The
underlying bedrock of the BCA is Malmesbury Group shale (the Tygerberg
Formation). The Malmesbury shale is the oldest rock in the area and volcanic
rocks within it have been dated at Bloubergstrand at 555±5 million years old.
The Malmesbury shale is well exposed on Blaauwberg Hill, which rises 231 m above sea
level. Along with the Tygerberg, which rise to over 400 m above sea level to
the east, Blaauwberg Hill forms a prominent hill on the coastal landscape. The reason
the Tygerberg forms such prominent hills on the landscape is attributed to the
intrusion of Cape granite, which has hardened the shale rock and made it more
resistant to erosion. Blaauwberg Hill does not appear to have any granite intrusions
and its resistance to erosion may reflect a greater presence of sandstone
rather than shale beds.
In the
low-lying areas surrounding Blaauwberg Hill, the Malmesbury shale is covered with tens
of metres of dune sand. The sand dunes are clearly visible along the coast
looking west from the top of Blaauwberg Hill. These coastal sand dunes are recent and
formed in the last tens of thousands of years. In places, the leached shell
material has formed sub-surface limestone beds (calcrete). Older sand deposits
occur inland on the landward (eastern) side of Blaauwberg Hill, and in places these
sands have been intensely leached and form quartz-cemented silcrete and
iron-oxide-cemented ferricrete outcrops.
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