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 Seaton 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

Sinuous Channels at Seatown 1

Sinous channel being eroded in intertidal rock layers

Today I am mostly thinking about the way these seawater drainage channels are being formed in intertidal rock and what factors contribute to their sinuosity. They occur low on the beach at Seatown in Dorset, England, in the calcareous mudstones of the Belemnite Member of the Charmouth Mudstone Formation. More thoughts to follow later on the subject of this coastal erosion process.

Limpets as agents of coastal bioerosion

Limpets are tiny but they have a big bite. They have a tongue-like structure called a radula with rows and rows of replaceable teeth. The teeth are very strong indeed. Limpets use the radula to scrape micro-organisms from the surface of rocks for food. In the process they can also remove some of the rock surface itself where the rock is sufficiently soft. The quantity of rock which is removed in this way is small but, over great lengths of time, and given that there are so many individual limpets, it all adds up to a significant degree of wearing away of our coastal rocks. This type of coastal erosion comes into the category of bio-erosion.

Andrews and Williams (2000) describe work on the Upper Chalk on the East Sussex coast where limpets (Patella vulgata) living on the chalk shore platforms contribute to the down-wearing processes. In a series of experiments designed to estimate the rate of erosion by limpets, they found that adult limpets consume about 4.9 grams of chalk a year and that, overall, limpets were responsible for lowering the platform by an average of 0.15 mm a year. However, where limpets were particularly abundant, the rate might be as high as 0.49 mm annually. Taking into account all weathering and erosion processes, it is thought that limpets are responsible for an average of 12% of the total down-wearing in this geographical location but this can be as much as 35% of down-wearing in the areas where limpets live in higher densities. The figures obtained from this research have wider implications for the wearing away of other types of rock by limpets in other places.

The images in this post were taken on the beach at Seatown in Dorset, on the south coast of England. The rock to which the limpets are attached is calcareous mudstone that belongs to the Belemnite Member of the Charmouth Mudstone Formation of the Lias Group, and was deposited during the Jurassic Period . It comprises alternating light grey and dark grey layers which are full of trace fossil animal burrows and fossils such as belemnites and ammonites. The dark grey layers seem to be softer than the light grey ones and limpets live on both types, and on bedrock and boulders. The darker mudstone has a characteristic way of fracturing giving an almost polygonal pattern of cracks, from which small pieces easily break off, leaving regular-shaped shallow hollows across the surface. Limpets often settle in these natural hollows and further adapt them to suit their individual size and shape.

The destructive side-effect of the feeding activity of limpets is just one kind of bioerosion. Both feeding and resting habits of limpets can result in the wearing away of rocks. When the tide is in, limpets venture forth in their underwater world to feed mostly by scraping up microscopic food and sometimes by biting larger pieces from seaweed. When the tide goes out and limpets have to endure a dry world, they return to their home base to rest and batten down against moisture loss and desiccation. Limpets may take advantage of existing nooks, crannies and hollows to settle when exposed to air but, in order to ensure a secure fit as they clamp against the rock, limpets agitate and grind their conical shell into the rock surface, wearing it away to get an exact fit. Each circular home base is a depression that is custom-made for an individual limpet. When the home base is abandoned by a limpet, other younger limpets may take it over, either as individuals or in groups.

In the photographs you can see the places where the limpets have made their home bases in such hollows. There are unoccupied home bases, and re-occupied home bases as well. Surrounding many of the limpets and their home bases are typical patterns of grazing marks that trace the limpets’ foraging expeditions outwards from the base when water covers the rocks. It is also possible to see in some pictures the way that the radula teeth have actually carved into the mudstone. Burrows made by bristle worms living in mud tubes are an additional feature on the same rocks, and belemnite fossils lie close to the surface in places.

Coastal erosion & defence at South Beach, Studland

All around our British coasts we can see evidence of coastal erosion. It seems to be happening at an ever increasing rate in recent years. It is especially noticeable where the edge of the seashore is composed of softer rocks or sand dunes, for example. The coastline of Dorset in England, like many other places, is vulnerable to coastal erosion. Studland Beach near Poole Harbour is a case in point.

The shore of Studland Bay is divided into three stretches: South Beach, Middle Beach, and Knoll Beach. At South Beach, we have already seen how the burrows of small marine worms help to break up the surface of the chalk that underlies the beach in an almost imperceptible process called bio-erosion. However, the physical, hydraulic, and abrasive action of high energy destructive storm waves on the soft materials of the cliff is the coastal process responsible for immediately obvious damage with a wearing-away and break-up of the rocks and other materials on the upper margin of the seashore.

This destructive action of waves is most apparent from the number of small land slips, and trees that have collapsed to the beach, as cliff material [like London and Creekmoor Clays, and Broadstone and Oakdale Sandstones] has washed away from beach level. This has caused the undermining of the cliff deposits, and the eventual fall of material from higher up. There are a number of trees lying in a horizontal position at the base of the low soft cliffs on South Beach.

Where the ground level changes from cliff top to nearer beach level, the wearing away of the soil by the waves means that the trees now seem to be growing directly from the seashore with nothing but sand and pebbles around their roots and trunks. Just a short distance north of these beach-bound trees there are numerous beach huts on the slightly higher ground behind the shore. They are in a vulnerable position. Here the soft ground on which the huts stand is protected from the destructive force of wave action and flooding by the emplacement of stone-filled wire cages known as gabions. These are stacked to form a barrier wall of harder material that is more resistant to erosional coastal processes.

Small holes made by marine worms in Studland Chalk bedrock

In the corner of South Beach at Studland in Dorset, where the chalk cliffs that lead to Old Harry Rocks meet the seashore, the Studland Chalk Formation bedrock extends over the beach as a flat wave-washed platform. The smooth white rock surface is exposed at low tide but is frequently covered by sand, pebbles and decaying seaweed. On a recent visit a lot of the weed had cleared and I was able to observe the chalk platform closely. I realised that it is riddled with small holes and tunnels made by marine polychaete worms.

The holes on the surface of the rocks are roughly dumb-bell shaped and a few millimetres across. You can tell the worms are still living and occupying the burrows because the combined mucous and mud tube-linings remain intact. In locations where the rock has broken, the shape of the tunnels leading down into the rock from the holes on the surface is revealed. The tunnels or burrows are approximately U-shaped. The worm lies in a doubled-up position in the burrow with both the head and the rear end at the rock surface. When the tide is in, and water covers the burrow, the worm protrudes and vigorously agitates its two long, thin, ciliated palps (feelers) to gather particulate food floating by. Waste matter is expelled into the water but it is probably the overall acidic environment created by the metabolic waste products that gradually dissolves the calcium in the rock to create the burrow.

The accurate identification of these worms is problematical since the most diagnostic parts are usually discarded by the animal as soon as the creature is extricated from its burrow. However, it is likely that they are bristle worms of the Spionidae family, probably the Polydora genus, and possibly Polydora ciliata (Johnston).

Studland Beach Finds

Some of the things that caught my eye as I walked along the beach at Studland in Dorset, England, included interesting beach stones; stranded clumps of red, green, and brown seaweeds; an empty shell of a clam just eaten by a bird; and tubes of Sand Mason Worms.

Studland Soft Seaweeds

Brightly coloured seaweeds were washing ashore at Studland Bay in Dorset on 21st May 2017. Isolated clumps of vivid red, green, and brown soft seaweeds, that looked attractive floating in the clear shallow water, or scattered individually on the yellow ribbed sand, soon accumulated into thick solid multi-coloured mats undulating on the water’s edge. When a mat of algae like this is pushed high with the rising tide, and left stranded on the upper beach, it decays rapidly to become what the human eye perceives as a rather smelly, ugly mess. For every other organism large or small on the beach, rotting seaweed is a marvellous bonanza of food and shelter, which also helps to stabilise the sandy beach for further colonisation by plants.

Seatown Beach Boulders

As you walk east along the shore at Seatown in Dorset, you reach Ridge Cliff from which numerous boulders have fallen over the years, and accumulated across the beach and into the water. What is most interesting is the great variety of shapes, colours, textures, and compositions. They represent all the different strata that make up the 80 metre high cliffs.

Seatown Shattered Eype Clay

The 80 metre high cliffs on the east shore at Seatown in Dorset along the Jurassic Coast are subject to land slips and rock falls. Large lumps of shattered blue-grey clay are common on the beach. They come from cascades of Eype Clay Member material that forms the lower part of the cliff exposures.

Seatown Rock Crystals

A large rock that had rolled down from the top of the cliffs at Seatown in Dorset was lying on the pebbles of the beach. It was yellow and rusty coloured. At this point along the shore, called Ridge Cliff, the rocks belong to the Dyrham Formation of the Liassic/Jurassic period. The lower section of the cliff is the Eype Clay Member of pale, blue-grey micaceous silty mudstone and shale. Above that is the Down Cliff Sand Member mostly of silts and fine sands with thin lenticles of hard calcareous sandstone. On top of this is the Thorncombe Sand Member with yellow-weathering, heavily bioturbated sands, with several horizons of large rounded calcareously cemented concretions. This boulder obviously came from one of these upper sandstone layers but I cannot say which one. Its broken edge revealed lovely abstract patterns and beautiful crystals.