Fossils are not only found in the cliffs, the rocky ledges of the beaches, and the boulders strewn across the shore at Lyme Regis in Dorset. Even when you take a walk around the harbour, on the top of the massive sea defence known as The Cobb, there are fossils beneath your feet – but these shelly remains have come from further afield.
The Cobb is a great sinuous, almost organic-looking, sea wall enclosing a safe haven for boats. It is constructed from huge blocks of fossiliferous stone. You can easily see in the cut surfaces of the stone vast numbers of bivalve and gastropod shells preserved as fossils and moulds within the rock. The blocks are Portland Roach, or Portland Admiralty Roach, which is a shelly oolitic limestone coming from the uppermost part of the Portland Stone Formation of the Upper Jurassic Period on the Isle of Portland. These big blocks lie above smaller slabs of Blue Lias argillaceous limestone from the local reefs. The Roach stone itself is characterised by the presence of moulds left by the long pointed gastropod locally known as ‘The Portland Screw’ – Aptyxiella portlanica – that has something like a turret shell shape.
The large number of holes that riddle the Roach stone mean that it is not really suitable for building construction in the way that the famous Portland limestone is used. However, Roach is frequently used for seawalls and sea defences.
The Cobb has been in existance for many centuries but what is seen today is a reconstruction from 1820. I think that was the first time in its history that the blocks of stone were mortared together. Mortar is most visible between the stones at the landward end but towards the seaward end some extra measures have been introduced to reinforce the structure. It looks as if the patchwork of blocks have been curiously secured together with giant iron staples.
Revision of a post first published 10 May 2010
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