Romancing the Stone Workshop

Well, it has been a long hard slog but we have managed to break the back of it. Yesterday we went live with our new website, www.oldcanvasexpeditions.com. Now Old Canvas is the provider of some of the top specialist safaris in Southern Africa. More than just a game drive is our slogan and that is exactly what we hope to achieve.

Old Canvas Expedition's flagship geology workshop is out of the town of Clarens in the Eastern Free State - at an altitude of 1800 metres it is the second highest town in the country. The beautiful sandstone cliffs which surround the town and provide the magnificent backdrops for which it is famous, are fossilised sand dunes, harking back to a time 180 million years ago when the area was a desert, the realm of dinosaurs and small mammals.

Gondwana was stilled joined; a vast supercontinent including Africa, India, Australia, Antarctica and South America. Incontestable forces began to play out at the end of Gondwana times, and the supercontinent began to tear itself apart. As these ruptures took place, thousands of metres of red hot, incandescent lavas flooded out onto the Gondwana landscape, burying all in a fiery embrace.

Walking through the town of Clarens gives no inkling of this cataclysmic history, but looking south one can see the blue Maluti Mountains - a remnant of those thousands of metres of lava which burned everything before it. Dinosaur bones protrude out of rocky road cuttings, the ancient desert dunes lie preserved in the cliff faces, faults form large dislocations in Earth's crust and dykes traverse the landscape for tens of kilometres, once feeder pipes to the overlying basaltic outpourings.

We will marvel at the geological history of the region and the story of both wandering continents and wandering dinosaurs. We will drive the highest road in Africa to the Letseng Diamond Mine - the highest on Earth and the source of 4 of the 20 largest diamonds ever found. On the final day we will fly past the Tugela Falls, the second highest on Earth at 947 metres and enjoy views of the Drakensberg. This range is a proclaimed World Heritage Site - the uKhahlamba. From the air we will also be able to see the post-Gondwana landscape and the phenomenal gorges that the eastward-flowing rivers have cut into the ancient African bedrock.

Join us then on a journey through 300 million years of Earth history, witness the breakup of Gondwana, the outpourings of lava and see the highest diamond mine on Earth. Perhaps the highlight will be to fly along the Eastern Escarpment past the Tugela Falls and the towering ramparts of the Drakensberg.

These are experiences which cannot be missed, and so we invite you to journey not only to one of the most picturesque towns in South Africa with its art galleries and restaurants, but on a journey down through the aeons, to a time when dinosaurs once ruled the Earth and Clarens was at the heart of a vast continent called Gondwana.

Vredefort Impact Site - Ground Zero

Romancing the Stone

Winging its way in at 70 km per second from somewhere in the asteroid belt, the meteorite, at 15 km in diameter is the largest and deadliest object to ever strike Earth. At ground zero the destruction is unimaginable, as a concentric shock wave propagates outwards at supersonic speed for a distance of over 300 km. The ground heaves, and where there was once solid rock, now there is molten glass, where there was once stable atmosphere, there is now a column of dust and ejecta towering thousands of metres into the afternoon sky. Ejecta and ash billow in vast, debris laden clouds, casting a doom-black pall across the land. The sun, as if bending down to look onto the face of the Earth, gazes impassively at her blighted face, while hot winds toss the dark clouds across the crimson disk before going out with the light.

Wandering through the long grass around the village of Vredefort, Free State, South Africa, the casual observer would not have the slightest inkling that this is Ground Zero of the largest meteorite impact on Earth. Low quartzite ridges provide the topographical interest with valleys in between. From the air the picture becomes somewhat clearer; as a series of concentric topographic ridges with intervening valleys become apparent, best developed in the northwest of the area. In geological parlance this is a “dome” – an approximately circular area of Earth’s uplifted crust, dipping away from the centre, and in our case 80 km in diameter. To explain the enormity of this event a little stratigraphy is necessary at this point and perhaps a good analogy is a multicoloured cake. The adventurous chef has placed pink sponge at the base, and then covered with alternating layers of chocolate and vanilla sponge. The pink represents the ancient granite which underlies 1000’s of square kilometres of real estate around the dome, and the alternating coloured sponge layers the younger sedimentary cover. At Vredefort the layers of the cake are stood on end, with the pink granite sponge protruding through the overlying layers. Returning to the scale of the real world, this is the equivalent of 20 km thickness of rock being upturned and the granite punching through to the surface. This is, to use an overused but appropriate word, awesome. Back on the ground, evidence of some kind of cataclysmic event having occurred here is to be found in the form of two rock types known as pseudotachylite and granophyre. Shatter cones and spectacular fracturing the rocks also are witness to this event. Tachylite is a form of glass found in volcanic environments, which the material in the dome resembles, although this is clearly not a volcanic environment, hence the prefix ‘pseudo’. Pseudotachylite is almost ubiquitous throughout the dome and occurs in veins from less than 1 mm to up to 100 metres thick. The Vredefort granophyre, a brecciated rock, has as part of its composition a glassy matrix indicating instant melting and rapid cooling. In fact both of these rock types are the product of shock-induced melting. The granophyre occurs as large dykes, several kilometres long and many metres wide with a composition different to any other known rock and containing fragments of extraterrestrial material. Then there are the shatter cones – concentric, conical structures the result of the almost instantaneous shock wave so intense as to shatter and melt hard, competent rock. And in the process to overturn a thickness of 20 km of granite and lithified material.

The 80 km diameter of the dome represents only the uplifted, central part of the impact crater and by extrapolating from other known impact craters on the Earth and the moon, a figure of 300 km diameter for the Vredefort crater is arrived at – the largest impact crater on Earth. As a student in the late 1980’s there was much speculation as to its origin, and geologists, being essentially romantics at heart, adopted the meteorite impact theory. However, absolute, conclusive evidence was lacking but thanks to the careful work of Uwe Reimold and his team from the University of the Witwatersrand, conclusive evidence was provided to sustain this theory, enough for it to be declared a World Heritage Site in 2005, as Earth’s oldest and largest meteorite impact crater. How old is it? Just over 2 billion years, which is just under half the age of Earth. Fortunately life was still in its infancy then and the impact, although cataclysmic, did not cause the damage that better known and more recent collision events had on the biosphere.

Romancing the Stone

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