The oldest rocks in the Cape Town area, South Africa, are part of the Malmesbury Group,1 named after the town of Malmesbury, 60 km north-west of Cape Town. The Malmesbury Group covers a large area around Cape Town: 200 km to the north and 100 km to the east. Beyond this they are covered by the younger sediments of the Cape Supergroup.2 Unaltered exposures are rare because they weather quickly and are frequently covered by sand and soil.

The Malmesbury group consists of a collection of diverse rock formations that are separated from each other across the area by major faults. Further, the formations have been folded and eroded, all of which prevents the relationships between the different formations being worked out, or their thicknesses. A tentative estimate for the thickness of the group is more than 6 km.3

The rocks that are exposed around Cape Town are part of the Tygerberg Formation, named after the Tygerberg Hills north of Bellville. The formation consists of alternating beds of greywacke (a muddy sandstone), siltstone, mudstone, and quartzite.4 There are many features of these rocks that point to the beds being rapidly deposited from flowing water….

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