The Charmouth Mudstone Formation is a

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

. It preserves fossils dating back to the early part of the

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

(

Sinemurian In the geologic timescale, the Sinemurian is an age and stage in the Early or Lower Jurassic Epoch or Series. It spans the time between 199.3 ± 2 Ma and 190.8 ± 1.5 Ma (million years ago). The Sinemurian is preceded by the Hettangian and ...

–

Pliensbachian The Pliensbachian is an age (geology), age of the geologic timescale and stage (stratigraphy), stage in the stratigraphic column. It is part of the Early Jurassic, Early or Lower Jurassic epoch (geology), Epoch or series (stratigraphy), Series an ...

). It forms part of the lower

Lias Group The Lias Group or Lias is a lithostratigraphic unit (a sequence of rock strata) found in a large area of western Europe, including the British Isles, the North Sea, the Low Countries and the north of Germany. It consists of marine limestones, sh ...

. It is most prominently exposed at its type locality in cliff section between

Lyme Regis Lyme Regis is a town in west Dorset, England, west of Dorchester and east of Exeter. Sometimes dubbed the "Pearl of Dorset", it lies by the English Channel at the Dorset–Devon border. It has noted fossils in cliffs and beaches on the Heri ...

and

Charmouth Charmouth is a village and civil parish in west Dorset, England. The village is situated on the mouth of the River Char, around north-east of Lyme Regis. Dorset County Council estimated that in 2013 the population of the civil parish was 1,31 ...

(alongside the underlying

Blue Lias The Blue Lias is a geological formation in southern, eastern and western England and parts of South Wales, part of the Lias Group. The Blue Lias consists of a sequence of limestone and shale layers, laid down in latest Triassic and early Jurassi ...

) but onshore it extends northwards to

Market Weighton Market Weighton ( ) is a town and civil parish in the East Riding of Yorkshire, England. It is one of the main market towns in the East Yorkshire Wolds and lies midway between Hull and York, about from either one. According to the 2011 UK cen ...

Yorkshire Yorkshire ( ; abbreviated Yorks), formally known as the County of York, is a historic county in northern England and by far the largest in the United Kingdom. Because of its large area in comparison with other English counties, functions have ...

, and in the subsurface of the East Midlands Shelf and

Wessex Basin The Wessex Basin is a petroleum-bearing geological area located along the southern coast of England and extending into the English Channel. The onshore part of the basin covers approximately 20,000 km2 and the area that encompasses the Englis ...

. The formation is notable for its fossils, including those of ammonites and marine reptiles and rare dinosaur remains. The formation played a prominent role in the history of early paleontology, with its Lyme Regis-Charmouth exposure being frequented by fossil collectors including

Mary Anning Mary Anning (21 May 1799 – 9 March 1847) was an English fossil collector, dealer, and palaeontologist who became known around the world for the discoveries she made in Jurassic marine fossil beds in the cliffs along the English Channel ...

Stratigraphy

Shales With Beef Member

The Shales With Beef Member is around 28–30 metres thick in the Lyme Regis-Charmouth region and predominantly consists of thinly bedded medium to dark grey mudstone, blocky calcareous pale-weathering mudstone and brown-grey organic-rich mudstones with frequent

bedding Bedding, also known as bedclothes or bed linen, is the materials laid above the mattress of a bed for hygiene, warmth, protection of the mattress, and decorative effect. Bedding is the removable and washable portion of a human sleeping environm ...

parallel

veins Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated b ...

of fibrous calcite ("beef"), that are usually less than 10 centimetres thick. Several beds of nodular and tabular limestone are also present. It is the lowest unit of the formation and directly overlies the Blue Lias Formation, with the boundary being marked by a prominent bioturbated horizon. Notable persistent marker beds within the member include the laminated calcareous siltstone "Fish Bed", "Table Ledge", which consists of lens beds of limestones with mud content with nests of rhynchonellid

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

s, the Devonshire Head and the Spittles limestones and the Birchi Nodules (which are septarian concretions) The upper boundary with the Black Ven Marl Member is marked by the prominent laterally persistent limestone Birchi Tabular Bed.

Black Ven Marl Member

The Black Ven Marl Member is around 43 metres thick consists of thinly bedded dark mudstones, with several laterally persistent cementstone horizons, notable horizons include the Lower and Upper Cement beds and the Stellare nodules.

Belemnite Marl Member

The Belemnite Marl Member is around 20 to 27 metres thick, and consists of interbedded pale and dark grey calcareous mudstone, with numerous belemnites, hence the name. The top of the member is marked by the Belemnite Stone Bed

Green Ammonite Member

The Green Ammonite Member is up to 31 metres thick predominantly consists of medium grey mudstones, with 3 limestone horizons, Lower Limestone; Red Band, and Upper Limestone, it is conformably overlain by the

Dyrham Formation The Dyrham Formation is a geologic formation in England. It preserves fossils dating back to the early part of the Jurassic period (Pliensbachian). See also * List of fossiliferous stratigraphic units in England See also * Lists of fossil ...

in some areas, but in the Charmouth area there is an erosive

unconformable An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

boundary with the much younger Early Cretaceous (

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0 ...

) aged

Gault The Gault Formation is a geological formation of stiff blue clay deposited in a calm, fairly deep-water marine environment during the Lower Cretaceous Period (Upper and Middle Albian). It is well exposed in the coastal cliffs at Copt Point in ...

clays.

Paleobiota

Ammonites

Sauropterygia

Ichthyosauria

Pterosauria

Dinosauria

Fish

Numerous fish species are known from the Charmouth Mudstone and underlying Blue Lias, from such horizons as the "Fish Bed" of the Shales With Beef Member.

Insects

Numerous species of insect are known from concretions, predominately in the Black Ven Marl Member.

References

{{Reflist Geologic formations of England Jurassic England Jurassic System of Europe Sinemurian Stage Pliensbachian Stage