The Witwatersrand (, ; ; locally the Rand or, less commonly, the Reef) is a , 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 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 plateau (or ridge) which rises about 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 .
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 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.
Geology
thumb|left|A diagrammatic north–south cross section through the Witwatersrand ridge / plateau, below the Johannesburg city center. Only the major subgroups of rocks are shown, each of which is made up of many separate layers of varying composition. The harder layers form the hills, and the softer layers the valleys. The oldest rocks of the Witwatersrand Supergroup form the Orange Grove Quartzite layer. This forms the scarp in the north, from which the Witwatersrand derives its name. The granite to the north of the Orange Grove Quartzite scarp is around 3.2 billion years old, and is an exposed portion of the underlying [[Kaapvaal craton on which a large portion of South Africa rests. (See illustration below.) To place this diagram into its broader context, see the diagrammatic geological cross section through the Vredefort impact structure at the end of the article.]]
The Witwatersrand plateau consists of a 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 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. Gold mining in these buried portions of the Witwatersrand Supergroup is sometimes carried out at depths of below the surface.
Witwatersrand Basin
thumb|upright=1.75|Witwatersrand Basin and major goldfields
thumb|Carbon Leader Gold Ore, Blyvooruitzicht Gold Mine, Carletonville Goldfield, West Witwatersrand. The Carbon Leader is a blackened, hydrocarbon-rich stromatolitic interval richly impregnated with [[native gold and uraninite. This is a paleoplacer deposit, part of an ancient alluvial fan succession.]]
thumb|High-grade gold ore from the Witwatersrand near [[Johannesburg.]]
The Witwatersrand Basin is a largely underground geological formation which surfaces in the Witwatersrand. It holds the world's largest known gold reserves and has produced over , which represents about 22% of all the gold accounted for above the surface.
Geological origin
The Witwatersrand basin was created during the Archean Eon, and is therefore amongst the oldest geological structures on Earth. It was laid down in two stages, over the course of 260 million years starting just short of 3 billion years ago. The first phase, lasting 60 million years, consisted of sedimentary deposits in a shallow sea, conveniently named the "Witwatersrand Sea". The resulting 2500–4500 m thick layer of sediments is termed the "West Rand Group" of Witwatersrand rocks. The second phase, which lasted for 200 million years, followed on from the first phase, with on-land deposits, resulting from the retreat of the Witwatersrand Sea, leaving a wide almost flat coastal plain over which rivers from the north formed wide braided river deltas, into some of which rich deposits of gold were deposited. The resulting 2500 m thick layer of rock is termed the "Central Rand Group". The "West Rand Group" and "Central Rand Group" of rocks together form the "Witwatersrand Supergroup", the full horizontal extent of which is termed the Witwatersrand Basin.]]
thumb|left|upright=1.5|A diagrammatic representation of the position and size of the [[Kaapvaal craton, outlined in red, in relation to present-day Southern Africa. The blue area depicts the portion of the craton that subsided below the "Witwatersrand Sea", about 3 billion years ago. It is in this sea that the sediments accumulated and would ultimately form the "West Rand Group" portion of the "Witwatersrand Supergroup" of rocks. The younger "Central Rand Group" of rocks accumulated on the low, flat coastal plain (see diagram below) after the Witwatersrand Sea had retreated southwards as a result of uplifting of the craton, especially in the north. CT indicates Cape Town, D Durban, B Bloemfontein, J Johannesburg, and K Kimberley.]]
thumb|upright=1.5|left|A diagrammatic presentation of the rivers that flowed into the Witwatersrand sea after 43000 m of sediment had already been deposited in the basin. The fast flowing rivers cascading down the mountains to the north now flowed over a wide flat coastal plain to form broad deltas of sluggish braided rivers, where the heavy materials (cobbles, gold, uranium and iron pyrite etc.) carried down from the mountains settled out, to form the gold bearing "Central Rand Group" deposits. Today's gold ore is confined to the fossil river deltas.
thumb|upright=1.5|The position of the Kaapvaal craton (khaki coloured area) beneath the South African landscape, and the shrunken, shallow Witwatersrand Sea (light blue) at the time that gold was deposited in the broad, river deltas of six rivers that flowed into that sea, dropping all their heavier materials (cobbles, gold, uranium iron pyrite etc.) in the braided rivers of the deltas (see illustration on the left). Most of these gold deposits are deep under the South African surface, but form outcrops (exposures at the surface) along the Witwatersrand ridge. The six gold fields thus established are, in order from the west, moving clockwise, to the northern shore of the Witwatersrand sea, [[Welkom, Klerksdorp, Carletonville, West Rand, East Rand and Evander. but island arcs did form. It was the coalescence of several of these island arcs that led to the formation of the Kaapvaal craton, one of the first microcontinents to form on Earth about 3.9 billion years ago. Within 60 million years, up to 4500 m of sediment had accumulated on the granite base, to become the "West Rand Group" of rocks that contribute over 60% of the total thickness of the Witwatersrand Supergroup.
The gold in the Witwatersrand Basin area was deposited in Archean river deltas having been washed down from surrounding gold-rich greenstone belts to the north and west. Rhenium-osmium isotope studies indicate that the gold in those mineral deposits came from unusual 3 billion year old mantle-derived intrusions known as komatiite, present in the greenstone belts.
Consequences of mining the ancient Witwatersrand rocks
Apart from the obvious hollowing out of the rocks below southern Johannesburg, causing unpredictable sinkholes, surface instabilities and earth tremors, the bringing to the surface of rocks that had been laid down in oxygen-free conditions had unforeseen effects. Iron pyrite (FeS<sub>2</sub>), which is relatively plentiful in the gold ores of the Witwatersrand, oxidises to insoluble ferric oxide (Fe<sub>2</sub>O<sub>3</sub>) and sulfuric acid (H<sub>2</sub>SO<sub>4</sub>). Thus, when mine waste comes into contact with oxygenated rainwater, sulfuric acid is released into the ground water. Acid mine drainage, as the phenomenon is called, has become a major ecological problem, because it dissolves many of the heavy elements, such as the uranium, cadmium, lead, zinc, copper, arsenic and mercury found in the mine dumps, facilitating their passage into surface water and ground water. The tailings ponds contain an average of 100 mg/kg of U<sub>3</sub>O<sub>8</sub>, and uranium levels are measureable in human hair.
Sulfuric acid also erodes concrete and cement structures, resulting in structural damage to buildings and bridges.
! rowspan=2|Year !! rowspan=2|No. of<br />mines !!rowspan=2|Gold output<br />fine (troy) ounces !!colspan=2|Value (million GBP)
|-
!In given year!! Revalued to 2010
|-
| 1898 || 77 || || £15.14 ||
|-
| 1899 (Jan–Oct) || 85 || || £14.05 ||
|-
| 1899 Nov – 1901 Apr|| 12 || || £2.02 || £908
|-
| 1901 (May–Dec) || 12 || || £1.01 || £441
|-
| 1902 || 45 || || £7.18 ||
|-
| 1903 || 56 || || £12.15 ||
|-
| 1904 || 62 || || £15.54 ||
|-
| 1905 || 68 || || £19.99 ||
|-
| 1906 || 66 || || £23.62 ||
|-
| 1907 || 68 || || £26.42 ||
|-
| 1908 || 74 || || £28.81 ||
|-
| 1909 || 72 || || £29.90 ||
|-
| 1910 || 63 || || £30.70 ||
|-
|}
Harrison declared his claim to the government of the Zuid Afrikaanse Republiek (ZAR), and in September 1886 President Paul Kruger issued a proclamation declaring nine farms public mining diggings, starting on 20 September 1886. The park is on the busy Main Reef Road, immediately west of Nasrec Road.