At the southern (Ashton) end of Cogmans Kloof, yellowish-brown metamorphosed shales (phyllites) of the Malmesbury Group underlie the steeply tilted and folded younger sandstones of the Table Mountain Group that dominate the scenery of the Kloof. This is in contrast to the horizontal contact between these two groups in Cape Town (Table Mountain), this plane (called an angular unconformity) at Cogmans Kloof is vertical, because it was folded along with the other strata.
The rock sequence surrounding the site is dominated by sandstone of the Peninsula Formation and Nardouw Subgroup. Sandy sedimentation was briefly interrupted by a period of glaciation and mud deposition. These sediments were deposited in coastal settings 400 – 450 millions of years ago when Africa was part of the supercontinent Gondwana. The beds have been thoroughly lithified by processes associated with deep burial, compaction and cementation.
Originally the layers of sand, now sandstone, would have been almost flat-lying. If you look at the site, you will see that these layers are now for the most part steeply inclined. This is because from 280 to 220 million years ago the whole mass was subjected to tremendous compressional forces, which had been directed from the south and occurred in several pulses, as controlled by the movments of the earth’s crust during that time.
It is an interesting fact that if it were not for the Table Mountain Group with it thick quartz-rich sandstones, we would not have had any Cape Ranges, at least not on their present scale. The sandstones are often very pure and these quartz-rich rocks are extremely resistance to weathering, being chemically relatively inert. Most of the chemical attack is related to the lichens which typically coat the outcrops and generally impart a grey tone, but as you can see, also colourful tints on some of the high rock faces. Because of their inertness to weathering the Table Mountain Group tends to occupy the high ground in landscapes which nature is constantly trying to reduce to sea level. What erosion we occasionally observe, is usually of a mechanical nature, involving rock-falls, sometimes triggered by earthquakes. It is probable that this is the dominant mechanism operating to reduce the relief of the Cape Mountains.
The most striking feature of Cogmans Kloof is the way it cuts through the mountain range. Exactly which processes acted to form these features are unknown, but several possibilities have been put forward. The first of these is antecedent drainage. This implies that rivers existed before the uplift of the mountain range and that their erosional power was able to keep pace with the rise of the land, that is, it could cut down as fast, or faster than the rate of uplift. The second possibility is that the modern drainage is as the result of superpositioning of streams onto the folded mountain range from some earlier elevated landscape which is what one sees in the Grand Canyon in Arizona USA. This however seems less likely than the former. The third and most feasible possibility recognises that all the great east-west ranges of the Cape Fold Belt possess deep canyons cut by streams draining their flanks e.g. the north bank tributary of the Breede River eroding northwards towards the Montagu area. These do not penetrate the ranges, but pairs originating on opposite flanks may intersect. If at some time in the past this intersection occurred, and the land north of the range was higher than in the south, then it is possible that, by aggressive headward erosion of the southern canyon, the drainage from the north may have been tapped and a poort formed through the process of river capture.
Info from pamphlet issued by Western Cape branch of the Geological Society of SA.
Please note that there is box on the back of one of the signs - it is labelled Cape Mountain Passes Geocache - this is not an official Geocaching.com (Groundspeak) cache and is not part of this earthcache.
To claim this earthcache you need to mail me the answers to the following :
Please tick the box to send along your e-mail address with your answers
1. From the waypoint look along the road in the direction of Montagu at that vertical folded mountain slope on the right above the road - What is the predominant colour of the lichen on the rock face?
2. Sandstone is mentioned in the description – describe how this type of stone is formed.
3. From the Geological Society plaque - what is the site Code and Number and what was the name of one of the largest faults which passes near the southern end of the kloof. (seems the plaque has gone missing - see spoiler picture.)
4. Walk through the tunnel to S33 48.193 E20 05.658 look across the river to your right and estimate the angle of the rock layers to the horizontal.
5. OPTIONAL logging task: It would be great to have log a photograph that shows your GPS in front of the folding mountain, you can include people in the photograph, but that is also OPTIONAL and is NOT required.