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The Drakensberg (derived from the Afrikaans name ''Drakensberge'' meaning "Dragon Mountains") is the name given to the eastern portion of the Great Escarpment, which encloses the central Southern African plateau. The Great Escarpment reaches its greatest altitude in this region ( – ). The Drakensberg escarpment stretches for over from the Eastern Cape Province in the South, then successively forms, in order from south to north, the border between Lesotho and the Eastern Cape and the border between Lesotho and KwaZulu-Natal Province. Thereafter it forms the border between KwaZulu-Natal and the Free State, and next as the border between KwaZulu-Natal and Mpumalanga Province. It winds north, through Mpumalanga, where it becomes known as Blyde River Canyon or Three Rondavels. It moves north again as far as Tzaneen in Limpopo Province, where it becomes the Wolkberg Mountains or Iron Crown Mountain, at above sea level, the Wolkberg becomes the highest mountain range in Limpopo. It veers west again and at Mokopane it is known as the Strydpoort Mountains.〔''Atlas of Southern Africa''(1984). p. 13, 190-192. Readers Digest Association, Cape Town〕〔''Encyclopædia Britannica'' (1975); ''Micropaedia'' Vol. III, p. 655. Helen Hemingway Benton Publishers, Chicago.〕 == Geological origins == About 180,000,000 years ago, a mantle plume under southern Gondwana caused bulging of the continental crust in the area that would later become southern Africa.〔McCarthy, T. & Rubidge, B. (2005). ''The Story of Earth and Life''. pp. 16-7,192-195, 245-248, 263, 267-269. Struik Publishers, Cape Town.〕 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.〔〔Truswell, J.F. (1977). ''The Geological Evolution of South Africa''. pp. 151-153,157-159,184–188, 190. Purnell, Cape Town.〕 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.〔''Encyclopædia Britannica'' (1975); ''Macropaedia,'' Vol. 17. p. 60. Helen Hemingway Benton Publishers, Chicago.〕 An almost uninterrupted period of erosion has continued to the present, resulting in layers several kilometers thick having been lost 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, further massive upliftment, especially in the East, has taken place in Southern Africa. As a result, most of the plateau lies above despite the extensive erosion. The plateau is tilted such that its highest point is in the east, and it slopes gently downwards towards the west and south. 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.〔〔''Atlas of Southern Africa.'' (1984). Readers Digest Association, Cape Town〕 The upliftment of the central plateau over the past 20 million years and erosion resulted in the original escarpment being moved inland, 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 approximately 150 km inland from the original fault lines which formed the walls of the rift valley that developed along the coast 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, which occurred over 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 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 to its present position, means that the rocks of the coastal plain are, with very few and small exceptions, older than those that cap the top of the escarpment. Thus the rocks of the Mpumalanga Lowveld below the Mpumalanga portion of the Great Escarpment are more than 3000 million years old.〔 The rocks of the KwaZulu-Natal Midlands belong, in the main, to the Beaufort and Ecca Groups (of the Karoo Supergroup), aged 220-310 million years, and are therefore considerably older than the Drakensberg lavas (aged 180 million years) which cap the escarpment on the border between KwaZulu-Natal and Lesotho.〔 The entire eastern portion of the Great Escarpment (see the accompanying map) constitutes the Drakensberg.〔〔The Times comprehensive atlas of the World. (1999) p. 90. Times Books Group, London.〕 The Drakensberg terminate in the north near Tzaneen at about the 22° S parallel. The absence of the Great Escarpment for about 450 km (280 miles) 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 Low veld.〔 When most South Africans and visitors speak of ''the'' Drakensberg, they refer to the Great Escarpment that forms the border between Lesotho and KwaZulu-Natal, believing it to be a range of mountains extending into Lesotho, more correctly known as the Lesotho Highlands. (This narrow use of the name "Drakensberg" is analogous to the use of the name "America" to refer to just the United States of America, rather than to the continent in which the United States is situated.) This highest portion of the Great Escarpment is known as ''uKhahlamba'' ("Barrier of up-pointed spears")〔Pearse, R.O. (1973) ''Barrier of Spears. Drama of the Drakensberg''. p. i. Howard Timmins, Epping, Cape〕 in Zulu and ''Maluti'' in Sotho. 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「Drakensberg」の詳細全文を読む スポンサード リンク
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