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The Great Escarpment, which edges the central Southern African plateau,〔 Atlas of Southern Africa. (1984). p. 13. Readers Digest Association, Cape Town〕 is a major geological formation in Africa (See "Geological origins" below).〔McCarthy, T. & Rubidge, B. (2005). ''The Story of Earth and Life''. pp. 16-7,192-195, 202-205, 245-248, 263, 267-269. Struik Publishers, Cape Town.〕〔Truswell, J.F. (1977). ''The Geological Evolution of South Africa''. pp. 151-153,157-159,184–188, 190. Purnell, Cape Town.〕 While it lies predominantly within the borders of South Africa, in the east it extends northwards to form the border between Mozambique and Zimbabwe,〔(【引用サイトリンク】title=Great Escarpment )〕 and in the west it continues northwards into Namibia and Angola.〔 The Times comprehensive atlas of the world (1999). p. 89. Times Books Group, London.〕 Different names are applied to different stretches of the Great Escarpment, the most well-known section being the Drakensberg (diagram on the left). The Schwarzrand and edge of the Khomas Highland in Namibia, as well as the Serra da Chela in Angola, are also well-known names. == Geological origins == About 180 million years ago, a mantle plume under southern Gondwana caused bulging of the continental crust in the area that would later become southern Africa.〔 Within 10 – 20 million years rift valleys formed on either side of the central bulge, which became flooded to become the proto-Atlantic and proto-Indian Oceans.〔〔 The stepped steep walls of these rift valleys formed escarpments that surrounded the newly formed Southern African subcontinent.〔 With the widening of the Atlantic, Indian and Southern Oceans, Southern Africa became tectonically quiescent. Earthquakes rarely occur, and there has been no volcanic or orogenic activity for about 50 million years.〔Encyclopaedia Britannica (1975); Macropaedia, Vol. 17. p. 60. Helen Hemingway Benton Publishers, Chicago.〕 This resulted in an almost uninterrupted period of erosion, continuing to the present, which shaved off a layer, many kilometers thick, from the surface of the plateau.〔 A thick layer of marine sediment was consequently deposited onto the continental shelf (the lower steps of the original rift valley walls) which 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 , tilted so that it is at its highest in the east, sloping gently downwards towards the west and south. Thus the altitude of the edge of the eastern escarpments is typically in excess of . It reaches its highest point (over ) where the escarpment forms the Lesotho - KwaZulu-Natal international border.〔〔 The upliftment of the central plateau over the past 20 million years caused the original escarpment to be moved inland through erosion to its present position, creating the present-day coastal plain.〔〔McCarthy, T.S. (2013) The Okavango delta and its place in the geomorphological evolution of Southern Africa. ''South African Journal of Geology'' 116: 1-54.〕〔Norman, n. & Whitfield, G. (2006). ''Geological Journeys''. p.290-300. Struik Publishers, Cape Town.〕 The position of the present escarpment is therefore approximately inland from the original fault lines which formed the walls of the rift valley that developed along the coast-line during the break-up of Gondwana. The rate of the erosion of the escarpment, in the Drakensberg region is said to average per 1000 years, or per year.〔 Because of the extensive erosion of the plateau itself, during most of the Mesozoic and Cenozoic eras, none of its surface rocks (except the Kalahari sands) are younger than 180 million years.〔〔Geological map of South Africa, Lesotho and Swaziland (1970). Council for Geoscience, Geological Survey of South Africa.〕 The youngest rocks that remain cap the plateau in Lesotho. These are the Clarens Formation laid down under desert conditions about 200 million years ago, topped by a which erupted, and covered most of Southern Africa, and indeed large parts of Gondwana, about 180 million years ago.〔〔 These rocks form the steep sides of the Great Escarpment in this region, where its upper edge reaches an altitude in excess of . The erosional retreat of the escarpment from the coast-line to their present position, about inland from their original position, means that the rocks that are exposed on coastal plain are, almost without exception, older than the rocks that cap the adjacent escarpment. Thus the rocks found in the Mpumalanga Lowveld below the Mpumalanga portion of the Great Escarpment are more than 3000 million years old.〔 To the south and south-west the rift valleys that formed during the break-up of Gondwana, ran, as elsewhere, more or less along the present Southern African coast-line.〔 The rift valley to south of the continent separated the Southern Cape from the Falkland Plateau, which had been thrust up into a truly massive, Himalaya-sized range of mountains about 290-330 million years ago.〔〔〔Tankard, A.J., Jackson, M.P.A., Eriksson, K.A., Hobday, D.K., Hunter, D.R. & Minter, W.E.L. (1982). ''Crustal Evolution of Southern Africa''. p. 352-364, 407. Springer-Verlag, New York.〕 Sediments eroded from these Gondwana mountains buried the Cape Fold Belt and formed the thick Beaufort Group of rocks of the Karoo basin.〔〔 As the escarpment eroded, moving inland, the buried Cape Fold Mountains that had formed 150 million years earlier, were gradually re-exposed. Being composed of erosion resistant quartzitic sandstone they erupted through the eroding landscape, ultimately to form the parallel mountain ranges that protrude from the coastal plain of the south and south-west Cape.〔 The eastern portion of the Great Escarpment (the Drakensberg) goes as far north as Tzaneen at about the 22° S parallel, from where it veers west to Potgietersrust, where it is known as the Strydpoort Mountains.〔〔Encyclopaedia Britannica (1975); Micropaedia Vol. III, p. 655. Helen Hemingway Benton Publishers, Chicago.〕 The absence of the Great Escarpment for about to the north of Tzaneen (to reappear on the border between Zimbabwe and Mozambique in the Chimanimani Highlands), is due to a failed westerly branch of the main rift that caused Antarctica to start drifting away from Southern Africa during the breakup of Gondwana about 150 million years ago. The lower Limpopo River and Save River drain into the Indian Ocean through what remains of this relict incipient rift valley, which now forms part of the South African Lowveld.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Great Escarpment, Southern Africa」の詳細全文を読む スポンサード リンク
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