6,000 year old seashells at Cogden Beach

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Fragile remains of a 6,000 year old cockle shell from a peat deposit beneath the shingle bank at Cogden Beach, Dorset, UK - part of the Jurassic Coast (1) 

Not fossilised ….. but the actual fragile remains of a 6,000 year old cockle shell in a block of ancient peat flung up onto the shingle bank at Cogden Beach from a layer underlying the mobile deposits on the seashore. 

Ancient peat block thrown up by storms onto the shingle bank at Cogden Beach, Dorset, UK - part of the Jurassic Coast - from a peat bed underlying the mobile sediments of the seashore (2)

You might not think this a very interesting picture but that illustrates just how easy it is to overlook this repository of environmental evidence from the distant past. Contained within its dessicating layers are the clues from which it is possible to reconstruct the scene on this very spot as it was many thousands of years ago.

Leaves, twigs, stems, wood, gastropod and bivalve shells, and even beaver bones have been recovered fom these peat blocks. Small fragments of vegetation and shells are the most frequent finds as shown in the picture below. The plant and animal remains have been radio-carbon dated to 6,000 BP (before present).

Mostly gastropod shells found crushed between layers of an ancient peat block found on the shingle at Cogden Beach, Dorset, UK - part of the Jurassic Coast (3)

As well as the peat, there are blocks of clay on the shingle, also containing shells. In these sediments the shells are better preserved than in the peat. The peat shells are so fragile that they almost disintegrate on touch. Whilst the acid environment of the peat formation would aid the preservation of plant and animal soft tissue remains, over time this would denature or dissolve the calcium compounds in the shells (or bones). The less acid but similarly anaerobic environment of burial in the fine clay sediments would lead to better preservation of the shells.

The photograph below shows the paired valves of a cockle in situ as they would have been in life.

Paired valves of an ancient cockle shell well preserved (not fossilised) in a block of clay, thousands of years old, thrust up onto the shingle bank at Cogden Beach, Dorset, UK - part of the Jurassic Coast (4)

Included in these blocks of ancient clay sediment are blue and black pebbles which are larger and very distinct from the pebbles forming the shingle bank.

Blue and black pebbles together with cockle shells embedded in an ancient block of clay sediments found on the shingle at Cogden Beach, Dorset, UK - part of the Jurassic Coast (5)

More of these distinctive pebbles in the clay are shown below. The dark colour is caused by burial in anaerobic (without air) conditions in the same way as previously described for black oyster shells from Rhossili Bay.

Blue and black pebbles embedded in a clay block found on the shingle bank at Cogden Beach, Dorset, UK - part of the Jurassic Coast (6)

And lastly, here is a context shot to show location and scale for a typical clay block.

For the real expert explanation of the geological phenomena briefly described in this blog, and for references to original research and publications, please refer to Ian West’s Web Site on the geology of the Fleet Lagoon.

A typical ancient clay block (from deposits underlying the present seashore sediments) found on the shingle bank at Cogden Beach, Dorset, UK - part of the Jurassic Coast (7)

Revision of a post first published 27 April 2009

COPYRIGHT JESSICA WINDER 2011

All Rights Reserved

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