What is Table Mountain made of?

What is Table Mountain made of?

It may seem like a silly question, with an obvious answer: rock, right?

But dig a little deeper (sometimes literally), and you’ll find that there’s much more to Table Mountain than meets the eye.

For as long as humans have occupied this part of the world – since approximately 15 000 years ago – Table Mountain has been a constant and steady presence. Myths and legends have been told about the place, and even today it is seen as an important spiritual landmark.

There’s something comforting in the permanence of a mountain. While the world around us changes constantly, we seek solace in its familiar outlines and features. Table Mountain’s unusual flatness has also made it iconic around the world.

But humans have only been present on the planet for a mere fraction of its history. Geological time spans way, way back into the past, and the further you look, the less familiar Table Mountain becomes.

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A Landsat image of Cape Town topography and the surrounding area, courtesy of NASA

Basic ingredients

Hundreds of millions of years ago, this corner of the continent was unrecognisable. Rivers ran into the ocean, depositing silt and mud from further inland. Tectonic shifts also caused the continents to collide, compressing the mud and silt into a fine-grained sedimentary rock called shale. Today, this layer is known as the Malmesbury Group.

About 540-million years ago, magma flowed upwards from deep within the Earth’s core and pushed up into the tiny cracks within this layer of shale. When magma cools, it becomes granite, and this mix of rock types can be seen today along the Sea Point coastline.

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Magma intruding up into a layer of folding Malmesbury rocks, courtesy of John Compton

Further tectonic shifts, ice ages, rising and falling sea levels and all manner of climate changes eventually flattened out the surface of the rock. That allowed layers of sand to settle on top of it.

As sand gets compacted by continents crashing into each other, it eventually hardens and becomes sandstone. More and more layers of sandstone built up over time on top of the shale granite layer. We call this layer the Table Mountain Group.

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Two kinds of sandstone that are part of Table Mountain Group can be clearly seen from Ou Kaapse Weg. Photo courtesy of Oggmus

The only way is up

All these continents bashing around also affected the landscape above them. At the pressure points, rocks were forced upwards, rising above sea level and sometimes, much further. This resulted in folded mountains, the kind you see here at the Cape (as well as the Atlas Mountains in North Africa).

It also provided the lift needed to create Table Mountain. The whole area was thrust skywards (over millions of years, of course).

Now exposed to wind, rain and ice, the sandstone on top was eventually eroded, exposing the land surface we see today, and creating the unmistakable flat-topped mountain that locals and visitors know and love.

Geologists believe that Table Mountain was once the bottom of a very wide delta. It’s incredible to think that the highest point in Cape Town was once the lowest!

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Erosion of the Cape Fold Mountains may have resulted in the flat top of Table Mountain we see today. Image courtesy of Fred the Oyster

Do your own rock-spotting

Looking up at Table Mountain is like travelling back in time! See if you can spot the various layers of rock that make up the mountain:

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The three main layers that make up Table Mountain can be clearly seen from the City Bowl. Can you spot the different layers? Image courtesy of UCT’s Department of Geological Sciences

So yes, the short answer to the question is that Table Mountain is made of rocks. But the story of how it came to be reveals incredible changes to the landscape around it, over time periods we can barely fathom.

It’s fair to say that Table Mountain rocks.

(Thanks to Prof Chris Harris at UCT’s Department of Geological Sciences for assisting with the science. Visit their website for further info on Cape Town’s geology)

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