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.

Screening Orkney

Re-examining Orkney: a barium–rubidium and lithofacies screen of the Eday Group and outer islands as a candidate source for the Stonehenge Altar Stone

Third in a sequence applying the Screening the Orcadian Basin method (Daw 2026) to candidate ground beyond the primary study area

sarsen.org Altar Stone Sourcing Enquiry · working paper · 12 July 2026

Abstract

Bevins et al. (2024) excluded Mainland Orkney as the source of the Stonehenge Altar Stone on the basis of a limited set of Stromness- and Rousay-Flagstone field samples, leaving the outer islands and the Eday Group untested. We re-examine Orkney comprehensively, combining the barium–rubidium stream-sediment screen of Daw (2026) with a lithofacies analysis of the Eday Group and a re-reading of the one published Eday clay-mineral datum, and we test the specific fault-controlled high-barium corridors on Sanday that a parallel desktop analysis proposed as candidate ground. Orkney returns essentially no signal on the primary screen: a 0.03% barium-floor hit rate against 10.8% for the mainland Orcadian outcrop — a single cell, at Yesnaby, independently attributable to vein baryte. The proposed Sanday corridors fall below both the barium floor and the ratio threshold and do not register. On lithofacies, Sanday and the coarse Eday Sandstone are disqualified: the only Altar-Stone-compatible facies, the Eday Flags, thins northward from about 150 m at Deerness and South Ronaldsay to roughly 10 m of flaggy sediment on Sanday and the Calf of Eday, where the succession is dominated by coarse, pebbly, cross-bedded sandstone. The sole facies-plausible residual — the Eday Flags of Deerness and northern South Ronaldsay — is not a screen hit, shows no elevated barium consistent with the Altar Stone’s diagnostic baryte cement, lacks tosudite (absent from every Orkney sample measured), and remains gated on an untested Eday-Group detrital-zircon comparison. We conclude that the screening process does not identify Orkney as a candidate source. The result is independent of, and consistent with, the Bevins mineralogical exclusion, and extends it to the outer-island gap that direct sampling had not reached.

1.  Introduction

Clarke et al. (2024, 2026) established, from detrital-zircon and apatite/rutile U–Pb geochronology, that the Altar Stone’s detritus derives from the Orcadian Basin of northern Scotland. Bevins et al. (2024) then tested Mainland Orkney directly, using portable XRF, automated SEM-EDS mineralogy, and clay X-ray diffraction on field samples of the Stromness and Rousay Flagstone formations, and concluded those units do not match the Altar Stone — principally on their abundant detrital K-feldspar, the absence of the pervasive diagenetic baryte cement that characterises the stone, and the absence of its diagnostic tosudite clay. That is a well-evidenced exclusion of the sampled units. It does not, by itself, speak to the Orkney ground that was not sampled: the outer islands, and in particular the Eday Group, which crops out most fully on Eday and Sanday and only marginally on the Mainland localities Bevins examined.

This paper closes that gap. It is the third in a sequence that applies the desk-based barium–rubidium screen of Daw (2026) — developed to rank the Orcadian Basin, and extended nationally in that paper’s Appendix C — to candidate ground the primary study set aside. The first established East Caithness (near Sarclet, and the Lybster–Clyth flagstone coast) as the strongest candidate, independently corroborated by the Clarke zircon match. The second closed the Midland Valley of Scotland. Here the method is turned back on Orkney, prompted by a parallel desktop analysis that proposed a set of fault-controlled, high-barium corridors on Sanday — in the Upper and Middle Eday Sandstone, near the North Scapa Fault and the Cata Sand system — as a previously untested Orkney target. We assess those corridors, and Orkney more generally, on the same two axes the method rests on: stream-sediment geochemistry, and lithofacies.

2.  Data and methods

The geochemical screen is that of Daw (2026): a basin-relative composite condition on the BGS G-BASE 500 m kriged stream-sediment grids — a barium floor (1025 ppm) combined with a barium/rubidium ratio threshold (the 95th percentile, ≈ 14) — with each surviving cell bedrock-verified by point-in-polygon join against the BGS Geology 625k map. Rubidium substitutes for potassium and so tracks K-feldspar and mica; the ratio isolates the Altar Stone’s distinctive combination of high barium (baryte cement) with a deficit of K-feldspar. Two limits of the method are load-bearing here and are stated at the outset. First, stream-sediment values are not rock values, and cannot be compared directly against the Altar Stone’s measured rock geochemistry; the screen is calibrated against stream-sediment thresholds only. Second, formation identity is not facies identity: a cell on genuine Old Red Sandstone bedrock may still be the wrong lithofacies, so a geochemical hit is a necessary but not sufficient condition and must be read together with the sedimentology.

The lithofacies analysis draws on the regional survey of Mykura (1976), after the sedimentological work of Fannin (1970) and Ridgway (1974), which maps the Eday Group and its internal thickness variation along the 58 km north–south outcrop. The clay-mineral comparison uses Table 4 of Bevins et al. (2024); we re-read the single Eday Group entry (sample 5514) directly from the published table image, because a flattened transcription of that row misassigns its values (Section 4.4).

3.  Results

3.1  Orkney is effectively silent on the primary screen

On the barium floor alone, the Orkney archipelago returns a 0.03% hit rate against valid grid cells — a single cell, at Yesnaby, itself independently attributable in the literature to vein-hosted baryte rather than diagenetic cement — against 10.8% for the mainland Orcadian outcrop (Caithness, Sutherland, Moray, Black Isle) and 2.8% for Shetland: a roughly 350-fold contrast between Orkney and the mainland basin. Under the full composite condition, no Orkney cluster survives. The Deerness area of East Mainland and the island of South Ronaldsay — which, as Section 3.3 shows, carry the only Altar-Stone-relevant facies — both fall within this near-zero population. There is no Orkney screen hit to rank.

3.2  The proposed Sanday corridors fall below threshold

The parallel analysis reported its strongest Sanday barium pixels at roughly 670–675 ppm with barium/rubidium ratios of about 8.5–8.9. Both figures sit below the screen’s thresholds — the barium floor of 1025 ppm and the ratio threshold of about 14 — by a clear margin. On the method’s own terms these are not anomalies; they are ordinary background, and they do not register. The corridors were interpreted as fault-controlled secondary baryte along the North Scapa Fault trend, which is the same vein-baryte association that makes the single Yesnaby cell a documented false positive rather than a candidate. We note one mitigating caveat, developed in Section 4.3: Orkney is extensively covered by blown sand and till, and its G-BASE coverage is sparser than the mainland, so a stream-sediment null over Orkney is softer evidence than a null over open mainland ground.

3.3  Lithofacies: the fine facies has pinched out in the north

The Eday Group is roughly 1,000 m of dominantly fluvial sandstone (Lower, Middle and Upper Eday Sandstone), with red marls and two finer intervals; of its formations, only the Eday Flags contain the lacustrine, finely laminated, grey ‘quiescent-water’ facies that resembles the fine, well-sorted, ripple-laminated Altar Stone. Mykura (1976) maps the thickness of the Eday Flags along the whole outcrop, and the gradient is monotonic: thick in the south, effectively gone in the north.

 

The proposed Sanday corridors sit in the bottom row, and in the coarse Middle/Upper Eday Sandstone rather than the Eday Flags. On Sanday the Middle Eday Sandstone alone reaches some 400 m of reddish-purple, trough-cross-bedded, pebbly gritty sandstone, with conglomerates at Hegglie Ber — a fundamental lithofacies mismatch to the Altar Stone. The one fine interval on the island, at roughly 10 m, is too thin and too sandy to source a coherent monolith of the required dimensions, and is untested. Sanday, the Calf of Eday, and the fault-proximal Eday ground (where, along the North Scapa Fault, the Flags horizon passes entirely into sandstone) are therefore closed on facies.

3.4  The Eday Flags residual is not screen-supported

The lithofacies analysis leaves a single facies-plausible residual: the Eday Flags where they are thick and best developed, at Deerness and northern South Ronaldsay. This ground is not, however, a product of the screen — it carries no elevated barium (Section 3.1). Since the Altar Stone’s defining diagnostic feature is its pervasive baryte cement, which is precisely what generates a high-barium signal, the absence of any barium anomaly over the fairway is at best neutral and arguably mildly counter-indicative: it suggests the baryte cement is not developed in these Eday Flags. Bevins et al. (2024) found no diagenetic baryte in any Orkney sample, consistent with that reading.

3.5  Clay mineralogy: a partial match, missing the diagnostic phase

The only published Eday Group clay analysis is Bevins et al. (2024) sample 5514, from an undivided Eday Group exposure at Bu 1 on southern Mainland — not from the Sanday corridors, and not from the Deerness/South Ronaldsay fairway. Read directly from the table, its <2 µm assemblage is about 10% illite, 49% R1-ordered mixed-layer illite/smectite (expandability ~25%), and 41% kaolinite, with no tosudite and no dioctahedral chlorite. Against the Altar Stone (illite 14–19%, dioctahedral chlorite 12–13%, tosudite 15–21%, R1 I/S 26–33%, kaolinite 16–25%), this is a partial match: it shares the kaolinite and the R1-ordered illite/smectite — the closest any Orkney sample comes to the stone — but lacks the tosudite and dioctahedral chlorite that are the Altar Stone’s diagnostic phases. Tosudite is absent from every Orkney sample Bevins et al. measured.

4.  Discussion

4.1  Closing Orkney on the right grounds

It is worth being explicit about why Orkney closes, because the parallel analysis reached a superficially similar conclusion by arguments that do not hold. It compared a Sanday stream-sediment barium/rubidium ratio (≈8.6) against an Altar Stone rock-level ratio (≈105), an invalid cross-medium comparison that would equally ‘exclude’ the corroborated East Caithness lead (stream-sediment ratio 18.2); it invoked a basin-wide thermal ceiling to argue tosudite could not survive, an argument that would also exclude the Caithness flagstones, where tosudite is likewise unreported; and it rested partly on a clay transcription that misassigned sample 5514’s values (Section 4.4). None of those arguments is needed, and none is sound. The defensible closure is simpler and independent of them: Orkney produces no screen signal (Section 3.1–3.2), and the coarse Eday facies that the barium corridors actually sample is the wrong rock, while the only compatible facies has pinched out in the north (Section 3.3).

4.2  The residual, and the gate that remains open

Intellectual honesty requires that the Deerness/South Ronaldsay Eday Flags be recorded as a residual rather than a closed case. It is the one Orkney ground that is the right facies and carries a partial clay match. But it is a facies-and-clay inference the geochemistry does not support, and it is gated on a detrital-zircon question that has not been answered. The Eday Group is stratigraphically higher than the Stromness/Rousay flags on which Clarke’s Orcadian match was established, and it was fed from the south and south-west carrying rhyolitic and volcanic detritus, with contemporaneous Middle-Devonian volcanism at the base of the Eday Flags. Its provenance therefore cannot be assumed to reproduce the Altar Stone’s signature merely because it lies within the basin — the same reasoning that governed the Midland Valley Upper Old Red Sandstone. A young or distinct volcanic zircon population, if present, would be inconsistent with the Altar Stone’s youngest concordant grain of ~498 Ma. Resolving whether any Eday Group detrital-zircon dataset exists, and how it compares, is the one desk check that could either close this residual outright or promote it to a field target; until then it is untested, not eliminated.

4.3  The drift-cover caveat

A stream-sediment null over Orkney is weaker evidence than the same null over open mainland ground. Orkney carries extensive blown sand and till — Sanday especially, with the Cata Sand system — and its G-BASE sampling is correspondingly sparser, so the grid may in places be sampling superficial cover rather than bedrock. This is a genuine reason not to treat the Orkney null as a hard exclusion. It is not, however, a reason to read the null as encouragement: the absence of a barium signal remains consistent with the mineralogical absence of baryte cement that Bevins et al. measured directly, and the lithofacies closure of Sanday (Section 3.3) does not depend on the geochemistry at all.

4.4  Relation to Bevins et al. (2024)

This re-examination is independent of the Bevins exclusion — it uses different data (national stream-sediment geochemistry and regional lithofacies rather than local field mineralogy) — and reaches a consistent result by a different route. Where Bevins et al. excluded the sampled Mainland flagstone units on mineralogy, the present screen finds no Orkney signal at all, and the lithofacies analysis accounts for the outer-island Eday Group that direct sampling had not reached. The two lines together close the outer-island gap that the original Orkney rejection had left open, with the single, explicitly flagged residual of Section 4.2.

5.  Conclusion

Applied comprehensively to Orkney — including the previously untested outer islands and the specific fault-controlled corridors proposed on Sanday — the screening process does not identify a candidate source for the Altar Stone. Orkney is effectively silent on the barium–rubidium screen (a single vein-baryte cell against a 350-fold-higher mainland hit rate), the Sanday corridors fall below threshold, and Sanday together with the coarse Eday Sandstone is disqualified on lithofacies, the only Altar-Stone-compatible facies having thinned to a marginal remnant in the north. The sole residual, the Eday Flags of Deerness and northern South Ronaldsay, is facies-plausible but unsupported by the geochemistry, without the Altar Stone’s baryte cement or tosudite, and gated on an untested Eday-Group zircon comparison; it is recorded as untested rather than eliminated. This is a negative result, and a useful one: it removes Orkney from contention on the method’s own terms and, with the Anglo-Welsh Basin and the Midland Valley, leaves the enquiry’s signal concentrated where the screen, the lithofacies, and the independent zircon evidence agree — the East Caithness coast of the mainland Orcadian Basin.

Selected references

Bevins, R.E. et al. (2024). Was the Stonehenge Altar Stone from Orkney? Investigating the mineralogy and geochemistry of Orcadian Old Red sandstones and Neolithic circle monuments. Journal of Archaeological Science: Reports, 58, 104738.

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: Refining the Provenance and Transport Pathways of Stonehenge’s Altar Stone. Journal of Quaternary Science.

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

Fannin, N.G.T. (1970). The sedimentary environment of the Old Red Sandstone of western Orkney. PhD thesis, University of Reading (unpublished).

Hillier, S., Wilson, M.J. & Merriman, R.J. (2006). Clay mineralogy of the Old Red Sandstone and Devonian sedimentary rocks of Wales, Scotland and England. Clay Minerals, 41, 433–471.

Mykura, W. (1976). British Regional Geology: Orkney and Shetland. HMSO, Edinburgh.

Ridgway, J.M. (1974). The Sedimentology and Palaeogeography of the Eday Group, Middle Old Red Sandstone, Orkney. PhD thesis, University of London (unpublished).

Closing the Midland Valley of Scotland as a source for the Stonehenge Altar Stone

Project record · Midland Valley strand closure · second in the sequence applying the barium–rubidium screen to candidate ground beyond the primary Orcadian study area

Abstract

Clarke et al. (2026) excluded the Midland Valley terrane on the basis of a distinctive mid-Palaeozoic and Neoproterozoic detrital-zircon signature carried by its Lower Old Red Sandstone successions — a signature the Altar Stone lacks. A national barium–rubidium stream-sediment screen, however, also identified clusters within the Upper Old Red Sandstone Stratheden Group on the Clyde and Ayrshire coast. These lie stratigraphically above the measured Lower ORS datasets and were therefore untested by Clarke. This paper applies a four-test veto framework — direct zircon data search and statistical comparison if available, source-terrane geometry and recycling assessment if no data exist, facies/lithology comparison to the Altar Stone’s defining characteristics, and clay mineralogy focused on the diagnostic tosudite phase — to close that gap independently. No published detrital-zircon U–Pb dataset exists for the Stratheden Group or the Upper ORS of the northern Midland Valley. The source-terrane geometry and recycling analysis returns an unfavourable prior: even the northern-flank clusters carry forward the forbidden grain populations via demonstrable reworking of Lower ORS detritus. Lithofacies comparison reveals a clear mismatch — the Stratheden Group is coarsening-upward, conglomeratic, aeolian-influenced and locally volcaniclastic, whereas the Altar Stone is fine-grained, well-sorted, ripple-laminated and non-volcaniclastic. Clay mineralogy shows that the diagnostic tosudite phase of the Altar Stone is documented in the Midland Valley only within the Lower ORS (Strathmore Group) and is absent from the Stratheden Group. Three independent lines therefore converge on exclusion of the Stratheden clusters at high confidence, although the closure remains indirect because no zircon or clay measurement has been performed directly on the Ba/Rb cluster ground itself. The Lower ORS clusters are excluded by direct measurement. With the Midland Valley strand closed, the enquiry returns to the Orcadian Basin for refined search within the East Caithness candidate ground.

1. Introduction

Clarke et al. (2024, 2026) established from detrital-zircon and apatite/rutile U–Pb geochronology that the Altar Stone’s detritus derives from the Orcadian Basin, and further showed that the Midland Valley of Scotland carries a mid-Palaeozoic and Neoproterozoic zircon signature the Altar Stone conspicuously lacks. Their methods statement records that the comparison datasets “encompass all currently published detrital zircon U–Pb ages from Scottish Old Red Sandstone successions.” That compilation rests on McKellar et al. (2020, 2021) for the northern Midland Valley (Strathmore Basin) Lower ORS and Phillips et al. (2009) for the southern (Lanark Basin) pre-ORS Silurian. Both are Lower ORS or older; no Upper ORS data entered the discriminant.

The national barium–rubidium screen reported in Appendix C of Daw (2026) nevertheless fired on genuine Old Red Sandstone clusters both inside and outside the Orcadian Basin. Within the Midland Valley these resolved into two stratigraphic populations: Lower ORS clusters (Arbuthnott–Garvock Group, Strathmore Group, and the Silurian–basal Lanark ground) that sit squarely inside Clarke’s measured material, and Upper ORS Stratheden Group clusters on the Clyde and Ayrshire coast (17.8, 13.2 and 9.5 km² at 74–81 % bedrock purity) that do not. The Lower ORS clusters are therefore already excluded by direct measurement. The Stratheden Group clusters survived contact with the published zircon evidence only because the evidence was never pointed at them.

To decide whether the Stratheden Group clusters warranted further masking or fieldwork, a four-test veto framework was applied in cheapest-and-most-lethal order. Any one decisive failure shuts the strand; it remains open only if it passes or ties on all four. The tests are: (A) search for any published detrital-zircon U–Pb dataset for the Stratheden Group and, if grain-level ages are recoverable, run a two-sample KS test against the 56 concordant Altar Stone grains; (B) if no zircon data exist, assess source-terrane geometry and recycling for the specific Ayrshire–Clyde clusters (Highland/Grampian versus Southern Uplands routing, and whether Lower ORS detritus is reworked upward); (C) compare Stratheden lithofacies directly against the Altar Stone’s defining sedimentological characteristics; (D) test whether the diagnostic tosudite/aluminous kaolinite clay assemblage is documented in the Stratheden Group itself. The present paper executes those tests and records the closure. It is the second in the sequence that applies the desk-based barium–rubidium screen of Daw (2026) and its associated veto methods to candidate ground beyond the primary Orcadian study area. The first established East Caithness (Sarclet–Lybster–Clyth flagstone coast) as the strongest national candidate, independently corroborated by Clarke’s zircon match at Sarclet. This paper closes the Midland Valley strand. The third turns the same method back on Orkney to close the outer-island and Eday Group gap left by Bevins et al. (2024).

The Stratheden Group was initially a legitimate lead for three reasons. Its Famennian age is compatible with the Altar Stone’s youngest concordant detrital zircon (~498 Ma Cambrian); the zircons impose no maximum depositional age that forbids an Upper ORS host. The volcanic axis (Ochil–Sidlaw) that supplies Clarke’s Midland Valley discriminant was extinct by Stratheden time, so the direct arc input might not extend upward. And a clay-mineral observation (Wilson 1971, reported in Hillier et al. 2006) appeared to point the right way: the Upper ORS of the Midland Valley was described as mineralogically similar to the Upper ORS of the Orcadian Basin and the Middle ORS around the Moray Firth — the very ground already implicated by the Altar Stone’s aluminous kaolinite–tosudite assemblage.

Set against these considerations was a mechanistic prior drawn from source-terrane geometry and palaeodrainage. The Altar Stone’s zircon load is Mesoproterozoic and Archaean (Laurentian, overprinted by Grampian ~460 Ma magmatism) and lacks both a pronounced Neoproterozoic population and a mid-Palaeozoic (Ordovician–Silurian) one. Across Britain and Ireland the Upper ORS is characterised by precisely those missing populations, recycled from Southern Uplands–Longford–Down sources or introduced by contemporaneous volcanism. For the documented Borders–Solway outcrop of the Stratheden Group, granitic clasts and palaeocurrents indicate a Galloway Hills / Southern Uplands source and internal drainage to the Jedburgh area — a southern routing opposite to the Altar Stone’s northern, Grampian-overprinted character. Even for the northern-flank (Ayrshire–Clyde) clusters that survived the Ba/Rb screen, the Stratheden sits unconformably on older Midland Valley ORS and demonstrably reworks it, so the forbidden grain populations can be carried forward regardless of the extinct volcanic axis. The honest prior was therefore unfavourable, but it remained a prediction from terrane models rather than a measurement on the candidate ground itself. The project exists because a confident stratigraphic prediction (“Middle ORS”) turned out to be unverified; the same discipline requires that an unverified exclusion also be tested rather than assumed.

This paper therefore executes the four tests, documents each result, records one clarification to the initial clay reading for the Stratheden lead, and hands the enquiry back to the Orcadian Basin with every screened Midland Valley candidate now accounted for.

2. Data and methods

The four-test veto framework applied here is as follows. Four tests are applied in order; any decisive failure shuts the strand.

Test A — Direct zircon (if data exist). Trace any recoverable Stratheden Group or northern Midland Valley Upper ORS U–Pb detrital-zircon dataset. If grain-level ages are recoverable in the form provided by Strachan et al. (2021) for Orcadian localities, run a two-sample Kolmogorov–Smirnov test against the 56 concordant Altar Stone grains exactly as performed for Sarclet/Braemore. Verdict rule: p > 0.05 and no Neoproterozoic or mid-Palaeozoic mode → OPEN strongly. Any failure → SHUT decisively. This test dominates all others if data can be found.

Test B — Source-terrane geometry (if no data). Establish the sediment routing of the Ayrshire–Clyde Stratheden specifically: Highland/Grampian source versus Southern Uplands source, using palaeocurrents, clast petrography and heavy-mineral data. Unavoidably Southern-Uplands-sourced or demonstrably recycling-dominated → SHUT on prediction (flagged as such). Genuinely ambiguous or northern-sourced without recycling carry-over → stays OPEN pending sampling.

Test C — Facies / lithology. Compare the Stratheden Group against the Altar Stone’s defining characteristics: grey-green, fine-grained, well-sorted, K-feldspar-poor, calcite/baryte-cemented, non-volcaniclastic, with quiescent-water ripple lamination. Coarsening-upward, aeolian-influenced, conglomeratic or locally volcaniclastic character → can SHUT the strand on mismatch alone.

Test D — Clay mineralogy. Check whether tosudite and the associated aluminous kaolinite assemblage are documented in the Stratheden Group specifically (distinct from the Lower ORS Strathmore/Stonehaven Groups where Hillier et al. 2006 place it). Match on the diagnostic phase → corroborates OPEN. Absence → weakly negative, not decisive by itself.

Data sources comprise the published zircon compilations already used by Clarke et al. (McKellar et al. 2020, 2021; Phillips et al. 2009; Strachan et al. 2021), the BGS Geology 625k and 1:50k maps for cluster verification and formation attribution, the regional sedimentological and provenance syntheses of Bluck (1978, 1980), Paterson et al. (1990) and Mykura (1991), the clay-mineral compilation of Hillier et al. (2006) including the Wilson (1971) observations, and the BGS Earthwise Old Red Sandstone reviews together with the relevant Midland Valley memoirs (Greenock district and regional geology). The Ba/Rb screen thresholds and cluster statistics are those of Daw (2026). No new field or laboratory data were generated; the exercise is entirely a desk-based audit and synthesis of existing published and archival sources.

Intellectual hygiene requires that exclusions be stated only at the stratigraphic and geographic level the evidence actually supports, that prediction and measurement be kept in separate columns, and that anything asserted as closed must be closed by evidence pointed at the thing itself — not at its neighbour, its parent unit, or its terrane label.

3. Results

3.1 The Lower Old Red Sandstone Midland Valley clusters are excluded by direct measurement

The national screen flagged Lower ORS clusters within the Arbuthnott–Garvock Group (94.5 % and 77.5 % bedrock purity), small Strathmore Group clusters, and the Silurian–basal Lanark ground. These sit squarely inside the material Clarke et al. characterised. McKellar et al. (2020) provide ten zircon samples across a 9 km succession in the northern Midland Valley (Stonehaven, Arbuthnott–Garvock and Strathmore Groups) with a proximal easterly / SW-Baltican source. Phillips et al. (2009) cover the southern pre-ORS Silurian of the Lanark Basin. Both datasets contain the diagnostic mid-Palaeozoic (~490–420 Ma) and Neoproterozoic grain populations shed from the Ochil–Sidlaw volcanic axis and proximal sources — populations the Altar Stone’s 56 concordant grains lack (youngest ~498 Ma). The Lower ORS Midland Valley candidates are therefore excluded by direct measurement on the precise stratigraphic units concerned. No further masking or fieldwork is required for them.

3.2 Test A returns no data: no published detrital-zircon U–Pb dataset exists for the Stratheden Group

Following Clarke et al.’s own methods statement, an exhaustive search was made for any published or archived U–Pb detrital-zircon dataset for the Stratheden Group or for Upper ORS successions of the northern Midland Valley (Fife, Firth of Tay, Ayrshire–Clyde coast). None was found. The gap is real; it is not an omission by Clarke but a reflection of the published record at the time their compilation was assembled. Consequently Test A cannot strongly open the strand by direct statistical comparison, nor can it shut it on a failed KS test. The result of Test A is therefore recorded as “no recoverable data exist.” The strand proceeds to Test B.

3.3 Test B: source-terrane geometry and demonstrated Lower-ORS recycling render the prior unfavourable for the Ayrshire–Clyde Stratheden clusters

The mechanistic prior for the Stratheden Group as a whole is unfavourable. The Altar Stone lacks both a pronounced Neoproterozoic (~1000–540 Ma) population and a mid-Palaeozoic (Ordovician–Silurian, ~490–420 Ma) one. The Upper ORS across Britain and Ireland characteristically carries both, either recycled from Southern Uplands–Longford–Down volcanism and metasediments or introduced by contemporaneous arc input. For the documented Borders–Solway outcrop, BGS provenance work and clast petrography indicate granitic detritus from the Galloway Hills / Southern Uplands and drainage into internal basins in the Jedburgh area — a southern routing geometrically opposite to the Altar Stone’s Grampian-overprinted northern signal. This geometry alone predicts the introduction of the missing populations.

For the specific northern-flank clusters (Clyde and Ayrshire coast) that survived the Ba/Rb screen, the picture is not provenance-monolithic; a Highland/Grampian source remains conceivable in principle and would remove the Neoproterozoic and mid-Palaeozoic objection at a stroke. However, the escape hatch is closed by recycling. The Stratheden Group sits unconformably on the older Midland Valley ORS succession and demonstrably reworks it. The very mid-Palaeozoic and Neoproterozoic grains that Clarke used as the Midland Valley discriminant can therefore be carried forward into the Upper ORS regardless of whether the Ochil–Sidlaw volcanic axis was extinct by Famennian time. No published palaeocurrent, clast or heavy-mineral dataset for the Ayrshire–Clyde Stratheden overturns this carry-over. Test B therefore returns an unfavourable verdict on prediction, reinforced by demonstrated reworking. The strand is shut at this point on the strength of source geometry and recycling; direct zircon measurement on the cluster ground would be required to overturn it, and none exists.

3.4 Test C: clear lithofacies mismatch

Even if the zircon prior had been favourable, Test C would shut the strand on its own. The Altar Stone is a fine-grained (dominantly fine sand), well-sorted, grey-green, ripple-laminated sandstone with pervasive diagenetic baryte and calcite cement and negligible detrital K-feldspar — a quiescent-water lacustrine or marginal facies. The Stratheden Group (the lower unit of the Scottish “Upper Old Red Sandstone”, Famennian) is a coarsening-upward sequence of red sandstone and conglomerate with aeolian influence, trough cross-bedding, pebbly gritty horizons and local volcaniclastic input. On the Clyde–Ayrshire coast the Middle and Upper Eday-equivalent sandstones reach hundreds of metres of reddish-purple, trough-cross-bedded, pebbly gritty sandstone with conglomerates; the finer intervals are thin and sandy. None of this matches the Altar Stone’s defining sedimentological characteristics. A clear facies mismatch exists. Test C alone is sufficient to exclude the Stratheden clusters.

3.5 Test D: clay mineralogy — tosudite is a Lower ORS phase in the Midland Valley; absent from the Stratheden Group

Re-examination of the Wilson (1971) observations as synthesised in Hillier et al. (2006) shows that the noted mineralogical similarity between the Upper ORS of the Midland Valley and the Upper ORS of the Orcadian Basin / Middle ORS of the Moray Firth is limited to the kaolinite + illite/smectite component. The diagnostic tosudite (and associated aluminous phases) that characterise the Altar Stone’s <2 µm assemblage are documented in the Midland Valley only within the Lower ORS Strathmore Group; they are not reported from the Stratheden Group or other Upper ORS successions. On the Altar Stone’s defining clay phase, therefore, the proxy is a mismatch rather than a match. This reading corrects an initial interpretation in the opening analysis of the Stratheden lead, which had taken the Wilson (1971) similarity as more favourable than the full published evidence supports. The similarity is real but partial and does not extend to the diagnostic assemblage required by the Altar Stone.

Convergence. Tests B, C and D each return negative independently and on different axes (provenance/recycling, facies, clay). Test A could not strongly open the strand because no data exist. No axis rescues the candidate. The Stratheden Group clusters of the Clyde and Ayrshire coast are therefore closed by convergent inference at high confidence. The closure is strong but remains indirect: no detrital-zircon U–Pb or matched clay separation has been performed on the specific Ba/Rb cluster ground itself.

The unscreened Stratheden outcrops (Fife, Arran, Kintyre) produced no Ba/Rb hit and are excluded by null result, subject to the same 500 m-resolution and superficial-cover caveats noted for other null areas in the national screen.

4. Discussion

4.1 The verdict at its honest register

The temptation to state a clean universal negative (“nowhere in the Midland Valley ORS could match the Altar Stone”) is resisted. That phrasing would repeat, in the opposite direction, the error the enquiry was built to catch — a confident claim outrunning the measurement that licenses it. What is actually supported is more precise and remains defensible:

•             Lower ORS and Silurian successions: excluded by direct measurement (Clarke et al.’s zircon signatures obtained on those exact Groups).

•             Upper ORS Stratheden clusters (Clyde/Ayrshire): excluded by convergent inference — a direct facies mismatch, a recycling-reinforced unfavourable zircon prior, and a clay proxy that fails on tosudite. High confidence, but no zircon or clay measurement performed on the cluster ground itself.

•             Every screened Midland Valley candidate is now accounted for; the unscreened Stratheden outcrops are excluded by null Ba/Rb result.

What would convert inferred to measured is straightforward: a single direct sample from one Clyde/Ayrshire Stratheden cluster, with U–Pb detrital zircon run against the 56 concordant Altar Stone grains (KS test) plus a matched clay separation tested for tosudite. Nothing found in this investigation suggests such measurement would reverse the verdict, but it has not been done and the paper records that fact explicitly.

4.2 Relation to Clarke et al. (2026)

It would be inaccurate to record that “Clarke was right without doing the full analysis.” Clarke closed the terrane on Lower ORS data and, by their own methods statement, had no Upper ORS zircon to test; the Stratheden Group was the genuine gap in their reasoning, not something they had already closed. What this strand supplied was the missing independent work — facies comparison, recycling audit and clay re-reading — and arrived at the same terrane-level verdict Clarke had assumed but could not yet support for the Upper ORS. The correct record is therefore that Clarke reached the right answer for the terrane on the data available to them; the work that makes the answer right for the Upper ORS is work they had not done and that is now complete.

4.3 Clarification of the initial clay reading for the Stratheden lead

As noted under Test D, an initial reading of the Wilson (1971) observations (as synthesised in Hillier et al. 2006) had logged the mineralogical similarity between Midland Valley Upper ORS and Orcadian/Moray Firth ORS as a favourable clay signal for the Stratheden lead. That reading was too generous. The similarity covers only the kaolinite + illite/smectite component; it does not extend to tosudite, which in the Midland Valley is a Lower ORS Strathmore phase and is not documented in the Upper ORS. On the Altar Stone’s diagnostic phase the clay proxy is therefore a mismatch, not a match. The same intellectual-hygiene rule applied throughout the enquiry — that conclusions must not outrun the measurements that license them — requires this clarification here.

4.4 No further work warranted on present evidence

“Closed” commits the project only to the following: every screened Midland Valley candidate is accounted for; no axis favours a match; the single residual (direct measurement on cluster ground) has not been performed but is not expected to alter the outcome on present evidence. No further field sampling or laboratory allocation is warranted for the Midland Valley strand. Stated at this register, the conclusion is reproducible and defensible in review.

4.5 Intellectual hygiene maintained

The Midland Valley episode illustrates the same discipline applied in reverse to the Middle ORS episode that prompted the enquiry. In both cases an unverified label (first “Middle ORS is established fact”, later “Midland Valley is ruled out”) threatened to do more work than the measurements licensed. The rule is identical in both directions: state exclusions at the level the data support (Lower ORS: excluded by measurement; Stratheden: untested until this strand, now closed by convergent inference), keep prediction and measurement in separate columns, and require that anything asserted as closed be closed by evidence pointed at the thing itself.

5. Conclusion

The Midland Valley of Scotland is closed as a source for the Altar Stone. The Silurian and Lower Old Red Sandstone successions are excluded by Clarke et al.’s directly measured detrital-zircon signatures. The Upper Old Red Sandstone Stratheden Group clusters of the Clyde and Ayrshire coast — the only Midland Valley candidates that survived contact with the published zircon evidence — are excluded by convergent inference from three independent lines: a clear facies mismatch, an unfavourable zircon prior reinforced by demonstrated Lower-ORS recycling, and failure of the clay proxy on the diagnostic tosudite phase. Every screened candidate is accounted for. The Stratheden closure is high-confidence but indirect; no zircon or clay measurement has been made on the cluster ground itself, and that limitation is stated explicitly.

With the Midland Valley strand closed, the enquiry returns to the Orcadian Basin for the more detailed search within the East Caithness (Sarclet–Lybster–Clyth) candidate ground that both the original barium–rubidium screen and Clarke’s independent zircon corroboration identify as the strongest remaining lead. 

Status: Midland Valley strand CLOSED. Lower ORS — excluded by measurement (Clarke et al. 2024, 2026). Upper ORS Stratheden clusters — excluded by convergent inference (facies + recycling-reinforced zircon prior + clay proxy), high confidence; direct measurement on cluster ground not performed. Key sources: Clarke et al. 2024, 2026; Daw 2026; McKellar et al. 2020, 2021; Phillips et al. 2009; Strachan et al. 2021; Hillier et al. 2006 (Wilson 1971); Bluck 1978/1980; Paterson et al. 1990; Mykura 1991; BGS memoirs and Earthwise ORS reviews. Next: detailed Orcadian Basin search within East Caithness candidate ground.

Selected references

Bevins, R.E. et al. (2024). Was the Stonehenge Altar Stone from Orkney? Investigating the mineralogy and geochemistry of Orcadian Old Red sandstones and Neolithic circle monuments. Journal of Archaeological Science: Reports, 58, 104738.

Bluck, B.J. (1978). Sedimentation in a late orogenic basin: the Old Red Sandstone of the Midland Valley of Scotland. In: Tectonic evolution of the Caledonides. Special Publication of the Geological Society of London.

Bluck, B.J. (1980). Evolution of a strike-slip fault-controlled basin, Upper Old Red Sandstone, Scotland. In: Sedimentation at oblique-slip margins. Special Publication of the International Association of Sedimentologists.

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: Refining the Provenance and Transport Pathways of Stonehenge’s Altar Stone. Journal of Quaternary Science.

Daw, T. (2026). The Stonehenge Altar Stone: Screening the Orcadian Basin. sarsen.org / repository. (Includes national Ba/Rb run in Appendix C.)

Hillier, S., Wilson, M.J. & Merriman, R.J. (2006). Clay mineralogy of the Old Red Sandstone and Devonian sedimentary rocks of Wales, Scotland and England. Clay Minerals, 41, 433–471.

McKellar, R.C. et al. (2020). Detrital zircon provenance of the Lower Old Red Sandstone, northern Midland Valley, Scotland. Journal of the Geological Society.

McKellar, R.C. et al. (2021). Further detrital zircon data from the Lower Old Red Sandstone of the Midland Valley. (Follow-up dataset.)

Mykura, W. (1991). British Regional Geology: The Midland Valley of Scotland (3rd edn). HMSO, Edinburgh.

Paterson, I.B., McAdam, A.D. & MacPherson, K.A.T. (1990). Geology of the Greenock district. Memoir of the British Geological Survey, Sheet 30W (Scotland).

Phillips, E.R. et al. (2009). Detrital zircon geochronology of Silurian–Devonian sedimentary rocks, southern Midland Valley, Scotland. Journal of the Geological Society.

Strachan, R.A. et al. (2021). Detrital zircon U–Pb ages from the Old Red Sandstone of the Orcadian Basin: implications for provenance and the Stonehenge connection. (Dataset as used in Clarke et al. re-analysis.)

Waldron, J.W.F., Floyd, J.D., Simonetti, A. & Heaman, L.M. (2008). Ancient Laurentian detrital zircon in the Southern Uplands, Scotland: implications for regional tectonics. Journal of the Geological Society.

Wilson, M.J. (1971). Clay mineralogy of the Old Red Sandstone of the Midland Valley. (Unpublished data cited in Hillier et al. 2006.)

BGS Earthwise Old Red Sandstone reviews and 1:50k/625k digital geology (accessed via BGS GeoIndex and associated memoirs).

The Altar Stone - Maybe Not The Orcadian Basin?

 

Refining the provenance of the Stonehenge Altar Stone: multi-criteria screening of the Orcadian Basin, the exclusion of its highest-priority target on measured thermal data, and first-pass results from the Midland Valley

Tim Daw July 2026

Abstract

A UK-wide stream-sediment geochemical screen identified an East Caithness coastal cluster (cluster 15) as the strongest onshore barium and Ba/Rb anomaly over continuous Old Red Sandstone. Application of the primary thermal-maturity dataset (Hillier & Marshall 1992) shows that the cluster core lies entirely within a region of measured vitrinite reflectance >3 % R₀ and spore colours 10/11 — maximum palaeotemperatures in excess of ~250 °C on the authors’ calibration. This is incompatible with the Altar Stone’s preserved clay assemblage — mixed-layer illite/smectite (26–33 % of the clay fraction), tosudite, dioctahedral chlorite and kaolinite (Bevins et al. 2024, Table 4) — which records low-grade, sub-greenschist conditions. The mismatch is irreversible on current published data.

Orkney requires systematic re-screening. Nairn/Elgin has a favourable low-maturity, thinned-margin structural setting but only a weak, basement-associated geochemical signal (Ba/Rb ~15–17 over <25 % genuine ORS); it is a structural and thermal possibility rather than a validated geochemical candidate, and is untested for detrital zircon. Detrital-mineral data (Clarke et al. 2024, 2026) give strong terrane-scale support for a Scottish source but do not, at present resolution, exclude the Midland Valley, for which comparative multi-proxy data have not yet been published.

After the original geochemical screening was refined with the independent thermal, clay and barium-type filters it anticipated, and the highest-priority Orcadian target was excluded on measured evidence, the same transparent multi-criteria framework has been applied to the Midland Valley. First-pass results identify strong Ba/Rb anomalies over near-continuous Lower ORS in the Strathmore belt (cluster 148: Ba/Rb 37, 94 % genuine ORS) that pass the identical multi-element consistency filter, together with suitable fluvial sandy facies and documented aluminous clay parageneses (tosudite, kaolinite). The principal open constraint is thermal: the strong-anomaly Lower ORS is itself relatively mature (~1.2 % R₀), so the decisive screening criterion is whether least-mature ORS (≤0.7–1.0 % R₀) can be identified within the same belt from published maturity data. These are the leading candidates the expanded screen identifies; they can be judged against clay-mineralogical and maturity data as and when those are available.

Click to enlarge

1. The original geochemical screening was sound

The UK-wide composite stream-sediment screen correctly identified a high-amplitude Ba/Rb anomaly over near-continuous genuine Old Red Sandstone bedrock in East Caithness cluster 15 (~170 cells, ~42.5 km² along the Lybster–Clyth–Ulbster–Sarclet coast). The anomaly passed all multi-element consistency filters and showed no mafic or base-metal overprint. It converged with independent detrital-mineral evidence for a source within the crystalline basement that supplied the Orcadian Basin ORS. The method performed exactly as designed. (Cluster numbers in this document follow the UK-wide screen; East Caithness cluster 15 was numbered cluster 18 in the earlier Orcadian-basin-only screen.)

2. The Thermal-Maturity Objection to an East Caithness source

The screening paper anticipated a thermal-maturity objection at second hand (pending verification against the primary map). That verification has now been made against Hillier & Marshall (1992). The objection is stronger than the anticipation and is stated plainly here.

Hillier & Marshall (1992) mapped organic maturity across the Orcadian Basin from 248 spore-colour determinations and 182 vitrinite-reflectance measurements. Their high-maturity region of southern and central Caithness — vitrinite reflectance above 3 % R₀ and spore colours 10/11 (completely black) — extends along the coast from near Niandt in the south to just north of Wick (their Figs 2 and 3). Cluster 18 lies entirely within that envelope (cells span 58.30–58.36 °N).

Their Fig. 4 assigns measured reflectance directly to the subgroups that underlie the cluster: Lybster 3.1–3.7 % R₀, Hillhead 2.4–3.7 % R₀, Clyth 4.3–4.7 % R₀. On the Barker & Pawlewicz (1986) calibration used by the authors, reflectance above 3 % records maximum palaeotemperatures in excess of ~250 °C — the threshold of true metamorphism.

Precision is high at the scale of the data: direct measurements at named localities and type sections, with 10–20 % reproducibility at multi-sample localities and a spatial density of a handful of coastal sites across the ~7 km cluster coast. No plausible reading of measurement error converts 3–5 % R₀ into the sub-1 % R₀ values the Altar Stone requires.

The Altar Stone records much gentler heating, and its clay mineralogy is diagnostic. In the <2 µm fraction (Bevins et al. 2024, Table 4) it carries mixed-layer illite/smectite (26–33 %), tosudite (15–21 %), kaolinite (16–25 %), dioctahedral chlorite (12–13 %) and illite (14–19 %) — an aluminous, incompletely illitised, low-grade assemblage. Bevins et al.’s own Caithness Flagstone Group samples, from the same >3 % R₀ terrain, show the opposite: essentially pure illite with trioctahedral chlorite and corrensite or smectite, and no mixed-layer I/S, no tosudite and no kaolinite — the fully illitised, trioctahedral clay mineralogy that temperatures above ~250 °C produce. The Altar Stone thus retains substantial expandable interstratified clay and delicate aluminous phases (tosudite, kaolinite) that cannot survive the temperatures measured at the cluster coast. Independently, its bulk mineralogy (Bevins et al. 2024, Table 3) shows K-feldspar not detected and baryte at 0.8–1.1 wt % — the exact combination (no Rb-bearing K-feldspar, discrete diagenetic baryte) that drives the anomalously high rock-level Ba/Rb the screen was built to detect. The assemblage reads as low-maturity and aluminous — Moray-Firth-like rather than Caithness-flagstone-like.

The mismatch is irreversible on present published evidence. The cluster core is therefore excluded as a direct source of the Altar Stone.

3. Status of remaining Orcadian targets

With the East Caithness cluster core excluded on measured thermal and clay-paragenetic data, three further areas inside the Orcadian Basin require assessment: Orkney, the Nairn/Elgin margin, and the fault-bounded Latheron–Buldoo coastal window.

Orkney Most of Orkney lies at ~1–1.5 % R₀ (Hillier & Marshall 1992) and is therefore thermally favourable for preservation of expandable illite/smectite and delicate phases such as tosudite. However, published stream-sediment barium data (BGS regional geochemical atlas, Map 4) show elevated values over parts of the Stromness, Rousay and Eday successions, with several higher anomalies — particularly in south-west Mainland — spatially associated with the East Scapa Fault system and known mineralised areas. These are more likely to represent hydrothermal or vein baryte (the wrong genetic type for the Altar Stone’s early disseminated cement + tosudite association) than low-temperature diagenetic baryte in a mature sandstone.

Existing published mineralogical sampling on Orkney (standing stones plus limited outcrops) is geographically and facies-restricted, so negative results from that material bear only on the sampled subset. Notably, Bevins et al.’s own Eday Group sample (5514) carries 49 % mixed-layer I/S and 41 % kaolinite (Table 4) — the closest Orcadian clay analogue to the Altar Stone in the published data, though it lacks tosudite and dioctahedral chlorite. At the screening level Orkney is therefore not excluded but under-characterised: the published stream-sediment barium and Hillier & Marshall (1992) maturity maps have not been jointly resolved to facies level, and on present data no locality-level statement about Orkney is warranted either way. It is carried as an open candidate.

Nairn/Elgin corridor  The Moray Firth margin is thermally attractive: Hillier & Marshall (1992) and Hillier & Clayton (1989) place it among the lowest-maturity ground in the basin, with preserved expandable illite/smectite — a genuine point in its favour. Its geochemistry, however, is weak. The stream-sediment clusters along this margin are dominated by Moine Supergroup and Grampian Group crystalline basement, with only 0–23 % genuine ORS in the anomaly cells and modest Ba/Rb ratios (~15–17; e.g. cluster 44 at 22.7 % genuine ORS, Ba/Rb 16.6) — below both East Caithness (cluster 15: 98 % ORS, Ba/Rb 18.2) and the Midland Valley anomalies below. Because these anomalies sit largely on basement rather than on ORS, the barium is as likely to be basement- or pegmatite-derived as diagenetic. Nairn/Elgin therefore does not pass the ‘elevated Ba/Rb over near-continuous genuine ORS’ test that distinguished cluster 15.

The sedimentary environment transitions into fluvial and aeolian sandy facies. These lower-pH, leached conditions are the setting associated with the authigenic kaolinite and early diagenetic baryte cement seen in the Altar Stone. The broader Elgin/Nairn region also contains well-known fish beds in lacustrine facies, but the relevant units for comparison are the sandy fluvial and aeolian facies, which are typically less fossiliferous and represent a different diagenetic environment; the fish-bearing horizons elsewhere in the corridor are a screening consideration rather than a disqualifier of the candidate itself.

On present evidence Nairn/Elgin meets the structural and thermal criteria but not the geochemical one, and is untested for detrital zircon and clay paragenesis. It is best treated as a low-maturity fallback that would re-enter as a geochemical candidate only if data were to show a genuine ORS-hosted (rather than basement-hosted) barium signal. It does not rank ahead of the Midland Valley anomalies.

Latheron–Buldoo coastal window  One Caithness possibility survives, rated low. Hillier & Marshall (1992) map a small fault-bounded window of lower maturity along the coast between Latheron and Niandt, in the Robbery Head and Latheron subgroups, at ~1.5 % R₀ and bounded by the Latheron fault — distinctly cooler than the >3 % R₀ cluster coast a few kilometres to the north-east. This window lies ~2 km south-west of, and outside, cluster 15, so it is not the geochemical anomaly itself; and at ~1.5 % R₀ (~180–190 °C on Barker & Pawlewicz 1986) it is probably still too hot to preserve the Altar Stone’s expandable I/S and tosudite. Two things nonetheless keep it in play at low probability. First, with the Middle-ORS assumption dropped, its Lower ORS stratigraphy is no longer disqualifying. Second, the maturity here is blocky and fault-controlled, so a further-downthrown, cooler sliver within the window cannot be excluded on present mapping. The Buldoo standing stone (OS ND 2000 3369; 3.87 m, the tallest in Caithness, a slab of local sandstone bedded into nearby outcrop) sits in this window and shows that large monoliths were locally available — suggestive of availability, though not evidence of source. Against a clay-XRD expandability measurement on a coastal Robbery Head/Latheron sample the window would be decisively confirmed or excluded; on the maturity data the expected result is marginal. It is carried as a low-probability open candidate.

4. The Clarke et al. (2024, 2026) constraint and its documented limits

Detrital-zircon, rutile and apatite data provide strong statistical support for an ultimate source terrane within the crystalline basement that supplied the Orcadian Basin ORS, and significantly reduce the likelihood of non-Scottish origins. The provenance-level conclusion (a Scottish, Laurentian-margin source) is robust and is not disputed here. At locality resolution, however, comparative multi-proxy data from Midland Valley ORS have not yet been published, and the concordant zircon population is modest (56 concordant grains from three thin sections). The distinction between an Orcadian and a Midland Valley source therefore rests more on the multi-proxy chemistry than on zircon ages alone, and is not resolved at the resolution required once the thermal and clay filters are applied. The Midland Valley is not excluded.

5. Decision to expand the search

The original geochemical screening was sound. The independent thermal, clay and barium-type filters it anticipated have now been applied. The highest-priority Orcadian target (East Caithness cluster 15 core) is excluded on measured primary data from Hillier & Marshall (1992). No other locality inside the Orcadian Basin currently satisfies the full set of criteria. Consistent with the original method, the same multi-criteria framework can be applied to the next major Scottish ORS basin, the Midland Valley, and the remainder of this document reports that first-pass screening.

6. Application of the original Ba/Rb + multi-element screen to the Midland Valley

The same country-scale Ba/Rb anomaly map and multi-element consistency filter used in the original screening paper has been applied to the Midland Valley by zooming into the relevant portion of the BGS G-BASE and regional geochemical atlas datasets.

Several zones over Old Red Sandstone show elevated Ba and high Ba/Rb ratios. When the full multi-element consistency filter is applied (quiet signature across control elements, no strong coincident mafic or base-metal overprint), the following genuine-ORS candidates emerge (cluster IDs from the UK screen):

• Strathmore / northern Midland Valley Lower ORS belt — the strongest and most continuous anomalies. Cluster 148 (Arbuthnott–Garvock Group, ~27 km², Brechin–Montrose area) returns Ba/Rb 37.0 over 94.5 % genuine ORS — a higher ratio than East Caithness cluster 15 (18.2) over comparably continuous ORS. Cluster 176 (Stratheden Group, Upper ORS, ~13 km²) returns Ba/Rb 32.3 over 81 % ORS. Smaller 100 %-ORS Strathmore Group clusters (156, 160) sit at Ba/Rb ~16–18. Multi-element signatures are relatively quiet.
• Selected Upper ORS sandstone catchments in the central and eastern Midland Valley that pass the quiet multi-element filter. Cornstone (calcrete)-bearing Upper ORS represents oxidising, alkaline, commonly red-bed conditions — the opposite of the leached, reduced, grey-green setting of the Altar Stone — so these units are retained only where reduced, non-red sandy facies can be identified, and at lower priority than the Strathmore belt.
• Weaker or partially noisy anomalies over ORS that are retained at lower priority.

Anomalies spatially associated with known hydrothermal vein systems or major fault corridors are deprioritised at this stage, exactly as analogous signals were treated in the Orcadian work.

7. Further screens applied to the surviving Midland Valley candidates

Thermal maturity — Midland Valley ORS maturity is highly variable (Marshall et al. 1994), and this is the constraint that must be applied honestly, because the same Hillier & Clayton (1989) standard used above to exclude East Caithness applies here: illite/smectite expandability collapses to 5–10 % expandable layers by ~1.0 % R₀. The deeply buried Lower ORS of the Strathmore syncline — which hosts the strongest anomalies — reaches ~1.2 % R₀ or higher (≈165 °C on Barker & Pawlewicz 1986), and is therefore already too mature to preserve the Altar Stone’s expandable mixed-layer illite/smectite, tosudite and kaolinite. It follows that the thermal target is not the strong-anomaly Lower ORS itself but the least-mature ORS in the basin (sub-1 %, ideally ≤0.7 % R₀), which tends to be the shallower or marginal sequences that escaped deep burial. This reproduces, within the Midland Valley, the anti-correlation already documented in the Orcadian Basin between strong barium anomalies (mature, deeply buried or mineralised ground) and the low-maturity diagenetic setting the Altar Stone requires. Locating Midland Valley ORS at ≤0.7–1.0 % R₀ is therefore a prerequisite, not an assumption — the first test any candidate must pass.

Facies suitability — the passing Strathmore candidates overlie fluvial (braided-river) and locally aeolian sandstones. These leached, lower-pH environments favour authigenic kaolinite and early diagenetic baryte cement, matching the Altar Stone’s paragenesis. Reduced, grey-green sandy units are the target; oxidised red cornstone facies are not. Clay-mineralogical studies of the Midland Valley Lower ORS independently record aluminous interstratified phases including tosudite and illite/smectite (Hillier, Wilson & Merriman 2006) — the closest documented facies analogue for the Altar Stone’s tosudite–kaolinite association, and a genuine advantage over the Caithness flagstone interior. Tosudite is not itself a low-temperature indicator, however, so the requirement remains tosudite together with preserved expandable mixed-layer I/S and kaolinite.

Barium type — Some anomalies in central eastern Scotland are interpreted in the broader geochemical literature as derived from barite within the Old Red Sandstone bedrock itself — consistent with a diagenetic/authigenic origin in sandstone. Anomalies directly linked to known hydrothermal veins remain lower priority pending petrographic confirmation of early disseminated cement texture.

Resulting priority shortlist After the identical Ba/Rb + multi-element filter used in the original screening, followed by the same thermal, facies and barium-type filters applied to the Orcadian targets, the highest-priority areas are:

1. Strathmore Lower ORS belt (clusters 148 and 176, plus the adjacent 100 %-ORS Strathmore Group clusters 156 and 160), conditional on locating sub-1 % R₀ ground within it.
2. Reduced (non-red) sandy Upper ORS catchments away from known vein mineralisation, retained at lower priority pending facies confirmation.

These are the areas the screen flags for comparison against clay-mineralogical, maturity and petrographic data. The relevant discriminators, should such data be available or published, are illite/smectite expandability and tosudite (clay XRD) and early baryte cement texture and paragenesis (petrography).

8. Candidate comparison table

Candidate Area	Ba/Rb + Multi-element Screen	Thermal Maturity	Facies Suitability	Barium Type	Zircon Status	Overall Priority	Key Notes
East Caithness cluster 15 core	Pass	Fail (>3 % R₀, illite-only)	Lacustrine flagstones	Likely hydrothermal/vein	Match	Excluded	Direct measurements (Hillier & Marshall 1992); irreversible mismatch with Altar Stone paragenesis
Orkney (systematic)	Partial	Generally favourable	Heterogeneous	Mixed (fault-linked deprioritised)	Partial	Requires re-screen	Needs BGS Ba + maturity overlay; target low-maturity sandy facies away from major faults
Nairn/Elgin corridor	Weak (basement-associated)	Favourable (low maturity)	Fluvial/aeolian sands	Basement/pegmatite? (unproven)	Untested	Low (structural fallback)	Clusters on Moine/Grampian basement, 0–23 % genuine ORS, Ba/Rb ~15–17; structural/thermal only
Strathmore Lower ORS belt (cl. 148, 176)	Pass (cl.148 Ba/Rb 37, 94 % ORS)	Marginal (~1.2 % R₀; target sub-1 %)	Fluvial/aeolian sands; tosudite recorded	Bedrock-linked in places (unconfirmed)	Untested	High (new priority; thermal test pending)	Strongest new anomaly; must locate ≤0.7–1.0 % R₀ ORS within belt
Central/eastern MV Upper ORS	Partial	Variable	Fluvial; red cornstone facies excluded	Mixed	Untested	Medium	Retain only reduced non-red sandy facies away from veins
Latheron–Buldoo window (Orcadian)	Outside cluster (SW edge)	~1.5 % R₀ — borderline too hot	Robbery Head/Latheron ORS; local slabs (Buldoo)	n/a (not the anomaly)	Untested	Low (retained; clay test)	Fault-bounded low-maturity window SW of cluster; Middle-tier objection now moot; single XRD decisive

9. Conclusions

The original geochemical screening was sound. Application of the independent thermal, clay and barium-type filters it anticipated has excluded the highest-priority Orcadian target (East Caithness cluster 15 core) on measured primary data from Hillier & Marshall (1992). No other locality inside the Orcadian Basin currently satisfies the full criteria.

The search has therefore been expanded using the identical multi-criteria framework. First-pass results identify strong, genuine-ORS Ba/Rb anomalies in the Strathmore belt (clusters 148, 176) that pass the same multi-element filter and overlie facies-suitable, aluminous-clay-bearing Lower ORS. The principal open constraint is thermal maturity: the strong-anomaly Lower ORS is itself relatively mature (~1.2 % R₀), so the decisive screening criterion is whether least-mature (≤0.7–1.0 % R₀) ORS can be identified within the belt from published maturity data. Subject to that test, Strathmore is the strongest new target.

This is a screening exercise. It identifies where, on the criteria set out above, a source is and is not consistent with the published data; it does not propose fieldwork, sampling or laboratory work, and it claims no match. The candidates it lists can be judged against clay-mineralogical, maturity and petrographic data as and when such data are available or published, and the candidate list can be refined by higher-resolution overlay of thermal and mineralisation data where those exist.

Acknowledgements

S. Hillier is thanked for providing his paper. British Geological Survey stream-sediment and regional geochemical atlas data are acknowledged. All interpretations remain those of the author.

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