Prince Charlie’s Cave Fossil Hunting

1829 – Early geological record
Roderick Murchison recorded the Lower Jurassic rocks of this part of the Trotternish coast. This is an early scientific reference to the succession later studied in detail between Prince Charlie’s Cave and Holm.

1873 – The coastal succession described
James Bryce published a description of the local Jurassic rocks, adding to the early geological record of this otherwise difficult-to-reach coast.

1873 – A fossil gastropod named from the cave locality
Ralph Tate named Valvata praecursor, with Prince Charles Cave at Portree recorded as its type locality. Its historical horizon was described as “Infra-Oxfordian”; this record should not be assigned to the Lower Jurassic shore beds without further specimen and stratigraphic evidence.

1920 – Shales logged and ammonites documented
George W. Lee’s geological memoir published the results of survey work, including Tait’s excavated shale-and-ironstone section between the cave and Holm. S.S. Buckman studied the ammonites and proposed the name Portree Shale. The trench exposed the ironstone and most of the underlying shale, although its lowest 4 m remained unseen.

1966 – Survey account consolidated
Frank W. Anderson and Kenneth C. Dunham’s memoir brought the northern Skye geology together in a further regional account. It remained one of the principal published descriptions of the cave-to-Holm Lower Jurassic succession, although it added comparatively little new detail to that part of the section.

1989–1994 – New field studies improved the section
MSc work by Neville Brookes in 1989 and Bruce Farrer in 1994 added observations from this poorly documented coast. Farrer’s measurements of the Portree Shale and Raasay Ironstone later supplied much of the detailed bed-by-bed succession used to interpret the site.

2004 – Detailed composite log and proposed type section
Nicol Morton brought together the measured sections, earlier survey work and collectors’ ammonite records into a numbered composite succession. He proposed the shale exposure about 1 km south of the cave as the type section of the Portree Shale Formation, while highlighting unresolved ages at the sandstone–shale boundary and within the ironstone.

2005 – Rare lobster occurrence summarised
A published geological synthesis recorded the lobster Pseudoglyphea foersteri from the Scalpay Sandstone south of Holm Island. The species had been named from Raasay material in 2002; the Holm record extends its documented occurrence to this Skye coast and should not be confused with the original type discovery.

The numbered succession below follows the published composite log, read from oldest to youngest. It combines exposures south-west of Holm Island (NG 520 506), about 1 km north of the cave (NG 518 490), and about 1 km south of the cave (NG 515 471). The last lies just outside the southern boundary of the defined Lower Jurassic conservation-review site and supplies much of the shale detail. These are measured historical exposures, not a claim that every bed is currently accessible or visible at the cave entrance. Thicknesses apply to this composite, with local variation and missing exposure retained.

LIAS GROUP

Scalpay Sandstone Formation

The exposed upper part is late Pliensbachian to early Toarcian in age. Fine- to medium-grained, mottled, calcareous sandstone dominates. Widespread bioturbation records animals disturbing the marine sediment; identifiable trace-fossil forms were not recorded in the detailed site description. Only the upper part of the formation is exposed in the logged sections.

Bed 1 (3.00 m)
Grey, slightly silty, extensively burrowed sandstone, weathering pale brown, with scattered large calcareous concretions (doggers). The illustrated thickness is the exposure above sea level at Holm, not the entire formation. Greater exposures near the cave contain patches of crinoid debris, locally in small channels or representing collapsed colonies, and large pectinid bivalves, recorded as Pseudopecten equivalvis. Amaltheus gibbosus indicates the Gibbosus Subzone of the Margaritatus Zone.

Bed 2 (0.50 m)
A softer, pale, cross-bedded sandstone recognised beneath the main dogger band at Holm. It cannot readily be separated from the surrounding sandstone farther south; the bed is therefore a local subdivision rather than an everywhere conspicuous marker.

Bed 3 (1.00 m)
A conspicuous band of large calcareous doggers, traceable from Holm towards Prince Charlie’s Cave and forming the roof of an overhang to its north. Pleuroceras spinatum and Pseudoamaltheus engelhardti support an Apyrenum Subzone age within the Spinatum Zone. Individual doggers in this sandstone succession can reach about 1 m across.

Bed 4 (7.00 m)
Grey sandstone, weathering pale yellow, with scattered doggers and local cross-bedding. Its top is not exposed at Holm. Farther south it contains large pectinid bivalves, recorded as Pseudopecten equivalvis, nests of the brachiopod Tetrarhynchia tetrahedra, belemnites and lenses of crinoid debris, principally Hispidocrinus schlumbergeri with some Balanocrinus donovani. Belemnites have varied orientations, including near-vertical examples, consistent with extensive disturbance by burrowing organisms. Pleuroceras spinatum occurs at several levels; the bed is assigned to the Hawskerense Subzone. North of the cave its upper part becomes finer and siltier.

Bed 5 (1.50 m)
Micaceous, silty sandstone that fines upwards into coarse siltstone, with occasional nodules. Dactylioceras (Orthodactylites) cf. tenuicostatum establishes the early Toarcian Tenuicostatum Zone, although the subzone is unresolved.

Bed 6 (0.25 m)
Calcareous sandy siltstone, shown as calcareous silty sandstone in the published log, with a strongly iron-rich upper layer and Dactylioceras. The sandstone–shale boundary is placed at its top. Its traditional Tenuicostatum Zone assignment remains uncertain: a proposed correlation with a younger ammonite-bearing bed south of Portree Bay would instead place it in the Serpentinum Zone. That correlation is unconfirmed; the fossils from the southern locality must not be treated as a proven assemblage from this bed here.

Portree Shale Formation

The shale succession is unusually thick and comparatively complete here. In 2004 the section south of the cave was proposed as its type section. The current BGS Lexicon also retains a reference section on Raasay; the historical proposal should not be confused with an unequivocal modern type designation. The transition from sand to mud reflects marine flooding. Much of the mud accumulated in a restricted offshore setting: some fissile layers suggest oxygen-poor bottom waters, while bivalves and blockier beds show that conditions were not uniformly devoid of bottom-dwelling life. Fossils are concentrated on particular surfaces, and shale ammonites are commonly crushed.

Bed 7 (2.00 m)
Claystone, silty and sandy at the base and progressively finer and more fissile upwards. Small calcareous nodules occur within the upper 0.5 m. Belemnites occur at several levels, and Dactylioceras with Lytoceras was recorded about 1 m above the base. These ammonites do not settle whether the basal shale belongs to the Tenuicostatum or Serpentinum Zone; the latter was provisionally preferred in the detailed site account.

Bed 8 (0.64 m)
Light grey-brown claystone with iron- and sulphur-stained layers, interpreted as weathered pyritic material. Belemnites occur, but no ammonites were recorded from this individual bed.

Bed 9 (0.10 m)
A thin band of iron-stained calcareous nodules separating the claystones below from the more continuous limestone above.

Bed 10 (0.24 m)
Hard, silty, nodular limestone containing shell debris, a distinct carbonate-rich layer in the lower shale succession.

Bed 11 (0.25 m)
Claystone containing ammonites and belemnites. This is one of the concentrated ammonite-bearing levels rather than evidence that the entire shale succession is equally fossiliferous.

Bed 12 (0.15 m)
A nodular limestone bed, more continuous than some of the isolated nodule bands above and below.

Bed 13 (0.32 m)
Slightly micaceous claystone with ammonites and belemnites. The lower fossiliferous interval, beds 8–13, yields Eleganticeras elegantulum, Cleviceras exaratum and Dactylioceras anguiforme, supporting the Exaratum Subzone. This is an interval-level assemblage, not an assertion that every species occurs in every bed.

Bed 14 (0.80 m)
Claystone with two bands of calcareous nodules, beneath the thicker fissile claystone of Bed 15.

Bed 15 (3.13 m)
Medium- to dark-grey fissile claystone with iron-stained and sulphurous layers, bivalves and ammonites. The proposed Exaratum–Falciferum subzone boundary is placed near the base; the higher shale fauna is dominated by Dactylioceras, which gives less precise age control. The published log records Dactylioceras toxophorum at several levels in this part of the succession.

Bed 16 (0.25 m)
Nodular limestone forming a discrete resistant bed above the thick fissile claystone.

Bed 17 (2.23 m)
Medium-grey claystone, becoming harder towards its top, with occasional bivalves and ammonites.

Bed 18 (0.13 m)
A thin band of calcareous nodules at the top of the thicker claystone interval. Its position and thickness distinguish it from the separate nodule band of Bed 20.

Bed 19 (0.35 m)
Claystone between two close-set nodule bands. Ammonites are locally concentrated; the published log records Dactylioceras and Lytoceras in this interval.

Bed 20 (0.17 m)
A second thin band of calcareous nodules, above the fossiliferous claystone of Bed 19.

Bed 21 (0.55 m)
Claystone with a further band of calcareous nodules, forming the highest logged interval below a break in exposure.

Bed 22 (c. 1.00 m)
An unexposed interval, marked as a gap in the published composite log. This is a gap in observation, not a demonstrated erosional hiatus. The upward continuation was recorded in a parallel shallow gully about 15 m to the south; its concealed lithology and fossils are not known.

Bed 23 (0.50 m)
Fissile claystone with belemnite-rich layers. Together with Bed 24 it contains scattered iron ooids, recording the transition towards the overlying ironstone. The uppermost shales have yielded Dactylioceras and Lytoceras.

Bed 24 (0.50 m)
Claystone with small nodules and iron ooids. This is the highest shale bed below the first hard ironstone; belemnite concentrations also occur in the upper shale interval.

Raasay Ironstone Formation

The three beds total about 1.49 m in the composite section. The transition begins with scattered iron ooids in the upper Portree Shale, but the formal boundary is placed at the first hard ironstone. The local ammonite evidence raises a possible Bifrons Zone age for part of the formation; poor preservation and differences from Raasay prevent a simple, fully resolved zonal assignment.

Bed 25 (0.49 m)
Hard, massive, crystalline calcareous ironstone or ferruginous limestone, with a claystone parting and belemnites. The first hard ironstone defines the formation boundary. Harpoceras subplanatum was reported from this basal interval, although the age significance of the sparse, imperfectly preserved ammonites requires caution.

Bed 26 (0.50 m)
Mainly claystone with two bands of ironstone nodules. Belemnites are abundant, especially at the top. Recorded ammonites include Harpoceras and Peronoceras fibulatum in the lower part and Phylloceras heterophyllum above; Dactylioceras was also reported from the middle ironstone interval. Reports of H. subplanatum and P. fibulatum suggest the Bifrons Zone if the identifications are confirmed. Loose Hildoceras semipolitum near the cave supplies supporting, but unprovenanced, evidence. A precise zone should not be assigned to every ironstone bed from these records.

Bed 27 (0.50 m)
Light- to medium-grey, massive to ooidal ironstone with a clay parting, weathering rusty or yellow-brown. This upper bed most closely resembles the characteristic ooidal ironstone of central Raasay; the local formation as a whole is thinner and more limestone- and clay-rich. Fossils other than belemnites are uncommon, and many belemnites are worn or corroded. The outcrop does not resolve the debated depositional setting of the ironstone.

Bearreraig Sandstone Formation

This is a stand-alone formation, not part of the Lias Group in the BGS framework. It forms much of the higher coastal cliff. Its basal mudstone belongs to the Dun Caan Shale Member; the younger sandstone succession is much more fully described at Bearreraig Bay and is not transplanted into the cave’s bed log.

Dun Caan Shale Member

Bed 28 (base only)
Dark-grey, micaceous, fissile shale at the base of the Dun Caan Shale Member. Only its base is drawn in the Lower Jurassic composite log, so no total thickness should be read from that figure. No ammonites were recorded here near the cave in that account; at Holm, Pleydellia occurs 5.5 m above the base and indicates the latest Toarcian Aalensis Zone. The boundary above the ironstone represents a substantial hiatus, unlike the mere exposure gap labelled Bed 22.

Higher cliff succession and locality limits
The overlying Bearreraig succession extends into the Middle Jurassic. Below Fiurnean, the upper formation includes the Garantiana Mudstone Member (called Garantiana Clay Member in older accounts), succeeded by the Cullaidh Shale and Elgol Sandstone formations of the Great Estuarine Group. These higher-cliff observations are separate from the accessible Lower Jurassic composite. Member thicknesses and individual beds measured at Bearreraig Bay should not be presented as measurements at Prince Charlie’s Cave.

Structure, intrusions and exposure
The strata dip about 5°–10° towards just north of west; the coast runs broadly along their strike. Minor NNW–SSE faults have recorded displacements of 1 m or less. Erosion along dykes helps explain caves on this coast, with local baking generally extending less than 1 m into adjacent sediment. The larger dolerite sill at Holm Island occupies the ironstone level and has altered approximately 2 m of underlying Portree Shale and 3 m of overlying Dun Caan shale. Scree, vegetation, landslips and fallen blocks conceal parts of the section; the geological log is not a route or a guarantee of safe exposure.

References
Morton, N. (2004), Prince Charles’ Cave to Holm, in Simms, M.J., Chidlaw, N., Morton, N. and Page, K.N., British Lower Jurassic Stratigraphy, Geological Conservation Review 30, including Figure 8.12 (JNCC; available under the Open Government Licence 3.0). Cox, B.M. and Sumbler, M.G. (eds, 2002), British Middle Jurassic Stratigraphy, GCR 26, Bearreraig Bay account. British Geological Survey, Lexicon entries for Scalpay Sandstone, Portree Shale, Bearreraig Sandstone and Dun Caan Shale; Barron, A.J.M., Lott, G.K. and Riding, J.B. (2012), Stratigraphical framework for the Middle Jurassic strata of Great Britain and the adjoining continental shelf, BGS Research Report RR/11/06.