Abereiddy Bay Fossil Hunting

1875 – Early description of the local rocks and fossils
Hicks described the geology and faunas of the district, including shelly material from the inland Llanvirn Quarry. These published records are distinct from the main beach’s familiar graptolite finds.

1881 – The Llanvirn name
Hicks proposed the Llanvirn Group from the succession around Abereiddy, giving this small area an enduring role in the development of Ordovician stratigraphy.

1915 – A detailed structural interpretation
Cox described the local stratigraphy and proposed thrusting to explain repetition of the succession. Subsequent work revised that interpretation as structural and fossil evidence improved.

1971 – Evidence for the syncline
Structural investigation and faunal and sedimentological studies supported repetition around a syncline, an important change from the earlier thrust-based explanation.

1973 – Locating the fold axis
Rickards used relationships between bedding and cleavage to locate the synclinal axis, helping establish the order and inversion of the exposed rocks.

1979 – Revision of the sedimentary and volcanic succession
Jenkins’s unpublished revision and igneous reinterpretation by Bevins and Roach developed the understanding of the interbedded shales and volcanic rocks. The unpublished status of the former work remains important when tracing the evidence.

1982 – An integrated field account
Hughes, Rickards and Jenkins published an excursion account bringing together the revised succession and its structural interpretation.

1984 – Submarine volcanic-debris interpretation
Work by Kokelaar and colleagues developed the interpretation of volcanic deposits emplaced and reworked in a submarine setting, including sediment flows from unstable volcanic slopes.

1987 – Distorted graptolites explained
Jenkins showed that supposed multiple pendent graptolite species were strain-modified forms of Didymograptus murchisoni, and used them to quantify three-dimensional deformation.

1990 – A radiometric constraint
A uranium–lead zircon determination from tuff immediately below the Cyffredin shales at Trwyn Castell yielded 464.6 ± 1.8 million years. This historical determination constrains that sampled horizon; it is not a modern numerical date for every fossil-bearing bed in the bay.

1992 – Geochemical study of the tuff
Bevins and colleagues investigated the geochemistry of the Abereiddi Tuff, adding evidence to the interpretation of its volcanic origin and emplacement.

1999–2000 – Conservation-review syntheses
Detailed igneous and stratigraphic conservation accounts brought together the sections, fossils and structures that make Abereiddy scientifically important.

2025 – A revised regional hierarchy
A preliminary stratigraphic framework placed Caerhys and Castell in the Drefach Subgroup of the Montgomery Group and argued for inclusion of the overlying historical Dicranograptus Shales within an expanded Mydrim Shales Formation.

Local sections and order. The succession below follows its interpreted older-to-younger order, with northern and southern exposures explicitly separated. The Llanrian Syncline repeats units around the bay and overturns the northern limb; some contacts are faulted. Measurements from one locality are not a single composite thickness for the whole bay, and descriptions of quarry exposures are scientific context rather than access directions.

FOEL CWMCERWYN GROUP

This group assignment for the inland Aber Mawr Formation follows the preliminary BGS framework of 2025. It is separate from the coastal volcanic and younger sedimentary groups described below.

Aber Mawr Shale Formation: inland historical context

Llanvirn Quarry. The historical ‘bifidus Shales’, assigned to the lower Abereiddian artus Biozone, are exposed inland at Llanvirn Quarry rather than as the lowest coastal beds. Blue-grey shale weathers buff or orange. The quarry’s historically described type fauna includes Barrandia homfrayi, Ormathops llanvirnensis, Placoparia cambriensis, Dinobolus hicksii, Modestospira llanvirnensis, ‘Theca’ caereesiensis and Acrograptus nicholsoni. These names are retained as historical source assignments; they are not a list of expected main-beach finds, and no individual museum accession is inferred.

FISHGUARD VOLCANIC GROUP

Llanrian Volcanic Formation

The volcanic succession is also spelt Llanrhian in later accounts and is related to the western part of the Fishguard volcanic succession. The regional Fishguard Volcanic Group thickness is not a measurement of Abereiddy Bay. The detailed member names below retain their original section-specific usage.

Lower Crystal / Lower Rhyolitic Tuff Member

Northern and southern lower tuffs. The two labels reflect differing historical descriptions rather than two separately stacked formations. Northern crystal-rich volcaniclastic sandstones are normally graded and described as up to 100 m thick in the igneous conservation account. Fine silicic rocks at Trwyncastell, once called rhyolitic lavas, contain shards supporting a tuff interpretation. The southern section at Pwll Caerhys has approximately 12 m of crystal tuff. The northern and southern quantities must remain separately scoped. The northern silicic deposits have been interpreted as subaqueously emplaced ash flows; recrystallisation has modified their original textures, so that interpretation should be distinguished from the older lava description.

Northern intratuff shale. Near Carn Llwyd, a 2 m interval of massive black shale with matrix-supported angular volcanic debris occurs between gabbros. No fossils were recorded from it, and assignment to the artus or murchisoni Biozone remains unresolved. It is not an additional verified fossil horizon.

Cyffredin Shale Member

Basal tuffaceous shale and fauna. Above the lower rhyolitic tuffs, the old tramway cutting at the north-west corner of the flooded quarry exposes tuffaceous basal shale with a low murchisoni Biozone fauna. Recorded northern fossils include Didymograptus murchisoni, Cryptograptus latus, Diplograptus sp., an unnamed Lasiograptus, Cnemidopyge cf. nuda, Segmentagnostus and Protolloydolithus. Similar southern Cyffredin faunas are reported at Melin Abereiddi Quarry and Pwll Caerhys, but that does not individually voucher every listed taxon at every outcrop. No local total member thickness is established.

Abereiddi Tuff Member

Northern limb. Fine grey vitric-lithic crystal tuffs overlie the Cyffredin shales. A minor intrusive body is mapped as andesite in the igneous account and described as vesicular keratophyre in the stratigraphic account, and the upper part was described on a raised quarry bench. This is a description of the scientific section, not permission to enter the flooded quarry or its benches.

Southern thicknesses and their scope. The stratigraphic conservation account describes 28 m of coarse pumiceous lapilli tuff near Melin Abereiddi. The igneous account gives about 60 m of tuffs in the wider Melin area and 95 m in two fining-upward units north-east of Aber Creigwyr. These describe different section extents and cannot be substituted for one universal member thickness.

South-western lower and upper tuff units. The lower unit is poorly to moderately bedded, becoming finer upwards with better-defined coarse bed bases and coarse-tail grading. The upper unit is less obviously bedded but also graded. Angular scoriaceous basalt lapilli and vesicular blocks or bombs up to 40 cm occur in finer matrix. Two sharp planar bases are described, with the upper unit conformable on the lower. Original volcanic particles were redistributed in submarine sediment flows; their presence does not make the whole deposit an undisturbed lava flow. North-east of Aber Creigwyr, these beds dip north at about 45°. The basaltic tuffs thicken and coarsen westwards and become less distinctly bedded; these are scoped lateral changes rather than uniform properties of every tuff around the bay.

Melin lower tuff unit. This unit lies conformably above tuffaceous mudstone. Poor sorting, crude coarse-tail grading and indistinct bedding characterise its lower part, followed upwards by thinner, finer beds. The vertical change records declining delivery of coarse volcanic debris.

Melin upper tuff unit. A distinct lapilli tuff marks the base, followed by beds 3–25 cm thick. Some show normal grading, parallel lamination and ripple-drift lamination; bases are mostly planar but locally scoured. The succession passes upwards into tuffaceous mudstone and mudstone. Debris flows and turbidity currents from unstable basaltic volcanic flanks explain this range of textures and the diminishing coarse supply.

MONTGOMERY GROUP

DREFACH SUBGROUP

The following sedimentary formations are placed in this hierarchy by the BGS preliminary framework of 2025. It refines the older regional terminology while retaining the importance of local fossil zones and section descriptions. Proposed revisions should not be mistaken for completed remapping of every exposure.

Caerhys Shale Formation

Northern succession. Tuffaceous shale at the base becomes increasingly silty upwards. A prominent graded tuff occurs approximately 9 m above the base, with another about 3 m below the top. Calcareous beds increase towards the overlying Castell Limestone. The murchisoni and teretiusculus biozones are represented, but their faunal change does not correspond to a sharp lithological boundary.

Southern basal beds. The lowest 1–2 m are tuffaceous, with redeposited tuff bands 1–2 cm thick and massive beds up to 20 cm. Bentonites occur 3 m above the base and 2 m below the top. Pyritous nodules in the basal 5 m sometimes contain pyritised Didymograptus murchisoni. These small-scale horizons are part of the southern section and must not be merged with the northern tuff heights as one measured log.

South-eastern graptolite surface. At the classic southern foreshore exposure, abundant D. murchisoni occurs where bedding and cleavage nearly coincide. Tectonic strain has altered colony shapes. The demonstration that supposed multiple pendent species were strain morphs of one species is important both for identification and for interpreting the fossils as markers of rock deformation.

Northern Caerhys fauna. The lower fauna includes D. murchisoni, Cryptograptus latus and an unnamed Lasiograptus. Higher beds lack pendent didymograptids and include Protolloydolithus sp., Segmentagnostus sp., Ogygiocarella cf. angustissima, Platycalymene cf. tasgarensis simulata, lingulate brachiopods, Modestospira and bivalves. These are documented section faunas, not an assurance that each fossil can be found loose on the main beach.

Castell Limestone Formation

Northern quarry entrance. Shale is interbedded with impure calcareous beds rather than a uniform massive limestone. Basal graptolites include Dicellograptus cf. sextans, Pseudoclimacograptus riddellensis, Orthograptus cf. acutus and Isograptus sp., alongside trinucleid trilobites. The source discusses a conodont assemblage of the low tvaerensis Zone and a late Llandeilian comparison while its section figure uses basal-Caradoc terminology; a falsely precise single boundary age is therefore avoided.

Northern microfauna. Recorded ostracods include Vitella fecunda, Ceratopsis, Conchoprimitiella, Laterophores and Pseudulrichia. Conodonts include Amorphognathus tvaerensis, Drepanoistodus suberectus, Phragmodus sp., cf. Icriodella sp. and Plectodina flexa. These specialist records are not a promise of visible hand specimens or permission for acid sampling.

Southern calcareous beds. South of the cave, immediately beneath the synclinal-core shales, the account records 9 m of calcareous or decalcified beds, including 2 m of micritic limestone. Fragmentary Trinucleus, Telaeomarrolithus and bryozoans occur. This is a local southern measurement rather than a thickness for every Castell exposure.

Mydrim Shales Formation: proposed expanded usage

Post-Castell shale. Northern beds historically yielded Nemagraptus sp., Diplograptus foliaceus, Trinucleus fimbriatus, Telaeomarrolithus cf. intermedius and Cnemidopyge sp., associated with the gracilis Biozone. Inverted shale near the synclinal axis yielded poorly preserved Dicranograptus sp. The 2025 framework argues for including the historical Dicranograptus Shales in an expanded Mydrim Shales Formation. Early Caradoc shale beneath the alluvium of the central bay is inferred rather than continuously exposed, so no measured core section is implied.

Folds, faults and younging. The Llanrian Syncline has an overturned northern limb. Near the axis on the southern side, beds are inverted; farther south they become right-way-up across a prominent fault. Closely spaced faults, gullies and breccias further interrupt the succession. This explains repeated units and apparently reversed order more adequately than treating the bay as a simple northward progression through younger strata.

References and evidence limits. Rushton and colleagues (2000), Geological Conservation Review 18, Abereiddi, pp. 143–147; Bevins in Stephenson and colleagues (1999), Geological Conservation Review 17, Castell Coch to Trwyncastell; Burt and colleagues (2012), Fishguard Sheet Explanation; Davies (2025), BGS OR/25/018, pp. 89–90 and plate 7. The GCR accounts provide the detailed section descriptions used here. The original 1982 excursion account and Jenkins’s 1979 thesis were not recovered for direct inspection. The inconsistent printed coordinate attached to one northern post-Castell locality has not been reproduced as navigation.