Beach Boulders at Eype 3

Another gallery of rock pictures from Eype beach. I hope the photographs in this and preceeding posts on the same subject will prove a useful resource for students of geology as well as those interested in the aesthetics of geology.

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Eype Beach Stream 4

Beach Boulders at Eype 2

Sedimentary rock boulders on the seashore extending into the sea

Another gallery depicting the amazing rock boulders on the seashore at Eype in Dorset, England, and the contexts in which they are found on the beach.

The cliffs at Eype, and the boulders on the beach below them, are made of Middle Lias sedimentary rocks from the Jurassic Period. It is difficult to judge the scale of the cliffs from looking at the photographs, so you might like to know that the silty sand and hard sandstone bands of the Three Tiers layers at the base can reach a maximum of 9m thick (although much of that is buried in this location); the clays and sandstones of the Eype Clay Member above that are 60 metres deep; on top of that the soft sands of the Downcliff Sands Member are 30 metres deep; and above that the soft sandstones of the Thorncombe Sands Member are 21 metres thick. This makes for a sequence of rock strata measuring a potential 120 metres (390 feet) in height – the measurements are approximate as the depth of the layers varies a lot.

In their book Classic Landforms of the West Dorset Coast, Brunsden and Goudie state that

The coastal cliffs of West Dorset owe their shape to the relief and orientation of the coastline, the variable properties or lithology of the rocks, the geological structure, the history of relative land and sea movements, the sequences of environmental change, the difference in erosional energy of the sea between the more exposed and sheltered parts of Lyme Bay, and to the complex sub-aerial processes which currently act on the cliffs themselves.

At Eype, the boulders on the beach demonstrate not only the variable lithology of the rocks in the cliffs above but also their structure. The alternating bands of different rock composition and colour are evidence for the cyclical nature of changing environmental conditions throughout geological history, including changes in sea level relative to land; while their presence on the seashore illustrates the ongoing erosional processes affecting the coastline to this day.

COPYRIGHT JESSICA WINDER 2014

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Beach Boulders at Eype 1

Stratification in a beach boulder

As the soft rocks of the exposed Eype Clay Member mudstones in the cliffs at Eype are eroded, the harder sandstone and limestone rocks (that were laid down at a later date above them) are under-mined. The unsupported rocks then break under their own weight, slide down the cliff and eventually come to rest on the orange gravel and pebbles of the shore. At the western end of Dorset’s Eype beach, part of the Jurassic Coast World Heritage Site, the boulders extend continuously from the base of the cliff out into the sea. Elsewhere along the shore, the boulders are clustered at the cliff foot adjacent to the soft micaceous mudstone layer. Most of the images shown here were photographed in a stretch of shore just a couple of hundred metres long.

The boulders tend to be very large, some about two metres in height and width. Their shapes are endlessly variable and their colours are generally complementary shades of yellow and grey. They can be patterned by layers of contrasting width, colour, and composition; or by scattered fossils and iron nodules. This is the first of several posts showing the fascinating variety of composition and form of these boulders, and the contexts in which they can be found. The boulders in these photographs are the same ones from which I photographed the rock textures in an earlier post. Every boulder seems unique in its appearance.

COPYRIGHT JESSICA WINDER 2014

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Eype Beach Stream 2

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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Eype Beach Stream 3

Little pebble bridges over a beach stream

On the western half of the shore at Eype in Dorset, England, the cliff is basically made up of porous yellow sandstones and limestones, belonging to the Middle Jurassic Down Cliff Sand Member and Thorncombe Sand Member, overlying the pale, blue-grey micaceous silty mudstone and shale known as the Eype Clay Member. All three members belong to the Dyrham Formation.  Rainwater soaks down through the upper porous rocks but, when it reaches the lower clay-based strata, it seeps out to the surface and drains away down the cliff face to the shore in numerous small streams.

The picture above, and the short video clip below, show one of these little streams running over the clay where someone has artistically constructed small pebble bridges over the flow. The last image in the post illustrates the general appearance of the cliff face with the small streams issuing from the lower layers.

Blue-grey clay and yellow sandstone strata at Eype Beach

COPYRIGHT JESSICA WINDER 2014

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