Lilstock Fossil Hunting

1980 – the Lilstock Formation was formally introduced
The Lilstock Formation was formally introduced in modern British stratigraphy in 1980, with Lilstock giving its name to this important latest Triassic unit. This cemented the site’s importance not just for fossil collecting, but for the formal geology of the Triassic–Jurassic boundary beds in Britain.

1986 – Excalibosaurus costini described from Lilstock
Lilstock produced the holotype of Excalibosaurus costini, described by Christopher McGowan in 1986. This unusual Early Jurassic ichthyosaur is one of the locality’s best-known fossils and is famous for its elongated upper jaw, giving it a swordfish-like appearance.

2003 – anatomy of Excalibosaurus clarified by a second specimen
Christopher McGowan described a more complete skeleton, ROM 47697, adding anatomical information about the genus originally named from Lilstock material. The Royal Ontario Museum catalogue records this second specimen as coming from Lyme Regis, so it should not be treated as a second confirmed Lilstock discovery.

2018 – giant Late Triassic ichthyosaur jaw described from Lilstock
A lower-jaw fragment collected by Paul de la Salle in 2016 was scientifically described in 2018. Recovered high in the Westbury Formation, it demonstrated that very large ichthyosaurs survived into the latest Triassic. Body-length estimates exceeding 20 metres were based on comparisons with other ichthyosaurs and remain uncertain because the fossil is incomplete.

2024 – new work on Rhaetian faunas reinforced the importance of the Lilstock section
Recent study of the Rhaetian succession at Lilstock has shown that its fish-dominated bone-bed faunas are unusual and may reflect deeper-water conditions than at many equivalent sites elsewhere in south-west England. This has added further importance to Lilstock as both a fossil and stratigraphic locality.

2024 – Lilstock jaw assigned to Ichthyotitan severnensis
Comparison with a newly described giant jaw from Blue Anchor allowed the Lilstock specimen to be referred to the new ichthyosaur Ichthyotitan severnensis. The Blue Anchor jaw is the holotype; the Lilstock jaw provides a second record of the species from the late Rhaetian.

MERCIA MUDSTONE GROUP

Blue Anchor Formation (Uppermost Norian To Rhaetian)

Upper Blue Anchor Formation / Grey Marls

The low cliff and foreshore beneath the Penarth Group at Lilstock are formed by the upper part of the Blue Anchor Formation, the old Tea Green and Grey Marl interval. Lithologically these beds are pale green-grey dolomitic silty mudstones and siltstones with thin argillaceous and arenaceous laminae, together with a few hard pale cream to buff porcellanous mudstone and siltstone bands. In west Somerset the uppermost part becomes more varied and includes grey, black, green and rarely red-brown dolomitic mudstones, laminated siltstones, yellowish-grey dolostones, mudcracks, scarce halite pseudomorphs and locally gypsum, with bivalves and bioturbation in the higher beds.

Williton Member

At Lilstock these upper marine-influenced beds are especially important because recent work has shown a Rhaetian-style vertebrate and bivalve fauna in the Williton Member, including teeth or denticles of Lissodus, Gyrolepis, Sargodon and Saurichthys, indicating that marine influence had already reached the basin before deposition of the classic black Westbury shales. Environmentally this was a mudflat to lagoonal and sabkha-margin succession passing upward into increasingly marine conditions. The top of the formation is commonly burrowed and bored and is locally slightly unconformable beneath the Westbury Formation.

PENARTH GROUP

Westbury Formation (Rhaetian)

Basal Westbury Bone Bed And Erosional Surface

The base of the Westbury Formation at Lilstock is exposed only when storm-beach cover is low, but where visible it rests sharply on the Blue Anchor Formation across an irregular erosional or non-sequential surface. This basal transgressive lag may contain reworked Blue Anchor mudstone clasts, quartz grains, phosphatic pebbles and abundant vertebrate debris. It marks the onset of the main Rhaetian marine flooding of the west Somerset basin and is one of the key surfaces in the whole section.

Lower Westbury Black Shales And Bone-Rich Arenaceous Beds

Above the basal lag lie dark grey to nearly black mudstones and shales with thin limestones, sandstones and fossiliferous arenaceous units. The lower part includes the classic bone-rich Westbury horizons, with fish scales, teeth, spines and other phosphatised vertebrate debris. Thin-shelled marine bivalves such as Rhaetavicula contorta, Eotrapezium concentricum, Chlamys valoniensis and Tutcheria cloacina are characteristic of the formation. At Lilstock, recent taphonomic work suggests that the basal and higher bone beds are dominated by osteichthyan remains, with chondrichthyans much less abundant than at some shallower Rhaetian sites farther east, implying a somewhat deeper-water setting. Sedimentologically these beds record a rapid shift into a low-oxygen marine shelf or embayment in which fine black mud accumulated under dysaerobic to anoxic bottom conditions.

Upper Westbury Shelly Black Shales

The upper Westbury Formation continues as dark fossiliferous mudstones and shales with subordinate thin limestones and sandstones. Shell beds and additional bone-rich seams may occur within it, but the general lithological character is still that of black, marine, organic-rich mudstone. The upper contact passes into the overlying Cotham Member across a non-sequence rather than through a simple conformable bedding transition. At Lilstock Bay the Westbury is an essential but not always fully visible part of the section, because beach cover can obscure its base while the higher beds are more persistently exposed in the cliff.

Lilstock Formation (Rhaetian)

Conventional BGS Eastern-Cliff Log

This bottom-to-top partial log was measured at ST 1797 4527, east of the headland at the eastern end of Lilstock Bay. The source uses Richardson bed numbers, with its stated boundary revisions. It is separate from the landslipped-block log at ST 1768 4512 and from Gallois’s alternative C-number scheme; thicknesses are not combined.

Westbury Formation

Bed10 black shale (1.35m)

Dark shale forms the lowest recorded interval of this partial cliff log.

Bed9 limestone (0.20m)

Hard fine limestone varies laterally into lenses.

Bed8 shale (2.01m)

Dark shale is partly blocky and locally contains a central fibrous-calcite seam.

Beds5b–7 limestone (0.28m)

Shell-bearing argillaceous limestone includes a central shale parting and calcite seams at its margins.

Black shale (0.64m)

A separate dark mud interval divides the limestones.

Calcite-seamed limestone (0.10m)

Dark earthy limestone contains several fibrous-calcite seams.

Upper black shale (0.91m)

This shale separates the two highest logged limestones.

Bed5a shelly limestone (0.15–0.23m)

Earthy limestone has local greenish colour, lensing and a fibrous-calcite band.

Lilstock Formation

Cotham Member

Basal silty mudstone (0.13m)

Green-grey mudstone contains silt and carbonate.

Hard siltstone (0.15m)

Grey siltstone includes faint, locally contorted lamination.

Upper micaceous mudstone (1.47m)

Greenish-grey silty mudstone is mostly blocky, laminated near its top, and includes harder nodular siltstone in its upper half.

Langport Member

Basal limestone package (0.24m)

Fine, marly, partly laminated limestone contains an internal dark mudstone layer.

Dark marly mudstone (0.02m)

A thin muddy separation lies above the basal limestone.

Porcellanous marly limestone (0.04m)

Fine carbonate forms a thin hard horizon.

Laminated greenish marl (0.05m)

Slightly fissile marl separates the limestone beds.

Dark splintery limestone (0.02m)

A very thin marly limestone precedes the thicker marl above.

Silty marl package (0.31m)

Grey marl becomes greener towards its base and includes silt bands.

Bed3 rubbly limestone (0.10–0.18m)

Pink-grey fine limestone has an uneven lower surface.

Bed2 massive limestone (0.20m)

Hard pink-grey limestone is poorly jointed.

Bed1 upper limestone (0.08–0.10m)

Grey silty limestone has a slightly uneven top.

Capping mudstone (0.13m)

Speckled green-grey mudstone completes this measured member log.

Lateral Member Variation And The Basal Westbury

The BGS landslipped-block log at ST 1768 4512 preserves a longer Westbury section, including vertebrate-bearing beds15 and23; the latter was historically included in the Sully Beds but reassigned to the Westbury Formation. Farther east, at ST18464542, the Cotham contains cracked and contorted silt-rich beds, while basal Langport limestones locally swell into large lenticular bodies. These measured lateral sections are retained as separate evidence and are not a single stack. Details and full source tables are linked below.

BGS Landslipped-Block Log At ST17684512

The following entries run from the lowest recorded horizon upwards. They describe this named exposure only; bed numbers are those retained by the BGS from Richardson. Unnumbered lithological entries are not assigned invented bed identifiers.

Westbury Formation

Basal recorded limestone (0.23–0.25 m)

Massive earthy carbonate has an uneven lower surface.

Bed24 mudstone (0.18–0.20 m)

Shaly dark mudstone overlies the basal limestone.

Bed23 limestone (0.05–0.08 m)

Dark carbonate contains fibrous calcite; the source records vertebrates here and reassigns this historical Sully bed to Westbury.

Beds18–22 mudstone (1.78 m)

Dark shaly mudstone has thin silty partings and iron-stained joints.

Nodular limestone (0–0.08 m)

A lenticular carbonate horizon with fibrous calcite can be absent.

Black shale (0.25–0.30 m)

A separate shale interval divides the limestones.

Wavy limestone (0.15 m)

Hard grey limestone displays undulating bedding.

Beds16–?17 shale (0.86–0.91 m)

Black shale forms the interval below bed15.

Bed15 limestone (0.10–0.15 m)

Earthy limestone contains calcite seams and locally much sand; vertebrate remains are recorded.

Bed14 shale (0.41 m)

Dark shale separates beds15 and13.

Bed13 limestone (0.13 m)

Dark carbonate contains fibrous calcite locally.

Beds10–12 mudstone (1.55 m)

Shaly mudstone is strongly iron-stained near its top.

Bed9 nodular limestone (0.13–0.20 m)

Earthy grey limestone varies into nodules and lenses.

Bed8 lower mudstone (1.14 m)

Shaly mudstone includes an internal calcite seam and an iron-stained upper part.

Lower limestone (0.08–0.10 m)

A thin earthy carbonate succeeds the mudstone.

Thin shale (0.05 m)

Shale separates the two thin carbonate beds.

Upper limestone (0.05–0.10 m)

Another earthy grey limestone forms a distinct horizon.

Bed8 upper shale (0.66 m)

Shale has an iron-stained uppermost layer.

Beds5b–7 lower limestone (0.05–0.08 m)

Earthy carbonate begins the higher alternation.

Mudstone parting (0.10 m)

Black mudstone separates the limestones.

Splintery limestone (0.10–0.13 m)

Hard grey carbonate forms a resistant layer.

Fibrous calcite (0.04 m)

A discrete mineral seam succeeds the limestone.

Higher mudstone (0.71 m)

Dark shaly mudstone forms the next interval.

Earthy limestone (0.05–0.08 m)

A thin dark carbonate lies below the uppermost mudstone package.

Shale seam (0–0.01 m)

The very thin intervening shale is locally missing.

Fibrous calcite seam (0.05–0.08 m)

Calcite separates the adjacent sedimentary intervals.

Shaly mudstone (0.89 m)

Dark mudstone forms most of the upper package.

Bed5a upper mudstone (0.15 m)

Less fissile mudstone has a conspicuous ochreous band at its base.

Top calcite seam (0–0.05 m)

The final fibrous-calcite horizon is locally absent.

Lilstock Formation

Cotham Member

Basal marl (1.22 m)

Green-grey marl includes lenses of laminated siltstone.

Lenticular silty limestone (0.05–0.08 m)

A hard dark carbonate bed separates the marls.

Upper blue-grey marl (0.38–0.43 m)

Marl becomes more fissile near its top.

Langport Member

Lowest laminated limestone (0.10–0.15 m)

Fine porcellanous brown-grey limestone begins this member log.

Lenticular porcellanous limestone (0.15–0.23 m)

The overlying fine carbonate varies laterally and is locally missing.

Marl seam (0.01 m)

Marl separates successive hard beds.

Dark splintery limestone (0.08 m)

Fine carbonate forms a separate resistant layer.

Marl parting (0.01 m)

A second very thin marl divides the limestones.

Pale fine limestone (0.04 m)

Fine hard carbonate caps the lower limestone alternation.

Silty marl with hard layer (0.15 m)

The source reports a hard layer as 0.2 m thick within this 0.15 m interval. Those values are internally inconsistent; the hard layer is retained descriptively without accepting the apparent thickness error.

Softer silty marl (0.08–0.10 m)

A softer muddy interval follows.

Hard grey marl (0.06 m)

More indurated marl precedes the rubbly limestone.

Bed3 rubbly limestone (0.18 m)

Carbonate has spheroidal and rubbly weathering.

Bed2 massive limestone (0.09–0.10 m)

Pale massive carbonate forms the next bed.

Marl separation (0.01 m)

A narrow marl seam separates the upper limestone pair.

Bed1 massive limestone (0.09 m)

The upper limestone has a nodular, irregular top.

Upper mudstone (0.15 m)

Greenish-grey mudstone with limestone lenses completes the log.

BGS Eastern Cliff Log At ST18464542

The following entries run from the lowest recorded horizon upwards. They describe this named exposure only; bed numbers are those retained by the BGS from Richardson. Unnumbered lithological entries are not assigned invented bed identifiers.

Lilstock Formation

Cotham Member

Lowest seen marl, bed4 (0.30 m seen)

Blocky silty marl is the lowest recorded unit; its base was not fully seen.

Contorted silty limestone (0.20–0.28 m)

Irregular carbonate is harder near its base and shows disturbed bedding.

Contorted siltstone (0.76 m)

Green-grey siltstone retains folded or disrupted fine layers.

Beds2–3 upper silty marl (0.38 m)

Sandy lenses occur low in the bed; sediment-filled cracks descend roughly 0.46 m into underlying sediment.

Langport Member

Basal laminated limestone group (0.25–0.28 m)

Two or locally three fine carbonate beds occur, with a comparatively persistent basal bed.

Silty marl (0.41 m)

Harder laminated bands occur within this grey muddy interval.

Bed3 conglomeratic limestone (0.15–0.20 m)

Rubbly carbonate rests on an uneven surface.

Mudstone parting (0–0.02 m)

A thin silty parting is locally absent.

Beds1–2 upper limestone (0.18–0.20 m)

Pale limestone is roughly divided in two and has irregular upper and lower surfaces.

Capping mudstone (0.10 m)

Silty mudstone with limestone lenses completes the exposure.

Gallois (2009) Alternative C1–C10 Correlation

Published C1–C10 numbering below belongs to Gallois’s Cotham Formation scheme, not the narrower BGS Cotham Member. BGS places the base of the Langport Member at the level correlated with C7. The descriptions are retained as an alternative correlation and must not be added to the conventional log above. The general lithological character of each bed is known from that coastal correlation, but the Lilstock-specific features most worth noting are the basal erosion surface, several levels of soft-sediment slumping, the much reduced ripple-sand interval in C5, and the burrowed hardground at the top of C10.

Bed C1 — Basal Lenticular-Bedded Mudstones, Slumps And Fish-Debris Lag

The base of the Cotham interval in Gallois’s scheme at Lilstock rests on an irregular erosion surface and lag rich in fish debris, sand grains and small black phosphatic pebbles. The bed itself is a greenish-grey calcareous mudstone and silty mudstone unit in which lenticular bedding, internal disruption and local soft-sediment slumping are conspicuous. At Lilstock these disturbed beds are particularly important because they pass laterally into more normal lenticular mudstones, showing that the deformation is primary and syndepositional rather than later tectonic crushing. The bed records abrupt shallowing and local instability on the sea floor after deposition of the black Westbury shales.

Bed C2 — Muddy Limestone Lens

C2 is a strongly cemented muddy limestone equivalent of the surrounding Cotham mudstones. It is lenticular rather than perfectly tabular and may itself be affected by local slumping. In the field it acts as one of the first useful marker bands above the base of the member, but it should not be mistaken for the White Lias; it remains wholly within the argillaceous Cotham facies.

Bed C3 — Thinly Bedded Greenish-Grey Mudstones With Silt Lenses

This bed is composed of weakly calcareous, greenish-grey laminated to thinly bedded mudstones with scattered silt lenses and isolated coarse-silt ripple structures. Fossils are sparse, in keeping with the generally impoverished west Somerset Cotham fauna. The bed records quiet-water deposition in a very shallow restricted lagoon or tidal-flat pond system, with intermittent traction transport of fine silt.

Bed C4 — Slumped Interval In The Lilstock Correlation

Gallois’s Lilstock column shows conspicuous slumping through much of this interval, below the major desiccation surface at the base of C5. It differs from the cemented muddy limestone described under the same bed number at St Audrie’s Bay. The correlation label links the sections but does not imply identical lithology or a continuous hard marker bed at every locality.

Bed C5 — Ripple-Laminated Sandstone And Calcarenite With Mudcracks

Elsewhere on the west Somerset coast C5 is a fine- to medium-grained sandstone or calcarenite with shell debris, wave-ripple lamination, mudstone partings and large desiccation cracks descending from its base. At Lilstock it is much reduced and is represented only by two, locally three, ripple laminae separated by mudstones. Even in this attenuated form it is a very important bed, because it records a short-lived higher-energy incursion across the restricted Cotham mudflat surface and shows that intermittent emergence and desiccation occurred during deposition of the member.

Bed C6 — Upper Cotham Greenish-Grey Calcareous Mudstones

C6 consists of thinly bedded to laminated, weakly calcareous greenish-grey mudstones with somewhat thicker more calcareous beds. It returns to the quieter, muddier style of Cotham sedimentation above the rippled C5 event bed. Depositional conditions were still very shallow and restricted, with repeated shifts between subaqueous settling and minor desiccation or reworking on a broad tidal-flat to lagoonal plain.

Bed C7 — Porcellanous Laminated Micritic Limestone

C7 is a thin-bedded, laminated, porcellanous micritic limestone that weathers into splintery slabs. Older authors linked this level with a Cotham Marble equivalent, and it is certainly one of the most distinctive carbonate marker beds in the member. At Lilstock, however, it is overlain by further mudstones of unmistakably Cotham lithology, which led Gallois to retain it in his Cotham Formation. In contrast, the BGS scheme places the Langport base at this level. It represents a brief interval of strong carbonate precipitation and early cementation within an otherwise muddy, restricted succession.

Bed C8 — Nodular And Lenticular Carbonate-Variable Beds

C8 is laterally very variable and may range from weakly cemented mudstone to muddy limestone or porcellanous limestone in alternating lenticular and nodular beds. At Lilstock this lateral instability is especially marked and contributes to the bed’s uneven weathering on the foreshore. The bed records rapidly fluctuating conditions of carbonate production, mud accumulation and early diagenetic cementation in a very shallow-water restricted setting.

Bed C9 — Upper Weakly Calcareous Mudstone

This is a relatively simple greenish-grey weakly calcareous mudstone bed forming the topmost muddy phase of the Cotham interval in Gallois’s scheme. Fossils remain few. Its main significance lies in separating the variable nodular-carbonate interval below from the hard burrowed marker limestone above.

Bed C10 — Burrowed Argillaceous Limestone Hardground

C10 is a highly bioturbated argillaceous limestone with abundant Thalassinoides and other burrow systems. Its upper surface is irregular, mineralised and locally carries encrusting oysters, making it one of the clearest omission surfaces in the whole Lilstock Bay section. This hardground is the key marker at the top of the Cotham interval in Gallois’s scheme and is immediately overlain by the porcellanous White Lias bed. In environmental terms it records a pause in sedimentation and firmground development on a shallow marine or marginal-marine floor.

Thickness Note: Do Not Apply The Conventional BGS Cotham Thickness To The More Extensive Gallois C1–C10 Interval

White Lias And Watchet Mudstone Above C10: Gallois Scheme

Gallois White Lias Porcellanous Limestone Above C10 (c. 0.21–0.22 m)

At Lilstock Bay the limestone facies of the Langport Member is reduced to a single thin bed of dense, laminated porcellanous limestone, the local White Lias. It is pale grey to cream, very hard, micritic and laterally persistent, and may split into one or two leaves with a thin calcareous mudstone parting between them. This is a crucial marker bed because it is far thinner than the thicker White Lias limestones of mid-Somerset and south Devon, yet it preserves the same basic lithological identity. It represents renewed shallow-marine carbonate deposition after the restricted muddy Cotham phase, but under conditions of extreme condensation on the west Somerset basin margin.

Watchet-Type Upper Langport Calcareous Mudstones And Muddy Limestones

Above the porcellanous limestone lies a very thin but important mudstone-rich interval. In older Somerset usage this was termed the Watchet Beds or Watchet Mudstone facies; in current BGS nomenclature the Langport Member includes porcellanous limestone below and calcareous mudstone above, so this attenuated upper unit belongs within the Langport Member. At Lilstock it is only a few decimetres thick and may be as little as about 0.15 m in some exposures, but it changes laterally over a short distance into shelly calcareous mudstone, muddy limestone and local ripple-marked calcareous sandstone or siltstone. Crushed bivalves such as Liostrea, Modiolus and other shells may occur. The top is marked by a sharp change to the laminated organic-rich paper shales of the Blue Lias, recording an abrupt shift from an aerobic shelly sea floor to dysaerobic or anaerobic early Jurassic basin conditions.

Thickness Note: The Thin White Lias And Watchet Interval Above C10 Is Only Part Of The Conventional BGS Langport Member

LIAS GROUP

Blue Lias Formation (Basal Boundary Section; Higher Beds In Separate Local Exposures)

The regional west Somerset Lower Lias bed numbering begins at the base of the fissile mudstone above the top of the Langport Member. The two divisions below summarise the basal boundary interval. Higher Blue Lias beds are also recorded locally: the BGS cliff section at ST 1785 4527 extends from bed 1 through beds 43–44 into the Liasicus Zone.

Basal Paper Shales

The base of the Lias Group at Lilstock is marked by a sharp planar contact above the top of the Langport Member and the appearance of laminated brownish-grey organic-rich fissile mudstones, the classic paper shales. Fish scales are common, and the finely fissile texture records very low-energy mud accumulation under dysaerobic to anaerobic bottom-water conditions. This sharp lithological shift marks the formation boundary; it is not automatically the Triassic–Jurassic time boundary.

Lowest Blue Lias Limestone–Mudstone Rhythms

Above the paper shales the first argillaceous limestone bands appear within grey calcareous mudstones and silty mudstones, beginning the familiar Blue Lias rhythm of limestone and mudstone couplets. On the wider west Somerset coast the first c. 5 m of the succession above the Langport Member are largely devoid of ammonites but contain a diverse bivalve fauna with abundant Liostrea hisingeri; only higher up do Psiloceras and the classic Planorbis-zone faunas become common. This basal interval should not be mistaken for the full local Blue Lias succession: separate Lilstock cliff and foreshore exposures extend into younger Hettangian and lower Sinemurian beds.

BGS Blue Lias Cliff Log At ST 1785 4527

This separate bottom-to-top log follows the BGS section at ST 1785 4527, east of Lilstock. It retains the published bed packages and unnumbered layers; it is not added to the Penarth logs above or to the broader coastal composite. The memoir groups beds 1–15 as pre-planorbis, beds 16–42 in the Planorbis Zone, and beds 43–44 and the capping limestone in the Liasicus Zone. Its historical Triassic age label for the pre-planorbis beds is not adopted here: that name alone does not locate the modern Triassic–Jurassic boundary.

Bed 1 — oyster-bearing shale (1.40 m)

Shale contains Liostrea.

Bed 2 — limestone (0.12 m)

A thin carbonate bed overlies the basal shale.

Bed 3 — shale (0.23 m)

A shale interval separates the limestone beds.

Bed 4 — massive limestone (0.41 m)

A shale parting separates a 0.08 m limestone at the top of this otherwise massive bed.

Bed 5 — shale and limestone (0.42 m)

Shales include a 0.10 m limestone layer.

Unnumbered lower limestone between beds 5 and 6 (0.25–0.36 m)

This limestone forms the lowest of three separately logged, unnumbered layers in this interval.

Unnumbered oyster-bearing shale between beds 5 and 6 (0.13 m)

Shale contains abundant Liostrea.

Unnumbered upper limestone between beds 5 and 6 (0.23–0.30 m)

The limestone locally divides into three beds; numerous Liostrea occur on its upper surface.

Bed 6 — oyster-rich shale (0.13 m)

Shell-rich bands within shale contain Liostrea.

Bed 7 — mudstone–limestone alternation (1.46 m)

Three limestone bands alternate with shale and mudstone.

Bed 8 — oyster-bearing limestone (0.20–0.28 m)

The limestone contains Liostrea.

Beds 9–15 — shale and limestone (0.72 m)

Shales enclose a 0.09 m limestone layer.

Bed 16 — limestone (0.30 m)

A limestone bed begins the interval placed in the Planorbis Zone in the published log.

Beds 17–19 — shale and blocky mudstone (0.56 m)

Blocky mudstone occurs at both ends of the shale interval.

Beds 20–22 — limestone (0.25 m)

This carbonate package lies below the ammonite-rich shale.

Beds 23–24 — ammonite-rich shale (2.08 m)

Shale with mudstone layers yields abundant Psiloceras planorbis.

Beds 25–35 — six limestone bands and mudstone (1.98 m)

Six distinct carbonate bands alternate with muddy intervals.

Beds 36–37 — Caloceras-bearing shale and mudstone (1.37 m)

The upper 0.41 m becomes mudstone, with limestone lenses in the upper 0.30 m. Caloceras occurs throughout.

Beds 38–40 — lenticular limestone (0.05–0.10 m)

A thin, lens-shaped carbonate horizon forms this package.

Bed 41 — mudstone (0.08 m)

A narrow muddy interval separates the adjacent limestones.

Bed 42 — limestone with uncertain ammonite (0.19 m)

A poorly preserved impression was tentatively identified as Caloceras.

Beds 43–44 — Liasicus Zone mudstone (2.79 m)

The published log assigns this mudstone package to the Liasicus Zone.

Unnumbered capping limestone (0.08 m)

A thin limestone lens completes the published section.

Higher Hettangian To Lower Sinemurian Lower Lias In The Wider Lilstock Foreshore (Composite Context Interval)

Higher Lower Lias beds are present along the broader Blue Anchor–Watchet–Lilstock coast and in faulted foreshore blocks near Lilstock, but they do not form one simple, continuously measurable Lilstock Bay cliff section. Regionally they comprise a thick mudstone-dominated Blue Lias succession with subordinate limestones, passing upward into lower Sinemurian mudstones, and the broader coastal tract between Blue Anchor and Lilstock exposes 160–200 m of Lower Lias. Because faulting, repetition and local structural complexity become especially important toward Lilstock, it is more honest to treat those higher beds as part of the wider west Somerset composite Lower Jurassic succession rather than to force a fictitious continuous Lilstock-only log.

Stratigraphic Significance

Lilstock Bay is one of the key British reference localities for the latest Triassic and earliest Jurassic because it shows the upper Blue Anchor Formation, the full Penarth Group and the base of the Lias Group in one coastal section. It is also the type area of the Lilstock Formation, with its two constituent members, the Cotham and Langport. The section is especially valuable because the White Lias is so attenuated, the Watchet-type upper Langport mudstone is still present, and the basal Blue Lias paper shales lie directly above, allowing unusually clear study of the latest Rhaetian to earliest Jurassic transition.

Depositional Environment

The Lilstock succession records repeated environmental shifts across the end of the Triassic. The upper Blue Anchor Formation represents dolomitic mudflats, lagoons and sabkha-influenced marginal-marine settings with increasing marine influence upward. The Westbury Formation records full Rhaetian marine transgression into a relatively quiet, low-oxygen basin where black muds, shell beds and bone beds accumulated. The Cotham Member marks renewed shallowing into very restricted lagoonal, tidal-flat and intermittently emergent conditions with soft-sediment instability, ripple events and desiccation. The Langport Member represents renewed shallow marine carbonate deposition, but in highly condensed form on the west Somerset basin margin. The basal Blue Lias then records a return to more open offshore marine mud accumulation with organic-rich paper shales and the first regular Jurassic limestone–mudstone rhythms.

Structural Style And Exposure

Lilstock must be treated as a faulted and partly slumped composite foreshore locality, not as a single undisturbed measured cliff. Across the broader Blue Anchor–Lilstock coast both normal and reverse faults occur, and structural complexity is greater in the Lilstock area than farther west. Within the Cotham Member at Lilstock, slump structures occur at more than one level and pass laterally into undisturbed lenticular mudstones and siltstones, strongly suggesting syndepositional deformation, probably linked to basin-margin instability and perhaps seismic shocks. The modern foreshore is also a classic macrotidal washboard-like platform, so beach cover and tidal state strongly affect what can be examined on any visit.

Total Thickness Covered Here

The permanent Lilstock Bay boundary section includes many metres of upper Blue Anchor Formation, the exposed upper part of the Westbury Formation, the Cotham and Langport members of the BGS scheme, including the thin White Lias and Watchet-type intervals distinguished in Gallois’s alternative scheme, and the basal paper-shale and limestone interval of the Blue Lias. Because the base of the Westbury is exposed only when storm-beach levels are low, and because foreshore structure and cover change along the bay, the locality is best understood as a detailed latest Triassic–earliest Jurassic composite section rather than as one single fixed measured cliff log.

References

Richardson, L. (1905, 1906, 1911) on the Rhaetic and White Lias succession of west Somerset, including Lilstock and St Audrie’s Bay.
Warrington, G., Audley-Charles, M.G., Elliott, R.E., Evans, W.B., Ivimey-Cook, H.C., Kent, P.E., Robinson, P.L., Shotton, F.W. & Taylor, F.M. (1980). A Correlation of the Triassic Rocks in the British Isles.
Hamilton, D. & Whittaker, A. (1977). Coastal exposures near Blue Anchor, Watchet and St Audrie’s Bay, north Somerset.
Whittaker, A. & Green, G.W. (1983). Geology of the Country around Weston-super-Mare.
Warrington, G. & Whittaker, A. (1984). The Blue Anchor Formation (late Triassic) in Somerset.
Mayall, M.J. (1981). The late Triassic Blue Anchor Formation and the initial Rhaetian transgression in south-west Britain.
Warrington, G. & Ivimey-Cook, H.C. (1995) on the Penarth Group and basal Lias of west Somerset.
Swift, A. (1995) on the White Lias / Langport Member facies variation in Britain.
Gallois, R.W. (2009). The lithostratigraphy of the Penarth Group (Late Triassic) of the Severn Estuary area, UK.
Lole Durbin, O., Duffin, C.J., Hildebrandt, C. & Benton, M.J. (2024). Onset of the Rhaetian transgression in deep waters at Lilstock, north Somerset.
British Geological Survey Lexicon entries for Blue Anchor Formation, Westbury Formation, Lilstock Formation, Cotham Member, Langport Member and Blue Lias Formation.
JNCC / Geological Conservation Review accounts for the Blue Anchor–Watchet–Lilstock coast and the British Penarth Group sites.

Whittaker & Green (1983), BGS Weston-super-Mare memoir
Gallois (2009), Severn Estuary Penarth stratigraphy
Lole Durbin et al. (2024), Lilstock microvertebrate faunas
BGS Lexicon: Lilstock Formation
GCR30 BlueAnchor–Lilstock