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 m 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 and it is an interesting but fairly obscure fact that the African hinterland, and Southern Africa in particular, has an average altitude of 1000 m. This is significantly higher than areas underlain by similar geology elsewhere on Earth, where average elevations of 300 m are the norm. Consider too that the highest point in Southern Africa, Thaba Ntlenyane, is located approximately 160 km from the coast. The descent from Southern Africa’s highest point to the littoral is therefore abrupt – from 3300 m to sea level over a horizontal distance of 160 km – this is nothing less than phenomenal and is almost unheard of except in very mountainous terrains. 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 approximately 160 million years ago. In lay person’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 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 Umsundusi and Umgeni Rivers flow through some spectacular, steep-sided valleys and the same can be said for the rest of the rivers of the province – Umkomaas, Umzimkulu, Umhlatuze and the great granddaddy of them all, the Thukela. The existing rivers, originally flowing on a high altitude plain prior to Gondwana breakup, 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 remarkable geological event. From the lofty viewpoint atop Town Hill one gets some inkling of the spectacular scenery of the Umsundusi 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. Perhaps 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 Kwa Ngcolozi 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 writ 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 of Pietermaritzburg; glacial deposits lie dumped by retreating ice everywhere; 160 million year old lava and ash flows outcrop lie 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 a myriad of 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 soil, the residue of the aeons, nourishes our crops.
As our Dusi paddlers can attest, the Umsundusi and Umgeni Rivers flow through some spectacular, steep-sided valleys and the same can be said for the rest of the rivers of the province – Umkomaas, Umzimkulu, Umhlatuze and the great granddaddy of them all, the Thukela. The existing rivers, originally flowing on a high altitude plain prior to Gondwana breakup, 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 remarkable geological event. From the lofty viewpoint atop Town Hill one gets some inkling of the spectacular scenery of the Umsundusi 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. Perhaps 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 Kwa Ngcolozi 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 writ 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 of Pietermaritzburg; glacial deposits lie dumped by retreating ice everywhere; 160 million year old lava and ash flows outcrop lie 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 a myriad of 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 soil, the residue of the aeons, nourishes our crops.
1 comment:
A fascinating read - well done and keep up the good work.
Regards
G
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