Let’s backtrack about 350 million years to when the then
continents were happily moving around on the slightly plastic
mantle of the Earth, each being carried by structures known as
tectonic plates. They began to collide into one grand continent
known as Pangea (meaning Entire Earth), with the
southern part called Gondwana. Along the ridges where the
future continents of Africa, South America and Antarctica were to
rift roughly 220 million years later, mountain ranges were being
pushed up, one of which we know today as the Cape Fold Belt
which you, amongst others, traverse between here and Cape Town.
These mountains, together with those now in Antarctica, drained
their rivers inland, not out to sea. As a result, for hundreds of
millions of years, water and its erosion products (mud, sand and
rocks) washed down from the mountains, steadily poured into a great
inland sea, the Karoo Basin. These sedimentary deposits
would eventually reach a thickness of over 6 km at its southern
edge.
At the position of this EarhCashe the inland sea was probably
only a few tens of meters deep. Here sediments were being laid down
at the mouth of a very large north-flowing river, part of the river
delta, depositing mud onto the bottom of the sea. Occasionally, the
slightly angled build-up, being very fluid, would slip as an
underwater avalanche, sporadically causing the fine layers of laid
down mud to be mixed together and the water was temporarily turbid.
This produced alternating strata with thicknesses of less than a
centimetre to tens of centimetres. Eventually, under the weight of
later deposits, it all solidified into the layers of mudstone and
turbidites that you can see in this magnificent road cutting
today.
To make it even more interesting, at the time this was happening
there were large volcanoes active along the region where South
America was splitting away from South Africa, about 1000 km to the
west, and the prevailing northwesterly winds were blowing the
volcanic ash this way. The ash sifted down on the surface of the
western side of the inland sea, sank to the bottom, and produced
interleaved layers with the mud deposits. The turbidites we see,
therefore have colours varying from the grey of pure mud through to
yellow of pure ash.
Then, about 130 million years ago, the single large continent
began to split up into Gondwana and Laurasia, then
into various smaller pieces as they drifted apart. All of the Karoo
deposits were originally laid down in horizontal strata, but later
pressures, including from the splitting of the continents, caused
folding of some of them. The term folding is used in geology
when one or a stack of originally flat and planar surfaces, such as
sedimentary strata, are bent or curved as a result of plastic (i.e.
permanent) deformation. In the process of continental drift,
pressures build in various parts of the Earth's crust, and slowly,
over thousands of years, the crust is transformed as layers of rock
are forced to buckle, warp or bend.
Where you are standing now was about 2 km below the surface when
all this happened. Since the break-up of Gondwana, southern
Africa has been a rising continent, lifted up by vertical currents
in the mantle beneath. We thus live on an eroding surface where all
this material had been weathered away and transported by rivers out
to sea. If you want to be at the level where the dinosaurs lived
you will need to get in a helicopter and rise about a kilometer
above the present surface. The Karoo around you here is roughly 230
million years old.
(Source: Prof Brian
Warner
Department of Astronomy, University of Cape Town , South Africa
AND
School of Physics and Astronomy, University of Southampton,
UK.)
To claim “Found it” you must email me satisfactory
responses to the following:
Any logs not accompanied by an email will be deleted.
- Find safe parking near the designated coordinates. Locate the
formation where the greatest amount of folding took place and send
me a picture of you/your party and your navigation device, showing
this feature in the background. (Note: There are two sections of
folding, you may choose either one.)
- Send me a close-up picture where you point out an example layer
which formed during a long period of pure mud
deposition.
- Send me a close-up picture where you point out an example layer
which formed during a period of high volcanic activity.
- Although sedimentary layers always form horizontally, between
the two folding features they are at a quite a steep angle. When
and how could the strata have tipped over like this?
- Pace out the distance between the two folding features (i.e.
the tipped section). Assuming a sedimentary rate of
20000years/metre, calculate roughly how many years it must have
taken for this (tipped) section to be laid down.
Note: Do not post your pictures,
replies or hints to this page, even if
encrypted.