Misadventures in Comprehending Table Mountain's Geology
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, below the super-continent where 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.
The Karoo Seabed hardens
Tectonic forces caused the super-continent to drift apart. During the following many millions of years, over land where the Karoo is now, water-borne sedimentary sand built up in layers. It 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 sandstone-covered land buckled, folding concertina-like; at the same time the bolt of granite anchored the sandstone-laden earth above it, forcing the land on either side to rise.
credit: Wikipedia (creative commons)
Is the Table Mountain flat top really an Ancient Riverbed?
After first encountering this diagram, and letting the up-welling reminder of our minimal significance within such time frames subside, I noted the diagram's label. It describes a 'possible ancient landscape.' So, this is informed and scholarly guesswork. I entered into the spirit of it by filling in my own much broader knowledge gaps.
I figured that the sandstone valley, where the top of Table Mountain is now, was probably flattened by running water. And placing oneself there and giving it some further thought, I suddenly felt very lost. Those familiar comforting vistas we know so well would have been blocked from sight by giant valley walls. Apart from lurking terror, I’d probably feel pretty lonely up there. There’d be no other humans. In the mountain's valley days, any life forms familiar to us now would probably have been absent.
The familiar emerges
The famous views only became distinct 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 shapes that dictate several factors unique to life in Cape Town. These range from the angle and ferocity of the weather - through to societal and residential planning, and traffic dynamics.
In older books prior thinking was that the flat part of the mountain was formed by ice sheets.
Not so simple
Looking more closely at the diagram above I noticed that it is a west - east cross-section. The ‘possible' valley runs north - south. My imaginary valley floor 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 much of the earth was under water.
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 is now out there in some pretty credible sources. It makes some kind of sense to my interested but inadequately equipped understanding of geology. And so as we tramp the mountain and until some better story comes along, it will be how I try to stumble through my explanations of the past six hundred million years of the ground beneath our feet.
Much of the thinking outlined above comes from work done by John Compton. His YouTube channel has some videos.