The Triassic/Jurassic Coast of North West Somerset, UK Content from the guide to life, the universe and everything

The Triassic/Jurassic Coast of North West Somerset, UK

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There is a stretch of coast running along north west Somerset, which points to an extraordinary episode in the life of the Earth. Along this coast is evidence of:

  • A Jurassic sea invading a Triassic desert in the form of fossilised ripples of sand.

  • The recession of the sea preserved in cliffs composed almost entirely of alabaster.

  • The resurgence of the sea, as evidenced by thousands of iridescent ammonites, tiny scallops, fish, and a myriad of warm, shallow sea fossils.

Unfortunately, the exact location will have to remain obscure, as fossil 'collectors' have had a deleterious impact over the years. A general location, however, will probably cause no harm. The following beaches and their fossils can be found between Hinkley Point and Porlock. Look across the Bristol Channel and you will be able to make out the south coast of Wales.

The Modern Coast

Other than the odd flat section caused by shallow river valleys, this coast is a mass of cliffs which end abruptly, diving straight down on to beaches that shelve off into the deep waters of the Bristol Channel. The coast here is exposed to the Atlantic weather, which is why, in part, it is so attractive to the fossil hunter. Based on observations by the renowned geologist Dr Peter Hardy of The University of Bristol, it has been estimated that the water erosion on this stretch totals about a millimetre a year (for the hard cliff rock). There are other, lower, cliffs composed of red mudstone which erode at a much faster rate, often causing a lethal environment for the unwary walker.

One of the reasons for this quick mudstone erosion is the height of the landscape behind the cliffs. The Quantocks are at one end, and Exmoor begins a little further on. Rain thunders down on to both, and percolates down, saturating the porous and mudstone rocks. Mudstone is similar to clay, in that it is composed of microscopic sheets of crystal lattice. When pressure is applied to these lattices, they slide off each other, like a surf board over water. This is why clay is so slippery. Destabilise these mudstone cliffs with lots of water, and things begin to slide.

Pillars of Alabaster

The alabaster cliff fall in 1998 was a tremendous sight. Five or six house-sized squares of cliff had fallen to the beach and jammed up against each other to form a room. These squares were composed of pink, orange and red alabaster, striated (marked with striate) with a shale-like rock. Where they had bashed against each other on the tumble down from the cliff, thousands of shards of alabaster had been knocked off, and lay scattered across the beach like a carpet. Within a couple of months there was not a single shard left. A mixture of high tides and tourists had scoured the beach.

Today, the fall is even more impressive. Several more blocks of alabaster, weighing at least a hundred tons apiece, have fallen from the cliff to join the other pioneers. Now it is easy to see that the entire cliff behind is composed of the same material. One of the layers of alabaster has slipped out, slid sideways, and now rises 35ft into the air from the beach. From the sea side of the beach it resembles a pillar. Erosion of the alabaster is fast. Since 1998, the sea has eroded the lower portions of the blocks, so they have 'waists'. Although interesting to look at, this feature has rendered the blocks even more unsafe than before. Be very careful when close to the blocks, and get ready to run at the faintest of noises.

Climactic Climatics

This coast records the turning point between the Triassic and Jurassic eras. It used to be mountainous desert around here and you can still see evidence of it. The soil is stained red by haematite (oxidised iron ore, usually in particulate form), laid down when the desert was young. There is also more obvious evidence. Among the two hundred million-year-old rocks on the beach you can see fossilised ripples - identical to those found on the beach sand after the tide has gone out. There are also rocks that look like the cracked mud you would get at the bottom of a dried up puddle or pool, and this is precisely what they record - an incursion of sea into a desert, followed by a retreat.

This happened many times, perhaps over a period of millions of years. There is other evidence of a turbulent past; jumbled conglomerates lie here and there. Conglomerates usually gather where a vigorous, fast flowing river or stream connects with a slower moving body of water or lake. The vigorous river then dumps its detritus (wood, gravel, rocks, etc) into the slow body, where it might compact over years and turn into a conglomerate. The higgledy-piggledy patterns of the conglomerate rocks on this beach point to violent flooding and recession as the seas washed in and out of the ancient desert. The desert was eventually covered in ocean, marking the beginning of the Lower Jurassic period. The iridescent ammonites found here (Caloceras johnstoni) are the 'zone fossils' for the Lower Jurassic. A zone fossil marks definitive borders between one age and the next. When a fossil is particularly common worldwide, strata can be identified and dated by their presence.

Sea-Shells, Bone-Beds and Iridescent Ammonites

Among the more famous fossils along this stretch are the bone beds. These are, as the name would suggest, beds of fossils composed of bones, mainly of fishes, but also containing scales and teeth. Entire fishes have been found a little further on. There are also what appear to be miniature scallops in profusion, captured in a dark, hard rock. But the most obvious of fossils in this area have to be the aforementioned iridescent ammonites. Some are over two feet across, and when wet, shine like mother of pearl. This Researcher has a 12" by 10" slate covered edge to edge with ammonites of this nature, none larger than an inch across. People have been down to this beach with pneumatic drills and other heavy equipment to dig these prizes out of the rock. This is a huge shame, as some of the largest ammonites are otherwise too heavy to move, and would have given the casual passer-by enjoyment for years to come.

Is There Somewhere Else You Could Go?

The short and happy answer is yes. Britain is brimming with fossils, some of which are easier to find than others. The easiest of all are in gravel paths and driveways. Have a good look in among the stones and, if you find one that has a patterned surface (rather like someone has pushed a sponge into it) you have found yourself a fossil coral. Also, everyone has heard about Lyme Regis. Again, caution is advised under the cliffs, but if you keep your head down, and walk along the beach, you should come away with an ammonite or two. One tip is to visit your local museum - they frequently run days out, where children of all ages can go to a site accompanied by a professional palaeontologist or geologist.

If this entry has piqued your interest in Somerset, geology, or fossils, then try the entries below, which deal with all three subjects in varying degrees:

  • A Deep-Time Sketch of the Mendips, Somerset, UK
  • Glastonbury Town and Tor, Somerset, UK
  • The City of Wells, the Cathedral and Bishop's Palace, Somerset, UK
  • The Reverend Toplady and the Rock of Ages, Somerset, UK
  • The Ichthyosaurs of Street, Somerset, UK

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