The Natal Group Sandstone

Standing on the edge of the precipitous cliffs, with the views to the distant krantzs and the cool winds tousling the hair, one cannot help but be moved by the magnificence of the scenery, or wonder how this all came to be. The rocks on which you stand are the result of a complete change in the geological conditions which had prevailed during the formation of our Himalayan-sized mountain range which we visited two weeks ago.

Six hundred million years have flitted by since our mountain chain was thrust up into the teeth of icy gales and snow. Inexorably the agents of erosion have ground down the high peaks of our mountain range into nothing more than mud and sand – transporting the detritus in rivers to be dumped ignominiously in some forgotten backwater, their magnificent provenance forgotten. All that remains of our high peaks is a level plain, floored by ancient granites and gneisses, with localised hills adding some interest to the landscape. It is difficult to grasp the concept of time – geologists glibly refer to millions of years or hundreds of millions of years, so to put this into some kind of perspective, the Alps were formed 60 million years ago and are still subject to the compressional forces as Africa continues to drive forward into the underbelly of Europe. Six hundred million years is sufficient time therefore to reduce a mountain range from towering peaks into nothing more than thick accumulations of sediment dumped in the low-lying regions of the Earth.

Crustal forces had changed from collision to tension since the days of our mountain building episode, and tensional stresses had begun to accumulate to the south of the supercontinent of Gondwana. These tensions began to tear apart the greater landmass to form rift valleys identical in many aspects to the well known Great Rift Valley system of East Africa, the rifts extending along the southern, western and then more latterly the eastern coasts of Southern Africa. Bear in mind though that Africa did not exist at this time – it was centrally located within Gondwana and I use the name only to give some kind of geographical reference to our journey. The rifting process continued long enough to develop valleys deep enough and wide enough for the southern ocean to inundate the existing landmass, forming a seaway which has been called by geologists as the Agulhas Sea. This new ocean, with marine life, currents and waves not unlike our own, became a great repository of a thick pile of sediment – sand, mud and gravel - which were deposited in the depths, recording the vagaries of ocean currents, floods and storms over the millennia. In the Cape the sediments attain a thickness of 8 km or more in places.

Here in KwaZulu Natal, rivers carried sand and detritus southwards towards our local arm of the Agulhas Sea, whereupon they were dumped into deeper water, to be reworked by ocean currents. The climate was apparently semi arid at this time, although there must have been enough rain to fill the rivers and transport the sediments to the coast. If you are able to read the rocks, almost any outcrop of the Natal Group Sandstone shows evidence of strong currents and ripples, indicating transport in river systems way back in the Ordovician. Moving ‘upstream’ around Melmoth and Eshowe the original stream valleys, dating back nearly 500 million years, are still to be seen, choked with boulders and gravel derived from the weathering of the surrounding mountains. These ancient palaeo valleys can be seen if one takes the Jameson’s Drift or Middle Drift roads from the northern rim of the Thukela Valley.

Ongoing accumulation of sediment continued until, with the passage of time, those tensional stresses in the crust which had lead to rifting were reversed and the whole accumulation of sediments were squeezed, folded and uplifted into what is now the spectacular scenery of the Cape Fold Belt. The sediments here in KwaZulu-Natal were spared the vice-like embrace of this tectonic mill, but were uplifted nonetheless, to form the magnificent scenery of Kloof Gorge, the rim of the Valley of a Thousand Hills, the Kop at Kranskop and the spectacular Oribi Gorge. Next time you are driving from Pietermaritzburg to Durban, the cliffs on your left just after the Shongweni off ramp and before the toll reflect an environment where rivers carried vast accumulations of sediment into an ancient sea, 490 million years ago.

Although not as dramatic as the mountain building event which preceded it, the formation of the Natal Group Sandstone was a significant event in our geological history and was the substrate on which the Gondwana Glaciation took place when South Africa (as part of the larger Gondwana landmass) found itself at the South Pole.