The palaeoenvironmental interpretation of colluvial sediments and palaeosols of the late pleistocene hypothermal in Southern Africa

Abstract

Colluvial deposits, reaching thicknesses of more than 10 m, mantle the pediment slopes and choke the streams and river valleys of an area covering about 20% of Africa south of the Zambezi. The sediments are usually deeply incised by gully systems, termed dongas, which have exposed abundant archaeological remains in the form of stone tools dating from the African Middle Stone Age. While the bedrock types in the different areas exhibiting colluvium vary greatly, the range in present mean annual rainfall is very narrow, being between 600 and 800 mm. The grain size characteristics of 55 samples of colluvium show a broad similarity in the percentages of sand-, silt- and clay-sized material. In the majority of cases the percentage of sand ranges from 45 to 65%, silt from 15 to 25% and clay from 10 to 35%. The 14C ages of palaeosols in the sediments in Swaziland, Natal and Zululand, based upon the dating of calcium carbonate nodules and charcoal from cave sediments containing identical stone tool assemblages, indicate that colluviation took place between about 30,000 and 12,000 years ago. The palaeosols and colluvial sediments of southern Africa are indicative of semi-arid conditions in which the regolith was stripped from poorly vegetated hill slopes and laid down on gently inclined pediment slopes. The age of the deposits supports the contention of several recent workers that the area experienced dry climatic conditions during the Last Glacial Maximum as a result of restricted circulation and reduced sea surface temperatures in the Southern Ocean.