Neale Monks (UK)
Fig. 1. The sites and geology of West Somerset.
The West Somerset coastline, between Blue Anchor and Lilstock, exposes an almost complete succession of strata from the latest Triassic well into the Early Jurassic (Fig. 1). Fossils are abundant throughout, mostly invertebrates such as ammonites, brachiopods and oysters, but also with vertebrate remains at certain horizons.
Most of the exposures are well suited to family visits, with good parking and obvious pathways down to the beach. Indeed, I have fond memories of collecting at Kilve, Quantoxhead and Lilstock (Fig. 2), these being the places I visited on family holidays as a child.
Fig. 2. These trace fossils were produced by worms or burrowing shrimps (Lilstock).
One thing worth noting though is that the entire coastline between Blue Anchor and Lilstock is classified as a Site of Special Scientific Interest, so hammering at the cliffs is prohibited. However, there is so much material on the foreshore that this isn’t a major problem. Casual collectors can simply turn over the slabs and stones until they find specimens worth keeping, while more expert collectors – using appropriate safety gear – may want to hammer open some of these rocks to find better the preserved specimens within.
There are three formations exposed along the West Somerset coast, the Westbury, Lilstock and Blue Lias Formations. Together, the Westbury and Lilstock Formations make up what is know as the Penarth Group.
The Westbury Formation is the oldest and represents the first half of the Rhaetian age of the Late Triassic. It is mostly a series of dark grey to black mudstone, shale and limestone beds, apparently deposited in a shallow sea. This formation isn’t particularly fossiliferous in West Somerset, though some fossils may be found at certain horizons. By contrast, where the Westbury Formation is exposed in North Somerset and South Gloucestershire, there are some very fossiliferous sandy units, the so-called Rhaetic Bone Beds, which contain numerous disarticulated marine, freshwater and terrestrial vertebrate remains.
The Lilstock Formation represents the second half of Rhaetian age. It is divided into two members the mostly shale and siltstone Cotham Member at the bottom and the mostly limestone and mudstone Langport Member at the top. While there are fossils to be found in the Lilstock Formation, their abundance and diversity is remarkably low, and it is generally believed that the Lilstock Formation was deposited in a region where salinity varied, preventing those invertebrates intolerant of salinity changes, such as ammonites and brachiopods, from becoming established.
The Blue Lias Formation contains sediments from the latest Rhaetian through to the early Sinemurian age of the Early Jurassic. Despite its name, it isn’t necessarily blue, and, at many places, is brown instead. It is very thick, up to 30m in some places, and obviously divided into alternating bands of pale limestone and dark mudstone or shale. (The word ‘lias’ was originally a quarryman’s word meaning ‘layers’.) Fossils are abundant throughout this formation and include both invertebrate and vertebrate remains. Ammonites are common and used for biostratigraphy. The Blue Lias Formation is sometimes identified with the Lower Lias, though, strictly speaking, the Lower Lias only includes the Jurassic part of the Blue Lias Formation.
Broadly speaking, the Westbury, Lilstock and Blue Lias formations represent a transition from shallow to deep water. The Westbury Formation appears to have been a shallow marine environment and, unlike the situation further north, in West Somerset the influx of freshwater and terrestrial material was relatively slight. So, while there are some occasional, non-marine vertebrate remains found here, mostly the fossils recovered from the Westbury Formation are marine animals, including sharks (Fig. 3) and ichthyosaurs (Fig. 4). Bivalves are common in the shaley beds, most notably Rhaetavicula contorta and Chlamys valoniensis.
Fig. 3. Part of a shark fin found at Blue Anchor.
Fig. 4. Ichthyosaurus vertebra from Quantoxhead.
Whereas the Westbury Formation was largely deposited under normal marine salinity conditions, the lower part of the Lilstock Formation, the Cotham Member, has features characteristic of hypersaline conditions. Mound-like structures called stromatolites (formed by photosynthetic algae and mud) are found at localities including Cotham, near Bristol. Modern stromatolites form only in hypersaline lagoons because the bacteria that form them are too vulnerable to grazing animals to exist anywhere else. Elsewhere, the Westbury Formation contains a sparse fauna of bivalves and crustaceans indicative of brackish water conditions. The upper part of the Westbury Formation, the Langport Member, sees the return of normal marine conditions. The diversity of bivalves is considerably higher and their overall abundance is also much greater.
By the time the Blue Lias was being laid down, West Somerset was thoroughly submerged under a tropical sea. Water depth varied though, with the limestones being deposited when the water was relatively shallow, and the mudstones and shales when the water was deeper. The marine life here was abundant and diverse. At the top of the food chain were the numerous ichthyosaurs (Fig. 5) and plesiosaurs that consumed smaller predators, such as ammonites, belemnites and fish.
Fig. 5. In 2002, Deposits’ Technical Controller, Alister Cruickshanks, discovered three quarters of an Ichthyosaurus skeleton at Hinkley Point. It is taking many years to prepare and this is the progress so far on the jaw section.
On the seafloor were oysters, including Gryphaea, bivalves large and small, nests of rhynchonellid brachiopods, stalked crinoids (Fig. 6), and any number of worms and shrimps only apparent now thanks to the burrows they left behind.
Fig. 6. Crinoid stems (Pentacrinus sp.) from Watchet.
Every bit of the coast between Blue Anchor and Lilstock is worth exploring and likely to yield some fossils. Watchet and Quantoxhead are probably the places where the more charismatic fossils, including ammonites, are most abundant, though Kilve and Lilstock are both very good too. Blue Anchor is a bit less fossiliferous than these, but the Westbury Formation is better exposed here making it the place to go for fish remains.
Starting with the Westbury Formation, the most common fossils found in the West Somerset part of this formation are bivalves of various types. Rhaetavicula contorta is probably the most distinctive fossil. It is a small, thin-shelled, scallop-like bivalve with a distinctive twisted shell, hence the contorta part of its name. It is most often found pyritised in shaley beds, often in considerable numbers, which leads geologists to believe that this bivalve was peculiarly well adapted to what was likely a muddy, oxygen-poor environment that other bivalves couldn’t tolerate. Chlamys valoniensis is another common fossil and, where it is found, it can again be found in great numbers. Indeed, some of the thick, shaley beds seem to be packed with the remains of this medium-sized scallop.
Fish remains tend to be disarticulated teeth, spines and small bones, and identifying these is generally difficult. Suffice it to say that the diversity is considerable and includes a variety of sharks, lungfish and bony fish. Ichthyosaur (Fig. 7) and plesiosaur bones, typically vertebrae, can be found as well, but these are not common and you need to be lucky to find one before a professional collector does.
Fig. 7. Ichthyosaurus rib found in a foreshore boulder on Watchet beach.
For the most part, the green-grey Cotham Member (part of the Lilstock Formation) isn’t fossiliferous for the reasons mentioned earlier, but the so-called White Lias facies, which makes up the Langport Member part of this formation, contains numerous shallow water bivalves. Mussels (Modiolus spp.) (Fig. 8) and various oysters are particularly common.
Fig. 8. Modiolus brachiopods from the Lilstock Formation at Blue Anchor.
The base of the Blue Lias starts with thin shale beds that yield to the alternating beds of shales, mudstones and limestones so characteristic of this formation. Over the years, geologists have argued about precisely where the base of the Jurassic should be. The problem is that the ammonite that supposedly indicates the start of the Jurassic in the UK, a species called Psiloceras planorbis (Fig. 9), isn’t actually found at the base of the Blue Lias.
Fig. 9. Psiloceras planorbis ammonite from Kilve.
Consequently, the beds beneath its first appearance have become known as the Pre-Planorbis Beds. Depending on your point of view, these Pre-Planorbis Beds are either the uppermost part of the Triassic or the lowest part of the Jurassic. In any event, the Pre-Planorbis Beds may lack ammonites but they do contain other types of fossil, including an extremely common oyster species, Liostrea hisingeri. Some geologists actually call these beds Oyster Beds because of this.
Psiloceras planorbis itself is a common ammonite and relatively easy to identify. Its species name comes from a genus of pond snail, Planorbis, which have smooth, spiral shells. So, if you find an ammonite that looks like a pond snail, chances are that it is Psiloceras planorbis. In fact, this ammonite wasn’t completely smooth, but was instead ornamented with very fine ribs.
A succession of ammonites can be found in higher beds, including Alsatites liasicus, Schlotheimia angulata, Arietites bucklandi and Arnioceras semicostatum, each of which is the zone fossil for a particular stratum. Alsatites liasicus (Fig. 10) is a species with a loose coil, much like Psiloceras planorbis, but with much stronger ribs.
Fig. 10) Large ammonites, such as Alsatites liasicus, will rarely be found whole (Lilstock).
Schlotheimia angulata is more tightly coiled and its ribs are slightly curved, and running around the ventral surface (effectively the outer edge of spiral) is a distinctive groove. Arietites bucklandi is one of the bigger ammonite species, often more than 30cm in diameter and, like Schlotheimia angulata, has a ventral groove. However, unlike Schlotheimia angulata, its ribs are straight and quite widely spaced. Arnioceras semicostatum (Fig. 11) is similar but has a very distinctive ventral surface with a sharp keel bounded with grooves on either side.
Fig. 11. Ammonites are common at Helwell Bay (just east of Watchet Harbour). This specimen is an Arnioceras, which can be easily found in the ledges along the foreshore.
Belemnites are found in the Blue Lias, but are not particularly common. On the other hand, bivalves of various types are abundant, especially oysters. One of the most distinctive is Plagiostoma gigas (Fig. 12), a scallop-like species that occasionally attained about the same size and shape as a rugby ball. It has a slightly rough shell texture and the group of bivalves to which it belongs are sometimes known as ‘file shells’.
Fig. 12. Giant bivalve (Plagiostoma giganteum) from Quantoxhead.
The peculiar Devil’s toenail, Gryphaea arcuata, can also be found in Blue Lias. This oyster is unusual in being a free-living species – most oysters, including all the modern species, live attached to something else, typically rocks or corals. Some geologists have argued that Gryphaea was derived from Liostrea, as the latter adapted to a muddier environment. Through the remainder of the Jurassic Period, further species of Gryphaea evolved, steadily becoming better adapted to life on muddy substrates, for example, by having wider, more boat-like shells that sit more stably on the substrate.
Brachiopods are not diverse, but they are numerous. The small rhynchonellid, Calcirhynchia calcarea, is especially common and cracked specimens often reveal sparry calcite crystals within them. In fact, cobbles with calcite blobs 5mm or so across stand a good chance of being nests of these brachiopods.
Plesiosaur and ichthyosaur vertebrae are usually found in situ rather than loose amongst the cobbles and scree. As it is against the law to hammer away at the cliffs here, you have to wait until after a heavy storm and then go looking for ones that have become exposed. With luck, you’ll be able to winkle these out without any hammering.
Care needs to be taken to wrap each fossil in newspaper after collection. Limestone is quite abrasive, and loose specimens bouncing around in rucksacks and pockets will quickly abrade any detail from one another. Pyritised specimens will need to be kept in a dry, airtight box or tin otherwise they will eventually decay.
Fig. 13. Various large, solitary corals in a Carboniferous Limestone boulder (Lilstock beach car park).
Fig. 1. The sites and geology of West Somerset.
The West Somerset coastline, between Blue Anchor and Lilstock, exposes an almost complete succession of strata from the latest Triassic well into the Early Jurassic (Fig. 1). Fossils are abundant throughout, mostly invertebrates such as ammonites, brachiopods and oysters, but also with vertebrate remains at certain horizons.
Most of the exposures are well suited to family visits, with good parking and obvious pathways down to the beach. Indeed, I have fond memories of collecting at Kilve, Quantoxhead and Lilstock (Fig. 2), these being the places I visited on family holidays as a child.
Fig. 2. These trace fossils were produced by worms or burrowing shrimps (Lilstock).
One thing worth noting though is that the entire coastline between Blue Anchor and Lilstock is classified as a Site of Special Scientific Interest, so hammering at the cliffs is prohibited. However, there is so much material on the foreshore that this isn’t a major problem. Casual collectors can simply turn over the slabs and stones until they find specimens worth keeping, while more expert collectors – using appropriate safety gear – may want to hammer open some of these rocks to find better the preserved specimens within.
Stratigraphy
There are three formations exposed along the West Somerset coast, the Westbury, Lilstock and Blue Lias Formations. Together, the Westbury and Lilstock Formations make up what is know as the Penarth Group.
The Westbury Formation is the oldest and represents the first half of the Rhaetian age of the Late Triassic. It is mostly a series of dark grey to black mudstone, shale and limestone beds, apparently deposited in a shallow sea. This formation isn’t particularly fossiliferous in West Somerset, though some fossils may be found at certain horizons. By contrast, where the Westbury Formation is exposed in North Somerset and South Gloucestershire, there are some very fossiliferous sandy units, the so-called Rhaetic Bone Beds, which contain numerous disarticulated marine, freshwater and terrestrial vertebrate remains.
The Lilstock Formation represents the second half of Rhaetian age. It is divided into two members the mostly shale and siltstone Cotham Member at the bottom and the mostly limestone and mudstone Langport Member at the top. While there are fossils to be found in the Lilstock Formation, their abundance and diversity is remarkably low, and it is generally believed that the Lilstock Formation was deposited in a region where salinity varied, preventing those invertebrates intolerant of salinity changes, such as ammonites and brachiopods, from becoming established.
The Blue Lias Formation contains sediments from the latest Rhaetian through to the early Sinemurian age of the Early Jurassic. Despite its name, it isn’t necessarily blue, and, at many places, is brown instead. It is very thick, up to 30m in some places, and obviously divided into alternating bands of pale limestone and dark mudstone or shale. (The word ‘lias’ was originally a quarryman’s word meaning ‘layers’.) Fossils are abundant throughout this formation and include both invertebrate and vertebrate remains. Ammonites are common and used for biostratigraphy. The Blue Lias Formation is sometimes identified with the Lower Lias, though, strictly speaking, the Lower Lias only includes the Jurassic part of the Blue Lias Formation.
Ecology
Broadly speaking, the Westbury, Lilstock and Blue Lias formations represent a transition from shallow to deep water. The Westbury Formation appears to have been a shallow marine environment and, unlike the situation further north, in West Somerset the influx of freshwater and terrestrial material was relatively slight. So, while there are some occasional, non-marine vertebrate remains found here, mostly the fossils recovered from the Westbury Formation are marine animals, including sharks (Fig. 3) and ichthyosaurs (Fig. 4). Bivalves are common in the shaley beds, most notably Rhaetavicula contorta and Chlamys valoniensis.
Fig. 3. Part of a shark fin found at Blue Anchor.
Fig. 4. Ichthyosaurus vertebra from Quantoxhead.
Whereas the Westbury Formation was largely deposited under normal marine salinity conditions, the lower part of the Lilstock Formation, the Cotham Member, has features characteristic of hypersaline conditions. Mound-like structures called stromatolites (formed by photosynthetic algae and mud) are found at localities including Cotham, near Bristol. Modern stromatolites form only in hypersaline lagoons because the bacteria that form them are too vulnerable to grazing animals to exist anywhere else. Elsewhere, the Westbury Formation contains a sparse fauna of bivalves and crustaceans indicative of brackish water conditions. The upper part of the Westbury Formation, the Langport Member, sees the return of normal marine conditions. The diversity of bivalves is considerably higher and their overall abundance is also much greater.
By the time the Blue Lias was being laid down, West Somerset was thoroughly submerged under a tropical sea. Water depth varied though, with the limestones being deposited when the water was relatively shallow, and the mudstones and shales when the water was deeper. The marine life here was abundant and diverse. At the top of the food chain were the numerous ichthyosaurs (Fig. 5) and plesiosaurs that consumed smaller predators, such as ammonites, belemnites and fish.
Fig. 5. In 2002, Deposits’ Technical Controller, Alister Cruickshanks, discovered three quarters of an Ichthyosaurus skeleton at Hinkley Point. It is taking many years to prepare and this is the progress so far on the jaw section.
On the seafloor were oysters, including Gryphaea, bivalves large and small, nests of rhynchonellid brachiopods, stalked crinoids (Fig. 6), and any number of worms and shrimps only apparent now thanks to the burrows they left behind.
Fig. 6. Crinoid stems (Pentacrinus sp.) from Watchet.
Where to go
Every bit of the coast between Blue Anchor and Lilstock is worth exploring and likely to yield some fossils. Watchet and Quantoxhead are probably the places where the more charismatic fossils, including ammonites, are most abundant, though Kilve and Lilstock are both very good too. Blue Anchor is a bit less fossiliferous than these, but the Westbury Formation is better exposed here making it the place to go for fish remains.
Fossils to look for
Starting with the Westbury Formation, the most common fossils found in the West Somerset part of this formation are bivalves of various types. Rhaetavicula contorta is probably the most distinctive fossil. It is a small, thin-shelled, scallop-like bivalve with a distinctive twisted shell, hence the contorta part of its name. It is most often found pyritised in shaley beds, often in considerable numbers, which leads geologists to believe that this bivalve was peculiarly well adapted to what was likely a muddy, oxygen-poor environment that other bivalves couldn’t tolerate. Chlamys valoniensis is another common fossil and, where it is found, it can again be found in great numbers. Indeed, some of the thick, shaley beds seem to be packed with the remains of this medium-sized scallop.
Fish remains tend to be disarticulated teeth, spines and small bones, and identifying these is generally difficult. Suffice it to say that the diversity is considerable and includes a variety of sharks, lungfish and bony fish. Ichthyosaur (Fig. 7) and plesiosaur bones, typically vertebrae, can be found as well, but these are not common and you need to be lucky to find one before a professional collector does.
Fig. 7. Ichthyosaurus rib found in a foreshore boulder on Watchet beach.
For the most part, the green-grey Cotham Member (part of the Lilstock Formation) isn’t fossiliferous for the reasons mentioned earlier, but the so-called White Lias facies, which makes up the Langport Member part of this formation, contains numerous shallow water bivalves. Mussels (Modiolus spp.) (Fig. 8) and various oysters are particularly common.
Fig. 8. Modiolus brachiopods from the Lilstock Formation at Blue Anchor.
The base of the Blue Lias starts with thin shale beds that yield to the alternating beds of shales, mudstones and limestones so characteristic of this formation. Over the years, geologists have argued about precisely where the base of the Jurassic should be. The problem is that the ammonite that supposedly indicates the start of the Jurassic in the UK, a species called Psiloceras planorbis (Fig. 9), isn’t actually found at the base of the Blue Lias.
Fig. 9. Psiloceras planorbis ammonite from Kilve.
Consequently, the beds beneath its first appearance have become known as the Pre-Planorbis Beds. Depending on your point of view, these Pre-Planorbis Beds are either the uppermost part of the Triassic or the lowest part of the Jurassic. In any event, the Pre-Planorbis Beds may lack ammonites but they do contain other types of fossil, including an extremely common oyster species, Liostrea hisingeri. Some geologists actually call these beds Oyster Beds because of this.
Psiloceras planorbis itself is a common ammonite and relatively easy to identify. Its species name comes from a genus of pond snail, Planorbis, which have smooth, spiral shells. So, if you find an ammonite that looks like a pond snail, chances are that it is Psiloceras planorbis. In fact, this ammonite wasn’t completely smooth, but was instead ornamented with very fine ribs.
A succession of ammonites can be found in higher beds, including Alsatites liasicus, Schlotheimia angulata, Arietites bucklandi and Arnioceras semicostatum, each of which is the zone fossil for a particular stratum. Alsatites liasicus (Fig. 10) is a species with a loose coil, much like Psiloceras planorbis, but with much stronger ribs.
Fig. 10) Large ammonites, such as Alsatites liasicus, will rarely be found whole (Lilstock).
Schlotheimia angulata is more tightly coiled and its ribs are slightly curved, and running around the ventral surface (effectively the outer edge of spiral) is a distinctive groove. Arietites bucklandi is one of the bigger ammonite species, often more than 30cm in diameter and, like Schlotheimia angulata, has a ventral groove. However, unlike Schlotheimia angulata, its ribs are straight and quite widely spaced. Arnioceras semicostatum (Fig. 11) is similar but has a very distinctive ventral surface with a sharp keel bounded with grooves on either side.
Fig. 11. Ammonites are common at Helwell Bay (just east of Watchet Harbour). This specimen is an Arnioceras, which can be easily found in the ledges along the foreshore.
Belemnites are found in the Blue Lias, but are not particularly common. On the other hand, bivalves of various types are abundant, especially oysters. One of the most distinctive is Plagiostoma gigas (Fig. 12), a scallop-like species that occasionally attained about the same size and shape as a rugby ball. It has a slightly rough shell texture and the group of bivalves to which it belongs are sometimes known as ‘file shells’.
Fig. 12. Giant bivalve (Plagiostoma giganteum) from Quantoxhead.
The peculiar Devil’s toenail, Gryphaea arcuata, can also be found in Blue Lias. This oyster is unusual in being a free-living species – most oysters, including all the modern species, live attached to something else, typically rocks or corals. Some geologists have argued that Gryphaea was derived from Liostrea, as the latter adapted to a muddier environment. Through the remainder of the Jurassic Period, further species of Gryphaea evolved, steadily becoming better adapted to life on muddy substrates, for example, by having wider, more boat-like shells that sit more stably on the substrate.
Brachiopods are not diverse, but they are numerous. The small rhynchonellid, Calcirhynchia calcarea, is especially common and cracked specimens often reveal sparry calcite crystals within them. In fact, cobbles with calcite blobs 5mm or so across stand a good chance of being nests of these brachiopods.
Plesiosaur and ichthyosaur vertebrae are usually found in situ rather than loose amongst the cobbles and scree. As it is against the law to hammer away at the cliffs here, you have to wait until after a heavy storm and then go looking for ones that have become exposed. With luck, you’ll be able to winkle these out without any hammering.
Care needs to be taken to wrap each fossil in newspaper after collection. Limestone is quite abrasive, and loose specimens bouncing around in rucksacks and pockets will quickly abrade any detail from one another. Pyritised specimens will need to be kept in a dry, airtight box or tin otherwise they will eventually decay.
| Don’t ignore the boulders! |
|---|
| West Somerset is blessed with a few outliers of the Carboniferous Limestone formation that makes up the nearby Mendip Hills, for example, at Cannington Quarry. However, these exposures are inland and generally not accessible to collectors. Nonetheless, it is often possible to find coastal revetments made up from locally quarried Carboniferous Limestone. Careful examination reveals abundant brachiopods, solitary corals and reef-building corals (Fig. 13). While it might be tempting to take a hammer to these rocks, damaging seawalls is illegal, so you’ll have to use a camera to record any interesting finds you come across. |
Fig. 13. Various large, solitary corals in a Carboniferous Limestone boulder (Lilstock beach car park).
| Quantock rocks |
|---|
| The Quantock Hills are largely composed of Devonian limestones and sandstones. These are easily seen along the banks of streams and on roadside cuttings, and dotted about the hills are numerous abandoned quarries as well. The formation known as the Hangman Sandstone includes the pink and reddish-purple building stones widely used in this part of Somerset. The only fossiliferous Devonian sediments are those that make up the limestones formerly quarried at places like Aisholt and Holwell, but fossils are not common. As the rocks have been somewhat metamorphosed, such fossils as they do contain, typically crinoids and corals, are poorly preserved and difficult to extract. |