236 THE GEOLOGY OF BRITISH EAST AFRICA part hi 



After the completion of the system of Rift Valleys, the 

 study of East Africa becomes the work of the archaeologist 

 and historian, instead of the geologist. The stone implements 

 of the old terraces of Lake Baringo (see pp. 323, 324) show 

 that man entered the region while yet the lakes were larger 

 than they are to-day. Volcanic eruptions still took place 

 and earth-movements continued, for some of the fault-scarps 

 are so bare and sharp that they must be of very recent 

 date. This continuation of earth-movements into the human 

 period is one of the most striking features of the district. 

 Whereas, according to the old view, British East Africa was 

 supposed to have acquired a condition of stable equilibrium at 

 a very early age, it has, on the contrary, been in a continual 

 state of change since the time of the formation of our Oolitic 

 limestones. During the eras between those of the Archean and 

 of the Chalk, the country may have enjoyed comparative rest ; 

 but in the age of the latter, there began one of the two greatest 

 of the series of volcanic outbursts known in the world's history. 

 This and the resultant series of earth - movements have kept 

 the region ever since in a condition of disorder and unrest. 

 One region has been raised and another depressed ; in one 

 place a fiord has been opened from the sea, and then separated 

 from it ; elsewhere a line of movement has reversed the direction 

 of rivers, and transferred lakes from one river system to another ; 

 while differences in elevation have caused variations in climate 

 and rainfall. The evidence of these changes is apparent on every 

 hand. Scars of great earth-movements, extinct volcanic craters, 

 dried lake basins and old river beds, show the extent and 

 recent date of these events, and the structural instability of the 

 region of the Great Rift Valley. 



In later chapters we shall see that this structural instability 

 has had a most important influence on all branches of the 

 natural history of British East Africa, for its results have 

 affected the development of both animals and plants, and 

 helped to mould the character of the people. 



