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

Rock Patterns & Textures at Tenby – Part 5

Green and red biofilm encrusting cave walls at Tenby

It was exciting to discover all the caves at South Beach in Tenby. The rock layers of the cliffs, which were originally laid down in horizontal layers at the bottom of ancient seas millions of years ago, have been subsequently pushed on-end by earth movements so that they now lie at very steep angles to the vertical. The waves have worked away in weaker areas between the strata and excavated small caves. I couldn’t wait to see inside them. They were variable in size but larger than I expected. Well worth exploring.

The floors were mainly sand, smoothed by the previous high tide. Sometimes pebbles were piled up against the back wall. I was mostly struck by how different they looked from one cave to the next. Some cave walls were almost polished, smooth, pale grey limestone, revealing irregular streaks of white calcite veining, occasionally with fossils. Others were roughly hewn with multiple broken facets.

Most intriguing of all were the mosaics of bright green and deep red organic encrustations coating some walls. I couldn’t work out the rationale for their seemingly ad hoc distribution. I am not sure what they are. Maybe they are cyanobacterial bio-films rather than encrusting algae – because of the location in which they are growing so high on the shore and away from light.

[There are in fact encrusting dark red forms of alga but these seem to be restricted to low shore situations in shallow water. Identification of these kinds of organisms is difficult, because they are not a distinct taxonomic group but are represented by a variety of different genera; and maybe I need to take some samples for examination under the microscope].

The pale grey Hunts Bay Oolite Subgroup limestone of the most western stretch of South Beach, which has most of the caves, eventually gives way to other rocks further east – like the Caswell Bay Mudstones which are more thinly bedded with a variety of colours and textures, and these house perhaps the largest cave – the last one of note before you reach Castle Beach and Castle Hill that act as a divider between South Beach and North Beach in Tenby.

COPYRIGHT JESSICA WINDER 2014

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Rock Patterns & Textures at Tenby – Part 3

This is the third in a series about the textures and patterns in rocks belonging to the Carboniferous Period and exposed in the cliffs at Tenby in South Wales. These photographs illustrate that erosion can happen on several size scales on the same rock surface, with tiny erosional pits (measuring in only millimetres and barely visible to the naked eye) superimposed on slightly larger scale pits (measuring in centimetres). [Don’t forget that you can click on a picture to enlarge it and see a description].

The pitted type of erosional surface, as shown in the images above and below, is probably the result of bio-erosion. However, in the red rocks, If I have identified the stratum and understood the textbooks correctly, then the fine erosional pitting is now taking place on top of fracturing and other features that may indicate exposure of the stratum to wave action and weathering an a much earlier geological time period.

Surface texture and pattern like elephant skin in Carboniferous Limestone

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Rocks & Pools on Worms Head Causeway

Looking down to the Worms Head at high tide

It was a sunny day with the prospect of a very low tide – just right for exploring the rocky causeway that links the Rhossili headland to the tidal island of Worms Head. I was really looking forward to it. The times when it is safe to venture out on the Causeway are always clearly indicated; and it is assuring to know that there is almost always a Coastguard Lookout monitoring the area through binoculars to render assistance if anyone gets into trouble out on the rocks. Most people seem merely to cross the causeway by the quickest route to reach the Worms Head. However, the Causeway itself is a source of great fascination for anyone like me and interested in natural history.

You have to be fairly fit to get down on the Causeway and need to have sensible footwear. The descent from the red-rimmed turf platform at the base of the headland, and the initial scramble over the tall projecting limestone strata of the first 50 metres or so, can be a challenge for some. However, it is well worth the effort because it is a different world out there. It is a unique experience. An alien landscape full of surprises awaits you.

When you look down on the Causeway from the headland , it might look a rough and barren expanse of dull buff and grey rocks. Boring, even. Once down of the rocks, a closer perspective reveals a wealth of detail with hidden pockets of colour, variations in texture and topography, strange natural sculpturings, ancient rocks with complex geological histories, embedded fossils such as entire Sea Lilies, tidal pools of every size, deep water gullies, multi-coloured seaweeds and myriads of seashore creatures. The variety and complexity of this beachscape might be perplexing but it is none the less inviting and exciting.

On this particular visit, I aimed for the central part of the Causeway that I hadn’t investigated before, and then slowly veered round in a more easterly direction before returning to base. I was interested not only in the geology and the seashore life as entities in their own right but was also intrigued by the way each component of the shore is influenced by the other – the way everything interacts. How the geology and landscape affect and facilitate the living organisms; and how the living organisms affect the landscape.

Once away from the very landward edge of the Causeway where the rocks can sometimes seem to be completely devoid of animal life, almost every rock surface is covered to a varying degree by small acorn barnacles of different types. The common mussel is abundant but not growing in such profusion as in previous years. Not many dog whelks were feeding on the barnacles and mussels along the route I was taking but, no doubt, they lurk in other lower shore areas. Large limpets cling to surfaces both wet and dry. Common Periwinkles and striped Top Shells are common. Even the smallest pool is home to red Beadlet and pink-tipped Snakelocks Anemones. Small fish and shrimps dart through the pools and hide beneath the seaweed. Large Balanus perforatus grow on the lower shore  – instantly recognisable with their volcano-shaped shells and beaky opercular plates.

While I was sitting eating my lunch, a large Common Green Shore Crab ventured out of the water right by my feet but soon made a hasty retreat. I made a little movie of him scuttling around the pool.

Even the most exposed rock surfaces out on the centre of the Causeway have some seaweeds growing on them. Bright green soft weeds of the Ulva species (like Sea Lettuce and Gutweed) seem to tolerate the dry rock as well as the pools. Red branching seaweeds make a dramatic colour counterpoint to them: they often grow together. Calcareous red seaweeds like the branching Coral Weed grow extensively, and patches of flat Corallinaceae crusts like Pink Paint Weeds line water-filled hollow basins and dips, and coat the water-line of large boulders in the gullies. The familiar Brown Fucoid seaweeds like Bladder Wrack and Toothed Wrack make an appearance further down the shore, while the large kelps such as Oarweed occupy deeper waters right on the shoreline and below. One interesting new alga that I spotted is an encrusting brown paint-like species covering the shells of limpets (probably Brown Limpet Paint, Ralfsia verrucosa).

The strange curvilinear shapes of some of the upstanding rocks, the deep gullies along bedding planes, and the numerous rounded hollows and depressions, are typical of coastal limestone karst topography. More extreme and more extensive examples can be seen elsewhere in Gower, such as around the tidal island of Burry Holms, and at Mewslade Bay and Caswell Bay. Many people assume that it is the impact of waves, acid dissolution by rain, and abrasion by sand-bearing winds, that are the combined means by which seashore rocks are worn away, slowly and steadily over the millennia. This is partly true; it does account for some of the erosion. However, there is another aspect to the erosion of seashore rocks which is equally, maybe more, important: bio-erosion.

It all starts in the smallest of ways on a microscopic level with organisms like bacteria, algae, fungi, and lichens – especially those that are capable of not only colonising the surface of the rock (endolithic organisms) but also of penetrating it (epilithic organisms), even if that is only to a depth of a few millimetres. The general effect of the rock penetration is a weakening of the substrate so that when grazing molluscs like periwinkles and limpets come along they can easily remove not only the bio-film on the surface but can also scrape off some of the surface rock as well.

For example, analyses of the gut contents of limpets shows that small particles of rock are ingested along with the food they obtain. Limpets also alter the rock in another way. They always return from foraging trips to the same position on the rocks – their home base. As a limpet adjusts its position on the home base, its shell mechanically grinds against the rock wearing away a circular depression; this depression is deepened and emphasised by the chemical effect of the limpet’s acidic waste products dissolving the rock. It has been calculated that over vast periods of time, the cumulative effects of limpets feeding on rocks can contribute the process by which they are reshaped or destroyed. Abandoned limpet home bases are common on the rocks of the Causeway where the animals have been dislodged by last winter’s stormy seas.

Another major bio-erosional component is the burrowing activity of marine polychaete worms, and of specially adapted rock-boring bivalved molluscs. It is amazing to see just how extensive is this kind of damage to the rocks on the Causeway. It is no wonder that there are so many pebbles and boulders with holes in them found on the shores all around the Gower Peninsula. Almost every damp patch, depression, hollow, pool, and gully has limestone riddled with these burrows. The burrowing activity of these marine invertebrates is made easier by the weakening of the rock by micro-organisms; and the burrows and holes then provide a greater surface area for the further colonisation by micro-organisms. The combined effects of all types of bio-erosion have a significant impact on the surface shape of the limestone and landscape.

The strata on the Causeway lie in parallel lines along an approximately northwest to southeast axis. Most of the rocks that you see are Black Rock Limestone Subgroup with some Shipway and Brofiscin Limestone. As you face the Causeway with your back to the Coastguard Lookout building on the Rhossili headland, then behind you and beneath the superficial loose deposits, lies first of all Gully Oolite and then High Tor Limestone as solid bedrock. In front of you, the Black Rock Limestone is bordered on the far side of the Causeway by strips of first Gully Oolite and then outermost High Tor Limestone solid bedrock. So there is a particular sequence to the layers of rock visible on the surface which reflects their history.

On the landward side of the Worms Head Causeway, the sharp projecting edge-on rock strata dip down and towards the Rhossili headland and lean at an angle in the direction of the open sea. On the seaward side of the Causeway, the lines of strata dip down and towards the open water with their free edges inclined in the direction of the land. Between these two areas of strata that point towards each other, there is a flatter, more eroded area, more severely cut away by wave action. The whole unit is the remains of an eroded geological feature called an anticline.

Imagine that the sedimentary rock layers were originally horizontal but later pushed upwards by earth movements into a mound or ridge; the resulting arched rock layers in the mound have been worn away by the elements over time until only the base of the mound remains with a characteristic layout in which stumps of the most recently formed younger rocks lie on the outside with the older layers on the inside of the feature.

You can visualise this process by thinking of a Swiss Roll. [If I am to persist with this analogy, perhaps we can go whole hog and imagine a chocolate one with cream filling?] If the cake were roughly cut  length-wise, the broken surfaces would have a pattern of longitudinal stripes with alternating sponge and cream. The layer which was original wrapped around the outside of the Swiss Roll cake would be seen as the two stripes of sponge on each side.

There is rocky shore zonation of the organisms that live on the Worms Head Causeway but this zonation is not so straightforward to recognise and interpret as on a normal stretch of shoreline. For a start, the Causeway is connected by beaches to the mainland at the Rhossili headland and the island at Worms Head. Elsewhere, the waters’ edge describes an irregular outline, the shape of which depends on the state of tide, and which more or less defies description. The surface is full of ups and downs on various scales.

Zonation is the way that organisms tend grow in associations on rocks depending on their tolerance for different degrees of exposure to the air – each type of organism having a physiological preference or need for more or less immersion in sea water. Typically, this zonation of organisms is seen on a rocky shore as different coloured bands – pale for barnacles, dark for mussels, yellow for lichens and so forth.

The best way to describe the zonation on the large scale out on the Causeway is by thinking of it radiating irregularly outwards, in a roughly concentric fashion, along a slight and highly disturbed incline from the highest to the lowest parts of the Causeway  – rather than a zonation with easily observable regular bands as on a normal rocky shoreline or cliff face. On a minor scale, there is zonation in the rock pools and in the water-filled gullies themselves.

At high tide the causeway is completely covered by the sea; sometimes the beaches are covered too. As the tide goes out, greater and greater areas of the causeway rocks are exposed to the air. It could be claimed that the water drains away from the perimeter and also from higher areas of the Causeway simultaneously. Water seems to continually make its way downwards from pools in the highest parts, through small cascades and gullies, to reach the sea. You can hear the continuous trickling sound of this water, merging with the noise of the wind, the breaking waves, the calls of birds. You are immersed in sound when you are out on the Causeway.

The tide seems to come in and go out in a very haphazard way. It is difficult for the occasional visitor to predict the direction in which the seawater will ebb or flow; or the speed with which it will rise and fall. This is what makes it potentially dangerous to be out on the Causeway when the tide is in flood – it could be difficult to decide which parts will be covered with water first, and therefore it is easy to get trapped by the water, with access denied to dry land. Swimming or even wading through the tide water is not a good idea because of the cross currents, water encroaching from three sides, and the hazardous sharp and barnacle-encrusted rocks beneath your feet.

Having kept an eye on the changing tide after spending a most enjoyable five hours out on the Causeway exploring and taking photographs, I looked for a safe, or easy way to get back to the Rhossili headland. The strata run in rows parallel to the headland and projecting higher and higher as you approach the beach. There are numerous pools between the layers of rock. This would make it hard work to traverse the last bit of terrain back to the beach from the location I was in. Luckily, north-south fault lines cross the rock layers. The areas of the fault lines tend to be worn down to lower levels than the surrounding projecting rocks because they are frequently filled with wide veins of softer white crystalline calcite and narrow veins of red haematite. Following these fault lines makes it much easier to negotiate a way back to the mainland.

The surfaces of these natural pathways are often worn smooth. Shallow streams of sea water flow along them and many small seashore creatures take advantage of the moist habitat they provide. The ‘stream’ beds and shallow pools along the fault lines are really colourful, often coated with a film of bright green microscopic algae that provides a vivid contrast to the red and white minerals, and to the purple striped Top Shells that love to graze there.

From out on the Causeway, not only can you view Worms Head from the most unusual angles and see it in a way that is completely different from the standard postcard perspectives – but there are also spectacular views of the Rhossili headland. The sixty metre high plateau of clearly stratified limestone is sketchily cloaked with turf which at its lowermost weathered edge reveals a vivid orange soil. This soil covers remnants of an ancient raised beach where seashells and pebbles from around 125,000 years ago, deposited in the Ipswichian Interglacial period, are cemented together by calcite and covered by glacial debris. The orange band contrasts dramatically with the bleached smooth pebbles and bizarre barren outcrops of the beach itself. This is the point to which I at last returned and was able to look back at the vast expanse of rocky causeway fully revealed by the now low tide. Next time I intend to venture out to the deep gullies of the far side of the Causeway and see what I can find there.

P.S. Don’t forget that you can click on any picture to enlarge it and see a description of the image

COPYRIGHT JESSICA WINDER 2014

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Rocks and Pools on Burry Holms

The fantastically sculptured Carboniferous limestone around the tidal island of Burry Holms, which lies at the northern end of Rhossili Beach on the Gower Peninsula in South Wales, provides a habitat for many intertidal species.

The exposed rocks between the highest and lowest tide levels are covered with a patchwork pattern of permanently attached dark mussels and pale acorn barnacles on which thousands of roaming dog whelks feed. Periwinkles and limpets graze on the algal films that cover the rocks and the shells. The curiously curving contours of the rocks supply numerous sheltered micro-habitats in the form of small hollows, crevices, gullies, overhangs, and pools.

Some of the pools are only just big enough to accommodate a couple of sea anemones and a few dog whelks. Some bigger pools are almost perfectly circular smooth basins dissolved into the stone, characteristically highlighted in summer by vivid green soft seaweeds concealing minute fish and multitudes of striped top shells and other gastropods. The occasional deeper pool  becomes a safe haven for clusters of common starfish and small shrimps; while wet overhangs and clefts display numerous beadlet sea anemones in a vast array of colours from pale khaki to bright red, together with rounded mounds of orange sponge.

All the organisms that live on the rocks in the inter-tidal zone contribute to the process by which the rocks are shaped. Frequently, this is done in a slow, subtle, and imperceptible way by the actions of epilithic and endolithic micro-organisms such as bacteria, fungi, algae, and lichens, and by the way these microscopic organisms are scraped from the surface and surface layers of the limestone by grazing seashore creatures.

Sometimes, the erosion is visible to the naked eye – as in the circular “home bases” that limpets have created by the continual grinding and wear of their shells against the rock as they settled in the same place each time after foraging trips; together with acid dissolution of the stone by their waste metabolic by-products. Another easily observable kind of bio-erosion damage is the burrowing activity of marine polychaete worms and boring bivalved molluscs. These small holes in rocks are often clustered in a band immediately above and below the water line of pools but also in any continually wet or damp grooves and channels. The overall persistent erosional activity of marine invertebrate organisms on intertidal seashore limestone over thousands and even millions of years contributes to the creation of fascinatingly sculptured karst topography like that seen around the island of Burry Holms.

COPYRIGHT JESSICA WINDER 2014

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