Cape Town Geology
An appreciation of the magnificent sea and mountain views of the "fairest cape" can be enriched by knowledge of its geological foundations. This page has been provided for schools and general public information and you need not be a geologist to recognise the layers of hard sandstone forming the steeper cliffs, or the crevices and forested ravines etched out by erosion along fractures and faults, or the rounded boulders of the crystalline granite basement exposed by wave erosion along the shoreline. Explanatory plaques have been erected at sites of geological interest and pamphlets are available from the Western Province Branch, Geological Society of South Africa (P.O. Box 572, Bellville, 7535; Tel. (021)-9484754). The Branch, is thanked for permission to use some of the pamphlet illustrations.
The late-Precambrian Malmesbury Group is the oldest rock formation in the area, consisting of alternating layers of dark grey fine-grained greywacke sandstone and shale, seen along the rocky Sea Point and Bloubergstrand shorelines. These sediments were originally deposited 560 Ma on an ancient continental slope by submarine slumping and turbidity currents. The sequence was subsequently metamorphosed by heat and pressure and folded tightly in a NW direction so that the rock layers are now almost vertical. Many tall buildings in the Cape Town CBD are founded on these rocks, which were in most places scoured by wave action during past periods of higher sea level.
The Peninsula Granite is a huge batholith that intruded into the Malmesbury Group about 540 million years ago as molten rock (magma) and crystallized deep in the earth, but has since then been exposed by prolonged uplift and erosion. The characteristic rounded shapes of granite boulders are a result of preferential weathering along intersecting joint fractures and are well displayed around Llandudno and Simons Town. Close up, the granite is a coarse-grained rock consisting of large (2-8 cm) white blocky feldspar crystals, glassy quartz and flakes of biotite, and inclusions (xenoliths) of dark, baked Malmesbury hornfels. In some places, intense weathering has altered the granite to kaolin clay that can cause slope stability problems in road cuttings. High quality kaolin is mined near Fish Hoek and Noordhoek.
The contact zone where the Malmesbury Group was intruded by molten granite can be seen at the Sea Point Contact and was made famous by Charles Darwin during his voyage of scientific discovery on H.M.S. Beagle in 1836. Here, beds of dark coloured Malmesbury rock, altered by intense heat are intermingled and folded with the light coloured intrusive granite.
Though initially intruded at great depth, prolonged uplift and erosion eventually exposed the granite at the surface, forming the basement rock.
The sedimentary rocks of the Table Mountain Group were deposited on the eroded surface of Malmesbury and granite basement rock. Deposition occurred in braided stream channels and tidal flats of a coastal plain and delta environment that extended across the region about 500-440 million years ago. The spectacular Chapman's Peak roadway was constructed along the contact unconformity between granite and the overlying Table Mountain Group sequence. The sand, silt and mud deposits of the Table Mountain Group were lithified by pressure and then folded in the Cape Fold Belt, extending 800 km along the southern coast to Port Elizabeth and 300 km to the north as the Cederberg Mountains.
The basal Graafwater Formation (80-100m thick) consists of interlayered pale brown sandstone, laminated pink siltstone and dark maroon coloured mudstone. It is best seen in road cuttings on the slopes of Table Mountain and along Chapmans Peak drive. Closer examination shows deposition cycles from current-bedded channel sandstones to increasing proportions of fine-grained maroon mudstone at the top, deposited in flood plains and lagoons.
The overlying Peninsula Formation (800-1500 m thick) consists of hard, light grey quartz arenite sandstone and dominates the steep mountain cliffs. Current bedding and pebble layers suggest that it was originally deposited as migrating sand bars in broad river channels.
The Pakhuis Formation is a lithified glacial outwash deposit and occurs on the highest points of Table Mountain, such as Maclears Beacon. It contains clusters of angular boulders and pebbles and was deposited at a time when the Gondwana continent, of which Africa is a part, was situated close to the South Pole.
Faults cut across and displace the rock layers. These more easily eroded zones are marked by ravines, for instance, cross-cutting faults separate multiple peaks of the Twelve Apostles. Some fault zones of crushed rock (breccia) are re-cemented by dark brown coloured iron and manganese oxide minerals and the Hout Bay museum displays samples of the rich manganese ore that was mined there last century.
The present landscape is due to prolonged erosion having carved out deep valleys, removing parts of the once continuous Table Mountain Group sandstone cover from the Cape Flats and leaving high residual mountain ridges.
Almost 50% of the Cape Peninsula and Cape Flats area is blanketed by a thin veneer of eolian and marine sands. Sea-levels fluctuated between -120 to +25 m from present mean sea level during the Pliocene and subsequent Pleistocene ice-age between 2.6 million and 18,000 years ago as a result of fluctuating global temperature and variable amounts of water accumulated in polar ice caps. At times the sea covered the Cape Flats and Noordhoek valley and the Cape Peninsula was then a group of islands. Beach sands with shell fragments and estuarine muds were deposited and later overlain by calcrete-cemented dune sands as the sea retreated. "Dune rock" that was deposited during a Pleistocene interglacial period about 120 000 years ago is now being eroded in the sea-cliffs near Swartklip where fossil remains of the extinct giant Cape Horse (Equus capensis) have been found.
For more information on the geology of Cape Town refer to John Compton’s book ‘The Rocks & Mountains of Cape Town’ (www.johnscompton.com)