44 PRESIDENTIAL ADDRESS SECTION T!. 



loids are vestiges, also at present undergoing erosion. It cannot 

 have been long, geologically, since this protective capping was 

 removed along the margins of these deposits, and thus a pre- 

 Karroo landscape has been, and is being, revealed, and is now 

 open to the sunshine after ages of burial. 



We can thus gather some idea of the physiography of this 

 old land surface, and are struck by the remarkable evenness and 

 maturity to which it had been brought in pre-Karroo times. 



This uniformity is evident in many places — between Tabas 

 Induna and llillside. for example. During vour visit to the 

 Bushman's paintings you will be near an outlier of forest sand- 

 stone that, in the form of loose sand, chips of agate and 

 chalcedony, covers the southern side of the Hillside kopjes. 

 From this position looking north can be seen Tabas Induna, 

 which is the receding edge of the Karroo covering, while in 

 between is the lately exposed pre-Karroo landscape. 



In North-Western Rhodesia the open plain stretches mile 

 after mile, the horizon is as of the ocean, and the few hills, of 

 not more that 300 feet in height, emphasise this contrast all the 

 more. 



The influence of the Karroo covering in shaping the features 

 of Rhodesia is supreme. As the mantle was gnawed back by 

 the Sabi and Umpopo river systems, erosion into the archaean 

 floor was commenced. On the north side of the plateau crest 

 the Zambesi basin still contains a great depth of clastic rocks, 

 the porosity of which tends to pass the water down through the 

 beds rather than over tlie surface. 



During the middle Karroo and at its close, movements took 

 place by which the Zambesi and Limpopo basins arrived at a level 

 lower than the plateau ridge. The gentle nature of the resultant 

 anticline may be judged by the fact that there is but a fall of 

 fifteen feet per mile between Bulawayo and the Limpopo. 



Dotted over the country are eminences and masses of rocks 

 that are slow of decay, and it is notable that nowhere do they 

 rise remarkably above each other. One might, as it were, place 

 a straight-edge along their tops and on the summit of the plateau, 

 and find that, with the allowance for the anticline above stated, 

 they would all approach a general level. I therefore look upon 

 these hills and mountains as bench-marks that indicate the level 

 of the old pre-Karroo land-surface — the flat areas between them 

 having been removed, by erosion subsequent to the removal of 

 the Karroo covering. All sculpture of the plateau has taken 

 place since that event, and thus we find that regions that were 

 earliest bared would now present a deeper incised appearance 

 (such as the Mazoe and Sabi rivers) than those near the margins 

 of the sandstone covering, as at Bulawayo and Gwelo. 



As examples of my point we may take the Matopos. They 

 are of the same general altitude as the plateau summit near 

 Bulawayo, yet in between is the Malemu, which has eroded a 

 valley in the contact zone of granite and schist, and cleaves the 

 INIatopos in its passage to the south. Yet to do this it must at 



