The western shore of Red Wharf Bay exposes coral-bearing Carboniferous limestone, with brachiopod sections and crinoid fragments in its ledges. North towards Trwyn Dwlban, striking sandstone-filled pipes record ancient dissolution and river activity between episodes of marine deposition.
FIND FREQUENCY: ♦♦ – Corals, worn brachiopods and crinoid pieces can be seen in ledges and occasional loose pebbles; recognisable finds are scattered.
CHILDREN – Supervise children around slippery ledges, channels and the tidal northern shore.
ACCESS – The village car park is beside the shore. Northern exposures require suitable tides and careful route planning.
TYPE – Carboniferous limestone ledges and coastal cliffs on the western side of Red Wharf Bay.
DIRECTIONS
♦ Use the council’s Pentraeth Red Wharf Bay car park beside the western shore. It is listed as free parking, but follow current signs and do not obstruct nearby properties.
♦ From the car park, descend by the public shore approach and walk north towards the limestone ledges and Castell-mawr only while the tidal route remains safe. Plan for a falling tide with ample time to return.
♦ Original village-shore reference: 53.30569, -4.20778. Do not enter gardens or adjoining land beyond the houses to bypass a flooded or obstructed shore route.
FOSSIL HUNTING
Inspect the limestone ledges and naturally exposed pebble surfaces on the western shore. Fossil outlines often become easier to distinguish when the rock is damp, but take care on wet or weed-covered surfaces. Brachiopods are commonly seen as worn curved sections rather than complete shells; crinoid pieces may appear as small discs or short segments.
Corals are less frequent but can form conspicuous patterned patches in the ledges. Look for repeated corallites or internal radiating structures rather than identifying a fossil by its overall lump-like shape. A section through a colony can look very different from its upper surface, and a loose pebble may be too worn for a confident species identification.
The documented Upper Dwlban interval contains silicified colonies of Lonsdaleia duplicata between the two prominent pipe-bearing levels. That is a particular scientific record, not an identification for every colonial coral photograph from the bay. Keep those in-situ colonies undamaged and use photographs to record their setting.
Some finer layers preserve burrows described as Planolites, while the upper cherts contain microscopic sponge spicules and silicified shell or crinoid remains. Burrows are traces of animal activity, not body fossils; the microscopic chert records do not promise whole extractable sponges on the beach.
The sandstone pipes are geological structures, not giant fossil trunks. Their shape reflects ancient solution cavities and sediment filling. Observe them from safe ground without approaching unstable faces or climbing the exposures. Record the ledge or loose-pebble context of any fossil photograph, since a transported pebble cannot automatically be assigned to the bed beside it.
Significant discoveries, geological research and conservation milestones at Red Wharf Bay.
1899 – Sandstone-pipe investigations presented
Abstracts of Greenly’s investigations were presented at the British Association meeting in Dover, preceding the full published account of the Dwlban Point pipes.
1900 – Sandstone pipes described in print
Greenly described sandstone-filled pipes in the Carboniferous limestone at Dwlban Point. He acknowledged earlier observations and accompanying mapping work, so this publication is not a claim that he was the first person to see the structures.
1983 – Multiple phases of solution and filling explained
Walkden and Davies analysed the pipes as products of repeated dissolution, sediment infilling and related processes, helping explain their complex relationship to the limestone and sandstone boundaries.
2009 – Limestone and capping chert re-evaluated
A modern stratigraphical framework defined the Red Wharf Limestone Formation and recognised the possible Pentre Chert affinity of the upper bedded cherts. The local chert correlation and precise age remained uncertain.
GEOLOGY
The western shore exposes the Brigantian Red Wharf Limestone Formation, part of the Clwyd Limestone Group. Skeletal limestone accumulated in a shallow Carboniferous sea, but marine conditions were repeatedly interrupted. Emergence allowed dissolution of the limestone and rivers cut channels that later received sand and gravel.
The most distinctive evidence is the sandstone filling of pipes and irregular cavities beneath eroded surfaces. Successive episodes of solution, filling, cementation and renewed erosion produced complex shapes. When the sea returned, marine limestone accumulated above the terrestrial interruption. The shore preserves four recognised depositional cycles separated by three sandstone-marked boundaries.
Chert nodules occur within the limestone, and the upper succession includes bedded chert. The latter was formerly called the Castell Mawr Chert Member but may instead correlate with the Pentre Chert Formation of the Craven Group. That modern comparison is tentative: the local chert is not securely dated, and a precise Carboniferous stage boundary cannot be fixed here from the available evidence.
About 55 m is described in the type cliffs above a faulted base. This is not a complete formation thickness visible from every ledge, and the lower contact is inferred rather than exposed. The chart shows the recognised local units and interruptions schematically; it does not turn the tidal shore into a single measured, continuously accessible section.
The Dwlban and Castell-mawr limestone cycles, sandstone pipes and uncertain capping-chert correlation.
CLWYD LIMESTONE GROUP
Red Wharf Limestone Formation
The formation is Brigantian, within the Viséan. Sandy skeletal limestone, nodular chert and intervening siliciclastic deposits record shallow marine sedimentation interrupted by emergence, dissolution and channel cutting. ‘Red Wharf Cherty Limestone’ is older terminology for the formation, not an additional unit. Approximately 55 m is exposed in the type cliffs above a faulted base; the contact with the Cefn Mawr Limestone is inferred rather than exposed here.
Lower Dwlban Beds. The lowest locally described package consists of irregularly bedded skeletal packstone with chert nodules. Its base is unseen. It forms the lower marine limestone interval beneath the first major sandstone-marked interruption.
Trwyn Dwlban Palaeokarst and Dwlban Sandstone. An eroded limestone surface is cut by channels and solution pipes filled with sandstone and conglomerate. A northern low-tide channel at SH53168213 is described as 7–8 m deep, with a tributary and pipe exposure at SH53258189. These are separately located field observations, not the thickness of a continuous sandstone bed. The sandstone top is bioturbated and is followed by mudstone bearing Planolites burrows and marine brachiopods higher in the interval. The lower foreshore pipe horizon includes pits up to 1.5 m wide and 3 m deep in the conservation account; these pit dimensions are distinct from the deeper channel measurement.
Upper Dwlban Beds. Irregular to massive skeletal limestone with chert follows the lower sandstone and muddy interval. Silicified colonies of Lonsdaleia duplicata are recorded between the two pipe-bearing levels. This locality-specific occurrence should not be extended to every coral-bearing pebble in the bay.
St David’s Sandstone. A second erosion surface is associated with sandstone and large downward-extending pipes in the underlying limestone, including the prominent features near SH53218172. The giant pipe shown in the locality figures is a sediment-filled dissolution structure, not a fossil tree. The reported approximately 2.5 m vertical spacing between pipe levels is a particular field observation and is not used to calibrate the schematic regional log. The upper pipe horizon contains pits up to 3 m across and 5 m deep in the conservation account.
St David’s Beds. A substantial interval of thick skeletal limestone with nodular chert overlies the second sandstone boundary. The detailed local account gives approximately 21 m for these beds. This value belongs to that described unit, not to the whole formation or the height of every coastal exposure.
Castell-mawr Sandstone. The third sandstone-marked cycle boundary is recorded near SH531815. It represents another interruption of marine carbonate deposition and separates the St David’s limestone from the upper Castell-mawr interval. No numerical thickness is inferred from the drawing of the log.
Castell-mawr Beds. Upper limestone follows the sandstone, with bedded chert beginning approximately 5 m higher in the detailed field description. The older log included the upper chert in the Castell-mawr succession; the modern possible correlation of that chert is treated separately below rather than hidden within a confidently named limestone member.
Repeated dissolution and filling. Four depositional cycles and three sandstone boundaries are recognised. The process model distinguishes white sand plugs, conglomeratic filling, channel and breccia development, shale and buff sand, and subsequent Quaternary erosion. Repeated cementation and lithification separated the sediment-filling phases. These are episodes of cavity development and sediment infilling, not five formal stratigraphic members. Their relationships explain the complex pipe outlines and the return to marine conditions above them.
Historical Dwlban section, published in 1900. Greenly’s figure 2 records, from bottom to top, piped limestone, about 2 ft of sandstone, about 7 ft of dark shale and 8–10 ft of upper limestone. These are historical measurements of the illustrated exposure. They are kept separate from the modern composite succession and are not assigned to a modern named bed by resemblance alone.
Historical inland crag south of the point. Figure 3 shows a crag approximately 230 yards south of Dwlban Point, with cherty limestone below sandstone normally about 8 ft thick and upper limestone about 10 ft thick. A pipe extends at least 12 ft downwards, with its bottom unseen. This is a separately located historical section, not an additional thickness to add to the shore log.
Early pipe observations and locality limits. The historical account describes pits 1–7 ft in diameter, surviving sandstone plugs 4–5 ft high, and larger pits at the point approximately 6 ft wide and traced 5–6 ft deep. These observations differ in exposure and date from modern measurements. Its discussion of three regional pipe horizons includes one north of Benllech; it does not establish three pipe-bearing horizons at Dwlban itself.
Historical fossil comparisons. Greenly’s suggested identification of a poorly preserved shell as Spirifera ovalis was tentative and is retained only as a historical interpretation. Plant occurrences at Penmon were comparative evidence, not plant finds from Dwlban Point.
CRAVEN GROUP — TENTATIVE UPPER CORRELATION
Possible Pentre Chert Formation
Approximately 6 m of bedded chert caps the local succession. Formerly termed the Castell Mawr Chert Member, it may belong to the Pentre Chert Formation rather than the Red Wharf Limestone Formation. A probable earliest Namurian correlation has been suggested, but the local chert remains undated. The boundary and age are therefore tentative, not a securely established stage transition on this shore.
Chert microfossils. Thin-section observations include sponge spicules and silicified brachiopod and crinoid remains. These microscopic records help interpret the deposit but do not imply that whole sponges or unaltered shells can be extracted from the shore’s chert.
References. Adams, A. E. and Cossey, P. J. (2004), Trwyn Dwlban, in British Lower Carboniferous Stratigraphy, Geological Conservation Review 29, pp. 378–380, figures 8.9–8.11; Waters and colleagues (2009), BGS RR/09/01, section 4.6.6; BGS Lexicon, Red Wharf Limestone Formation; GeoMôn, Red Wharf Bay RIGS geological statement; Walkden, G. M. and Davies, J. (1983), Sedimentology 30, pp. 861–878, as discussed in the conservation account. Greenly, E. (1900), On sandstone pipes in the Carboniferous Limestone at Dwlban Point, East Anglesey, Geological Magazine 7(1), pp. 20–24, figures 1–3.
SAFETY
Check tide times and the weather before entering the northern shore. Headlands and channels can cut off the return before the broad sand flats are covered. Turn back early and never depend on crossing private land as a tide escape.
Wet limestone, seaweed, loose blocks, mud and channels can all be hazardous. Keep clear of cliffs and overhangs, do not climb Castell-mawr or inspect pipes beneath unstable faces, and supervise children closely. Avoid the shore in rough seas.
EQUIPMENT
Take a camera, hand lens, scale, tide information and footwear with good grip. Observation of the fossil-bearing ledges and pipes requires no hammer. If the landowner expressly permits taking loose material, carry small padded containers and record each specimen’s actual find position.
CLEANING AND TREATING
Wrap lawfully obtained specimens separately and retain their loose-pebble or bed provenance. Start with minimal dry brushing. Hard cherty limestone can fracture sharply, while partly silicified fossils may still contain vulnerable carbonate material.
Avoid acid extraction, aggressive splitting, polishing and routine consolidants. These can destroy shells, diagnostic surfaces and the relationships that make a specimen useful. Photograph unusual material and seek specialist advice before destructive preparation.
IDENTIFY YOUR FINDS
Need help identifying a fossil? Share clear photos, where you found it and its size with the community.
FURTHER READING
Geological Conservation Review: Trwyn Dwlban
GeoMôn: Red Wharf Bay geological site
BGS: Red Wharf Limestone Formation
BGS: Carboniferous stratigraphical framework
Isle of Anglesey Council: Red Wharf Bay car park
ACCESS RIGHTS
Use the public shore approaches and follow current notices. The northern rock exposures include the protected Trwyn Dwlban geological site. Do not assume that the whole bay has identical ownership, protected boundaries or collecting conditions.
Public access and free parking do not give permission to remove fossils or rock. Ask the landowner before collecting and check any conservation consent required. Leave fossils in the ledges, sandstone pipes, chert beds and cliff faces undisturbed; photograph them instead. Do not enter gardens or adjoining land to reach exposures.
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 2008, Updated 2017, Edited by Jon Trevelyan 2016
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