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.

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).