Rock Bottom

Comprehending Table Mountain's Geomorphary

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 many millions of years, over the earht's surface where the Karoo is now, water-borne sedimentary sand built up in layers. It hardened into sandstone several kilometers thick, 8km, according to some estimates.

Continental Pushback

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 8 km-thick 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?

The diagram's label describes a 'possible ancient landscape.' So, informed and scholarly guesswork.

Was the sandstone valley, where the top of Table Mountain is now, flattened by running water?

The familiar emerges

Familiar vistas blocked from sight by giant valley walls, which once they started to recede and the modern mountain shaped itself round the mainly subterranean granite base, then emerged.

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 - from tourism to the angle and ferocity of the weather; and societal and residential planning; traffic dynamics, a keen group of mountain rescue experts.

Not so simple

So why is Table Mountain flat shaped?

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

Looking more closely at that valley diagram, I've noticed that it is a west -east cross-section.

The ‘possible' valley runs north - south.

I think. (My geology always feel like shakey ground). Geology, like the material it describes is a long-ranging constantly evolving thing.

How long will the theory of the iconic flattop being the remnant of an old valley hold up? It is credibly sourced. It makes some kind of sense as we tramp the mountain, and until some better story comes along, it will be how I try to stumble through my explanation of the ground beneath our feet's past six hundred million years, or so..

Much of the thinking outlined above comes from work done by John Compton. His YouTube channel has some videos.