The geology of the Natal Drakensberg is relatively simple because the various rock layers consist almost entirely of igneous and sedimentary types which usually lie horizontally.
The Upper Beaufort Series forms the gently rolling country of the Natal midlands and the bottom of the lower berg valleys. It consists of fine to medium-grained yellow sandstones and red, green, blue and purple shale or mudstone.
Lying on top of the Upper Beaufort Series is the Molteno Formation, which consists of coarse, sometimes glittering, blue to grey sandstones. They are thought to have formed from the sediment which accumulated in a vast inland delta. These form the ledges and terraces at the foot of the lower berg, with the best examples to be found in the south.
Above the Molteno Formation lies the Elliott Formation (Red Beds) which consist of fine-grained red or purple shale or mudstone. These were formed from sediment as areas of shallow water dried up about 200 million years ago, and the footprints of dinosaurs from the end of the Triassic Period (245 to 208 million years ago) can still be seen in this layer in some areas of the berg (e.g. Dinosaur Cave). Some scientists speculate that the increasing aridity of the climate was responsible for the disappearance of the Triassic dinosaurs. This climate change may have been the result of a meteor impact (it has been almost conclusively proven that another meteor impact brought the Cretaceous period (144 to 65 million years ago) and the age of the dinosaurs to an end).
Above the Red Beds is the Clarens Formation (commonly referred to as cave sandstone), which is very soft, fine-grained and white or cream in colour. It forms the most prominent cliffs and overhangs of the lower berg, hence the name “cave” sandstone. This layer is believed to have been laid down by wind during the dry period at the end of the Triassic period. Rivers later cut into this layer, forming valleys and leaving behind undercut cliff sections which we call the “caves” of the lower berg. It is in these caves that the Bushmen later lived for thousands of years and left their paintings.
About 180 million years ago and 20 million years after the start of the Jurassic period (208 to 144 million years ago), the tectonic plates of the super-continent of Gondwana, consisting of Africa, Antarctica, South America, Australia and India, started drifting apart. This resulted in the extension (or stretching) of the tectonic plates, causing magma (molten rock) to flow up onto the surface through rifts in the Earth’s crust in a process called fissure eruption. The Drakensberg rifting resulted in a flood of igneous lavas called the Lesotho Formation solidifying to a depth of a kilometre or more over the region now known as Lesotho, and the flow may even have reached the present-day Natal coastline. Over millions of years, erosion eventually formed the peaks, passes and valleys which we recognise today as the upper berg and Lesotho highlands. Pockets of gas trapped in the lava also formed caverns which, when exposed by erosion, became the caves found on the escarpment (e.g. Ngwangwe Cave, Corncob Cave, Didima Cave, Roland’s Cave and the Ndumeni Dome caves).
During the period of fissure eruption, some lava flows forced their way up into underground cracks and cavities. These intrusive lavas form the common blue dolerite of Natal. Once the surrounding rock is eroded away they form hard, erosion-resistant prominences called dykes (if they are mainly vertical) or sills (if they are mainly horizontal and cover a large surface area). Dolerite dykes are especially common in the southern berg, Fingall’s Rock being just one example. Most dolerite dykes are easily recognised by their characteristic columns which collapse to reveal a more-or-less six-sided cross-section.
There are only a few types of minerals and crystals found in the Natal Drakensberg, and these are a result of the magmatic activity associated with the Drakensberg rift:
- Amygdales are the white or green spots or veins found in basalt, and are formed when zeolites, quartz or chalcedony percolate into cavities in the basalt in solution and then crystallise. Basaltic rocks and boulders containing amygdales are commonly found in rivers below the escarpment, where these have been washed down from the high berg.
- Zeolites are a group of hydrated aluminium silicates of potassium, sodium or calcium. In the berg the most common zeolite is natrolite, which occurs as glassy, slender crystals radiating from a point. They are formed in cavities in lava flows.
- Quartz is the crystalline form of silicon dioxide, and in its purest form, the slender, six-sided crystals are colourless and transparent. It is found in amygdales, veins, vugs (regions of swelling in veins) and geodes (hollow cavities in which the crystals point inwards).
- Chalcedony (pronounced “kal-said-on-ee”) is a type of quartz which occurs as a lining or filling of cavities in rocks (e.g. basalt). In the berg, chalcedony is often pale blue with a waxy appearance, occurring either as a thin layer lining amygdaloidal pipes or in hardened form in chert and agate.
- Chert (pronounced “shirt”) is the whitish to grey, opaque, waxy quartz found all over the Drakensberg. It often occurs in variegated layers, in which case it is called agate. Chert is similar to flint but more brittle, and was used by the Bushmen to make arrowheads and spear tips.
- Agate is an impure form of quartz consisting of banded chalcedony.
- Gibson R. L. and Reimold W. U. (2005). The Danger from Space and South Africa’s Mega-Impact the Vredefort Structure. Chris van Rensburg Publications.
- Irwin P, Akhurst J; Irwin D. and Irwin P. (1980). A Field Guide to the Natal Drakensberg. Published by the Natal branch of the Wildlife Society of Southern Africa.