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The Witwatersrand (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 "ridge of white waters" in Afrikaans. This east-west-running scarp can be traced with only one short gap, from Bedfordview (about 10 km [6 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. The entire plateau-like structure is also often called the Witwatersrand. The plateau's elevation above sea-level is between 1700 and 1800 metres (5600–5900 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. == Geology of the Witwatersrand ridge == 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. SOURCESWikipedia