The ancient of days

2008-03-14 00:00

White, snow-capped peaks project jaggedly into the blue rarefied air of the jet stream, which draws out long plumes of snow from those high summits. In the valleys below the snow-flanked shoulders of the mountains, glaciers brood, grinding and plucking the encompassing rock which holds them in their icy realm. Further down-slope, fast flowing rivers, milky with the rock flour derived from glacial mills above, cascade seawards through valleys strewn with sand and boulders. These sediments are the debris of the aeons, carried here by inexorable action of water and ice. There are no birds or animals, no plants or trees; the only sound is the roar of the rapids, which is occasionally punctuated by the crash of landslides, or the muffled thunder of avalanches falling from the snowfields above. This is a primal world set in a time when life was in its infancy and a mountain range of Himalayan proportions straddled an ancient supercontinent called Rodinia. Forming part of the inner core of this mountain range were rocks which now find themselves, 1,1 billion years later, exhumed and exposed to the glare of the African sun, the footfalls of Nguni cattle and the impetuous ebb and flow of our mud-laden rivers.

How do we know this? Geologists, plying their trade in the Alps, Rockies and then the Himalaya, had dissected and described the rock types which underlay these massive mountain ranges. Geologists here at home realised that the rocks exposed in the Valley of a Thousand Hills or in the valleys of the Thukela or Umkomaas rivers had uncanny resemblances to those found in more modern examples.

It was therefore no great conceptual leap to realise that the geological processes which formed the Alps and the Himalaya were identical to those which created the granites and gneisses* which underlie our province. By inference, the rocks of what is known as the Natal Metamorphic Province, had their origins in the roots of an enormous mountain belt. The modern-day Himalaya is the result of India colliding with greater Asia due to plate tectonics and, in a similar vein, our ancient range formed 1,1 billion years ago, as Ur and Atlantica, two relatively small continents, collided with a larger continent known as Nena (acronym for North America-North Europe) to form Rodinia.

I can hear you gasp, but if one accepts that the Himalaya and the Alps are the result of continental collision, then an ancient mountain range in KwaZulu-Natal caused by the same mechanisms is not in the realm of fantasy. Collisions of this magnitude throw mountains up to great heights, but also carry crust to great depths, sometimes up to 70 kilometres or more, where temperatures and pressures melt, cook and torture the buried sediments into metamorphic rocks which at first glance belie their origins.

Some fine examples of the original core of our ancient mountain range are to be found in the valley of the mighty Thukela river. Augmented by a myriad tributaries, this river has cut down through the younger geological cover to exhume the ancient bedrock of our province.

Fantastic folding may be seen in the road cuttings on the southern approaches to the steel bridge at Jameson’s Drift. Granites and gneisses are exposed in the river beds of most of our rivers throughout the province and provide a competent substrate for socketing the foundations of our dams which have become essential to our urban life.

One particularly distinctive rock type, called “augen” gneiss, is to be seen in the road cuttings on the descent to Nagle Dam via the abattoir road. The term “augen” describes perfectly the large, eye-shaped feldspar crystals which “schiller” in the harsh sunlight. The more adventurous might continue the drive to the dam and negotiate access to the spillway below the diversion weir. Blasted out of the living rock, the gneisses exposed in the floor of the cut are classic examples of a high-grade metamorphic rock.

Light and dark banding characterise the rock mass and closer inspection will show this material to be melted, contorted and stretched into tight, convoluted folds under those immense pressures and temperatures which we discussed earlier.

It is here then, in the deep tectonic mill that our rocks found themselves 1,1 billion years ago —contorted and melted into the gneisses and granites exposed in KwaZulu-Natal’s river beds.

Since then, those towering peaks have been ground down by the inexorable action of a billion years of rain, wind and ice, to leave only a remnant of a once magnificent, Himalayan-sized mountain chain which stretched for hundreds of kilometres across a now disintegrated supercontinent. So next time when you stand on the crystalline basement of our province be aware that snow-clad peaks towered above your head, 1,1 billion years ago.

* Gneisses — coarse-grained metamorphic rock that is bonded and foliated.

• Allan Davie runs weekend geological workshops covering the fundamentals of geology and which include visits to relevant sites. He can be contacted via e-mail at geologist@netactive. co.za

Guide: Plate tectonics

Our Earth has been likened to a honey-filled paper bag, swung around one’s head on a piece of string. We delude ourselves that the so-called Earth on which we conduct our lives is permanent, but our terra firma is nothing but an illusion, with Earth’s crust comprising a thin layer atop a mass of molten magma.

Divided into a number of segments called plates, the crust floats on the surface of this molten mass, jostling and colliding in an interminable, grinding dance.

In some instances the crust is destroyed when one plate margin is forced beneath the edge of another, to be remelted and assimilated back into the hot mantle.

In other instance the colliding plates throw up mountain ranges as in the case of the Alps or the Himalaya. And in some instances they slide past one another, causing earthquakes and mayhem, as in the San Andreas Fault of California.

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