Geology of the Drakensberg

Long, long ago, the place where the Drakensberg mountains now stand was a vast inland sea, covering thousands of square kilometres. Very slowly, that sea filled up with layers of sand, pebbles, silt and clay, carried in from distant mountains by huge meandering rivers.

Plants and animals that lived and died in this changing environment fell into and were buried by these sediments. Eventually, sandy deserts spread over the area, and then it was all covered over by huge volumes of molten lava, poured out over quite a short time.

Today, these huge layers, which once covered nearly all of South Africa, are wearing away. What is left behind are the most resistant parts, which we call mountains. The relentless pull of gravity, the constant wind, the driving rain, the snow and ice of winter – all these forces form, shape and chisel the rock away, sending the grain of the mountains by grain back into the ocean.

The rivers of KwaZulu-Natal, with its high rainfall, are steadily eating away at these mountains and, one day in the distant future, they will be gone.

Cliffs almost a kilometre high

The Drakensberg mountains are huge and include Southern Africa’s highest point. Some of the cliffs are almost a kilometre high. When we look at these imposing mountains and the rolling slopes beneath them, the caves and the rocky streams, we see several kinds of rocks, the remains of ancient layers of sand, silt, mud and lava.

Each layer of rock was formed in a particular environment. By looking closely at the rocks, seeing what they are made of and how they are composed, we can read a series of ‘stories in the stones’. The most visible rocks are both the highest and the youngest (deposited most recently).

These are the thick layers of Basalt which cap the mountains, and on which the Kingdom of Lesotho stands. These are almost a kilometre thick in places.

Basalt is formed when molten lava pours out onto the surface and cools down quickly. Directly below the basalt is a layer of pale sandstone. These Clarens or Cave sandstones are like fossilised sand dunes, formed when the area was a vast desert.

Undercut by erosion, these rocks form the huge overhanging caves of the little Berg. Beneath these sandstones are layers of mudrock, harder to see, but the oldest known dinosaurian nesting site was found in these layers.

Under them are found sandstones with pebbles that may have travelled 1000 km from the Cape, carried by huge rivers. In this section, we also find large pieces of fossilised tree trunks and insect fossils.

Below this are the oldest exposed rocks in the Drakensberg, a sandstone and mudstone layer containing fossils of some of the oldest known land-living reptiles, as well as several mammal-like reptiles.

Interesting facts about the Drakensberg Mountains

1. Basalt often has pockets or holes in it from the gas bubbles trapped inside when it cools down rapidly. These holes later fill with dissolved minerals which form small, often layered, crystalline pockets when they solidify and cool. Agates, often found in Drakensberg streams, are formed in this way.
2. Cape Vultures and Bearded Vultures nest on the high and inaccessible cliffs of these mountains. Without the safety of the cliffs, these birds would not survive.
3. There are several endemic plants and animals found in the Drakensberg mountains. These unique organisms are found nowhere else on Earth.
4. Two of South Africa’s largest rivers, The Orange and the Tugela, begin in the Drakensberg. With a drop of 948 metres, the Tugela Falls is the highest waterfall in the world.
5. Thabana Ntlenyana, in Lesotho, is Southern Africa’s highest point, at 3482 metres above sea level.
6. The highest point in South Africa is Mafadi, 3446 metres above sea level.

• This article by John Roff was first published in The Drakensberg Magazine 2023.