This is the final (so far) draft of the paper - also to be found 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_BasinSARSEN.ORG · ALTAR STONE SOURCING ENQUIRY
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.
Tim
Daw ORCID
0000-0002-6377-2177
Cannings Cross Farm, Wiltshire SN10 3NP, United Kingdom
tim.daw@gmail.com · www.sarsen.org
COMPUTATIONAL ANALYSIS & CODE Claude (Anthropic) INDEPENDENT REPLICATION Grok (xAI)
PUBLISHED July 2026
DOI
10.13140/RG.2.2.10365.12008 LICENCE
CC 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 Old Red Sandstone bedrock), 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. https://www.bgs.ac.uk/download/g-base-for-the-uk-barium_grid/
●
BGS G-BASE rubidium grid: as above. https://www.bgs.ac.uk/download/g-base-for-the-uk-rubidium_grid/
●
BGS Geology 625k: national bedrock,
fault, and superficial-deposit polygon/line GIS layers. https://www.bgs.ac.uk/download/bgs-geology-625k-gis-line-and-polygon-data-shapefile-format/
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.
This extreme
ratio is not a statistical curiosity but a direct consequence of the Altar
Stone's own diagenetic history. Bevins et al. (2023) describe the Altar Stone
as cemented by diagenetic baryte with a near-total absence of K-feldspar — the
principal host mineral, along with clay, for both potassium and rubidium in
sandstones generally; their own comparison dataset shows Rb tracking K-feldspar
content across samples in the same way. A rock cemented this way is, in effect,
pushed toward high Ba and low Rb simultaneously by the same diagenetic process,
which is what produces a ratio an order of magnitude above ordinary sedimentary
rock. Because baryte is chemically resistant to weathering (Bevins et al.
2024), the barium component of this signature does carry through into stream
sediment reasonably faithfully; but a stream-sediment sample integrates
material eroding across an entire catchment, not one bed, so even a catchment
partly draining Altar-Stone-type facies dilutes that signature with sediment
from every other, geochemically unremarkable rock unit in the same drainage
area. The ceiling a stream-sediment sample can reach is therefore set by how
much of a catchment's sediment supply one facies can ever represent, not by the
facies' own end-member chemistry — consistent with the basin's highest observed
stream-sediment ratio (39.2) sitting at roughly a third of the Altar Stone's
own mean.
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. The core grid-screening component of this method — grid loading, ratio
calculation, threshold derivation, and clustering — was independently
replicated by a separate AI system (Grok, xAI) using the same source data and
parameters, without access to the authors' own code (Appendix B).
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
|
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% confirmed
genuine Old Red Sandstone bedrock, mean Ba 1453 ppm (maximum 1577 ppm), mean
ratio 18.2, centred at 58.33°N, 3.26°W. The mapped formations across the
cluster belong to the Middle ORS Caithness Flagstone Group; that is stated here
as a fact about the bedrock, and about the facies the Altar Stone most closely
resembles, not as the Altar Stone’s own stratigraphic assignment, which is
unpublished (Section 4.1). 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.
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 — the up-sequence transition into the flagstone facies described above.
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.
(Photo:
P. Stone.) https://geoguide.scottishgeologytrust.org/p/gcr31/gcr31_sarclet
Shetland (Melby/Walls). 85% confirmed genuine Old Red Sandstone bedrock, 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 Old Red Sandstone conglomerate/sandstone (mapped as Middle ORS), 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.
A related limit must be stated plainly, because it constrains
how every bedrock label in this paper may be read. The Altar Stone itself
carries no published assignment to the Lower or Middle Old Red Sandstone: the
primary literature establishes an Orcadian Basin source terrane for its
detritus (Clarke et al. 2024, 2026) and a grey, fine-grained, K-feldspar-poor,
calcite- and baryte-cemented facies (Bevins et al. 2020, 2023, 2024), but not a
stratigraphic tier. The stone resembles the flagstone facies developed
principally in the Middle ORS, and this paper uses ‘Middle ORS’ only in that
sense — as a description of mapped bedrock and of the facies being sought. It
follows that the tier recorded by a bedrock polygon is not a discriminant in
either direction: a Middle ORS polygon does not imply a match (Loch
Duntelchaig, above), and a Lower ORS polygon does not imply a mismatch, since
detrital zircon spectra constrain the source terrane feeding a basin, not the
depositional unit sampled. Nor can the tier be recovered from the stone’s
composition: within the Orcadian Basin both Lower and Middle ORS are
plagioclase-dominant, so the Altar Stone’s near-absence of K-feldspar excludes
only the Upper ORS. Facies and composition, not stratigraphic tier, carry the
whole discriminating burden of this search.
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. As a direct test of this choice, Appendix
C reports the identical screen run over the entire UK with no study box: the
East Caithness and Shetland results survive unchanged under the slightly
stricter national threshold.
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:
|
Locality
|
BGS sample ID
|
NGR
|
Thin section
|
|
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.
Burollet, P.F., Byramjee, R. & Couppey, C. (1969) Contribution
à l'étude sédimentologique des terrains dévoniens du Nord-Est de l'Écosse.
Notes et Mémoires No. 9. Compagnie Française des Pétroles, Paris, 85 pp.
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
Hillier, S. (1993) Origin, diagenesis and mineralogy of
chlorite minerals in Devonian lacustrine mudrocks, Orcadian Basin, Scotland. Clays
and Clay Minerals, 41, 240–259.
Hillier, S. & Clayton, T. (1989) Illite/smectite
diagenesis in Devonian lacustrine mudrocks from northern Scotland and its
relationship to organic maturity indicators. Clay Minerals, 24, 181–196.
Hillier, S. & Marshall, J.E.A. (1992) Organic maturation,
thermal history and hydrocarbon generation in the Orcadian Basin, northern
Scotland. Journal of the Geological Society of London, 149, 491–502.
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.
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. https://www.bgs.ac.uk/download/g-base-for-the-uk-barium_grid/
https://www.bgs.ac.uk/download/g-base-for-the-uk-rubidium_grid/
British Geological Survey. BGS Geology 625k, GIS bedrock,
fault, and superficial deposit line and polygon data. https://www.bgs.ac.uk/download/bgs-geology-625k-gis-line-and-polygon-data-shapefile-format/
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.
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 of
106 analyses >1025 ppm Ba; mean Ba >2750 ppm across the in-situ subset;
Sr = 0.0092·Ba + 91, r = 0.71) 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.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. Forty-six 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. In particular, one cluster
comprises 170 cells (42.5 km²) with a mean Ba of 1452.96 ppm (maximum 1577.2
ppm) and a mean Ba/Rb ratio of 18.221. Its centroid lies at approximately
easting 326,097 m, northing 938,147 m (OSGB36), with the cluster bounding box
spanning easting 322,000–330,000 m and northing 934,500–941,500 m. This places
the anomaly in East Caithness, with the centroid approximately 9.7 km southwest
of the precise zircon sample location reported by Strachan et al. (2021) at
Sarclet Harbour (ND 3470 4270). The nearest cell in the cluster is ~6 km from
that point and the farthest ~13.4 km — all figures match those reported in
Section 3.3.
This hotspot was identified from the geochemical grids and
the published Altar Stone pXRF 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 Sections 3.2–3.3.
B.5 Assessment
The grid-screening component of the method is judged fully
reproducible: independent implementation reproduces the P95 threshold, the East
Caithness cluster's cell count, area, mean and maximum Ba, and mean ratio to
within rounding, and correctly identifies it as the same cluster reported in
Section 3.3. This replication validates both the general workflow and the
specific East Caithness result (170 cells, mean Ba 1453 ppm, mean ratio 18.2,
~9.7 km from the Sarclet Harbour zircon sample) 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.
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, any period column)
|
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).
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 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 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.
Appendix
E
A multi-element consistency check on
the East Caithness cluster
Purpose. The national screen (Appendix C) selects on a single, geochemically
unusual property of the Altar Stone: an extreme Ba/Rb ratio produced by
disseminated diagenetic baryte cement together with a near-total absence of
K-feldspar. A reasonable objection is that a screen tuned to one ratio might
select a locality that is anomalous in ways the Altar Stone is not. This
appendix tests that objection directly. It is a consistency check, not a
discriminant: for a suite of elements in which the Altar Stone is ordinary,
the retained cluster should also be ordinary. The test can weaken confidence in
a target (by revealing an unexpected anomaly) but cannot by itself confirm one.
Prediction (stated before the result). In the pXRF data of Bevins et al.
(2023; supplementary Table S1), the Altar Stone is not anomalous in Cr, V, Ni,
Zn or Pb — all sit within, or below, the range of ordinary Old Red Sandstone.
It is only mildly elevated in Zr, consistent with its heavy-mineral laminae. If
East Caithness cluster 18 is a plausible source region, its catchment
stream-sediment chemistry should therefore be unremarkable in every one of
these elements, with no signal of a mafic/ultramafic or mineralised provenance.
Method. The BGS G-BASE / TellusNI compiled UK stream-sediment grids (Everett et
al., 2019; kriged, 500 m cell, <150 µm fraction; Open Government Licence)
were sampled at each of the 170 cells of cluster 18, using the same OSGB grid
registration as the Ba and Rb layers underpinning the primary screen. For each
element we report the cluster mean and range, its position in the national distribution
(UK percentile) and within a Caithness–East Sutherland regional box (regional
percentile), and the mean kriging standard-deviation over the cluster as an
uncertainty flag. All per-cell values are released in cluster_cell_element_values.csv for reproducibility.
E.1
Cluster 18 across six elements
|
Element
|
Altar Stone (pXRF)
|
Cluster 18 mean (range), ppm
|
UK %ile
|
Regional %ile
|
Kriging σ
|
Reading
|
|
Cr
|
56 ± 14
|
54 (40–66)
|
P7
|
P39
|
±73
|
consistent
|
|
V
|
46 ± 10
|
70 (52–87)
|
P17
|
P44
|
±19
|
consistent
|
|
Ni
|
~61 †
|
34 (29–44)
|
P40
|
P71
|
±15
|
consistent
|
|
Zn
|
97 ± 24
|
100 (72–126)
|
P33
|
P83
|
±115
|
consistent
|
|
Pb
|
34 ± 12
|
31 (15–54)
|
P33
|
P79
|
±294
|
consistent
|
|
Zr
|
316 ± 184
|
632 (496–756)
|
P76
|
P37
|
±101
|
consistent (see note)
|
† Ni is not among the summary elements of Bevins et al. (2023);
the ~61 ppm figure is computed here directly from the mmc2.xlsx supplementary
data and includes moisture-affected 2021 light-element readings, so it is
low-confidence and carries no weight. Kriging σ is the mean over-cluster
standard deviation of the kriged surface, in ppm.
Result. The prediction holds. Five of the six elements sit at or below the
national median; the tight cell-to-cell ranges (e.g. Cr 40–66 ppm across 170
cells) indicate a coherent, low-variability signal rather than an averaging
artefact. There is no evidence of a mafic/ultramafic catchment (which would
raise Cr and V together) and no evidence of base-metal mineralisation (which
would raise Zn and Pb). Where the numerical agreement with the Altar Stone is
close (Zn 100 vs 97; Pb 31 vs 34; Cr 54 vs 56), this is reported as consistency,
not as a match: rock pXRF on a weathered surface and the <150 µm
stream-sediment fraction are different media, and exact agreement is partly
fortuitous.
The Zr exception. Zr is the one element above the national median (P76). This
is expected and non-diagnostic. Sandy Orcadian catchments concentrate detrital
zircon hydraulically, so the whole terrain runs Zr-rich in stream sediment —
within the regional box cluster 18 is actually below the median (P37). Zr is
also the one element in which the Altar Stone itself is mildly elevated, so the
direction is if anything favourable; but per the framing above, Zr is given no
evidential weight in either direction.
E.2
The check also separates the candidate clusters
Applied across the largest clusters returned by the national
screen, the same elements do a little more than pass cluster 18 — they
reproduce, from an independent data layer, the same ordering already reached on
bedrock grounds. Several high-Ba clusters that were deprioritised for
geological reasons also carry clear geochemical signals that are in tension
with the Altar Stone:
|
Cluster
|
Location / status
|
Ba mean
|
Cr
|
V
|
Ni
|
Zn
|
Zr
|
|
4
|
NW Sutherland —
rejected (basement)
|
1277
|
133
|
151
|
52
|
49
|
827
|
|
11
|
N Sutherland — rejected
(basement)
|
1775
|
117
|
159
|
42
|
76
|
748
|
|
28
|
Lairg/Shin — rejected
(basement)
|
1610
|
106
|
124
|
32
|
98
|
771
|
|
1
|
Shetland — rejected
(Zn–Pb mineral.)
|
1285
|
76
|
95
|
27
|
335
|
368
|
|
31
|
Nairn corridor —
rejected (bedrock)
|
1415
|
63
|
45
|
18
|
62
|
928
|
|
37
|
Elgin corridor —
rejected (bedrock)
|
1301
|
55
|
56
|
20
|
66
|
1020
|
|
18
|
East Caithness /
Sarclet — retained
|
1453
|
54
|
70
|
34
|
100
|
632
|
Values are cluster-mean ppm. Red numerals flag
values markedly above the Altar Stone and its ordinary-ORS comparators: a
coupled Cr–V mafic/basement signature in the Sutherland clusters (4, 11, 28),
and a Zn–Pb mineralisation signature at Shetland (cluster 1). Cluster 18
(green) is the only large cluster that is simultaneously top of the Ba/Rb
screen and quiet across all consistency elements.
The Sutherland clusters (4, 11, 28) show mean V of 124–159
ppm against the Altar Stone's 46 ppm, coupled with elevated Cr — the coherent
mafic/basement signature expected where the screen has caught cells draining
Moine metasediment or basic intrusions rather than clean ORS. This is the same
contamination already inferred from the 1:625k and 1:50k bedrock inspection,
now visible independently in two elements. Shetland (cluster 1) carries a Zn–Pb
signal (Zn 335 ppm) pointing to base-metal mineralisation in its catchments.
The Nairn and Elgin corridor clusters (31, 37) pass the chemistry cleanly; they
were set aside on bedrock grounds (Moine basement and fragmented ORS strips),
and this check neither resurrects nor further condemns them — it simply
confirms the chemistry was not the reason they failed. East Caithness is left
as the one large cluster that is both geochemically anomalous in the intended
sense (Ba/Rb) and unremarkable in every unintended sense.
E.3
Why the coastal strip looks different: a directed uranium and gravity check
Motivation. Much of the Caithness Old Red Sandstone is of high thermal
maturity — large parts of the flagstone sequence in both Caithness and Orkney
carry illite-only clay assemblages (Hillier & Clayton, 1989) — and that
maturity is attributed partly to contact heating from a concealed intrusion,
the ‘Caithness Granite’, inferred to underlie Caithness east of a
Dounreay–Berriedale line (Hillier & Marshall, 1992), a footprint that on
the plain reading includes cluster 18. The Altar Stone, by contrast, retains
smectite-bearing interstratified clays — an R1-ordered mixed-layer
illite/smectite together with tosudite (Bevins et al., 2024, Table 4) — which,
taken at face value, records less severe heating than the illite-only
assemblages typical of much of the Caithness Flagstone. Because clay maturation
is irreversible, this contrast is a potential exclusion argument against any
strongly heated part of the basin. Two independent data layers test whether
that granite roof is in fact shallow beneath the target: stream-sediment uranium,
because granite–Devonian contacts in this region are uranium-mineralised (e.g.
the Helmsdale granite contact), and Bouguer gravity, because granite is less
dense than crystalline basement and a thick, shallow pluton produces a closed
gravity low.
Uranium. The cluster-18 catchments are uranium-quiet: mean ~3.5 ppm in the G-BASE
kriged grid, regional percentile P29, with no sign of the granite-contact
enrichment visible where the screen caught granite-influenced ground elsewhere
(cluster 11, North Sutherland, ~11 ppm). The kriging uncertainty (σ ≈ ±11 ppm)
makes this a soft, percentile-level signal only — but it points against a
shallow granite roof directly beneath the target.
Gravity. Bouguer anomalies were computed directly from 646 BGS GB land-gravity
survey station observations across East Caithness (no interpolated or licensed
grid used). There is no gravity low beneath the target. The coastal strip sits
on a positive Bouguer high: mean 10.4 mGal within 10 km of the cluster-18
centroid (n = 68 stations), 13.0 mGal at Sarclet (n = 43), 8.1 mGal at Lybster
(n = 59), with the anomaly rising across the East Caithness box from ~12 mGal
inland to ~18 mGal at the coastal edge. The signature a thick, shallow
concealed granite would impose is absent under the retained cluster; the
worst-case reading of the thermal objection — that the target sits directly
over the shallow roof of the heat source — is positively contradicted rather
than merely softened.
What this says about the coastal strip. The most natural reading of a coastal
Bouguer high is dense Caledonian basement at shallow depth beneath a thin Old
Red Sandstone cover: the coastal strip is geophysically distinct from the basin
interior because it is the structurally thinned margin of the basin. This is
consistent with the search logic of the main text — a marginal block whose
thinly bedded flagstones are kept continuously exposed, and detached as loose
blocks, by marine erosion — and it is the setting in which a relatively thin
cover succession is expected.
The caution that must accompany this. Present-day gravity constrains
present structure; it does not reconstruct burial history. Clay maturity
records maximum palaeotemperature, and uplift that post-dates peak burial does
not ‘un-cook’ the rock. These two checks therefore dispel the concealed-granite
worry and mark the coastal strip as structurally marginal; they are structural
corroboration, never a maturity proof, and they leave the clay-maturity
objection itself untouched — that is addressed directly next.
The direct clay evidence. The most detailed clay-mineralogical survey of the
Caithness Devonian — Burollet, Byramjee & Couppey (1969), with the X-ray
diffraction work carried out by J. Ferrero at the Compagnie Française des
Pétroles laboratory in Bordeaux — sampled the Middle Old Red Sandstone
flagstones locality by locality across East Caithness and found their clay
assemblage to be, in the authors’ own word, monotonous: roughly equal illite
and chlorite, with well-crystallised illite, and smectite (montmorillonite)
essentially absent from the Wick, Passage and Thurso groups (noted only as a
trace at the base of the Passage group at Papigoe and as a minor component in a
‘particular episode’ at Achanarras; at Castletown illite reaches ~100%).
Kaolinite is rare and confined to higher-energy, sandier facies. Where the
survey does record smectite and irregular illite–montmorillonite
interstratifieds alongside poorly-crystallised mica, it is not in the
flagstones at all but in the overlying Upper Old Red Sandstone (John O’Groats
and Dunnet Head), which Ferrero attributes to intensified surface weathering
under a warmer, wetter climate rather than to low burial temperature.
What the modern diagenetic framework adds. Two later syntheses put numbers and a
thermal framework onto Ferrero’s description and sharpen the comparison.
Hillier & Clayton (1989) show that where mixed-layer illite/smectite occurs
in the Caithness–Orkney flagstones it is of low expandability — 10–15%, and
only in the least-mature areas — falling to illite-only as maturity rises; at a
comparable maturity (~1% Rₒ) the Caithness–Orkney I/S carries roughly 5–10%
expandable layers against 20–25% in the Moray Firth. That is significant
because the one Orcadian field sample Bevins et al. (2024) found to resemble
the Altar Stone — an Eday Group sample of R1-ordered I/S with ~25%
expandability and abundant kaolinite — was likened by those authors to the
Cromarty Firth, i.e. the Moray Firth margin, and sits well above the entire
Caithness–Orkney flagstone field. On present evidence the Altar Stone’s clay
maturity looks Moray-Firth-like, not Caithness-flagstone-like. The tosudite is
a second, independent tension. Hillier, Wilson & Merriman (2006) document
tosudite (regularly interstratified dioctahedral chlorite/smectite) in the
Lower ORS of the Midland Valley (Strathmore Group), the Lower ORS of the Welsh
Borderland, and offshore in the Clair Field — and tie it to an aluminous,
kaolinite-bearing paragenesis (kaolinite → tosudite → sudoite); they do not
report it as characteristic of the onshore Caithness flagstones, whose
assemblage is trioctahedral chlorite + illite + dolomite. The Altar Stone’s own
assemblage (Bevins et al., 2024, Table 4: ~14–19% illite, 26–33% R1 illite/smectite,
15–21% tosudite, 12–13% dioctahedral chlorite, 16–25% kaolinite) is exactly
that aluminous, kaolinite–tosudite–dioctahedral-chlorite association —
mineralogically closer to a Lower-ORS or offshore-type signature than to the
Caithness flagstone interior. We state this plainly because it is the first
question a clay-mineralogical reviewer will raise, and the paper should be seen
to have anticipated it rather than to have overlooked it. Two honest
qualifications attach to it: the 2006 review is candid that provenance versus
diagenesis is unresolved (its two lead authors disagree in print in the same
paper), so ‘aluminous facies’ is a facies/diagenetic signature and not a fixed
geographic label; and Burollet’s coastal sampling in this sector centred on the
Lower ORS Sarclet horst, so the coastal Middle ORS block that is the actual
search target — the Lybster–Clyth Flagstone — has not, on the published record,
been clay-sampled directly.
Two axes, not one. The clay evidence carries two independent signals which the
discussion above keeps distinct. The first is maturity: R1-ordered
illite/smectite with retained expandability records moderate maximum
palaeotemperature — less severe heating than the illite-only assemblages of the
flagstone interior. The second is composition: kaolinite, tosudite, and
dioctahedral chlorite form an aluminous paragenesis (Hillier, Wilson &
Merriman, 2006) that the flagstone interior’s trioctahedral
chlorite–illite–dolomite assemblage does not develop at any maturity — a
less-buried block of ordinary flagstone would show more expandable I/S but
still no kaolinite and no tosudite. A viable source must therefore satisfy both
axes at once: an aluminous facies of moderate burial. Within the search area,
both axes point the same way — to the structurally marginal coastal strip that
the uranium and gravity evidence above independently identifies. Read this way,
the Altar Stone’s clay assemblage is not an anomaly to be explained but the
sharpest part of the target specification: it does not argue against East
Caithness in general, it specifies which East Caithness — and predicts
what a fresh sample from it must contain. It should be said plainly that no
published source reports tosudite from the Caithness flagstones: its presence
in the coastal target facies is a prediction of this framework, not a
datum, and the framework stands or falls with it.
Why this sharpens the target rather than settling it. The same framework that raises the
bar also supplies a coherent, testable way through, and it is the physically
grounded version of the ‘small area’ idea. In Hillier & Clayton’s (1989)
model, I/S expandability is governed by thermal history — burial depth and
residence time — region by region, not by a single basin-wide clock; the low
Caithness–Orkney expandabilities are attributed specifically to longer, deeper
burial. A structurally marginal, less-deeply-buried block would therefore be
expected to be less mature — more expandable, more Moray-Firth-like — than the
basin interior. That is precisely the block the uranium and gravity evidence
above argues the coastal strip to be: a thinned basin margin over shallow
basement. The clay data and the geophysics thus make a single joint prediction
rather than sitting in tension — the coastal target, if it is the source,
should be measurably less mature than the flagstone interior. The honest limit
on this is that Hillier & Clayton’s own Caithness points do not themselves
demonstrate a high-expandability window within Caithness (their lowest values
are ~10%, at the top of the succession); the marginal-block expectation is a
physically reasonable extrapolation beyond their sampling, not something their
data show, and it does not by itself deliver the tosudite. All of this reduces
to one field measurement whose result we can state in advance: an
illite/smectite expandability and full clay-assemblage determination on a fresh
coastal East Caithness (Lybster Flagstone) sample — ideally run alongside a
re-determination of the Altar Stone’s own expandability by the same method,
since the stone’s percent-expandable value is not itself published (Bevins et
al., 2024, Table 4 reports mineral proportions; the ‘moderate maturity’ reading
here is an inference from the R1 ordering and the kaolinite–tosudite
coexistence), so that the maturity comparison is like-for-like. Expandability
of ~25% or more together with tosudite would strongly support the target and
would place it, as the gravity predicts, at the marginal, less-mature edge of
the basin; expandability of ~5–10% in an illite–chlorite assemblage with no
tosudite would exclude it. Either way the outcome is decisive, and either way
it leaves the paper’s primary evidence untouched: neither the Ba/Rb screen nor
the detrital-zircon corroboration depends on the clay question. Burollet et al.
(1969) and the Hillier syntheses do not pre-empt that test; they tell us
exactly what it must measure.
E.4
Limits of this check
●
It
is negative evidence. A
pass removes an objection; it does not identify a source. Many localities
nationwide would also pass. High catchment Cr, V, Zn or Pb can disfavour a
cluster; low values can only fail to disqualify it.
●
Media
are not equivalent. The
Altar Stone figures are rock pXRF on weathered surfaces; the grids are kriged
<150 µm stream sediment integrating whole catchments. Comparisons are
percentile-level and directional, not analytical matches.
●
Kriging
uncertainty is severe for the mineralisation-prone elements. Mean over-cluster σ is ±294 ppm for Pb
and ±115 ppm for Zn — larger than the values themselves — so those two rows
support only percentile statements, never ppm comparisons. V and Ni are the
tightest (σ ≈ ±15–19 ppm); Cr, Zr and the national-scale nugget in Cr (±73 ppm)
sit in between. Relative and regional-context statements are robust; absolute
ppm claims are not.
●
Bulk
Cr does not test the detrital chrome-spinel content. Chrome spinel and chromite are reported
in the Altar Stone at trace level (of order 0.02–0.03 modal %, grains ~5–60 µm;
Bevins et al., 2020, 2023). A population this small contributes only of order
tens of ppm to bulk Cr — below the regional stream-sediment background and its
kriging noise — so a bulk-Cr pass is fully compatible with either the presence
or the absence of detrital chrome spinel in the source. Detrital heavy-mineral
abundance is a separate line of evidence, resolvable only by like-for-like automated
SEM-EDS heavy-mineral counts on East Caithness Middle ORS thin sections, and is
not addressed by this check.
E.5 Conclusion
Across the six elements in which the Altar Stone is not
anomalous, East Caithness cluster 18 is not anomalous either. The single-ratio
screen has not selected a locality that is unusual in unintended ways, and the
same elements independently reproduce the deprioritisation of the
mafic-influenced Sutherland clusters and the mineralised Shetland cluster
already reached on bedrock grounds. Stated at the level the data support: the
retained cluster is unremarkable in every element in which the Altar Stone is
unremarkable. This strengthens confidence in the screen's specificity
without being claimed as positive evidence of source. The directed uranium and
gravity checks (E.3) add structural corroboration from two further independent
layers: no concealed-granite signature underlies the target, and the coastal
strip has the geophysical character of a thinned basin margin — corroboration
of structure, not of maturity.
Data and code
Grids: BGS G-BASE / TellusNI compiled UK stream-sediment geochemistry (Cr, V,
Ni, Zn, Pb, Zr), kriged 500 m ASCII rasters and per-element kriging-uncertainty
layers, Open Government Licence. Outputs: cluster18_multielement_summary.csv
(per-element cluster statistics), per_cluster_multielement_means.csv
(cross-cluster contrast), cluster_cell_element_values.csv (all 3,545 screened
cells, six elements, for reproduction). Uranium and gravity: BGS G-BASE
kriged U stream-sediment grid (as above); BGS GB land-gravity survey station
observations (landgrav_csv, Open Government Licence), 646 East Caithness
stations, Bouguer statistics computed directly from station values.
