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Fossils of the Blue Lias Formation -- A quick guide

Liostrea hisingeri

Liostrea hisingeri

Each 40mm long

Psiloceratid ammonite

Psiloceratid

ammonite

FOV=100mm

Schlotheimiid ammonite

Schlotheimiid

ammonite

FOV=100mm

Arietitid ammonite

Arietitid ammonite

FOV=300mm

Plagiostoma giganteum

Plagiostoma

giganteum

FOV=120mm

Cardinia

Cardinia sp.

FOV=40mm

Gryphaea

Gryphaea sp.

FOV=100mm

Crinoid fragments

Crinoid fragments

FOV=10mm

Branch (driftwood)

Branch (driftwood)

width 50mm

The Blue Lias Formation is exposed at a number of coastal areas in South West England including Lyme Regis in Dorset and Lilstock, Kilve, Quantoxhead and Watchet on the Somerset coast. The formation was deposited offshore in water below wave base and shows a characteristic repetition of beds of limestone and shale when seen in a cliff section. A diverse fossil record is preserved in these beds, most famously the large ammonites that can be seen on the foreshore at Lyme Regis, and a number of the well documented ichthyosaur fossils that have been discovered in its uppermost beds in the same area. In fact, remains of large reptiles such as this are certainly not uncommon, and it's by no means unusual to find teeth, ribs or vertebra at most exposures of the formation. Other vertebrates are well represented, too, including a number of different species of fish. However, the full palaeontological diversity is outside the scope of this short piece.

What I intend to do here is give a brief introduction to what can be most commonly found, and provide a very rough guide to where in the formation you'll find it. This is particularly useful on the Somerset coast where the Blue Lias is around 175m deep; both faulting and folding of the beds mean most of those 175m can be examined somewhere along the shore between Blue Anchor and Hinkley Point.

The Blue Lias formation actually starts in the Triassic with several metres of beds above the Lilstock Formation of the Penarth Group. From here it runs through the Hettangian Stage of the Jurassic, and into the Semicostatum Zone of the Sinemurian Stage where the cycles of limestone and shale gradually change into the predominantly mudstone lithology of the Charmouth Mudstone Formation. These zones are based on ammonite species, and these can further be broken down into subzones, but that's a level of detail not required here. It is, however, useful to state the relevant zones to give some context for the next few paragraphs. Within the Hettangian Stage, and starting with the oldest, the zones are Planorbis, Liasicus and Angulata. In the base of the Sinemurian Stage they are Bucklandi and Semicostatum

The beds below the Planorbis zone at the base of the Blue Lias Formation are called the pre-planorbis beds. Ammonites are absent from these, and typically they contain bedding plains covered with the bivalve Liostrea hisingeri. Ichthyosaur remains are recovered from this level, too, but although not uncommon, they are more likely to have been picked up by collectors. The Planorbis Zone is where you'll find the first ammonites, typically Psiloceras planorbis; this zone is the first of the Hettangian Stage, and marks the start of the Jurassic Period. Psiloceras planorbis along with various other psiloceratids dominate the zone, and are characteristically either smooth, or very weakly ribbed. Specimens are usually crushed in the shale units and preserved in 2D, often having a striking iridescent sheen when freshly exposed. The Planorbis zone may also contain the small bivalve Cardinia sp. and the larger Plagiostoma giganteum both of which occur through the remainder of the formation.

The Liasicus Zone sees the first Schlotheimiid ammonites appear with their more prominent ribbing. Waehnoceras sp. occurs first, and is replaced by Schlotheimia sp. in the Angulata zone. The first examples of the bivalve Gryphaea sp. can be found, too, in this zone, and higher in the succession can be extremely abundant at certain levels. The next noticeable change occurs at the base of the Bucklandi zone where arietitid ammonites with strong ribbing, keels, and often larger size join the schlotheimiids. One of these is Metophioceras sp. that can be seen on the foreshore at Lyme Regis in large numbers on limestone beds exposed at low tide.

Other common fossils that occur throughout the formation are pieces of crinoids, with their characteristic 5-fold symmetry, and lengths of fossilised driftwood. Fish scales are often present, too, and can sometimes be seen after splitting blocks of shale, though you'll need a close look! Belemnites are not common in the lower part of the formation, but can be found preserved more frequently near the top. Another interesting point is that a number of classes, or species, show a size increase as they evolve through the time taken for deposition of the formation. Ammonites are one example, but the same can be said for the genus Gryphaea and the species Plagiostoma giganteum

I hope that's of use to some people. Don't get the impression, though, that the fossils I've stated above are the only ones likely to be found. I've just given a quick overview based on a number of visits I've made, and the literature I've read relating to these areas. A good starting point to learn more is the Geologists' Association Guide No.22 listed in the bibliography below.

For anyone wishing to see an outcrop of the Blue Lias formation in the South West of England then the beach to the West of Lyme Regis provides the most readily accessible location. The photo on this page titled "Blue Lias Formation, Dorset" was taken there. However, note that you're also likely to find material from the Charmouth Mudstone Formation which is the next formation in the succession, as well as Cretaceous material that sits above this over an unconformity. In fact, for fossil collecting the Charmouth Mudstone Formation is more productive, and the ammonites usually better preserved, but I'll write about that another time.

2006

Blue Lias Formation, Dorset

Blue Lias Formation, Dorset.

Typical limestone and shale beds

Typical limestone and shale beds

Position of Blue Lias Formation in Jurassic

Position of Blue Lias Formation in Jurassic

Bibliography

Dean, W.T., Donovan, D.T. and Howarth., M.K. 1961. The Jurassic ammonite zones and subzones of the North Western European Province: Bulletin of the British Museum of Natural History, v.4, pp. 435-505.

House, M.R. 1993. Geology of the Dorset Coast. Geologists' Association guide No.22. 164pp

Simms, M.J., Chidlaw, N., Morton, N. and Page, K.N. 2004. British Lower Jurassic Stratigraphy, Geological Conservation Review Series, No.30, Joint Nature Conservation Committee, Peterborough.

Whittaker, A. and Green, G.W. 1983. Geology of the country around Weston-super-Mare. Mem. Geol. Surv. G.B., Sheet 279 with parts of 263 and 295.