The geology of the Western Cape is dominated by the Ordovician to Lower Devonian Cape Supergroup (CSG). The rocks involved are generally sandstones and shales, with the shales (Bokkeveld group) persisting in the valley floors and the erosion resistant sandstones forming the parallel ranges, the Cape Fold Belt. (CSG). This originally consisted of as much as 8 000 – 10 000 m of layered sandstones and shales, deposited in a rift valley on a subsiding continental margin, and lying uncomfortably on top of Precambrian basement sandstones, siltstones and thick mudstones.
These older rocks are mostly found in the vicinity of Cape Town and the far west of the Western Cape, but also outcrop at the foot of the Swartberg Mountains as the Cango Cave Group, and comprises limestones, turbidetes and conglomerates.
An extensive fault system runs through the Western Cape parallel to the Cape Fold Belt, with evidence of older faults being reactivated into thrusts due to compression. Along the southern edge of the Swartberg Mountains the Cango Fault, which extends for 320 km eastwards from Cape Town, forms the boundary between the Cretaceous sediments of the Uitenhage Group in the Oudtshoorn Basin and the uplifted older rocks of the Swartberg Mountains, with rocks of the Precambrian Cango Caves Group exposed at their base.
The CSG rocks underwent extensive erosion during this period and were partially buried by Karoo sediments, but re-emerged during the tectonic upheavals which resulted from the breakup of Gondwana about 150 Ma to form the Cape Fold Belt. This runs predominantly east-west for about 1,000 km across the southern tip of present-day Africa, with the westernmost arm trending north-south parallel to the west coast. Some Cretaceous sedimentation occurred in the Western Cape area, but much has been eroded except in the Oudtshoorn Basin south of the Swartberg Mountains, although it is believed to be widespread offshore.
The Cape Fold Belt is a fold and thrust belt of late Paleozoic age, which affected the sequence of sedimentary rock layers of the Cape Supergroup in the southwestern corner of South Africa. It was originally continuous with the Ventana Mountains near Bahía Blanca in Argentina and other fold and thrust belts in Antarctica and eastern Australia. The rocks involved are generally sandstones and shales, with the shales (Bokkeveld group) persisting in the valley floors and the erosion resistant sandstones forming the parallel ranges, the Cape Fold Mountains.
The Swartberg consist of two officially named ranges – the Klein Swartberg and the Groot Swartberg. the Gamka River cutting a gorge directly through the range, forming the natural dividing line between the two ranges.
The Swartberg range, reaching a maximum height of 2 325 m at Seweweekspoortpeak in the Klein Swartberg and part of the Cape Fold Belt, stretches from the town of Laingsburg, 200km east of Cape Town, eastwards for 230 km, with the Klein Karoo to the south and the vast expanse of the Great Karoo to the north. The range is traversed by three dramatic roads: the tarred Meiringspoort Pass to the east, the gravel Swartberg Pass and Seweweekspoort pass/gorge to the west. Most of the Swartberg range is a UNESCO World Heritage Site, stretching from Meiringspoort to the west of Towerkop with its split peak, towering over Ladismith at a height of 2 189 m. The peak is so named for its cleft peak that, according to legend, was split by a spell and subsequent bolt of lightning.
The contortions in the rock in all three passes display astonishing anticlines and synclines, and the vivid coloration of the surrounding Quartzite is remarkable. At the Northern end of the Swartberg pass seven hundred metre high quartzite cliffs of the upper Table Mountain Group can be seen, and these are often tilted through 90 degrees (sometimes even more).