Have you ever wondered how the Knysna heads came into existence? We asked a geologist, and this is what we learned....
Just like every square centimeter of the earth’s surface as we know it, the geology within the surroundings of Featherbed Nature Reserve is also subjected to two major forces. These forces can be classified into two groups: firstly, those below the crust in the mantle where thermodynamic forces result in plate tectonics, and secondly, the forces of nature above the surface where the geology is exposed to wind, water, ice, etc. The forces below the crust are typically responsible for shearing, bending, folding, thrusting, lifting, and tension actions. Whereas the geology above the surface is subjected to eroding, deposition, and gravity sliding.
From Figure 1 (above) we can see the area and surrounding area of Featherbed Nature Reserve is underlaid by rock formations of the Peninsula Formation, which form the base of the Table Mountain Group and the base of the Cape Supergroup. The Peninsula Formation is classified as Ordovician in age (± 480 Ma). We know that the Peninsula Formation (although not visible within this area specifically) was deposited on top of tilted and folded rocks of the Malmesbury Group and granites of the Cape Granite Suite. What we also know in general about the Peninsula Formation is, that it comprises at least half of the Table Mountain Group and its thickness varies from 1800 - 2150m. It is composed of a monotonous succession of medium to coarse-grained, thickly bedded, greenish grey sandstone which weathers whitish. Thin layers of conglomerate or individual pebbles are present here and there. In layman’s terms, these cross-bedded sandstone layers represent beach sand deposited within a shallow, but extensive intracratonic, basin which is oriented in an east-west direction as part of the bigger Gondwanaland.
The deposition was mainly from the north into the fairly stable basin. With time and as more and more layers of rock formations were deposited on top, and due to pressure and heat, these beach sands were metamorphosed into sandstone and eventually into quartzites. What we see today on the surface of the Peninsula Formation are the beautiful quartzite formations forming the Knysna heads. A view of Table Mountain (as seen from Cape Town city center) also exposes the Peninsula Formation, which it is lying horizontally on top of the Cape Granites.
Fast forward a couple hundred million years later, during the early phases in the process of breaking up of the Gondwanaland, a number of basins defined as normal faults with downthrows to the south were formed. These basins along the southern Cape coast are rift basins that originated, just prior to the separation of South America from Southern Africa. The Algoa and Gamtoos basins (which represent the bigger basins) continued eastward onto the Agulhas Bank. The smaller basins at Plettenberg Bay, Knysna and Mossel Bay are the landward continuation of larger basins towards the south-east and onto the Agulhas Bank. These basins were filled mainly with continental and marine deposits of the Late Jurassic to Cretaceous age (145 - 66 Ma). These deposits are called the Enon Formation and consist of layers of yellowish and red conglomerate with lenticular beds of red, yellow and green sandstone and predominantly red claystone. A beautiful example and exposure of the Enon formation is along the northern side of the main highway (N2) entering Knysna from the west.
Most of the surface area at Featherbed and further towards the west are covered in Tertiary (± 66Ma in age) to Quaternary (± 2.6Main age) dune rock and sand deposits.
Structural deformation/development.
On both sides of the Knysna heads the remnants of an anticlinal structure, dipping towards Featherbed Nature Reserve, is slightly visible within the Peninsula Formation. This anticlinal structure (Figure 1) forms part of the Cape fold belt (orogenesis) and is the result of compression from south to north. Which again was the product of continuous Gondwana orogeny defined by four surges in the earth’s crust. The third surge, which took place during the Lower to Middle Triassic (± 220 Ma) was the most spectacular and resulted in the compression and forming of the Southern Cape’s folded mountain ranges.
Figure 2 shows the development of cross joints which are perpendicular to the fold’s axis. These fractures or joints are very prominent and clearly visible throughout the area.
This was followed by an extensive period of erosion until the Late Jurassic to Cretaceous period when the rift basins were developed and the Enon Formation was deposited within and preserved. The Knysna Lagoon’s (forming part of the Knysna basin) connection or exit to the Indian Ocean was probably further to the west of the current Knysna heads. A later phase of deformation (centimeters to meters in scale), and probably Late Tertiary in age, was responsible for lateral movement with an NNW – SSE orientation.
At Featherbed Nature Reserve along the hiking trial this latest phase of deformation is beautifully exposed at the surface (Figure 3). Here it is clearly visible where the lateral movement is cutting through the joint sets as described above. In fill material of clay, probably associated with the Enon Formation, is proof that this phase of deformation was later and probably late Tertiary or Early Quaternary in age. These latter deformation zones created the pathways or weak zones for water and chemical erosion to infiltrate and eventually the development of the spectacular connection between the Knysna Lagoon and the Indian Ocean, known as the Knysna Heads.
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