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.
Here are some more pictures of the boulders at the eastern end of Charmouth Beach in Dorset, England, all exhibiting natural fracture patterns in sedimentary rock belonging to the Jurassic Charmouth Mudstone Formation. I’m not sure which particular layer they come from but it could be the Black Ven Marl Member. Perhaps someone can help me out with the identification? These images show the boulders at the foot of the cliff adjacent to the landslip or mud slide. In contrast to the dark boulders at the water’s edge shown in the previous post, these are dry and therefore lighter in colour.
I wonder if these boulders could have been the inspiration for an artwork in the sculpture park in Tout Quarry on the Isle of Portland featured in an earlier post.
The shoreline at Charmouth looked particularly dramatic on this April visit as storm clouds periodically burst and blue skies were only intermittent. Charmouth Beach lies on the World Heritage Jurassic Coast in Dorset, England. The rocks are mainly Jurassic Period Charmouth Mudstone Formation. The character of the cliffs changes as you walk from west to east because the sedimentary rock layers gently slope and disappear beneath the beach surface level while new rock strata are freshly revealed at eye level. The predominance of softer rocks has led to a great deal of cliff slippage, and this means that the chronological sequence of the layers is frequently obscured by fallen debris; it makes it difficult to distinguish which rocks are which. The numerous rockfalls regularly contribute to the boulders on the beach and in this post I feature some boulders that exhibit some interesting fracture patterns. Of course these are not the only rock type on the beach, and I will post some more photographs of other patterns and textures in boulders and in the cliff face on the eastern half of Charmouth Beach in due course.
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.
There is an island called Spencer’s Island just off shore in the north Minas Basin in Nova Scotia, Canada. The same name also applies to the small community on the mainland opposite the island. It is the rocky outcrops along the beach of the mainland that feature here. While the island itself is mainly igneous basalt extruded in the Jurassic period, the steeply dipping strata along the mainland shore are composed of Carboniferous period Cumberland Group rocks (approximately the same type as are found at the famous Joggins Fossil Cliffs further north).
At the moment, I have not managed to find information to explain the details of these rocks but they certainly are interesting to look at. I know that they are composed of sandstones, siltstones, and shale (I didn’t notice any conglomerate or coal which are typical of this rock group). They are rapidly wearing away. They are severely impacted by strong tides and currents especially over the winter months. The outcrops reveal multiple layers of sedimentary deposits which occur in a cyclical way with harder more solid bands alternating with softer finely laminated ones. This reflects the depositional environment which was changing on a regular basis. All the rocks are subject to fine fracturing on a massive scale so that the slightest touch would cause the crumbling fragments to cascade to the beach. Sometimes the weathering process has led to falls of large boulders. Sometimes the outcrop is reduced to fine scree.
It was a very cold, dull, windy, wet day in late May when we visited Spencer’s Beach and we were very glad to discover a delightful beach cafe where we filled up on coffee and clam chowder. The beach was once a busy ship-building yard – famous as the place that built the Mary-Celeste.