and other soft pyroclastics. The Nandi or west wall is the 

 loftier, and originally the fault-scarps must have been more conspicuous 

 there; but erosion has been so active on this side that it has worn down 



