The Edinburgh Geologist
Issue no 41

Geological Tables
Table Mountain

by Professor David Reid

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An appreciation of the magnificent sea and mountain views of the 'fairest cape' can be enriched by knowledge of its geological foundations. But 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.

Precambrian sedimention and igneous activity

The late-Precambrian age Malmesbury Group is the oldest rock formation in the area, consisting of alternating layers of dark grey fine-grained greywacke sandstone and slate, seen along the rocky Sea Point and Bloubergstrand shorelines. These sediments were originally deposited on an ancient continental slope by submarine slumping and turbidity currents. The sequence was subsequently metamorphosed and folded tightly in a NW direction so that the rock layers are now almost vertical. Many tall buildings in Cape Town are founded on these rocks, which were, in most places, scoured by wave action during past periods of higher sea level.

About 630 million years ago, a huge batholith was intruded into the Malmesbury Group and this is now known as the Peninsula Granite. This crystallized deep in the earth but has, since then, been exposed by prolonged erosion. Preferential weathering along intersecting fractures results in the characteristic spheroidal shapes of the granite boulders. These are particularly well displayed around Llandudno and Simonstown. Close up, the granite is a coarse-grained rock consisting of large (2-5 cm) white or pink feldspar crystals, glassy brown quartz and flakes of black mica containing xenoliths of dark Malmesbury hornfels. In some places, intense weathering has altered the granite to kaolin clay soils that 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 Sea Point and was made famous by Charles Darwin during his voyage of scientific discovery on H.M.S. Beagle in 1844. Here, slivers of dark coloured rocks of the Malmesbury Group, altered by intense heat, are intermingled and folded with the pale coloured intrusive granite to form a complex migmatite. Large feldspar crystals occur in both the granite and dark hornfels layers.

Though initially intruded at great depth, prolonged erosion eventually exposed the granite at surface and it now forms a basement upon which younger sedimentary rocks of the Table Mountain Group were deposited.

Ordovician sedimentation

Table Mountain Group sandstones were deposited on this eroded surface of granite basement, in the stream channels and tidal flats of a coastal plain and delta environment that extended across the region about 450 million years ago. The sand, silt and mud deposits were lithified by pressure and then folded in the Cape Fold Belt, extending along the southern coast. The spectacular Chapman's Peak roadway has been constructed along the contact unconformity between granite and the overlying Table Mountain sequence.

The basal Graafwater Formation (300-450 m thick) consists of interlayered pale brown sandstone, laminated pink siltstone and dark maroon coloured shale. It is best seen in road cuttings on the slopes of Table Mountain and along Chapman's Peak drive. Closer examination shows deposition cycles from current-bedded channel sandstones to increasing proportions of fine-grained maroon shales at the top, deposited in flood plains and lagoons.

The Peninsula Formation (800-1500 m thick) consisting of hard, light grey, coarse pebbly quartz sandstone, 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 tillite (a lithified glacial outwash gravel) 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.

Later modification of the landscape

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. In some fault zones the crush breccia is re-cemented by dark brown coloured iron and manganese oxide minerals. This provided a rich manganese ore that was mined in and around Hout Bay in the last century.

The present landscape is formed by 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 half of the Cape Peninsula and Cape Flats area is blanketed by weakly cemented marine sands. Sea-levels fluctuated between -120 to +200 m from present mean sea level during the Pliocene and subsequent Pleistocene ice-age 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.

Further information

This article is based on a web page on the University of Cape Town (UCT) web site, which was produced for schools and general public information and can be accessed on www.uct.ac.za/depts/geolsci/cape.html..
 
 

Read next article: The Atherton Tablelands

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Figures

Sandstone layers form ramparts overlying Precambrian shales and granite

Simplified geological cross-section through Table Mountain

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