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.

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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

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Lyme 8

 

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Lyme 7

 

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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.

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