ROCK BOTTOM

MISADVENTURES  IN COMPREHENDING

THE GEOLOGY OF TABLE MOUNTAIN

 

Geological theorists don’t seem to have quite figured out how Table Mountain’s flat top was formed. That’s what I get from a layman’s reading of it. Generally the idea has been that about 540 million years ago, from deep below where the mountain later rose, a magma up-welling intruded into the existing metamorphic rocks. On cooling, it formed a massive bolt of igneous granite.   

 

Tectonic forces caused water to cover this area over the following many millions of years. Water-borne sedimentary sand built up in layers and hardened into sandstone, several kilometers thick according to some estimates.  

 

Later, the land-forms we now know as Antarctica, Australia and South America’s Falkland Plateau pushed up against Africa. This was due to continental drift. Where the southern and western Cape currently is, the buckling land ‘folded’, concertina-like; the bolt of granite anchored the sandstone-laden earth above it, forcing the land on either side to rise.  

A west-east (left to right) geological cross section through Table Mountain. on the Cape Peninsula indicating how the Cape Fold Mountains have been eroded in this region, leaving what was once the bottom of a valley to form Table Mountain with its flat table-top
credit: Wikipedia (creative commons)

Is the Table Mountain flat top really an Ancient Riverbed?

 

On first encountering this diagram, after calming my boggled thoughts and the up-welling reminder of utter personal insignificance within such time frames, my imagination took over. I started filling in my gaps of knowledge by thinking that the sandstone valley, where the top of Table Mountain is now, had probably been flattened by running water. And that the valley walls would have blocked the familiar comforting vistas.

 

Apart from lurking terror, you’d probably feel pretty lonely. There’d be no other humans.  

 

In the mountain's valley days, any life forms familiar to us now would probably have been absent. A googled guestimate had me imagining some proto-fynbos. Maybe shrew-like creatures would be clinging to the valley walls, trying to avoid any intimidating reptiles you’d come across if you managed to get to the old landscape's higher ground.

 

Table Mountain Rocks

In the mountain's valley days, any life forms familiar to us now would probably have been absent. A googled guestimate had me imagining some proto-fynbos. Maybe shrew-like creatures would be clinging to the valley walls, trying to avoid any intimidating reptiles you’d come across if you managed to get to the old landscape's higher ground.

 

The famous views only emerged after those high valley walls started to recede around the shape of the granite base. It kept the core intact of what was to become the mountain's recognizable shape.

 

It took longer than most massifs to form. Table Mountain is thought to consist of the most durable exposed rock around, weathering away at between 2 - 7 millimeters per 1000 years.  

 

Slowly the geological shapes that we know as Lion’s Head and Signal Hill also started taking form. 

View of Lion's Head from India Ve nsterhiking route on Table Mountain

It will be interesting to see if this theory of the iconic flattop being the remnant of an old valley holds up over time. It comes from credible sources. Kirstenbosch Botanic Gardens has a public copy of the diagram above. Another is the Table Mountain Wikipedia page, which has included it for long enough for the voluntary team who police the website to have pulled it if they didn’t think it empirically worthy.

 

But in some older books one can still find prior thinking that the flat part of the mountain was formed by ice sheets. Perhaps neither theory is mutually exclusive.

Table Mountain winter ice
Table Mountain winter ice

In time geologists will build on these ideas. I was recently surprised to find out that the notion of plate tectonics is only around several decades old. This serves to remind us that the story of the earth's formation is constantly unfolding. And that the diagram's label describes a 'possible ancient landscape.'

 

When I looked more closely at the diagram I noticed that it is a west - east cross-section. The ‘possible' valley runs north - south. It would have intersected the famous flat top, not run along it.   

 

So why is Table Mountain flat shaped? It seems no one is entirely certain. Geology, like the material it describes is a long-ranging constantly evolving thing.


When I mentioned my geologic musings to a more knowledgeable friend, he threw cold water on them. Literally, like a whole ocean. 'What you are talking about probably happened when the earth was under water. Five hundred million years ago when all this action round here started there was no life. Anywhere. That's why you don't find fossils on Table Mountain. And by the way. In those days the sea was probably orange.'

 

an upside down orange sea of cloud

For a book on the geology around the Cape Peninsula and a little beyond, John S. Compton's The Rocks & Mountains of Cape Town is a good place start.  I think he may be the originator of the diagram.