Lithofacies review of the Nairn and southern Moray Firth Devonian Old Red Sandstone

Abstract

The national barium–rubidium screen and subsequent Orcadian prioritisation (Daw 2026) identified East Caithness (the Sarclet–Lybster–Clyth flagstone coast) as the strongest candidate ground for the Altar Stone, independently corroborated by the detrital-zircon match of Clarke et al. (2024, 2026). The only other coastal belt within the Orcadian Basin that returned any screen signal, and merited a stratigraphic check, is the Nairn–Findhorn–Elgin margin on the southern shore of the Moray Firth. A lithofacies audit of the published BGS mapping, memoirs and Geological Conservation Review accounts confirms that this belt is a marginal expression of the same Lake Orcadie system: the Middle Old Red Sandstone here (Inverness and Black Isle Sandstone groups) is predominantly fluvial red sandstone deposited along the lake margin, with subordinate lacustrine flag intervals, and the Upper Old Red Sandstone (Nairn Sandstone Formation and the Elgin beds) is a mixed fluvial–lacustrine, red-to-grey calcareous, cornstone-bearing succession. Its fine lacustrine flag intervals are the same facies family as the Caithness flagstones and carry the same Achanarras-assemblage fish. That last point is decisive in an unexpected direction: because the East Caithness lead is itself built on the fish-bearing quiet-water flag facies, the presence of fish beds cannot be used to exclude the Moray margin. We therefore correct the provisional “too many fossils” dismissal rather than endorse it. What genuinely separates the Moray margin from East Caithness is not the fine facies but its marginality and oxidation, the subordinate and heterogeneous development of its flags (poor monolith potential), and above all its weak, poorly bedrock-verified barium signal — the proxy for the Altar Stone’s diagnostic baryte cement — against the strongest national signal at East Caithness. The diagnostic baryte–tosudite assemblage is untested at Nairn, as it is still under validation at East Caithness. The Nairn strand is therefore recorded as deprioritised and untested rather than eliminated on lithofacies. The Permian–Triassic New Red Sandstone of the coast (Hopeman and Burghead sandstones) is excluded on age and requires no facies argument.

1.  Why the Moray margin was the remaining Orcadian candidate to check

Clarke et al. (2024, 2026) place the Altar Stone’s detrital-mineral source in the Orcadian Basin and exclude the Midland Valley, the Anglo-Welsh Basin and Mainland Orkney. The barium–rubidium stream-sediment screen of Daw (2026) ranked East Caithness — the Sarclet–Lybster–Clyth flagstone coast — as the strongest national hit, with independent zircon corroboration at Sarclet. Within the Orcadian Basin the only other coastal belt that returned any screen signal was the southern Moray Firth margin around Nairn, Findhorn and Elgin, although the signal was weak and, as Section 5 notes, poorly bedrock-verified. This belt exposes Middle and Upper Devonian rocks laid down in the southern, marginal part of Lake Orcadie. It was provisionally set aside on the informal observation that its fine facies “had too many fossils.” This note tests that dismissal formally, and finds it must be replaced: the fossil criterion does not survive scrutiny, but the belt is nonetheless a low-prior, untested candidate for other, sounder reasons.

2.  Data and methods

The audit uses published sources only: BGS 1:50 000 and 1:625 000 digital mapping; the BGS Earthwise accounts for the Devonian of the Grampian Highlands and the Northern Highlands; the BGS memoir for Fortrose and eastern Inverness (Sheet 84W); and the Geological Conservation Review volume on the Old Red Sandstone of Great Britain (including the Tynet Burn and related Moray fish-bed sites). No new field data were collected. Lithofacies were evaluated against the Altar Stone criteria established in the screening and mineralogical work (Bevins et al. 2024; Clarke et al. 2024, 2026; Daw 2026): fine- to very fine-grained, well-sorted sandstone; ripple or planar lamination indicating quiescent water; grey-green colour; negligible detrital K-feldspar; pervasive baryte (with calcite) cement; and a tosudite / aluminous-kaolinite clay assemblage. Two method limits are carried throughout: stream-sediment values are not rock values, and a barium anomaly is a proxy for baryte cement, not a measurement of it; and formation identity is not facies identity.

3.  Lithofacies of the Nairn–Moray Devonian succession

3.1  Middle Old Red Sandstone — the marginal facies of Lake Orcadie

Around the southern Moray Firth the Middle ORS is represented by the Inverness and Black Isle Sandstone groups, which BGS characterises as predominantly fluvial red sandstone successions deposited along the lake margins — the marginal counterpart of the deep-water Caithness Flagstone Group at the centre of the basin. Within this dominantly marginal, sandier and more oxidised succession, subordinate lacustrine flag intervals occur — the Inshes Flagstone, Nairnside and Hillhead sandstones — as grey and purple flaggy micaceous sandstones and dark calcareous flags with laminated shaly mudstones and limestone nodules. These finer intervals carry the Achanarras (and post-Achanarras Eifelian) fish assemblage, the same faunal marker that defines the lacustrine flagstones of Caithness. The Hillhead Sandstone, with its post-Achanarras fish, is unconformably overlain in the Ardersier–Cawdor area by the Nairn Sandstone Formation (Section 3.2).

3.2  Upper Old Red Sandstone — Nairn Sandstone Formation and the Elgin beds

The Nairn Sandstone Formation, the oldest Upper ORS unit of the district, comprises an irregular basal reddish conglomerate overlain by red, grey and yellow calcareous cross-bedded and flaggy sandstones with thin conglomerate beds and soft limestone-bearing mudstones; it is a mixed fluvial–lacustrine sequence carrying a Givetian fish assemblage. In the Findhorn area desiccated mudstones (clay galls) and a calcrete horizon (the Cothall Limestone) are recorded. The overlying Whitemire, Alves and Scaat Craig beds are grey-to-reddish siliceous pebbly sandstones and fine conglomerates with marly intervals and cornstone (calcrete) palaeosols, and the Upper ORS subdivisions here are themselves defined on six successive fossil-fish assemblages (Asterolepis, Psammolepis, Bothriolepis, Holoptychius and others). The succession records shallow-water and periodically dry-bed conditions, with rapid lateral facies changes and local overstep along the margins of fault-bounded sub-basins. Fine flaggy and shaly intervals are present but are subordinate, laterally impersistent, and interbedded with pebbly and pedogenically modified beds.

3.3  The New Red Sandstone of the coast is not Devonian

The Hopeman and Burghead sandstones of the Hopeman–Burghead–Lossiemouth coast are post-Devonian and rest unconformably on the Upper ORS. They are not a single unit and are not both aeolian: the Hopeman Sandstone Formation is a Late Permian to Early Triassic aeolian dune sandstone (the ‘Elgin Reptile’ beds, with Chelichnus trackways), while the overlying Burghead Sandstone Formation is Triassic and fluvial (waterlain), part of the New Red Sandstone. Their historical confusion with the Old Red Sandstone — the very controversy that the 1851 discovery of Leptopleuron (Telerpeton) at Spynie brought to a head — is a caution rather than a candidacy: on age alone neither is relevant to the Altar Stone, and no facies argument is required to set them aside.

4.  Comparison with the Altar Stone criteria

Assessed against the Altar Stone benchmark, the Moray margin divides into points of genuine similarity and points of genuine difference — with the single criterion the provisional dismissal relied upon, fossil content, belonging to neither.

●        Grain size, sorting and structures. The fine, well-sorted, ripple- and planar-laminated intervals of the Moray succession are the lacustrine flag intervals of Section 3.1–3.2. Texturally these are comparable to the East Caithness flagstones — they are the same quiet-water facies — but here they are subordinate to marginal fluvial red sandstone and are laterally impersistent.

●        Colour and composition. The succession is dominated by red, grey and yellow calcareous sandstones with cornstone palaeosols; a clean, grey-green, K-feldspar-poor sandstone of the Altar Stone type is not specifically reported. This oxidised, pedogenically modified, marginal character is a real point of difference from the reduced, deep-water grey flagstone facies — though the grey lacustrine flags of Section 3.1 show the reduced facies is locally present.

●        Monolith potential. The fine intervals are thin, impersistent and interbedded with coarser and nodule-rich beds — a poorer prospect for a coherent monolith of the required dimensions than the thick, laterally persistent Caithness flag sequences.

●        Diagnostic diagenesis (baryte, tosudite). No published clay or cement data exist for the Nairn–Moray flags. On the screen, the belt returned only a weak and poorly bedrock-verified barium signal — the proxy for the Altar Stone’s pervasive baryte cement — in contrast to the strong, well-verified signal at East Caithness. This is the most substantive point against the belt, and it is a proxy, not a measurement.

5.  Why the fossil criterion does not discriminate

The provisional dismissal rested on the observation that every fine lacustrine facies in the Moray belt is fish-bearing. That observation is correct but non-discriminating, because the East Caithness ground on which the whole enquiry rests is itself the fish-bearing flag system. The Lower Caithness Flagstone Group is built from the Clyth and Lybster subgroups above the Sarclet Group, with the Achanarras Fish Bed within it; the quiet-water fish-bed facies of the Clyth and Lybster subgroups are locally carbonate-rich, approaching dolomitic limestone; and the basin’s fish beds carry bituminous residues from oil generation. The Achanarras assemblage that marks the Moray fish beds is the same marker found at the Niandt Limestone of east Caithness, the Sandwick Fish Bed of Orkney and the Cromarty and Edderton fish beds of Easter Ross. The fish beds are the correlatable quiet-water phase of a single lacustrine system, not a property that distinguishes one part of it from another.

It follows that fossil content cannot exclude the Moray margin without also excluding the preferred East Caithness lead — an argument that proves too much. The Altar Stone is barren of macrofossils simply because it derives from the barren sandstone phase of a depositional cycle rather than from the thin fish-bed phase at the cycle base; both phases are present in every cycle, in Caithness and in the Moray margin alike, and a monolith is by definition drawn from the sandy phase. If anything, the presence of the Achanarras-assemblage flags on the Moray margin is evidence that the correct lacustrine facies family is developed there — a point of similarity, not the decisive difference the provisional dismissal took it to be. The criterion is therefore withdrawn.

6.  Assessment: deprioritised and untested, not eliminated

The honest position is narrower than a lithofacies closure but is sufficient for the enquiry’s purposes. The Nairn–southern Moray Firth belt is a marginal, more oxidised, more heterogeneous expression of the same Lake Orcadie lacustrine system that reaches its deep-water optimum in East Caithness. Its fine lacustrine flags are the right facies family and cannot be excluded on facies or fossils; what places the belt well below East Caithness is the combination of a marginal and oxidised overall character, subordinate and impersistent flag development with poor monolith potential, and — most substantively — a weak, poorly bedrock-verified barium signal where the Altar Stone’s defining baryte cement should produce a strong one. The diagnostic baryte–tosudite assemblage is untested here, exactly as it remains under active validation at East Caithness.

The belt is therefore recorded as deprioritised and untested, not eliminated. What would resolve it is the same test that will confirm or refute East Caithness: direct sampling of a fine grey lacustrine flag interval — clay XRD for tosudite, modal K-feldspar, baryte-cement habit and rock geochemistry — benchmarked like-for-like against the Altar Stone. On present evidence the prior is low and the enquiry’s effort is better spent on the stronger East Caithness lead; but recording the Moray margin as a proven facies exclusion would overstate the evidence, and would rest on an argument that also excludes the lead.

7.  Conclusion and implications for the enquiry

A systematic lithofacies audit of the Nairn–Findhorn–Elgin margin does not reproduce the provisional “too many fossils” dismissal; it replaces it. The fine lacustrine flag intervals of the belt are the same fish-bearing quiet-water facies as the East Caithness flagstones, so fossil content cannot discriminate between them. The belt is instead deprioritised on its marginal and oxidised character, its subordinate and impersistent flag development, and its weak barium signal, with the diagnostic clay and cement assemblage untested. This distinguishes the Moray margin, within the enquiry’s ledger, from the Midland Valley and Orkney: those were screened out; the Moray margin is present but deprioritised and untested. With that distinction stated honestly, the enquiry’s signal remains concentrated on the East Caithness flagstone coast — the only ground where a strong, well-verified barium signal, a compatible and thickly developed flag facies, and independent detrital-mineral geochronology converge, and where the diagnostic baryte–tosudite assemblage is under active validation. The next phase is detailed target refinement within that East Caithness fairway.

Status

Nairn / southern Moray Firth Devonian ORS strand: DEPRIORITISED and untested — a marginal expression of the same lacustrine system as East Caithness; the fine flag facies is present and shares the Achanarras fish assemblage, so it is not a facies or fossil exclusion; deprioritised on marginality, poor monolith potential and a weak barium signal, with the diagnostic baryte–tosudite clays untested. Not eliminated. New Red Sandstone (Hopeman aeolian, Burghead fluvial) excluded on age. Enquiry focus remains on East Caithness (Sarclet–Lybster–Clyth) refinement.

Selected references

Bevins, R.E. et al. (2024). Was the Stonehenge Altar Stone from Orkney? Journal of Archaeological Science: Reports, 58, 104738.

British Geological Survey. Devonian, Grampian Highlands; Middle Old Red Sandstone, Northern Highlands of Scotland; Bedrock Geology UK North — the Old Red Sandstone Supergroup. BGS Earthwise.

British Geological Survey. Fortrose and eastern Inverness (Sheet 84W), memoir for the 1:50 000 geological map.

Clarke, A.J.I. et al. (2024). A Scottish provenance for the Altar Stone of Stonehenge. Nature.

Clarke, A.J.I. et al. (2026). From Highlands to Henge. Journal of Quaternary Science.

Daw, T. (2026). The Stonehenge Altar Stone: Screening the Orcadian Basin. sarsen.org.

Dineley, D.L. & Metcalf, S.J. (1999). Fossil Fishes of Great Britain. Geological Conservation Review Series 16 (incl. Tynet Burn).

Trewin, N.H. & Thirlwall, M.F. (2002). The Old Red Sandstone of Scotland (in The Geology of Scotland, ed. Trewin).