THE AMAZING VAAL

An Evolutionary Perspective on one of the world's oldest rivers

by Graeme Addison

Walk along the banks of the Vaal, or paddle its cloudy waters, and you are taking a trip back in time through a vast span of planetary evolution, from the origins of life to our brief moment of human history.

In the Vredefort Dome, the Vaal looks like a young river. It has steep rapids and a rocky bed, with canyons forming around it. It is, in  fact, an old river turned young. The Vaal occupies what is arguably the area in which some of the world's oldest river basins were centred, and the river itself is one of the oldest continuously flowing rivers on the planet.Yet despite its longevity, the Vaal that we know is only 1/10th as old as the Vredefort Dome (aged 2023 million years), which in turn is more than 1500 million short of the origins of life itself some 3600 million years ago. The Earth itself is about 4500 million years old. So the birth of life preceded the formation of the Dome. While water has been an essential constituent of life since the beginning, the Vaal is a relative newcomer as a lifegiving channel on the surface of southern Africa.

Let's go back in time, back and further back, to explore the amazing story. This article is only a sketch for a broader canvas on the evolutionary perspective applied to the Dome.

VAAL AND ORANGE RIVERS

The Vaal is at least 230-280 million years old, the age of the Karoo Supergroup which the river both helped to build and later started eroding. It may be even older than that, since rivers must have been responsible for the deposition of the Karoo sediments in a vast sea before erosion began. The early Vaal could have been one of these rivers.

By the time the Karoo was in existence, the Vaal is thought to have been a larger river than the Orange. It was older and bigger, but the Orange has grown and the Vaal has shrunk - captured, in fact, by the Orange, and turned into one of its tributaries. This probably happened as the headwaters of the Orange cut back into the newly formed volcanic cap of Lesotho, consisting of flood basalts which covered much of the central interior. This higher ground attracted good rains and added to the volume of the Orange. The Vaal was further away and flowed in a flatter, less well-watered bed. (The only thing to change this picture has been water transfer from the Tugela and Upper Orange to slake the thirst of Gauteng, swelling the influx to the Vaal and, ironically, reversing the course of evolution).

EMERGENCE OF LIFE

But the evolution of the river systems is not the end of the story, or rather, it's not even the beginning of it. The beginning stretches far back into our Earth's origins. Back into the hellish Hadean period, the Earth was a cooling rock spinning in space and being bombarded by asteroids and comets, which are thought to have brought the water in the seas. In the following period of geological time, the Archaean, the atmosphere formed and water fell as rain, feeding the first rivers. Water was essential for the emergence of life, and the right conditions existed in the deep seas, and in lakes and ponds.

There is evidence in rock surrounding the Dome of the very first appearance of life itself. By about 3.6 billion years ago (3600 million years) the very first single-celled forms of life had developed in the form of what we can today identify as stromatolites, or fossilised  algae. These tiny algae, or cyanobacteria, needed water to emerge and survive. Life, then, existed in a primitive form probably when the rivers of the region created vast shallow swamps in which algae could flourish. Several satellite sites of the Vredefort Dome have been identified where stromatolites can be seen. The presence of the fossilised evidence of life's origins is one of the major scientific assets of the Dome area.

GOLD WEALTH

Another major asset - both scientific and wealth-creating - is gold. There must have been a high range of mountains around the basin of what we now know as the Witwatersrand Supergroup, a system of rock strata formed up until about 2.7 billion years ago. At that time, steep, high-energy rivers carried gravel down into deltas around the edge of the Witwatersrand lake, an inland see fed by many rivers. The ancestors of the Vaal were already at work in this basin.

The gold itself was created, along with other heavy minerals, in the explosions of stars called supernovae. Earth obtained its share of all elements when our solar system coalesced out of star dust. Somehow, great concentrations of gold must have built up in the mountains around the Witwatersrand lake. Once deposited in the lake and buried by later strata, the gold was further concentrated by hydrothermal action - basically hot underwater rivers that settled seams of gold in relatively dense quantities.

At left is a picture of the gold ore which I picked up on a visit to one of the deepest mines on Earth, Mponeng, near Carletonville about 70km north of the Vaal in the Witwatersrand system. It's a hard, lumpy conglomerate called "banket" after the nuts-and-raisins pudding enjoyed by Scots miners. Today the ore is mined extensively and was the original source (along  with the diamonds of the Vaal and Orange) of South Africa's mineral wealth.

Following the formation of the gold-bearing Witwatersrand strata, the layers settled and hardened and rivers eroded the surface. Soft sediments became hard quartzites, buried deep under the weight of later deposits.

ERODED REMNANTS OF CRATER

Then - 700 million after the Wits Supergroup was formed - a sudden mighty blast took place in this region in what was the largest energy release ever to occur on the surface of our planet - at least that we know of. That event was the explosion that caused the Vredefort Dome, Ring, or Structure.

The central crater is called a "Dome" because the rock that surged up to fill the hole was rather like a champagne cork popping out of a bottle - a plug that domed upwards. The full extent of the crater today stretches from Johannesburg in the north in a great semicircle to Welkom in the southwest, with much of the eastern side buried under the more recent Karoo Supergroup. The crater is very, very old, much older than the current Vaal. But as soon as the crater and its rings were formed, flows of water across the surface began the work of erosion. Over many aeons, rivers shaped the landscape, wearing the crater down until it was deeply scoured and its complex innards were exposed.

I run Vredefort Dome excursions on the river and off it, introducing visitors to the remarkable evidence of the blast which can be clearly seen in the rocks exposed by river erosion recently and in the distant past. What's interesting if you paddle the river and step off it onto any island is that you will actually see the physical evidence of the blast, right there in the rocks. The picture at left shows granite chunks embedded in melted rock, black glass actually, or obsidian. This sample lies right next to the river in a pile of boulders. It's called pseudotachylite, or false volcanic rock. From island viewsites you can see the surrounding crater hills and begin to appreciate how the river has interacted with the crater over millions and millions of years to expose features in the rocks.

RING STRUCTURE & FAULTING

The Vaal's course through the Dome is a product of the unique blast geology. Two things happened to create canyons and islands. First the river carved its way into the ring structure of the crater, forming small canyons. Secondly, the bed of the river has taken its shape from the underlying cracks in the Dome.

The river is still cutting canyons through the the Dome's ring structure. The so-called collar of the Dome, which we see as a semicircular range of mountains called the Bergland, comprises the strata of the Witwatersrand which were capsized or upturned by the blast. The rings are cut through by macro fault lines which the Vaal now follows, deepening them as canyons. The large cracks run outwards like the spokes of a  bicycle heel. The centre of the structure is roughly where the saltpan called the Inlandsee lies today, with the Vaal crossing the Dome across the northern part.

The Vaal has found its way into the Dome core through one of the macro-faults in the Bergland at Smilin' Thu, and goes out again at another macro-fault at Kommandonek. After Kommandonek it turns and runs between the Witwatersrand strata for a while, past the village of Venterskroon, then exits the Bergland finally past Elgro lodge. Roads also follow convenient macro-faults through the hills where there are passes. These passes were once used by ox-wagons and horse transports following rustic tracks, but today roads sweep through them bringing traffic to Parys and Vredefort.

SUPERIMPOSED RIVER

The Vaal is what is called a "superimposed" river, an old channel that has cut down into a rising landscape. Since the blast around 2000 million years ago, and down to the present, the river basin in the Vredefort Dome area has passed through lengthy eras of erosion, deposition  and more erosion. The Dome structure was worn away then covered by the 10km thick Karoo system, which in turn began to be eroded, a process  that continues to this day.  

So the Vaal, from its inception as a Karoo river, to its aged modern self, became a very old, mature river. It developed long, looping meanders as it found its way across the flat Karoo.  The southern African subcontinent had been rising steadily as the river sliced downwards, and the combined effect has been to create canyons which are typical of a young river. The canyons of the Vaal in this area, though, are by no means typical of canyons elsewhere - in fact they  are exceptional. They have come about through the unique combination of the crater and the erosional processes.  Both the canyons and the islands owe their existence and their profiles to faulting in the structural geology of the underlying Dome rocks.  

ISLANDS ON MICRO-FAULTS

The macro-faults in the crater go out radially from the centre, but within the granite core and the Witwatersrand quartzites are micro-faults which cut in all directions. It is these micro-faults, exposed by millennia of erosion and exploited by the river, that have provided the interconnected channels running between islands of the Vaal. The river has exploited and widened the micro-faults, and the has been to fashion the bed of the river into what is called anabranching. The term anabranch comes from physiology, as medical people use it to describe the tiny branching capillaries in the veins under our skin.

Erosion of the granite by water, lightning, heat stress and chemical action breaks down the hard rock into gravel. This takes place by stages. As the river washes over and around rocks, its hydraulic forces exert a kind of suction that eats away at the cracks and "plucks" the broken boulders off the solid granite bedrock. The boulders are washed downstream particularly at flood times when you can hear them bounding and booming along the floor of the river. The granites steadily crumble away and the gravel collects organic material which enriches it as soil and makes it hospitable to pioneer plants. It is remarkable to walk on the islands and see this progression of the breakdown of bare rock and the build-up into plant-filled soil happening before one's eyes.

Like any young river, the Vaal has cut sharply and its bed has become filled with rapids. Those visiting the area to paddle the peaceful channels or run the whitewater by raft, generally have little idea of the incredible past belonging to these features. But once you have read this you will understand a lot more of what you are seeing!

NEGLECTED ASSET

It has to be said that the Vaal today has been seriously neglected. It is, of course, valued and managed as a water supply (by the Dept of Water Affairs) and recognised as a popular venue for picnicking, fishing and canoeing. But it has been overlooked as a special scientific and historical heritage feature of the Vredefort Dome.

Currently, from Otters' Haunt our home on the Vaal, Karen and I are running a campaign to save the islands of the Vaal. We hope to see the original highveld and bushland vegetation restored with the removal of Australian eucalypts and Amazon water hyacinth as well as many other invader species. It is important to publicise the uniqueness of the Vaal and its islands because there is nothing like them anywhere else on Earth.

HUMAN HISTORY

Where there is water, there are human settlements. Geology has made the Vredefort Dome world famous. But there is much more to tell about the region and its river. The Vaal  has been a drawcard for wandering tribes and gold-hunting prospectors alike. It was a crossroads of migrations, and hence, warfare. The archaeological record is replete with this prehistory which is still being uncovered by the experts. Stone Age and Iron Age artefacts are being unearthed, along with an astonishing record of trade with distant parts of Africa and the East.  Through it all, the river has continued its uninterrupted flow.

MARVEL AMONG WATERWAYS

Steadily accumulating research is revealing that the Vaal River and its islands are a truly unique feature of the Dome. The ancient river basins are intimately connected with the Dome's formation and later erosion, and the islands indicate micro-faulting in the rocks originating from the blast. Yet no special mention was made of the Vaal in the proposals for the World Heritage Site, probably because nobody recognised the river features for what they are. As I have studied and written about rivers, lived on rivers and adventured down them all my life, perhaps I could see what others could not. Let's all do something to draw attention to this marvel in the world of waterways.

Copyright, the author, 2007.