Stories in the Rocks

The Drakensberg has dramatic scenery, but a relatively simple structure. Except for the lowest slopes, the whole of the Berg is made up of only two rocks.

From 1500 metres altitude (The Cavern) up to about 2100 metres is the Cave Sandstone. It forms conspicuous pale yellow-brown kranzes capped by an undulating plateau – the Little Berg. The name “Cave” refers to its pattern of erosion. The bases of big rock faces are undercut to form overhangs or shelters, rather than caves. This is where the Bushmen lived and painted, secure from wind and rain.

The Cave Sandstone was formed during the Triassic Period, about 200 million years ago. This region was then a desert. Examination of the sand grains in the rock shows them to be angular, a feature of wind-blown sand. Eventually the shifting dunes compressed and fused under their own weight. It rained occasionally. We know this because fossil footprints of small bipedal dinosaurs can be found. Rain washed finer sand into hollows where it stayed damp long enough to record footprints. As the prints dried out they were preserved by new layers of sand blown on top. Finally, perhaps yesterday, the ancient rock cracked at this plane of weakness, typically in a cave ceiling collapse, revealing the footprints.

Overlying the sandstone is 1000 metres of basalt, best seen in the Amphitheatre in Royal Natal National Park. This igneous rock was extruded by the biggest volcanoes of all time in the Jurassic Period 140 million years ago. The lava flowed smoothly, and the top of the Berg is still fairly flat. The “Dragon’s teeth” appearance from below is due to the differing speeds with which river valleys cut back into the basalt wall. Originally much thicker, this layer covered most of southern Africa, as well as Australia, India, Antarctica and South America. In those days all these land masses were joined together in the supercontinent Gondwana. It split apart during these eruptions, the pieces drifting on the Earth’s semi-solid mantle to their present-day positions. The piece of the geological jigsaw that fits against our eastern shores is now on the far side of Antarctica.

Basalt crystallizes in neat hexagonal columns. In side view these form vertical stripes. A nice example forms a thin layer capping the hill above Echo Cave. Geodes can be found beneath the eroding basalt. These look like large rough pebbles, but are hollow, and are steam bubbles that were frozen into the rock as it solidified. Violet crystals of amethyst line the inside of the hollow. Undamaged geodes still contain their original water which can be heard swishing about if the geode is shaken.

David Johnson

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