Table Mountain Sandstone
Table Mountain Sandstone, 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.
File:Cederberg geology.jpg|thumb|260px|Schematic diagram of an approximate 100 km west-east geological cross-section through the Cedarberg portion of the Cape Fold Belt. The rocky layers belong to the Cape Supergroup. Not to scale. The green layer is the Pakhuis diamictite formation which divides the Table Mountain Sandstone into a lower and upper portion. It is the lower portion that is particularly hard and erosion resistant, and, therefore, forms most of the highest and most conspicuous peaks in the Western Cape, as well as the steepest cliffs of the Cape Fold Mountains. The Upper Table Mountain Sandstone Formation, above the Pakhuis tillite layer, is considerably softer and more easily eroded than the lower Formation. In the Cederberg Mountains This formation has been sculpted by wind erosion into many fantastic shapes and caverns, for which these mountains have become famous. The Graafwater Formation forms the lowermost layer of the Cape Supergroup in this region, but is, for simplicity, incorporated into the Table Mountain Sandstone Formation in this diagram.
File:Geological cross section through Cape Peninsula.jpg|thumb|300 px|Schematic geological west-east cross section through Cape Peninsula, based on a section through the Back Table just south of Table Mountain. Not to scale. On the Peninsula the basement layer consists in the main of Cape Granite. The Table Mountain Sandstone forms the steep escarpments that surround the approximately 5 km-wide central plateau. It consists of the layer below the "Pakhuis diamictite", of which there is only a trace at the highest point on Table Mountain at 1085 m above sea level. The lowermost formation of the Table Mountain Group is the "Graafwater Formation", which rests unconformally on the Cape Granite base, as opposed to the Malmesbury Formation base in most of the rest of the extent of Cape Supergroup in the Western Cape. Kirstenbosch Botanical Gardens, and all the major wines estates on the Peninsula are situated on the fertile granite slopes on the east side of the mountain. The soils derived from Table Mountain Sandstone are very poor in nutrition.
Composed primarily of quartzitic sandstone, Table Mountain Sandstone was deposited between 510million years ago and 400million years ago. 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 330million years ago, shaping the landscape from Clanwilliam to Port Elizabeth. 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.
Origins
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. 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. 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 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.
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 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".
File:Erosion A5.svg|thumb|right|300 px|A west-east geological cross section through Table Mountain on the Cape Peninsula, the Cape Flats and the Hottentots-Holland Mountains on the mainland, indicating how much of the Cape Fold Mountains has been eroded away.
Structure
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, as well as the sheer rock faces of upper 600 m of Table Mountain.
A small patch of Pakhuis tillite occurs on the top of Table Mountain at Maclear's beacon, but most of the Pakhuis Formation is found as a thin layer in the Table Mountain Sandstone Formation of the more inland mountains to the west of a line between Swellendam and Calvinia. These diamictite rocks are composed of finely ground mud, containing a jumble of faceted pebbles. They can easily be recognized at a distance as this formation readily erodes into fertile, gently sloping, green swaths in a landscape where this contrasts starkly with the bare rocky surfaces of the quartzites above and below. In several locations the quartzites immediately below the glacial horizon have been rucked into a series of folds. This is believed to have been caused by the movement of ice ploughing into the underlying unconsolidated sands. A good example of this can be seen on a ridge of rocks near Maclear's Beacon on Table Mountain, close to the edge of the plateau overlooking the Cape Town City Bowl and Table Bay.
The Upper Table Mountain Sandstone Formation, above the Pakhuis and Cederberg Formations, consists of much softer sandstone than the Lower Table Mountain Sandstone Formation, and is often referred to as the Nardouw Formation. In the Cederberg this formation has been eroded by the wind into a wide variety of "sculptures", caves, and other fascinating structures for which these mountains have become well-known.