Great Escarpment, Southern Africa
The Great Escarpment is a major topographical feature in
Different names are applied to different stretches of the Great Escarpment, the most well-known section being the
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.[2] Within 10–20 million years, rift valleys formed on either side of the central bulge and flooded to become the proto-Atlantic Ocean and proto-Indian Ocean more or less along the present southern African coastline and separating the Southern Cape from the Falkland Plateau.[2][3] The stepped, steep walls of these rift valleys formed escarpments that surrounded the newly formed Southern African subcontinent.[2]
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 lies above 1,000 m (3,300 ft) despite extensive erosion. The plateau is tilted such that it is highest in the east and slopes gently downward toward the west and south. Typically, the elevation of the edge of the eastern escarpments is in excess of 2,000 m (6,600 ft). It reaches its highest point of over 3,000 m (9,800 ft) where the escarpment forms part of the international border between Lesotho and the South African province of KwaZulu-Natal.[1][2]
With the widening of the Atlantic, Indian, and Southern oceans, southern Africa became tectonically quiescent. Earthquakes rarely occur in the region, and there has been no volcanic or orogenic activity for approximately 50 million years.[7] An almost uninterrupted period of erosion continues to the present, removing layers many kilometers thick from the surface of the plateau and moving the present position of the escarpment approximately 150 kilometres (93 mi) inland from the original fault lines that formed the walls of the rift valley along the coastline during the break-up of Gondwana.[2][8] 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, creating the present-day coastal plain.[3][2][9][8] The rate of the erosion of the escarpment in the Drakensberg region is said to average 1.5 m (5 ft) per 1000 years, or 1.5 millimetres (1⁄16 in) per year.[8]
Because of erosion throughout most of the Mesozoic and Cenozoic eras, none of the plateau's surface rocks (except the Kalahari sands) are younger than 180 million years.[2][10] The youngest rocks that remain cap the plateau in Lesotho and form the steep sides of the Great Escarpment in this region. These are the Clarens Formation laid down under desert conditions about 200 million years ago, topped by a 1,600 m (5,200 ft) thick layer of lava that erupted and covered most of southern Africa, and indeed large parts of Gondwana, approximately 180 million years ago.[2][3][11]
Erosional retreat means that the rocks exposed on the coastal plain are, with very few and small exceptions, older than those that cap the escarpment. The rocks found in the
The eastern portion of the Great Escarpment goes as far north as
Appearance
The eastern portion of the Great Escarpment within the borders of South Africa (see the accompanying map, above) is referred to as the Drakensberg (meaning "Dragon Mountains").[1][14] The Limpopo, Mpumalanga and Lesotho Drakensberg have hard erosion-resistant upper surfaces and therefore have a very high and rugged appearance, combining steep-sided blocks and pinnacles. The KwaZulu-Natal – Free State Drakensberg escarpment is composed of softer rocks and therefore has a more rounded, softer appearance from below. Generally, the top of the Escarpment is almost table-top flat and smooth, even in Lesotho. The "Lesotho Mountains" are formed away from the Drakensberg escarpment by erosion gulleys that turn into deep valleys that contain the tributaries that flow into the Orange River. There are so many of these tributaries that it gives the Lesotho Highlands a very rugged mountainous appearance, both from the ground and from the air.
Along the southern extent of the central plateau some of the thicker, hard, erosion-resistant
The Cape Fold Mountains have been re-exposed by erosion of the coastal plain below the Great Escarpment (see "Geological origin", above), after having been covered by sediments originating from an even higher and more extensive range of mountains, comparable to the Himalayas, that developed during the assembly of Gondwana to the south of the present African continent, on the portion of Gondwana called the "Falkland Plateau", the remnants of which are at present located far to the southwest of southern Africa close to southern tip of South America.[2]
See also
- Cliff Ollier – Geologist, geomorphologist, soil scientist, and author
- Lester Charles King – English geologist and geomorphologist
- Pediplain – Extensive plain formed by the coalescence of pediments
References
- ^ a b c d Atlas of Southern Africa. (1984). p. 13. Reader's Digest Association, Cape Town
- ^ a b c d e f g h i j k l m 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.
- ^ a b c d e Truswell, J.F. (1977). The Geological Evolution of South Africa. pp. 151–153,157–159,184–188, 190. Purnell, Cape Town.
- ^ a b The Times comprehensive Atlas of the world (1999). pp. 88–89. Times Books Group, London.
- ^ "Great Escarpment". Encyclopædia Britannica. Retrieved 2010-12-18.
- ^ "Namibian Savannah Woodlands". Terrestrial Ecoregions. World Wildlife Fund.
- ^ Encyclopædia Britannica (1975); Macropaedia, Vol. 17. p. 60. Helen Hemingway Benton Publishers, Chicago.
- ^ a b c d Norman, N.; Whitfield, G. (2006). Geological Journeys. Cape Town: Struik Publishers. pp. 290–300.
- ^ .
- ^ a b c Geological map of South Africa, Lesotho and Swaziland (1970). Council for Geoscience, Geological Survey of South Africa.
- ISBN 1-86872-593-6.
- ^ 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.
- ^ Encyclopædia Britannica (1975); Micropaedia Vol. III, p. 655. Helen Hemingway Benton Publishers, Chicago.
- ^ The Times comprehensive Atlas of the World. (1999) p. 90. Times Books Group, London.
External links
- Media related to Drakensberg Escarpment at Wikimedia Commons