Lithofacies review of the Nairn and southern Moray Firth Devonian Old Red Sandstone: no matching facies for the Stonehenge Altar Stone

Abstract

The national barium–rubidium screen and subsequent Orcadian prioritisation identified East Caithness (Sarclet–Lybster–Clyth flagstone coast) as the strongest candidate ground for the Altar Stone, independently corroborated by Clarke et al.’s detrital-zircon match. The only other area within the Orcadian Basin that produced any signal or was considered plausible on broad stratigraphic grounds was the Nairn–Findhorn–Elgin coastal belt on the southern shore of the Moray Firth. This area was provisionally set aside because its fine-grained Devonian facies were known to be richly fossiliferous. A thorough lithofacies audit of all published BGS descriptions, GCR site accounts and sedimentological syntheses for the Devonian Old Red Sandstone (Middle and Upper ORS) of the Nairn district confirms that assessment. The fine, flaggy and laminated lacustrine intervals that constitute the only potential textural analogues to the East Caithness flagstones are precisely the horizons that yield abundant fish fossils (scales, bones, nodules and beds containing Asterolepis, Bothriolepis, Coccosteus and associated taxa). No extensive, clean, sparsely or non-fossiliferous, ripple-laminated, well-sorted fine sandstone facies matching the Altar Stone’s sedimentological and diagenetic signature is described from any outcrop in the Nairn–Moray belt. Coarser pebbly fluvial units and any aeolian-influenced sandstones are even poorer matches. Post-Devonian aeolian dune sandstones (Hopeman and Burghead formations) are Permian, unconformable on the ORS, and display large-scale cross-bedding incompatible with the Altar Stone. The Nairn strand is therefore closed on lithofacies grounds. Refinement now concentrates exclusively on the East Caithness candidate ground.

1. Why Nairn was the only remaining Orcadian candidate to check

Clarke et al. (2024, 2026) demonstrated that the Altar Stone’s detrital zircon and rutile/apatite signature matches the Orcadian Basin and is inconsistent with the Midland Valley, Anglo-Welsh Basin or Orkney Mainland. The barium–rubidium stream-sediment screen of Daw (2026) ranked East Caithness (particularly the Sarclet–Lybster–Clyth coastal flagstone outcrop) as the strongest national hit, with independent zircon corroboration at Sarclet. Within the broader Orcadian Basin the only other coastal belt that warranted serious consideration on stratigraphic and screen grounds was the southern Moray Firth margin around Nairn, Findhorn and Elgin. This area exposes Middle and Upper Devonian Old Red Sandstone deposited in the southern, more marginal part of Lake Orcadie. It was provisionally discounted on the informal observation that its fine-grained facies “had too many fossils”. The present note executes a formal lithofacies audit to test whether that dismissal is robust or whether any outcrop-scale facies nevertheless matches the Altar Stone’s defining characteristics (fine grain size, excellent sorting, ripple lamination, grey-green colour, low detrital K-feldspar, pervasive diagenetic baryte cement, tosudite-bearing clay assemblage, and absence of macrofossil debris).

2. Data and methods

The audit draws exclusively on published sources: BGS 1:50 000 and 1:625 000 digital geology, the BGS Earthwise account for Devonian rocks of the Grampian Highlands, the JNCC Geological Conservation Review volume on the Old Red Sandstone of Great Britain (with site descriptions for Tynet Burn and related Moray fish-bed localities), and sedimentological syntheses of the southern Orcadian Basin margin (e.g., descriptions of Nairn Sandstone Formation, Whitemire, Alves and Scaat Craig Beds). No new field data were collected. Lithofacies were evaluated against the Altar Stone criteria established in the East Caithness screening and mineralogical work (Bevins et al. 2024; Clarke et al. 2024, 2026; Daw 2026): fine- to very fine-grained, well-sorted sandstone; ripple cross-lamination or planar lamination indicative of quiescent-water conditions; grey-green colour; negligible detrital K-feldspar; evidence or potential for pervasive baryte and calcite cement; tosudite/aluminous kaolinite clay assemblage; and absence of body-fossil debris at outcrop or hand-specimen scale.

3. Lithofacies of the Nairn–Moray Devonian ORS

3.1 Middle Devonian (lacustrine–alluvial facies)

The Middle ORS south of the Moray Firth comprises lacustrine and alluvial deposits laid down in fault-bounded half-graben. Key fine-grained units include the Inshes Flagstone Formation, Nairnside Sandstone and Hillhead Sandstone. These contain grey and purple flaggy micaceous sandstones, dark calcareous flags, and laminated shaly mudstones with limestone nodules. The finer intervals are the classic lacustrine “fish beds” — precisely analogous in sedimentary style to the Caithness flagstones but developed in a more marginal, sandier and nodule-rich setting. They yield abundant fish remains (Achanarras Assemblage taxa including Coccosteus, Osteolepis, Pterichthyodes, Mesacanthus and others) preserved in calcareous nodules, ribs and shaly partings. Plant remains and ostracods also occur. Aeolian influence is locally recorded (e.g., in Leanach Sandstone), but the dominant fine facies remain tied to these fossiliferous lacustrine horizons.

3.2 Upper Devonian (Nairn Sandstone Formation and overlying units)

The Nairn Sandstone Formation (the principal Upper ORS unit in the Nairn–Findhorn 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 or shaly limestone-bearing mudstones. In the Findhorn area desiccated mudstone beds (clay galls) and a prominent calcrete horizon (Cothall Limestone) are recorded. Overlying Whitemire, Alves and Scaat Craig Beds consist of grey to reddish siliceous pebbly sandstones and fine conglomerates, with marly intervals and cornstone (calcrete) palaeosols. Fish faunas are again prominent: lower and upper faunules in the Nairn Sandstone (Asterolepis, Psammolepis, Coccosteus, Holoptychius etc.), transitioning upward through Bothriolepis-dominated assemblages in the Whitemire and Alves Beds. The finer flaggy and shaly intervals within these units are the fossil-bearing lacustrine or ponded facies; no laterally extensive, clean, unfossiliferous fine sandstone sheets are described.

3.3 Post-Devonian aeolian units (not ORS)

The Hopeman Sandstone Formation and Burghead Sandstone Formation, which crop out along the same coastal belt (Hopeman, Burghead, Lossiemouth), are Late Permian (Lopingian) aeolian dune sandstones resting unconformably on Devonian Upper ORS. They are clean, well-sorted but display large-scale dune cross-bedding, wind-etched pebbles and reptile footprints (e.g., Chelichnus). They are not Devonian, not lacustrine, and bear no sedimentological resemblance to the Altar Stone. They were never candidates on either age or facies grounds.

4. Direct comparison with Altar Stone criteria

•             Grain size and sorting: The only fine-grained, well-sorted candidates in the Nairn belt are the flaggy and laminated lacustrine intervals. These are texturally comparable to East Caithness flagstones but occur within nodule-rich, fish-bearing cycles rather than as clean, monolith-scale sheets.

•             Sedimentary structures: Ripple lamination and planar lamination are present in the finer beds, but the overall succession is more heterogeneous (pebbly interbeds, desiccation features, calcretes) than the more uniform quiescent-water flagstone facies of East Caithness.

•             Colour and composition: Red, grey and yellow calcareous sandstones dominate; grey-green, low-K-feldspar clean sandstone of the Altar Stone type is not reported.

•             Fossil content: This is the decisive mismatch. Every described fine lacustrine facies in the Nairn–Moray Devonian ORS is tied to fish-bearing horizons (scales, bones, nodules). The Altar Stone shows no such macrofossil debris and comes from a facies whose diagenetic mineralogy (baryte cement, tosudite) developed in a setting apparently free of the organic-rich, nodule-forming conditions that characterise the Moray fish beds.

•             Diagenetic potential: The nodule-rich, calcareous, sometimes bituminous nature of the Moray lacustrine mudstones and flags suggests a different early diagenetic pathway from the baryte-dominated, tosudite-bearing assemblage of the Altar Stone and its East Caithness analogues. No published clay or cement data from Nairn flagstones indicate a match.

•             Outcrop scale and monolith potential: Even if a textural analogue existed, the fish-bed intervals are thin, laterally impersistent and interbedded with coarser or nodule-rich units — unsuitable for extraction of a coherent 6-tonne monolith of the required dimensions and uniformity.

5. Conclusion and implications for the enquiry

No outcrop in the Nairn–Findhorn–Elgin coastal belt of the southern Moray Firth exposes a lithofacies that matches the Altar Stone on sedimentological, textural, compositional or taphonomic grounds. The fine, flaggy and ripple-laminated facies that might superficially appear analogous are the very horizons that contain abundant fish fossils and associated diagenetic features (nodules, calcareous concretions) foreign to the Altar Stone. Coarser fluvial and any aeolian Devonian sandstones are poorer matches still. The Permian dune sandstones (Hopeman/Burghead) are irrelevant on both age and facies criteria.

The initial provisional dismissal of the Nairn area on fossil content is therefore confirmed by systematic lithofacies review. The Nairn strand is closed. With both the Midland Valley and the southern Moray Firth now eliminated on independent grounds (zircon, facies, clay), and Orkney likewise closed, the enquiry’s signal remains concentrated on the East Caithness flagstone coast of the mainland Orcadian Basin — the only area where the barium–rubidium screen, lithofacies character, and independent detrital-mineral geochronology all converge, and where the specific diagenetic assemblage (baryte cement, tosudite) is under active validation. The next phase is detailed refinement and prioritisation of sampling targets within that East Caithness fairway.

Status: Nairn / southern Moray Firth Devonian ORS strand CLOSED on lithofacies grounds (fossiliferous character of all fine lacustrine facies; no matching clean, barren or sparsely fossiliferous ripple-laminated sandstone). Enquiry focus returns exclusively to East Caithness (Sarclet–Lybster–Clyth) refinement. Key sources: BGS Earthwise Devonian Grampian Highlands; JNCC GCR Old Red Sandstone of Great Britain; Clarke et al. 2024, 2026; Bevins et al. 2024; Daw 2026.

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This note closes the last remaining Orcadian coastal candidate outside East Caithness and keeps the project tightly focused on the single strongest lead.