It is surely one of the most iconic views in the world, and yet never fails to impress the visitor: Table Mountain rising steeply and dramatically from the very centre of the city of Cape Town. To the south, the land narrows rapidly towards the famous Cape of Good Hope, constantly battered by wild seas and fierce winds. The whole area is one of fascinating diversity in flora and fauna as well as scenery, and at the root of this is, of course, the geology.
With the exception of some Quaternary and Holocene deposits, the rocks of the Cape are all very old. They comprise three main groups; the Cambrian-Carboniferous Table Mountain Supergroup, predominantly sandstones; the Cambrian Cape Granite; and the Precambrian Malmesbury Group, composed of dark grey mudstones and lighter sandstones. Although originating in ancient times, the way these rocks and the landscape appear today is the result of millions of years of constant erosion, deposition, earth movements and transformations.
Much of Cape Town and the coastal plain is underlain by the 560 million year old Malmesbury Group of rocks, which are ancient slump and turbidity current deposits. They still exhibit classic features such as graded bedding and ripples, despite their great age and the subsequent metamorphism and tight folding they have undergone. They form the base of Table Mountain and rarely outcrop at the surface, but can be seen along the rocky Sea Point shoreline on the western coast of Cape Town.
The Malmesbury Group also contains volcanics, seen as reddish-brown rocks among the usually dark slates and sandstones, indicating that the sediments probably accumulated on a tectonically active margin, with volcanoes developing as oceanic crust was pushed down into the earth under lighter continental material.
At one time Nelson Mandela and fellow inmates of the prison on Robben Island, 10km north of Cape Town and easily seen from Table Mountain, had a chance to view these rocks at close quarters. The core of the island is a Malmesbury Group anticline, smoothed to a platform by Pleistocene wave action, and overlain by wind blown sand. Van Riebeeck's Quarry, on the southern part of the island, was positioned at the crest of the fold so that the subhorizontal hard sandstone layers could be easily worked into stone slabs, which were used for many of the early buildings in the city.
During the early Cambrian, Malmesbury Group sediments were disturbed by the intrusion of magma, the result of the continental collision which eventually formed the super continent of Gondwanaland. The magma cooled and crystallised slowly, in the process melting and deforming the surrounding sediments. Although this occurred at great depth, subsequent erosion exposed the granite at the surface, forming a base for the deposition of the rocks which produced the dramatic cliffs of Table Mountain and Cape Point.
The effect of hot magma intruding into Malmesbury Group shales can be clearly seen near Sea Point, south-west of Cape Town. Over a mere 150m of coastline, the transition from undisturbed Malmesbury Group, through a complex mixed rock known as migmatite, to homogenous granite, is beautifully exposed on a wave cut platform. In 1836, Charles Darwin visited this location, and it had a major impact on his thoughts on the origin of rocks, as the initially fluid nature of the granite is hard to deny.
Nine kilometres of sand
The intrusion of the Cape Granite marked the beginning of the uplift and erosion of the Cape Town area, as the deeply buried Precambrian rocks and granite were forced to the surface. The hiatus between these and the thick layers of mudstones and sandstones which make up the overlying Table Mountain Group lasted over 30 million years, and the resultant disconformity is very clear, particularly along the spectacular Chapman’s Peak Drive, south of Cape Town.
The Table Mountain Group, part of the Cape Supergroup, was deposited between 510 and 340 million years ago, and the oldest rocks, the Graafwater and Peninsula Formations, form the 600m high sandstone cliffs of Table Mountain. The Graafwater, up to 60m thick, is composed of fining–up cycles of interbedded iron-rich maroon mudstones and lighter sandstones, passing conformably into the more massive, relatively iron-free sandstones of the Peninsular Formation, which are 700m thick in this area.
These are the only Cape Supergroup rocks found in the Cape Peninsula, except at the highest point of Table Mountain, where there is a very small remnant of the Late Ordovician Pakhuis glacial tillite, left by the tail end of a melting glacier. At one time, however, the Group probably covered a large percentage of Southern Africa, with up to 9,000m of sandstones deposited on a passive continental margin in a variety of terrestrial and shallow marine depositional environments. Tectonism in the Permo-Triassic caused them to buckle and fold, forming the Cape Fold Belt, which extends 600 km from the eastern side of False Bay to beyond Port Elizabeth, but at one time was much more extensive.
Folding exposed softer layers to the surface, so extensive erosion removed much of the once continuous Table Mountain Group layer. The nearly horizontal sandstones of Table Mountain are the sole remnant of the Fold Belt in the Cape Peninsula, probably representing the trough of a deep fold, where the horizontally lying beds were less easily eroded.
Prolonged periods of uplift and erosion mean that there is a huge gap in the geological record of much of the Cape area, from the end of deposition of the Table Mountain Group in the early Carboniferous right up to the present day. Igneous activity about 130 million years ago (Early Cretaceous) resulted in a number of black dolerite dykes which cut through the older rocks, but there was little further sedimentation on land until fluctuating sea levels during the Pliocene and Pleistocene ice-ages resulted in the deposition of weakly cemented marine sands over parts of the Peninsular and the Cape Flats area. Evidence of these sea level changes can be seen as wave cut terraces along the coast around Cape Town.
Much of the Cape Town landscape has been altered to accommodate the demands of man, and the unique biota of the region has also suffered major changes. But stand in the incessant wind at Cape Point, looking over the gloriously rugged landscape towards Table Mountain, and you realise that over much of the Cape Peninsula, geology and nature still have the upper hand.
Acknowledgement: Thanks to Prof John Compton of the University of Cape Town for assistance with this article.