GEOLOGY of Fernkloof
The geological history of the southern tip of Africa is complex. It started about 540 million years ago with the intrusion of Cape granite into older rocks such as the Malmesbury shales. During the ensuing millions of years there were periods of subsidence under extensional (pulling apart) tectonic forces, and periods of uplift under compressional (collisional) tectonic forces. These forces pushed the African continent around, so that at times it was placed at warm and humid low latitudes near the equator while at other times it moved nearer to the South pole under a thick cover of ice.Deposition of a seven km thick pile of muds and sands from eroding highlands into the Agulhas Sea led to the formation of the c.450 year old Cape Supergroup. Burial and compression of these sediments formed the hard sandstones and shales of this important system. By the end of the depositional period, which was brought on by a change from an environment of stretching, thinning and subsidence of the crust, to one of compression and crustal thickening, these rocks were subjected to intense folding and fracture. The results can be seen in the unique folds for which the Cape Fold Mountains are named.Today we see these sandstones in the Overberg area, occurring either as mountains, or as cliffs along the coast. Differential erosion shapes the surface so that we have resistant rocks forming the highlands, as in FNR, whilst the valleys result from the removal of more easily eroded material. The mountains of the FNR represent remnants of the basal portions of the Cape Supergroup. Faults occur throughout the area and can be seen as linear breaks which cut through the folded sandstones. These faults are being exploited by Hermanus for groundwater, which is used to supplement the surface flow collected in the De Bos Dam.The town of Hermanus occupies a flat plain between the mountains and the sea. This is a wave-cut platform and is clear evidence of an earlier (Tertiary) high stand of sea level. Subsequent lowering of sea level led to the formation of the cliffs which dominate our coastline and which are so popular with fishermen and the whale watching community.Erosion of the sandstones produces the exceptionally poor quality soils (often little more than sand) that characterise our area and, together with the prevailing Mediterranean climate, shape the fynbos biome. Within valleys and in selected pockets, soils with a higher clay content occur and these more nutritional environments give rise to denser vegetation and, occasionally, forests.
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