The old northern quarry at Trefil opens a window into a Carboniferous tropical sea. Weathered limestone preserves corals and shells, while the surviving tracks and quarry faces tell the story of limestone working for the South Wales iron industry.
FIND FREQUENCY: ♦♦♦ – Fossils occur in the limestone, although grey shells and corals can be difficult to distinguish from their matrix.
CHILDREN: ♦♦♦ – Uneven ground, large blocks and quarry faces make this unsuitable for very young children; supervise older children closely.
ACCESS: ♦♦♦ – An approximately one-kilometre approach on foot from the village barrier leads to the old northern workings.
TYPE – Disused limestone quarry, with exposed rock and displaced quarry waste.
DIRECTIONS
♦ From the A465 Heads of the Valleys Road, take the junction signed for Tredegar and Trefil, then follow the road north towards Trefil. From Tredegar, the A4048 leads to the same junction.
♦ Continue through Trefil towards the cattle grid and barrier. The original guide’s parking reference is SO 11975 12932, just before the barrier; check the current signs and choose a lawful space without blocking gates, passing places or the road. Do not drive beyond the barrier.
♦ Walk north along the quarry road past the barn, then follow the rocky track on the right towards the junction by the working-quarry entrance. Do not enter the working quarry: cross the main track towards the gap between low mounds leading into the old northern quarry. Follow current waymarks and signs.
♦ The old fossil quarry is at SO 11746 13944, roughly one kilometre from the village barrier.
FOSSIL HUNTING
The most useful rock to examine is the dark-grey limestone, especially its naturally weathered surfaces on the old quarry floor and low exposures. Fossils may stand slightly proud of the matrix or appear as darker shapes against a pale weathered surface. Concentrate on places you can inspect without climbing a face or unstable scree; a hand lens and a photograph taken in low-angle light often reveal details that are missed at first glance.
Corals are a particular attraction. The original field account records Syringopora, Siphonodendron and Siphonophyllia. Colonies of closely spaced tubes look very different in side view and cross-section: the same colony may show long parallel corallites on one surface and a cluster of round or oval openings on another. Solitary rugose corals can have a horn-like outline, with radiating internal partitions visible where naturally exposed. These features help separate corals from ordinary pale patches in the rock, but an incomplete section rarely supports a confident species name.
The original guide also records a coral-rich layer visible around the northern quarry. This is a useful local observation, rather than a formally named or measured bed. View accessible examples in place and compare them with the photographed colonies below. Loose quarry waste has lost its original position, so a coral in a displaced block cannot be assigned to that layer merely because it looks similar.
Brachiopods commonly have ribbed shells with a bilateral outline; productid and spiriferid forms are recorded in the original field account. Coiled gastropod shells and occasional bivalves add variety. Check the overall shape, repeated ribs and the way a fossil meets its surrounding rock before deciding what it is.
Trilobites are much less frequent. A small tail shield or other fragment may show a segmented central axis with ribs to either side, rather than the radial pattern of a coral. The original photographs include material interpreted as a trilobite fragment, but do not justify adding a species name. Photograph both a close detail and the surrounding rock, with a scale beside the fossil.
Pale quartz-rich sandstone and conglomerate blocks belong to a different part of the geological story. Rounded quartz pebbles and visible sand grains distinguish them more reliably than colour alone, because limestone also weathers pale. These blocks generally offer fewer conspicuous marine fossils than the limestone. Examine surfaces already exposed without digging, levering or splitting; see ACCESS RIGHTS before removing anything or using tools.
Significant discoveries, geological research and conservation milestones at Trefil.
1794 – Quarrying begins
The Kendall brothers opened the Trefil quarry to supply limestone to Beaufort Ironworks. It subsequently served other ironworks, establishing the industrial role of the exposed Carboniferous limestone.
1829–1919 – Successive northern quarry workings
The surviving northern quarry faces, tramways and earthworks represent successive phases of extraction during this period. Their layout preserves the connection between the limestone workings and the iron industry.
1989 – The geological succession documented
The Abergavenny district memoir described the fossiliferous limestone and the younger, unconformable sandstone of the Trefil area. It provides the geological framework for understanding the contrasting rocks seen in the quarry waste.
13 August 2003 – Buried tramroad sleepers revealed
Drought conditions made buried tramroad sleepers visible in aerial photographs of the northern quarry, adding evidence for its historic transport system.
30 March 2007 – Northern quarry remains scheduled
Trefil Quarries North was scheduled as monument MM338, recognising the importance of its quarry faces, tramways and earthworks. This protection is relevant to any proposal to disturb the site.
GEOLOGY
Trefil’s old northern quarry exposes the Dowlais Limestone Formation, part of the Pembroke Limestone Group. It formed during the Holkerian part of the early Carboniferous, when this area lay beneath a warm, shallow sea. Shells, coral skeletons and carbonate mud accumulated on a broad marine platform. Coral colonies grew within that setting, but their presence does not make every quarry face the remains of a single continuous coral reef.
The limestone described in the geological memoir is grey, well bedded and locally bituminous, with tabular beds separated by dark shale partings. These contrasts reflect changes in the balance between carbonate sediment and finer mud. Weathering attacks softer material and exposes the relief of some shells and corals, while fresh-looking limestone can conceal fossils of almost the same colour as the matrix. That is why slowly examining weathered surfaces is often more revealing than judging a block at a distance.
The younger Twrch Sandstone Formation, belonging to the Marros Group, accounts for the pale sandstone and quartz-pebble conglomerate reported among the quarry blocks. Between the limestone and the younger siliciclastic deposits lies an erosional break. Elsewhere in the Trefil quarry complex, sandstone fills hollows cut into the limestone surface, showing that exposure, erosion and local dissolution interrupted marine carbonate deposition. Those neighbouring junction exposures provide context; they are not a measured section of the northern fossil quarry.
Quarrying has brought these contrasting rocks together on the floor. The faces retain some original layering, but the spoil does not form a reliable oldest-to-youngest sequence. A block’s rock type is informative; its present position alone cannot identify the original bed or establish an exact age for the fossil inside it.
Detailed geology and stratigraphy of Trefil Quarry
This account concerns the old northern quarry at SO 11746 13944. The order below separates the older limestone, the erosional break and younger sandstone, but the loose quarry waste is displaced and does not form a measured vertical section.
PEMBROKE LIMESTONE GROUP
Dowlais Limestone Formation
The Holkerian limestone exposed in the quarry was deposited on a shallow marine carbonate platform. The geological memoir describes well-bedded grey, bituminous, tabular limestone with dark shale partings in the Trefil area. Carbonate mud, shells and coral skeletons accumulated under varying conditions; the presence of coral colonies does not establish a continuous framework reef through every bed.
Limestone beds and shale partings. Harder limestone alternates with finer, more easily weathered material. Fresh surfaces can be dark grey, whereas weathered limestone becomes pale. Shells and corals may stand out in relief where the surrounding matrix weathers away. The original northern-quarry account identifies dark-grey limestone as the most useful fossil-bearing material.
Unnumbered coral-rich horizon. The original inspected guide records a coral-bearing layer traceable around the northern quarry. It is retained here as a local field observation, without a formal bed name, numerical thickness or member assignment. Syringopora, Siphonodendron and Siphonophyllia are reported from the quarry, together with productid and spiriferid brachiopods, gastropods, occasional bivalves and rare trilobite material. Individual loose fossils cannot be assigned to the visible coral layer merely by resemblance.
Erosion at the limestone top. The broader Trefil succession records erosion and local dissolution before the younger quartz-rich sediment was deposited. Published junction exposures elsewhere in the quarry complex illustrate sandstone occupying irregular hollows in limestone. The detailed section southeast of this guide’s northern quarry is contextual evidence, not an additional stop or a bed log for SO 11746 13944. The contact represents missing time rather than an uninterrupted passage from limestone into sandstone.
MARROS GROUP
Twrch Sandstone Formation
The original guide records pale sandstone and conglomerate blocks alongside the limestone waste, consistent with the Twrch Sandstone overburden mapped in the quarry area. Quartz-rich sand and rounded quartz pebbles distinguish these rocks from pale-weathering carbonate. Marine macrofossils are generally less conspicuous than in the limestone, but the rocks should not be described as wholly barren. The formation’s regional Pendleian–Yeadonian range is not an independently measured age range for each block at this quarry.
Quarry waste and limits of correlation. Extraction has mixed blocks from different parts of the succession. No independently verified measured bed-by-bed log, named member or thickness for the northern quarry’s coral horizon has been established. Nearby logs at Pant Mawr, Cwar yr Hendre and other exposures are separate sections and are not reproduced here as if they belonged to this quarry. Record a fossil’s actual rock type and position in spoil rather than inventing its original bed.
References
Barclay, 1989: Geology of the Abergavenny district
BGS Lexicon: Dowlais Limestone Formation
BGS Lexicon: Twrch Sandstone Formation
British Regional Geology: Wales
SAFETY
Keep well away from steep faces and the ground immediately below them: rock falls remain possible in an abandoned quarry. Avoid climbing loose scree or large blocks, and watch for concealed gaps, vegetation-covered stones and uneven old tramways.
The approach crosses exposed upland ground. Wear grippy boots, take weather protection and supervise children closely. Watch for livestock and vehicles near the separate operating quarry; obey signs and keep out of the working area.
EQUIPMENT
Bring a camera or phone, a hand lens, a small photographic scale and a notebook for recording visible fossils. Sturdy footwear, waterproof clothing and drinking water are more useful here than extraction equipment. A charged phone is sensible, though reception should not be relied upon.
CLEANING AND TREATING
Leave in-situ fossils uncleaned: scraping, chemical treatment or aggressive brushing can damage both the specimen and its setting. A photograph under oblique light is the appropriate way to record a surface without altering it.
For a limestone specimen already held lawfully, keep supporting matrix around thin shells and coral tubes. Gentle dry cleaning under magnification may be sufficient. Avoid acids, including vinegar, because fossil and matrix may both be calcite; difficult mechanical preparation is best assessed off-site by an experienced preparator. This advice does not establish permission to take new material from the quarry.
IDENTIFY YOUR FINDS
Need help identifying a fossil? Share clear photos, where you found it and its size with the community.
FURTHER READING
British Geological Survey: Geology of the South Wales Coalfield, Abergavenny district
BGS Lexicon: Dowlais Limestone Formation
BGS Lexicon: Twrch Sandstone Formation
Cadw: Trefil Quarries North
Coflein: Trefil Quarries North
ACCESS RIGHTS
The old northern quarries are accessible on foot via the public route and common land, but this does not establish a right to remove fossils. The quarry faces, tramways and earthworks form the scheduled monument Trefil Quarries North (MM338).
Before taking material or using tools, obtain the landowner’s permission and check with Cadw whether scheduled monument consent is required. In the meantime, enjoy the fossils where they are and leave the ground, faces and historic remains undisturbed. Keep out of the separate working quarry and obey local signs. Motor vehicles are not permitted beyond the village barrier.
It is important to follow our ‘Code of Conduct’ when collecting fossils or visiting any site. Please also read our ‘Terms and Conditions‘
Barry Taylor. Last inspected 2019, Updated 2019, Edited by Jon Trevelyan 2018
LINKS
♦ Fossil Discussions
♦ Fossil Articles
♦ Buy Fossils, Tools and Equipment
♦ Buy Crystals, Meteorites, and Artefacts
♦ Join Fossil Hunts
♦ UK Fossils Network
