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, on the super-continent where the Table Mountain would later rise, a magma up-welling intruded into the existing metamorphic rocks. On cooling, it formed a massive bolt of igneous granite.   

 

Tectonic forces caused the super-continent to drift apart. Water covered the area during the many following millions of years. You could think of it as the 'Karoo Sea'. 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 back up against what we now call Africa. Where the southern and western Cape currently is, the land buckled, folding 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?

 

After first encountering this diagram and shaking the up-welling reminder of our minimal significance within such time frames, I noted the diagram's label. It describes a 'possible ancient landscape.' So, an informed and scholarly guess.

 

I entered into the spirit of it by imaginatively filling in my own broad knowledge gaps. The sandstone valley, where the top of Table Mountain is now, were probably flattened by running water. And placing oneself there, any familiar comforting vistas would have been blocked out by the giant valley walls.

 

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

Table Mountain Rocks

In the mountain's valley days, any life forms familiar to us now would probably have been absent.

 

The famous views only emerged after those high valley walls started to recede, and the modern mountain shaped itself round the mainly subterranean granite base. 

 

It took its own long time. Table Mountain is thought to consists of the most durable rocks 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 as well as the rest of Table Mountain became the familiar shape that dictates several factors unique to life in Cape Town. These rage from the winds we live with, through to societal and residential spacing, and traffic dynamics.

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 copy of the diagram (above) on public display. So too Table Mountain's Wikipedia page.

 

In older books prior thinking was that the flat part of the mountain was formed by ice sheets.

Table Mountain winter ice
Table Mountain winter ice

Looking 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? No one seems entirely sure. 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 may have been 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.