THE GEOLOGY OF BRITISH EAST AFRICA part hi 



of faults and earth -movements still dominated the scenery. 

 An hour after entering this valley, we reached the edge of the 

 Great Rift Valley, which, like the former, must be directly due 

 to earth-movements. Once the plateaux of Mau and Kikuyu 

 were continuous across the site of the Rift Valley ; a double 

 series of north and south cut through the plateaux, and 

 allowed the block of material between them to subside. This 

 left a great open Rift Valley (or, to use Prof. Suess's term, 

 a " Graben "). This method of valley formation is illustrated 

 by Fig. 9 ; strips of country have fallen owing to a series of 

 parallel cracks or " faults," and thus a valley has been formed 

 with precipitous, and sometimes step-like sides. Such valleys 



Fig. 9. — Section across Rift Valley. (F = Faults. ) 



have long been known in America, and the extraordinary 

 steepness of their bounding walls may be seen in photographs 

 of the Yosemite Canon in California. 



Block Mountains. — A third feature in this region of Africa 

 is mountains constructed on a different plan from those typical 

 of Europe ; there they are formed by actual elevation owing 

 to the intrusion of igneous rocks or to the folding of beds 

 once laid down as horizontal sheets of sediment ; some of the 

 mountains of the Rift Valley, on the other hand, are formed of 

 layers which are still horizontal (as in Fig. 10) ; each mountain 

 consists of a huge block of material ^ that has been left standing, 

 while the rest around it has fallen to a lower level. 



^ A " SchoU " in the terminology of Professor Siiess. 



