Geology of the Little Karoo is quite different from that of the Great Karoo, the Little Karoo being an integral part of the Cape Fold Mountain belt.
At one stage of its evolution, the Little Karoo was an inland sea of approximately 14 500 square kilometres. It would have been a quarter the size of Lake Victoria (68 800km2); 87times the size of the Sea of Galilee (166km2) and 24 times the size of the Dead Sea (605km2).
“NS cross section Southern Cape” by Oggmus – Created 29 April 2015
A diagrammatic 400km North-South cross-section through the Southern portion of the country at approximately 21ᵒ 30’E (i.e. near Calitzdorp in the Little Karoo), showing the relationship between the Cape Fold Mountains (and their geological structure) and geology of the Little and Great Karoo, as well as the position of the Great Escarpment.
The heavy black line flanked by opposing arrows is the fault that runs for nearly 300km along the Southern edge of the Swartberg Mountains. The Swartberg range owes some of its great height to upliftment along this fault line. The subsurface structures are not to scale.
The geology of the region bears no resemblance to that of the Great Karoo (see the diagram of a NS geological crass section through the Little Karoo).
The Little Karoo valley is an integral part of the Cape Fold Mountain Belt, with the two ranges on either side composed of extremely hard, erosion resistant, quartzite sandstone belonging to the 450-510 million year old Table Mountain Group (i.e. the oldest layer of the Cape Supergroup)
The valley floor is covered, in the main, by the next (younger) layer of the Supergroup, namely the much softer Bokkeveld shales. The dolerite of the Great Karoo did not penetrate these rocks, and so Karoo Koppies are not seen in the Little Karoo.
The Little Karoo contains two other geological features that give the landscape a special character.
1.During the erosion of the African interior following the bulging of the continent during the massive lava outpourings that ended the Karoo sedimentation 180 million years ago, some of the eroded material was trapped in the valleys of the Cape Fold Mountains, especially during the Cretaceous period, about 145+- 4 to 66 million years (MA) ago.
These “Enon Conglomerates”, as they know, were deposited by high energy, fast flowing rivers, and found between Calitzdorp and Oudtshoorn, were they form the strikingly red “Redstone Hills.”
2.The second special geological feature that marks the Little Karoo, is the 300km long fault line along the Southern edge of the Swartberg Mountains.
The Swartberg Mountains were uplifted along this fault, to such an extent that in the Oudtshoorn region the rocks that form the base of the Cape Supergroup are exposed.
These are locally known as the “Cango Croup”, but are probably continuous with the “Malmesbury Group” that forms the base of the Table Mountain on the Cape Peninsula, and similar outcrops in the Western Cape. In the Little Karoo the outcrop is composed of limestone, into which an underground stream has carved the impressively extensive Cango Caves