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

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.

Rip-Rap Rocks at Seatown

View looking west at Seatown beach, Dorset, England, showing rip-rap sea defences in foreground

The cliffs are eroding in many places along the Dorset coast, particularly where the rocks are soft. This results in land slips and mud slides. It has always been going on but in recent years the erosional processes seem to have accelerated along with changing weather conditions. At Seatown on the coast near Chideock in Dorset, large boulders have been imported to protect the shore from the sea adjacent to the Anchor Inn that sits at the mouth of the River Char. I cannot name the rock types represented in the rip-rap for certain since they are not local to the area and have been chosen specifically because they are harder and more resistant than the cliffs on this beach. I am not even sure that they were quarried in Britain. However, some of them remind me a lot of Carboniferous limestone with fossils, calcite and haematite inclusions. Anyway, they are really interesting and well worth a closer look. The patterns, colours, and textures are amazing. I would like to visit again when the rocks are wet and the more subtle colour variations would be highlighted.

Sand Dune Erosion at Whiteford

Sand dune erosion at Whiteford Sands on the north Gower coast in South Wales

Shorelines evolve. Changes happen – sometimes slowly and sometimes quickly. The winter of 2013 to 2014 brought severe storms and winds that impacted on all our British coastlines. Whiteford Sands on the north shore of the Gower Peninsula in South Wales was no exception. By May 2014 a dramatic change in the long line of dunes bordering the sands was clear to see. The dunes have been fixed for a long time with the outer slopes stabilised with marram grass, and a turf covering further inland. Small changes had been occurring steadily for many years with a gradual wearing away of the dunes.  High tides and extreme weather events had been nibbling at the seaward faces. The erosion process has not been continuous but interspersed with periods of accretion both by water-borne and wind-borne sand.

The sand was originally deposited by a melting ice sheet in Carmarthen Bay, including the Loughor estuary on which Whiteford Sands is situated, and at Pendine Sands and Rhossili Bay. The first direct evidence for glaciation on Wales dates to 480,000 years ago in the Anglian Stage of the Pleistocene in the Quaternary Period when ice sheets enveloped Wales and the adjacent sea. The last ice coverage in the region was the Late Devensian Era about 24,000 years ago. There seem to be only minimal additions to the sand deposits from local sources since then because the Carboniferous limestones of the area dissolve rather than disintegrate into particles or grains. The sand is a therefore a finite resource albeit one that is controversially exploited locally by dredgers on the Helwick Bank just off the tip of Gower.

The sand is basically mobile within the area on the shorter and longer timescales. A useful and interesting research report on this subject is that by V J May on Carmarthen Bay in the Geological Conservation Review in which the sediment transport around the region is discussed. Figure 11.12 presents a sketch map of the key geomorphological features and sediment transfers of Carmarthen Bay. Figure11.13 depicts variations in accretion and erosion since 1950 in Carmarthen Bay. Figure 1.17 illustrates geomorphological features of Rhossili bay and Whiteford Burrows. The report records how and in which directions the sand is being shifted by river/estuarine currents, onshore and longshore drift; and where attrition and accumulation of sand is most marked. It is an intriguing read and gives much to elucidate the field observations I have been making in the area over the last decade.

Hopewell Rocks, New Brunswick

Red cliffs at Hopewell Rocks in New Brunswick, CanadaYou don’t have to be a rockhound to be impressed by the spectacular scenery at The Hopewell Rocks. Tall cliffs of sloping red strata rise high above the Bay of Fundy shore, with an abundance of naturally worked shapes, caves, arches, and free-standing pillars of rock called sea stacks. At high tide, people can kayak around the stacks, also known locally as “Flower Pots” because of the groups of full-grown trees that grow on top of them – as they also do right to the cliff edges, with their root systems often clearly visible.  At low tide, it is possible to descend a staircase to the ocean floor itself and explore these geological phenomena close up. Viewing time on the seashore is limited by the enormous and potentially dangerous rise and fall of the tides in this narrower northern neck of the Bay, where in some places, and at certain times, the sea can rise by as much as 56 feet.

At one time, about 600 million years ago, this part of Canada’s New Brunswick Province started its life near the Equator. Here it was subjected to uplifting earth movements that incorporated it into the Appalachian Oregon, an ancient mountain chain that now stretches from New Foundland to Florida. By 360 million years ago, the Appalachian building activities had ended and were followed by predominantly erosional processes.

The rocks exposed at Hopewell originated specifically in that part of the Appalachians called the Caledonian Mountains. Erosion by water and wind about 350 million years ago, in the Lower Carboniferous Period,  steadily wore down the mountains, creating massive volumes of boulders, stones, gravel, sand and mud. Near the highland areas, flash floods tore through the valleys and canyons, washing away loads of eroded sediment and depositing it as stony and gravelly debris. Further from the highlands, sediment formed alluvial plains with sorted layers of sand and mud. The region covered by these terrestrial deposits in present day Atlantic Canada is called the Maritime Basin.

Over time, the coarser material in the erosion deposits on the flood plain became consolidated and cemented together with finer sand and silt. Because the land lay near the equator, the climate was hot and dry. Iron-bearing minerals became oxidised, and the rocks turned into redbeds. The series of red rock layers is now known as the Hopewell Cape Formation; this is the rock exposed in the cliffs and sea stacks at Hopewell today – eventually brought to its current position by Continental Drift, the tectonic movement of continental crustal plates.

In the first instance, the variably-textured sedimentary strata were deposited in horizontal layers. However, earth movements tilted them to angles between 30 and 45 degrees. The tilting of the rocks caused horizontal cracks to form parallel to the bedding planes, and also vertically at right angles to the strata. These lines of weakness in the rocks have become the points of entry for weathering agents – glaciers, tides, snow, ice, and winds. Erosion by these forces widens the cracks and steadily works away at the softer horizontal strata. The expansion of water as it changes to ice is a significant factor in the enlargement of cracks and crevices, and the breaking up the rock. Sandstone is softer than the conglomerate and easy for waves to wear away. The overall result is that broad columns of rock are carved into the cliff face. Undercutting at the cliff base creates caves and arches. Eventually, some columns are completely separated from the cliff face and become sea-stacks or “flower pots”.

Redbeds of alternating tilted layers of conglomerate and sandstone from the Hopewell Cape Formation of the Lower Carboniferous Period in Canada.The erosion activities are on-going. Extreme weather events and storms of recent years may accelerate the processes. The cliff face is gradually receding. Sea stacks eventually collapse and new ones are formed. A sea stack can last as little as 100 years or as long as a thousand. However, there is no need to panic about seeing the sights at Hopewell as soon as possible for fear that they will all disappear into the sea – geologists have calculated that there is enough conglomerate in the Hopewell Cape Formation to make “flower pots” for the next 100,000 years.

Erosion of the tombolo at Dogs Bay

View of Dogs Bay, Connemara, Ireland

Looking northeast from the offshore island towards the white calcareous sandy beach and dune system, known as a tombolo, that connects the island to the mainland at Dogs Bay in Connemara on the west coast of Ireland.

Dogs Bay is a famously beautiful sandy beach in Connemara on the west coast of Ireland. The beach is actually part of a geomorphological feature known as the Dogs Bay/Gorteen Bay tombolo. A tombolo is a spit or bar of sand or gravel connecting an island to the mainland or another island. In general terms, persistent winds from the southwest have meant that waves meeting the island and wrapping around it, slow down as they converge on the northeast and landward side of the island, where force and speed of the waves decreases and they deposit their load of sediment. Over time the sediments gradually accumulate to such an extent that they rise as a bar above the water. The sediment bank eventually stretches all the way from the island to the mainland, and the connecting bar is termed a tombolo.

The sand has become stabilised by the growth of vegetation; and at the present time is a very special and rare type of habitat known as machair. Machair only forms on calcareous soils. At Dogs Bay the sand is  composed mainly of minute fragments of the carbonate skeletons of marine animals such as sea urchins and their spines, sponge spicules, bryozoa, seashells, snails, and most remarkable of all, the intricate microscopic skeletons of one-celled creatures called Foraminifera. Dogs Bay is one of only a few beaches in the world with predominantly Foraminiferan sand.

Machair, and the surface of the Dogs Bay tombolo, is unlike many of the coastal dune systems that I have visited in England, where the dunes are full of peaks and troughs, and where marram grass dominates. Marram is often a major initial factor in the stabilisation of the loose grains. Here, however, a grassland vegetation of low species diversity is encouraged to grow in a moist, cool, windy, oceanic climate on the fairly level and compacted alkaline soil of a mature sand dune system, and grazing by animals is vital to the maintenance of the habitat. Useful information about the features in this area is available from an on-line field guide produced by the Irish Quaternary Association. Click here for details of tombolo formation and machair habitat (pages 13-17).

Arriving at Dogs Bay, it was clear to see the impact of the earlier winter waves. Storms in the first few months of the year had ripped up and washed away the road and the car park at the entrance to the beach. A sign post now lying on the shore showed where it had been. On scrambling down to the beach, a close inspection of the wonderful curve of the dunes at the top of the shore revealed that the leading edges had been sliced away leaving hanging sheets of machair turf and huge clumps of vegetated dune material on the shore. Wooden posts, perhaps fencing from the top of the dunes delineating boundaries and preventing grazing animals from falling over the edge, were lying loose on the beach below, sometimes apparently supporting  the dune-top hanging mats of vegetation.

The tombolo is a vulnerable feature of the landscape. It seems that there is a history of natural damage to it in this location. In pictures 4 and 5 of the gallery below, the cross-section through the eroding dune shows a narrow horizontal dark brown band about half way down the vertical surface. This is a richly organic ancient soil level (palaeosol) that is associated with archaeological remains such as shell middens and the remains of a settlement, showing that people in the past used the site and exploited its marine resources to augment their diet. The palaeosol  is present at each end of the line of dunes but is absent in the central part. Its absence in this part could indicate that either the dunes at that time in that place were not stabilised by vegetation, or that the centre of the tombolo has been severely eroded in the past and recovered from the damage.

Apparently, about ten years ago, local people feared that the tombolo would be totally breached. Maybe it was at that time that steps were taken to prevent destruction of the spit. It is clear from the presence of rock-filled metal gabion cages on the beach that conservation measures were in place prior to last winter’s storms that battered much of the coastline around Ireland and Britain. However, at the time of my visit on a cold, wet, mostly dull day at the end of March 2014, it was evident that more of these measures might be required in future to prevent further damage.