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The Namib and Kalahari deserts preserve evidence of a long history of aridification in southern Africa. The earliest sedimentation within the Kalahari Basin dates to the Late Cretaceous, but the first onset of arid conditions probably occurred in the Early Tertiary; other arid episodes recurred throughout the Tertiary but the development of the Kalahari Sand Sea, with its linear dune system, is unlikely to predate the Late Pliocene. This, the largest single body of aeolian sand in the world, is now largely stabilized by vegetation, but evidence exists for its periodic reactivation throughout the Quaternary. Although episodes of dune building are evident within the Namib region as early as the Eocene, the Namib Sand Sea, represented by the now consolidated Tsondab Sandstone Formation and the overlying active dunes of the Sossus Sand Formation, does not predate the Upper Miocene. Evidence from the Upper Pleistocene suggests, however, that there were significant fluctuations in the degree of aridity which prevailed during and after the last glacial period.