Rock Bottom

Comprehending Table Mountain's Geomorphary

The general idea: 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 earth's surface - kind of present day Karooish? Anyway - water-borne sedimentary sand built up in layers. Hardened into sandstone several kilometers thick, 8km, according to some estimates.

Continental Pushback

Later, 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, some say, the bolt of granite anchored the sandstone-laden earth above it, forcing the land on either side to rise. An argument against this last theory is that this did not happen at other spots with granite under-girds.

credit: Wikipedia (creative commons)

Is the Table Mountain flat top really an Ancient Riverbed?

The diagram's label describes a 'possible ancient landscape.' Some say the sandstone valley, where the top of Table Mountain is now, was flattened by running water.

The familiar emerges

The modern mountain shaped itself round shale and the subterranean granite base that emerges over A Very 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, and keen mountain rescuers.

Not so simple

So why is Table Mountain flat shaped?

Geology, like the material it describes is a long-ranging constantly evolving thing.

Older geology books offer a flattening formed by ice sheets.

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.