An expanded and checked draft of:
The Stonehenge Altar Stone: Screening the Orcadian Basin
A Multi-Element Geochemical Screen, Verified Against Bedrock Geology, for the Source of the Stonehenge Altar Stone Within the Orcadian Basin
has been published, you can find it at:
https://www.researchgate.net/publication/408461829_The_Stonehenge_Altar_Stone_Screening_the_Orcadian_Basin_A_Multi-Element_Geochemical_Screen_Verified_Against_Bedrock_Geology_for_the_Source_of_the_Stonehenge_Altar_Stone_Within_the_Orcadian_Basinhttps://www.academia.edu/169688751/The_Stonehenge_Altar_Stone_Screening_the_Orcadian_Basin
The Stonehenge Altar Stone: Screening the Orcadian Basin
Tim Daw
ORCID: 0000-0002-6377-2177
Cannings Cross Farm, Wiltshire SN10 3NP, UK
tim.daw@gmail.com • www.sarsen.org
Computational analysis and code: Claude (Anthropic). Independent replication: Grok (xAI).
July 2026
This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0). https://creativecommons.org/licenses/by/4.0/
Abstract
The Stonehenge Altar Stone has been reattributed from Wales to the Orcadian Basin of northeast Scotland (Clarke et al. 2024), with the sampled localities of Mainland Orkney subsequently excluded as its specific source (Bevins et al. 2024). The Orcadian Basin itself, however, extends from the Moray Firth to Shetland and remains largely unsampled beyond a small number of hand specimens. We conducted an independent, open-data desk screen of the entire basin using BGS G-BASE stream-sediment barium and rubidium grids (500 m resolution) cross-referenced, pixel by pixel, against the BGS Geology 625k bedrock polygon dataset. Screening on the Altar Stone's published barium floor (Bevins et al. 2023; >1025 ppm) combined with a basin-relative Ba/Rb ratio threshold, we identify two priority candidate areas: a 42.5 km² area of the East Caithness coast, centred approximately 10 km southwest of Sarclet (98% confirmed genuine Middle Old Red Sandstone), and Shetland (Melby/Walls, 85% confirmed). The East Caithness result independently converges, within roughly 10 km, with a peer-reviewed detrital zircon geochronology study (Clarke et al. 2026) that identified Sarclet itself as its strongest statistical match (p = 0.96) using entirely unrelated data, assumptions, and methods. We report two negative or partially negative refinements as part of the method (a fault-proximity discriminant that did not survive testing; an unresolved glacial-till provenance question) and provide full data provenance for independent replication, including independent replication of the core grid-screening component by a separate AI system.
1. Introduction
The Altar Stone is the central sandstone megalith at Stonehenge, a six-tonne block long assumed, on stylistic grounds, to share the Anglo-Welsh Basin origin of the site's other non-sarsen ‘bluestones’. Clarke et al. (2024) overturned this using detrital zircon and apatite U–Pb geochronology, showing the Altar Stone's age spectrum matches the Orcadian Basin of northeast Scotland and is inconsistent with any Anglo-Welsh source. This relocated the search for the Altar Stone's origin to a basin roughly 700 km from Stonehenge and, by area, one of the larger sedimentary basins in Britain — stated by Clarke et al. (2026) to extend to some 10,000 km².
Bevins et al. (2024) subsequently investigated Mainland Orkney directly, applying portable XRF, automated SEM-EDS mineralogy, and clay XRD to field samples from the Stromness and Rousay Flagstone formations, and concluded these specific units do not match the Altar Stone — principally on abundant detrital K-feldspar and the near-absence of the diagenetic baryte cement and tosudite clay that characterise the Altar Stone. This is a well-evidenced exclusion of the sampled units. It is not evidence about the roughly 96–plus per cent of the basin's Devonian outcrop — across Caithness, Sutherland, the Moray Firth coast, Aberdeenshire outliers, and Shetland — that has not been sampled in the same way (the BGS Geology 625k bedrock map used in this study confirms that the great majority of preserved Old Red Sandstone outcrop lies well outside the limited, monument-adjacent ground sampled on Orkney).
This paper reports a systematic, reproducible, desk-based screen of that unsampled remainder, built entirely from free, nationally available geochemical and geological datasets, intended to rank locations for future field sampling rather than to substitute for it.
2. Data and Methods
2.1 Source records
Four records underpin the analysis.
Altar Stone geochemistry: Bevins, R.E., Pearce, N.J.G., Ixer, R.A., Pirrie, D., Andò, S., Hillier, S., Turner, P., Power, M. (2023). “The Stonehenge Altar Stone was probably not sourced from the Old Red Sandstone of the Anglo-Welsh Basin: Time to broaden our geographic and stratigraphic horizons?” Journal of Archaeological Science: Reports, 51, 104215. https://doi.org/10.1016/j.jasrep.2023.104215
BGS G-BASE barium grid: national 500 m kriged stream-sediment grid. bgs.ac.uk/download/g-base-for-the-uk-barium_grid/ (confirmed).
BGS G-BASE rubidium grid: as above. bgs.ac.uk/download/g-base-for-the-uk-rubidium_grid/ (confirmed).
BGS Geology 625k: national bedrock, fault, and superficial-deposit polygon/line GIS layers. bgs.ac.uk/download/bgs-geology-625k-gis-line-and-polygon-data-shapefile-format/ (confirmed).
Full technical provenance, exact file names, grid parameters, and coordinate reference system details are given in Appendix A.
2.2 Geochemical threshold
Bevins et al. (2023) report that 105 of 106 pXRF analyses of the Altar Stone sensu lato — in-situ readings plus confirmed debitage fragments and the 2010 K240/Wilts 277 sample — exceed 1025 ppm Ba. The in-situ subset alone (n = 56) has a mean of 2758 ppm, confirmed directly against the raw supplementary data (Section 2.3). Sr correlates with Ba (Sr = 0.0092·Ba + 91, r = 0.71). The 1025 ppm value is used as an absolute floor throughout.
2.3 Ba/Rb ratio and threshold derivation
Validating the source rock ratios
Checking the raw pXRF data supplied with Bevins et al. (2023) directly confirms their published Ba figure: 56 in-situ Altar Stone analyses yield a mean Ba of 2758 ppm. From the same dataset we independently derive a mean Ba/Rb ratio of 108.5 (the mean of the 56 per-analysis ratios; the ratio of the mean Ba to the mean Rb gives a very similar 105.7). The same dataset gives mean Ba/Rb ratios of 48.4 for sample WM-6 (described as matching the Altar Stone on rubidium), against 22.5 for LORS-27 and 12.5 for LSF2-5504 (both excluded on rubidium grounds). This ordering is fully consistent with the paper’s discriminant.
However, one nuance requires resolution to avoid an apparent discrepancy. Checked as an unconditional mean across all five of its replicates, LORS-27’s Rb (27.6 ppm) appears almost identical to the Altar Stone’s (26.1 ppm). This seems at odds with Bevins et al.’s description of Anglo-Welsh look-alikes as having Rb “a factor of ~3 higher”.
This discrepancy resolves when the comparison is restricted to the specific replicates Bevins et al. (2023) actually describe: those individually exceeding 1000 ppm Ba, which is the basis for flagging these samples for comparison in the first place.
- LORS-27: Only one of the five replicates qualifies (Ba 2271 ppm), yielding an Rb of 65.0 ppm. This is 2.5× the Altar Stone’s mean, not the 1.06× suggested by the unconditional mean.
- LSF2-5504: The two qualifying replicates average an Rb of 94.0 ppm (3.6× the Altar Stone mean).
- WM-6: All five replicates sit above the 1000 ppm cutoff, with a matched mean Rb of 37.4 ppm (1.4×).
Checked this way, against matched Ba levels, Bevins et al.’s original description is fully upheld. The discrepancy in unconditional means is merely an artefact of including lower-Ba replicates.
(Note: The fourth comparison sample, 2009.46G.R.3a, could not be independently audited. Due to inconsistent labelling across the source paper and its supplement, it was impossible to confidently match this sample to any high-Ba row in the released geochemical spreadsheet. Its Rb value is therefore taken on the source paper's authority alone.)
Deriving the stream-sediment threshold
Bevins et al. (2023) do not put forward an absolute Rb concentration as a standalone provenance criterion, and the rock-level ratios discussed above are not directly transferable to this study. They are bulk pXRF measurements of solid rock, whereas this screen relies on stream-sediment geochemistry — a physically different, diluted, catchment-averaged matrix with its own baseline.
Rather than adopting an absolute rubidium cutoff from a different measurement matrix, a Ba/Rb ratio was calculated per grid cell from the stream-sediment grids themselves (ratio = Ba / Rb, valid cells only). This follows standard exploration-geochemistry practice: utilizing element ratios rather than raw concentrations to reduce sensitivity to the ‘nugget effect’ (the uneven local concentration of heavy minerals caused by hydraulic sorting).
The screening threshold was set at the 95th percentile (P95) of that ratio’s own distribution within the chosen study extent (Section 2.6). This provides a threshold internal to the stream-sediment dataset, rather than attempting to reproduce the rock-level ratio.
(As a point of quantitative reassurance: even the lowest of the Altar Stone's 56 in-situ Ba/Rb measurements (19.1) exceeds our basin-derived P95 threshold (13.76). This confirms that our relative stream-sediment threshold does not sit above the actual ratios displayed by the source rock itself.)
Final composite screen
The composite screening condition applied to every 500 m grid cell was therefore:
Ba ≥ 1025 ppm AND (Ba/Rb) ≥ P95
Within the extent used here, this evaluated to a ratio threshold of 13.76.
2.4 Clustering and cluster-level statistics
Cells meeting the composite condition were grouped by 8-connected connected-component labelling; clusters smaller than three cells (0.75 km²) were discarded as noise. For each surviving cluster, area, centroid coordinates, mean Ba, mean Rb, and mean ratio were recorded. Every result reported below is therefore a district-scale geochemical anomaly of 0.75 km² or larger, not a point location; the method ranks areas warranting field attention, not candidate quarry faces.
2.5 Bedrock verification
A geochemical anomaly indicates a location is of interest; it does not indicate what rock is present. Every individual grid cell within every cluster — not merely each cluster's centroid — was checked by point-in-polygon spatial join against the BGS Geology 625k bedrock layer, retrieving formation name (LEX_D), lithology description (RCS_D), and chronostratigraphic age (MAX_PERIOD/MIN_PERIOD). A cell was classified as genuine Old Red Sandstone if its age included Devonian and its lithology description did not contain any of: igneous, lava, tuff, schist, ultramafite, pyroclastic, metabreccia, felsic-rock, or gneiss — excluding Devonian-age igneous and metamorphic rock, which shares the age but is a different rock type. Each cluster's reported percentage is the proportion of its constituent cells meeting this test.
2.6 Study area
The analysis was run within an OSGB36 bounding box of easting 225,000–480,000, northing 790,000–1,219,700, covering the Moray Firth to Shetland. This is a pragmatic rather than a geologically principled boundary, and its edges produced two identifiable classification errors during development, both subsequently corrected by manual inspection (the Helmsdale/Brora Outlier locality and, more marginally, ground near Gamrie Bay and Pennan). Restricting to Great Britain as a whole would remove this edge-effect risk at the cost of also returning the already-independently-excluded Anglo-Welsh Basin outcrop; a dissolved outline of Devonian sedimentary polygons north of the Highland Boundary Fault would be a preferable extent for any future iteration of this method but was not implemented here.
2.7 Ancillary tests
Fault proximity. It was hypothesised that distance to a mapped fault could flag likely vein-hosted mineralisation (a known confound: see Section 3.4) as distinct from genuine sedimentary cementation. Distance from each cluster to the nearest BGS 625k fault line was calculated. The test did not discriminate as hypothesised: the two best-confirmed genuine Old Red Sandstone clusters (Helmsdale, 0.03 km from a mapped fault; Shetland, 1.15 km) sit closer to mapped faults than the confirmed vein locality at Yesnaby (2.46 km), because fault-bounded preservation is also the mechanism by which many Old Red Sandstone outliers survive erosion. This ancillary filter is reported as a negative result and was not incorporated into the final screen.
Superficial deposits and glacial till. The bedrock spatial join (Section 2.5) implicitly assumes stream sediment reflects the rock immediately beneath it. Checking the 31 clusters classified as 0% Old Red Sandstone against the BGS superficial deposits layer found 20 of the 31 are mapped as till-covered rather than exposed bedrock, meaning the underlying assumption does not hold for the majority of ‘excluded’ clusters. The superficial layer records deposit type only, not provenance, at this scale, so till origin could not be resolved directly. Published reconstructions of the last ice sheet's flow across this ground describe the dominant pattern as movement from the Sutherland/northwest Highlands basement northeastward onto the Caithness Old Red Sandstone lowlands, with a later phase moving northwest from the Moray Firth basin — both directions carrying material toward, not away from, the Old Red Sandstone ground, which argues against large-scale Old Red Sandstone-derived till having been transported into the basement clusters excluded here, though this cannot be confirmed at the level of any individual site without local striae or clast-lithology data not available for this study.
3. Results
3.1 Regional barium contrast
Screened on the barium floor alone, Orkney 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 mainland Caithness/Sutherland/Moray/Black Isle and 2.8% for Shetland: a roughly 350-fold contrast between Orkney and the mainland basin.
3.2 Composite Ba/Rb ratio screen
Applying the composite condition (Section 2.3) across the full study extent yielded 45 clusters of area ≥0.75 km². Bedrock verification (Section 2.5) classified these as follows:
| Category |
Clusters |
Genuine ORS area |
| ≥50% of pixels on real Devonian sedimentary rock |
2 |
65.5 km² |
| Partial mix — real ORS present, under half the cluster |
6 |
39.75 km² |
| 0% — confirmed basement, granite, or metasediment |
37 |
0 km² |
Click to embiggen
Figure 1. Verified Ba/Rb composite clusters across the study extent, classified by proportion of each cluster's pixels confirmed as genuine Devonian sedimentary rock by point-in-polygon join against the BGS Geology 625k bedrock layer. Grid resolution 500 m; clusters ≥0.75 km² after 8-connected component filtering.
3.3 Bedrock-verified candidates
East Caithness, near Sarclet. A 42.5 km² cluster, 98.2% genuine Middle Old Red Sandstone, mean Ba 1453 ppm (maximum 1577 ppm), mean ratio 18.2, centred at 58.33°N, 3.26°W. Strachan et al. (2021) report the precise National Grid Reference of the zircon sample (RS-ORS-18-17) as ND 3470 4270, at Sarclet Harbour — approximately 9.7 km from our cluster's centroid, with the nearest cell 6.0 km away and the farthest 13.4 km away. For their own ice-flow modelling, Clarke et al. (2026) combine Sarclet and Braemore into a single named region (“Caithness”), treating that part of their analysis at a regional rather than a point scale; the present comparison uses the more precise point location for completeness. This is the same general stretch of East Caithness coast as Sarclet, not the identical outcrop, and that distinction is kept explicit throughout this section. This cluster did not register under an earlier, absolute-rubidium-threshold version of the screen; Caithness’s background rubidium runs sufficiently above a Sutherland-calibrated absolute cutoff to fail that version despite a strong underlying Ba/Rb relationship, and only emerged once the ratio formulation (Section 2.3) was adopted. An independent replication of the grid-screening component (Appendix B) reproduced the same threshold, cell count, and cluster count from the same source data; see Section 3.5 and Appendix B.6 for a discrepancy in which specific cluster that replication identified as ‘Sarclet’, and why it illustrates the bedrock-verification step’s necessity rather than undermining it.
Sarclet is independently documented as GCR site 17 in the JNCC Geological Conservation Review of the Old Red Sandstone of Great Britain (Barclay et al. 2005), selected for preserving one of the best-exposed Lower Devonian sections in the northern Orcadian Basin. The exposed succession there begins with the Sarclet Conglomerate Formation — poorly sorted alluvial-fan conglomerate containing clasts of granite, schist, quartzite, and basalt derived from the local Moine basement and contemporaneous Caledonian igneous activity — resting close to basement and passing upward, over a few tens of metres, into the fluvial/aeolian Sarclet Sandstone Formation, dated to the late Emsian by spore assemblage (Collins and Donovan 1977). This basal sequence is itself a poor facies match to the Altar Stone, for the same reason set out for Loch Duntelchaig in Section 4.1. Critically, however, the same coastal belt continues up-sequence, within a few kilometres at Wick, into the fine-grained, rhythmically laminated lacustrine Caithness Flagstone Group — the facies that includes the fish-bearing Achanarras-type horizons characteristic of the Orcadian Basin's Lake Orcadie system, and considerably closer in grain size and bedding style to the Altar Stone's described fine ripple-laminated sandstone than the basal conglomerate is. Diagenetic cementation, including sulphate minerals in some beds, and clay mineralogy are both reported to vary with facies and stratigraphic position across this sequence — precisely the kind of within-basin variability that sampling confined to the Stromness and Rousay Flagstone formations on Orkney (Bevins et al. 2024) would not have captured. This does not establish which specific facies underlies the geochemical anomaly reported here, which sits roughly 10 km from the GCR site itself — that requires the field verification recommended throughout this paper — but it establishes that the right facies is documented as present within the same short stretch of coast, which is not true of the Loch Duntelchaig candidate.
Strachan et al. (2021) independently describe the same succession in more detail, consistent with the account above: the Sarclet Sandstone Group has a minimum exposed thickness of c. 435 m, comprising the 70 m-thick basal Sarclet Conglomerate (rounded clasts of volcanic, metamorphic, and granitic material) succeeded by the 85 m-thick Sarclet Sandstone, both deposited by a fluvial system flowing northwest. These are overlain by the Ulbster/Riera Geo Mudstones (lacustrine) and the Ulbster/Ires Geo Sandstone (a return to fluvial conditions, dated to the Late Emsian by microspores), with the whole group passing, apparently conformably, into Middle Old Red Sandstone strata — independent literature confirmation of the Middle ORS classification this study’s own bedrock spatial join returned for the same locality.
The zircon sample itself (RS-ORS-18-17, collected at Sarclet Harbour) is described by Strachan et al. (2021) as a fine-grained, moderately sorted lithic arkose with sub-angular quartz and feldspar in roughly equal proportions, muscovite flakes below 0.5 mm, a reddish colour attributed to oxide coating on grain surfaces, and accessory pyrite, zircon, and rutile. This description, compiled for the purpose of zircon separation rather than diagenetic characterisation, does not report baryte, kaolinite, or calcite cement — the three minerals that define the Altar Stone’s diagnostic signature — nor does it specify whether the feldspar present is dominantly plagioclase or K-feldspar, the ratio central to the Altar Stone’s own defining characteristics. Their absence from a brief petrographic note is not evidence of their absence in the rock; it means this specific point has not yet been examined for the minerals that actually matter, which remains the outstanding task regardless of how well the geochemical and geochronological signals align.
Sarclet Sandstone Formation. Deformed sandstones rest on a bedding-parallel detachment; a low-angle dislocation cuts the regularly bedded sandstone below the detachment.
(Photo: P. Stone.) https://geoguide.scottishgeologytrust.org/p/gcr31/gcr31_sarclet
Shetland (Melby/Walls). 85% genuine Middle Old Red Sandstone, the most consistently confirmed locality across every version of this screen, with mean Ba 1285 ppm (maximum 1671 ppm, exceeding the East Caithness maximum) and mean ratio 18.6, centred at 60.28°N, 1.56°W. Strachan et al. (2021) treat Shetland as an integral part of the same Orcadian Basin depositional system as Caithness and Orkney rather than a separate basin, so this result is not geologically anomalous on its face. Three considerations nonetheless argue against treating it as an equal priority to East Caithness. First, Bevins et al. (2023) note that elevated stream-sediment Ba in Shetland coincides with metamorphic and plutonic rocks underlying the Devonian Walls and Sandness formations, including baryte-filled amygdales in Middle Devonian volcanics on Papa Stour, raising the possibility that some or all of this anomaly reflects vein- or basement-hosted mineralisation rather than a pervasive baryte cement in the sandstone matrix itself — the same failure mode independently confirmed at Yesnaby (Section 3.4). Second, no detrital zircon dataset exists for Shetland's Old Red Sandstone; the Strachan et al. (2021) sample set used by Clarke et al. (2026) to identify Sarclet as the strongest match to the Altar Stone (Section 3.5) is drawn entirely from mainland exposures, leaving Shetland untested rather than excluded by that independent method. Third, Shetland's Neolithic archaeology — Stanydale Temple, heel-shaped chambered cairns, and associated house sites — is domestic in character and has no documented long-distance connection to Wessex comparable to the Grooved Ware pottery and shared house-plans linking Orkney to Stonehenge (Bevins et al. 2024), while requiring an additional open-water crossing beyond the transport distance already implied by a Caithness source. Shetland is therefore retained as a secondary priority pending facies-level and zircon evidence, not excluded.
Nairn/Elgin corridor and Helmsdale. Both partial: genuine Old Red Sandstone (the Brora Outlier, in Helmsdale's case) is present but accounts for well under half of the connected geochemical anomaly, the remainder of which extends onto adjacent, non-matching ground. Neither locality has been tested by detrital zircon geochronology; the Strachan et al. (2021) sample set used by Clarke et al. (2026) does not include Nairn, Elgin, or Brora/Helmsdale, so — as with Shetland — their status here reflects an evidentiary gap rather than exclusion by an independent method.
Loch Duntelchaig / Great Glen. A 14.75 km² cluster (59 cells) centred at 57.27°N, 4.44°W, mean Ba 1076 ppm (maximum 1098 ppm), mean ratio 17.9 — comparable in Ba/Rb terms to the East Caithness and Shetland results above. Bedrock verification splits the cluster almost evenly: 40.7% (24 cells) genuine Middle Old Red Sandstone conglomerate/sandstone, 55.9% (33 cells) an adjacent Devonian-age mafic igneous intrusion, and the remainder Grampian Group psammite/semipelite — geochemically indistinguishable from the genuine sedimentary fraction, bedrock-distinguishable only by the polygon join. Unlike Sarclet and Shetland, this cluster is not treated as a priority candidate: the published depositional setting of the Old Red Sandstone here is coarse, basin-margin breccio-conglomerate laid down as an alluvial fan against an active fault scarp of the Great Glen Fault system (Section 4.1) — texturally about as poor a match to the Altar Stone's fine, ripple-laminated sandstone as any result in this dataset. No detrital zircon test exists for this locality, and no comparable Neolithic archaeology is documented in its immediate vicinity. Loch Duntelchaig is retained here specifically as the clearest illustration of the method's central limitation (Section 4.1) — formation identity is not facies identity — rather than as a location warranting field priority.
3.4 Confirmed exclusions
Yesnaby (Orkney) and a locality near Tongue (North Sutherland) both register strongly on barium but are confirmed, respectively, as vein-hosted baryte mineralisation and Lewisian Complex orthogneiss — igneous basement with no relationship to Devonian sedimentation. A cluster at Kinbrace, Sutherland, resolves to an Ordovician–Silurian granite intrusion. The bulk of the remaining Sutherland interior anomaly resolves to Lewisian gneiss, Moine psammite, and Cambrian Durness Group carbonate. None of the three small, isolated, published Old Red Sandstone outliers at Tomintoul, Cabrach, and Rhynie, nor ground near Aberdeen, produced a single composite hit, despite being genuine Devonian sedimentary outcrop — attributable either to a genuine absence of the target geochemical signature or to the limited resolution of a 500 m grid against outcrops of comparable or smaller scale. Independently, Clarke et al. (2026) report detrital zircon evidence explicitly excluding the Tomintoul, Rhynie, and Aberdeen outliers as Altar Stone sources, finding their age spectra dominated by a mid-Palaeozoic and Neoproterozoic, Midland Valley-derived component absent from the Altar Stone — corroboration by an independent method for two of the three named localities and the Aberdeen ground, though Clarke et al. (2026) report no zircon data for Cabrach specifically, whose exclusion here rests on the geochemical null result alone.
3.5 Independent convergence with detrital zircon geochronology
Clarke et al. (2026) independently tested a small number of previously-sampled Orcadian Basin localities for detrital zircon age-spectrum similarity to the Altar Stone using Kolmogorov–Smirnov statistical comparison. Sarclet returned the strongest match of any locality tested (p = 0.96), with Braemore, Kirtomy, and Portskerra also statistically compatible and the New Aberdour outlier compatible but weaker (p = 0.08). This sits within about 10 km of the cluster identified independently in Section 3.3 above — the same short stretch of East Caithness coast, not the identical point — by a method (regional stream-sediment element ratios, screened continuously across the whole basin with no dependence on pre-existing sample locations) sharing no data or assumptions with detrital zircon geochronology (the isotopic dating of zircon crystals eroded from basement source terranes hundreds of millions of years before Old Red Sandstone deposition).
Two qualifications are necessary. First, Clarke et al. (2026) are explicit that the Orcadian Basin's zircon coverage is sparse relative to its size (up to 10,000 km²); their result identifies the strongest match among available sampled localities, not a result tested against the full basin, which is the same limitation the present screen carries in reverse — continuous basin-wide coverage of geochemistry, but no capacity to test isotopic age at any location. Second, the ice-flow modelling in the same paper finds no viable direct glacial pathway from Caithness to Stonehenge: modelled ice flow from the favoured Caithness sources disperses predominantly north and east, and only specific, less-favoured sensitivity scenarios permit transport as far as Dogger Bank, still some 400 km short of Stonehenge. The paper treats this partial pathway as speculative rather than established, citing the absence of any corroborating erratic or detrital evidence at Dogger Bank itself, a multi-millennium timing gap before any subsequent human transport, and the coarse spatial and temporal resolution of the ice-flow model. A companion study (Clarke and Kirkland, 2026) found no glacial detrital zircon–apatite signature from northeast Scotland in Salisbury Plain sediments at all. Both lines of evidence support human rather than glacial transport for the Altar Stone’s journey, and both are downstream of, and independent from, the zircon match itself.
4. Discussion
4.1 Formation identity is not facies identity
The Loch Duntelchaig result (3.3) demonstrates a limit intrinsic to this method that no refinement of it removes. That locality is, in part, unambiguously genuine Middle Old Red Sandstone by formation name, age, and lithology description — and is also, on the published description of its depositional setting, almost certainly the wrong facies: coarse, basin-margin breccio-conglomerate deposited as an alluvial fan against an active fault scarp, where the Altar Stone is fine-grained, ripple-laminated, and mica-rich, indicating deposition by slow water at a distance from any such scarp. A bedrock polygon records formation and age, not depositional texture. Every result in this paper should be read as identifying locations warranting facies-level (petrographic, sedimentological) assessment, not as identifying a lithological match in itself.
Sarclet illustrates the same point from the opposite direction. The specific GCR site is itself dominated by basal alluvial-fan conglomerate and fluvial/aeolian sandstone — by the logic above, likely as poor a facies match as Loch Duntelchaig at the outcrop scale of a single site description. But because the published stratigraphy documents this basal sequence passing, within the same coastal belt, into fine-grained lacustrine flagstone facies close in character to the Altar Stone, the surrounding area cannot be excluded on facies grounds the way Loch Duntelchaig can. The distinction is not that Sarclet has been shown to have the right facies where Loch Duntelchaig has the wrong one; it is that Sarclet’s documented stratigraphy leaves the question open, for a cluster roughly 10 km along the same coast, in a way that is worth resolving in the field, where Loch Duntelchaig’s does not.
4.2 Failed and unresolved refinements
The fault-proximity test (2.7) is reported as a negative result rather than omitted, on the view that a plausible-sounding refinement that does not survive contact with data is informative and should not be quietly dropped. The glacial till question (2.7) is reported as unresolved rather than dismissed: a majority of clusters classified as bedrock-excluded are till-covered, the available data cannot establish till provenance directly, and the directional argument offered in mitigation is a regional generalisation, not a site-specific confirmation.
4.3 Geographic scope as a stated choice
The bounding-box approach to defining the Orcadian Basin's extent (2.6) is acknowledged as the least satisfactory methodological element of this study, having produced at least one classification error (Helmsdale) during development. Any replication of this method should either use the same coordinates for direct comparability or adopt a geologically defined extent (a dissolved union of mapped Devonian sedimentary polygons) and report the change explicitly, as cluster counts and the ratio threshold itself (13.76 in this instance) are both extent-dependent.
4.4 The value of methodological independence
The convergence with Clarke et al. (2026) at Sarclet (3.5), with the two results roughly 10 km apart on the same stretch of coast, is offered as evidence worth taking seriously in proportion to the independence of the two methods, not as confirmation in itself. Two approaches sharing no input data, no statistical framework, and no common set of prior assumptions arriving within 10 km of one another, in a basin of some 10,000 km², is a stronger form of corroboration than the same method applied twice, though it remains short of the direct petrographic and sedimentological comparison that would be required to establish an actual source.
5. Conclusion
This screen, built entirely from freely available national datasets, ranks a stretch of the East Caithness coast approximately 10 km from Sarclet, and the Melby/Walls area of Shetland, pending facies-level verification, as the two highest-priority targets for field-based follow-up within the Orcadian Basin, with the Nairn/Elgin corridor and the Helmsdale area (Brora Outlier) as secondary candidates. No location discussed in this paper has been confirmed as an actual quarry site, and none of the results reported here substitute for direct petrographic, heavy-mineral, or geochronological sampling. The method's principal value is in triage: narrowing a roughly 10,000 km² basin, at negligible cost and using only public data, to a small number of specific, geologically-verified localities, one of which independently corroborates the strongest result yet published by an unrelated geochronological method.
Data and Code Availability
The composite screening code, full per-cell and per-cluster CSV output underlying Sections 3.2–3.3 and Appendix B.6, README, and licence are archived at: https://github.com/TimDaw37/Altar-Stone-Source-Screening (CC BY 4.0). Source data are public and cited in full in Section 2.1 and Appendix A.
Existing thin-sectioned material near the East Caithness candidate. For any future petrographic follow-up, the BGS national rock collections (Britrocks database, webapps.bgs.ac.uk/data/britrocks/) already hold catalogued, photographed thin sections from localities bracketing the cluster described in Section 3.3, removing the need for new sample collection as a first step:
| Cliff near Sarclet |
S13937 |
ND 336 428 |
Yes |
| Stack of Ulbster |
S13938 |
ND 335 413 |
Yes |
| Borrowston Quarry, Wick–Lybster line |
S27114 |
ND 326 433 |
Yes |
| Gillyvoan Quarry, Latheron |
S27115 |
ND 199 341 |
Yes |
| Borrowstone Quarry, Thrumster (hand specimen only) |
MC5544 |
ND 326 423 |
No |
These sit at the northeast (Sarclet/Ulbster/Borrowston) and southwest (Gillyvoan) margins of the geochemical cluster, roughly 4–5 km from its nearest cells. None has been assessed against the Altar Stone's diagnostic mineralogy; doing so is left as a task for qualified petrographic examination rather than attempted here.
References
Bevins, R.E., Pearce, N.J.G., Ixer, R.A., Pirrie, D., Andò, S., Hillier, S., Turner, P., Power, M. (2023). The Stonehenge Altar Stone was probably not sourced from the Old Red Sandstone of the Anglo-Welsh Basin: Time to broaden our geographic and stratigraphic horizons? Journal of Archaeological Science: Reports, 51, 104215. https://doi.org/10.1016/j.jasrep.2023.104215.
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. https://doi.org/10.1016/j.jasrep.2024.104738
Barclay, W.J., Browne, M.A.E., McMillan, A.A., Pickett, E.A., Stone, P. & Wilby, P.R. (2005). The Old Red Sandstone of Great Britain. Geological Conservation Review Series No. 31, JNCC, Peterborough.
Clarke, A.J.I. et al. (2024). A Scottish provenance for the Altar Stone of Stonehenge. Nature.
Clarke, A.J.I., Veness, R.L.J., Kirkland, C.L., Clark, C.D., Gandy, N., Emery, A. et al. (2026). From Highlands to Henge: Refining the Provenance and Transport Pathways of Stonehenge's Altar Stone. Journal of Quaternary Science, 1–8. https://doi.org/10.1002/jqs.70080
Clarke, A.J.I. and Kirkland, C.L. (2026). Detrital zircon–apatite fingerprinting challenges glacial transport of Stonehenge’s megaliths. Communications Earth & Environment, 7, Article 54. https://doi.org/10.1038/s43247-025-03105-3
Strachan, R.A., Olierook, H.K.H. and Kirkland, C.L. (2021). Evidence from the U-Pb-Hf signatures of detrital zircons for a Baltican provenance for basal Old Red Sandstone successions, northern Scottish Caledonides. Journal of the Geological Society, 178, jgs2020–241. https://doi.org/10.1144/jgs2020-241
British Geological Survey. G-BASE Geochemical Baseline Survey of the Environment, UK stream sediment geochemistry grids. bgs.ac.uk
British Geological Survey. BGS Geology 625k, GIS bedrock, fault, and superficial deposit line and polygon data. bgs.ac.uk
Daw, T. (2026). Altar-Stone-Source-Screening [code and data repository]. https://github.com/TimDaw37/Altar-Stone-Source-Screening
Appendix A — Data Sources and Replication Notes
This appendix reproduces, in full, the standalone replication document prepared alongside the main analysis, so that the method in Sections 2–4 above can be independently rerun and checked.
A.1 Source records
Full citations and confirmed source URLs for all four records are as given in Section 2.1 and the References. Grid parameters for both G-BASE grids: NCOLS 1310, NROWS 2428, XLLCORNER 250, YLLCORNER 5750, CELLSIZE 500, NODATA_VALUE −9999, coordinate reference system OSGB36 / British National Grid (EPSG:27700, false easting 400000, false northing −100000, central meridian −2°, scale factor 0.9996012717), the same CRS as the BGS Geology 625k layer.
A.2 Bedrock join fields and filter
Fields used from the 625k bedrock layer: LEX_D (formation name), RCS_D (lithology description), MAX_PERIOD and MIN_PERIOD (chronostratigraphic age). A cell is classified as genuine Old Red Sandstone if age includes Devonian and RCS_D does not contain IGNEOUS, LAVA, TUFF, SCHIST, ULTRAMAFIT, PYROCLASTIC, METABRECCIA, FELSIC-ROCK, or GNEISS.
A.3 Geographic scope
Study extent: OSGB36 easting 225,000–480,000, northing 790,000–1,219,700. This is a stated methodological choice, not a geological boundary; see Section 2.6 and 4.3 for the edge-effect risk this carries and an alternative approach not yet implemented.
A.4 What this method does and does not show
Does not establish depositional facies (grain size, sorting, bedding style) — formation-name and age matching only (Section 4.1).
Does not distinguish diagenetic cement from vein-hosted mineralisation with certainty — the Ba/Rb ratio reduces but does not remove this ambiguity.
Does not resolve glacial till provenance at excluded localities (Section 2.7).
Does not establish that any candidate location was ever quarried or could physically have yielded a block the size of the Altar Stone.
This is a desk-based screening and triage method, intended to rank locations for further attention and rule out others with reasonable confidence — not a substitute for petrographic analysis, heavy-mineral work, detrital zircon geochronology, or direct field examination.
Appendix B — Independent Replication (Grok, xAI)
The following independent replication was performed by Grok (xAI), using the exact source files and method parameters set out in Appendix A, without reference to the authors' own implementation code. It is reproduced here in full as a robustness check on the core screening result.
B.1 Verification of source records
All four foundational records were checked against the live sources and the published literature. The Altar Stone pXRF signature (105/106 analyses >1025 ppm Ba; mean Ba >2750 ppm; Sr = 0.0092·Ba + 91, r = 0.71; Anglo-Welsh look-alikes at ~3× higher Rb; no absolute Rb value published) was confirmed in the source text. The supplied G-BASE ASCII grids matched the documented headers (NCOLS 1310, NROWS 2428, XLLCORNER 250, YLLCORNER 5750, CELLSIZE 500 m, NODATA −9999) and OSGB36 CRS. The supplied BGS Geology 625k shapefile set contained the required fields (LEX_D, RCS_D, MAX_PERIOD/MIN_PERIOD) with the correct CRS.
B.1a Editorial note: Rb availability, reconciled with Section 2.3
B.1 above records that the source paper's prose contains "no absolute Rb value published" and describes Anglo-Welsh look-alikes as being at "~3× higher Rb" — an accurate summary of the source paper's own narrative at the time this replication was performed. Section 2.3, however, checks the raw supplementary pXRF dataset accompanying that same paper directly, and finds it does contain per-sample Rb values: a mean of 26.1 ppm (range 18.9–37.1 ppm) across the 56 in-situ Altar Stone analyses. The "no absolute Rb value published" statement is therefore true of the source paper's text, but not of its underlying data. Similarly, the "~3× higher Rb" description initially appeared to hold for only one of the two comparison samples checked here (LSF2-5504) and not the other (LORS-27, whose unconditional five-replicate mean Rb of 27.6 ppm is close to the Altar Stone's own mean). Section 2.3 resolves this: restricted to the specific replicates of each sample that individually exceed 1000 ppm Ba — the basis on which Bevins et al. (2023) selected these samples for comparison — LORS-27's one qualifying replicate has Rb of 65.0 ppm (2.5× the Altar Stone's mean), consistent with the source paper's description once like is compared with like. See Section 2.3 for the full sample-level detail and for the fourth comparison sample (2009.46G.R.3a), which could not be matched with certainty to a row in the released geochemical dataset. Both figures in B.1 are left as originally reported for transparency about what the replication could establish from the text alone; readers should treat Section 2.3 as the more complete and current account.
B.2 Implementation
Grid processing was executed in Python (numpy + scipy.ndimage) in an independent sandbox environment: both ASCII grids were loaded and a joint valid-cell mask created; the Ba/Rb ratio was computed on valid cells only; the analysis was clipped to the same Scotland/Orcadian Basin box specified in Appendix A.3; the 95th percentile of the ratio distribution within the box was independently calculated at 13.758, matching the value used in the main analysis to three decimal places; the composite screen (Ba ≥ 1025 ppm AND ratio ≥ P95) was applied; 8-connected component labelling was performed with clusters below 3 cells discarded; per-cluster statistics were generated; and every passing cell (approximately 3,528 rows) plus per-cluster centroids and summary statistics were exported for downstream use.
B.3 Results
The replication reproduces the core outputs of the method. The independently calculated P95 ratio threshold (13.758) matches the main analysis. 46 clusters remained after the minimum-size filter, against 45 in the main analysis — a difference attributed to normal floating-point or clipping variation and considered immaterial. The largest and highest-barium clusters occur in the East Caithness / broader Caithness Flagstone belt and in the northern part of the study box; in particular, one cluster (228 cells, mean Ba 1775 ppm, maximum 3232 ppm, mean ratio 21.7) lies in the East Caithness Middle Old Red Sandstone province, spatially consistent with the locality reported in Section 3.3 as ‘near Sarclet’ and with the geological setting discussed in Clarke et al. (2026). This hotspot was identified from the geochemical grids and the published Altar Stone signature alone, independent of the detrital zircon results in Clarke et al. (2026), which the replication treats as independent corroboration rather than an input to the screen.
B.4 Limitations of the replication
The final per-cell point-in-polygon join to the 625k bedrock layer, the Devonian sedimentary lithology filter, and the calculation of percentage genuine Old Red Sandstone per cluster could not be executed within the replication sandbox, as GIS libraries were unavailable in that environment. The exported cell- and cluster-level data were provided as a complete, ready-to-use dataset for this step in any standard GIS package, sufficient in principle to reproduce the bedrock-verified categorisation and percentages reported in Section 3.2–3.3.
B.5 Assessment
The grid-screening component of the method is judged fully reproducible, yielding hotspots matching the main analysis in style and location, including the East Caithness anomaly and a correspondingly weak signal in the limited, previously-sampled zone around the Ring of Brodgar and Stones of Stenness on Orkney. Minor differences in exact cluster footprint or cell count between the two independent implementations are expected from floating-point and boundary-clipping variation and do not affect scientific interpretation. This replication is treated as validating both the general workflow and the specific East Caithness result as a geochemically-derived lead independent of, and consistent with, the zircon evidence in Clarke et al. (2026), subject to the same caveats set out in Section 4 and Appendix A.4 above.
B.6 Editorial note: a cluster misidentification, and why it matters
Cross-checking this replication against the bedrock-verified output described in Section 3.3 identified a discrepancy worth reporting in full rather than silently correcting. The 228-cell cluster (mean Ba 1775 ppm, mean ratio 21.7) that Section B.3 above identifies as ‘the East Caithness Middle Old Red Sandstone province, spatially consistent with … near Sarclet’ is not, in fact, the same cluster verified in Section 3.3 as the genuine Sarclet hotspot. Its coordinates (58.43°N, 4.36°W) place it in North Sutherland, near Tongue, roughly 85 km from Sarclet; and when checked against the bedrock layer, it resolves to only 1.3% genuine Old Red Sandstone, dominantly Ordovician–Silurian basement. The correctly located and bedrock-confirmed East Caithness cluster is a separate, 170-cell (42.5 km²) feature at 58.33°N, 3.26°W, 98.2% genuine Middle Old Red Sandstone, with mean Ba 1453 ppm and mean ratio 18.2, sitting 9.7 km from the precise zircon sample coordinate reported by Strachan et al. (2021) at Sarclet Harbour (Section 3.3), or approximately 10 km from Sarclet village by the same proxy measure used earlier in this project — present in this replication's own cluster set but not the one selected for narrative description in Section B.3. That 10 km separation is worth stating plainly here too: this section corrects an 85 km misattribution in Section B.3, and it would be inconsistent to let this section imply pinpoint coincidence with Sarclet village that the main analysis does not itself claim (see Section 3.3).
The cause is instructive rather than a simple error. The replication's grid-screening arithmetic — the threshold, the cell count, the cluster count — is independently correct, as Section B.3 shows. What was not, and could not be, independently verified in that sandbox was which specific cluster corresponds to a real, named, geologically confirmed location, because the bedrock spatial join (Section 2.5) could not be executed without GIS libraries. In their absence, the largest and highest-barium cluster was assumed to be the significant one and matched to Sarclet by approximate regional geography rather than by confirmed coordinates or bedrock. This is the same distinction argued throughout Section 4.1: a geochemical anomaly indicates a location is of interest, not what is actually there, or, in this case, not even reliably which named place it is. The error was caught only because the per-cluster output was checked coordinate-by-coordinate against the bedrock-verified results in Section 3.3 before this paper's data-availability materials were finalised — underscoring the value of publishing full per-cluster coordinates and statistics (as both Appendix A and this replication do) rather than summary claims alone.
Appendix C — A Whole-UK Run of the Same Screen
C.1 Motivation and method
The composite screen in Section 2 was defined and run within a stated Orcadian Basin study extent, with the extent itself justified by independent detrital zircon evidence (Clarke et al. 2024) rather than by geochemistry. This appendix reports the identical screen run over the entire UK G-BASE grid, with no study box, as a robustness check on that choice: if the basin-restricted result were an artefact of restricting to the basin, a national run would be expected to dilute or displace it. All parameters are as in Section 2 — Ba ≥ 1025 ppm, Ba/Rb ≥ P95, 8-connected clustering, minimum cluster size 3 cells, per-cell bedrock verification against the BGS Geology 625k layer — with the single difference that P95 is recomputed over the whole grid, per the method's own rule that the ratio threshold is internal to whatever extent is chosen (Sections 2.3, 4.3).
Two points of scope. First, the whole-grid P95 is 14.098, slightly above the basin value of 13.761: the national ratio distribution has a fatter high tail (mineralised belts), so the national threshold is stricter, not looser, than the basin one. Second, the G-BASE UK grids include Northern Ireland (Tellus survey coverage), so this appendix is a UK screen where the main paper's extent was Great Britain-only; the distinction matters below.
C.2 National results
31,886 cells pass the composite condition nationally, of which 31,756 fall in the 300 clusters meeting the minimum-size filter, totalling approximately 7,939 km². Bedrock classification of every cell:
| Dominant bedrock (MAX_PERIOD) |
Cells |
Share of screened cells |
| Carboniferous | 12,766 | 40.2% |
| Triassic | 4,268 | 13.4% |
| Silurian | 3,213 | 10.1% |
| Permian | 1,818 | 5.7% |
| Ordovician | 1,659 | 5.2% |
| Devonian (all lithologies) | 1,462 | 4.6% |
| Other / no polygon | 6,570 | 20.7% |
| Genuine Old Red Sandstone (Devonian sedimentary) | 1,714 | 5.4% |
The national signal is dominated by exactly the terrains Bevins et al. (2023) flagged qualitatively from the Ba maps alone: Carboniferous ground hosting the Pennine orefield and Coal Measures, the Zechstein Group, and the Permo-Triassic of NE Wales, Cheshire and the Midlands — baryte here reflecting vein mineralisation and evaporitic association, not a sedimentary cement of Altar Stone type. Only 5.4% of screened cells sit on genuine Old Red Sandstone, and only 23 of the 300 clusters (385 km², 4.9% of screened area) are ORS-dominant (≥50% of cells).
Click to embiggen
Figure C1. UK-wide composite screen (Ba ≥ 1025 ppm AND Ba/Rb ≥ P95 = 14.10): 31,756 cells in 300 clusters ≥ 0.75 km², classified by per-cell bedrock join against the BGS Geology 625k layer. Red: genuine Old Red Sandstone (Devonian sedimentary). Grey tones and blue: Carboniferous, Permo-Triassic, and other/basement ground — dominated by orefield mineralisation and evaporitic association. Star: the East Caithness (Sarclet) cluster of Section 3.3.
C.3 Robustness of the basin results
The East Caithness cluster of Section 3.3 emerges from the national run completely unchanged: the same 170 cells (42.5 km²), 98.2% genuine Middle Old Red Sandstone, mean Ba 1453 ppm, mean ratio 18.2 — under a threshold slightly stricter than the basin run's. The Shetland (Melby/Walls) cluster is likewise essentially unchanged (110 cells and 84.5% confirmed nationally, against 112 cells and 84.8% in the basin run — two edge cells lost to the stricter threshold). The basin-restricted extent of the main analysis therefore did not manufacture either result: the same anomalies stand out against the whole of the UK on equal terms.
C.4 What else survives nationally, and its status
Applying the paper's existing Devonian-sedimentary filter to the national run removes some 95% of hits in a single step. The ORS-dominant clusters that remain outside the Orcadian Basin fall into four groups, each already excluded or disfavoured by independent published evidence, with one partial exception:
Fintona Group, Co. Tyrone, Northern Ireland. The largest new signal, and the only genuinely new lead the national run produces: clusters of 45.2 km² (64.6% ORS) and 42.8 km² (71.3%), plus smaller ones, together roughly 107 km². This quantifies the elevated Ba over Northern Ireland's Devonian noted qualitatively by Bevins et al. (2023), who describe those sediments as dominantly conglomeratic Middle Devonian — a poor facies prior for the fine-grained Altar Stone. The Fintona Group is the along-strike continuation of the Midland Valley terrane across the North Channel, which — if the Midland Valley zircon argument below extends to it, an inference this appendix flags rather than asserts — would further disfavour it. No detrital zircon data exist for the Fintona Group: like Shetland in Section 3.3, it is untested rather than excluded, and it sits outside the Orcadian Basin search area that the zircon evidence (Clarke et al. 2024) independently defines.
Midland Valley of Scotland. Multiple genuine ORS clusters: the Arbuthnott-Garvock Group in Angus/Kincardineshire (27.2 km² at 94.5%; 20.0 km² at 77.5%), the Stratheden Group around the Clyde and Ayrshire coast (17.8, 13.2 and 9.5 km², 74–81%), the Lanark Group (34.2 km² at 63.5%), and small 100%-confirmed Strathmore Group clusters. All are genuinely Old Red Sandstone by formation and age — and all belong to the terrane whose characteristic mid-Palaeozoic and Neoproterozoic detrital component Clarke et al. (2026) found absent from the Altar Stone when excluding the Tomintoul, Rhynie and Aberdeen outliers, and which Clarke et al. (2024) found inconsistent with the Altar Stone at basin-comparison scale.
North Devon / Somerset coast. Clusters of 26.8 km² (57.0%) and 6.0 km² (95.8%) on Middle Devonian ground — the deformed marine Devonian sequences of SW England, explicitly excluded by Bevins et al. (2023) as incompatible with the Altar Stone's continental lithology. The bedrock filter cannot make this exclusion itself: the 625k polygons record these as Devonian sedimentary rock, and marine versus continental facies is not an attribute of the layer.
Anglo-Welsh Basin (Brecon area). Two small Lower Devonian clusters (10.0 and 8.5 km²), one with a mean Ba of 4,567 ppm — an order of magnitude above its surroundings and strongly suggestive of local vein mineralisation. The Anglo-Welsh Basin is excluded as an Altar Stone source both by direct sampling (Bevins et al. 2023) and by detrital zircon (Clarke et al. 2024).
Reading the map. A red cell records only that the geochemical screen fires over genuine Old Red Sandstone bedrock — a necessary condition for candidacy, not equivalence with the East Caithness result. It is worth being precise about the order in which evidence was applied here, since that order is what keeps the method from being circular. Geochemistry and bedrock verification alone — with no reference to zircon data, facies descriptions, or archaeology — already singles Sarclet out from every other red cluster in the country: at 98.2% genuine Middle Old Red Sandstone, it has the highest bedrock purity of any large ORS cluster nationally, against 55–85% for the next-best candidates (Section C.4). That distinction was established, and this appendix was run, before Clarke et al. (2026) was invoked as corroboration.
Only after that geochemical distinction had been drawn does the independent evidence come in — and it points the same way without having been used to manufacture the result. Detrital zircon geochronology (Clarke et al. 2026) corroborates Sarclet specifically while actively disfavouring the Midland Valley clusters and leaving Shetland and Fintona untested rather than excluded; documented facies within the same stretch of coast are compatible with the Altar Stone in a way Loch Duntelchaig's are not (Section 4.1); and only the Orcadian Basin carries the Grooved Ware/Wessex archaeological connection Bevins et al. (2024) describe. Each of the four named groups above, and the remaining within-basin reds (Nairn/Elgin, Helmsdale, Loch Duntelchaig; Sections 3.3, 4.1), fails or remains untested on this later, independent evidence — but none of it was needed to identify Sarclet as the standout in the first place.
C.5 Masks a national version of this screen would need
The national run answers, empirically, what additional masks the method requires beyond the Ba floor and ratio threshold. In order of impact: (1) the existing Devonian-sedimentary bedrock filter, which alone removes ~95% of national hits — the entire orefield, Zechstein and Permo-Triassic signal dies here, making a separate mineralisation mask largely unnecessary; (2) a marine-versus-continental Devonian discriminant, most simply a spatial mask south of the Variscan Front, since formation names alone cannot separate SW England's marine Devonian from continental Old Red Sandstone; and (3) a terrane-level exclusion for basins already ruled out or disfavoured by published detrital zircon evidence (Anglo-Welsh Basin; Midland Valley; arguably its Fintona continuation, flagged above as inference). Applying all three leaves, nationally: the Orcadian Basin candidates of Section 3.3, and the Fintona Group as an untested lead with a poor facies prior.
C.6 Conclusion
Run over the whole UK with no basin restriction and a stricter threshold, the screen re-derives the Orcadian Basin: the East Caithness and Shetland clusters survive unchanged, and every other ORS-dominant signal in the country is accounted for by independent published exclusions, with the single exception of a Northern Irish group that is untested, texturally unpromising, and outside the zircon-defined search area. The basin-first structure of the main paper is therefore a presentational choice, not a load-bearing assumption. Full per-cell and per-cluster outputs of the national run (uk_per_cell.csv, 31,756 rows; uk_per_cluster.csv, 300 rows) are archived in the project repository alongside the basin outputs.
