The Okavango Delta is southern Africa' largest wetland ecosystem and probably the most pristine large wetland ecosystem in the world. Alex du Toit was the first to recognize the role of faulting in the origin of the Delta, proposing that the Delta lies within a graben structure related to the East African Rift Valley system. The history of its rivers is more ancient, extending back to the breakup of Gondwana. Two mantle plumes initiated the breakup of Gondwana: the 140 Ma Karoo plume and the 130 Ma Parana-Etendeka plume. Domical uplift and rifting associated with these plumes created two major river systems: the Okavango-Zambezi-Limpopo system and the Vaal-Orange system. The climate at the time of breakup was hot and humid and the interior experienced extensive erosion, so much so that by the Oligocene (ca. 30 Ma), the sub-continent had been planed to base level, rising only a few hundred metres above sea level and mantled by thick, leached soils (now known as the African Erosion Surface). Warping in the continent interior created uplifted arches and depressions, most notably the Kalahari basin. Arching severed the link between the lower Limpopo and its central African headwaters (Zambezi-Okavango), and a large lake formed in the Kalahari depression (Lake Palaeo-Makgadikgadi). This lake gradually disappeared, partly due to sedimentation but mainly due to the increasing dry climate.