Two major colluvial deposits have been identified on the slopes of the Iran
gi Hills in north central Tanzania. The oldest of these occurs in shallow d
epressions on the pediment slopes. The time of deposition has been dated by
the use of optical stimulated luminescence (OSL) dating techniques to Late
Pleistocene. The OSL dates were 14,700+/-1600, 14,200+/-1500, and 11,400+/
-1300 years. In Late Pleistocene time the climate changed from dry to wet c
onditions throughout much of the tropics and this time period is frequently
reported as a period characterised by enhanced soil erosion. It is likely
that the older colluvial unit in the Irangi Hills was formed as a result of
climatic change when rainfall increased ahead of a vegetation change and t
hereby provided conditions favourable for soil erosion. The younger colluvi
al unit yielded deposition ages of 400-600 years and is part of a number of
successive erosion and deposition periods during the last millenium, proba
bly as a result of anthropogenic impact on the landscape.