Water-worn Limestone 3

Natural sculpturing of limestone on the Worms Head Causeway in Gower, South Wales.

You would think that all the limestone strata on the Worms Head Causeway in Gower would be worn down equally to a smooth, flat, even surface – but not so. Upstanding at various points on what I suppose is really a wave-cut platform (albeit eroded by acid rain and seashore creatures as well), isolated areas remain standing. They look like giant teeth embedded in the worn surface strata. I do not know why these areas are more resistant, however, I have read that some parts of the limestone become harder by dolomitisation, a process in which the calcium carbonate is converted to magnesium carbonate by the intrusion of seawater (I think before the original sediments harden and compact). Maybe that is the explanation.

Water-worn Limestone 2

Weathered limestone rock layers on the Worms Head Causeway in Gower, South Wales.

The limestone further east along the Worms Head Causeway shore, towards Tears Point, displays the results of a number of erosion agents leading to some curious formations. The clean, smooth surfaces of mounded and hollowed shapes result from mechanical abrasion where the rock is pounded by stones carried in the waves; by chemical and physical erosion caused by micro-organisms and marine invertebrates (bio-karst surfaces made by such organisms as lichens, limpets, and sea urchins); and acid dissolution by rainwater when the tide is out, especially around the edges of pools, in natural fissures like joints and bedding planes, and areas where water constantly drains – resulting in what is called karst topography. Small circular pits (image 48) of dissolved limestone readily connect with each other, soon enlarging into bigger pools that are known as kamenitzas – which in turn can interconnect with other pools as seen in the images below (particularly images 49, 50, and 51).

Water-worn Limestone 1

Water-worn limestone rock layers on the Worms Head Causeway in Gower, South Wales.

The Carboniferous sedimentary strata outcropping on the landward shore of the Worms Head Causeway at Rhossili show differential erosion by the sea. Some areas of the Black Rock Limestone Subgroup are clean, smooth and worn down whilst others are sharp and jagged with encrusting biofilms and barnacles. This is partly due to the varying compositions and relative hardness of the different strata, and partly to the way in which the waves with their rock-bearing loads seek lines of least resistance in the shore with each tidal ebb and flow. Areas of weakness, for example, between bedding planes and in minor faults with veins of soft white crystalline calcite and red haematite, are more vulnerable to repeated abrasion. This has led to the formation of numerous channels, gullies, and basins among other more resistant rock outcrops. Rounded pebbles and cobbles frequently lying within the hollowed areas evidencing their role in wearing the bedrock away. Mechanical abrasion allied to varying rock resistance is not the only way that the limestone is altered. Elsewhere on the causeway, limestone acid dissolution and marine organisms are the most common agents of natural change in surface texture and sculpturing, creating karstic and bio-karstic limestone topography.

Velella velella (By-the-wind Sailor) at Rhossili

By-the-wind sailor (Velella velella Linnaeus) on the beach at Rhossili, Gower, South Wales.

By-the-wind Sailors (Velella velella Linnaeus) have been washing ashore at Rhossili Beach over the past week. The sandy strandline has been dotted with dozens of the dark blue jellyfish-like creatures with their characteristic translucent sails. They are not actually jellyfish but pelagic (free-floating) colonial hydroids. [See also the earlier posts about this marine invertebrate in Sailing by Rhossili Bay and Bright Blue Blobs on the Beach.]