Marine geology of the Cape Peninsula and False Bay

The Cape Peninsula is a peninsula that juts out into the Atlantic Ocean at the south-western extremity of the African continent. At its tip is the Cape of Good Hope. The peninsula forms the west side of False Bay. The two main rock formations are the late-Precambrian Malmesbury group (metamorphic rock) and the Peninsula granite, a huge batholith that was intruded into the Malmesbury Group about 630 million years ago.

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.

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.

The Peninsula Granite is a huge batholith that was intruded into the Malmesbury Group about 630 million years ago as molten rock and crystallised deep in the earth, but has since then been exposed by prolonged erosion. 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 (2–5 cm) white or pink feldspar crystals, glassy brown quartz and flakes of black mica, and containing inclusions of dark Malmesbury hornfels.

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