Witwatersrand

The Witwatersrand (UK: /wɪtˈwɔːtərzrænd, -rɑːnd/, US also /ˈwɪtwɔːtərz-/; Afrikaans pronunciation: [ˌvətˌvɑːtərsˈrant]; locally the Rand or, less commonly, the Reef) is a 56-kilometre-long (35 mi), north-facing scarp in South Africa. It consists of a hard, erosion-resistant quartzite metamorphic rock, over which several north-flowing rivers form waterfalls, which account for the name Witwatersrand, meaning 'white water ridge' in Afrikaans. This east-west-running scarp can be traced with only one short gap, from Bedfordview (about 10 km [6.2 mi] west of O.R. Tambo International Airport) in the east, through Johannesburg and Roodepoort, to Krugersdorp in the west (see the diagram at left below).

The scarp forms the northern edge of a 7-to-10-kilometre-wide (4–6 mi) plateau (or ridge) which rises about 200 m (660 ft) above the surrounding plains of the Highveld. A number of picturesque Johannesburg suburbs, including Observatory, Linksfield Ridge and Upper Houghton are located along the scarp, overlooking the rest of northern Johannesburg with views up to the Magaliesberg (although locals refer to segments of the scarp using area-specific names, such as Linksfield Ridge, Parktown Ridge or Observatory Ridge). The entire plateau-like structure is also often called the Witwatersrand. The plateau's elevation above sea-level is between 1,700 and 1,800 metres (5,600–5,900 ft).

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.

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 upon the declaration of the republic.

Witwatersrand and the Rand are names for the conurbation that developed along the range, although the terms are falling into disuse^{[citation needed]} and Witwatersrand was the "W" in PWV (Pretoria-Witwatersrand-Vereeniging), the initial name of Gauteng province. In this context, it has lent its name to institutions including the University of the Witwatersrand (Wits University) and the defunct Rand Afrikaans University (RAU, now part of the University of Johannesburg), and to towns and regions such as the East Rand, West Rand and Randburg.

The Witwatersrand plateau consists of a 5000-to-7000-metre-thick (3.1–4.3 mi) layer of mainly sedimentary rocks laid down over a period of about 260 million years, starting approximately 2.97 billion years ago. The entire series of rocks, known as the "Witwatersrand Supergroup", consists of very hard erosion resistant quartzites, banded ironstones and some marine lava deposits, interspersed with softer, more easily eroded tillites, mudstones and conglomerates. The oldest rocks (laid down 2.97 billion years ago) form the northern scarp of the Witwatersrand plateau; the youngest (laid down 2.71 billion 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, locally referred to as banket. The abundance of this gold is without a natural equal anywhere else in the world. Over 40,000 tonnes (44,000 short tons) have been mined from these rocks since this precious metal was first discovered here in 1886. This accounts for approximately 22% of all the gold that is accounted for today.

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 (0.04 in) in size, called "flyspeck carbon", or as continuous layers about 10–20 mm (0.4–0.8 in) 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.