WALK REPORT

Beach GeoHike with Dr John Rogers

– Merrilee Berrisford

Icy wind blew along Silversands Beach, chilling to the bone assorted naturalists, amateur geologists and just beach people following John the geologist. Anyone know how to recognise a yardang? It’s a coppice dune eroded (deflated in geology speak), and there they were! Barbara Jenman and I thought we’d never noticed them before. Were they the work of the storm? And a coppice dune is a small dune on its own, with grass growing on the top.

Walk Report John Rogers

Then there was the elaborate wind-etching on the side of a dune. At first sight it seemed to be the work of children having fun. John said this pattern could be seen in future sandstone.

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Ever wondered how plants on dunes get enough moisture? Well quartz doesn’t transmit heat; the sand is quartz so the sunshine heats and dries only the shallow top layer.

What about movement of sand dunes? The size of the grain of sand/quartz is critical and it seems that dune sand grains at a diameter of ¼ mm are just right for excellent mobility. At this point John quoted:  “’See the world in a grain of sand’ – who wrote that?” “William Blake” said Karin Fry, quick as a wink. Afterwards she confessed that she’d seen it in a Veld and Flora!

Afternoons are the best time to watch dune movement. Our sand was still damp from rain, but where it had dried, grains were flowing downwards at the soft touch of a moving grass. No wonder our dunes march across the beach.

Individual rocks sticking out of the sand were, unsurprisingly, sandstone – belonging to the Peninsula Formation of the Table Mountain group. Patterns in the rocks show cross-bedding and parellel laminations, giving clear clues as to how they were formed in layers.

Then we came to the rocky outcrop at the western end of Silversands, Aasbank to the fishermen. Quite a different scene. Here all the rocks have, as John says, just been dumped, no layering; it is darkish brown with pebbles embedded, and strips of quartz, some quite wide.

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This is Pakhuis Formation tillite (glacial diamictite for the experts) and is not usually found at sea-level. In Betty’s Bay you find it at the top of the Leopard’s Kloof waterfall. Downfaulting has resulted in Pakuis Formation running from Aasbank to Rooi Els  – see if you can spot it.

There was much more, the effect of wave action on the beach; swash, backwash, rips; kelp holdfasts raising stones. Another GeoHike another year will be great.