Rock Textures at Eype 2

 

View of the beach at Eype in DorsetThe shore at Eype is littered with large boulders, several tons in weight, that have broken from the strata high on the cliff and then slip-slided down the lower mudstones and clays to the beach. They are all rocks belonging to the Jurassic Dyrham Formation. that includes a fascinating assortment of mudstones, sandstones, and limestones, some with ironstone nodules or carbonate concretions, and lots with fossils. I cannot with confidence identify the specific rock types illustrated in all the close-up photographs I took. It is quite a complicated geology at this coastal location. However, a general picture of the represented rock types follows. Fossils are found in more or less all the strata, ammonites are said to be common, but the ones I saw were mostly fragmentary shells and bullet-shaped belemnites

An accurate and up-to-date source of information about the geology of this locality is the British Geological Survey’s Geology of south Dorset and south-east Devon and its World Heritage Coast, published in 2011 by the Natural Environment Research Council. All the information that follows has been obtained from this book.

The Dyrham Formation is comprised of three members. At the base of the cliff is the Eype Clay Member which is a pale, blue-grey micaceous silty mudstone and shale. The base of the Eype Clay Member is marked by The Three Tiers  about a metre thick with three prominent sandstone beds separated by shales and mudstones. Higher up is a band of calcareous nodules, the Eype Nodule Bed. At the top of the band is Day’s Shell Bed with a rich fauna of juvenile bivalves and gastropods.

Above the Eype Clay member is the Down Cliff Sand Member made up of silts and fine sands with thin lenticles of hard calcareous sandstone. At its base is a fossil-rich layer known as the Starfish Bed, with abundant brittle-stars. At its top is the Margaritatus Stone which is hard, grey, iron-shot limestone.

At the top of the Dyrham formation is the Thornecombe Sand Member, sitting on the Down Cliff Sand Member. The bottom-most layer is the blue-grey Margaritatus Clay, above which are yellow-weathering, heavily bioturbated sands, with several horizons of large rounded calcareously cemented concretions. There is an impersistent band of limestone running through the middle of this, and a shelly Thornecombiensis Bed sealed by sandy mudstone atop it.

So you can see that there are many different rock layers and types in the stratified cliff, often obscured by land slips, and it is quite difficult for an amateur like myself to correctly identify pieces of these strata when they are lying on the shore.

However, one noticeable feature in the beach boulders was the occurrence of bioturbation: this is defined as a disruption of sediment by organisms, seen either as a complete churning of the sediment that has destroyed depositional sedimentary structures, or in the form of discrete and clearly recognisable burrows, trails, and traces (trace fossils). The most easily recognisable trace fossils are the largish burrows of Crustacean Thalassinoides – which you can see in images 4 and 9.

View of the beach at Eype in DorsetAnother phenomenon that is responsible for some of the more unusual colouration and patterning of the rocks, is the transformation of blue-grey rock to yellow by the weathering process on exposure to air, which oxidises iron minerals in the stone. Iron staining, iron nodules (often in association with fossil fragments), and veins of iron, also contribute to rich colour patterns both within and on the surface of the boulders. Sometimes the colours are exhibited as a thin outer layer that is exfoliating into abstract patterns of contrasting hues on the rock.

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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.

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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.

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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

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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.

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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

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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.

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