Table Mountain Sandstone

Table Mountain Sandstone (TMS), formally known by its geological name the Peninsula Formation Sandstone, is a group of rock formations within the Cape Supergroup sequence. While the term "Table Mountain Sandstone" remains widely used, it is no longer formally recognized; the correct geological name is "Peninsula Formation Sandstone," which is part of the Table Mountain Group. The name originates from Table Mountain, the famous landmark in Cape Town, South Africa. For consistency with common usage, the term "Table Mountain Sandstone" will continue to be used throughout this article.

Composed primarily of quartzitic sandstone, Table Mountain Sandstone was deposited between 510 million years ago (Cambrian Period) and 400 million years ago (Silurian Period). It is the hardest and most erosion-resistant layer of the Cape Supergroup, making it responsible for the highest peaks and steepest cliffs of the Cape Fold Belt. Despite being the oldest and lowermost layer of the Cape Supergroup, its resistance to erosion has allowed it to persist as the dominant rock formation in many prominent landscapes across the Western Cape.

The folding of the Cape Supergroup into the parallel mountain ranges of the Western Cape began approximately 330 million years ago, shaping the landscape from Clanwilliam (about 200 km north of Cape Town) to Port Elizabeth (about 650 km east of Cape Town). Beyond these points, the Cape Supergroup sediments are not folded into mountain ranges but instead form steep cliffs and gorges, where surrounding sediments have been eroded away, as seen in locations like Oribi Gorge in KwaZulu-Natal.

The Cape Supergroup rocks were laid down as sediments in a rift valley that developed in southern Gondwana, just south of Southern Africa, during the Cambrian-Ordovician Periods (starting about 510 million years ago, and ending about 330-350 million years ago). An 8 km thick layer of sediment accumulated on the floor of this rift valley. Closure of the rift valley, starting 330 million years ago, resulted from the drift of the Falkland Plateau back towards Africa, during the Carboniferous and Early Permian periods. This caused the rucking of the Cape Supergroup into a series of parallel folds, running mainly east-west, but with a short section running north-south in the west (resulting from the collision with the eastward movement of Patagonia into southern Africa). Continued subduction of the paleo-Pacific Plate beneath the Falkland Plateau and the resulting further compression of the latter into Southern Africa, raised a mountain range of immense proportions to the south of the former rift valley. The folded Cape Supergroup formed the northern foothills of this towering mountain range.

Sediments, eroded from this immense Falkland Mountain range to the south, buried the folded Cape Supergroup rocks, and the plains beyond (to the north) to ultimately form the Karoo Supergroup, a sequence of sediments that eventually covered most of southern Africa and other parts of Gondwana. The Cape Supergroup re-emerged as mountains when uplift of the subcontinent, about 180 million years ago, and again 20 million years ago, started an episode of continuous erosion that was to remove many kilometres of surface deposits from Southern Africa. Although the tops of the original Cape Fold Mountains were eroded away, the hard Table Mountain Sandstone component eroded much slower, forming the backbone of the Cape Fold Mountains, with the younger, but very much softer Bokkeveld shales remaining only in the valleys (see diagram on the left).

The Falkland Mountain range had probably eroded into relative insignificance by the mid-Jurassic Period, and started drifting to the south-west soon after Gondwana began to break up 150 million years ago, leaving the Cape Fold Belt to edge the southern portion of the newly formed African continent. Even though the mountains are very old by Andean and Alpine standards, they remain steep and rugged due to the Table Mountain Sandstone's quartzitic sandstone geology, making them very resistant to weathering.

The degree to which the original Cape Fold mountains (formed during the Carboniferous and early Permian Periods) have been eroded is attested to by the fact that the 1 km high Table Mountain on the Cape Peninsula is a syncline mountain, meaning that it once formed part of the bottom of a valley when the Cape Supergroup was initially folded. The anticline, or highest elevation of the fold between Table Mountain and the Hottentots-Holland Mountains to the east, on the opposite side of the isthmus connecting the Peninsula to the mainland, has been eroded away. The Malmesbury shale and granite basement on which this anticline mountain rested also formed an anticline; but being composed of much softer rocks, readily weathered into a 50 km wide sandy plain, called the "Cape Flats" (see diagram below, on the right).

Table Mountain Sandstone Formation is, barring the Graafwater Formation, the oldest component of the Cape Supergroup. It was laid down as sandy deposits, with a maximum thickness of 2000 m, in a flooded rift valley. It contains no fossils. Its subsequent burial under the other Formations in the Cape Supergroup, and thereafter under the sediments brought down from the Falkland Mountain range, compressed and partially metamorphosed the original sandy deposits into very hard quartzitic sandstones, which in their folded configuration form the peaks, steep cliffs and rugged crags of the Cape Fold Mountains. During the deposition phase a short period of glaciation left a layer of tillite, called the Pakhuis Formation, which today divides the Table Mountain Sandstone Formation into a lower and upper layer. It is particularly the lower layer which is now extremely hard and erosion resistant, causing it to form most of the summits, crags and high cliffs that characterize the Cape Fold Mountain ranges (see illustration second from the top left), as well as the sheer rock faces of upper 600 m of Table Mountain.