Contact!!
VulcanSpirit
Richard & Alison Brunstrom
Sun 3 Jan 2016 16:20
Under Lion’s Head, on the beach in the posh Cape Town suburbs, lies one of the wonders of the geological world - the Seapoint Contact. This site was first described in 1818, and visited by Charles Darwin in 1836. Darwin later wrote “A man must for years examine for himself the great piles of superimposed strata, and watch the sea at work grinding down old rocks and making fresh sediment before he can hope to comprehend anything of the lapse of time, the monuments of which we see around us” 'On The Origin Of Species', 1859. At Seapoint metamorphosed sedimentary rocks of the Malmesbury Group are exposed on the shore. These are 580 million years old, and cover a huge area around Cape Town. About 540mya a huge upwelling pluton of magma burst into these rocks, melting them and disturbing their structure. The boundary between these two types of rock is starkly evident on the beach, and even casual observation shows some of what must have happened. This site played a significant role in the development of modern geology. Here I am, standing on the yellowish rounded granite:  Just yards to the north, the beach is composed of dark flat shales:  Here is part of the boundary. This is not a fault, it’s a contact point, the edge of the pluton butting up against the shale into which it has intruded (deep underground when this happened, and now exposed by erosion, uplift etc):  The actual boundary is quite complicated, with magma melting the shale, breaking bits off and enveloping them to form xenoliths. Here are a couple:   An amazing sight/site, and one which is known to have been influential on Darwin. “Shortly" after this event, at about 510mya the huge sedimentary formations of the Table Mountain Group began to be laid down. The Malmesbury shales and the granite were worn flat by erosion and Table Mountain laid down horizontally on top of them. Above Table Mountain (1080m) were four further formations now all eroded away except for traces of some ancient glacial tillite(!) on the summit plateau. Truly like this one can begin to comprehend the lapse of geological time. |