CHAP. XII AN EXTINCT LAKE 233 



Lake Suess must have lasted for a considerable period, 

 gradually dwindling as the climate became more arid, and as 

 earth-movements cut off its extension to north and south. One 

 of these earth-movements formed the ridge crowned by the 

 volcano Longonot, which separates the Kedong and the Naivasha 

 basins. That this ridge is later than the lake is shown by the 

 fact that the terraces on its flanks are uptilted. It is also 

 certain that the lake was earlier than the last of the lava flows 

 from Longonot, for these have spread out over the alluvium, a 

 fact illustrated in PI. IX. (p. 97). Volcanic action, however, 

 took place in the district while the lake was in existence, for 

 Doenyo Nyuki (the Red Mountain) once rose above its surface 

 as a volcanic island. 



In the later stages of the history of Lake Suess volcanic 

 activity of a violent type was renewed, new craters being 

 formed along lines running north and south. Kibo — the 

 highest mountain in Africa — was a member of this series, to 

 which also belong the Kyulu Mountains (a line of craters in 

 Kikumbuliyu to the west of the Uganda road), Elgon and 

 Lekakisera to the north-east of the Victoria Nyanza, and the 

 most recent craters on the floor of the Rift Valley, such as 

 Suswa and Longonot. These volcanic disturbances were 

 followed or accompanied by another series of earth-movements. 

 The chain of volcanoes from Elgon to Lekakisera apparently 

 marks a line of elevation, which cut off the connection between 

 the Nyanza basin and Basso Narok (Lake Rudolf). Parallel 

 to this was a line of depression, which lengthened the Albert 

 Nyanza Rift Valley, and formed the gorge between Wadelai 

 and Lado, through which the waters of the Central Basin of 

 Africa gained an outlet to the Nile. 



Mention of this event reminds us that in Equatorial Africa 

 there is not merely one Rift Valley, but a series, which ramifies 

 through the country like the rill systems of the moon. Suess 

 has shown that these " valleys of dislocation " often end by 

 splitting into a series of smaller ones, which he calls a " virga- 

 tion," from its resemblance to the loose ends of a bundle of 

 twigs. The Great Rift Valley presents a typical case of this 

 in Palestine. In Africa, however, it branches occasionally, 

 and forms an open system. The members of this have not 

 been geologically surveyed, and in many cases the topographical 



