The Karoo Sea

This is swampland on a giant scale. Teaming, pushing, living, scrambling forest grows riotously along the muddy banks of lagoons and stinking back swamps. Trees, club mosses, cycads and ferns compete for space, water and light, while those that have seen their days out collapse back exhausted to the forest floor, to be rapidly colonised by fungi and bacteria. Beetles scramble through the leaf litter or fly ungainly through the canopy, while cockroaches scuttle and scratch in the leaf mould. In the creeks and backwaters amphibians populate the mud flats and dark pools, while out on the sunny waterways fish leap and squadrons of giant dragonflies keep station above the languid waters.

Times have clearly moved on from the icy grip of the Dwyka Ice Age. Gondwana has wandered northwards and South Africa now finds itself at approximately 30° south where temperatures are more conducive to life, which has seized the day, colonising the land to form vast subtropical forests along the northern rim of an ancient sea. But now we need to step back a little and look at the geological processes which have led to this change of circumstances. We have looked at the great ice age known as the Dwyka glaciation of 300 million years ago, but now we must take our focus off KwaZulu Natal for a short time and look at what was going on in South Africa as a whole. Many of us will have travelled through the passes of the Cape Mountains and wondered at the spectacular folding which characterise this mountain chain. These mountains began to form approximately 330 million years ago during the late stages of the Dwyka glaciation. The mountains formed the southern boundary of an ancient sea which covered the majority of Southern Africa – bearing in mind of course that we were part of a larger supercontinent. This sea had a low tidal range and is thought to resemble the modern Black Sea - open to the larger ocean, but without the dynamics of a full-blown ocean. To the north, stretching from the Northern Cape through Gauteng to Mpumalanga, were the Cargonian Highlands – a range of hills which defined the northern limits of this sea. Our Dwyka ice sheets have been bumping and grinding their way southward across these northern highlands before being cast adrift as icebergs on the turbulent waters. As the ice scoured the landscape, Gondwana continued to drift northwards, ultimately bringing us to warmer latitudes and creating conditions conducive for the burgeoning of life.

With decreasing latitude, the ice melted, to be replaced by southward flowing rivers which began to drain the Cargonian highlands, depositing vast quantities of sediment along the northern margins of the Karoo Sea to form large, meandering deltas. Like deltas everywhere, they supported the forests and swamp life where today’s narrative began. The trees that dominated these deltas were a Gondwana classic known as Glossopteris - the distribution of this species another nail in the coffin for the naysayers of continental drift and plate tectonics – it is found in South America, South Africa, Australia, India and Antarctica, strong evidence indeed for the supercontinent of Gondwana. Glossopteris, ferns, horsetails and club mosses thrived within these vast deltas, to the extent that thick accumulations of wood and organic matter were unable to rot and were buried and ultimately converted to coal. These deposits form our coal reserves of KwaZulu-Natal and the Highveld.

The southern margins of the Karoo Sea were characterised by deep water and a narrow littoral, set against the towering peaks of the Cape Mountains. Small, fast flowing rivers typical of mountainous regions deposited their load along a narrow littoral on the southern margin of the Karoo Sea. Every so often some of this accumulation of sediments would slump into the depths to form large, fan like deposits on the sea floor called turbidites, and these may still be seen fossilised in the hills of the Eastern Cape. But this is treatise on the local geology, so we will stay focussed on what is in our backyard. The accumulation of sediment in the Karoo Sea continued for in excess of 150 million years, eventually filling the basin. Karoo rocks have the widest distribution across the subcontinent and reflect the changing latitude and depositional environments as the basin began to fill. It is an ongoing tail of evolution and change and we will continue with the Karoo story in the next article. However any journey inland from Pietermaritzburg will traverse sediments once laid down either on an ancient sea floor or on the swampy margins thereof, where verdant forests thrived in a magnificent outburst of life. Karoo rocks extend to the crest of van Reenen’s Pass and beyond – a truly magnificent heap of sediment which reflects in part the geological history of a subcontinent.

Romancing 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 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.