Overberg Geoscientists Group
440 Mya
Formation of Pakhuis Tillite
During the late part of the Ordovician and Early Silurian Period most of the Gondwana landmass experienced an ice age and glacial period. At that time, what is now Cameroon in central Africa was located over the South Pole. The main ice cap was centred over most of Africa. The glacial period of 12 million years, that followed the Peninsula Formation deposition, is represented by the Pakhuis Formation reaching a maximum thickness of 150 m. The accumulation of the overlying Cedarberg Formation followed the retreat of the ice sheets. The Pakhuis Formation occurs as a thin unit (on average only about 60 m thick) midway in the Table Mountain Group rocks of the inland mountains to the west of a line between Calvinia and Swellendam.
Geology. The products of the late Ordovician glaciation are the Pakhuis Tillites. This being a lithified sedimentary rock that consists of non-sorted to poorly sorted sediment formed by material eroded from the land by the ice flows and glaciers. The quartz-rich tillite contains particles that range in size from clay to boulders, suspended in a matrix of mudstone and sandstone. Boulders and pebbles dragged along the base of the moving ice sheets and glaciers could end up with flattened surfaces and parallel linear markings called striations. The pebbles mainly consist of white quartz, as the last remnants of a very abrasive process.
Ice flow evidence. At Pakhuis Pass near Clanwillian, from which the formation derives its name, we see surfaces identified as glacial pavements. Linear groove marks were cut into bedrock by glaciers slowly dragging boulders along that scoured the surfaces. In several locations the quartzites below the glacial horizon have been rucked into a series of folds believed to have been caused by the movement of ice ploughing into the underlying unconsolidated sands.
Ice Ages. The Ordovician Pakhuis glacial activity and event was part of earth’s natural processes and periodicity. It being one of at least five major ice ages or periods in earth’s history over the past 3 billion years. The Late Cenozoic Ice Age began 34 million years ago, its latest phase being the Quaternary glaciation in progress since 2.58 million years ago, with us now experiencing an expected warmer inter-glacial period.
Further Reading
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Hiller, N. 1992. The Ordovician system in South Africa: a review. In: Webby & Laurie (eds.). Global perspectives on Ordovician geology. Balkema.
https://www.history.com/topics/pre-history/ice-age
MacRae, C.S. 1999. Life etched in stone. Fossils of South Africa. Geological Society of South Africa.
Norman, N. 2013. Geology off the beaten track exploring South Africa’s hidden treasures. Struik Nature.
Rogers, J. 2018. Geological Adventures in the Fairest Cape: Unlocking the Secrets of its Scenery: 1–320. Pretoria: Council for Geoscience.
Visser, J.N.J., 1989. Episodic Palaeozoic glaciation in the Cape-Karoo basin, South Africa. In: Oerlemans, J. (ed.), Glacier fluctuations and climatic change, 1-12. Kluwer Academic Publishers.