Great Ormes Head

The basement beneath the Great Orme limestone succession is not exposed. Extensive dolomitisation affects the lower part of the headland succession, including the Llanarmon Limestone. Replacement of the original carbonate can obscure bedding and destroy shell textures, although pockets of crinoidal limestone survive. Regional formation thicknesses are not measurements of the Great Orme.

Historical Pier Dolomite. This name describes altered rock in older accounts and is not a safe one-to-one synonym for the entire modern Llanarmon Limestone Formation. The limestone immediately beneath the Tollhouse Mudstone Bed is assigned to the Loggerheads Limestone in the current definition. Dolomitisation crosses stratigraphic divisions, so the alteration must be distinguished from the age and original sedimentary unit of the rock.

Historical Pier Dolomite type section and fossiliferous lenses. The eastern section near the Grand Hotel and pier, continuing north beyond the toll gate, was described as about 140 m of mainly medium- to coarse-grained dolomite. Dark-grey limestone survives in thin bands and lenses, commonly rich in crinoid debris; elsewhere replacement has destroyed the original rock texture. A thin crinoidal limestone lens about 7.6 m below the top yielded Gigantoproductus of the maximus group and Megachonetes cf. papilionaceus. The 7.6 m figure is its position below the top, not the lens thickness. Morton also recorded Syringopora ramulosa in limestone beds beyond the pier. These are historical records from the dolomitised succession, not a measured thickness or fauna list for the modern Llanarmon Formation alone.

Southern and northwestern dolomite exposures. The historical account describes probably about 90 m on the southern slopes overlooking Loreto College and Gogarth Abbey, mostly massive dolomite with clearer bedding in its higher parts and small-scale cross-bedding visible on some weathered surfaces. At the northwestern coast, about 12 m was recorded at high-water mark, including a conspicuous red band 0.6–1.0 m thick. These are separate local exposures, not additional intervals to total with the eastern type section or present-day access recommendations.

Pale, shell-rich limestone forms repeated shallow-water packages. Rubbly intervals and exposure or soil-related surfaces interrupt more massive limestone. The historical Great Orme Limestone cycles described around Happy Valley preserve this repetition in much greater detail than a formation-level summary. They are not a claim that every bed is currently exposed or safely accessible on a visitor route.

Tollhouse Mudstone Bed. This calcareous mudstone and argillaceous limestone interval, up to 2 m thick, lies within the Loggerheads Limestone Formation. Its type section is on the west side of Marine Drive, about 100 m north-northwest of the tollhouse, and on the shore below. The muddy interval records a temporary change from dominant carbonate accumulation and contains a historical shell and ostracod fauna. This is geological locality information, not advice to stand in the road or scramble to the exposure. The old assignment as a member of the Leete Limestone is superseded.

Tollhouse type-section fauna and lateral thinning. The historical type section at SH 7815 8326 contains 1.2–2.0 m of dark-grey calcareous mudstone and earthy limestone. Its recorded fauna includes Gigantoproductus of the maximus group, Linoprotonia, Megachonetes cf. papilionaceus, Schizophoria, Bellerophon, Aviculopecten and ostracods. Below the lighthouse the interval is about 1 m thick and thins westwards. The memoir’s regional marker Daviesiella llangollensis was not found on Great Orme and is not added to the locality’s fossil record. The old local formation terminology is superseded by the modern bed rank used above.

Happy Valley historical measured cycles, youngest to oldest. The following component descriptions retain the order of the published 1984 log, descending from Cycle 9 to Cycle 1 and the Tollhouse Mudstone. Covered intervals are retained because they are part of the measured record. The digitised Cycle 2 table has an ambiguous break between a “5” entry and a 14.63 m continuation; these are reported explicitly rather than combined into a newly invented thickness. The Cycle 1 record Koninckophyllum θ is retained in its historical notation without inferring a modern species identification.

Cycle 9, logged component 1 (3.35 m). Limestone, grey to pale grey, thin-bedded, slightly porcellanous; Lithostrotion junceum, Syringopora cf. distuns, Delepinea?, Eomarginifera sp., Gigantoproductus sp., Megachonetes sp., Pugnax pugnus, Straparollus sp.

Cycle 8, logged component 1 (6.1 m). Limestone, pale grey to brown, rubbly to massive; Lithostrotion portlocki, Gigantoproductus sp.

Cycle 8, logged component 2 (13.1 m). Limestone, pale, in 0.5 to 1-m beds; Lithostrotion portlocki, Athyris expansa, Gigantoproductus sp. maximus group, Megachonetes sp.

Cycle 7, logged component 1 (6.1 m). Limestone, grey, rubbly in part; Dibunophyllum bourtonense, Syringopora cf. geniculata, Antiquatonia sp., Gigantoproductus sp., Megachonetes sp.

Cycle 7, logged component 2 (6.1 m). Limestone, grey, thick-bedded; Chaetetes depressus, Gigantoproductus sp. maximus group, Linoprotonia sp., Naticopsis?.

Cycle 6, logged component 1 (2.74 m). Limestone, pale grey, with pink rubbly beds; Lithostrotion maccoyanum, L. portlocki.

Cycle 6, logged component 2 (7.31 m). Limestone, poorly exposed.

Cycle 6, logged component 3 (1.83 m). Limestone, grey, massive; Axophyllum vaughani, Lithostrotion pauciradiale, Palaeosmilia murchisoni, Syringopora cf. geniculata, smooth spiriferoids.

Cycle 5, logged component 1 (2.74 m). Limestone, grey, rubbly.

Cycle 5, logged component 2 (1.83 m). Limestone, grey, well bedded; Gigantoproductus sp., Megachonetes cf. papilionaceus, smooth spiriferoid, Bellerophon sp.

Cycle 4, logged component 1 (3.05 m). Grass-covered slope.

Cycle 4, logged component 2 (1.83 m). Limestone, pale grey.

Cycle 4, logged component 3 (14.63 m). Limestone, poorly exposed, pale grey; clisiophylloid indet., Megachonetes cf. papilionaceus, Gigantoproductus sp. [juv.], pectinoid indet., trilobite cephalon indet.

Cycle 3, logged component 1 (11.89 m). Limestone, grey, rubbly; Koninckophyllum?, Lithostrotion aff. martini, Palaeosmilia murchisoni, Syringopora cf. geniculata, Linoprotonia cf. hemisphaerica.

Cycle 3, logged component 2 (17.37 m). Limestone, grey; Caninia aff. juddi, Chaetetes depressus, Dibunophyllum bourtonense, Koninckophyllum sp. [juv.], Lithostrotion aranea, L. junceum, L. pauciradiale, L. cf. sociale, Palaeosmilia murchisoni, Syringopora cf. geniculata, S. cf. reticulata, Gigantoproductus sp., smooth spiriferoids.

Cycle 2, logged component 1 (10.67 m). Grass-covered slope with poor exposure of grey limestone.

Cycle 2, limestone entry with ambiguous thickness. Limestone, pale grey to grey; Caninia sp., Koninckophyllum sp., Lithostrotion junceum, L. martini, L. pauciradiale, Palaeosmilia murchisoni, Syringopora cf. ramulosa, Gigantoproductus sp. maximus group, Pleuropugnoides sp., Schizophoria sp. The digitised table prints “5” beside this entry; the following fauna continuation is printed beside 14.63 m. The layout is ambiguous and no corrected combined measurement is inferred.

Cycle 2, continuation printed beside 14.63 m. Macrochilina sp., Meekospira sp., Naticopsis sp., Straparollus sp., Streblopteria sp., Weberides sp. This continuation is retained with its printed measurement, subject to the table-layout uncertainty explained above.

Cycle 1, logged component 1 (2.43 m). Grass-covered slope.

Cycle 1, logged component 2 (3.66 m). Limestone, pale grey; Aulophyllum fungites cf. redesdalense, Dibunophyllum bourtonense, Koninckophyllum θ [historical notation], Lithostrotion martini, L. sociale, Syringopora cf. ramulosa, Athyris expansa, Gigantoproductus sp., Linoprotonia sp., Megachonetes cf. papilionaceus, Bellerophon sp., Koninckopora inflata.

Cycle 1, logged component 3 (7.62 m). Limestone, grey; Caninia sp. subibicina group, Lithostrotion martini, L. aff. martini, L. pauciradiale, Syringopora cf. ramulosa, Megachonetes cf. papilionaceus, Productus sp., Schizophoria sp.

Cycles 1 and 2 were historically grouped as the Lithostrotion affinephillipsi beds. The alternation of hard, massive limestone and softer rubbly cycle tops gives parts of the headland a stepped profile. Such historical coral-zone terminology is not an additional modern formation or a separate proven bed at Bishop’s Quarry.

Lighthouse horizon. The historical Lighthouse horizon is a rubbly, nodular limestone band about 2 m thick near the lighthouse. The 1984 memoir traced it into the rubbly top of Cycle 5 in the section north of Happy Valley, then along Craig Rofft towards Ogof Deuben. Its local thickness near the lighthouse does not replace the 2.74 m rubbly component measured in the Happy Valley cycle log. This is a historical correlatable horizon, not a new formal member or an invitation to approach the sea cliffs.

Caninia Band and cliff-forming limestone. The succession comprising Cycles 6–9 at Cregiau Cochion contains Smyth’s historical Caninia Band; the memoir also describes the same upper-cycle sequence in cliffs and scars above the lighthouse. The account does not pin the band to a numbered component of the Happy Valley table, so no new bed number or thickness is assigned. Lower in the succession, Cycle 3 is Smyth’s historical “massive white limestone” and forms the cliffs at Pen-trwyn and is also described below the lighthouse and around Cregiau Cochion. These outcrop relationships help explain the headland’s stepped relief without joining separate exposures into an invented continuous log.

Repeated muddy limestone and mudstone packages alternate with shell-rich limestone. Marine deepening phases brought muddier conditions than those typical of the shallower Loggerheads packages. The historical Bishop’s Quarry Limestone interval is the principal fossil-viewing target of this guide; its large productoid shells occur alongside smaller shelly fossils in the more argillaceous beds.

Bishop’s Quarry northwest face: historical measured log, oldest to youngest. The logged sequence begins with at least 0.22 m of dark limestone. The nine components below are followed by 4 m seen of the historical Summit Limestone. These are face measurements, not the whole formation thickness. The south face overlaps this record and reaches slightly lower strata; it must not be appended as though it extended the northwest face into one continuous stack.

Northwest face, component 1 (seen to 0.22 m). Limestone, dark grey.

Northwest face, component 2 (0.60 m). Mudstone and shaly limestone; Antiquatonia sp., Eomarginifera sp., Lingula sp., Megachonetes sp., Rugosochonetes sp., smooth spiriferoids, Spirifer bisulcatus, Posidonia becheri, Streblochondria sp.

Northwest face, component 3 (0.90 m). Limestone, dark grey.

Northwest face, component 4 (0.15 m). Mudstone with limestone lenses.

Northwest face, component 5 (2.43 m). Limestone, grey; Gigantoproductus edelburgensis, G. cf. tulensis, Martinia sp., Productus hispidus, orthocone nautiloid.

Northwest face, component 6 (0.52 m). Mudstone and limestone, dark grey; Amplexizaphrentis derbiensis, Diphyphyllum lateseptatum, Rhipidomella michelini.

Northwest face, component 7 (1.13–1.28 m). Limestone, grey.

Northwest face, component 8 (0.22 m). Mudstone, black, with lenses of earthy limestone; Amplexizaphrentis derbiensis, Fenestella sp., Eomarginifera cambriensis, E. cf. lobata, Megachonetes sp., Productina margaritacea, Rhipidomella michelini, trilobite pygidium.

Northwest face, component 9 (3.65 m). Limestone, dark grey, with mudstone partings: Chaetetes depressus, Gigantoproductus crassiventer, G. edelburgensis, G. cf. moderatus, G. cf. tulensis, G. sp.[latissimoid], Pugilis pugilis, Rugosochonetes sp.

Bishop’s Quarry south face and bedding. The southern face at SH 7668 8300 overlaps the northwest-face succession and reaches slightly lower beds. Additional historical records comprise Lithostrotion maccoyanum, Fistulipora incrustans, Penniretepora, trepostomatous bryozoans and Rugosochonetes speciosus. The limestones range from fine or very fine grained to coarser where skeletal remains are abundant; ordinary beds are usually 15–20 cm thick, exceptionally about 0.5 m. Closely spaced joints in some fine limestone can resemble cleavage, with recorded dips of 70–90° and strikes ranging from east-southeast to north-northeast. These structures are distinct from bedding and add to the need for caution around faces.

Historical correlation and nearby Bishop’s Quarry Limestone exposures. Earlier palaeontological work compared the quarry beds with the Hardraw Limestone of the Yorkshire Dales, using the historical P1c zonal framework; that is a published correlation, not a reason to import Yorkshire beds or thicknesses. Southwest of the quarry, basal beds of the historical local unit are grey to dark-grey, thinly bedded limestone with silicified productoids and a little pale chert. A separate exposure north of the summit buildings at SH 7667 8356 shows 3 m of dark-grey to black, partly flaggy limestone with abundant productoids, while the small quarry east of the summit at SH 7721 8312 shows 1.5 m of dark-grey, well-bedded limestone with thin shale partings. These isolated measurements remain separate from the principal quarry-face log.

Porcellanous sponge-bearing limestone above Craig Rofft. Immediately above the sandstone, the historical quarry account records 0.30 m of pale grey-brown porcellanous limestone. Smyth described a meandrine form of Chaetetes from this band in 1925; the 1984 memoir referred it to Chaetetipora. That published change of identification is retained as historical taxonomic context, not a new specimen determination. The memoir considered a limestone band in the nearby section at SH 7736 8300 probably equivalent, rather than proving an exact one-to-one correlation.

Separate historical section southwest of Craig Rofft, SH 7736 8300: oldest to youngest. This seven-component limestone log lies across the road and tramway from the sandstone quarry. It is described here with the overlying limestone discussion, not as components of the Craig Rofft Sandstone Member. Its porcellanous horizon was tentatively correlated with the sponge-bearing cap described above; that probable correlation does not independently fix every component against a modern formation boundary. The measurements belong to this short exposure alone: they are not added to either the sandstone thickness or the Bishop’s Quarry log. The location identifies the historical scientific record and is not a recommended roadside examination point.

Southwest Craig Rofft section, component 1 (0.79 m). Limestone, pale, sandy.

Southwest Craig Rofft section, component 2 (0.99 m). Limestone, grey and pale grey, intraclastic.

Southwest Craig Rofft section, component 3 (0–0.15 m). Limestone, porcellanous, pale brown, with earthy limestone and red mudstone.

Southwest Craig Rofft section, component 4 (0.10–0.25 m). Limestone, grey, rubbly; nodules of porcellanous limestone up to 15 cm across at base locally.

Southwest Craig Rofft section, component 5 (0.91 m). Limestone, grey, crinoidal, rubbly; a few brachiopods; Chaetetipora at base.

Southwest Craig Rofft section, component 6 (2.36 m). Limestone, pale grey to buff, intraclastic and partly sandy.

Southwest Craig Rofft section, component 7 (2.74 m). Limestone, grey to pale grey, rubbly.

Cherty or sandy skeletal limestone and mixed carbonate–siliciclastic cycles occur in the younger headland succession. The historic Summit Limestone includes 4 m seen above the northwest-face Bishop’s Quarry log. Other summit quarries expose cherty limestone or dolomite; an old quarry west of the summit was described with 10 m of dolomite containing irregular pale chert nodules. These are distinct local observations, not the complete thickness of the modern formation or assurance of present-day safe access.

Historical Summit Limestone: the 4 m quarry-top interval. At the northwest face of Bishop’s Quarry this interval is pale-grey or brown limestone, with abundant predominantly white chert in its upper half. The historical fauna includes Caninia cambrensis, C. juddi, Diphyphyllum lateseptatum, Koninckophyllum?, Antiquatonia, Gigantoproductus crassiventer, G. cf. moderatus, G. cf. tulensis, Linoprotonia, Pugilis pugilis and Spirifer. The list belongs to this historical local interval and is not transferred to every exposure of the Red Wharf Limestone. A quarry east of the summit at SH 7710 8338 also contains abundant, poorly preserved brachiopods in the historical Summit Limestone.

Other summit exposures: chert, brecciation and dolomite. The quarry at SH 7713 8332 shows grey to pale-grey, fine-grained limestone with pale chert in nodules and veins, locally brecciated and patchily dolomitised. A separate exposure at SH 7703 8342 shows about 8 m of fine-grained, thinly bedded cherty limestone; the cliff section north-northwest of the summit at SH 7671 8345 exposes about 12 m of similar but thicker-bedded rock. The 10 m of chert-nodular dolomite west of the summit belongs to the distinct quarry at SH 7648 8338. None of these measurements is added to the 4 m Bishop’s Quarry interval to manufacture a complete formation thickness.

Historical headland thickness estimates. Using its older local terminology, the 1984 memoir estimated about 70 m of the Gronant Group on Great Orme, incomplete at the top, with about 20 m of Bishop’s Quarry Limestone and a maximum of roughly 50 m of Summit Limestone. The Geological Conservation Review separately describes about 140 m visible in the Happy Valley type section out of an estimated 175 m of the historical Great Orme Limestone. These estimates concern different historical units and exposure scopes, not an additional stack to sum. They are retained as context, not equated to the complete thicknesses of the modern Cefn Mawr or Red Wharf formations and not substituted for the explicitly scoped measured sections above.

The Red Wharf Bay type-section thickness and the Castell Mawr Chert Member belong to Anglesey and are not imported as a Great Orme bed stack. Similarly, channel fills described elsewhere in the formation are not automatically features of Bishop’s Quarry. The local record is best understood from the measured headland sections and their explicitly stated exposure limits.

Structure, alteration and displaced blocks. Tilting and faulting affect the succession, while dolomitisation and silicification change the rock’s mineralogy and the visibility of its fossils. Modern work relates these changes to the wider evolution of the basin, rather than one simple reef history. The displaced cherty block at Gorsedd Uchaf has been discussed in terms of both glacial transport and collapse; its origin should not be presented as settled.

Additional uncorrelated field observations. Grundy Ridewood’s 2013 field guide, based on an unpublished undergraduate project, describes a lower grainstone interval at Happy Valley about 10 m thick, with an undulating clast-supported breccia about 2.5 m above its base, and strongly bioturbated red marls between limestone packages. At Craig Rofft it records lateral changes in colour, grain size and sandstone development. These are attributed observations, not a correlation with particular numbered components of the historical logs. The guide’s alternative formation mapping and thickness estimates are not adopted here in place of the modern framework and its defined contacts.

Historical museum records: separate headland horizons. The 1982 Trinity College Dublin catalogue preserves specimen-based evidence from several Great Orme exposures. Its historical names, locality descriptions and age qualifiers are retained here without turning them into modern bed correlations, new thicknesses or present-day collecting directions. “Figured” and “referred” records are not automatically type specimens, and material from elsewhere on the headland is not assigned to the principal Bishop’s Quarry.

Shale at the base of the historical Lithostrotion affine-phillipsi beds. Records from 300 yards west of Pen-morfa Lodge include figured Carcinophyllum vaughani (TCD F77), Dibunophyllum aff. muirheadi (F73–F75, with additional referred material R10), and Lonsdalia duplicata (F76; the catalogue flags the generic spelling as historical). Figured Lophophyllum material (F81) carries a historical taxonomic symbol whose meaning is not resolved here, rather than a guessed species name. These series include both figured pieces and associated unfigured material. Other headland records from the affine-phillipsi beds, without that precise shale locality, comprise referred Tabulipora howsei var. nov. (R11) and Productus sp. nov. (R12); the provisional labels are those of the historical record.

Caninia Band, beds above it and Lighthouse horizon. Caninia benburbensis material (F100) from the headland’s historical Caninia Band includes a specimen figured by Lewis in 1927. The catalogue tentatively gives an Asbian/Brigantian age, which is not converted here into a precise modern bed age. Figured Dibunophyllum (Histiophyllum) sp. nov. (F78) comes from beds above the Caninia Band, 300 yards south of the lighthouse; figured Rhodophyllum sp. nov. (F79) is recorded from the Lighthouse horizon, 190 yards southwest of it. These are distinct historical collection positions and provisional identifications, not additional fossils proved in Bishop’s Quarry.

Other quarry and headland records. Figured Zaphrentis cf. oystermouthensis (F80) is labelled from the historical Bishop’s Quarry Beds, but its recorded quarry is 390 yards east of the summit; the unit name is not evidence that it came from the principal quarry. Referred Productus “x” (R13) has only a headland locality within those historical beds. The catalogue also records figured Chaetetes septosus material (F82) from Great Orme and associated Chaetetes sp. material (F71) from an abandoned quarry described as 680 feet east of the semaphore, 1,200 feet southeast of the lighthouse and 486 feet above Ordnance Datum. F71 comprises unfigured parts associated with the figured specimen BM R24997; it must not be relabelled as a series of figured types. These old reference points are collection provenance, not newly verified navigation.