The Swartberg Mountains in the Western Cape province of South Africa are considered to be one of the finest exposed fold mountain chains in the world. Two dramatic passes afford the geologist unparalleled views of Paleozoic quarzitic sandstones folded spectacular fashion.
The Swartberg (Black Mountains in Afrikaans) range, part of the Cape Fold Belt, stretches from the town of Laingsburg, 200 km east of Cape Town, eastwards for 230 km, with the Klein Karoo to the south and the vast expanse of the Groot Karoo to the north. The easternmost end of the range rises over 2,000m and is traversed by two dramatic roads: the paved Meiringspoort Pass and unpaved Swartberg Pass. It is possible to drive a circular route encompassing both these passes – a round trip from the town of Oudtshoorn of about 170 km – in a day, allowing for frequent stops to gaze at the breathtaking views and the awe-inspiring rocks, and also the beautiful flowers, vegetation and bird life. Much of the Swartberg range is a UNESCO World Heritage site.
Folds and Thrusts
The geology of the Western Cape region is dominated by the Ordovician to Lower Devonian Cape Supergroup (CSG). This originally consisted of as much as 10,000m of layered sandstones and shales, deposited in a rift valley on a subsiding continental margin, and lying unconformably 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 Caves Group, and comprise limestones, turbidites and conglomerates.
About 330 million years ago, as the continents collided to form the super continent of Pangea, a subduction zone formed in the south, causing the CSG to undergo uplift and compression, which deformed and folded the previously horizontal layers, forming a mountain range up to 7 km high. The weight of the buildup of these mountains caused subsidence, and a large basin formed to the north. Between the Late Carboniferous and the Early Jurassic this filled with the sediments partially derived from the CSG rocks to form the Karoo Supergroup, which form the Karoo Plateau that now lies over much of the center of South Africa, stretching from the northern Cape area over 1,000 km to Johannesburg.
Geology in the Swartberg Pass region. (Source: Norman and Whitfield, 2006)
North-south geological cross-section through the southern Cape Fold Belt, from the southern Karoo to the complex of offshore basins formed during the breakup of Gondwana. In the center it crosses the arch of metamorphosed sediments that lie beneath the Cape Fold Mountains and the Cretaceous sediments around Oudtshoorn. Vertical scale exaggerated. (Source: Norman and Whitfield, 2006)
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.
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.
Traversing the Swartberg
The Swartberg Pass (the westernmost route through the mountains) is a national monument, honoring the amazing achievement of Thomas Bain, the engineer who (with help from convict labor and lots of gunpowder) created the 24 km-long road, which was opened in 1888. It travels up the southern side of the mountains, predominantly composed of massive quartzites of the lower Table Mountain Group (TMG), the oldest rocks of the CSG, which rise almost vertically over 1,000m from the valley floor. It then winds down through the crevasses and folds of younger TMG sediments, to Prince Albert and the Groot Karoo. The road is gravel, but unless there has been a lot of rain it does not require a four-wheel drive car.
Meiringspoort Pass is now traversed by the main N12 north-south road from Beaufort West to the coast, but the first, rather rough road through the pass opened in 1858. It is very different to the Swartberg Pass, as the route follows the path of the Groot River, which has eroded through the surrounding mountains to form a deep canyon. The soaring cliffs on either side show more fantastic exposures of folded and refolded TMG rocks. The route crosses the river 25 times, a fantastic feat of engineering – but this meant that the road was susceptible to flooding and eventually, to avoid this, the alternative western route was constructed.
References
Norman, N. and Whitfield, G.; Geological Journeys: A Traveller’ s Guide to South Africa’s Rocks and Landforms. Struik Nature Publishers and Council for Geoscience, 2006.
Compton, J. S.; The Rocks and Mountains of Cape Town. Double Storey Books, 2006
