Gateway to the Knysna Estuary
The historic coastal town of Knysna lies on the N2 highway and is situated on the Northern side of a shallow lagoon-like estuary. The rocky headlands, called The Heads, are located nearby on the coastline.
Access to the Eastern Heads, with its excellent views, is via the George Rex Drive. Access to the Western Heads is restricted and is part of the private Featherbed Nature Reserve.
In 1804, George Rex, a well-connected English gentleman, settled along the North shore of the estuary and is considered to be the founder of the town. Later, the Norwegian Thesen family played a major role in the booming timber and shipping business, utilising the protected harbour provided by Knysna’s shallow estuary.
The tide (estimated at Spring tide at 19 x 106 m3) enters Knysna’s estuary by surging through a narrow channel, flanked by the two spectacular headlands. In 1818 the first British ship sailed safely through the rocky Knysna Heads into what became a useful harbour. Although deep, the channel narrows to less than 200m and, with strong currents, it has always been a treacherous passage in which ships have foundered and lives lost.
[Scuba divers will be interested in the Paquita, a German vessel, which sank on the Eastern side of the Knysna Heads way back in 1903.
The wreck, which is easily accessible to qualified divers, remains in excellent condition. Its iron plates still glimmer brightly and its anchors are clearly visible, despite sitting at the bottom of the ocean for over a century.]
Regional Geology
Knysna’s geology displays a complex history based on the geological foundations of Gondwana during the Proterozoic period (more than 600 million years ago).
In paleogeography, Gondwana, also Gondwanaland, is the name given to the more Southerly of two supercontinents (the other being Laurasia) that were part of the Pangaea Supercontinent that existed from approximately 300 to 180 million years ago (Ma).( https://en.wikipedia.org/wiki/Gondwana )
There was further substantial change, but by the Middle Pleistocene (0.8 Ma) the gross geomorphology was well established. Since then, the current landscape has been modified through the Late Pleistocene and into the Holocene through a series of fluctuations in both climate and sea level change.
The headlands are composed of strongly jointed south-dipping strata of hard Table Mountain Group quartzite [TMQ: Ordovician / Silurian Age 400 – 500 Ma] that extends Eastwards along the coastline to Cape St Francis, and are part of the Cape Supergroup that forms the remarkably resistant Cape Fold Belt.
These quartzites are the same rocks that make up the cliffs of Table Mountain!
In the greater Knysna area the Table Mountain Group is represented by three formations:
- The lower Peninsula Formation is an extremely thick package of sands generally >1000m which stretches from the Cape Peninsula to the Eastern Cape. It consists mainly of clean white weathering sandstones which were deposited in a high energy barrier and shelf environment. These rocks form the Knysna Heads and are also exposed along the pass between Simola and Gouna.
- Overlying these sands is a thin band of shale known as the Cederberg Formation. These rocks are rarely more that 40m thick and consist of thinly laminated shales with occasional dropstone layers. Deposition occurred in a post- glaciation transgression.
- Overlying the shales is the Tchando Formation, which is another sequence of medium to course sandstones with similar lithological and sedimentary characteristics to the Peninsula Formation.
Landscape and Landforms
The Heads are remnants of the extensive coastal plain that is present throughout the Southern Cape. They originally form part of a wave-cut platform that was later elevated more than 200m above sea level.
On their Northern side is the wide shallow estuary of the Knysna River, often referred to as a “lagoon.” At one time the headlands flanked a narrow river gorge, a feature that is common along the Southern Cape’s coastal rivers, the Groot, Bloukrans and Storms rivers to the East of Knysna.
The river eroded a deep cleft through layers of hard quartzite when the sea was at a lower level, but the gorge has since been flooded by the sea.
Rock Formations
Beds of hard, well-jointed Peninsula Formation quartzite of the Table Mountain Group dip at around 20° to the South and are well exposed in the cliffs near sea level.
It is speculated that the Western Head was once an island, but the upper part is now covered by much younger dune rock and windblown sand.
At the base of the Eastern Head a good geological explanation can be seen for the formation of the spectacular passage between the headlands:
- Highly jointed quartzite: There is a very strong set of vertical joints in the quartzite that allows it to break more easily; in places it has been shattered to form zones of brecciation (broken rock) and consequent weakness.
- Numerous fractures and thin quartz veins of similar orientation, which are probably fault related and have contributed to a wide zone of structural weakness.
- The result is that intensely jointed and shattered quartzite at this particular site weathered and eroded more easily, and the weakened fracture zone provided an ideal pathway for a young coastal river to cute a gorge.
- North of the quartzite ridge is Knysna estuary, underlain by sediments of the Enon Formation, conglomerates that are the basal unit of the Uitenhage Group.
In order to qualify for this Earthcache, you will need to email the CO, luanshya, with your answers to the following questions:
1. Take a seat on the bench at GZ and enjoy the spectacular view of The Heads. To your right (and immediately left of the signboard) you will notice a quartz vein in the rock. How wide would you estimate this vein to be?
2. As you approached GZ you will have passed a sign board detailing the sinking of the Paquita. How deep is the channel at its deepest point?
3. The Spring tide surges through the channel at The Heads displacing approximately 19,000,000m3 of water. What speed can this tidal surge reach? (Now you know why swimming across the channel is strongly discouraged!)
As you take a walk along the path to the viewpoint (Waypoint 1) you will pass a cave to your left that was occupied by Stone Age people as long as 120,000 years ago. More recently (about 3,000 years ago) the ancestors of the San people and later the Khoekhoe made a home here.
4. Looking behind you (North) from the bench at the viewpoint, what quality of the rocks is quite evident that assisted the early river in breaking through the hard quartzite to get to the sea? Note: You really, really do need to read the cache description to answer this question correctly!!
5. [Optional] Take a selfie of yourself and GPS with the Western Head in the background.
References:
50 Must-See Geological Sites in South Africa, Gavin Whitfield, Struik Nature, 2015.
Final Knysna Environmental Impact Report – November 2009; Chapters 6 – 9 ( http://www.nra.co.za/content/Final_EIR_20_Nov_09-Chapt_6-9.pdf )