 Consequently, a thick layer of marine sediment was deposited onto the continental shelf (the lower steps of the original rift valley walls) that surrounds the subcontinent.During the past 20 million years, southern Africa has experienced further massive uplifting, especially in the east, with the result that most of the plateau, despite the extensive erosion, lies above , is tilted so that it is at its highest in the east, sloping gently downward toward the west and south.Thus the altitude of the edge of the eastern escarpments is typically in excess of .