How fire and ice shaped our landscape

2008-02-27 00:00


the stone

From Hilton’s lofty viewpoint one can almost imagine the blue smudge that is the Indian Ocean out towards the east, particularly after rain when the visibility is at its best.

I often marvel at the amazing altitude difference between the city and the misty heights and how comfortable we are with the 500-metre ascent, which many of us make on a daily basis. This may be partly due to a long and familiar association with this “hill” which defines to some extent our town and which we effortlessly beetle up and down thanks to the internal combustion engine.

However, there is more to all this than initially meets the eye. It is an interesting but fairly obscure fact that the African hinterland, and Southern Africa in particular, has an average altitude of 1 000 metres. This is significantly higher than areas underlain by similar geology elsewhere on Earth, where average elevations of 300 metres are the norm.

Consider too that the highest point in southern Africa, Thaba Ntlenyane, is located approximately 160km from the coast. The descent from Southern Africa’s highest point to the littoral is therefore abrupt — from 3 300 metres to sea level over a horizontal distance of 160 kilometres. This is nothing less than phenomenal and is almost unheard of except in very mountainous terrains. The question is, why?

Current thinking has it that southern Africa is elevated due to ongoing mantle-plume activity beneath the subcontinent, which initiated the break-up of the supercontinent of Gondwana about 160 million years ago. In layperson’s terms, this “hot spot” beneath the Earth’s crust has led to a localised upwelling of buoyant, molten material (the mantle plume) which in turn has led to the upward bulging of the overlying crust and associated elevated topography.

Ongoing tensional stresses in the Earth’s crust, due to continental rifting, led to the formation of seaways (the infant Indian and Atlantic oceans) on the southern African continental margins. This resulted in faulting and the stepping down of the landscape from the original land surface of the interior (what is now the highland regions of the Drakensberg) to the ocean margins.

As our Dusi paddlers can attest, the Umsunduzi and Umgeni Rivers flow through some spectacular, steep-sided valleys. The same can be said for the rest of the rivers of the province — the Umkomaas, Umzimkulu, Umhlatuze and the great granddaddy of them all, the Thukela.

The existing rivers, originally flowing on a high-altitude plain prior to the Gondwana break-up, then took the most direct route to the newly formed Indian Ocean, cutting down through the ancient bedrock to the sea to form the spectacular kloofs and valleys which we know so well.

The verdant growth of strelitzia and acacia and the unique ecosystems of these valleys are a direct result of this geological event.

From the lofty viewpoint atop Town Hill, one gets some inkling of the spectacular scenery of the Umsunduzi and Umgeni Valleys, but for some real mind-blowing scenery, follow the Comrades Marathon route to Durban and keep your eyes peeled for views of the Valley of a Thousand Hills out towards the east. Even more spectacular vistas are to be had on the road from Hillcrest to Inanda Dam where a stop on the edge of the escarpment prior to the descent to KwaNgcolozi will reward you with some of the finest views in Africa.

In the following weeks we will venture forth on a geological journey, beginning on the edge of the great ocean basins, then on to the fiery birth of the Drakensberg, down through the age of deserts, temperate swamps and dinosaurs, ice ages and ultimately to the ancient exposed bones of the Earth — the remnants of an ancient Himalayan-sized mountain chain which once straddled our province.

Earth history is written large in our own back yard — 300-million-year-old glacial pavements where the ice ground across the landscape are to be found just outside Pietermaritzburg. Glacial deposits lie dumped by retreating ice everywhere, 160-million-year-old lava and ash flows outcrop in the cuttings of Sani Pass, the body imprints of crocodile-sized amphibians lie preserved in the Karoo mudstones outside Estcourt, and Durban’s Berea is part of an ancient cordon of sand dunes which accumulated when sea levels were higher than they are now.

We walk, in a sense, on hallowed ground, where the history of the Earth is recorded in the rocks beneath our feet, affecting us all in myriad ways. Our prosperity and survival is intimately entwined with the underlying geology — it provides us with metals and fossil fuels vital to modern life, forms the substrate on which we found our structures, and the soil, the residue of the aeons, nourishes our crops.

Series: Who is Allen Davie?

Today we bring you a new 12-part series titled “Romancing the stone — adventures in Earth history” on the geological make-up of KwaZulu-Natal. Allen Davie will be writing on a fortnightly basis about various topics around this subject.

Davie’s interest in matters geological began with a fascination with dinosaurs at a young age and names like Stegosaurus and Dimetrodon featured large in his vocabulary. This led to a degree in geology from the University of Natal, an honours degree from Rhodes University and a Masters degree from the University of Leeds in England.

All these qualifications have taken him around portions of Europe, the Middle East and Africa, working on a variety of projects ranging from tunnels in the Italian Dolomites to the big dams in Lesotho. Currently he heads up a branch of a firm of consulting geologists and geo­technical engineers and finds himself travelling throughout KwaZulu-Natal and occasionally elsewhere.

Davie is often astonished how little people know about their physical world, particularly the Earth on which they walk, so if you are in the near vicinity when some interesting landscape heaves itself above the horizon or some rock is found in the path, prepare yourself for a discourse on origins and the significance thereof.

People are often amazed to learn that we had a Himalayan-sized mountain range passing right through KwaZulu-Natal way back when, or that South Africa lay over the South pole 300 million years ago and we were buried under hundreds of metres of glacial ice.

The aim of our series of 12 articles is to provide a broad overview of the geological history of our province, along with GPS co-ordinates of some of the geosites which will allow the more interested readers to go and see these for themselves.

Should you want to know more, Davie runs weekend geological workshops covering the fundamentals of geology and which include visits to some of these sites. He can be contacted via e-mail at ­

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