Tour Guide, Vaal River, South Africa
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The Vaal was born on the Karoo sedimentary system. Do not confuse the geological term Karoo – a supergroup of rock types – with the semi-desert in the centre of South Africa. Although the desert forms part of the Karoo supergroup the geological formations actually stretch much further, into Mpumalanga province, the Eastern Cape and KwaZulu/Natal.
The Vaal more or less skirts the Karoo supergroup to the north. The Vredefort impact crater or structure lies partly under the Karoo rocks and partly exposed by erosion.
The Vaal is thought to be around 230 million years old. Rivers are difficult to date because they both build rock formations through deposition, and then take them away through erosion. So it is hard to find rocks that date the origins of the river.
The Vaal is one of the oldest rivers in the world. It is also the river of diamonds and of gold. It is the only major river on Earth that flows right through an impact crater – and the crater itself is the largest we know of.
The Vredefort Dome impact site, marking the centre of the crater, is between 50-60 km across; while the whole crater stretches from Johannesburg to Welkom 360 km to the south. The impact took place 2 billion years ago.
The basin of the Vaal, however, is much older. Ancestral rivers formed the Witwatersrand Basin more than 3 billion years ago. The basin is among the oldest on Earth.