The Tertiary and Post-Tertiary Lacustrine Sediments of the Nyasan Rift-Valley

Abstract

In the course of their examination of the country lying along the north-western shore of Lake Nyasa in 1906-1909, Dr. A. R. Andrew & Mr. T. E. Gr. Bailey recognized two main groups of sediments resting upon the crystalline rocks: the older of these sediments was found to be of Karroo age, whereas the younger, in virtue of its general incoherency and the apparent absence of fossils, apart from certain mollusca, was regarded as belonging to Recent times. Although later work has shown the necessity for a revision of certain of these conclusions, we are none the less indebted to these earlier writers for their valuable pioneer work on the geology of Nyasaland. The uppermost ‘Karroo’ of the Nkana area and part of the ‘Recent’ deposits of other areas are now known to constitute the Deinosaur Beds of late Jurassic or early Cretaceous age, which have been followed over a distance of 80 miles along the northwestern shore of Lake Nyasa; moreover, the remaining ‘Recent’ deposits fall into six groups that range through later Tertiary and post-Tertiary times. The second oldest of these groups, designated below ‘the Chiwondo Beds’, but first mentioned by Andrew & Bailey, has recently yielded remains of Mastodon and of Hippopotamus , and these mammalian fragments show that the beds are of Pliocene age; moreover, a new series of lacustrine shells has been found in association with the mammalian bones. Lake Nyasa lies within the Great Rift-Valley that runs down the eastern side of Africa from the