Cogmans Kloof Geology

At the southern end of Cogman’s Kloof (Ashton), yellowish-brown phyllites of the Malmesbury Group (‘metamorphosed shales’) underlie the steeply tilted and folded younger sandstone of the Table Mountain Group that dominate the scenery of the kloof. In contrast to the horizontal contact between the Malmesbury and Table Mountain Groups at Cape Town, this plane (called an angular unconformity) at Cogman’s Kloof is vertical, because it was folded along with the other strata.

The rock sequence surrounding the site is dominated by sandstones of the Peninsula Formation and the Nardouw Sub-Group. Sandy sedimentation was briefly interrupted by a period of glaciation (Pakhuis formation diamictite) and mud deposition (Cedarberg Formation shale). These sediments were deposited in Coastal settings 400-450 millions of years (Ma) ago when Africa was part of the super-continent Gondwana. The beds have been thoroughly lithified by processes associated with deep burial, compaction and cementation.

Originally the layers of sand (now sandstones) would have been almost flat-lying. If you look around you at the site, you will see that these layers (beds) are now for the most part steeply inclined. This is because from 280 to 220 Ma 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 interaction between major plates of the earth’s crust during that time.

The geological cross-section provided shows a large anticlinorium (arch) consisting of smaller folds in the SW and a syncline (trough) at Montagu. Tight, small folds that decorate the northern, steep limb of the anticlinorium are beautifully revealed on the steep mountain slopes north of the site. It has been suggested that these folds represent collapse of folded strata under the influence of gravity.

The geological cross-section shows that the southern entrance of the kloof is marked by a major crustal dislocation (fault) – the Worcester Fault – which extends for a great distance along the southern margin of the Langeberg Mountains in the foothills immediately north of Ashton, Robertson an Worcester. The formations to the south of the fault have been lowered by several kilometres and major earthquakes would have accompanied the faulting as Gondwana started to break up.

This fault is as geologically important to the region as the San Andreas Fault is to California. You will also see a smaller fault at the northern (Montagu) end of the kloof. In 1969, a subsidiary fault between Tulbagh and Ceres was reactivated, leading to a destructive earthquake with a magnitude of 6.5 on the Richter Scale. Rock falls were reported at the time.
It is an interesting fact that were it not for the Table Mountain Group with its 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 resistant to weathering, being chemically relatively inert. Most of the chemical attack is related to 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 and thorough induration, 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 rock falls provided source materials for the gravels found in the modern river channels.

The most striking feature of Cogman’s Kloof is the way it cuts through the mountain range. It is one of several such features found in the Cape Ranges, perhaps the best known being the Seven Weeks Poort which slices through the Great Swartberg Range to the northeast. Exactly which processes acted to form these features is unknown, but several possibilities need to be considered. The first of these is antecedent drainage.

This implies that rivers existed prior to 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 that the rate of uplift. If this model is correct, we must be dealing with very ancient drainage indeed. In fact, if it is appropriate, the early kloof must have been a very grand feature because it is probable that the range through which it cuts may originally have been several thousand meters high.

The same problem of great antiquity applies to the second possibility which is that the modern drainage is the result of super-positioning of streams onto the folded mountain range from some earlier elevated landscape, the sort of process seems less likely than the former.

A third and probably the 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. a north-bank tributary of the Breede River eroding northwards from the Ashton are towards the Montagu area. These often do not penetrate the ranges, but pairs originating on opposite flanks may intersect.
At some time in the past this intersection occurred, and if the land north of the range was higher that 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.
Whatever processes were involved in the formation of the kloof, it must be accepted that its aspect today is in a sense unique, because it is undergoing continuous modification. Although in any single lifetime little or no change may be apparent, over a longer period, say millions of years, many aspects of the kloof would be expected to alter.

Broadly these would involve down-cutting of the active channel and widening by sidewall retreat. If for any reason, climatic or tectonic, down-cutting was reduced or halted, the stream would tend to cut sideways, widening the valley. Renewed down-cutting would leave parts of the previously-cut floor abandoned and these would remain as terraces. It is upon one of these terraces that you are standing whilst reading the plaque.

A reminder of the powerful forces of erosion was provided in January, 1981, when heavy rains in the southern Karoo caused the Keisie and Kingna Rivers to flood and to meet at their confluence in Montagu, before rushing through Cogman’s Kloof to Ashton and the Breede River, with considerable loss of life and damage to property.

This description was produced by the Geological Society of South Africa and is taken from their brochure and plaque located in Cogman’s Kloof Pass.

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