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, the Pensacola Mountains (East Antarctica), the Ellsworth Mountains (West Antarctica) and the Hunter-Bowen orogeny in eastern Australia. The rocks involved are generally sandstones and shales, with the shales (Bokkeveld Group) persisting in the valley floors while the erosion resistant sandstones (belonging to the Peninsula Formation) form the parallel ranges, the Cape Fold Mountains, which reach a maximum height of 2325 m at Seweweekspoortpiek ('Seven Weeks Defile Peak' in Afrikaans).
The Cape Fold Mountains form a series of parallel ranges that run along the south-western and southern coastlines of South Africa for 850 km from the Cederberg 200 km to the north of the Cape Peninsula, and then along the south coast as far as Port Elizabeth, 650 km to the east (see the two maps one above the other on the right).
Geological origin
The rocks were laid down as sediments in a rift valley that developed in southern Gondwana, just south of Southern Africa, during the Cambrian-Ordovician Periods (starting about 510 million years ago, and ending about 330–350 million years ago). (See the yellow block labeled C on the Earth's geological timeline diagram on the right.) An 8-km-thick layer of sediment, known as the Cape Supergroup (see below), accumulated on the floor of this rift valley. Closure of the rift valley, starting 330 million years ago, resulted from the development of a subduction zone along the southern margin of Gondwana, and the consequent drift of the Falkland Plateau back towards Africa, during the Carboniferous and early Permian periods. After closure of the rift valley, and rucking of the Cape Supergroup into a series of parallel folds, running mainly east-west (with a short section running north-south in the west, due to collision with eastward moving Patagonia), the continued subduction of the paleo-Pacific Plate beneath the Falkland Plateau and the resulting collision of the latter with Southern Africa, raised a mountain range of immense proportions to the south of the former rift valley. The folded Cape Supergroup formed the northern foothills of this towering mountain range.
The weight of the Falkland-Cape Supergroup mountains caused the continental crust of Southern Africa to sag, forming a retroarc foreland system, into which the Karoo Supergroup was deposited. Eventually much of the Cape Supergroup became buried under these Karoo deposits, only to re-emerge as mountains when upliftment of the subcontinent, about 180 million years ago, and again 20 million years ago, started an episode of continuous erosion that was to remove many kilometers of surface deposits from Southern Africa. Although the tops of the original Cape Fold Mountains were eroded away, they eroded much slower than the considerably softer Karoo deposits to the north. Thus the Cape Fold Belt "erupted" from the eroding African landscape to form the parallel ranges of mountains that run for 800 km along the southern and south-western Cape coastline today. In fact, they form the coastline, either sloping steeply directly into the sea, or are separated from it by a relatively narrow coastal plain.
The Falkland Mountain range had probably eroded into relative insignificance by the mid-Jurassic Period, and started drifting to the south-west soon after Gondwana began to break up 150 million years ago, leaving the Cape Fold Belt to edge the southern portion of the newly formed African continent. Even though the mountains are very old by Andean and Alpine standards, they remain steep and rugged due to their quartzitic sandstone geology (see below) making them very resistant to weathering. The famous Table Mountain forms part of the Cape Fold Belt, being made up of the local lowest (oldest) strata of the Cape Supergroup, composed predominantly of quartzitic sandstone which forms the impressive, almost vertical cliffs which characterize the mountain and the rest of the range which constitutes the backbone of the Cape Peninsula.