Above Porthgain’s harbour, loose slate waste records the village’s quarrying past. Fossils are uncommon, but occasional graptolites reward a careful look at natural surfaces, while the cliffs and surviving stone hoppers reveal a much wider geological and industrial story.
FIND FREQUENCY: ♦ – Sparse graptolites in loose industrial slate waste; a visit may produce no identifiable fossils.
CHILDREN – Steep steps and unguarded cliff edges require close supervision; keep away from the edge and unstable heaps.
ACCESS – Signed village parking and a short but steep stepped path above the harbour; check current charges.
TYPE – Loose quarry waste on the clifftop, with uncertain original bed provenance.
DIRECTIONS
♦ Follow signs to Porthgain and use the village’s signed parking area. Check the current signs and charges; toilets are near the village car park.
♦ Walk along the south side of the harbour below the large brick hoppers. The information board explains the village’s slate and roadstone industries.
♦ Climb the steps beside the white building at the end of the hoppers. At the top, follow the signed coast path towards the loose slate waste, approximately 50 m beyond the original path entrance. Current waymarkers take precedence over the old stile description.
♦ Keep to safe ground beside the path. The waste extends towards an unguarded cliff edge; do not follow it to the drop.
♦ Original locality reference: 51.94788°N, 5.18499°W.
FOSSIL HUNTING
The material described in this guide is the loose slate waste above the harbour hoppers. It is an industrial deposit rather than a continuous fossil bed, and much of it is barren. The original field account and photographs record occasional graptolites, but there is no securely established species list for this particular waste heap. Treat a visit as patient observation of scattered natural surfaces rather than a search for a guaranteed number of specimens.
Graptolites were colonial marine animals. In these rocks their remains may appear as flattened, narrow traces, sometimes showing a fine repeated pattern along an edge. A hand lens and light falling obliquely across the surface can help distinguish such detail from cracks, mineral streaks and uneven weathering. A photograph with a scale is useful before attempting an identification; the examples illustrated here should not be taken as evidence for a named species or biozone.
Look at the bedding surfaces of already loose pieces in a safe area where the landowner allows inspection. Slate also breaks along cleavage, a fabric formed later under pressure. Cleavage need not coincide with the original layers, so repeatedly splitting the easiest plane can cut through a fossil rather than reveal it. Avoid moving large blocks or excavating the waste to seek fresher rock.
The industrial history matters to identification. Workings at Porthgain, Trwyn Llwyd and Abereiddy were connected, and the origin of an individual loose fragment cannot be read directly from the geological map beneath the heap. Record the actual find spot, the date and ‘loose industrial slate waste; original bed uncertain’. Do not label a specimen as an Abereiddy species simply because historical accounts sometimes called the flooded quarry at Abereiddy ‘Porth Gain Slate Quarry’. That is a different collecting locality.
Significant discoveries, geological research and conservation milestones at Porthgain.
1840 – Development of the slate industry
Quarry leases and investment established the industrial setting that later produced Porthgain’s extensive waste and transport infrastructure.
1851 – Harbour expansion
Harbour, breakwater and pier development improved shipment from the slate workings, linked to the coast by railways and inclines.
1883 – Renewed slate operations
The United Welsh Slate Company renewed operations with transport equipment including inclines, aerial ropeways and a tunnel, adding to the industrial landscape visible around the harbour.
1904 – The roadstone hoppers
Forest of Dean United Stone Firms developed the surviving hoppers for crushed roadstone. Their role was broader than storing slate and helps explain the range of material handled at the harbour.
1910 – End of slate working
Slate production ended, leaving waste from the earlier industry alongside the infrastructure of the continuing stone trade.
Early 1930s – Closure of the stone industry
The final closure left the hoppers and other structures as evidence of the village’s exploitation of its rocks. Historical accounts differ between 1931 and 1932 for the end of operations.
1982 – Heritage conservation
Purchase by the National Park and Village Association was followed by conservation of the harbour’s industrial remains.
1997 – Conservation area designation
Porthgain Conservation Area was designated, recognising the village and harbour’s historic character and providing a framework for conserving its industrial landscape.
2022 – Formal geological appraisal
The Porthgain–Traeth Llyfn coastal strip was described in the National Park’s geodiversity planning guidance, highlighting the Cambrian–Ordovician succession, erosion surface and igneous activity.
2025 – Refined stratigraphic framework
A preliminary revision placed the Penmaen Dewi shales in a new regional hierarchy and distinguished the neighbouring historical Porth Gain Beds as the Abercastle Formation. This refined formation terminology without resolving the provenance of individual pieces of quarry waste.
GEOLOGY
At the saved locality point, the geological map identifies the Penmaen Dewi Shale Formation, an Ordovician unit of dark blue cleaved mudstone. The fine sediment accumulated in the sea before burial and later deformation turned it into the layered and cleaved rock seen today. ‘Arenig’, used on the map, is a traditional regional subdivision of the Ordovician; it should not be read as a precise numerical date for an unprovenanced piece of quarry waste.
The neighbouring Porthgain–Traeth Llyfn coast preserves a more complicated history than a simple stack beneath the spoil. Cambrian sedimentary rocks were eroded before early Ordovician deposition, and the coastal succession records further marine sedimentation, volcanic activity and later Ordovician basic intrusions along bedding. Folding and faulting also affect the rocks. The headlands therefore cannot usefully be described as either wholly igneous or wholly ‘Lingula Flags’. Diorite was worked for roadstone and historically sold as ‘granite’, a commercial description rather than a precise rock identification.
Stratigraphic terminology has developed. Early Ordovician sandstones historically called the Porth Gain Beds were included under Ogof Hen in older mapping and the online Lexicon, whereas the 2025 BGS preliminary framework distinguishes these St Davids-district rocks as the Abercastle Formation. That refinement does not change the map identification of Penmaen Dewi at the original spoil-search point, nor does it identify the source of every heap.
The village’s industries added another layer to this natural history. Slate was quarried and transported through a linked system of inclines, railways and coastal workings; later, crushed roadstone passed through the surviving brick hoppers. The loose material is consequently valuable evidence of industrial activity, but it cannot serve as a measured stratigraphic section. The chart separates the mapped bedrock, neighbouring coastal context and displaced waste rather than presenting them as one exposed succession.
Explore Porthgain’s mapped bedrock, changing formation names and the uncertain origin of the fossil-bearing quarry waste.
Scope. The original search point above the harbour hoppers is mapped as Penmaen Dewi Shale Formation at 1:50,000 scale. The surrounding coast and industrial waste require separate interpretation. No complete measured succession through the visitor’s spoil area has been established; regional formation thicknesses are not substituted for a local log.
FOEL CWMCERWYN GROUP
EASTERN CLEDDAU SUBGROUP
These group and subgroup assignments follow the BGS preliminary framework of 2025, within its Preseli Supergroup. The framework is a proposed refinement, while the existing digital map and online Lexicon retain the familiar Penmaen Dewi Shale name. The hierarchy does not establish the origin of individual loose pieces.
Penmaen Dewi Shale Formation
Mapped mudstone beneath the locality. BGS describes dark blue cleaved mudstones and assigns the mapped unit to the Arenig Series of the Ordovician. Davies’s 2025 framework uses the plural ‘Penmaen Dewi Shales Formation’. Fine marine sediment was compacted and subsequently cleaved; original bedding surfaces and later cleavage are distinct structures. Neither a detailed fossil biozone nor a numerical age has been established for the loose waste itself.
Boundaries and thickness. The online Lexicon gives approximately 300 m as a regional estimate for Sheet 209 and adjacent ground, with formal boundaries undefined. This is not a measured thickness above the Porthgain hoppers. No bed numbering, contact geometry or local thickness is inferred from that regional value.
TRIFFLETON SUBGROUP: NEIGHBOURING COASTAL CONTEXT
The 2025 framework assigns the Abercastle Formation to the Triffleton Subgroup. This contextual unit is separate from the Eastern Cleddau Subgroup assignment of Penmaen Dewi above; the two are not presented as a measured vertical stack at the waste heap.
Abercastle Formation: neighbouring coastal context
Historical Porth Gain Beds. The older Ogof Hen usage includes sandstone and sandy mudstone, locally with basal conglomerate and an unconformable lower contact. The 2025 framework, following earlier specialist work, distinguishes the early Whitlandian transgressive sandstones of the St Davids district as the Abercastle Formation rather than extending Ogof Hen indiscriminately into this area. These names describe a nomenclatural revision, not two units to stack one above the other at the spoil heap.
Coastal succession and igneous rocks. The Porthgain–Traeth Llyfn geodiversity site records erosion of Cambrian strata before early Ordovician deposition, subsequent marine sedimentation and volcanic activity, and later Ordovician basic sills intruded along bedding, whose slow cooling produced readily visible mineral grains and textures. These observations apply to the documented coastal strip, not to a continuous accessible log beneath the search area. The quarried diorite was marketed as granite; the trade name does not make it a true granite. Folding and faulting further complicate the present arrangement.
Industrial slate waste and fossils. Loose material is displaced from its original bed, and linked workings at Porthgain, Trwyn Llwyd and Abereiddy complicate provenance. The original locality photographs and field account document uncommon graptolites, but no securely published species-level voucher or bed assignment from this exact heap has been established. The mapped substrate cannot be used to label every fragment, nor may the much richer Abereiddy fauna be transferred here.
Evidence limits. Cox’s extensive 1915/16 district paper and the 1982 Dyfed excursion account are important historical sources, but their complete original Porthgain sections were not available for this revision. No detailed log has been reconstructed from titles or regional summaries. The chart is a geological context guide, not a measured column.
References. British Geological Survey, Penmaen Dewi Shale and Ogof Hen Lexicon entries and St Davids 1:50,000 mapping; Davies (2025), BGS OR/25/018, pp. 77–79, preliminary framework; Pembrokeshire Coast National Park (2022), RIGS supplementary planning guidance, Porthgain–Traeth Llyfn entry; Porthgain Conservation Area Appraisal (2022).
SAFETY
The waste lies close to an unguarded cliff edge. Stay well back, especially in wind, and avoid unstable heaps, loose blocks, old workings and industrial structures. The steps are steep and can be slippery; children need close supervision.
Tell someone your route and expected return time. Mobile reception may be unreliable. This guide does not direct visitors down the cliffs or into quarries, and a safe clifftop path does not make the adjoining waste or historic structures safe to explore.
EQUIPMENT
Wear footwear with a good grip and bring a hand lens, camera, scale, notebook and weather protection. Where removal is permitted, use soft wrapping and a rigid box for thin shale pieces. A hammer is not necessary for looking at natural surfaces and should not be treated as permission to disturb the waste or protected rocks.
CLEANING AND TREATING
Compressed fossils can be lost through scraping. Remove only loose dry dirt with a soft brush, and stop if the rock flakes or the fossil film begins to lift. Avoid acids, aggressive brushing, soaking, oils and varnish.
Keep matching halves together and wrap each piece separately. Preserve the actual find-spot and uncertain-provenance label. Leave a fragile or potentially important specimen unprepared and seek specialist advice before consolidation or further work.
IDENTIFY YOUR FINDS
Need help identifying a fossil? Share clear photos, where you found it and its size with the community.
FURTHER READING
BGS Lexicon: Penmaen Dewi Shale Formation
BGS: preliminary revised Lower Palaeozoic stratigraphic framework (2025)
Pembrokeshire Coast National Park: Regionally Important Geodiversity Sites
Porthgain Conservation Area Appraisal
National Trust: Porthgain and Abereiddi coastal walk
ACCESS RIGHTS
Keep to the paths, leave the industrial remains undisturbed and follow current site notices. Ask the landowner before collecting or disturbing material, and comply with any conservation restrictions. The conservation area, scheduled industrial remains and geological interest do not in themselves grant permission to remove fossils.
Do not dig into heaps, dismantle structures or work the cliff. Official visitor pages differ over parking charges, so check the signs at the parking area you use.
It is important to follow our ‘Code of Conduct’ when collecting fossils or visiting any site. Please also read our ‘Terms and Conditions‘
Joe Shimmin. Last inspected 2007, Updated 2017, Edited by Jon Trevelyan 2016
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