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Geology of the Drakensberg Mountains
February 16, 2014
The geology of the Drakensberg Mountains is fairly simple. What is difficult to grasp is the huge time scale that this drakensberg geology took place in. The Drakensberg Escarpment and high peaks are the capping ontop numerous horizontal layers of the Karoo Supergroup of rocks which cover some two-thirds of South Africa, much like the icing on a tiered wedding cake. The time in which the layer of minerals was deposited is not the same as when the sediments were formed.
The Karoo Period began about 250 million years ago before the continental shift in Gondwanaland. This Drakensberg area was covered by a vast sheet of ice. When this continental glacier melted, it left behind a sludge of eroded material which solidified into the Dwyka Group lying in a basin that stretched from the south-western Cape to Mpumalanga and into Zimbabwe.
This is the oldest and deepest layer of the Karoo Supergroup. The basin started off as an immense inland sea, and, as it filled up with further deposits and dried, the Ecca~Group, containing vast forests of primitive trees, was laid down as mainly shales.
Then came the Beaufort Group, largest and deepest of all, comprising mudstones, sandstones and shales. Over the tens of millions years of the Beaufort Period, conditions varied greatly but that era was characterised by swampy landscapes where the first amphibians crawled out of the Triassic seas to colonise the emerging land masses. They in turn evolved into mammal-like reptiles and later the earliest mammals. After about 30 million years of watery conditions the Karoo Basin filled up, with sediments which in time became rocks, and dried out, lakes becoming swamps and dry plains and then the wind-swept deserts of the Stormberg Group.
It is the rocks of the Stormberg Group that make up the Drakensberg. The first, or lowest, layer is the Molteno Beds comprising mainly blue-grey sandstones which form ledges and terraces at the base of the Little Berg. Plant fossils are abundant, but no animal remains have been found. The Red Beds form the second layer, with their typical red to purple mudstones and shales, which form the steep grassy slopes of the mid-Little Berg. These rocks are seldom exposed in today's landscape, but the increased aridity which caused their deposition is thought to have brought about the slow demise of the mammal-like reptiles of the Karoo dynasty (not to be confused with the dinosaurs of the later Jurassic Period).
Large, dinosaur-type fossils are abundant in this layer. The large-grained, yellowish Cave Sandstone (or Clarens) layer appears to be largely aeolian (wind deposited). These rocks are extremely soft and erosive, which accounts for the cliffs that define the Little Berg, and the many caves and overhangs found in this very visible band of rock. Fossils are rare, mainly reptiles and fish.
Towards the end of the Cave Sandstone time, Gondwanaland began to heave and buckle and split apart. What followed was a long period of fireworks: about 190 million years ago the land opened in huge cracks and fiery lava poured out to cover most of what today is KwaZulu-Natal, Lesotho and the Free State When solidified they formed the relatively soft, aerated basalt 'icing' on our mountain cake.
With the break-up of Gondwanaland, continental uplifting and the climatic influence of the sea initiated a cycle of erosion which continues today. What we see from below as the Drakensberg Mountains is really a giant step that is being continuously worn back by water and gravity.