Robben Island Geology

THE GEOLOGY OF ROBBEN ISLAND
The ferry tour to Robben Island focusses mainly on the old prison compounds, formerly occupied by anti-apartheid political prisoners. The island tour guides are prepared to allow special groups to inspect the excellent geological exposures both around the coast and in the several quarries that were worked in the past.

Here is a photo gallery of some of the geological exposures that are accessible from the road that circumnavigates the island, and which is followed by the bus tour.


The stunning view of Table Mountain towering above the Cape Town waterfront, from which the ferry crosses the bay to Robben Island	The low flat profile of Robben Island is the product of wave action during a higher sea level stand	Established structures of the original prison wardens village; Faure jetty on the left, hospital centre and officer housing to the right. Light house built on the highest point (Minto Hill, 30m asl)

Lime Quarry (now usually referred to as Mandela's Quarry), developed in fossilised sand dunes of Pleistocene age. Calcareous soil (calcrete or caliche) has formed above the shell-bearing dune sand that was deposited above the basement metasediments of the Neoproterozoic Malmesbury Group	Entrance to a cave cut by prison labour into the limestone of Mandela's Quarry (Photo: J. Rogers)	Differential weathering of the friable calcrete since the lime quarry became inactive in the late 1980s has led to the ledges and overhangs

Vertical solution pipes cut across the calcrete layers, Mandela's Lime Quarry	Sketch map of Robben Island showing the prominent buildings, facilities and access roads (from Robben Island by Charlene Smith)	Schematic cross section through Roben Island showing its rock basement (Malmesbury Group metasediments) and the thin veneer of Quaternary dunes (from a poster by De Beers Marine)

The distinctive profile of Table Mountain and the Cape Peninsula is built from hard resistant sandstones of the Ordovician Table Mountain Group, resting unconformably on folded metasediments of the Neoproterozoic Malmesbury Group, seen along the Robben Island shore in the foreground	Steep storm beaches composed of large cobbles of Malmesbury greywacke characterise most of the shoreline of Robben Island, being exposed to the south-westerly swells of the Atlantic Ocean	Van Riebeeck's Quarry exploited the hard greywacke sandstone of the Malmesbury Group, which provided excellent building material for early colonial buildings, such as the Cape Town Castle

Van Riebeeck's Quarry was strategically positioned at the crest of a broad anticlinal fold within the Malmesbury Group strata, so that the subhorizontal layers could be easily worked into building slabs	Close up view of the Malmesbury strata in Van Riebeeck's Quarry reveals distinctive graded bedding, with the coarse-grained greywacke sandstone forming the darker base, grading up into paler coloured siltstone	Section through graded beds of Malmesbury greywacke, with the lighter-coloured silty top of one cycle developing current induced ripples (Photo: J. Rogers)

Exposed pavement in the floor of Van Riebeeck's Quarry showing the development of ripple patterns in what is the top of a graded bed	Part of the vertical orthophoto of Robben Island that shows Rangatira Bay (named after the ship that wrecked on these shores in 1916), with the quarry and two prominent fold closures exposed in the wave-cut platform (axis marked with black arrows)	Close-up of the fold closure exposed at the SW end of Rangatira Bay - anticline plunging NW

Long Bay, developed by differential erosion of the softer dolerite in a dyke that intruded the Malmesbury Group during the Cretaceous break-up of Gonwanaland and the opening of the South Atlantic Ocean	At low tide the head of Long Bay exposes dark rounded outcrops of dolerite, which are easily distinguishable from the mid-grey boulders of Malmesbury greywacke (Photo: J. Rogers)	Vertical orthophoto view of Long Bay, with black arrow marking the NW-SE trend of the dolerite dyke

Quarry located at Rangatira Bay, excavated in shallow-dipping greywackes of the Malmesbury Group, along the NW shore of Robben Island. Sea water seeping through the seawall has mixed with bird droppings and sea weed, evaporated and developed variable colours through algal growth in the resulting highly saline brine	Opposite limbs of the NW-plunging anticline exposed in Rangatira Bay (Photo: J. Rogers)	The exposed western shore of Robben Island faces the strong Atlantic storm swells, which often brings with it the wrecks of disabled vessels

The juxtaposition of a wrecked fishing vessel with her sister ships off shore emphasises the narrow margin for error that faces those that work these wild coastal waters	Fierce Atlantic storms are quite capable of ripping even large Abalone shellfish from their rocky perches and strewing their remains along the stony western shore of Robben Island	Wave-cut platform developed over dipping Malmesbury strata, western shore of Robben Island

Geological map of the island, which is about 2km across, showing the basement metasediments of the Neoproterozoic Malmesbury Group (brown) exposed around the shore, while the flat interior of the island is covered by Pleistocene calcretised dune sands (coastal limestone - yellow)	Line-drawing geological map of Robben Island	Oblique aerial view of Robben Island looking approximately south across Table Bay towards the Cape Peninsula (Photo: Postcard by Alain Proust)