Road Trip Kirstenbosch National Botanical Garden

The most prominent of the geological formations to be found in the Garden are the sedimentary rocks of the Table Mountain Group. The sediments that formed these mountains are thought to have accumulated, from some 520 million years ago, along the coastline of the ancient supercontinent called Gondwana.

The Table Mountain Group is represented on the Cape Peninsula by two formations -Graafwater and Peninsula. In Kirstenbosch the Graafwater Formation can be observed along the upper contour path, where its soft, red sandstone sediments have been eroded in some places to form small caverns. The Peninsula Formation is a much thicker, coarser layer of sediments resistant to erosion, and results in the impressive precipices below Fernwood Buttress and the rugged backdrop of Castle Rock.

Road Trip Kirstenbosch National Botanical Garden Photo Gallery

About 250 million years ago a mountain-building period preceded the breaking up of Gondwana. It is believed that a small oceanic continent drifted into Gondwana, pushing up the deep sediments of the Table Mountain Group to form the Cape Fold Mountains. Erosion followed the mountain-building period, and the thick cap of sandstone over Table Mountain was reduced to half its original height. Even greater erosion events stripped away the landscape between the Peninsula and the other Cape mountains, creating the Cape Flats.

The recent history of the Cape Peninsula saw the sea level rise and fall during the Pleistocene Ice Age of the last two million years. Fluctuations of over 130 metres in sea level resulted in the exposure of a much wider coastline around the southern Cape, with the Agulhas plain extending over 150 kilometres further south than it does today. Conversely, sea level rise during the warmer cycles of the Ice Age caused the Cape Flats to be flooded, creating an island of the Cape Peninsula, and isolating Kirstenbosch and the whole of Table Mountain from the continent.

The Kirstenbosch climate is typical of the Mediterranean regions of California, Chile, southern Australia and the Mediterranean basin – warm, dry summers and cool, wet winters. The high, east-facing ridge rising above Kirstenbosch accentuates the temperature and rainfall gradients of the Garden’s climate, giving it a unique set of microclimates.

But it has not always been so. The process of continental drift had dramatic consequences for climates across Africa. After breaking away from South America 130 million years ago, Africa drifted northwards: today, the Cape is 20 degrees of latitude closer to the equator than at the time of separation. Some 12 million years ago, the glaciated Antarctic finally broke away from South America, giving rise to the circumpolar ocean and the cold, upwelling Benguela Current along the west coast, stabilising weather patterns in the region.

A display ofparticularly forbidding mammatocumulus clouds looms over the Cape as a cold front passes by. A winter northwester heralds the passage of a cold front, with clouds pouring through the gap between Fernwood Buttress and Devil’s Peak. Condensation of fog on restios such as Ischyrolepis subverticillatus is a key source of water on Cape mountains during the summer. A stationary subtropical high-pressure cell hovers over the South Atlantic. During summer, low-pressure cells pass eastwards to the south of the Cape, while in winter, the low-pressure cells shift northwards by five degrees of latitude, and so pass over the Cape. As the low-pressure cells develop and approach the Cape, converging masses of cold air from behind the front mix with the warmer air preceding it, resulting in condensation and belts of clouds, and forming the regular cold fronts (about 20 per year) that bring the Cape its stormy, wet winter weather.

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