June 14, 2011

Chapman’s Peak Nonconformity

Posted by Evelyn Mervine

Chapman’s Peak Nonconformity, South Africa, May 2011.

On my recent trip to South Africa (see also my recent posts Sevilla Rock Art Trail, The Maltese Cross, The “Peace in Africa” in Port, and Cape Peninsula in Pictures), I spent my very last day touring around Cape Town with my fiance, my future sister-in-law, and her boyfriend. One of the things that we did just before I headed to the airport was drive along the Chapman’s Peak Drive, a road just outside Cape Town that stretches from Noordhoek (which means “North Corner” in Afrikaans) to Houtbaai (which means “Wood Bay” in Afrikaans; the Afrikaans-English hybrid “Hout Bay” is commonly used). The drive follows the rocky coastline and provides breathtaking views. First constructed during World War I (and considered an incredible feat of engineering for the time), the drive was closed for a time in the 1990s due to safety concerns. The winding road is built on quite steep terrain and is frequently covered by rockfalls. However, the drive was re-opened in the 2000s (with reinforced safety features) as a scenic toll road catering to foreign tourists. However, there are many residents of Hout Bay and the surrounds who travel along Chapman’s Peak everyday. Occasionally, the drive is closed because of weather or rockfall.

Google map showing location of Chapman’s Peak Drive. Click to enlarge.
Google map showing location of Chapman’s Peak Drive (zoomed out).
Note the location of Cape Peninsula just below the drive. Click to enlarge.

I highly recommend that every visitor to Cape Town go on the Chapman’s Peak Drive, which has gorgeous views of the coastline and ocean. If you are a geologist visiting Cape Town, you absolutely have to go on the Chapman’s Peak Drive. The reason is simple: the road follows a spectacular nonconformity between ~550 million year old Cape Peninsula granite and ~450 million year old sandstone of the Table Mountain Sandstone Group. The basal (bottom-most) layer of this sandstone group is called the Graafwater*. The Chapman’s Peak road was built by carving out the cliffside into the softer sandstones; the road itself rests on top of the harder granite.

Same picture as above, but with the nonconformity marked,
South Africa, May 2011. Click to enlarge.

For those of you who are not geologists, a nonconformity is a place where there were igneous or metamorphic rocks that were eroded are in contact with overlying sedimentary rocks. Nonconformities form when, after a period of erosion, sedimentary rocks are deposited on top of igneous or metamorphic rocks. There is missing time (the time of erosion) in the rock record. Thus, there is a nonconformity of geologic time. At the Chapman’s Peak nonconformity, about 100 million years of time is missing in the geologic record. The word nonconformity is closely related to the word unconformity, which is a similar contact between two sedimentary rocks (or rock groups).

Here are some figures (taken from University of Cape Town Geology Department websites here and here) that give you a sense of the geology of the Cape Town area, including along the Chapman’s Peak Drive:

Postcard view of Cape Town. Taken from here. Click to enlarge.
Similar view to above postcard, but with rock types colored in. Figure taken from here.
By the way, one of my favorite things to do in Cape Town is take a moonlight hike up
Lion’s Head, the little knob of sandstone sticking out of the Cape Granite.
Note the location of Hout Bay. Click to enlarge.
Rock types of Table Mountain, Cape Town. The red link marks the
unconformity/nonconformity. Figure taken from here.
Click to enlarge.

This University of Cape Town website has a good description of the geologic history of the Cape Town area. The oldest rock around Cape Town is the Malmesbury Group (alternating graywacke sandstone and slate), which was intruded by the Cape Granite (and some later dykes). Then there was erosion, and then the impressive Table Mountain Sandstone Group was deposited. I look forward to learning more about Cape Town geology when I move to Cape Town in August, but the above figures should give you a rough idea of the geology in which the Chapman’s Peak nonconformity is found.

Below are some more pictures of Chapman’s Peak Drive. These pictures are from my recent trip and also from my first visit to the Chapman’s Peak Drive in January 2008.

Sign for the Chapman’s Peak Drive toll road, South Africa, January 2008.
A trio looking over Hout Bay, South Africa, May 2011.
Another Hout Bay view, South Africa, May 2011.
Viewing platform at Hout Bay, South Africa, May 2011.
Cliffs along Chapman’s Peak Drive, South Africa, May 2011.
Another viewing platform at Hout Bay, South Africa, May 2011.
Jackie contemplates some geology (and scenery), Hout Bay, South Africa, May 2011.

Hout Bay Sentinal, South Africa, January 2007.
Beach view, Chapman’s Peak Drive, South Africa, January 2008.
Looking across Hout Bay, South Africa, January 2008.
Another spectacular Chapman’s Peak Drive view, South Africa, January 2008.
Can you spot the nonconformity?
Chapman’s Peak Drive, South Africa, January 2008.
Roadcut through the Graafwater formation.
Chapman’s Peak Drive, South Africa, May 2011.
A perilous place to investigate the Graafwater– but we did anyway!
Chapman’s Peak Drive, South Africa, May 2011.
Jackie takes a closer look at the Graafwater. Do you see the cross-bedding?
Chapman’s Peak Drive, South Africa, May 2011.
Found something interesting…
Chapman’s Peak Drive, South Africa, May 2011.
What is it?
Chapman’s Peak Drive, South Africa, May 2011.
It’s a trace fossil! A filled-in burrow.
Chapman’s Peak Drive, South Africa, May 2011.
More trace fossils in Graafwater.
Chapman’s Peak Drive, South Africa, May 2011.
These fossilized burrows were filled with either sand or organic material.
Chapman’s Peak Drive, South Africa, May 2011.
More trace fossil burrows!
Chapman’s Peak Drive, South Africa, May 2011.
Another view of the road along the nonconformity.
Chapman’s Peak Drive, South Africa, May 2011.
Yet another view of the road and nonconformity.
Chapman’s Peak Drive, South Africa, May 2011.
Obligatory cute (and slightly sunburnt) geologist couple shot in
front of the nonconformity. Chapman’s Peak Drive,
South Africa, May 2011.

*I asked my fiance what “Graafwater” means. He says: “directly translated [from Afrikaans] it’s rubbish, ‘spade-water.’ “