North of Portmarnock, the Robswalls rocky shore exposes fossil-rich Lower Carboniferous limestone beside Coast Road. Between the coast south-east of Malahide and the Carrickhill Martello tower, weathered platforms reveal corals, crinoids, bryozoans and shells. Faults and folds repeat parts of the succession, making this a rewarding place to study fossils in their original rock.
FIND FREQUENCY: ♦♦♦♦ – Fossils are readily visible on weathered limestone platforms, including large coral colonies and crinoid fragments. Photograph them in place on this protected shore.
CHILDREN: ♦♦♦♦ – Suitable for carefully supervised children in calm conditions; broken, uneven and slippery platforms require care.
ACCESS: ♦♦♦♦♦ – Short approaches from the Coast Road footpath by established steps. Foreshore travel is much rougher than the pavement above.
TYPE: Fossiliferous limestone and shale exposures, with loose pebbles between the platforms.
DIRECTIONS
♦ Follow the R106 Coast Road south-east from Malahide or north from Portmarnock. The guide covers the rocky Robswalls shore as far as Carrickhill Martello tower, north of Portmarnock’s main seafront and Velvet Strand.
♦ Use established sets of steps from the public coastal footpath. Avoid unmade cliff paths and informal scrambles: some are unstable and provide poor routes back from the foreshore.
♦ Park only in permitted spaces and check the local signs. Do not assume the whole Coast Road is free parking. The named Coast Road Car Park at Malahide has a ticketed initial free period and subsequent charges during signed operating hours; this does not describe every lay-by.
♦ Plan a short out-and-back shore visit around low water, keeping your chosen steps in view. Use the coastal pavement to move between access points when the tide or rock conditions make the foreshore route unsuitable.
FOSSIL HUNTING
The main attraction is the limestone surface itself. Weathering and wave washing reveal pale fossils against the darker rock, often in enough detail to recognise their broad groups without removing them. Look across a clean bedding surface first, then examine promising patches with a hand lens. The local assemblage includes brachiopods, corals, bryozoans, crinoids and gastropods; shells and broken skeletal debris can be crowded together in a single bed.
Crinoid stems appear as small round discs in cross-section or as segmented lengths where the rock cuts along a stem. A patch of such fragments records disarticulated material rather than a complete sea lily. Bryozoan colonies can show delicate branching or a fine repeated pattern of small openings. Compare their texture with the much larger repeated sections through a coral colony, rather than assuming that every pale branch is the same kind of fossil.
Colonial corals can form broad, rounded masses cut by the present rock surface. Depending on the angle, their corallites appear as groups of circles or as closely spaced elongated tubes. Solitary rugose corals instead represent individual cups or horns, commonly seen here in cross-section. Smyth’s classic study described colonies of Syringopora growing on earlier shells: some in the northern block were attached to shells he identified as Conocardium, and repeated beds farther south contain colonies on Euomphalus. These are useful records of organisms using shells as a firm base, rather than simply a collection of unrelated fossil shapes.
Brachiopods commonly present curved shell outlines, ribs or sections through paired valves; gastropods show coiled outlines or cuts through successive whorls. A single polished section can be deceptive, so photograph several views with a scale. The mapped visitor trail north of the tower highlights a shell-rich limestone bed near a small anticline and coral-and-gastropod-rich rock farther north near a concrete outlet pipe. These are local observation points, not a promise that each fossil group occupies one end of the entire coastline.
There is more to read than fossils alone. Look for changes in the direction of bedding, narrow shale partings and veins cutting the rock. Similar fossil-bearing layers recur across the faults, so seeing the same association twice need not mean that two separate episodes of deposition have been identified. Do not try to trace a bed through algae-covered or submerged ground: the continuity is a geological interpretation, not a safe walking route.
Loose pebbles between the platforms also contain coral, bryozoan and crinoid fragments, but they have lost their precise bed position. Record whether a photograph shows bedrock or a loose stone and include a wider view of its setting. Removal or disturbance requires the appropriate prior consent and landowner permission; neither loose material nor a large detached block is automatically available to collect or hammer.
Significant fossil discoveries, geological research and conservation milestones at Portmarnock Robswalls shore.
1920 – Faults, fossil beds and a coral described
Louis B. Smyth published a detailed account of the coast, recognising three major faults and using recurring fossil beds to identify repeated parts of the succession. He described a new coral under the name Endophyllum irregulare from his M2 and M4 bed groups in the northern block. His paper appeared on 11 February 1920, following its presentation in November 1919; these are publication and presentation dates, rather than dates of collection.
1974 – Microfossils refine the age of Block E
T. R. Marchant published a 123.5 m measured section through the northern fault block and distinguished two microscopic fossil assemblages supporting an upper Tournaisian age. The highest samples gave poor or inconclusive foraminiferal faunas, so the study did not establish a precise age boundary in every exposed bed.
1982 – Historic specimens catalogued at Trinity College Dublin
John R. Nudds documented type and figured material from the classic section, including Smyth’s M2 coral specimens and Marchant’s microfossil slides. The catalogue preserves the link between published descriptions, museum material and recorded bed provenance; its publication does not represent a new collection from the shore.
2007 – Geological heritage significance documented
The Fingal geological heritage audit recorded the Robswalls coast’s fossiliferous Lower Carboniferous succession and three main faults, assessing its national scientific importance. County Geological Site recognition highlighted its research and educational value, without itself granting permission to remove fossils.
2019 – Statutory SAC protection formalised
The statutory designation of Malahide Estuary Special Area of Conservation covered this rocky foreshore. Its provisions require the appropriate prior consent for fossil and rock removal or disturbance within the mapped protected area.
GEOLOGY
The rocky shore exposes limestone and shale of the Malahide Formation, deposited in the Lower Carboniferous and principally associated here with late Tournaisian fossil assemblages. Much of the limestone consists of marine skeletal debris in fine carbonate mud, with muddier beds and shale recording a changing supply of clay. Regional studies interpret the formation as a shelf or ramp succession; individual beds preserve different textures and fossil concentrations within that broader setting.
A walk along the shore does not follow one simple stack of progressively younger beds. The rocks generally dip northwards, but faults divide the classic section and folds change the local attitude. Smyth named the blocks E, F, G and H from north to south. This guide covers E, F and the part of G extending to Carrickhill Martello tower. Southern G continues a short distance beyond the tower to the Carrickhill fault, with H farther south. Fossil-marker beds show that parts of F and G repeat the same succession. This is why fossils and bedding are more informative together than either is alone.
The northern Block E contains Marchant’s 123.5 m measured section. Its alternation of clay-rich and cleaner bioclastic limestone includes an oolitic interval between 85 and 100 m above the logged base. Ooids are small coated grains, while the skeletal material records marine life; together these textures show that the limestone is not a uniform mass. Lower parts have been irregularly altered to dolomite, locally damaging the preservation of fossils. Veins and broken rock near faults record later changes after the original sediment became solid.
Much younger processes shape what can be seen now. Glacial till occurs above the limestone along parts of the coast, while groundwater emerging near the till–bedrock junction can deposit porous tufa. Modern waves uncover and polish some fossil surfaces, redistribute pebbles and leave other areas hidden beneath sediment or marine growth. The chart isolates the measured northern block; the expandable account explains the repeated beds and the limits of matching historical labels to modern measurements.
The fault blocks, measured section and fossil-marker beds of the Robswalls shore.
Malahide Formation
The principal local unit comprises argillaceous bioclastic limestone and shale, with cleaner limestone beds and lenses. ‘Argillaceous’ indicates clay content; ‘bioclastic’ identifies broken skeletal material. Geological Survey Ireland uses the regional age term Courceyan, and studies of the classic shore assemblages place them in the late Tournaisian. The name ‘Malahide Limestone Formation’ in modern literature refers to the same unit. No formal member hierarchy is assigned here.
Structural framework and historical bed labels. Smyth’s 1920 coastal map divides the section into fault-bounded blocks E, F, G and H from north to south. The retained Robswalls-to-tower stretch includes E, F and most of G. Smyth’s map carries southern G, including M12x, beyond the tower to the Carrickhill fault; H/M13 lies beyond that fault. His M1–M13 labels denote groups of beds used to record fossil provenance, rather than modern formal members. The general northward dip is modified by local folding, and deformation or cleavage can make bedding difficult to read in the muddier limestone.
Fault-zone alteration. Near the northern fault, dolomitisation and brecciation modify the limestone, sometimes obliterating fossils. Smyth also described calcite veining and disturbed beds near the Coastguard and Carrickhill faults. Later heritage descriptions illustrate en-echelon mineral veins. A pale vein should not be identified as one particular mineral solely from its colour, and the fault blocks must not be added together as an uninterrupted vertical thickness.
Block E: measured section and differing surveys. Marchant’s 1974 northern-block log totals 123.5 m, although the plotted upper label is rounded to 123 m. It records predominantly biomicritic limestone, in which skeletal particles occur in fine carbonate mud. Smyth had reported 290 ft for upper E (M1–M4a–c) and 180 ft for lower E (M4d–M5). These older measurements and subdivisions are retained as historical observations; the two surveys are not assumed to have identical endpoints, and no exact conversion of M labels into Marchant’s metre positions is established.
Lower Block E: irregular dolomitisation and lower microfauna. Dolomite replacement is extensive but uneven in the lower limestone and affects microscopic fossil preservation. Marchant’s samples 32–37 cover a 16 m sampled interval, with the first plotted sample near 16 m above the logged base. That sampled interval is not the same as the basal 0–16 m of rock. His lower assemblage includes forms recorded as Tournayella discoidea and Tournayella gigantea var. minoris, used in comparison with upper Tournaisian assemblages elsewhere.
Middle and upper Block E: limestone alternations and the 85–100 m interval. Clay-rich and cleaner bioclastic limestones alternate through the section. Between 85 and 100 m above the logged base, Marchant recorded oobiosparite, a limestone containing ooids and skeletal grains in sparry calcite. This is a lithological interval in one measured block, not a separate formal member or a numerical age boundary.
Upper sampled fauna and the highest samples. Marchant described an upper assemblage of smaller tournayellids, including Tournayella kisella and forms recorded as Spinotournayella? aff. michoti. His prose gives samples 37–46 over 75.5 m, whereas the occurrence-table heading gives 38–46: sample 37 cannot therefore be treated as an unambiguous boundary from this account alone. Samples 47–55 yielded poor or inconclusive foraminiferal assemblages. Neither these higher samples nor the top of the oolitic interval establishes a new formal zone in the local section.
What the microfossil table actually records. In Marchant’s two Malahide columns, the historical records shared by the lower and upper assemblages include Archaesphaera inaequalis, tournayellids, Tournayella sp. and Endothyra sp. The lower column also records Tournayella gigantea var. minoris and Septaglomospiranella sp.; the upper includes Earlandia minor, Tournayella kisella and Spinotournayella? aff. michoti. These are the paper’s microscopic identifications, rather than a modern checklist of fossils identifiable on a visitor’s hand specimen. The neighbouring Lane and Holmpatrick columns describe different localities.
Upper E: shell-encrusting corals and the M2/M4 record. In the lower layers of his upper E succession, Smyth described domed colonies of Syringopora, commonly growing on large shells identified as Conocardium. Some preserved the encrusting base, showing how the coral occupied an earlier shell surface. He also described the coral Endophyllum irregulare from M2 and M4; the figured type material recorded in the museum catalogue comes from M2. These names are historical determinations, not newly verified identifications of the guide photographs.
M4 and the top of M5: colonial coral horizons. Smyth recorded colonies identified as Michelinia aff. megastoma in M4 and near the top of M5. A bed near the top of M5 contained small young colonies alongside larger ones. This is a useful description of a particular coral-bearing horizon, but it does not justify inventing a formal ‘Michelinia Bed’ or assigning it an unsupported thickness.
Lower E: the small-shell layer. The lower E account includes a layer with shells historically identified as Seminula cf. ambigua, considered by Smyth to be restricted to one bed. The original uncertainty expressed by ‘cf.’ is retained. This localised shell concentration is part of his faunal subdivision, rather than a presently verified species range or a direct tie to one of Marchant’s numbered samples.
Blocks F and G: repeated strata. Block F comprises M6–M9 in Smyth’s scheme, with M7 repeating M6 through folding. Block G comprises M10–M12 and repeats much of the F succession across faulting. He reported 310 ft for F and 300 ft for G, but those historical block totals include repeated strata and must not be added to make an independent combined thickness. Similar rock types initially disguised the repetition; the order and spacing of fossil markers supplied the more persuasive evidence.
Repeated marker package, youngest to oldest. The following five horizons are the successive markers recognised by Smyth in both blocks. Their fossil names retain his historical usage. They are not formal lithostratigraphic units, and their local thicknesses were not given in the account used here.
1. Syringopora on Euomphalus. At the top of the repeated marker package, colonies grew on shells identified as Euomphalus. Smyth recorded the association in M6–M7 near the centre of the syncline, and again in M10 immediately south of the Coastguard fault, where the beds’ strike made them well displayed along the coast. The coral-on-shell relationship helps distinguish the marker from an isolated loose coral.
2. Camarotoechia beds. Below that coral-bearing marker, Smyth matched shell-bearing beds in M7 with those in M11. Their position between the coral-on-gastropod horizon above and the upper Schelwienella marker below formed part of the repeated faunal pattern.
3. Upper Schelwienella bed. The next shell horizon is recorded in M7 and M12β. Smyth distinguished it on the map from the lower bed carrying the same historical fossil name. It is the upper of two comparable shell markers, rather than a duplicate entry for one bed.
4. Athyris bed. Between the two Schelwienella markers, a shell-bearing horizon in M7 was correlated with one in M12. This intervening bed is important because the sequence of fossil associations, not just the recurrence of one shell type, identifies the structural repetition.
5. Lower Schelwienella bed. The lowest of the five listed markers occurs in M7 and M12α. Together with the upper occurrence and intervening Athyris bed, it completes the repeated package documented in Smyth’s account. He also noted Syringopora growing on shells identified as Schelwienella in M12.
M8–M9 and adjoining M12x: lower shell horizons. Below the five-marker package, M8 contained abundant shells identified as Syringothyris carteri, followed lower in the section by Athyris glabristria in M9. The corresponding M12x exposures lie in southern G just beyond this guide’s tower endpoint and are included here only to explain the historical correlation. Algae and other growth obscured much of the rock. Smyth found two shell layers yielding the former identification and a lowest layer with the latter, uncovered in the kelp zone during a spring tide. This records the historical observation conditions, not a guarantee that those surfaces are exposed or safely accessible now.
Other coastal observations north of the tower. Parkes’s visitor map identifies finely laminated limestone interpreted as probably very shallow-water or tidal deposits, a small anticline and a shell-rich bed immediately farther north. Nearer the Coastguard fault, it records coral-and-gastropod-rich surfaces and tufa where groundwater leaves the till–limestone junction. These are useful local features without a secure published conversion to Smyth’s M labels. The till and tufa belong to much younger parts of the landscape history, outside the Carboniferous formation hierarchy.
Broader-coast coral context and the southern endpoint. A modern synthesis lists solitary rugose corals such as Caninia, Cyathaxonia, Fasciculophyllum, Sychnoelasma and Zaphrentites, with rarer Caninophyllum and Rylstonia, and tabulate corals including Syringopora, Michelinia and Palaeacis from the broader classic Malahide assemblage. It does not locate each genus in E–G. Smyth’s M13/Block H, south of the tower, supplied his new Michelinia gracilis and Zaphrentis junctoseptata; that adjoining type-locality history must not be mistaken for finds within this guide’s retained northern stretch.
Tober Colleen Formation: wider mapped context. The geological heritage audit also records this formation within its broader Malahide Coast site. Regionally it comprises dark calcareous mudstone, commonly bioturbated, with subordinate thin micritic limestone, and is associated with the older name Rush Slate. It overlies the Malahide Formation where Waulsortian limestone is absent. A modern exposure-level contact has not been tied here to the retained guide endpoint, so it is not added as a measured top bed in the chart and Smyth’s Block H is not automatically equated with it.
Age and correlation limits. Regional revisions of the Malahide coral assemblages, including reassessment of their correlation with coral zone RC3, are not bed-by-bed correlations of this foreshore. Smyth’s historical Z/C1 terminology and later micropalaeontological samples should therefore not be recast as a precisely located modern stage boundary. Loose pebbles on the beach also lack the bed position needed for such correlation.
References
Smyth, L. B. 1920. The Carboniferous Coast-Section at Malahide, Co. Dublin. Scientific Proceedings of the Royal Dublin Society 16, 9–24, plates I–II. Historical fault blocks, bed groups, fossil markers and coral descriptions.
Marchant, T. R. 1974. Preliminary note on the micropalaeontology of the Dinantian Dublin Basin, Ireland. Annales de la Société géologique de Belgique 97, 447–461. Block E measured section and Malahide microfaunal columns.
Geological Survey Ireland. Current Bedrock Geology 1:100,000 map and lexicon entries for the Malahide and Tober Colleen formations; Clarke, A., Parkes, M. & Gatley, S. 2007, Fingal geological heritage audit, Malahide Coast, DF006.
Parkes, M. 2012. Islands, Coast and Quarries: The Geological Heritage of Fingal. Fingal County Council, pp. 28–30. Local visitor map and field observations; its older collecting advice is superseded by current consent requirements.
Nudds, J. R. 1982. Catalogue of type, figured and referred fossils in the Geological Museum of Trinity College, Dublin: Part 1. Historic specimen and slide provenance.
Somerville, I. D. & Rodríguez, S. 2010. A new genus and species of colonial rugose coral from late Tournaisian (Waulsortian) mud-mounds in Ireland: its ecological associations and depositional setting. Palaeoworld 19, 414–425. Regional framework and broader Malahide coral context; the new coral described in that paper is from Sutton.
Poty, E., Devuyst, F.-X. & Hance, L. 2006. Upper Devonian and Mississippian foraminiferal and rugose coral zonations of Belgium and northern France: a tool for Eurasian correlations. Geological Magazine 143, 829–857. Regional correlation context.
SAFETY
Visit on a falling tide towards low water and in calm weather. The sea can reach the cliffs at high tide, closing the route between platforms and steps. Check local tide times and conditions beforehand, keep a clear return route, and turn back before the water forces you towards the cliff or an unsafe scramble.
Algae, wet limestone, shale surfaces and loose boulders can all be slippery. The platforms are uneven and fissured, and waves may wash across apparently dry ledges. Wear grippy footwear, supervise children closely and avoid working directly below unstable cliff or till faces. Use the established steps rather than unmarked cliff paths.
EQUIPMENT
A hand lens, camera or phone, scale and notebook are the most useful equipment for observing the fossils and recording their beds. Take current local tide information and wear sturdy footwear. Leave hammers and chisels out of an ordinary observation visit; splitting a loose block is also a disturbance of protected material.
CLEANING AND TREATING
Do not clean or prepare fossils in the rock platforms. Their weathered surfaces and bed relationships are part of the interest of the site. For material already lawfully held, start with gentle dry brushing under magnification and keep the locality and loose-or-in-situ provenance with the specimen.
Stable limestone may be rinsed carefully, but shale-rich or fractured material should not be soaked. Let wetted specimens dry slowly. Avoid acids, bleach, routine varnish and attempts to force hard matrix from calcitic fossils: fossil and limestone may dissolve or break together. Leave an unusual or potentially significant specimen unprepared until specialist advice has been obtained.
IDENTIFY YOUR FINDS
Need help identifying a fossil? Share clear photos, where you found it and its size with the community.
FURTHER READING
UCC: fossils of the Malahide–Portmarnock coast
Fingal geological heritage: Malahide Coast, pages 28–30
Geological Survey Ireland: Malahide Coast site report
Marchant’s measured section and microfossil study
Smyth’s original coastal study
NPWS: Malahide Estuary SAC
ACCESS RIGHTS
The Robswalls rocky foreshore lies within Malahide Estuary Special Area of Conservation. Removing or disturbing fossils, rocks or sediment within the SAC requires the appropriate prior consent. The provisions are broader than a restriction on hammering bedrock and contain no general loose-pebble exemption.
Photograph fossils in place unless the necessary consent and landowner permission have been established. Contact NPWS about the proposed activity and exact area through its Activities Requiring Consent guidance. The public coastal path, heritage recognition and older collecting advice do not provide that permission.
It is important to follow our ‘Code of Conduct’ when collecting fossils or visiting any site. Please also read our ‘Terms and Conditions‘
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