Rock Clefts in Rhossili Cliffs

Extreme low tides at Rhossili on Gower allow access to parts of the cliffs less frequently inspected. The distal north face of the Rhossili headland shows a complex mixture of natural and man-made features, on both a minor and major scale, often impossible to distinguish. The Carboniferous limestone was at one time quarried and this has resulted in changes of shape in the cliff face. However, most of the characteristic cracks, crevices and caves are the result of the waves and the weather working upon naturally-occurring fissures and cavities in the exposed bedrock surface.

On a small scale, it is possible to see where fractures in the Black Rock Limestone, sometimes infilled by white crystalline calcite and red haematite, are the lines of weakness along which erosion is progressing. These natural clefts are typically wider towards the outside, narrowing down to the width of the crack. The wearing away and widening of the break in the rock is initiated by mechanical and chemical action of waves and rain. Once the crevice has started to widen, it becomes a habitat for small organisms which add to the erosion process, again by both mechanical action and acid erosion.

In the photographs accompanying this post, it is possible to see some conical limpets, small acorn barnacles, and spots of bright orange sponge taking advantage of the relative shelter of these natural clefts; while minute blue-black mussel spat frequently settle (at least temporally) in the narrow confines of the crack itself. The clefts tend to retain moisture to a greater degree than the main cliff face, and may act as channels for water to drain downwards to the beach. These damp locations make ideal places for microscopic algae and bacteria to grow, and for mud-tube dwelling marine worms to thrive, especially low down, near the mobile beach sediments. Mostly, it is the former presence of the worms that is indicated. The level of the sand and pebbles fluctuates and affects the survivability of the worms. They leave a strange texture on the rock surface – composed of thousands of burrows or tunnels, just a few millimetres in width, that were incidentally etched by their acidic metabolic waste products.