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

All Rights Reserved

Rock Patterns & Textures at Tenby – Part 4

This is the fourth part of the series of rock texture pictures from Tenby. All so far have been from South Beach where the Carboniferous strata range from Hunts Bay Oolite, to High Tor Limestone, to Caswell May Mudstones, and Gully Oolite. Many of these close-up images have shown erosion patterns, caused sometimes biologically and sometimes chemically, or a combination of both. The first four photographs in this post show the fine, and approximately-linear ridges and grooves (click the pictures to enlarge them for a better view), that seem to be restricted to the otherwise smoother, un-pitted, darker patches on the surface of the rock. I am thinking that whereas the pits are probably caused by various effects of bio-erosion or bio-erosion plus solution, the almost microscopic grooves here could be the result of chemical erosion which sometimes occurs from contact with acid rain. If so, these micro grooves and ridges are microrills, and like miniature rillenkarren – a feature of karst topography – and they are evidence for relatively recent erosional activity.

The patterns of grooves and fissures in the four images below, could also be a karstic type of solution feature. I am not sure – but they are certainly intriguing and look to my eye rather like the tough wrinkled hides of elephant or rhinoceros.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

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

Rock Patterns & Textures at Tenby – Part 2

Black lichen growing on pitted limestone cliff surface

More natural patterns and textures in rocks from South Beach cliffs at Tenby in South Wales. Organisms like bacteria and lichen that grow on the surface of rock (as shown in some of these photographs) can be agents of erosion, especially those species capable of penetrating the first few millimetres of the surface of the substrate: their growing habits can result in a weakening of the rock surface. Small gastropod molluscs such as periwinkles feed on the bio-film created by the bacteria, lichens, and other organisms like algae and fungi. Those molluscs with particularly hard radula teeth, for example limpets, actually remove small particles of the weakened rock along with their food. This minor activity over long periods of time contributes to the wearing down of the rock surface and the production of a pitted surface. Erosion of rock by biological phenomena is referred to as bio-erosion and it occurs in conjunction with other chemical and mechanical erosional processes.

White-veined limestone with pitting in a cliff face

Patches of black lichen on naturally fractured and erosionally pitted limestone

Triangular patterns of natural fractures in limestone

Gastropod fossils embedded in limestone cliffs at Tenby

Bacterial discolouration on an eroding limestone surface

Dark patches of rock colonised by bacteria on the eroding surface of naturally fractured limestone

Dark patches of rock colonised by bacteria on the naturally-fractured and eroding surface of limestone

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

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

Rock Patterns & Textures at Tenby – Part 1

Detail of veining and fractures in limestone cliffs

The Carboniferous Limestone cliff rocks at South Beach in Tenby, South Wales, are covered with numerous pock marks, hollows, grooves and holes, making honey-comb, lace-like, linear, heiroglyphic, and random patterns. These variations in surface texture in the Hunts Bay Oolite Subgroup strata shown here are thought mostly to be caused by different forms of weathering and erosion activities acting in unison to degrade and remove the surface of the rock.

The effects of erosion on the surface of limestone in seashore cliffs

The effects of erosion on the surface of limestone in seashore cliffs

The effects of erosion on the surface of limestone in seashore cliffs

The effects of erosion on the surface of limestone in seashore cliffs

The effects of erosion on the surface of limestone in seashore cliffs

The effects of erosion on the surface of limestone in seashore cliffs

The effects of erosion on the surface of limestone in seashore cliffs

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

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

All Rights Reserved

Rock Textures at Eype 1

View of the cliff on the western shore at Eype showing stratification

View of the cliff on the western shore at Eype showing stratification

The interesting thing about Eype Beach is that you don’t have to climb the cliffs to see the rocks in detail – the rocks come to you! Boulders of sandstone and limestone from high in the cliffs regularly collapse to the beach, which becomes strewn with them, and affords an opportunity to examine the composition of, and the fossil content of, the variety of rock types represented in the strata above shore level. Even the low, thick band of softer mudstones and shales slips down on a fairly regular basis, and liquified by small streams, oozes over the shingle of the upper beach.

It’s going to take me a while to work out which rock is which. However, I can say that Eype Beach has two different geologies more or less separated at Eypesmouth where a small stream cuts its way down a steep-sided valley through the predominantly soft rocks. If you turn right and westwards where the stream breaks through to the shore, and walk towards Thorncombe Beacon as I did, then on your right-hand side are cliffs made up of several virtually horizontal rock strata of different types of sedimentary rock. The lowermost layer, nearest to the level of the shingle beach, is a 55 foot depth of blue-grey Eype Clay Member made up from micaceous silty mudstone and shale – also called  the Micaceous Beds – from the Middle Jurassic Period.

Above the blue-grey mudstone, are the yellow layers of silts and sandstones of the Down Cliff Sand Member and the Thorncombe Sand Member – with sporadic fossil beds, and thinner bands of calcareous sandstone and ironshot limestone. You can easily see the contrasting colours of the different rocks in the cliff face.

I had hoped to find some brittle star fossils, that was the main aim of the visit, but I wasn’t lucky on this occasion. It was rather hot on the day and I don’t think I walked far enough along the shore to be in the most likely location. The Starfish Bed with Palaeosoma egertoni is at the very base of the Down Cliff Sand Member which itself overlies the Eype Clay Member. Large blocks of this rock fall to the beach – but you have to hope that the block has fallen the right way up for you to see the brittle star fossils, and also hope that a professional fossil hunter has not got there before you! I’ll have to keep on keep searching.

View looking west toward Thorncombe Beacon from the base of the cliff at Eype

View looking west toward Thorncombe Beacon from the base of the cliff at Eype

It was clear that many types of rock were identifiable on the beach; even the modern mud-slicks and clay seepages were interesting because they demonstrate and replicate the same  processes that would have contributed to the textures and patterns of the ancient rocks. As the soft muds dried out in the sun, the surfaces were beginning to form a crazy paving patchwork of cracks – the same as could be observed in nearby slabs of rock. As the liquified clays dribbled outward from the base of the cliff rock exposure, they incorporated assemblages of small pea-sized pebbles and who-knows-what man-made objects that might end up in rock strata of the future.

So the gallery of pictures today just shows details of a small selection of the rocks and sediments to be found on Eype Beach with a range of the natural textures and patterns they exhibit. It’s the starting point for the Eype geological learning journey.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved.

Sweyne’s Howse

 

In the earlier  discussion about the strange stone that might be an ancient axe – which was found on the Worm’s Head Causeway on the Gower  Peninsula – I mentioned the nearby Neolithic chambered tombs. The most frequently found ancient tools in this area are the polished stone axes from the Neolithic. There are some earlier tools – but these are flint, and were probably made on a flint-working site that existed on the tidal island of Burry Holms at the north end of Rhossili beach. (The Worm’s Head Causeway is at the southern end of Rhossili beach).

The Neolithic period started about 6,000 years ago and was marked by an influx of farming communities who cleared the land for grazing and cultivation. These people left tools behind them, the ones most frequently found being the polished stone axes that have been recovered from sites such as Paviland, Oystermouth and Barland. The most noticeable remains of these people are the large megalithic structures. On Rhossili Down are the remains of chambered tombs belonging to this category: the best known of which are Sweyne’s Howses (there are two of them).

The images in this post show the location of Sweyne’s Howse on Rhossili Down – positioned on the land the Neolithic settlers cleared, at the junction between freely-drained and poorly drained soils, recognisable by the heathland vegetation on the one hand and the fertile cultivated fields on the other. The upland is covered with vibrant pink flowering Ling and Heather in the summer, and the ground is still grazed by sheep, cattle, and roaming wild ponies.

The tomb is now located amongst cleared bracken on the lower slopes of the Down. It looks very different from various angles, being comprised of massive slabs of local Old Red Sandstone Conglomerate – slabs that look like pillars when viewed end-on. The rocks are covered with a thick layer of lichens.

From higher up on Rhossili Down, it is possible to look down towards the north-west and see the island of Burry Holms where there was an earlier Mesolithic flint-working site. Looking down to the south-west, you can see the Worm’s Head and its Causeway where the strange stone was found.

I now have an appointment to take the stone to an archaeologist who is a specialist in stone tools at the National Museum of Wales; and a geologist will also be on hand to give their opinion. I am hoping that the stone is an axe – but I am also prepared to be disappointed because I know how easy it is for an amateur like myself to be mistaken about this kind of thing.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved

A Curious Beach Stone

A stone found on the Worm's Head Causeway

I’m always looking at pebbles and beach stones but I wasn’t the one who first spotted this curiously shaped stone. My companion picked it up from where it lay in a shallow tide pool out on the Worm’s Head Causeway, which is at the end of the Gower Peninsula. It is a fairly symmetrical and flattened leaf-shape; and measures approximately 12 by 7 cm. It seems to be made of limestone – but I could be wrong about that.

One edge is smooth and rounded. The other is thinner and sharper. Overall, it is well worn and smoothed – it has been rolling around on the shore for a considerable time. The surface has evidence of both infesting and encrusting organisms. There are small burrows made by marine worms and also by sponges – I’m not sure what types they are. At the broad end is a larger hole that perforates the stone. It looks a lot like part of a tunnel that might have been bored by a bivalved mollusc such as a Flask Shell or a Wrinkled Rock Borer. Within the hole, small acorn barnacles have attached their plates. Over the flat surfaces of the stone are minute lace-like Sea Mats and the occasional calcareous tube made by a worm. The whole stone feels balanced and comfortable in the hand.

I’m quite excited about this stone because I think it might be an ancient hand axe! I’m going to send these pictures to experts at the National Museum of Wales for their opinion. Maybe you, the reader, knows something about this object and can tell me something about it. I have read that a really old Neanderthal flint axe was once found at Rhossili; and Palaeolithic stone axes have been recovered from some of the local caves. However, most of the axe heads discovered in this area have been Neolithic; and physical evidence for Neolithic occupation of the locality can still be easily seen in the megalithic chambered tombs – like Sweyne’s Howes on Rhossili Down.

If this piece of rock is not just an oddly shaped beach stone, and it is in fact an axe head, then its most curious feature of all must be the perforation. From a naturalist’s point of view, it seems most unlikely that a rock boring mollusc would have burrowed into such a thin section of rock as presented by a lost hand axe. That being so, it raises the possibility that the rock was chosen for making into a hand axe because it already had the hole in it. Microscopic examination of the inner surface of the hole, beneath the encrusting barnacles, could reveal whether the hole is naturally made by some organism or if it is man-made. Surely a most unusual phenomenon in ancient axe-making.

I’ll keep you posted about developments. Fingers crossed that it really is something special – but maybe it is only in my imagination.

Flat surface of a stone found on the Worm's Head Causeway

Flat surface of a stone found on the Worm's Head Causeway

Possible worked sharp edge to the strange beach stone

Possible worked sharp edge to the strange beach stone

Blunt, rounded edge to the odd beach stone

…….and where the stone was found:

COPYRIGHT JESSICA WINDER

All Rights Reserved

More about Joggins Fossil Cliffs

View of Joggins Fossil Cliffs inclined strata

As you walk northwards along the shore at Joggins Fossil Cliffs, you are walking backwards through time. As you look straight-on at the inclined rock strata exposed in the cliff face, the strata are rather like the pages of an open book, with the beginning of the story (or the chapter might be a better simile) told by the pages furthest to the left; and the the end of the chapter to the right. Each layer of rock has a different thickness and composition. Each stratum represents a different event or episode in geological history. Altogether, the Joggins Fossil Cliffs site comprises a 14.7 km coastal section with the majority of detailed stratigraphical and sedimentological studies being concentrated in about a 4 km section of the Joggins Formation.

Originally, the layers of rock were laid down in horizontal beds but, over time, earth movements have tipped them up so that now these are all lying at an angle of about 20 degrees. The conditions under which these sedimentary rock layers were laid down changed in a cyclical way. The location in which the sediments were formed was basically a coastal environment.  Periodically the sea moved in and out. This meant that the place became either drier or wetter, more marine, more freshwater, or more terrestrial depending on the prevailing sea level.

When the sea level was high, the area was covered in shallow sea. This is when the limestone rocks were formed. When the sea level fell, the area become more terrestrial. The sea level changed according to such major phenomena as the movement of tectonic plates in the earth’s crust or due to eustatic land/sea level changes linked to the build-up or the thawing of ice sheets. The terrestrial environments were not always absolutely dry but were muddy swamp-like places with stands of vegetation including large tree-like plants.

The coastal strip was traversed by rivers that cut channels in the sediments and brought down large quantities of sand and other materials that could bury the coastal plants in times of flooding. This is where the sediments that formed the sandstone layers came from. In-filled river channels are preserved in the cliff face and are also visible at low tide as long lines of rock projecting from the surface of the shore as they extend out to sea. Mudstones and shales were also formed. Rapidly buried rotting vegetation and trees were sometimes converted under pressure to coal.

The rocks at Joggins belong to the Cumberland Group of the Langsettian in the Upper Carboniferous Pennsylvanian period. The rocks along the total 14.7 km stretch of cliffs are made up of four formations: from The Ragged Reef Formation,  (the youngest to the south), then the Springfield Mines Formation, next the Joggins Formation itself, and then to the Little River Formation and Boss point Formation in the north. I was only able to walk from Dennis Point (SMF) to Coal Mine or Hardscrabble Point (JF) because of the state of tides and weather.

On my visit I found it impossible to work out which strata were which. However, since returning home and doing a bit of research, I have discovered that a sedimentological log has been recorded for the length of the Joggins Formation by Davies et al 2005. Each stratum was measured to the nearest centimetre and its composition, structure, and associated fossils described. This means that when I visit again, I will be well equipped to ‘label’ each stratum that I photograph – with a little help from the published diagrams, and hopefully an expert guide from the Joggins Fossil Cliffs Visitor Centre.

It has been possible to find out what some of the rock patterns, structures and textures represented. For example, there were many examples of preserved wave and current ripples patterns on beach stones and boulders as well as in bedrock layers:

Ironstone or siderite nodules formed of iron carbonate by precipitation in evaporating shallow water; and clay galls derived from dried mud polygons incorporated into sandstones are common.

Cross-bedding of sedimentary layers made by deposition from migrating streams and rivers; patterns of cracks in sun-baked muds; and conglomerates  of various sorts with small pebbles and rock fragments brought down by running water and cemented into a matrix, were also seen.

Wrapped up within these layers of sedimentary rock are numerous, sometimes spectacular fossils, preserved in situ where they lived and died. The plant fossils include huge ‘tree’ trunks still rooted in their life position; fossil stems, roots, leaves, and diverse carbonised items of vegetational debris abound. Invertebrate fossils such as bivalved and gastropod molluscs are also found, as are pieces of giant millipede, dragonflies and whip spiders. Trace fossils of tracks left by Arthropleura millipedes and reptiles occur. In fact, the actual skeletons of small reptiles, such as Hylonomus lyelli – one of the first reptiles to evolve on earth, have been recovered from hollow tree trunks at this site. More about the fossils in the next post.

REFERENCES

Calder, John (2012) The Joggins Fossil Cliffs: Coal Age Galapagos, Province of Nova Scotia, Department of Natural Resources, Crown Copyright, ISBN 978-1-55457-473-5.

Davies, S. J., Gibling, M. R., Rygel M. C., Calder, J. H., and Skilliter D. M. (2005) The Pennsylvanian Joggins Formation of Nova Scotia: sedimentological log and stratigraphic framework of the historic fossil cliffs, Atlantic Geology, 41, pp 115 – 142.

Joggins Fossil Institute, The Joggins Fossil Cliffs Field Guide.

Nova Scotia Department of Natural Resources, Nova Scotia Geological Highway Map.

COPYRIGHT JESSICA WINDER 2014

All Rights Reserved