Great Escarpment, Southern Africa

"Great Escarpment" redirects here. For other similar features, see Great Escarpment (disambiguation).
A map of South Africa showing the central plateau edged by the Great Escarpment and its relationship to the Cape Fold Mountains in the south. The portion of the Great Escarpment shown in red is officially known as the Drakensberg, although most South Africans think of the Drakensberg as only that portion of the Escarpment which forms the border between KwaZulu-Natal and Lesotho. Here the Escarpment rises to its greatest height of over 3000 m.
Satellite image of South Africa. If this image is compared with the map on top, the Great Escarpment can easily be identified.

The Great Escarpment, which edges the central Southern African plateau,[1] is a major geological formation in Africa (See "Geological origins" below).[2][3] While it lies predominantly within the borders of South Africa, in the east it extends northwards to form the border between Mozambique and Zimbabwe,[4] and in the west it continues northwards into Namibia and Angola.[5][6]

Different names are applied to different stretches of the Great Escarpment, the most well-known section being the Drakensberg (diagram on the right). 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.[2] 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.[2][3] The stepped steep walls of these rift valleys formed escarpments that surrounded the newly formed Southern African subcontinent.[2]

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.[7] 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.[2] 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.[3]

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 1,000 metres (0.62 mi), 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 2,000 metres (1.2 mi). It reaches its highest point (over 3,000 metres (1.9 mi)) where the escarpment forms the Lesotho - KwaZulu-Natal international border.[1][2]

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.[2][8][9] The position of the present escarpment is therefore approximately 150 kilometres (93 mi) 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 1.5 metres (4 ft 11 in) per 1000 years, or 1.5 millimetres (0.0015 m) per year.[9]

An approximate SW-NE cross section through South Africa with the Cape Peninsula (with Table Mountain) on the far left, and north-eastern KwaZulu-Natal on the right. Diagrammatic and only roughly to scale. It shows the major geological structures (coloured layers) that dominate the southern and eastern parts of the country, as well as the relationship between the Central Plateau, the Cape Fold Mountains, and the Drakensberg escarpment. The south western escarpment (the Roggeberg escarpment) is also clearly visible on the left, but is not labelled. The significance and origin of the geological layers can be found under the headings "Karoo Supergroup" and "Cape Supergroup"

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.[2][10] 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 1,600 metres (0.99 mi) thick layer of lava which erupted, and covered most of Southern Africa, and indeed large parts of Gondwana, about 180 million years ago.[2][3][11] These rocks form the steep sides of the Great Escarpment in this region, where its upper edge reaches an altitude in excess of 3,000 metres (1.9 mi).

The erosional retreat of the escarpment from the coast-line to their present position, about 150 kilometres (93 mi) 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.[10] 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.[2] 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.[2][3][12] Sediments eroded from these Gondwana mountains buried the Cape Fold Belt and formed the 6 kilometres (3.7 mi) thick Beaufort Group of rocks of the Karoo basin.[2][12] 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.[2]

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.[1][13] The absence of the Great Escarpment for about 450 kilometres (280 mi) 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.[8]

Appearance

A stylized illustration of the Southern African Great Escarpment, based particularly on its appearance in the Great Karoo, where thick erosion resistant dolerite sills (represented by the thick black lines in the diagram) generally form the upper, sharp edge of the escarpment. In other parts of the Escarpment hard erosion resistant geological layers similarly form the upper, abrupt edge (see text). Note the island remnants of the earlier extent of the plateau on the plain below the escarpment, left behind as the escarpment has gradually eroded further inland.[8]
A view from the top of the Great Escarpment in the Karoo National Park near Beaufort West, looking south across the plains of the Lower Karoo. Note the remnants of the former extent of the central plateau on the plain below the escarpment (see diagram on the right). Also note the dolerite sills which top the escarpment and mountains in the distance, giving these structures their characteristic flat-topped appearance, along the east-west course of the escarpment, through the Great Karoo.
A view of the Mpumalanga Drakensberg portion of the Great Escarpment, from God's Window, near Graskop looking south. The hard erosion resistant layer that forms the upper edge of the escarpment here consists of flat lying quartzite belonging to the Black Reef Formation, which also forms the Magaliesberg mountains near Pretoria.[2][9]
The Amphitheatre in the Drakensberg region of the Great Escarpment on the border of KwaZulu-Natal and Lesotho. Here the top of the escarpment is composed of basalt (lava) that welled up on to the surface, instead of burrowing underground between the layers of sedimentary rocks, as it did in the Karoo as part of the same process, to form the dolerite sills seen in the photograph above. The Drakensberg lavas covered most of Southern Africa about 180 million years ago. This layer of lava is very much thicker (1400 m) than the dolerite sills (typically only 20-50 m) seen in the east-west running southern portion of the escarpment.

The eastern portion of the Great Escarpment within the borders of South Africa (see the accompanying map, above) is referred to as the Drakensberg[1][14] (meaning “Dragon Mountains”), because of the Escarpment’s appearance from below. 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. The top of the Escarpment is generally almost table-top flat and smooth, even in Lesotho. The "Lesotho Mountains" are formed away from the Drakensberg escarpment by erosion gulleys which turn into deep valleys which 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 dolerite sills form large parts of the upper edge of the escarpment (see illustration on the right), but where the sills are thinner or absent then, like the portion of the Drakensberg between KwaZulu-Natal - Free State, the escarpment is composed of softer rocks, aged between 250-300 million years old.[10] This means that in these regions the scarp has a more rounded appearance, or has eroded away to such an extent that the scarp may no longer be evident (for instance along the route taken by the N1 highway between Beaufort West, below the escarpment, and just beyond Three Sisters on the way to Richmond or Victoria West, on the plateau; and also where the Orange River has eroded a broad valley through the Escarpment in the west before it flows into the Atlantic Ocean). However, along most of its extent it forms a 400-800 m high mountain-like ridge which roads into the interior have to negotiate, sometimes up steep winding passes, such as the Burke's, Vanrhyns, Bloukrans, Gannaga, Ouberg, Verlatekloof, Teekloof, Molteno, Goliatskraal, Daggaboersnek, Katberg, Nico Malan and Barkly passes. Nevertheless, the Escarpment to the south and west of the plateau lacks the grandeur of the Mpumalaga and Lesotho Drakensberg, on the one hand, and the extremely rugged, intricately folded, ranges of Cape Fold Mountains which run parallel to the coast on the seaward side of the Great Escarpment. (The parallel ranges of mountains, to the south of the Escarpment, can clearly be seen on the accompanying satellite image of South Africa, especially when compared to the diagram on the left, which shows the course of the Great Escarpment.) The fold mountains were formed about 330 million years ago,[2][3] and therefore pre-date the formation of the escarpment by nearly 200 million years. The two events are geologically unrelated to one another. They also represent two very different geological processes: the Great Escarpment resulted from rifting, and the tearing apart of the Gondwana super-continent, whereas the Cape Fold Mountains resulted from the collision of tectonic plates, during the assembly of Gondwana, in the same way that the Andes Mountains in South America are being formed today.

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

References

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  1. 1 2 3 4 Atlas of Southern Africa. (1984). p. 13. Readers Digest Association, Cape Town
  2. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 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.
  3. 1 2 3 4 5 6 Truswell, J.F. (1977). The Geological Evolution of South Africa. pp. 151-153,157-159,184–188, 190. Purnell, Cape Town.
  4. ↑ "Great Escarpment". Encyclopædia Britannica. Retrieved 2010-12-18.
  5. ↑ "Namibian Savannah Woodlands". Terrestrial Ecoregions. World Wildlife Fund.
  6. ↑ The Times comprehensive atlas of the world (1999). p. 89. Times Books Group, London.
  7. ↑ Encyclopaedia Britannica (1975); Macropaedia, Vol. 17. p. 60. Helen Hemingway Benton Publishers, Chicago.
  8. 1 2 3 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.
  9. 1 2 3 Norman, n. & Whitfield, G. (2006). Geological Journeys. p.290-300. Struik Publishers, Cape Town.
  10. 1 2 3 Geological map of South Africa, Lesotho and Swaziland (1970). Council for Geoscience, Geological Survey of South Africa.
  11. ↑ Sycholt, August (2002). Roxanne Reid, ed. A Guide to the Drakensberg. Cape Town: Struik Publishers. p. 9. ISBN 1-86872-593-6.
  12. 1 2 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.
  13. ↑ Encyclopaedia Britannica (1975); Micropaedia Vol. III, p. 655. Helen Hemingway Benton Publishers, Chicago.
  14. ↑ The Times comprehensive atlas of the World. (1999) p. 90. Times Books Group, London.

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