The cutting of Chapman’s Peak Drive between 1915 and 1922 gives us a picture into how large parts of the Cape Peninsula were formed. The road was a dream of the Administrator of the Cape Sir Fredrick de Waal (after whom De Waal Drive is named) who felt a road here would attract visitors to the Cape. It was a sharp eyed mining surveyor called Charl Marais who spotted that the particular geology of the area made this road possible - he was a perfectionist and was seen working in extreme conditions including being roped to the cliffs at times and hiring help to cut foot-holds and platforms for his surveying equipment. He noticed that a large part of the mountain side was in fact the lower portion of the Table Mountain Group namely the Graafwater Formation which consists of relatively soft intermingled layers of brown sandstone pink siltstone and maroon coloured shale (laid down some 450 million years ago) overlaying the granite which forms the base layer of this part of the peninsula – the granite which forms an undulating platform dates back over 600 million years. Marais concluded that the road could be carved out of the cliff face at this geological interface with the cutting being done in the softer Graafwater Formation. The construction was started in 1915 from the easier Hout Bay end and fourteen months later from the more difficult Noordhoek end.
Then the more technical stuff which comes from a brochure published by the Geological Society of South Africa. An occasional feature of these rocks is the presence of steep faults that displace the beds. The contact between the granite and the overlying beds is irregular, with the sediments occasionally occupying depressions (as much as 3 m deep) on the granite surface. Good examples of this feature can be seen along the road towards the bottom of Chapman's Peak Drive just beyond Chapman's Point. Chapman’s Point is the headland below Chapman’s Peak as the road turns towards the long beach on the Noordhoek side. The sedimentary rocks above the granite platform were deposited on a vast coastal plain located between a highland area to the north and a sea to the south. The character of the coastal plain changed through geological time as the sediments were deposited. 1. Rivers draining the highlands flowed southwards towards a sea located to the southwest beyond the present-day Cape coastline. On reaching the coastal plain, where the slopes were low, current velocity slowed and the main river channels split up into smaller, shallower streams that flowed across the plain to the sea. These streams deposited the alternating layers of sand, silt and mud of the Graafwater Formation. Because of the warm, dry climate prevailing at this time, the subaerial surface frequently dried out and mudcracks developed that later became filled with sand. In time the muds were compacted, gradually dewatered, and the sand filling the cracks was squeezed and contorted. Excellent examples of these sand-filled cracks can be seen in the road cutting about halfway down Chapman's Peak Drive, between the northern lookout point and Chapman's Point. The contrast in colour between the sedimentary rocks in the road cutting is due to the formation of fine particles of red iron oxide shortly after deposition. The particles were concentrated in the finer-grained siltstones and mudstones. 2. Following the initial stage of development, the coastal plain was subjected to periodic flooding when large quantities of sand were deposited. The sandstones display a wide range of sedimentary structures including cross-bedding, scour and fill, flat bedding with parting lineations, ripples and animal burrows (trace fossils). Sandstone bedding surfaces are covered with a variety of ripple marks. These include interference ripples, flat-topped ripples, ladderback ripples and double-crested ripples. The presence of these ripple types, together with marks made by marine organisms, suggest that there was a periodic marine influence on deposition at this time. As the floods waned, mud was deposited on top of the sands in the form of thin layers and lenses. Some of this mud was eroded and reworked by the next flood event into small mudflakes which were then incorporated into the sand. Between flood events the exposed surface of the coastal plain dried out in places and cracked. However, the small size of the cracks, compared with those in the lower part of the succession, indicates that the surface was exposed for only a short time prior to the next flood. The sand-stones deposited at this time can be traced for some distance from the northern lookout point towards Chapman's Point. Close inspection of the bedding surfaces in the vicinity of Chapman's Peak lookout point reveals a host of different - sedimentary structures which testify to the fluctuating conditions of flow responsible for them. 3. The conditions prevailing at the beginning of coastal plain development re-established themselves as the coastline began to build seawards towards the southwest. On the landward side, the coastal plain passed into a much more extensive alluvial plain drained by wide, shallow river channels that deposited mainly sand and some gravel. These deposits were shaped into migrating ripples of various sizes which on burial and preservation produced structures called crossbedding in the resulting sandstone. These conditions were influenced by uplift in the source areas, which caused the alluvial plain to extend seawards and bury the coastal plain beneath sand and gravel. This seaward extension of the alluvial plain is seen in the abrupt change from thin-bedded coastal plain sandstones, siltstones and mudstones below, to the more thickly bedded, alluvial plain sandstones of the Peninsula Formation in the cliffs above. Between some of the major cliff-forming sandstone beds above the road are very thin, less resistant beds of sandstone and siltstone. These occasionally contain burrows and traces made by marine organisms, as well as mud cracks which formed when the upper parts of the deposits dried out on exposure to the atmosphere. Some of the traces are thought to have been made by primitive, extinct marine crustaceans called trilobites as they crawled across the soft, wet bottom in search for food. The finer-grained, softer beds of sandstone and siltstone were laid down under very shallow marine conditions when the sea periodically flooded the alluvial plain. Up to eight thin marine intervals can be recognised in the alluvial plain sediments above Chapman's Peak Drive.
References Rust, LC. (1967). On the sedimentation of the Table Mountain Group in the Western Cape Province. Unpublished Ph.D. thesis, University of Stellenbosch, IZO pp. Theron, JN. (1984). The Geology of Cape Town and environs. Explanation sheets 3318 CD and DC, and 3418 AB, AD and BA(I:50 000). Geological Survey of South Africa, 77 pp.
Requirements required to log as a find: Please tick the box to send along your e-mail address with your answers
1. Post a picture from the main viewsite looking south of yourself and your GPS with the rest of the drive along with your log - Note - this has become an optional logging requirement, but would still be nice to see!
2. E-mail me with the following information - Climb the little hill to your left and overlooking the road. Examine the rockface across the road to the right of the fault line
- a) How many large bands of sedimentary rocks can you see below the catch fence?
- b) The thicker bands are interspersed with narrower bands, why do you think this happened?