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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.
Lekker Links
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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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