514 



NATURE 



[October 6, 1923 



nightmare ; but some movement in that direction I 

 believe to be inevitable, and, with nationalisation of 

 the land, it might well romc more speedily than one 

 would venture to contemplate. None will question, 



at any rate, that, should such a day arrive, education 

 in the principles underlying the calling will loom as 

 largely as practical training in determining the standards 

 of admission to the use of the land. 



The Structure of the Great Rift Valley. 

 By Prof. J. W. Gregory, F.R.S. 



THE explanation that the lake chains of East 

 Africa lie in a system of tectonic valleys which 

 are a continuation of the basin of the Red Sea was due 

 to Suess (1891) in his contribution to the geological 

 results of Teleki's expedition. Suess regarded the 

 Great Rift Valley as made by a sudden rupture of the 

 crust of the earth owing to contraction, as preceded by 

 no upheaval, its age as Pliocene and Pleistocene, and 

 the height of the land beside it as due to an uplift^ in 

 consequence of the rupture ; and he considered that 

 as the East African Rift Valley is bounded by block 

 mountains and not by parallel horsts, it is different in 

 structure from that of the Rhine. The present writer, 

 after a visit in 1892-3 to the highest part of the Rift 

 Valley, supported Suess's view of its formation by earth- 

 movements due to lateral tension, but he considered 

 that the valley had a much longer and more complex 

 history than Suess recognised ; for the Rift Valley 

 was made by faulting repeated at intervals from at 

 least the Oligocene to the Pleistocene, it was initiated 

 by an uplift of a broad arch in the Upper Cretaceous, 

 and the infall of the top of that arch was probably a 

 consequence of the foundering of the floor of the Indian 

 Ocean. 



The Great Rift Valley in its course from Syria to 

 Mozambique varies greatly in structure. In some 

 places it consists of a single trench, and at others of 

 several branches. Its structure is geographically most 

 complex in Tanganyika Territory, where it was studied 

 with especial care when that area was part of German 

 East Africa. A valuable discussion of the combined 

 topographic, geological, and geodetic researches in that 

 region has now been prepared by Prof. Krenkel, of the 

 University of Leipzig.^ He shows that between the 

 Congo and the eastern coast of Africa three great 

 tectonic belts are now well established. That nearest 

 the coast forms the eastern front of the main African 

 plateau. As it is the oldest, and in the most exposed 

 position, its structures have been obscured by denuda- 

 tion. Hence the determination that this mountain 

 rampart was formed by faulting required close ex- 

 amination of its geology. The evidence available 

 shows that the central part of Tanganyika Territory is 

 traversed by a zone of fractures, which extends from 

 Lake Nyasa to the plateau front west of Mombasa. 

 This eastern zone consists in places of a series of step 

 faults, but includes, as in Uluguru, some rift valleys. 



The second belt is the continuation of the main 

 trunk of the Great Rift Valley southward from Kenya 

 Colony. It includes Lake Magadi, and forks at Lake 

 Natron ; one branch goes south-westward, and includes 

 Lake Eyasi^ and disappears near the town of Tabora. 



' [In 1891 be referred to the uprise as an Aytfw6V>ung; later as an 

 Aufwvislung.^ 



' Die Bruchzonen Ostafrikas: Tektonik, Vulkanismus, Rrdbeben und 

 SchwereanomaUen. Von Prof. E. Krenkel. Pp. viii + 184. (Berlin; 

 Gebriider Bomtraeger, 1922.) 7s. \d. 



NO. 2814, VOL. 112] 



The main trunk continues southward ; it is repeatedly 

 deflected south-westward by faults parallel to those of 

 the eastern fracture belt ; it becomes indefinite after 

 passing Kilimatinde on the railway from Dar-es- 

 Salam to Tanganyika. There is some evidence of the 

 extension of this fracture belt through the Ruaha 

 valley to Lake Nyasa. The only gap still uncertain in 

 the course of the Great Rift Valley is from thf» i ...... 



part of the Ruaha to near Kilimatinde. 



The westernmost tectonic belt follows the wisurn 

 branch of the Rift Valley, and includes the Albert 

 Nyanza and Lake Tanganyika. It forks near its 

 southern end : one branch breaks into splinters on the 

 southern coast of Tanganyika ; the longer branch goes 

 south-eastward past Lake Rukwa, joins the main trunk 

 at the Ruaha valley, and continues through Lake 

 Nyasa to south of the Zambezi, where it has been traced 

 by Teale and Wilson. The evidence of the tectonic 

 origin of the valley is especially clear around Lake 

 Tanganyika, the coasts of which show complex series 

 of faults, fault blocks, and secondary rift valleys. 

 Many of the faults are quite modem, as some of them 

 have dislocated recent conglomerates and have tilted 

 some of the lake beaches. The walls of this valley, 

 from the features noted in the original graphical 

 description of it by Burton, are young, and, as Prof. 

 Krenkel holds, the westernmost of the three tectonic 

 belts is probably the youngest. 



Between Suess's simple theory' that the Rift Valley 

 was formed from a single series of fractures in the upper- 

 most Kainozoic and my more complex classification 

 with its three different series of fractures separateH 

 by four volcanic periods, Prof. Krenkel adopts an int< r 

 mediate position. He accepts two periods of faulting 

 and three of volcanic activity for the Nyasa basin ; so 

 that his sequence of events is nearly as long as mine : 

 but he regards all the volcanic rocks as Miocene or lati : . 

 The evidence on which I referred the lava of the plains 

 near Nairobi to the Upper Cretaceous was admittedly 

 scanty ; but that age fitted in best with the general 

 history of that part of the world. Later a promising 

 clue to the age of the earlier volcanic eruptions wa> 

 offered by Dr. Oswald's work on the Victoria Nyanza ; 

 but the volcanic pebbles he collected in the pre-Miocene 

 conglomerates cannot be certainly identified. It is to 

 be hoped that some visitor to that area will make a 

 further collection of the volcanic pebbles from these 

 conglomerates, so that their position in the East 

 African volcanic sequence may be determined. 



The view that the Kapitian lava plains are Pliocene 

 has been held persistently ; but that view has now 

 been conclusively disproved by fossils collected by Mr. 

 Sikes from beds deposited in depressions in the surface 

 of these lavas. The fossils have been identified by 

 Mr. R. B. Newton as Pliocene, so that the lavas them- 

 selves must be Miocene or older. Their Cretaceous age 



