Otters and Gold

By Graeme Addison

First published in The Way magazine, 2008

An otter was sunning himself on one of our makeshift bridges across a rocky side channel of the Vaal River. He was a big specimen of the animal that populates African rivers, lakes, and ocean estuaries from South Africa to Ethiopia, the Cape Clawless otter.

Instead of fleeing underwater towards his holt, or den, hidden in the muddy bank somewhere, he dropped into the water and weaved aggressively to within a metre of me. I stood transfixed. He stared upwards insolently, his bewhiskered snout twitching with exasperation. I had disturbed him at his morning meal. Never having seen an otter this close, for a moment thought he was about to rip into my legs with strong sharp teeth to defend his territory or his females.

But he thought better of it, dipped out of sight like a dolphin, and emerged casually on a humpbacked rock about 15 metres away where he proceeded to clean his clawless hands of the remnants of a crab feast. I settled down to watch him and ponder the remarkable story of how he, and I, came to share this extraordinary river environment which vast planetary forces had helped to shape.

Here, some 2.023 billion years ago, an asteroid is thought to have hit the Earth, exploding with the force of trillions of nuclear bombs. The blast blew a hole 50km deep in the Earth’s crust. A vast plug of granite in the centre popped up like a champagne cork and this is called a dome, although in fact the structure is a crater.

The Vaal is broken up into many channels with rapids running between densely thicketed islands. Occasionally through the trees one catches a glimpse of the dramatic mountain rim of the Bergland – ridges that mark the inner edge, or collar, of the world’s largest and oldest visible blast crater, the Vredefort Dome. It is named after the town of Vredefort which lies closest to the centre, but the crater extends from horizon to horizon.

The story of how the Vaal has carved its course through the Dome is a fascinating and complex one. Its channel follows micro-faults in the underlying granite, cracks which are the product of the original blast and the resulting uprush and cooling of the rock.

The river is usually known as the River of Diamonds because of the diamond pipes and alluvial terraces bearing diamonds that occur much lower down, around Kimberley. But the Vaal in the Dome area might better be described as the River of Gold. Not because it carries gold in nugget or powder form, but because its erosional action has exposed the gold seams that were deeply buried in the Vredefort blast.

I am not a geologist nor a space scientist specialising in meteorites (the name given to hurtling rocks that actually hit the Earth). I am a writer living on the Vaal in the Dome, and a keen paddler with a passion for rivers worldwide. It was the attraction of the whitewater that drew me here eight years ago, and it is the extraordinary, still-emerging story of the Vaal’s origins that has kept me here.

When the asteroid plunged to Earth it punched a hole in the rock strata of the Witwatersrand Supergroup. Until then, this mass of solidified silt and pebbles had lain flat, layer upon layer, where rivers had deposited the sediment in a gigantic inland lake, the Witwatersrand lake.

Picture a series of dinner plates lying on top of each other: that’s how the strata might have looked, stacked one on top of the other by ages of deposition. Over tens of millions of years the sediments had been crushed by the weight above, heated and hardened and much of the stratified rock had turned into crystalline quartzites.

Most importantly for the generations of South Africans who were to appear billions of years later, gold dust had been deposited especially on the edges of the lake where river deltas had emptied into it. Gold is heavy, so along with pebbles it dropped out of the water first, making a conglomerate. The resulting mix was called “banket” by Scots miners, the name of plum pudding at Christmas with sparkly bits in it. The sparkles are the gold dust, placer gold, in the conglomerate.

Now picture how an asteroid crashed into the Earth, shattering the dinner plates or even melting and bending some of them, so great was the heat of the impact. Fragments of strata were hurled upwards and sideways, while others were bent down deep under the Earth’s surface.

The asteroid blast shattered the Witwatersrand Supergroup in the centre, folding, faulting and twisting the strata. If you drop a stone into water you will see that it makes a hole which quickly splashes closed, throwing up a column of water in the middle and sending out rings in concentric circles. The same thing happens when a very large rock travelling at an estimated 15-20km/sec punches into the Earth, as was this asteroid.

There was an upheaval in the middle and rings radiated outwards. So the Vredefort Dome was formed, a complex crater with a granite core and classic rings. The whole process took only four minutes and the outer ring may have stretched from north of Pretoria to south near Harrismith today – a distance of up to 360km.

But we don’t know for sure. What we do know is that the original surface was steadily worn down over a period of 1 500 million years or so. The surface today is probably about 10-15 km down – that is, we are deep down in the old crater.

The rings have long ago disappeared. Near the centre of the crater we are left with the remnants of Witwatersrand strata, turned near-vertical by the force of the blast. Towards the outside of the crater, which is now smaller, the cities of Johannesburg, Krugersdorp, Klerksdorp and Welkom sit on bits of the old river deltas, appropriately known as the Arc of Gold (see diagram).

The unassuming Vaal has been the agent of the Dome’s exposure to human ken. In the period between when the blast occurred and today, many things happened. There was movement in the crust, and another huge lake or inland sea formed with sedimentation covering the old Dome up to a depth of about 10km. This was the Karoo Supergroup.

The Vaal was born on the surface of the Karoo up to 280 million years ago, which makes it one of the oldest rivers flowing on the planet today. Patiently the Vaal has removed most of the Karoo surface in the area of the Witwatersrand, re-exposing bits of the gold-bearing reefs.

Much more lies deeply hidden down below, but at least we know it’s there. The world’s deepest mining operations at Western Deep Levels are reaching down to 4km to get at the gold.

If there had been no Vredefort explosion to warp and sink the Witwatersrand strata, and no Vaal River to uncover it later, there would have been no South African gold rush, no wealthy cities on the veld, no country as we know it with its history of land-grabs and conflict, and possibly neither apartheid nor the reconciliation that has followed.

All the gold would long ago have been carried away by erosion of the once flat-lying strata, and there would probably have been no sign of it anywhere in this country. By being tipped up and buried, it was preserved. Then the Vaal came along and showed it to us.

Visitors to the Vaal in the heart of the highveld of South Africa, with its mines, heavy industries and cities, are amazed at the river’s history and origins. What many regarded as an industrial ditch is one of the grand old ladies of watercourses in the world. It owes its extraordinary natural features – some 300-odd islands spread over a 30km distance – to a terrifying event in the Earth’s distant past.

The Vaal today is a very old river which has gained new youth by being superimposed, from above, on the even older structure of the Dome. The Vaal cuts through the mountain collar of the Bergland and crosses the granite core of the Dome before exiting the Bergland once again. As it does so, it exploits faults in the rock, running this way and that. Over time the islands have emerged with their rich vegetation and varied birdlife and wildlife.

Two centuries ago there would have been crocodiles basking on the rocks and hippos lurking in the pools, but they were all shot out.

Today what remains is still a slice of natural Africa. Porcupines rattle their quills in the night, caracal or rooikat stalk the vervet monkeys, and by day Fish Eagles cruise high overhead issuing their wild calls. Perhaps the rarest waterfowl on the Vaal is the Great Bittern, whose five-syllabled whooping can sometimes be heard from the reed beds though one almost never catches sight of the tall, tawny bird itself.

Then there are the otters. The Cape Clawless, as the name implies, has agile hands for catching crabs, lizards and insects which he hunts at dawn and dusk; and the smaller Spotted-Necked otter is a night-time swimmer who goes after fish with sharp claws. This island paradise needs to be protected – for the wildlife, certainly, but also because of what it tells us about the interactions between rivers, the planet, and the solar system we live in.

We occupy a dangerous neighbourhood, with asteroids wandering by with frightening frequency. Some are predicted to strike the Earth, and if they are as big as the Table Mountain-sized Vredefort rock they could cause our extinction. Meanwhile the River of Gold holds a special place in our history because it has played a central part in the discovery of gold and the supply of water to Gauteng.

·        Graeme Addison is a South African popular science writer. He is the author of The Edge Series on innovations in science and technology in SA (SAVI, 2005), and of White Water: The world’s wildest rivers (Struik-New Holland, 2001), as well as several other books.