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The Witwatersrand is a 56-kilometre-long north-facing scarp in the Gauteng Province of South Africa. It consists of a hard, erosion-resistant quartzite sedimentary rock, over which several north-flowing rivers form waterfalls, which account for the name “Witwatersrand”, meaning the “ridge of white waters” in Afrikaans.〔Truswell, J.F. (1977). ‘’The Geological Evolution of South Africa’’. p. 21, 27-28, 33-36, . Purnell, Cape Town.〕 This east–west-running scarp can be traced with only one short gap from Bedfordview (about 10 km west of O.R. Tambo International Airport) in the east, through Johannesburg and Roodepoort, to Krugersdorp in the west (see the diagram bottom-left below).〔Norman, N., Whitfield, G. (2006) ‘’Geological Journeys’’. p. 38-49, 60-61. Struik Publishers, Cape Town.〕 The scarp forms the northern edge of a 7- to 10-kilometre-wide plateau (or ridge) which rises about 200 m above the surrounding plains of the Highveld. The entire plateau-like structure is also often called the “Witwatersrand”. The plateau’s elevation above sea-level is between 1700–1800 m. The Witwatersrand plateau forms a continental divide with the run-off to the north draining into the Indian Ocean via the Crocodile and Limpopo Rivers, while the run-off to the south drains via the Vaal into the Orange River and ultimately into the Atlantic Ocean.〔〔’’Encyclopaedia Britannica’’ (1975). Micropaedia Vol. X. p. 720. Helen Hemingway Benton, Chicago.〕 The Witwatersrand lies within the province of Gauteng, formerly called the “PWV”, an acronym for Pretoria, "Witwatersrand" and Vereeniging. When used in this latter context, the term ''Witwatersrand'' refers to the entire Greater Johannesburg Metropolitan Area. This conurbation is oblong in shape and runs from Springs in the east to Randfontein and Carletonville in the west, as well as Soweto to the south. This area is also often colloquially referred to as “the Rand” or “Wits”. Because of the extraordinary quantities of gold that have been extracted from the Witwatersrand rocks, the South African currency was named the “Rand” in 1961. ==Geology of the Witwatersrand ridge== (詳細はquartzites, banded ironstones and some marine lava deposits, interspersed with softer, more easily eroded tillites, mudstones and conglomerates. The oldest rocks (laid down 2970 million years ago) form the northern scarp of the Witwatersrand plateau; the youngest (laid down 2710 million years ago) are those that form the southern edge of the plateau. Gold is found in the conglomerate strata of the younger members of the Supergroup. The abundance of this gold is without equal anywhere else in the world. Over 50 000 tons have been mined from these rocks since this precious metal was first discovered here in 1886. This accounts for approximately 50% of all the gold ever mined on earth.〔 Not all the conglomerates contain gold, and of those that do (known as “reefs” by the miners), the gold is not uniformly distributed throughout the layer, but tends to occur in streaks, where the pebbles that make up the conglomerate are larger than elsewhere. Here the gold is associated with other minerals, especially iron pyrite and uraninite, as well as carbon rich materials such as kerogen, or bitumen, which occurs in small balls less than 1 mm in size, called “flyspeck carbon”, or as continuous layers about 10–20 mm thick.〔〔 The gold-bearing conglomerates occur chiefly in the upper, younger layers of the Witwatersrand Supergroup of rocks, on the southern side of the Witwatersrand plateau. The Witwatersrand Supergroup strata which reach the surface in Johannesburg dip downwards to the south at an angle of about 30°.〔 From there on they are almost everywhere, with very few exceptions (see below), covered by younger rocks.〔Geological map of South Africa, Lesotho and Swaziland (1970). Council for Geoscience, Geological Survey of South Africa.〕 Gold mining in these buried portions of the Witwatersrand Supergroup is sometimes carried out at depths of below the surface.〔〔(National Geographic )〕 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「Witwatersrand」の詳細全文を読む スポンサード リンク
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