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).