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Blue Lias Formation — Blue Lias Formation
Blue Lias Formation — MichaelMaggs, Wikimedia Commons, CC BY SA 2.5

Formation · Jurassic

Blue Lias Formation

Early Jurassic unit, Dorset · Somerset · Warwickshire · Glamorgan

The Blue Lias and Charmouth Mudstone between Lyme Regis and Charmouth are Mary Anning's cliffs: ichthyosaurs, plesiosaurs, the fish Dapedium, and the golden pyritised ammonites (Promicroceras, Arnioceras) and Asteroceras that wash out after winter storms. Pyrite ammonites are cheap and lovely but decay in damp air, so store them dry. Marine reptile skulls and skeletons from the Black Ven mudslides are prepared by a handful of specialists and sell for large sums; ask for the finder and date, since the collecting code records important specimens.

CountryUnited Kingdom
RegionDorset, Somerset, Warwickshire, Glamorgan
AgeHettangian, Early Jurassic
Age range~201–190 Ma
GroupLias Group
Macrostrat rankFm· Macrostrat
Macrostrat age206–193 Ma· Macrostrat
PBDB collections73 collections, 448 occurrences· Paleobiology Database

Collecting and legality

The Jurassic Coast is a World Heritage Site with a voluntary code of conduct: collect from the beach and fallen material, never dig the cliffs, and report scientifically important finds through the West Dorset Fossil Collecting Code recording scheme. Beach-found fossils belong to the finder and may be sold; the Charmouth Heritage Coast Centre explains the code.

Lithology and setting

Alternating limestone and dark shale beds of a shallow, muddy Early Jurassic sea; the overlying Charmouth Mudstone holds the pyritised ammonites and Black Ven marine reptiles.

Commonly recorded taxa (PBDB)

Genera most often recorded from this unit in the Paleobiology Database (first 200 occurrences): Plagiostoma (13), Liostrea (12), Schlotheimia (11), Isocrinus (10), Cardinia (9), Pinna (9), Pseudolimea (9), Terquemia (9), Cenoceras (8), Modiolus (Cyranus) (8), Gryphaea (7), Mactromya (7), Camptonectes (6), Coroniceras (6), Antiquilima (Antiquilima) (5).

Source: Paleobiology Database

Age

Localities

Typical taxa

Referenced by

Elsewhere

Cite this article

Stratum Study. "Blue Lias Formation" (Formation). Stratum Study pack 2026.09, as of 2026-09-19. https://stratum.black/study/formations/blue-lias-formation. Accessed 2026-09-20.

BibTeX · CSL-JSON

@misc{stratum-blue-lias-formation,
  title        = {Blue Lias Formation},
  author       = {{Stratum Study}},
  howpublished = {\url{https://stratum.black/study/formations/blue-lias-formation}},
  year         = {2026},
  note         = {Formation article, Stratum Study pack 2026.09, as of 2026-09-19; sources: stratum, macrostrat, pbdb, wikidata, commons. Accessed 2026-09-20.}
}

Citations carry the pack version and the article's as-of date so they resolve to the same text later. The facts themselves belong to the sources listed below; cite those for the science.