Broughton Bay is a wide sandy expanse on the north shore of the Gower Peninsula in South Wales, facing the Loughor Estuary or Burry Inlet. A small promontory called Twlc Point at the western end of the beach has an interesting geology with an exposure of Hunts Bay Oolite from the Carboniferous Period. I have written about these strata in earlier posts such as:
On this particular visit I was content to appreciate the way that pebbles of many types and colours on the upper shore were clustered around outcrops and boulders of the limestone which were often pink-tinged and sometimes fossiliferous.
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).
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.
Little Tor cliff at the east end of Oxwich Bay in Gower, South Wales, is made of Carboniferous Limestone of the Hunts Bay Oolite Sub Group. In common with beach outcrops of the same type of rock at Broughton on the north Gower coast, and Tenby that lies further west in Pembrokeshire, the surface is marked on a small scale with scalloped depressions and branching runnels that are the result of acid erosion and sand abrasion, giving rise to interesting textures and patterns.
The small sinuous etchings are called microrills (Ford and Williams 2007). They are typically 1 mm wide, round bottomed dissolution channels that are found close together. The pattern is reminiscent of rain running down a window pane. On gentle rock slopes they have curving paths and divide and rejoin in a network-like pattern. On steeper gradients the channels are straighter. Some microrills are made by slightly acidic water flowing down the rock surface but in other instances they are caused by the “water moving upwards, drawn by capillary tension exerted at an evaporating front. Capillary flow is believed to explain much of their characteristic sinuosity”.
Ford, D. and Williams, P. (2007) Karst Hydrogeology and Geomorphology. John Wiley & Sons, Chichester, England. Revised Edition, p324. ISBN 978-0-470-84997-2.
The magnificent limestone outcrop that separates Three Cliff Bay from Oxwich Bay on the Gower Peninsula in South Wales is known as Great Tor. It is composed of steeply dipping layers of Carboniferous Period sedimentary rock called Hunts Bay Oolite Sub Group.
A further assortment of interesting rock patterns and textures in Carboniferous Limestone strata of the cliffs on the east and west sides of Threecliff Bay on the Gower Peninsula in South Wales. Of particular interest is the honey-comb texture of erosion along the bedding planes of some of the dipping layers, and the red colouring due to iron bearing minerals of other areas, on the east side of the bay. On the west side of the bay the rocks seem less weathered and are often encrusted with patches of black lichen.
This post provides a context for the earlier post of mostly close-up images in Rock Textures at Langland Bay 1. Langland Bay is a popular beach near Swansea in South Wales. It is located on the south coast of the Gower Peninsula. It has a wide stretch of lower sandy shore, and pebbles landward in the central part. There are also wide expanses of low-lying rock platforms with hundreds of shallow pools in which to hunt for seashore creatures. To each side of the bay low cliffs of Carboniferous period sedimentary rock are overlain with much more recent glacio-fluvial and later poorly consolidated deposits.
The distinct layers of Hunts Bay Oolite, High Tor Limestone, and Penmaen Burrows Limestone form the southwest limb of the Mumbles anticline that extends from east to west. Here at Langland the sea has breached the rocks to create the embayment. The strata are riven by numerous joints and minor faults that have allowed the sea to penetrate, eroding away the rock to form small coves, undercuts, caves, tunnels, and passages to explore. The photographs shown here probably do not do justice to the site, as it was a very dull and overcast day when I visited, but I hope they serve to illustrate that the geology of Langland Bay is interesting from many points of view.
Bridges, E. M. (1997) Classic Landforms of the Gower Coast, Series Editors R. Castleden and C. Green, The Geographical Association and The British Geomorphological Research Group, page 17. ISBN1-899085-50-5.
Langland Bay on the Gower Peninsula was new to me. In all the years that I have been travelling to Gower I had not previously paid it a visit – perhaps because it is one of those popular beaches close to Swansea where visitors tend to throng – and I like to have the beach to myself! On this particular dismal August day the visitors were few and far between but it was clear what a delight the beach must be on better days.
A wide expanse of sand is bordered by low cliffs and rocky outcrops of Carboniferous High Tor Limestone and Hunts Bay Oolite. The bay has been eroded into the seaward-dipping rock layers of the Mumbles anticline and there are many fault lines crossing the strata. Consequently, veins of red haematite and white calcite abound along with patches of brecciated rock. Langland is best known to geologists for the sequences of glacio-fluvial and later, mostly unconsolidated, deposits that lie above the limestone. However, for myself, it was the wonderful maze of small pebble-floored coves, arches, and caves that held the interest with their wide variety of patterns and textures in the rocks, and the natural fracture patterns dissecting weathered surfaces.
Threecliff Bay on the south coast of the Gower peninsula in South Wales is one of the most beautiful and interesting locations. The scenery is spectacular and the three rocky peaks that give the bay its name are clear to see. The Pennard Pill river follows great meandering loops as it approaches the sea and it flows down a valley created by weaknesses along a tear fault that skewed the alignment of the rock strata. The rocks on the east side of the valley do not line up with those on the west side. The strata in the east have been moved northwards.
The pictures in this post were taken where those displaced rocks outcrop in cliffs on the east side of the bay. They are composed of Carboniferous Limestone. I think they are from the Black Rock Limestone Subgroup – the only available geology map has out of date nomenclature for the various rocks types and calls this section of strata Penmaen Burrows Limestone (d1b). What fascinated me was the wonderful red tinge in the rocks due to the iron content and the intricate natural patterns of discontinuous white veins of calcite. I wonder if these veins are something to do with the pressures and heat resulting from the northwest to southeast tear fault that defines the valley. It looks as if a first set of cracks was infilled with calcite before a second set, cross-wise to the first, was formed in a subsequent event that generated yet more pressure and heat.
The rocks at Fall Bay are arrayed like the riffled pages of a book. Layer after layer of Carboniferous Limestone is sequentially spread out across the west side of the bay. Each layer has an observably different texture; some are bioturbated with bioclasts and fossils such as fragmentary crinoids and corals. The bedding planes of some strata have deeply sculptured surfaces from weathering and bioerosion. Lichens, barnacles and limpets colonise the rocks and take advantage of the meagre shelter offered by cracks, crevices, and solution hollows.