Pinhay Bay Fossil Hunting

1906 – Cotham Member fossils and White Lias structures recorded
Richardson described the Pinhay Bay Rhaetian beds, including the Cotham Marble, the “Estheria Bed” with Estheria minuta, ripple marks and the distinctive Sun Bed at the top of the White Lias. These observations helped establish Pinhay Bay as one of the key Devon sections for the latest Triassic coastal succession.

1924 – Pre-planorbis Beds measured above the White Lias
Lang recorded the basal Blue Lias at Pinhay Bay, including the Pre-planorbis Beds, also known as the Ostrea Beds because of their abundant oysters. This was important because the first appearance of Psiloceras planorbis lies above these beds, making the bay useful for discussing the Triassic–Jurassic boundary interval.

1960 – classic study of the Langport Member fauna
Hallam produced the main detailed account of the Pinhay Bay White Lias, listing a diverse Langport Member assemblage of bivalves, gastropods, rarer solitary corals and trace fossils. The work showed that many fossils occur in small dense patches within the limestones and marls, recording shallow, repeatedly disturbed marine conditions.

1961 – Cotham Marble interpreted as algal growth
Hamilton’s work on Cotham Marble helped explain the “landscape” structures seen in the Cotham Member at Pinhay Bay as algal or microbial growths rather than ordinary inorganic patterns. This gave the bay’s Cotham Marble a clearer biological significance within the late Triassic lagoonal setting.

1983 – deformed Cotham Member bed linked to seismic disturbance
Mayall recorded a deformed bed in the middle of the Cotham Member at Pinhay Bay and correlated it with nearby East Devon sections. The bed includes disturbed sediment, microfaulting and water-escape features, providing evidence that some fossil-bearing late Triassic lagoonal sediments were affected by earthquake-related movement soon after deposition.

2001 – Triassic–Jurassic boundary beds re-examined after new exposure
Newly exposed sections at Pinhay Bay allowed Wignall to document the White Lias–Blue Lias contact in detail, including bored hardgrounds, intraformational conglomerates, truncated burrows such as Diplocraterion, and rapid change into organic-rich basal Blue Lias shale. This made the bay especially important for interpreting sea-level change and environmental stress across the end-Triassic interval.

2020 – dewatering structures described in the oldest Blue Lias
Gallois and Thompson described unusual dewatering and diapiric structures in the oldest Rhaetian part of the Blue Lias at Pinhay Bay. Although mainly sedimentological, the work matters for fossils because it refines the unstable sea-floor conditions in which the earliest Blue Lias oyster-rich beds and associated marine fossils were preserved.

Section Architecture

Pinhay Bay is not a single simple cliff log. The key exposures are the low cliffs and foreshore in the middle and eastern part of the bay, where the upper Penarth Group and basal Blue Lias are seen, together with the faulted western end of the bay where Blue Lias is downthrown against the older Langport Member. Landslides, beach build-up and foreshore cover repeatedly obscure parts of the section, so the succession has to be treated as a composite of cliff, foreshore and temporary erosion windows rather than as one uninterrupted face.

Scope Note

The main PB log focuses on the classic Pinhay Bay boundary section: the Westbury Formation, Cotham and Langport members of the Lilstock Formation, and basal Blue Lias. The PB labels are editorial cross-references; published bed numbers retain their source-specific meaning. A separate account below records the higher Blue Lias and basal Charmouth Mudstone exposed elsewhere within Pinhay Bay. It is not joined to the boundary log as one uninterrupted measured column, and the farther continuation towards Seven Rock Point and Lyme Regis is identified separately.

MERCIA MUDSTONE GROUP

Blue Anchor Formation (Late Triassic)

The uppermost Blue Anchor mudstones form the substrate below the basal Westbury burrow fills exposed intermittently on the foreshore. Only this contact interval is included here; no complete local formation thickness is implied.

PENARTH GROUP

Westbury Formation (Rhaetian)

Bed PB1 – Basal Westbury Burrow-Fill Bone-Bed And Pebbly Transgressive Lag

The base of the Westbury Formation is exposed in the intertidal area at Pinhay Bay as dark mudstone and pebble-bearing infillings penetrating the top of the older Blue Anchor surface. Like the equivalent east Devon boundary beds elsewhere, this is a transgressive lag developed in burrows and shallow irregularities rather than a perfectly even sheet. The sediment includes dark grey mudstone, shell debris, fish and vertebrate remains, angular green mudstone clasts and quartz grains, and it records the first major marine flooding of the latest Triassic surface in this area. The bed is an important marker, but it is best treated honestly as a burrowed erosional base rather than as a tidy planar seam.

Bed PB2 – Main Westbury “Contorta” Shales (c. 5.0 m)

Above the basal transgressive lag, the Westbury Formation consists of dark grey to black fossiliferous mudstones with shelly intervals and local shell-detrital limestones. In older descriptions of the Pinhay Bay section this unit is simply the “Contorta Shales”, but the fauna and lithology are those of the standard Rhaetian Westbury Formation. Thin-shelled bivalves dominate, and the environment was a quiet-water brackish to shallow-marine embayment or restricted shelf. Compared with the more complex Cotham and Langport carbonates above, the Westbury succession is more monotonously muddy and distinctly darker in colour.

Lilstock Formation (Rhaetian)

Cotham Member

Bed PB3 – Cotham Member, Including The “Estheria Bed” And Cotham Marble (c. 1.5 m In The Historical Composite Log)

At Pinhay Bay the Cotham Member is less fully visible than the overlying Langport Member, but older descriptions record the “Estheria Bed”, black shales and the Cotham Marble in the section. In regional east Devon correlation, this interval represents thinly interbedded greenish-grey mudstones, laminated and nodular limestones and subordinate sandy or silty beds deposited in shallow lagoonal to brackish-water conditions. The Cotham Marble or “Landscape Marble” marks the top of the member and represents stromatolitic or algal-mat growth in very shallow water. Where exposed in east Devon the member may also show slumps, dewatering structures and ooid ripple trains, but at Pinhay Bay those details are commonly obscured by beach cover and intermittent exposure.

“Estheria Bed” and Cotham Marble
The historical Pinhay account identifies a pale concretion-bearing limestone with the crustacean recorded as Estheria minuta, as well as black shale and the algal “Landscape Marble”. These are local records, rather than a transfer of the more complete Culverhole bed numbering.

Middle Cotham deformed bed (about 0.4 m)
Mayall’s poorly exposed Pinhay horizon comprises calcitic mudstone with ripples, silty lenses and sediment deformation. Correlation with Culverhole was proposed, with possible earthquake disturbance as an interpretation rather than a demonstrated origin for every disturbed bed.

Langport Member

Langport Member Note

The Langport Member at Pinhay Bay is the classical thick east Devon “White Lias” development. Gallois (2007) reported the upper 7.0 m exposed, including the junction with the Blue Lias. His review distinguishes Hallam’s 7.84 m incomplete record, which omitted the basal beds and Cotham contact, from Mayall’s separate 8.0 m complete-section measurement. These are different historical observations, not a modern thickness range. The member is composed mainly of very pure off-white fine-grained limestone with subordinate grey porcellanous limestone, marl partings and a series of hardgrounds, slumps, channels and clast-rich remobilized beds. Published local Pinhay bed numbers 1a–12 are retained here where they can be used confidently.

Bed PB4 – Pinhay Beds 1a–1b, Lower Slumped Limestones And Porcellanous Hardgrounds

The lower exposed Langport beds at Pinhay Bay consist of slumped fine-grained limestones separated by distinctive porcellanous hardground surfaces. These are among the first strong indications that the “White Lias” here was not deposited as a simple undisturbed limestone sheet. The limestones were repeatedly lithified very early, fractured or destabilized, and then draped or overlain by renewed micritic sediment. Gallois’s composite begins above the unexposed basal part; beds 1a–1b must not be equated automatically with the base of the member.

Bed 1a
Lower slumped limestone, bounded above by a porcellanous hardground.

Bed 1b
A second slumped limestone above that surface, capped by another hardground below Bed 2.

Bed PB5 – Pinhay Beds 2–3, Nodular And Channelled Limestones With Marl Partings

Above the lower slumped units, the succession passes into nodular limestone with marl partings and then into the distinctive channelled base and channel infill of Pinhay Bed 3. The base is erosive and locally cut into the bed below; infill includes slumps and dewatering structures. This is one of the most revealing units in the whole bay because it shows that the middle White Lias surface was locally incised and re-filled, rather than merely accumulating by quiet parallel bedding. The member was clearly deposited in a very shallow, laterally variable marine setting where local relief could develop on the sea floor.

Bed 2
Nodular limestone with marl partings, above the hardground at the top of Bed 1b.

Bed 3
Channel-fill limestone with slump and dewatering structures; the channelled geometry varies laterally.

Bed PB6 – Pinhay Beds 4–6, Middle Slumped And Wavy-Laminated White Lias

Pinhay Bed 4 is a slumped limestone with abundant angular clasts, followed by Bed 5, a wavy-laminated limestone with load structures, and Bed 6, a more simply wavy-laminated limestone. Together these beds record repeated soft-sediment instability and short-distance remobilization of semi-lithified carbonate muds. The load structures and slump fabrics show that deformation took place early, while the sediment was still only partly consolidated. At the same time, the fine-grained micrite and sheet-like geometry show that the original depositional setting remained one of very shallow, low-energy carbonate sedimentation.

Bed 4
Slumped limestone containing abundant angular limestone clasts.

Bed 5
Wavy-laminated limestone with load structures.

Bed 6
Wavy-laminated limestone above the load-structured bed.

Bed PB7 – Pinhay Beds 7–8, Remobilised Clast-Rich Limestones And Hardground Surface

Pinhay Bed 7 comprises parallel-bedded remobilised limestones containing common clasts, shells and bored pebbles, and it is capped by the hardground surface of Bed 8. This interval is one of the clearest field signs that previously lithified or semi-lithified carbonate beds were being broken up, reworked and re-deposited in place or after only very short transport. The bored pebbles and shell debris indicate repeated pauses in sedimentation, colonization of firm seafloor and subsequent erosion. It is therefore a marker of intermittent condensation within the larger White Lias succession.

Bed 7
Parallel-bedded, remobilised limestone containing clasts, shells and bored pebbles.

Bed 8
The hardground capping that clast-rich interval, directly below the Main Slump Bed.

Bed PB8 – Pinhay Bed 9, Main Slump Bed (c. 1.37 m)

This is the most conspicuous single deformation unit in the Pinhay Bay White Lias. It consists of slumped limestone with abundant clasts, some up to boulder size, concentrated especially toward the base. The unit has long been recognised as the “Main Slump Bed” and is one of only two obvious marker beds that can be correlated readily between Pinhay Bay and Charton Bay. The large clasts and chaotic internal fabrics show failure and remobilization of already lithified or partly lithified carbonate sheets on a very shallow shelf or tidal-flat surface.

Bed PB9 – Pinhay Beds 10–11, Upper Finely Wavy Laminated Limestones

Above the Main Slump Bed, the succession becomes more regularly bedded again and is dominated by finely wavy laminated limestones, in places with marl partings and minor local clasts or hardground fragments. These upper beds are more uniform than the slump-rich lower part of the member, but they still preserve evidence of early lithification and interrupted sedimentation. Their fine lamination and purity show renewed quiet-water micrite deposition after the instability of the Main Slump Bed.

Bed 10
Finely wavy-laminated limestone with marl partings.

Bed 11
Finely wavy-laminated limestone, below the topmost Sun Bed interval.

Bed PB10 – Pinhay Bed 12, Sun Bed And Locally Replacing Scour Hollows

The top of the Langport Member is the Sun Bed, a thin wavy laminated limestone forming the final White Lias surface beneath the Blue Lias. At most localities this surface is marked by the classic Diplocraterion burrows of the Sun Bed, but at Pinhay Bay it may be locally absent and replaced by scour hollows infilled with brecciated limestone. Detailed sedimentological work has shown that the uppermost White Lias at Pinhay first developed a burrowed firmground and marine hardground, was then eroded and reworked into an intraformational conglomerate, underwent brief subaerial emergence with fissuring and pitting, and was finally drowned beneath the Paper Shale of the Blue Lias. It is one of the most important boundary surfaces on the British Triassic–Jurassic coast.

Langport Thickness Records: 7.84 m In Hallam’s Incomplete Log; 8.0 m In Mayall’s Complete Log; Upper 7.0 m Exposed In The 2007 Account

Historical measured detail: Hallam-based GCR log, oldest to youngest

The following preserves the separately published historical measurement in the GCR account. Rapid lateral change prevents a confident one-to-one match of every entry to Gallois’s 1a–12 scheme; the Main Slump Bed and Sun Bed are the secure principal ties. These are historical bed thicknesses, not current exposure measurements.

Historical interval 1 (1.83 m)
Thin limestones, locally porcellanous or rubbly, with slight surface relief, marl partings up to 0.01 m and rare bivalve pockets.

Historical interval 2 (0.58 m)
Limestone with minor slumping and a locally porcellanous top.

Historical interval 3 (0.51 m)
Thin limestone beds.

Historical interval 4 (1.37 m)
Limestone with subangular porcellanous clasts, minor slumps and a locally rubbly upper part.

Historical interval 5 (0.20 m)
Limestone with an irregular base.

Historical interval 6 (0.86 m)
Thin rubbly limestones, irregular marl partings and pockets of small bivalves.

Historical interval 7 (0.05 m)
Porcellanous limestone.

Historical interval 8 (1.37 m)
The Main Slump Bed.

Historical interval 9 (0.11 m)
Locally porcellanous limestone with an eroded top.

Historical interval 10 (0.02 m)
Shelly marl containing limestone lenses.

Historical interval 11 (0.23 m)
Thin rubbly limestones and marl partings with a shell band near the top.

Historical interval 12 (0.25 m)
An eastern limestone, locally irregular on top, replaced westwards by several thinner limestones.

Historical interval 13 (0.41 m)
Thin irregular limestones and brown marl partings, locally rubbly or perforated.

Historical interval 14 (0.05 m, locally thicker)
The Sun Bed, locally a limestone-pebble conglomerate in a marly matrix.

LIAS GROUP

Blue Lias Formation (Latest Triassic–Early Jurassic)

Blue Lias Note

At Pinhay Bay the base of the Blue Lias is one of the most critical boundary intervals in southern Britain. The formation begins sharply above the Sun Bed with the Paper Shale of Bed H1. The lowest 2.7 m of the Blue Lias at Pinhay Bay are the traditional Pre-planorbis or Ostrea Beds, but the first unequivocal psiloceratid ammonites do not appear until Bed H25, so the lowest Blue Lias beds here may in part still represent the very latest Triassic or earliest Jurassic before the first locally recorded ammonites. The modern global Jurassic boundary is defined at Kuhjoch, Austria, by the first occurrence of Psiloceras spelae tirolicum; it is not defined by either this lithological junction or the first British psiloceratids.

Bed PB11 – Bed H1, Paper Shale

The Paper Shale is a laminated, organic-rich, silty shale resting sharply on the irregular top of the White Lias and draping fissures, hollows and small erosional relief on the Sun Bed surface. It is dark, finely laminated and rich in pyrite, including abundant small framboids that indicate sulphidic bottom-water conditions. Fossils are almost absent except for a single bedding plane crowded with disarticulated valves of Modiolus minimus and echinoid spines. The bed has yielded articulated and associated Diademopsis material in the Pinhay Bay succession and is of exceptional sedimentological significance because it records the rapid marine transgression and abrupt establishment of an anoxic lower water column after brief emergence of the White Lias surface.

Bed PB12 – Beds H2–H5, Basal Limestone–Shale Couplets Above The Paper Shale

Bed H2
Limestone, laminated at its base and top but strongly bioturbated through its centre, with a shelly fauna dominated by Liostrea.

Bed H3
Laminated organic-rich shale, locally cut out over a dome rising from H2. The shale changes thickness across small growth structures; a thin limestone rib was recorded in one of Wignall’s sections.

Bed H4
Limestone with laminated margins and a bioturbated centre. At Wignall’s section 4 it overlies the deformation in H2–H3 without being similarly disturbed.

Bed H5
Finely laminated organic-rich shale containing a thin bioclastic limestone parting.

These first couplets show the beginning of the classic Blue Lias rhythm, alternating between more oxygen-poor laminated shale deposition and more oxygenated, bioturbated limestone phases. Synsedimentary folding and small westward-downthrowing growth faults affect the Paper Shale and, to a lesser extent, H2–H3 at several local sections; they are absent from Wignall’s easternmost section.

Bed PB13 – Beds H6–H24, Higher Pre-planorbis / Ostrea Beds (Total Pre-planorbis Succession c. 2.7 m)

The rest of the Pre-planorbis Beds continue the limestone–mudstone rhythm established in H1–H5. Oysters, especially Liostrea hisingeri, are abundant at several levels, which is why this package was long known as the Ostrea Beds. Articulated or associated Diademopsis tests and spines are known from H6 as well as H1 and H3, and these lowest Blue Lias echinoid-bearing levels lie below the first ammonites in the Pinhay Bay succession. The lithology and faunal evidence together suggest repeated alternation between anoxic laminated deposition and more oxygenated seabed colonization on a shallow but fully marine shelf. Because the first psiloceratids are not yet present, the higher part of this interval remains crucial to debate over the precise practical position of the Triassic–Jurassic boundary in east Devon.

Bed H6
An independently recorded Diademopsis-bearing horizon, alongside H1 and H3.

Beds H7–H23
The checked sources do not provide a complete individual lithological description of every intervening bed H7–H23, so those beds remain a grouped interval here.

Bed H24
Page records the first local Plagiostoma in this bed, just below the first psiloceratids.

Bed PB14 – Beds H25–H29, Earliest Ammonite-Bearing Boundary Interval

The earliest recorded psiloceratids in the Pinhay–Seven Rock section occur in Bed H25; Page’s detailed H25 observations are from the foreshore west of Seven Rock Point. They are small, crushed, poorly preserved forms associated with a burrowed horizon filled by greyish white marl just below Bed H26; they are thought most likely to be Neophyllites rather than the traditionally expected Psiloceras erugatum fauna. Small evolute psiloceratids are also present in Bed H26. Page’s 2002 placement of the system boundary within H25 reflects the historical British ammonite convention. H25 is not demonstrated here to coincide with the subsequently ratified global boundary; the burrowed horizon below its first ammonites may nevertheless mark a small non-sequence. Higher in the same lower ammonite-bearing package, about 0.18 m above the base of Bed H29, a typical plicatulum-level fauna including Psiloceras cf. plicatulum and P. ex grp. sampsoni marks a more secure early Hettangian ammonite horizon.

Bed H30, Intruder Bed
This conspicuous tabular, fine-grained limestone forms a ledge on the eastern Pinhay foreshore. Centimetre-wide fissures are partly filled by bioclastic limestone. It lies above the H29 ammonite interval and is a named local marker, rather than the global Jurassic boundary.

Additional deformation study: independent 2021 numbering

Laborde-Casadaban and colleagues logged a separate numbered succession through the Langport and basal Blue Lias. Their unit numbers below are not equated with Gallois’s 1a–12 labels or Lang’s H numbers. The figure places the Sun Bed near the top of unit 7, with unit 8 in the Blue Lias.

Units 3A and 3B
Together these form a 1.85 m debris-flow package with an erosive base and limestone-mud conglomerate. Unit 3B has conspicuous convolute bedding attributed to fluid escape.

Unit 3C
The limestone is almost wholly brecciated; liquefied material formed injections along small normal faults.

Unit 3D
At the 3C–3D boundary, small faults and changes in bed thickness indicate movement during sedimentation.

Unit 5B (1.50 m)
A basal accumulation of angular clasts passes into disrupted limestone with metre-scale ball-like bodies. Original slumped bedding survives better in the western part of the section.

Unit 6
Original bedding survives, although faults cut the unit.

Unit 7
This package also retains its bedding despite cross-cutting faults and includes the topmost Langport interval.

Unit 8E
Domed injections occur at fault tips in the basal Blue Lias, deforming the overlying bed. The authors interpret the linked faults and liquefaction structures as evidence of repeated seismic disturbance; this does not mean every slump in the bay has the same cause.

Higher Blue Lias Note

Higher Blue Lias continues eastwards around Seven Rock Point towards Lyme Regis, and higher levels also occur on the western side of Pinhay Bay. The detailed account above focuses on the Penarth–basal Blue Lias section. A separate upper Blue Lias–basal Charmouth exposure is documented within the bay, so locally derived Arnioceras blocks should not be dismissed as material imported from another locality.

Separate higher Lias exposure within Pinhay Bay

This is an Early Jurassic, higher Lias section in another part of the bay, not the immediate continuation of PB14 as a complete measured column. Gallois and Paul describe beds 41–49 and the succeeding basal Charmouth succession in the western cliffs and towards Seven Rock Point.

Glass Bottle (Bed 47)
Gallois and Paul record this Blue Lias marker locally as sparse limestone concretions, about 5–10 m apart; it is absent from their illustrated Pinhay section. This variability is why a continuous limestone band should not be assumed here.

Grey Ledge (Bed 49)
The uppermost conventionally defined Blue Lias limestone changes from widely spaced concretions in the western exposures to closer concretions and tabular beds east of the fault. Arenicolites burrows and large Arietites occur on its upper surface.

Charmouth Mudstone Formation (Sinemurian; separate upper section)

Shales-with-Beef Member

Mudstone above Grey Ledge and Picnic Ledge
About 0.3 m of mudstone with basal pyrite precedes the 0.35–0.4 m laminated Picnic Ledge limestone. Its crushed Arnioceras, oysters, Oxytoma and Nannobelus, and Calcirhynchia-filled Thalassinoides in fallen slabs, are documented at Pinhay.

Fish Bed (Bed 51)
This higher laminated marker is recorded intermittently west of the fault and in inaccessible high cliff exposures to the east. The local extra limestones make the lithological formation boundary more complex than a simple regional chart suggests; these beds are discussed using the conventional Grey Ledge boundary.

Table Ledge (Bed 53)
This higher Shales-with-Beef limestone forms a prominent pale-weathering marker in the Pinhay cliffs east of the fault. It is distinct from the laminated Fish Bed below; no thickness from another coastal section is assigned here.

Depositional Environment

The Pinhay Bay succession records a major environmental shift from latest Triassic marginal marine muds and lagoonal limestones into earliest Jurassic offshore mudstone–limestone rhythms. The Westbury Formation represents quiet-water brackish to shallow-marine mud deposition following transgressive reworking at its base. The Cotham Member records shallow brackish lagoonal conditions with stromatolitic and shell-bearing beds. The Langport Member, traditionally the White Lias, formed in a broad very shallow marine seaway of slightly abnormal salinity, with repeated early lithification of micritic carbonate mud, hardground development, burrowing, erosion, slumping, brecciation and brief emergence of the sea floor. The contact with the Blue Lias is highly dynamic, recording marine hardground formation, local reworking into intraformational conglomerate, short-lived subaerial exposure and then rapid transgression. The Paper Shale and lowest Blue Lias were deposited under an anoxic to euxinic lower water column, later passing into the repeated alternation of laminated, more oxygen-poor shales and more bioturbated limestones that typifies the basal Blue Lias rhythms.

Total Thickness Covered Here: Historical Logs Record About 5.0 m Of Westbury And 1.5 m Of Cotham; Langport Records Are Qualified Above. The Basal Blue Lias Includes The Approximately 2.7 m Pre-planorbis Interval And Higher Ammonite Beds. These Separate Records Are Not Added Into One Modern Measured Total

References

Benton, Cook and Turner (2002), GCR: Pinhay Bay
Gallois (2007), The stratigraphy of the Penarth Group of the east Devon coast
Wignall (2001), Sedimentology of the Triassic–Jurassic boundary beds in Pinhay Bay
Page (2002), Hettangian and Lower Sinemurian ammonite succession of the east Devon coast
Edwards, R. A. and Gallois, R. W. (2004), Geology of the Sidmouth district: a brief explanation of the geological map, Sheets 326 and 340
International Commission on Stratigraphy, Hettangian GSSP
BGS Lexicon: Lilstock Formation
BGS Lexicon: Langport Member
BGS Lexicon: Cotham Member
BGS Lexicon: Westbury Formation
BGS Lexicon: Blue Anchor Formation
BGS Lexicon: Blue Lias Formation
Page and Paul (2017), Articulated echinoids from the basal Blue Lias Formation near Watchet
Gallois and Paul (2009), Lateral variations in the topmost Blue Lias and basal Charmouth Mudstone
Barton et al. (2011), Geology of south Dorset and south-east Devon and its World Heritage Coast
Laborde-Casadaban et al. (2021), Do soft sediment deformations record seismic activity during the break-up of Pangea?