Anyone who lives in the Western Cape must be aware of the extraordinary mountain ranges we have to criss-cross every time we travel. We would have noticed the beautiful rock formations in passes such as Seweweekspoort, Swartberg or Meiringspoort. These striated, contorted rocks are part of the Cape Fold Mountains.
Our own Kleinriviersberge are another example right here on our doorstep. Look at the rocks of Mosselberg in Fernkloof (especially when the late afternoon sun turns it to gold) and you can not fail to wonder when and how these apparently solid rocks were so squashed, contorted and turned upside down.
This is my own, very lay interpretation of how the Cape got its magnificent mountains.
To explain what happened, we must look very briefly and simply at the structure of the earth. We know that the centre or core is solid (even though it is very hot) because of the huge pressure of everything above it. The mantle wraps around the core and is made of minerals that are also very hot – so hot that it’s almost like boiling porridge.
On top of that is the earth’s crust that makes up the land masses and the bottom of the sea. The crust is cooler and almost solid – like the solid surface of the porridge when it cools down. But underneath the crust the boiling porridge is moving (called convection currents) and the top crust floats with it. Hot porridge rises to the top and pushes the crust out to the edges. But the porridge crust is confined to the pot, so when it hits the solid side of the pot, it dips back under the surface, while more porridge rises over the flame under the pot.
We can look at the earth like that too. Molten stuff rises to the top in places on the earth’s surface forming a new crust, while older pieces of the crust (with its continents or seas on top) float away with it. These floating pieces of crust are called plates and the floating around is called plate tectonics. But the earth does not expand. So where does such a moving crusty plate go?
When it hits another plate, especially if it’s a thick continental plate, it dips underneath it. This is called subduction. Now you can imagine that if one part of the earth moves below another part of the earth, even if very slowly, this can be pretty dramatic. The drama can be enacted in the form of earthquakes, volcanoes or majestic mountains being thrust up into the sky. Petty awesome either way.
The restless Earth
Let’s backtrack a bit. As we now know the earth’s crust is made up of a whole lot of plates floating about its surface. Some of these plates carry continents (continental plates) and some carry oceans (oceanic plates). Millions of years ago the earth didn’t look anything like it does today. Over these millions of years, ancient continents have been repeatedly crashing into each other and again drifting apart. New continents formed and then ripped apart again, Oceans opened and closed again over aeons of time.
To explain the formation of the Cape Fold Mountains we only have to go back to 500 million years ago to a supercontinent called Pangaea. The name is derived from the Greek “pan” meaning all, and “Gaia” meaning Mother Earth. This supercontinent encompassed all the land masses of the time and it was centred in the southern hemisphere.
Skip a few millions years and convection currents in the mantle started pulling this landmass apart again. The tectonic plates carried with them what is now South America to the west, the Falkland Plateau to the south, East Antarctica to the east and India moved north-east. Between South Africa and the Falkland Plateau a sea opened up – called the Agulhas Sea.
The landmasses all around the Agulhas Sea were buffeted by rain and wind and the land eroded to be washed down rivers into this sea. Slowly, layer upon layer of sand and mud were deposited in the sea. Eventually, the weight of the deposits exerted enough pressure to turn the lower layers into sandstone (sand) and shale (mud).
But the earth is never static. After more millions of years the plates started to converge again. The Falklands Plateau and other landmasses drift inexorably back to southern Africa. Except, the Agulhas Sea is now filled with layers of sandstone and shale. No matter – the earth’s forces cannot be stopped.
As the Falklands Plateau slams into Africa, the oceanic crust behind the plateau buckles down and gets subducted under the land. It pushes up the Falklands Plateau into epic mountain ranges which in turn push, squash and buckle the rocks in the bottom of the Agulhas Sea.
What used to be sea bed, is elevated to mountain heights – mountains that show their violent origin in the folded and faulted layers of sandstone we see in the Cape Fold Mountains today. The supercontinent so formed by the landmasses of the southern hemisphere and India is called Gondwana, meaning the ‘Land of the Gonds’, a tribe in India.
But the earth has a never-ending story. After more millions of years the tectonic movement of the plates reversed again. Gondwana started to break up. The Falkland Plateau with its majestic mountains pulled away from Southern Africa, leaving the Cape Fold Mountains behind.
These mountains, composed mainly of sandstone and shale, eroded over more millions of years, leaving us with the present sandstone mountains. The shale, being softer, eroded more quickly and accumulated in valleys between ranges and to the north, where they formed fertile agricultural soils. On the other hand, sandstone soils, poor in nutrients, contributed to the amazing diversification of the fynbos plants that grown on them.
So that’s the very simplified story of our beloved mountains and the precious fynbos they are home to. When next you look at them, ponder the forces that formed them over hundreds of millions of years.
To learn more about the rock formations we see daily all around us in the Overstrand, come and listen to geologist Dr John Bristow, who will present the monthly talk at Whale Coast Conservation on 20 August.
About the Author
Whale Coast Conservation passionately lives by its slogan “Caring for your environment”.
Its small staff and volunteers are dedicated to
- raising community and visitor awareness of the unique, biodiverse natural resources of the Cape Whale Coast region and
- to projects that convert awareness into practical actions that lead towards living sustainably.
WCC ensures expert representation in public participation processes that contribute to environmental and developmental policies and legislation. We monitor regional development; and ensure compliance with legislation and guidelines.
WCC increases general public awareness of sustainability through environmental education, citizen-science research projects, community projects and campaigns.
WCC communicates with its audience through exhibitions, signage, technology demonstrations, workshops, talks, film shows, newsletters and articles.
WCC places emphasis on educating future generations through its Youth Environment Programme (YEP). YEP is offered to 24 schools in its target area with a total of over 10,000 learners.
WCC facilitates schools’ participation in special events such as Earth Day, Walking for Water, Arbor Day and Coastal Clean-ups.
WCC facilitates educator development programmes to improve the capacity of educators to offer informed environmental content in their lessons across all learning streams.