The most direct route from Knysna to the R62 is over the Prince Alfred’s pass (R339). Once a bridle path used for centuries by locals and forest elephants, work on this mountain pass commenced in 1860 and was completed by 1867 in time for a visit by Prince Alfred in person. The pass is one of the many works of the engineer Thomas Bain.
At 68.5 km it is the longest (publicly accessible) mountain pass in South Africa by a considerable margin, as well as being the second oldest unaltered pass still in use.
This all stands in testimony to the skill of Thomas Bain and it is his work, particularly the dry-stone walling, that catches the eye of todays road user as they journey along the pass.
Many users coming from Knysna drive through the tiny settlement of De Vlugt and enter the most scenic section of the pass completely unaware of the geological wonder that looks down upon them.
If you park carefully on the edge of the road at the listed coordinates and look across the valley your eyes should come to rest upon a striking rock formation that is aptly named Bain’s Pillar.
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- Approximately how far away from where you are standing ( GZ ) is the pillar ?
- What would you estimate the height to be of the freestanding portion of the pillar?
- What rock would you think the pillar is made of?
- Name two geological processes that contributed to the formation of the pillar.
- Optional – send a picture of yourself and / or your GPS taken at GZ with the pillar in the background.
So…… How on earth was this structure formed ?
The Geology.
The rock that we see today in this pass is of sedimentary origin and it is referred to as the Cape Supergroup. It was originally formed about 500 million years ago beneath an inland sea ( the Agulhas sea ) on a continent known as Gondwana. About 180 million years ago this rock was thrust upwards with enormous force to form what we now know as the Cape Folded Mountains.
About 150 million years ago as Gondwana started to split apart to form separate continents of South America, Antarctica and Africa the cape Folded Mountains were left to form the Southern boundary of the African continent. These mountains although old by geological standards are very resistant to weathering and erosion due to their quarzitic sandstone composition.
The Structure.
The pillar is known geologically as a Tor but can also be referred to as a Castle Koppie or Kopje. Tors can be made of many different types of rock, most commonly granite but also schists, dacites, dolerites, coarse sandstones and others.
Tor on Cornish moorland, United Kingdom
Geomorphological formation.
Bain’s Pillar was been formed over millions of years by the combined processes of erosion and weathering. Variations in the composition of the rock layers and their joints have resulted in parts of the surface eroding at different speeds. The area of rock surrounding the pillar has been weathered and eroded at a greater rate whereas the pillar has undergone the same processes at a reduced and less effective rate. Eventually it has allowed the pillar to separate from its surrounds and become the freestanding formation that we see today.
Sources:
https://serc.carleton.edu/41084
https://www.mountainpassessouthafrica.co.za/find-a-pass/western-cape/item/142-prince-alfred-s-pass-part-1.html
https://en.wikipedia.org/wiki/Cape_Fold_Belt
Prince Alfred’s Pass ( Spectacular and Diverse ) by Ilse Meyer
https://pixabay.com