When glaciers ground across Africa

2008-04-11 00:00

From Zululand in the north to the shores of the Cape in the south are to be found accumulations of a fine grained, blueish grey rock containing ground down lumps of granite, sandstone and other rocky debris. This might all seem esoteric and of little relevance to us, but perhaps if I were to tell you that these piles of blueish-grey rock represent nothing less than fossilised deposits of glacial till, perhaps you might sit up and take notice.

Glaciers? In Africa? Well, yes, but not last week, not last century, but 300 million years ago. At that time we were still part of the supercontinent of Gondwana, drifting through the polar regions, with conditions not much different to those which Antarctica is currently experiencing.

However, no one was around then to observe this, so what evidence is there to prove that we were in the grip of a pre-historic ice age? Many of the geologists who mapped the strata of South Africa 100 years back were from European schools and hence were familiar with modern glacial tills which occur in northern Europe.

The effects of the inexorable milling of the landscape by ice sheets during the last European ice age are no different to those experienced here 300 million years ago, except that our deposits have had time to consolidate and lithify. I would imagine that they were more than a little surprised to find evidence of glaciation while sweltering under the African sun. At that time there was no mechanism to explain to our pioneering geologists how the climate got to be so cold, but there was no refuting the evidence. We have the benefit of hindsight now that the theory of continental drift has become hard geological fact, but back then the very idea of wandering continents had not even been suggested.

To have put forward a crackpot theory that the immovable Africa had been part of a larger ancient continent, and moreover that this continent had once been centred on the South Pole would have led to derisive laughter at best or questions as to the state of their sanity at worst.

The fine-grained groundmass which makes up the bulk of the rock is fossilised rock flour — a pulverised matrix ground into being by the overbearing weight of tons of ice juddering across the ancient continental surface. Our local outcrops of Dwyka Tillite contain countless millions of fragments of Natal Group Sandstone and Granite Gneiss, torn from the underlying substrate by the bulldozing and grinding ice.

Perhaps even more spectacular are the occurrences of striated pavements in a number of places around the province. When they opened up the foundations for the new geography building at the then University of Durban-Westville, they found a striated pavement with grooves gouged into the hard underlying sandstone — grooves made by the bedload of the glacier as it ground its way across the sandstone substrate.

Other local examples occur on top of the Natal Table Mountain and on the Hopewell Farm near Thornville. Dropstones — boulders rafted out on ice floes and then deposited into the sea floor muds — can be seen in the road cuttings on Table Mountain — Nagle Dam Road.

So much for the direct evidence of glaciation, the question now remains as to how they worked out that southern Africa was drifting around down there in the high latitudes. Well, it was found that, as lavas pour out on to Earth’s surface, which they have been doing for 4,6 billion years, the iron in their minerals aligns itself with the prevailing magnetic poles.

As the rock cools the iron is frozen into that orientation and preserved for geophysicists to analyse millions of years later. Some of you might have noticed when driving up Sani Pass, round uniform holes drilled in the lavas in the road cuttings, evidence that the geophysicists have been there before you. Careful note is taken of the orientation of the sample in the field before it is removed from outcrop.

Back in the laboratory the specimen is then reorientated to its original field position and then the orientation of the iron is measured. This then tells our geophysicist in which direction the magnetic pole once lay, relative to our rock sample. Thousands of these measurements have been plotted on to base maps and over time a pattern has emerged of polar wanderings throughout Earth history.

So on a continental scale, vast ice lobes ground southwards across the ancient face of our continent, scarring and adding character to our African landscape. Vast deposits of tillite may be seen from the Dwyka River in the Cape to the far north of Zululand and are evidence of continental wanderings and an ice age which had us in its frigid grip 300 million years ago.

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