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When Somerset lay beneath the sea

A pebble of Carboniferous limestone from Vallis Vale, Somerset showing how it had been bored, as part of an ancient seabed, and then uprooted, tumbled, encrusted with oysters and then finally buried.University of Bristol

Press release issued: 29 May 2020

In a new study, University of Bristol geologists show how the Mendip Hills in Somerset were overwhelmed by the ocean more than 200 million years ago.

Research at a site in Vallis Vale, near Frome, shows how the ancient seas lapped up over the Mendips in stages, leaving behind evidence of the power of the waves.

The evidence consists of limestone pebbles that carry borings made by molluscs as well as oysters.

These pebbles were torn up from the underlying Carboniferous limestone which formed the basic landscape all over Somerset and across the Severn Estuary to South Wales.

Beginning at the end of the Triassic period, seas flooded over the whole area, leaving just the tops of the hills showing above the waves.

James Ronan, currently studying the Masters in Palaeobiology at the University of Bristol carried out this research last year as an undergraduate – the findings of which are published today in the journal Proceedings of the Geologists’ Association.

He said: “As the sea flooded around the edges of the Mendips, sea creatures made borings into the limestone seabed to find safety.

“Then storms followed and ripped up the limestone seabed, turning it into flat pebbles that contained the borings. The pebbles were dumped and then colonised by oysters before finally being buried.”

Professor Mike Benton, from Bristol’s School of Earth Sciences, led the project. He added: “Geologists had already noted ancient beaches like this at higher levels in the Mendips and dated as Jurassic and Cretaceous.

“So, we didn’t expect to find such an ancient example. This now shows that the Mesozoic seas flooded the landscape in the south-west in four or five steps from 200 to 100 million years ago, leaving a series of strandline deposits in sequence.

“Ours is the oldest and is at the eastern end of the Mendips, and the strandlines get younger and younger as you head westwards, going up the flank of the hills to the top.”

These Mesozoic seas were full of unusual marine reptiles such as ichthyosaurs and plesiosaurs, as well as numerous sharks and bony fishes.

Dr Chris Duffin, a fossil expert who worked on the samples, said: “We found huge numbers of teeth, scales and other fossils of these fishes.

“They were mainly predators, feeding on other fishes and invertebrates, and some of the sharks and bony fishes had broad teeth, adapted to crushing shellfish for food.”

Storm beds in the ancient Rhaetian seas that first flooded the Mendips over 200 million years ago, tore up the pebbles from the ocean floor, and also carried in teeth and bones of fishes and reptiles, and tumbled and sifted these to leave a concentration in the form of a fossil-rich bonebed.

The Rhaetian bonebeds are known all round north Somerset and South Wales, and further afield in central England, as well as in central Europe, showing that this flooding event was widespread, and perhaps related to major faulting of the Earth as the North Atlantic Ocean began to tear itself open.

The project was part of a programme run by the Bristol Palaeobiology Research Group called ‘At the Feet of the Dinosaurs’ and is available for student interns at the University of Bristol and operates in conjunction with Bristol City Museum.


'Beginning of Mesozoic marine overstep of the Mendips: the Rhaetian and its fauna at Hapsford Bridge, Vallis Vale, Somerset, UK' by J. Ronan, C.J. Duffin, C. Hildebrandt, A. Parker, D. Hutchinson, C. Copp and M.J Benton in Proceedings of the Geologists’ Association

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