It was exciting to discover all the caves at South Beach in Tenby. The rock layers of the cliffs, which were originally laid down in horizontal layers at the bottom of ancient seas millions of years ago, have been subsequently pushed on-end by earth movements so that they now lie at very steep angles to the vertical. The waves have worked away in weaker areas between the strata and excavated small caves. I couldn’t wait to see inside them. They were variable in size but larger than I expected. Well worth exploring.
The floors were mainly sand, smoothed by the previous high tide. Sometimes pebbles were piled up against the back wall. I was mostly struck by how different they looked from one cave to the next. Some cave walls were almost polished, smooth, pale grey limestone, revealing irregular streaks of white calcite veining, occasionally with fossils. Others were roughly hewn with multiple broken facets.
Most intriguing of all were the mosaics of bright green and deep red organic encrustations coating some walls. I couldn’t work out the rationale for their seemingly ad hoc distribution. I am not sure what they are. Maybe they are cyanobacterial bio-films rather than encrusting algae – because of the location in which they are growing so high on the shore and away from light.
[There are in fact encrusting dark red forms of alga but these seem to be restricted to low shore situations in shallow water. Identification of these kinds of organisms is difficult, because they are not a distinct taxonomic group but are represented by a variety of different genera; and maybe I need to take some samples for examination under the microscope].
The pale grey Hunts Bay Oolite Subgroup limestone of the most western stretch of South Beach, which has most of the caves, eventually gives way to other rocks further east – like the Caswell Bay Mudstones which are more thinly bedded with a variety of colours and textures, and these house perhaps the largest cave – the last one of note before you reach Castle Beach and Castle Hill that act as a divider between South Beach and North Beach in Tenby.
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