A major groundwater resource across the high African Plateau is shown to occur within the thick overburden derived by in situ chemical weathering of the underlying crystalline Basement Complex. The saprolite aquifer horizon is identified at the base of this weathered zone. The chemical weathering processes that produced this aquifer horizon are largely controlled by circulating groundwater, and this in turn is a function of the climate. The saprolite profiles tend to have similar characteristics over a wide variety of rock types. The geomorphological development of the plateau by cyclic erosion has resulted in a predictable distribution of the aquifer and the analysis of data from over one thousand wells shows it to be hydrogeologically very uniform.