Geology of Cape Town


lies at the south-western corner of the continent of Africa. It is bounded to the south and west by the Atlantic Ocean, and to the north and east by various other municipalities in the Western Cape province of South Africa.
The Cape Peninsula is a rocky and mountainous peninsula that juts out into the Atlantic Ocean at the south-western extremity of the continent. At its tip is Cape Point and the Cape of Good Hope. The peninsula forms the west side of False Bay and the Cape Flats. On the east side are the Helderberg and Hottentots Holland mountains. The three main rock formations are the late-Precambrian Malmesbury group, the Cape Granite suite, comprising the huge Peninsula, Kuilsriver-Helderberg, and Stellenbosch batholiths, that were intruded into the Malmesbury Group about 630 million years ago, and the Table Mountain group sandstones that were deposited on the eroded surface of the granite and Malmesbury series basement about 450 million years ago. The sand, silt and mud deposits were lithified by pressure and then folded during the Cape Orogeny to form the Cape Fold Belt, which extends along the western and southern coasts. 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 False Bay, and leaving high residual mountain ridges.
At times the sea covered the Cape Flats and Noordhoek valley and the Cape Peninsula was then a group of islands. During glacial periods the sea level dropped to expose the bottom of False Bay to weathering and erosion, with the last major regression leaving the entire bottom of False Bay exposed. During this period an extensive system of dunes was formed on the sandy floor of False Bay. At this time the drainage outlets lay between Rocky Bank Cape Point to the west, and between Rocky Bank and Hangklip Ridge to the east.

The geological structure of the region

Malmesbury group deposition

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, and from the Strand to Gordon's Bay. These sediments were originally deposited 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 during the Saldanian orogeny so that the rock layers are now almost vertical. These rocks were, in most places, scoured by wave action during past periods of higher sea level.

Appearance of the exposed Malmesbury group rocks

Most of the exposed shoreline Malmesbury rocks are steeply dipped, and weathered to form sharp edged ridges where more resistant layers stand out among the softer strata. The rocks are generally dark in colour where fresh rock has been exposed by erosion, and may be finely laminated.

Coastal areas where Malmesbury group rocks are exposed

  • Robben Island
  • Green Point to Sea Point
  • Gordon's Bay
  • Blousteen
  • Grotto Bay

    Cape granite intrusion

The Cape Granite suite is three huge batholiths that were intruded into the Malmesbury Group about 630 million years ago as molten rock and crystallised deep in the earth, but have since then been exposed by prolonged erosion. The Peninsula batholith underlies the Cape Peninsula and the west side of False Bay, The Kuilsrivier-Helderberg batholith is to the northeast and the Stellenbosch batholith a bit further to the north. The characteristic spheroidal shapes of granite boulders are a result of preferential weathering along intersecting fractures and are well displayed around Llandudno and Simonstown. Close up, the granite is a coarse-grained rock consisting of large white or pink feldspar crystals, glassy brown quartz and flakes of black mica, and containing inclusions of dark Malmesbury hornfels near the contact zone.
The climate of this region was warmer and wetter in the Cretaceous. This led to severe chemical weathering of the granite to saprolite rich in kaolin clays, decomposed from the large visible crystals of potassium feldspar that are so conspicuous in the granite.
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 Malmesbury rocks, altered by intense heat are intermingled and folded with the pale coloured intrusive granite to form a complex mixed rock. 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 the surface and it and what remains of the similarly eroded Malmesbury group now form a basement upon which younger sedimentary rocks of the Table Mountain Group were deposited.
Other large granite plutons of similar age are found in the Western Cape, but none of the others extend to the coastline in this area. The Stellenbosch pluton extends under the Helderberg and Hottentots Holland mountains. Cape Blue Rock, a dense Hornfels, was formed from the Malmesbury series rock by the Stellenbosch pluton, and was quarried as a building aggregate near Sir Lowry's Pass. The Blue Rock Quarry has been closed and is now flooded and used for water sports including diving and water skiing.

Appearance of the exposed Peninsula Granites

Almost all the exposed granite has been extensively weathered and is in the form of rounded corestones. The colour is generally pale to medium grey, and the surface is typically fairly rough, with clearly visible crystals, and no layered structure. As an intrusive rock, dip and strike do not apply, but the massive rock is cracked on jointing planes, which tend to be characteristic of the location, and weathering has accentuated these joints. The general direction and spacing of joints in some areas is fairly consistent over quite large areas, and the underwater landscape is often a continuation of the general trends above the surface, which can be useful for underwater navigational purposes.

Coastal and offshore areas where Peninsula Granite rocks are exposed

  • Sea Point to Chapman's Peak
  • Simon's Town harbour to Partridge Point
  • Roman Rock and adjacent reefs
  • Cape Point
  • Bellows Rock
  • Seal Island
  • Whittle Rock

    Table Mountain Group deposition

sandstones were deposited on the eroded surface of granite and Malmesbury series 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 during the Cape Orogeny to form the Cape Fold Belt, which extends along the western and southern coasts.
The basal Graafwater Formation consists of interlayered pale brown sandstone, laminated pink siltstone and dark maroon coloured shale. 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 consisting of hard, light grey medium to coarse grained 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 occurs on the highest point of Table Mountain, at Maclear's Beacon and on parts of the Hottentots-Holland range. It contains clusters of angular boulders and pebbles and was deposited at a time when the Gondwana continent, of which Africa was a part, was situated close to the south pole.
Graafwater, Peninsula and Pakhuis formations are from the Ordovician period.
Cederberg, Goudini and Skurweberg Formations from the Silurian period, and Rietvlei Formation from the Devonian period complete the Table Mountain group, and are found in the Hottentots Holland mountains to the East of False Bay. These strata are all well above the present sea level in the Cape Town area.
The Cederberg Formation includes dark grey siltstone and silty sandstones, and is darker than the overlying Goudini Formation which is light brown to light grey quartzitic sandstone, with interbedded reddish brown siltstone and shale. The Goudini formation is more feldspathic and finer grained than the Skurweberg Formation, which is characterised by fairly thick bedded medium to coarse grained, light grey, slightly feldspathic quartzose sandstone.
The Rietvlei Formation consists of alternating horizons of light grey quartzose and feldspathic sandstone, siltstone and shale.

Cape orogeny

The formation of the Cape Fold Belt is the result of a collision of tectonic plates that ended over 200 million years ago The accumulated strata of the Cape Supergroup and the older granites and Malmesbury group were raised and deformed by the pressure of the South American, Antarctic and African continental plates slowly moving together. The resulting fold mountains have been eroded to their present state over the ensuing period, and what exists today are the remnants of a much larger and higher mountain chain.
Faults cut across and displace the rock layers. These more easily eroded zones are marked by ravines; cross-cutting faults separate multiple peaks of the Twelve Apostles. Some fault zones of crushed rock are re-cemented by dark brown coloured iron and manganese oxide minerals.
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 in an example of inverted relief.