Sedimentary deposition across the stable granite-gniess basement commenced around 3 billion years ago. In stratigraphic terms the Witwatersrand sequence is divided into two divisions, the lower dominantly marine, slate rich West Rand Group and the upper dominantly aluvial sandstone rich Central Rand Group. Combined, these units make up a sedimentary package up to 7km thick.
Sedimentary deposition ended approximately 2,700 million years ago when widespread faulting resulted in extensive lavas of the Ventersdorp supergroup covering the Witwatersrand Basin. Reef groups located on the West Rand in order of oldest to youngest are the Main, South, Johnstone, Livingstone, Bird, Kimberley, Elsburg and Ventersdorp Contact Reefs.
The Supergroup consists of quartzites, shales (iron rich), conglomerates (pyrites), diamictite, pyritic sands and gold in quartz-pebble conglomerates.
The main areas underlain by this Supergroup are: Witwatersrand, Heidelberg-Nigel-Evander, Klerksdorp-Carltonville, Parys (Vredefort dome), Welkom, Piet Retief and Paul Pietersburg (Pongola).