Seeberg View Point
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Transcript of the plaque:
Geological Society of South Africa
The Seeberg viewpoint is located on exposures of the coarse-grained, porphyritic Seeberg Granite along the north-eastern shores of Langebaan Lagoon. The granite crystallised deep within the crust at the beginning of the Cambrian Period some 540 million years (uranium-lead radiometric dating) ago. Across the lagoon, the hills forming the northern part of the Langebaan Peninsula consist of ignimbrite, a rock made of hot volcanic ash ejected from fissures approximately 515 million years ago and deposited at such a high temperature that the edges of the constituent grains fused together.
Overlying the Cambrian bedrock in this area is a geologically young sedimentary succession of weakly cemented rocks. In the cliffs south of Kraalbaai, it comprises burrowed lagoonal sand overlain by horizontally laminated beach sand of the Velddrif Formation, which is about 130,000 to 120,000 years old (Late Pleistocene). This sequence is overlain by cross-bedded, cemented dune sands of the Langebaan Formation, which is approximately 117,000 years old and contain footprints of early modern human. Sea levels subsequently fell by as much as 130 m during the Last Glacial Maximum, when, some 20 000 years ago, ice sheets extended over much of northern Eurasia and North America. The lagoon would have been left high and dry, tens of kilometres from the sea at this time.
As the ice sheets melted and the sea rose again to a level around 3 m higher than today approximately 6-7 thousand years ago, two tidal inlets allowed flooding of Langebaan Lagoon, one south of Saldanha bay and a second west of Kraalbaai. The cliffs south of the Preekstoel, on the western side of the lagoon, began eroding away at this time, when sub-tidal channels in the lagoon hosted extensive oyster colonies. Heaps (middens) of discarded mussel and limpet shells on the peninsula, up to 6000 years old, show that this has long been a popular spot for human visitors.
The modern microtidal (tidal variation less than 2 m) lagoon has developed since the closing of the Kraalbaai inlet, around 2,000 years ago, when sea level fell to its present level. The subtidal channels shallow from a depth of 17 m beside Langebaan towards Geelbek in the south, where they are flanked by wave-rippled intertidal sand-flats that are intensively burrowed by sand prawns. The elevated supratidal salt marshes are cut by sinuous drainage channels and host a rich carpet of multy-coloured, salt-loving succulent vegetation. At high tide, the subtidal channels appear dark blue, whereas the intertidal zone has a turquoise hue, a beautiful site on a fine day, with Table Mountain far to the south on the horizon.