Traces of Ancient Changes of Climate in East Africa

Abstract

T he investigation of the traces of ancient changes of climate which the present writer started in British East Africa in 1927-1928 was partly carried out on high mountains there, partly in the eastern branch of the Great Rift Valley in the neighbourhood of the Equator (NILSSON, 1932). Traces of former pluvial periods in British East Africa (abbreviated B. E. A. below) are abundant. The heavier precipitation of these periods gave rise to large glaciers, the extension of which are marked by huge moraines far down the slopes of the highest mountains, and in the areas between the mountains we find almost everywhere evidence of such periods in the numerous river beds and ancient lake basins, now wholly or partly dried up. Especially in the basins of the Rift Valley, the series of ancient lake sediments and beaches give a very good idea of the sequence of pluvial and interpluvial periods and their subepochs which have followed each other up to the present time. During my work in B. E. A. it gradually became clear to me that traces of pluvial and interpluvial periods must no doubt occur also in adjacent parts of Africa where the conditions for their development were favourable. It seemed to me that Abyssinia must be, from my point of wiew, a very promising country, though HUME and CRAIG (1911) and other geologists consider that Abyssinia, even during the pluvial periods, had a drier climate than it has to-day. .In July 1932 I went to Kenya Colony in order to complete my former observations in the Nakuru-Naivasha Basin and on Mount Kenya, but the main object of my expedition was to extend the field of investigation to Abyssinia. During the first half of 1933 I travelled by caravan from Addis-Abbaba to the Blue Nile, Lake Tana and the high mountains of Semien in the northernmost part of the Abyssinian Plateau. Afterwards I visited the Zwai-Shalla Basin in the Rift Valley and also Mount Kaka, the highest point of the Somaliland Plateau, situated at the NW. corner of the plateau. In Fig. I Mount Kaka lies just N. of A in the word Alutu, E. of Lake Zwai.