Geological circumnavigation of the Cape Peninsula

posted: 2252 days ago, on Thursday, 2011 Sep 22 at 21:37
tags: events, Ed Foster.

by Edward Foster

13 September 2011

“When shall we lot meet again in a howling gale, midst biting sand, or in driving rain?

When we can tell a granites from dolerites, when we recognize xenoliths and aplites ”

My apologies to the Bard of Stratford for taking liberties with the First Witch in Scene 1, Act 1 of Macbeth.

On a wet and windy Thursday morning in the Waterfront parking lot, Garth and John checked and rechecked the list, consulted, counted and checked the list again and then counted heads once more. A few telephone calls later and very shortly after the appointed 08:00 Connie, our driver, piloted the coach out of the parking area heading for our first appointment with the Cape winter weather and the Geology of the Cape Peninsula. John switched to narrative mode and we were treated to the first round of entertaining Geological information mixed in with historical and personal miscellanea.

After the first stop the group, most of us more than just a little damp, boarded the coach for stop number two, the world renowned Sea Point Contact between the Malmesbury rock and the Cape Granite.

By now the wind had ratcheted up a notch or two and it was wetter, much wetter. But the brave band followed John down onto the rocks learning about Cape Granite, Malmesbury shale, Intrusions, Inclusions, Xenoliths, Anticlines and Synclines and quickly coming to grips with time dished up in hundreds of millions of year dollops. The look on Connies face, when his passengers in various stages of wet to very wet, traipsed on board very clearly said he was glad he was the driver and had to stay in the vehicle.

Figure 1: John on the rocks at the Sea Point Contact completely undeterred by his lack of headgear or absence of natural covering, Rumour has it he was humming something about raindrops falling on his head.

Figure 2: Wind and rain, lots of both coming in of a very grey and turbulent South Atlantic. Check Stephan’s!

Figure 3: Some of the stragglers in the parking area at the Sea Point Contact. The body language says wet, windswept and having second thoughts about the rest of the day.

Figure 4: The dark grey rock is Malmesbury rock which was deposited in the Adamastor Ocean while the light grey rock is Cape Granite that forced its way in much, much later.

Stop three was just a little further on and this time it involved boulder hopping on wet, slippery boulders, ploughing through wet sand and slogging up sandy slopes which insisted on taking you back to where you’d just been. In between doing all this, one had to listen to John, not get bowled over by the wind and keep an eye on the tide, which had by now turned and was on its way back in. It was also still raining, although most of us had reached the point where, short of falling into the water or being hit by a wave, we couldn’t really get much wetter.

After a dose of more Cape granite mixed in with pieces of Peninsula sandstone and some juvenile dolerite (a mere 130 million years old), we started the long trek back up to the road and the dry, windless haven of the of the coach interior.

Figure 5: The conclusion of the two-man consultation was, quite correctly, that these boulders were indeed granite, or more correctly, very wet Cape granite.

Figure 6: Against a backdrop of Cape granite, windswept, stormy sea, and rain shrouded expensive real estate, John waits patiently for his bedraggled troops.

Figure 7: The black rock is Dolerite, the lighter slightly yellowed rocks to its left are Peninsula Sandstone, while the rest is all Cape granite.

The next stop was Hout Bay with the exciting prospect of being able to purchase something hot to eat and drink! The trip along the coast was enlivened by John’s very entertaining running commentary on things Geological, as well as anecdotes from his extensive experience of taking student groups along this same route. Many of those students had eventually completed post-graduate work that contributed greatly to the scientific knowledge about the Peninsula’s Geology.

After the pie-stop we took to the road again heading for Chapman’s Peak. Once at the crest of the pass, we disembarked and made our way to the lookout point. The rain had stopped, but the wind speed had increased to the point where it was threatening to blow us off our feet or over the edge or possibly both. I got the impression this wind had aspirations of getting fairly high up on the Beaufort Scale before the day was done. I was also absolutely amazed that John was actually able to make himself heard above the gale.

Figure 8: Looking back across Hout Bay from the view point. The streaky sea and swarms of white caps tell a very windy tale.

Figure 9: The view toward Kommetjie from the view point, tells the same windy story.

Figure 10: Chapman’s Peak drive and in the distance the Graafwater formation can be seen resting on the Cape Granite base at sea level. Above the Graafwater formation, roughly were the catch-nets are, is the Peninsula formation.

From Chapman’s Peak we proceeded to Cape Point. After a pit stop and visit to the restaurant, John led us of down the path to Dias Beach. It was a long way down and once we were on the beach, we had to contend with waves of windblown sand that not only sandblasted anything that was exposed, but also found its way into places we didn’t even know we had places. John pointed out all sorts of interesting Geology: old wave cut platforms, cemented sand from ancient dunes; water ripples from ancient slow moving streams and cross bedding caused by equally ancient fast moving currents. He showed us evidence of a fault running across the beach that had resulted in the southern section dropping lower than the northern section.

Then it was time to slog our way over a dune and the tackle the long climb back up to the cliffside path. I just hope I wasn’t the only one that felt the years accumulate in my legs as I climbed!

Figure 11: Dias beach, spectacularly beautiful but, trust me, it is a long way down there and at least twice as far back up again.

Figure 12: Down we go! Note the figure on the beach to get an idea of the distance down to the sand.

Figure 13: Cemented dune sand, passed on the way down and on the way up, only much more slowly.

Figure 14: Wave cut platforms inspected and found to be correct as specified.

Figure 15: The rugged coast running from Dias beach toward Cape Point.

Once back on the cliffside path we headed for the coach, which Connie had moved from the upper parking area to the lower one. Initially we had the gale coming in from our right with a precipitous drop on our left and then during the last stages of the return journey we were walking head on into the wind. Some of us were actually knocked over and I know there were times on the last stretch when the wind literally stopped me in my tracks. It was quit an exhilarating experience although the possibility of being blown over the edge of the cliff was not really that appealing.

Figure 16: The group spread out along the path on the way back to the coach. Note the drop on the left.

Figure 17: View back toward the light houses.

A windblown but by now dry party thankfully boarded the coach for the second last lap. John has an extensive knowledge of the Geology of the Cape Peninsula, but he also has the gift of being able to convey hard scientific facts in a manner that makes them comprehensible to the lay person. Over and above his scientific knowledge he also has a seemingly unlimited supply of interesting anecdotes. Anyway, our next stop was the parking area just above Smitswinkel Bay. Here John pointed out the large Dolerite dike that had intruded into the granite but not made it all the way through to the formations above the granite.

Figure 18: From the left along the beach is all Cape Granite and just beyond the beach is the much younger Peninsula Sandstone so there is a fault running just beyond the beach.

Figure 19: Just to the left of the houses the very dark rock of the Dolerite intrusion can be seen.

The last lap down to Froggy Pond was a short one and once we had disembarked, we made our way across the sand and rocks to view John’s last set of Geological showpieces. A close-up look at a Dolerite dyke, some good examples of xenoliths and also an aplite. Although there was still a stiff breeze blowing everyone enjoyed the late afternoon sunshine, quite a contrast to our very wet start to the day.

Figure 20: The group, boulder hopping in hot pursuit of John who is headed for the dike he has been bragging about all afternoon.

Figure 21: Here it is, a real dyke up close and personal! The dark slice in the middle is Dolerite and the lighter material on either side is Cape Granite.

Figure 22: This prominent ridge on the Cape Granite boulder is an aplite. It was formed by granite intruding into pre-existing granite which had already cooled.

Figure 23: On our way back to the waterfront we saw whales in False Bay and, surprise, surprise, one of the elusive South African submarines. It wasn’t doing its submarine thing, just bobbing around on the surface outside Simonstown harbour.

We had travelled safely right around Adamastor’s petrified body, from his head down to his toes and back. At one stage I had thought about those early sailors in their barely adequate ships, rounding the Cape of Storms and wondered what it must really have been like on a pitching deck with the wind howling through the rigging, spray flying and foam fringed mountains of sea water breaking across the bows – scary I think, very scary!

nothing more to see. please move along.