Formation · Jurassic

Kimmeridge Clay Formation

Late Jurassic unit, Dorset · Wiltshire · Oxfordshire · Cambridgeshire

The Kimmeridge Clay is the Late Jurassic marine mudstone of Dorset's Kimmeridge Bay and the English Midlands, famous for pliosaurs, ichthyosaurs, marine crocodiles and flattened ammonites such as Aulacostephanus and Pectinatites. Trade material is mostly ammonites, fish scales and isolated reptile teeth and vertebrae; the Etches Collection at Kimmeridge shows what a lifetime of beach collecting can build. Kimmeridge fossils are pyritic and flatten on drying, so mounted specimens are usually consolidated.

Country	United Kingdom
Region	Dorset, Wiltshire, Oxfordshire, Cambridgeshire, Yorkshire
Age	Kimmeridgian, Late Jurassic
Age range	~157–149 Ma
Group	Ancholme Group
Macrostrat rank	Fm· Macrostrat
Macrostrat age	155–149 Ma· Macrostrat
PBDB collections	123 collections, 340 occurrences· Paleobiology Database

Collecting and legality

Kimmeridge Bay is on the Smedmore Estate within the Jurassic Coast World Heritage Site; loose beach material may be collected under the same code of conduct, cliffs are not to be dug, and the Etches Collection museum takes important finds. Inland exposures are working clay pits and require permission.

Lithology and setting

Dark, organic-rich mudstones and oil shales with cementstone bands, the main North Sea oil source rock.

Commonly recorded taxa (PBDB)

Genera most often recorded from this unit in the Paleobiology Database (first 200 occurrences): Pliosaurus (14), Sauropoda (7), Colymbosaurus (6), Grammatodon (6), Thalassophonea (6), Deltoideum (Deltoideum) (5), Venericyprina (5), Lepidotes (4), Protocardia (4), Pterosauria (4), Theropoda (4), Thracia (4), Corbulomima (3), Dakosaurus (3), Nanogyra (Nanogyra) (3).

Source: Paleobiology Database

Elsewhere

Cite this article

Stratum Study. "Kimmeridge Clay Formation" (Formation). Stratum Study pack 2026.09, as of 2026-09-19. https://stratum.black/study/formations/kimmeridge-clay-formation. Accessed 2026-09-20.

BibTeX · CSL-JSON

@misc{stratum-kimmeridge-clay-formation,
  title        = {Kimmeridge Clay Formation},
  author       = {{Stratum Study}},
  howpublished = {\url{https://stratum.black/study/formations/kimmeridge-clay-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.

Collecting and legality

Lithology and setting

Commonly recorded taxa (PBDB)

Age

Typical taxa

Elsewhere