There is a growing interest in “geotourism” in southern Africa. Hout Bay has some gems for the would-be geologist, not least of which is the famous nonconformity (photo a & c) between the light-coloured Cape Granite (intruded 540Ma) and the overlying maroon to brown thinly bedded mudstones and sandstones of the Graafwater Formation (518My) at the base of the Table Mountain Group. The nonconformity marks a period of 30 to 20 million between the granite and the overlying sediments. Excavation to expose the surface of the nonconformity provided a stable shelf on which to construct Chapman’s Peak Drive (photo b, d & g). Now a toll road, the drive has been further modified to protect users of the road from infrequent falling rocks (photo b).
It is possible that you may also identify dark-coloured dolerite dykes cutting through the Cape Granite and the sediments of the Table Mountain Group. The dykes were intruded about 130My ago when Gondwanaland was separating to form the African continent.
The mountain slopes in and around Hout Bay are subjected to weathering and erosion by a number of different processes. Rock falls occur periodically, and perhaps one day the Sentinel will be significantly altered when the seaward side finally slides and tumbles in to the sea. You will also notice a number of lighter-coloured stream-like streaks running down the mountain sides. These are debris flows which occur during exceptionally heavy down pours. The quartz-dominated Table Mountain Group provides beautiful white sands, such as those on Noordhoek Beach at the far end of the drive.
If you would like to know more I suggest you obtain a copy of John Rogers’ recently published book called “Geological Adventures in the Fairest Cape” (ISBN 978-1-920226-84-8) published by the Council for Geoscience. The book covers several main routes and is beautifully illustrated to help travellers to read the landscape and geology as they pass along them.