Being a curious person I recently got interested in the geology of the region. This was partly fueled by the fact that the view from my computer is of the Dassenburg in Fish Hoek, a most intriguing looking lump of rock. This is part one of a series of articles I am planning and is a brief journey back in time, written by a laymen, for the enjoyment of other laymen.
All good things are built on solid foundations, and the Cape Point Peninsula is no exception. We take for granted the very rocks on which everything is built, and though we are more aware than most of the towering lumps mountain that surround us (the mother city’s greatest icon is its mountain after all,) still we don’t really give them much thought. So it seems prudent to begin our look at the history of the Cape Point Peninsula by first stepping back to the very beginning and giving a laymen’s tour of the Geology of the area.
Cape Point is like a layered cake, a mille fille in geological terms, with layer upon layer of different rocks, having being laid down over millions of years in some very different processes.
The first of these layers is Malmesbury Shale, a term familiar to most Capetonians, and yet I suspect understood by few. Around 560million years ago the area that is today called Cape Point was an undersea area at the edge of a large continent. Over millions of years deposits of mud and sand were washed off the continent into the sea, where they began to settle. As the layers began to build the weight of the upper deposits began to compact the lower layers, forming sedentary rocks, which today underpin much of the CBD and Cape Flats. This process was interrupted around 540million years ago by volcanic activity which forced molten rock up into the Malmesbury deposits, which hardened into granite rocks. These underlie a lot of the peninsula south of Table Mountain, and are most obvious on the shoreline around Simons Town in the form of the large, smooth boulders at Boulders Beach.
The Earth’s surface is in a constant state of flux and when the movement caused the area to be pushed up above the seas surface, exposing it to the elements for around 35 million years, eroding the rocks into a flat plateau. Further tectonic activity saw the region become the bed of an inland sea and delta.
For the 170million years sand and mud was washed off the surrounding land into this system, depositing millions of tons of material on top of the Malmesbury shale and granite bedrock. As the sea floor continued to sink downwards, more and more material was deposited, resulting bands of deposits up to 7km thick. The upper layers compressed the lower layers creating sand stone deposits which would later become the Cape Supergroup of rock, of which Table Mountain is a part of.
The area was once more thrown into turmoil when the super continent Pangaea was formed between 280 and 235 million years ago. This upheaval gave us the patterns of strata that we are familiar with today. The Sandstone is deposited in nearly horizontal layers. Tectonic movement and volcanic activity cause these layers to be lifted in different areas. The result is the angular geography that we see in the rock strata today. In some areas the layers have been pushed up to such an extent that they are near vertical.
By this point the region that is now the Cape Point Peninsula was part of a vast mountain range. The sand stone has been eroding ever since; giving us the mountains we are so familiar with today.