Seatown Dissected Mudstone Layers

Coastal mudstone layers eroding into long fingers of rock separated by narrow sinuous channels

The rocks low on the beach at Seatown in Dorset are wearing away in a most peculiar fashion. In an earlier post I showed narrow sinuous channels cutting their way down through the mudstone between tide levels and I wondered how they had formed and what had influenced their shape. These are my thoughts and speculations about the processes contributing to these formations.

At one point along this stretch of shore, the narrow winding channels can be seen dissecting the rock layers into a number of adjoined parallel bars  (a bit like the fingers of a Kit-Kat). So what might be going on?

I have noticed by looking at the cliffs along the western and eastern shores at Seatown that there seems to be a propensity for this kind of medium to naturally form polygonal cracks or fractures once exposed to air and losing moisture. Below are three random examples of fracture patterns in cliff materials.

I think that the same phenomenon is a feature of the exposed layers of mudstone bedrock that outcrop inter-tidally. It is just possible to see the faint lines of these natural cracks in some of the close-up photographs below. Most of these original cracks are obscured because they have become the preferred location for Polydora bristle worms to occupy and create burrows. Although only a few millimetres across, the holes made by burrow-making activity have weakened the fracture lines, widened and extended them. At the same time as this bio-erosion activity is going on, continual swash and backwash by waves, and attrition by rolling gravel and pebbles, has smoothed and lowered the surface by physical destructive processes. (Chemical erosion plays a part too but will need a lot more explanation another time.)

As the combined physical and bio-erosion processes continue, the depressions where the worms burrow increase in size and can join up to form channels.

Once a channel is open, water and hard transported materials like rocks, pebbles gravel, and sand, can move rapidly through the channel in an upshore direction with each wave that breaks on the beach; and in a seaward direction as the water drains back down the shore. This physical action accelerates the erosion of the channels which speedily become deeper and wider to such an extent that they can carve the rock into distinct blocks. Smaller channels can form diagonally, at an angle to the shoreline, as they follow the conjoined outlines of the burrow-filled rock fractures. However the main force of the waves on the beach is perpendicular to the shoreline. This means that the channels formed by chance in that orientation are the ones that are most affected and enhanced by the swash and backwash of the waves.

The images also show the rock on the east Seatown shoreline is composed of alternating almost horizontal layers of pale (carbonate-rich and carbon-poor) mudstone, and darker (carbon–rich and carbonate-poor) mudstone from the Belemnite Marl Member of the Charmouth Mudstone Formation. The uppermost layers being weakened by various erosional processes that have effectively divided them up into strips, erode and break away more easily on the shoreward edge parallel to the shore.

There are more contributing factors to rock erosion on the coastline than I have been able to talk about here and I will explore them further in later posts.

All the photos are shown again below and you can click on any thumbnail to see a larger version of the image in a gallery format

Rock Formations at Langland Bay

Carboniferous limestone rock formation at Langland Bay

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.

REFERENCE

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.

Rocks at Caswell Bay

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Caswell Bay in Gower features a classic sequence of different rock types within the broader category of Carboniferous Limestone. As you walk in an easterly direction from the café and car park at the top of the beach, towards the sea with the rock outcrops on your left, you walk past a series of spectacular rock formations with marked stratification and jointing, weathering and erosion patterns, faults, thrusts and folds. A repeated sequence of Caswell Bay Mudstone, Caninia Oolite, Laminosa Dolomite, Crinoidal Limetones, and Seminula Oolite.

It is not a straightforward series because of the synclinal and anticlinal folding and thrusts – so I am still trying to fathom out which rock is which! Nevertheless, artistically and photographically there was much to enjoy and this Posting presents a range of the natural patterns and structures in the limestone. Some of the more interesting rock patterns have been photographed close-up and were shown in an earlier Posting Caswell Rock Patterns & Textures.

One of the sources of information I am using to try and understand the geology at Caswell Bay and to identify the rocks that I am photographing is the on-line Geological Society Field Guide to Caswell Bay.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

COPYRIGHT JESSICA WINDER 2013

All Rights Reserved

Cliffs at Mewslade Bay

Limestone cliff peak with raised beach behind it.

The cliffs at Mewslade Bay on the Gower Peninsula are spectacular. They are formed from High Tor Limestone from the Carboniferous Period. You can see a range of the wonderful shapes of the tall peaks and cliffs that fringe the bay in the gallery below. Click on any picture to enlarge it and to see the photographs as a slide show.

COPYRIGHT JESSICA WINDER 2013

All Rights Reserved