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.

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.

Rocks & Pebbles near Twlc Point

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:

Rocks on the west side of Broughton Bay Part 1

Rocks on the west side of Broughton Bay Part 2

Rocks on the west side of Broughton Bay Part 3

Brachiopod fossils in Hunts Bay Oolite at Broughton Bay

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.

Flints Embedded in Studland Chalk

Flint nodule embedded in chalk

Flint nodules embedded in the low cliff between two minor faults in Studland Chalk Formation exposures at South Beach on Studland Bay, Dorset, England.  At the top of the cliff face is a layer of ironstone and iron-stained flints that has caused the rusty stain on the chalk below. Elsewhere the rocks are covered with a fine coating of green algae.

A couple of useful references for the geology of the area in which these photographs of the chalk and flints were taken:

Barton, CM, Woods, MA, Bristow, CR, Newell, AJ, Weathead, RK, Evans, DJ, Kirby, GA, Warrington, G, Riding, JB, Freshney, EC, Highley, DE, Lott, GK, Forster, A, and Gibson, A. 2011. Geology of south Dorset and south-east Devon and its World Heritage Coast. Special Memoir of the British Geological Survey. Sheets 328, 341/342, 342/343, and parts of 326/340, 327, 329 and 339 (England and Wales), 9–100.

Cope, JCW, 2012 Geology of the Dorset Coast, Geologists’ Association Guide No. 22, Guide Series Editor SB Marriott, The Geologists’ Association, 191-194. A serious guide for the more dedicated amateur and professional.

Ensom, P and Turnbull, M 2011 Geology of the Jurassic Coast, The Isle of Purbeck, Weymouth to Studland, published for the Jurassic Coast Trust by Coastal Publishing, ISBN 978-1-907701-00-9, pages 96-117. A beautifully illustrated beginner’s guide to the geology of the area – one of a series of excellent publications by the Jurassic Coast Trust.

Swanage Solid and Drift Geology (map), British Geological Survey (Natural Environment Research Council) 1:50,000 Series, England and wales Sheets 342 (East) and part of 343

Flint nodule embedded in chalk

Low chalk cliff with row of embedded flints

Flint nodule embedded in chalk

Line of flint nodules embedded in chalk

Flint nodule embedded in chalk

Line of flint nodules embedded in chalk

Flint nodule embedded in chalk

Line of flint nodules embedded in chalk

Flint nodule embedded in chalk

Flint nodule embedded in chalk

Patterns in Pyroclastic Breccia near Louisbourg

Angular rock fragments embedded in a volcanic ash matrix from a pyroclastic flow in Cape Breton Island

The entire coastline north and south of Louisbourg on Cape Breton Island in Nova Scotia, Canada, is composed of Neoproterozoic volcanic rocks dating back 575 million years. A few hundred metres north of the Louisbourg Lighthouse along the Trail to Morning Star Cove and Gun Landing Cove, lies an area of seashore that offers the chance to take a close-up look at the compositions and natural patterns in rock made of pyroclastic breccia.

Pyroclastic literally means ‘fire-broken’ and is used to describe volcanic rocks made up of fragmented pieces that are normally the result of an explosive volcanic event. Clasts are pieces of broken down rock. According to the Oxford Dictionary of Earth Sciences edited by Michael Allaby (ISBN 978-0-19-921194-4) “breccia is a coarse clastic sedimentary rock, the constituent clasts of which are angular. Breccia literally means rubble and implies a rock deposited very close to the source area. The term may also be applied to angular volcanic rocks from a volcanic vent.

Rock of a similar type of origin, although not identical, has been used by the sculptor Emily Young in the creation of the carved heads that were recently on display on Neo Bankside in London. Stillness Born of History II is described as being made of “onyx with volcanic pyroclastic brecchia”.

Stillness Born of History II at Neo Bankside

Carved stone head by Emily Young displayed at Neo Bankside in LondonBeautifully textured and patterned onyx with volcanic pyroclastic breccia has been used by the famous sculptor Emily Young to create this fabulous head called Stillness Born of History II displayed (courtesy of Bowman Sculpture) at Neo Bankside in London, England, just south of the Tate Modern Gallery. Pyroclastic breccia is composed of fine-grained volcanic ash, pumice, and rock fragments larger than 2.5 inches (63.5 mm). When the fragments are smaller than this, the rock is called tuff.