Sedbury cliff Fossil Hunting

1845 – Early fossil-insect account
Peter Bellinger Brodie published observations supplied by Mr Higgins on the insect-bearing and landscape-patterned limestones at Sedbury. Richardson later identified this as the earliest account known to him.

1877 – Lower Lias fossils documented
Sir William Vernon Guise described the relatively extensive Lower Lias and recorded ammonites and bivalves, while noting that the Rhaetic bone bed was less conspicuous than at Aust.

1903 – Detailed cliff succession measured
Linsdall Richardson published the numbered Rhaetic section, fossil lists and a vertical profile, documenting lateral changes in the bone bed, ostracod-bearing shale and the conglomerate below the Lias. He acknowledged specific permission from the Marling family to examine the cliff.

1903 – Faunal ranges and the basal conglomerate examined
Arthur Vaughan analysed the lowest Lias beds with Richardson. He compared the abundance ranges of bivalves and ammonites and interpreted the basal conglomerate as reworked Cotham Marble. His faunal ranges distinguished a lower bivalve-rich succession from ammonite-rich beds above.

1961 – A later synthesis of the cliff succession
Welch and Trotter summarised the older Sedbury measurements, retaining a 37 ft Lower Lias interval with basal oyster-rich beds and higher planorbis and johnstoni ammonites. Their account also recorded the outlier’s faults and the patchy exposures then known.

1977 – Bone-bed succession reviewed
Sykes included a compact Sedbury section in his study of British Rhaetian bone beds, distinguishing fish-bearing sandstones and limestones above the basal conglomeratic layer.

1994 – Earlier ammonite occurrence reported
Peter Hodges reported Psiloceras planorbis from low beds at Sedbury and other south-west British sections, questioning the use of its local first appearance as a simple synchronous boundary marker.

2017 – Active tufa-forming seepages recorded
A British Geological Survey report documented groundwater seepages and active tufa formation on the unstable, scrub-covered Sedbury cliff, linking present-day water movement to its distinctive deposits and vegetation.

2019 – Mudstone depositional environments reassessed
Paul Milroy, V. Paul Wright and Michael Simms included the Sedbury succession in a sedimentological study of the Mercia Mudstone Group, recognising evidence of changing conditions on saline mudflats and in temporary lakes.

The main succession is given from oldest to youngest, following Richardson’s 1903 measured section. Modern formation headings provide a framework, but the historical bed numbers, fossil names and thickness scopes remain explicit. Grouped numbers stay grouped. The higher beds of this Lower Lias exposure were studied largely from fallen blocks, and this account is not a claim that every bed is continuously exposed or accessible today.

MERCIA MUDSTONE GROUP

II. Red Marls (56 ft). Red marls with angular fracture and blue-grey/yellow zones and patches. This is the exposed interval in Richardson’s section, not the full thickness of a formation. Modern work identifies Branscombe silty-mudstone facies at Sedbury; the historical red-marl row remains the measurement used here.

Blue Anchor Formation

I. Tea-green Marls (10 ft 3 in in plate; approximately 10¼ ft in text). Yellowish-green marls with a hard marlstone band. The marls grade down into the red beds but meet the overlying bone bed abruptly. The three subordinate measurements below sum to 10 ft 1 in–10 ft 5 in, reflecting the varying thickness of the hard band; they are not extra beds to add to the 10 ft 3 in total.

Tea-green Marls (c), lowest (7 ft 6 in). Yellow-green marl with hard nodular masses.

Tea-green Marls (b), middle (7–11 in). Hard marlstone band.

Tea-green Marls (a), highest (2 ft). Relatively soft yellow-green marl.

PENARTH GROUP

Westbury Formation

15. Bone-Bed (4 in). Coarse calcareous light-grey micaceous sandstone with quartz pebbles; locally conglomeratic with reworked Tea-green Marl. Richardson recorded Gyrolepis Alberti (scales and teeth?), Sargodon tomicus, Saurichthys acuminatus, Hybodus cloacinus, Hybodus minor, Acrodus minimus, Nemacanthus (spines), Modiola, Schizodus (?), Labyrinthodont-tooth, skin of Hybodus, coprolites, fish-vertebrae, piece of wood. Sandstone was more usual than an Aust-like conglomerate. Locally reworked marl clasts reached 8 in across. Richardson did not record Ceratodus teeth despite searching; a fragment rich in Sargodon disintegrated during removal. The named fossils retain his historical identifications.

Richardson counted fifteen Sargodon tomicus specimens within one square inch of a fragment that shattered during removal. This was an exceptional historical concentration within the sandy bone bed, rather than a prediction of present-day fossil abundance.

13–14. Sandstone-layers and shale (8 in combined). Alternating shale and calcareous micaceous sandstone, with small quartz pebbles. Richardson recorded Acrodus minimus, Gyrolepis Alberti (scales and teeth?), Saurichthys acuminatus, small coprolites. Ripple marks and indistinct surface markings were also described. Numbers 13–14 are retained as a combined local measurement, rather than split into invented thicknesses.

12. Shales (6 in). Black earthy shale.

11. Shales (1 ft 2 in). Firm finely laminated black shale. This firm shale formed a projecting ledge about 6 in above beds 13–14.

8–10. Shales (5 ft 4 in combined). Black shale with sandy streaks and selenite; concentrations of fossils at some levels. Richardson recorded Schizodus Ewaldi, Modiola minima, Pleurophorus elongatus, Pleurophorus angulatus, Protocardium rhæticum. Richardson grouped beds 8–10; separate thicknesses for those numbers are not supplied.

7. Limestone (6 in tabulated; 4–8 in reported range). Hard slightly pyritic blue-black limestone; fibrous calcite on its lower surface; lenticular exceptionally hard variant also described. Richardson recorded Acrodus minimus, Saurichthys acuminatus, Gyrolepis Alberti, Hybodus minor, Pecten valoniensis, Avicula contorta, Protocardium rhæticum, Modiola minima, Schizodus Ewaldi, coprolites, ichthyodorulite. The 6 in column value sits within the recorded 4–8 in variation. The high-level Acrodus occurrence is an historical identification, not a new taxonomic determination.

6. Shales (6 in). Earthy black shale rich in shell debris. Richardson recorded Schizodus, shell fragments.

5b. Limestone (5 in tabulated; 3–8 in reported range). Hard black-blue limestone weathering brown. Richardson recorded Pecten valoniensis, Gyrolepis Alberti.

5a. Shales (2 ft 4 in). Laminated black shale, weathered brown-black and grey-green locally. Richardson recorded Pecten valoniensis, Schizodus Ewaldi, Myophoria Emmerichi, Protocardium rhæticum. Richardson assigned this bed to the Lower Rhaetic provisionally. He expected Avicula contorta but did not actually record it here.

Lilstock Formation

Cotham Member

4. Shales (3 ft 5 in). Imperfectly laminated grey-green shale. Richardson recorded Myophoria, Pecten, shell fragments. The Cotham framework is a broad modern placement of the historical Upper Rhaetic interval, rather than a newly measured formation boundary.

3. Estheria-Bed or Cypris-Bed (8 in tabulated; 4–12 in range). Variable nodular/arborescent limestone, green fine-grained rock or cream laminated rock. Richardson recorded Estheria minuta var. Brodieana, Schizodus (?), Pleurophorus, fish scales (?). The branching or landscape-like markings vary. Estheria was rare, and Richardson did not record Naiadita here. The historical names do not establish a modern reassignment of every small shell.

The historical Estheria Bed varies laterally: Richardson described nodular rock with arborescent markings, greenish fine-grained rock without those markings, and cream-coloured laminated rock. Brodie’s earlier account, quoted by Richardson, recorded Cypris, minute plant fragments and a few small fish in the Landscape Stone. Richardson did not record Naiadita here. Brodie and Higgins distinguished the Insect Limestone from the Landscape or Cypris Bed; the four-foot intervening shale in that historical account was a lateral observation, rather than a universal additional thickness.

Sandy/gritty band over Estheria-Bed (¼–2 in). Sandy band directly above limestone or separated by thin clay. The gritty band lay directly on the limestone or was separated from it by thin clay. It is a subordinate horizon, not an additional formation or an independently additive whole-section thickness.

2. Shales (2 ft measured in plate; 2–3⅓ ft lateral range). Thinly laminated grey-green shale; ochre streak with ostracods. Richardson recorded Darwinula liassica and varieties. T.R. Jones identified the ostracods from an ochreous streak about 1 ft above the Estheria Bed in the thinner part of the section. Their occurrence is tied to that streak, not to every grey-green shale.

The yellowish streak carrying abundant Darwinula liassica and varieties lay one foot above the Estheria Bed where the intervening deposit measured two feet. It is a fossil horizon within Richardson’s Bed 2, rather than another separately measured bed.

LIAS GROUP

Blue Lias Formation

Cotham Marble remnants / basal Lias conglomerate contact (Conglomerate 0–2 in; remnant limestone unquantified). Lithographic limestone fragments in conglomerate; locally attached to remnant bored limestone, elsewhere lying on shale. Richardson recorded fish scales, shell fragments (Ostrea?). Remnant, locally bored Cotham Marble must be distinguished from reworked pieces in the conglomerate. No intact White Lias was reported; an exposed Langport Member is therefore not added.

The basal Lias conglomerate rested variously on remnants of Cotham Marble or on shale. In fallen masses it could adhere to the succeeding fissile, oyster-bearing limestone. Thin limestone remnants were locally bored, showing a discontinuous, laterally variable contact.

Lowest Lias: limestones with shale partings (1 ft 1 in). Limestone with shale partings. Richardson recorded Pleuromya crowcombeia, Ostrea liassica, Modiola minima, Lima valoniensis, Avicula cygnipes, Phasianella (?).

Lias: shales with limestone bands (1 ft 8 in). Shale with limestone bands. Richardson recorded Pleuromya crowcombeia, Ostrea liassica, Modiola minima, Avicula fallax, Protocardium rhæticum, Unicardium cardioides.

Lias: shale with a limestone band (1 ft). Shale and limestone. Richardson recorded Ostrea liassica, Modiola minima, Avicula fallax.

Lias: limestone (4 in). Limestone. Richardson recorded Ostrea liassica, Lima Hermanni, Lima pectinoides, Lima valoniensis, Anomia, Pecten of calvus and textorius types, Pseudodiadema spines.

Lias: fissile shales with two thin limestones (4 ft). Fissile shale with two limestone bands. Richardson recorded fish scales, Ostrea liassica, Lima pectinoides, Avicula fallax, Anomia, Pseudodiadema.

Lias: limestone with shale band (1 ft 4 in). Limestone with a shale interbed. Richardson recorded Ammonites (Psiloceras) planorbis, Modiola minima, Lima gigantea, Lima pectinoides.

Lias: shales (11 in). Shale. Richardson recorded fish scales, Anomia, Pseudodiadema.

Lias: limestones with thick shale deposits (2 ft 7 in). Limestone and comparatively thick shale. Richardson recorded Ammonites (Psiloceras) planorbis (abundant in top bed), Ammonites (Psiloceras) Johnstoni, Lima gigantea, Modiola minima, Cardinia, Anomia.

Higher part of the historical Lias exposure: shales and limestone bands (24 ft). Shale and limestone bands. Richardson recorded Ammonites (Psiloceras) Johnstoni, Lima gigantea. These beds were examined only in fallen shore blocks, not as a fully accessible in-situ upper log. “Upper” here means the higher part of this local exposure, not the Upper Lias subdivision.

SUPERFICIAL DEPOSITS

Gravel shown above Lias (No thickness). Superficial gravel shown in plate profile. The drawing alone supplies neither a terrace-formation assignment nor a reliable age or fossil assemblage for this superficial material.

Comparing the early Lias accounts

Richardson’s individual Lias rows sum to 36 ft 11 in–37 ft 1 in, depending on the 0–2 in basal conglomerate. Vaughan estimated about 35 ft overall and stated that detailed direct study was restricted to the lower 12 ft. The higher 24 ft in Richardson’s plate was described from fallen blocks. These differences should not be hidden by selecting one rounded thickness.

Vaughan’s 1903 range diagram distinguished the lower Pleuromya crowcombeia zone, about 4 ft thick, from roughly 30 ft assigned to his Ammonites psilonotus zone above. His graphic shows ranges across the faunal change, including 4 ft below and 13 ft above it, but is not a precision modern bed log. These were abundance-based local divisions, not modern formal members or globally synchronous first appearances.

Vaughan’s diagram additionally plots Astarte cf. obsoleta and Cardinia of the concinna group in the higher interval, approximately above six feet on its relative scale, and labels Anomia pellucida specifically. These historical determinations are not assigned to individual Richardson limestone rows.

Vaughan’s lowest approximately four feet. His Pleuromya crowcombeia zone was crowded with Ostrea liassica, Modiola minima and P. crowcombeia. Cardium rhaeticum and Unicardium cardioides were abundant in certain layers, and Lima valoniensis was very common. Avicula cygnipes occurred near the base, while A. fallax was uncommon. He found no Pholadomya glabra through the section he examined.

Fauna above the proposed division. Lima gigantea, L. Hermanni and L. pectinoides, associated by Vaughan with L. hettangiensis, became abundant. Pecten of the calvus and textorius types was common; shale contained plentiful Anomia and Pseudodiadema fragments. His prose identified only O. liassica and M. minima as persisting from the characteristic lower fauna, the latter sparsely and the former abundantly only near the base of the higher interval.

The ammonite occurrence and the historical faunal boundary. Abundant Ammonites planorbis appeared approximately four feet above Vaughan’s division, without a corresponding major change in the other fauna. That abundant entry was therefore not his zonal boundary. The specimens were flattened, iron-stained casts identified historically from their smoothness and growth rate. Richardson reported Johnstoni within the planorbis range, but Vaughan had not personally confirmed both in one bed; Johnstoni was abundant higher in the succession.

Limits of the range diagram. Continuous lines mark sustained abundance, whereas interrupted lines show sparse occurrences or intervals between concentrations. Their ends delimit readily detected occurrence, rather than precise evolutionary first or last appearances. Some graphic ranges differ from the generalised prose. Vaughan directly studied only the lower twelve feet in detail; his failure to find an Angulatus-zone fauna in fallen material from higher beds records the limits of that historical sampling.

Sykes’ 1977 abbreviated Rhaetian section

This separate column runs from youngest to oldest. It summarises the lower fossil-bearing succession and does not replace Richardson’s full section.

Black shales (0.7 m).

Limestone (0.12 m). 75–200 mm range; fish remains and bivalves.

Fissile black shale (0.15 m).

Pyritic limestone (0.15 m). fish remains and bivalves.

Earthy/fissile black shale (1.83 m).

Alternating pebbly sandstone and shale (0.2 m). fish remains and coprolites.

Conglomeratic coarse sandstone bone bed (0.1 m). fish, reptile remains and coprolites.

Tea Green Marl (thickness not given). base; no thickness.

Sykes’ 1.83 m earthy/fissile shale interval differs from Richardson’s combined 7 ft for beds 8–12, approximately 2.13 m. This older-source-derived abbreviation is not treated as a new independent full cliff log.

Welch and Trotter’s 1961 summary. The following twelve packages run from youngest to oldest. The memoir explicitly abbreviates Richardson’s account, with different grouping and fossil nomenclature; it is not an independent remeasurement or a replacement numbering scheme.

Lower Lias (37 ft). Alternating limestone and shale.

Cotham grey-green shale (2 ft). Darwinula.

Cypris Bed (8 in). Estheria and Pleurophorus.

Cotham grey-green shale (3 ft 5 in). Myophoria and Chlamys.

Westbury black shale (2 ft 4 in). Chlamys valoniensis, “Schizodus” ewaldi, Myophoria and Protocardia rhaetica.

Upper Pecten Bed (5 in). Hard blue-black limestone with Chlamys valoniensis.

Black shale (6 in). No additional fauna specified in this summary row.

Lower Pecten Bed (6 in). Hard blue-black limestone with Chlamys valoniensis, Pteria contorta, Protocardia rhaetica, Modiolus minimus, “Schizodus” ewaldi and fish remains.

Sandy-streaked black shale (5 ft 4 in). “Schizodus”, Modiolus minimus, Pleurophorus elongatus and Protocardia rhaetica.

Black shale (1 ft 8 in). A combined package in this summary, without a secure one-to-one match to an individual Richardson bed.

Alternating sandstone and shale (8 in). Fish remains.

Bone Bed (4 in). Coarse light-grey calcareous sandstone with Tea Green Marl fragments and Gyrolepis, Sargodon, Saurichthys, Hybodus, Acrodus and Nemacanthus.

Separate red- and green-marl description. The memoir quotes Lawrie’s nearly 60 ft of red marl beneath 10–12 ft of Tea Green Marl. Thin sandstone bands near the base of the red marl formed foreshore reefs. These figures describe a different historical account from Richardson’s 56 ft and 10 ft 3 in.

The 37 ft Lower Lias interval. Welch and Trotter could not divide Blue Lias from Lower Lias Clay here. They described a basal conglomerate with Cotham fragments, approximately eight feet of basal “Ostrea Limestones”, and the remainder with Psiloceras planorbis and Caloceras johnstoni. The eight feet belongs within the 37 ft total.

The 1961 memoir described the Sedbury outlier as approximately one mile long north–south and half a mile wide, with two small step faults near Offa’s Dyke. Much of the cliff was slipped or inaccessible and no inland exposures were known at that time. These are historical exposure observations.

Later bed records and depositional interpretation

Andrew, Howe and Paul report Hodges’ Sedbury bed 19 as yielding Ctenostreon philocles with Plagiostoma giganteum, Pseudolimea sp. and Pinna semistriata. Psiloceras occurs approximately 1 m higher in Hodges’ bed 23. Those numbers belong to Hodges’ later scheme and are not equated with Richardson’s numbered Rhaetic beds. The earlier local occurrence of Psiloceras planorbis also cautions against using the first ammonite seen in one section as a simple universal boundary marker.

Milroy and colleagues’ 2019 study interpreted alternating saline-mudflat and temporary-lake conditions in the upper Mercia Mudstone facies, without evidence of significant marine influence in the associations examined. Its Figure 13C caption identifies a dolomite bed with wrinkled lamination, interbedded with peloidal–intraclast packstone, in the Sedbury succession at a plotted-source depth of 20 m. Figure 12C separately shows irregular mottled silty-mudstone patches and lenses, small desiccation cracks and rare bioturbation, overlain by weakly stratified silty mudstone, at a source depth of 23 m. These are individually located facies illustrations within the study’s plotted succession, rather than heights above the present shore or a complete new local log.

Richardson recorded a gentle dip towards the south-south-east, not exceeding 3°, and a step fault about 30 yards north-east of Offa’s Dyke, with separately recorded displacements of 3 ft and 7 ft. Springs and fallen material complicated those historical observations.

Modern seepages and tufa. The BGS site study at ST 5557 9308 examined about ten metres of seeping cliff over low-permeability Triassic and Jurassic strata. Active tufa was associated with Eucladium verticillatum and stronger Palustriella commutata seepages, with fallen tufa blocks on the beach and soil or clay accumulating on the lower slope after slips. The mapped accessible H7220 patch was no more than one square metre. These are modern seepage and habitat dimensions, rather than bed thicknesses or evidence of a present safe shore route.

References
Richardson, L. (1903), The Rhaetic and Lower Lias of Sedbury Cliff, Quarterly Journal of the Geological Society 59, 390–395, Plate XXIV.
Vaughan, A. (1903), The lowest beds of the Lower Lias at Sedbury Cliff, ibid. 59, 396–402.
Welch, F. B. A. and Trotter, F. M. (1961), Geology of the country around Monmouth and Chepstow, memoir for sheets 233 and 250, especially pp. 118–124.
Sykes (1977), British Rhaetic bone-beds, Mercian Geologist 6, 197–239, especially p. 209.
Hodges, P. (1994), The base of the Jurassic System: new data on the first appearance of Psiloceras planorbis in southwest Britain, Geological Magazine 131, 841–844; later bed records reported by Andrew, C., Howe, P. and Paul, C. R. C. (2015), Previously unreported spiny limid bivalves from the Blue Lias Formation (Early Jurassic: Hettangian) of Lyme Regis, Dorset, UK, Geoscience in South-West England 13, 410–418.
Milroy, P., Wright, V. P. and Simms, M. J. (2019), Dryland continental mudstones: Deciphering environmental changes in problematic mudstones from the Upper Triassic (Carnian to Norian) Mercia Mudstone Group, south-west Britain, Sedimentology 66, 2557–2589.
British Geological Survey (1984), Lower Jurassic rocks of the Bristol district; BGS OR/17/020, Sedbury Cliff site account.