The Blue Lias comprises decimetre scale alternations of argillaceouslimestone and mudstone. These alternations are caused by short-term climatic variations during the Early Jurassic attributed to orbital forcing (Milankovitch cycles). These limestone-mudstone alternations pass up into a clay member formerly known as the Lower Lias Clay now the Charmouth Mudstone. This lithology consists of monotonous mudstones weathering to clay at the surface. Sparse thin limestone and nodule bands are seen where the rocks are exposed. The deposition of a clay-rich mudstone member normally indicates deposition in a deeper marine environment. The lowest beds of the formation are referred to as the "Pre-planorbis beds" in reference them being deposited before the first appearance of the ammonite Psiloceras planorbis.
Wilmcote Limestone
In certain restricted parts of Britain, the lowermost member of the Blue Lias is the Wilmcote Limestone. It lies above the Cotham Member of the Lilstock Formation and beneath the Saltford Shale Member.[2] The Wilmcote Limestone of central England was formerly quarried close to Stratford-upon-Avon, for example at Wilmcote, Temple Grafton and Binton. It is roughly 200 million years old, dating back to the dawn of the Jurassic Period.
Much of the Wilmcote Limestone is very fine-grained, blue-grey when fresh, and very finely layered. Fossils are quite rare, except in the lowest beds. It was formerly used for a variety of purposes, including walling, building, paving, gravestones, cement-making and as a source of agricultural lime. It is no longer quarried, and most of the old quarries are either infilled or overgrown.
Geologists think that the Wilmcote Limestone originated as layers of fine-grained mud on the floor of a sheltered, shallow muddy sea or lagoon that covered parts of central England at the dawn of the Jurassic Period. Very little life could tolerate the stagnant conditions on the seabed. As a consequence the mud was seldom disturbed, which is why the fine, paper-like layering is preserved.
Above the sea bed, the shallower waters supported ammonites, fish, and marine reptiles (ichthyosaurs and plesiosaurs). Their remains were discovered in the Wilmcote Limestone quarries during the nineteenth century. The Warwickshire Museum[3] houses a collection of these fossils and some are on display at the Market Hall Museum in Warwick.[4][5]
Blue Lias is useful as a building stone, and as a source of lime for making lime mortar. Because it is argillaceous, the lime is hydraulic. Since the mid-nineteenth century, it has been used as a raw material for cement, in South Wales, Somerset, Warwickshire, and Leicestershire. The cement plant quarry at Rugby, Warwickshire is probably the best exposure of the formation: more than 100 layers can be seen.
In areas where Blue Lias is quarried it has been used in buildings and churches as well as tombstones in cemeteries. An example of a Blue Lias town is Street, near Glastonbury. Other examples of Blue Lias buildings can be found in the nearby towns of Somerton and Ilchester.
It remains popular in more modern-day surroundings where it is used in the construction of new housing developments and extensions for existing buildings in conservation areas. Blue Lias is mainly used in flooring, walling and paving slabs – both coursed and layered. It is also used in the making of flagstones and cobbles.
There are only four quarries in Somerset quarrying Blue Lias at present. AR Purnell at Ashen Cross Quarry in Somerton have been mining blue lias stone since 1996. Hadspen Quarry Ltd. Hadspen Quarry operate one in Keinton Mandeville. Ham & Doulting Stone Co Ltd. operate one of these, Tout Quarry near Somerton.[7]
Paleofauna
The rock is rich in fossil remains from the Jurassic period. The blue-grey colour is provided by its iron content, enclosed to a large extent in pyrites.[8]
Dinosaurs
Theropods
Pterosaurs
Fish
Numerous fish species are known from the Blue Lias and overlying Charmouth Mudstone.
Ichthyosaurs
Plesiosaurs
Insects
Insect compression fossils are known from the localities of Binton in Warwickshire and Copt Heath near Birmingham.[23]
^Ambrose, K.; 2001: The lithostratigraphy of the Blue Lias Formation (Late Rhaetian–Early Sinemurian) in the southern part of the English Midlands Archived 7 March 2006 at the Wayback Machine, Proceedings of the Geologists' Association 112(2), pp. 97-110.
^"Wilmcote Limestone Member". The BGS Lexicon of Named Rock Units. British Geological Survey. Retrieved 20 April 2014.
^"Museum Service – Warwickshire Heritage and Culture". Archived from the original on 21 April 2014. Retrieved 20 April 2014.
^Ambrose, K., 2001. The lithostratigraphy of the Blue Lias Formation (Late Rhaetian - Early Sinemurian) in the southern part of the English Midlands. Proceedings of the Geologists' Association vol. 112, 97-110.
^Williams, B.J. & Whittaker, A., 1974. Geology of the Country around Stratford-upon-Avon and Evesham. Memoirs of the Geological Survey of Great Britain. HMSO, London.
^Wilson et al., 1990 Geology of the South Wales Coalfield, Part VI, the country around Bridgend Mem Br Geol Surv sheet 261 & 262 (England and Wales)
^"Strategic Stone Study: A Building Stone Atlas of Somerset and Exmoor" (PDF). English Heritage. p. 11. Retrieved 11 October 2011.
^Weishampel, David B; et al. (2004). "Dinosaur distribution (Early Jurassic, Europe)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 532–534. ISBN 0-520-24209-2.
^Baron, Matthew G. (29 April 2024). "A new name for old bones: A reassessment of Early Jurassic theropod remains from Dorset, England". Palaeontologia Electronica. 27 (1): 1–12. doi:10.26879/1346. ISSN 1094-8074.
^ a b c dBenson, Roger B. J.; Evans, Mark; Druckenmiller, Patrick S. (16 March 2012). Lalueza-Fox, Carles (ed.). "High Diversity, Low Disparity and Small Body Size in Plesiosaurs (Reptilia, Sauropterygia) from the Triassic–Jurassic Boundary". PLOS ONE. 7 (3): e31838. Bibcode:2012PLoSO...731838B. doi:10.1371/journal.pone.0031838. ISSN 1932-6203. PMC 3306369. PMID 22438869.
^Choiniere, Jonah N.; Wills, Simon; Bennett, S. Christopher; Barrett, Paul M. (2020). "A small theropod dinosaur from the Lower Jurassic Lias Group of Charmouth, Dorset". Proceedings of the Geologists' Association. 131 (6): 751–757. Bibcode:2020PrGA..131..751C. doi:10.1016/j.pgeola.2020.08.003. ISSN 0016-7878.
^ a bLomax, Dean R.; Massare, Judy A. (February 2017). Benson, Roger (ed.). "Two new species of Ichthyosaurus from the lowermost Jurassic (Hettangian) of Somerset, England". Papers in Palaeontology. 3 (1): 1–20. Bibcode:2017PPal....3....1L. doi:10.1002/spp2.1065. S2CID 89567182.
^Mcgowan, Christopher (1989). "Computed Tomography Reveals Further Details of Excalibosaurus, a Putative Ancestor for the Swordfish-Like Ichthyosaur Eurhinosaurus". Journal of Vertebrate Paleontology. 9 (3): 269–281. Bibcode:1989JVPal...9..269M. doi:10.1080/02724634.1989.10011762. JSTOR 4523265.
^Lomax, Dean R.; Massare, Judy A.; Mistry, Rashmiben T. (3 September 2017). "The taxonomic utility of forefin morphology in Lower Jurassic ichthyosaurs: Protoichthyosaurus and Ichthyosaurus". Journal of Vertebrate Paleontology. 37 (5): e1361433. Bibcode:2017JVPal..37E1433L. doi:10.1080/02724634.2017.1361433. ISSN 0272-4634. S2CID 90238537.
^Lomax, Dean R.; Evans, Mark; Carpenter, Simon (January 2019). Somerville, Id (ed.). "An ichthyosaur from the UK Triassic-Jurassic boundary: A second specimen of the leptonectid ichthyosaur Wahlisaurus massarae Lomax 2016". Geological Journal. 54 (1): 83–90. Bibcode:2019GeolJ..54...83L. doi:10.1002/gj.3155. S2CID 134448865.
^Martin, Jeremy; Vincent, Peggy; Suan, Guillaume; Sharpe, Tom; Hodges, Peter; Williams, Matt; Howells, Cindy; Fischer, Valentin (2014). "A mysterious giant ichthyosaur from the lowermost Jurassic of Wales". Acta Palaeontologica Polonica. doi:10.4202/app.00062.2014.
^"PV R 1158". NHM Data Portal. NHM. Retrieved 10 August 2022.
^McGowan, Chris (2003). "A new specimen of Excalibosaurus from the English Lower Jurassic". Journal of Vertebrate Paleontology. 23 (4): 950–956. Bibcode:2003JVPal..23..950M. doi:10.1671/1860-20. ISSN 0272-4634.
^McGowan, Chris (1986). "A putative ancestor for the swordfish-like ichthyosaur Eurhinosaurus". Nature. 322 (6078): 454–456. Bibcode:1986Natur.322..454M. doi:10.1038/322454a0.
^Smith, As (2015). "Reassessment of 'Plesiosaurus' megacephalus (Sauropterygia: Plesiosauria) from the Triassic-Jurassic boundary, UK". Palaeontologia Electronica. 18 (1): 1–20. doi:10.26879/504. ISSN 1094-8074.
^Benson, Roger B. J.; Bates, Karl T.; Johnson, Mark R.; Withers, Philip J. (May 2011). "Cranial anatomy of Thalassiodracon hawkinsii (Reptilia, Plesiosauria) from the Early Jurassic of Somerset, United Kingdom". Journal of Vertebrate Paleontology. 31 (3): 562–574. Bibcode:2011JVPal..31..562B. doi:10.1080/02724634.2011.572937. ISSN 0272-4634. S2CID 39844147.
^Kelly, Richard S.; Ross, Andrew J.; Coram, Robert A. (2018). "A Review of Necrotauliids from the Triassic/Jurassic of England (Trichoptera: Necrotauliidae)". Psyche: A Journal of Entomology. 2018: 1–12. doi:10.1155/2018/6706120. hdl:1983/06ae01fb-06bc-41d0-bcbd-f7519cb07df6. ISSN 0033-2615.
External links
"Explore the Jurassic Coast" at the National Trust
The Philpot Museum website
Geology of Whitby
Geology of Lyme Regis Area
Geology of the Wessex Coast
Ham & Doulting Stone Co Ltd
Conservation of Blue lias article
Fossils of the Blue Lias Formation -- A quick guide