Layer-Cake Karoo

Dolerite, Karoo geology

The distinctive flat-topped Karoo mountains like Teebus and Koffiebus near Steynsburg have their origins in dolerite (ironstone).

About 300 million years ago, what is now South Africa was frozen and mostly lifeless, drifting over the South Pole and landbound within the vast Supercontinent of  Gondwana.

As it inched towards the warmer north, boulders and general sedimentary muck picked up by the grinding glaciers dropped into a growing superlake of meltwater – the origins of the enormous Karoo inland sea.

Karoo geology

The Karoo has distinctive geology, including flat-topped hills and mountains.

The underwater debris slowly hardened to become the Dwyka Formation, bottom-most layer of the rock sequence geologists call the Karoo Supergroup.

Huge mountains to the south and north were slowly eroded away by massive rivers.

Year after year, epoch after epoch, the fertile layers of silt came down, pages and pages in the books of Ecca and Beaufort rock groups.

The mountains shrank, the silting grew, the sea contracted to a lake and later, a lively inland delta.

But 251 million years ago a mysterious, planet-wide disaster wiped out 96% of all life on Earth. Weird Permian lifeforms, water meadows, ferns, cycads and glossopteris trees all vanished.

For aeons, the vast Karoo basin was a desert covered by huge sand dunes – the Clarens Formation – and new life slowly evolved to suit the dryer landscape.

Then, 182 million years ago, hot lava poured out of the restless earth through cracks in the planet’s crust as Gondwana started to tear apart. Molten rock covered the sand dunes, baked them hard and capped them with rigid basalt.

Karoo geology

The Spekboom mountains from the Mountain Zebra National Park.

It oozed along underground cracks and fissures, slowly cooling into hard dolerite sills and dykes.

The dolerite – also called ironstone for its reddish colour and hardness – is the very reason you see level-headed hills like Teebus and Koffiebus near Steynsburg, Martha and Maria near Tarkastad, and the Three Sisters, all across the Karoo.

The rest has just quietly eroded away.

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3 Responses to Layer-Cake Karoo

  1. Mukhtar August 19, 2014 at 1:38 pm #

    Thank you for sharing this info with me. Always wondered about this. Now I know.

  2. Ingrid September 25, 2015 at 12:23 pm #

    LOVE this info – thank you for sharing

  3. Rob Wissing June 18, 2016 at 9:23 am #

    Thank you for a fascinating insight☺

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