Witwatersrand means Ridge of White Waters, a name given to the area by early Burish settlers. The Main Reef is a layer of gold-bearing bands of quartz pebble conglomerates on which the wealth of Johannesburg is based. The 120-km long gold deposit, the richest gold deposit of the world, is three billion years old. It has produced more than 50,000 tons of pure gold so far.

The gold was first discovered in 1886 by George Harrison. George Harrison was an Australian prospector who worked as a builder on the Highveld farm in Langlaagte. One Sunday morning on a hike he tripped over an outcrop of very old rock. He was fascinated, broke off a piece, took it back to the farm, crushed it, and panned it in a borrowed frying pan. The gleam of gold made him apply for a free prospector's license, which he obtained. But he sold his license immediately to Frank William Marsden for the sum of GBP 10. There is a legend, that George Harrison was killed by a lion soon after. Frank Marsden sold his claim on to Alfred Heppel for GBP 50.

The history of Johannesburg is based on the gold, as it was founded as a miners settlement immediately after the discovery of the gold. Beneath various show mines and working mines in the Witwatersrand ridge, there is an show mine right in the middle of Jo'burg.

The gold is found in the Black Reef Quartzite Formation, which has a maximum thickness of about 30 m. It may be subdivided into a lower Conglomerate Unit and an upper Quartzite Unit. The rocks were formed during the early Proterozoic. It is a very old placer deposit, gold which was relocated and enrichened by erosion and transport. Because of continuous subsidence of the area, a shallow sea followed a 600Ma long erosional phase, which produced deep valleys in the Archaean rocks. The old relief was filled with new sediments, now called the Wolkberg Group, which consist of quartzites, shists, and conglomerates of the Schelem, Selati and Mabin Shadows Formation. The subsidence continued and the shists of the Black Reef Formation and the Matmi Dolomit Subgroup were deposited. The complete pack of sedimentary rocks is called Transvaal Sequence in South Africa.