A simple test to see how effective copying Lidar images into AI engines could enhance them. Click any to enlarge them.
The prompt used was:
"Enhance this lidar view to bring out every detail you can, recolour in the yellow- brown spectrum"
"A transparent, multi-proxy desk screen—barium–rubidium
stream-sediment geochemistry (a proxy for the Altar Stone’s diagnostic baryte
cement and K-feldspar deficit), thermal-maturity mapping, clay mineralogy,
sedimentary facies, and detrital-zircon provenance—has been applied iteratively
across the UK Old Red Sandstone (ORS). It has narrowed a national-scale
problem, step by step and at the appropriate level of evidence, to a single
field-accessible target. The screen and Clarke et al.’s (2024, 2026) detrital-zircon
data agree that the source lies on the East Caithness coast; but the prime
barium cluster (Sarclet–Lybster–Clyth) sits in the high-maturity zone mapped by
Hillier & Marshall (1992), where vitrinite reflectance of 3–6% R₀
has driven the delicate expandable and aluminous clays of the Altar Stone past
preservation.
Two
independent lines then refine, rather than defeat, the result. The clay
evidence (Hillier & Clayton 1989; Hillier et al. 2006) shows the Altar
Stone’s tosudite–kaolinite–dioctahedral-chlorite assemblage to be an aluminous
sandstone diagenetic pathway. While classically expressed in the UK Lower ORS,
work on offshore Middle Devonian blocks (e.g., the Clair Group) demonstrates
that this signature is strictly facies- and fluid-controlled rather than
stratigraphically restricted, meaning it can be seamlessly accommodated within
porous sandstone bodies encased in lower-maturity segments of the Caithness
Flagstone Group.
One place is identified where such a rock could occur:"
I've included a photo of a megalithic monument that is there, but I need to just check it again before revealing the site.
The main problem is that the lack of data means that unsampled gaps in the record don't get filtered out, and that small areas are filtered out by being smeared in with the surrounding geology. Other data may highlight such areas, and suggest better screening. So such a desktop screening exercise can only suggest places worth further investigation, with a rock hammer.
Claims have been circulating that the idea of human transport for Stonehenge’s bluestones was “invented” by geologist Herbert H. Thomas around 1920–1923. The story goes that, in the aftermath of the First World War, there was a national need for a feel-good narrative about heroic, highly skilled ancestors — and Thomas obligingly supplied one, while ignoring or suppressing the glacial transport views supposedly held by his fellow geologists.
This version is itself a myth.
The debate did not begin with Thomas. In 1902, geologist J.W. Judd published his thoughts on the foreign stones at Stonehenge. Judd proposed they were glacial erratics — boulders carried by ice and left on Salisbury Plain. He was struck by the variety of rock types and especially by the abundance of bluestone fragments around the monument. This, he argued, suggested the stones had been worked and dressed on site from pre-existing glacial deposits rather than being laboriously brought from afar. Judd acknowledged challenges with the known limits of glaciation but suggested earlier, more extensive ice action could explain it.
H.H. Thomas’s 1923 paper (“The Source of the Stones of Stonehenge”, Antiquaries Journal) did what scientists are supposed to do: he reviewed the existing hypothesis, applied new petrographic analysis, and tested it against the geological evidence available at the time.
Thomas’s key points were straightforward:
Thomas did not invent human transport as a patriotic fable. He arrived at it by examining Judd’s hypothesis and finding the geological evidence against it stronger. This is normal scientific process — propose, test, refine or reject on the basis of data — not myth-making or morale-boosting.
Note on the 1921 discussion: In the discussion following Hawley’s interim excavation report (published 1921), two contributors — Mr. Dale and Rev. G.H. Engleheart — supported Judd’s glacial erratics view and mentioned possible striations on fragments. Neither was a professional geologist (Dale appears as a contributor to antiquarian/geological discussions of the period; Engleheart was a clergyman and local antiquary). Thomas, as Petrographer to the Geological Survey, was the qualified geological voice engaging directly with Judd’s ideas. Claims of Thomas ignoring a consensus of “fellow geologists” do not hold up.
The notion of a post-WWI patriotic conspiracy or deliberate ignoring of colleagues doesn’t hold up. Thomas engaged directly with Judd’s arguments in a scholarly journal. His work was published in a scholarly journal and focused on facts from rock samples and field geology, not morale-boosting narratives.
Of course, science moves on. Later researchers like Kellaway (1971) revisited glacial possibilities with new data, and the debate continues today with advanced geochemical fingerprinting, LiDAR, and field studies. Recent work has strengthened the case for human transport while refining exact source locations (e.g., Carn Goedog, Craig Rhos-y-felin).
But the 1923 paper was not myth-making. It was Thomas doing the unglamorous work of reviewing a prior hypothesis (Judd’s) and rejecting it on evidence.
The real myth is the one that turns a careful piece of geological reasoning into a conspiracy of patriotic invention. The evidence shows something much more ordinary — and more interesting: a scientist looking at the rocks and following where they led.
Abstract
The national barium–rubidium screen
and subsequent Orcadian prioritisation (Daw 2026) identified East Caithness
(the Sarclet–Lybster–Clyth flagstone coast) as the strongest candidate ground
for the Altar Stone, independently corroborated by the detrital-zircon match of
Clarke et al. (2024, 2026). The only other coastal belt within the Orcadian
Basin that returned any screen signal, and merited a stratigraphic check, is
the Nairn–Findhorn–Elgin margin on the southern shore of the Moray Firth. A
lithofacies audit of the published BGS mapping, memoirs and Geological
Conservation Review accounts confirms that this belt is a marginal expression
of the same Lake Orcadie system: the Middle Old Red Sandstone here (Inverness
and Black Isle Sandstone groups) is predominantly fluvial red sandstone
deposited along the lake margin, with subordinate lacustrine flag intervals,
and the Upper Old Red Sandstone (Nairn Sandstone Formation and the Elgin beds)
is a mixed fluvial–lacustrine, red-to-grey calcareous, cornstone-bearing
succession. Its fine lacustrine flag intervals are the same facies family as
the Caithness flagstones and carry the same Achanarras-assemblage fish. That
last point is decisive in an unexpected direction: because the East Caithness
lead is itself built on the fish-bearing quiet-water flag facies, the presence
of fish beds cannot be used to exclude the Moray margin. We therefore correct
the provisional “too many fossils” dismissal rather than endorse it. What
genuinely separates the Moray margin from East Caithness is not the fine facies
but its marginality and oxidation, the subordinate and heterogeneous
development of its flags (poor monolith potential), and above all its weak,
poorly bedrock-verified barium signal — the proxy for the Altar Stone’s diagnostic
baryte cement — against the strongest national signal at East Caithness. The
diagnostic baryte–tosudite assemblage is untested at Nairn, as it is still
under validation at East Caithness. The Nairn strand is therefore recorded as
deprioritised and untested rather than eliminated on lithofacies. The
Permian–Triassic New Red Sandstone of the coast (Hopeman and Burghead
sandstones) is excluded on age and requires no facies argument.
Clarke et al.
(2024, 2026) place the Altar Stone’s detrital-mineral source in the Orcadian
Basin and exclude the Midland Valley, the Anglo-Welsh Basin and Mainland
Orkney. The barium–rubidium stream-sediment screen of Daw (2026) ranked East
Caithness — the Sarclet–Lybster–Clyth flagstone coast — as the strongest
national hit, with independent zircon corroboration at Sarclet. Within the
Orcadian Basin the only other coastal belt that returned any screen signal was
the southern Moray Firth margin around Nairn, Findhorn and Elgin, although the
signal was weak and, as Section 5 notes, poorly bedrock-verified. This belt
exposes Middle and Upper Devonian rocks laid down in the southern, marginal
part of Lake Orcadie. It was provisionally set aside on the informal
observation that its fine facies “had too many fossils.” This note tests that
dismissal formally, and finds it must be replaced: the fossil criterion does
not survive scrutiny, but the belt is nonetheless a low-prior, untested
candidate for other, sounder reasons.
The audit
uses published sources only: BGS 1:50 000 and 1:625 000 digital mapping; the
BGS Earthwise accounts for the Devonian of the Grampian Highlands and the
Northern Highlands; the BGS memoir for Fortrose and eastern Inverness (Sheet
84W); and the Geological Conservation Review volume on the Old Red Sandstone of
Great Britain (including the Tynet Burn and related Moray fish-bed sites). No
new field data were collected. Lithofacies were evaluated against the Altar
Stone criteria established in the screening and mineralogical work (Bevins et
al. 2024; Clarke et al. 2024, 2026; Daw 2026): fine- to very fine-grained,
well-sorted sandstone; ripple or planar lamination indicating quiescent water;
grey-green colour; negligible detrital K-feldspar; pervasive baryte (with
calcite) cement; and a tosudite / aluminous-kaolinite clay assemblage. Two
method limits are carried throughout: stream-sediment values are not rock
values, and a barium anomaly is a proxy for baryte cement, not a measurement of
it; and formation identity is not facies identity.
Around the
southern Moray Firth the Middle ORS is represented by the Inverness and Black
Isle Sandstone groups, which BGS characterises as predominantly fluvial red
sandstone successions deposited along the lake margins — the marginal
counterpart of the deep-water Caithness Flagstone Group at the centre of the
basin. Within this dominantly marginal, sandier and more oxidised succession,
subordinate lacustrine flag intervals occur — the Inshes Flagstone, Nairnside
and Hillhead sandstones — as grey and purple flaggy micaceous sandstones and
dark calcareous flags with laminated shaly mudstones and limestone nodules.
These finer intervals carry the Achanarras (and post-Achanarras Eifelian) fish
assemblage, the same faunal marker that defines the lacustrine flagstones of
Caithness. The Hillhead Sandstone, with its post-Achanarras fish, is
unconformably overlain in the Ardersier–Cawdor area by the Nairn Sandstone
Formation (Section 3.2).
The Nairn
Sandstone Formation, the oldest Upper ORS unit of the district, comprises an
irregular basal reddish conglomerate overlain by red, grey and yellow
calcareous cross-bedded and flaggy sandstones with thin conglomerate beds and
soft limestone-bearing mudstones; it is a mixed fluvial–lacustrine sequence
carrying a Givetian fish assemblage. In the Findhorn area desiccated mudstones
(clay galls) and a calcrete horizon (the Cothall Limestone) are recorded. The
overlying Whitemire, Alves and Scaat Craig beds are grey-to-reddish siliceous
pebbly sandstones and fine conglomerates with marly intervals and cornstone
(calcrete) palaeosols, and the Upper ORS subdivisions here are themselves
defined on six successive fossil-fish assemblages (Asterolepis, Psammolepis,
Bothriolepis, Holoptychius and others). The succession records shallow-water
and periodically dry-bed conditions, with rapid lateral facies changes and
local overstep along the margins of fault-bounded sub-basins. Fine flaggy and
shaly intervals are present but are subordinate, laterally impersistent, and
interbedded with pebbly and pedogenically modified beds.
The Hopeman
and Burghead sandstones of the Hopeman–Burghead–Lossiemouth coast are
post-Devonian and rest unconformably on the Upper ORS. They are not a single
unit and are not both aeolian: the Hopeman Sandstone Formation is a Late
Permian to Early Triassic aeolian dune sandstone (the ‘Elgin Reptile’ beds,
with Chelichnus trackways), while the overlying Burghead Sandstone
Formation is Triassic and fluvial (waterlain), part of the New Red Sandstone.
Their historical confusion with the Old Red Sandstone — the very controversy
that the 1851 discovery of Leptopleuron (Telerpeton) at Spynie brought
to a head — is a caution rather than a candidacy: on age alone neither is
relevant to the Altar Stone, and no facies argument is required to set them
aside.
Assessed
against the Altar Stone benchmark, the Moray margin divides into points of
genuine similarity and points of genuine difference — with the single criterion
the provisional dismissal relied upon, fossil content, belonging to neither.
●
Grain size, sorting and structures. The fine, well-sorted,
ripple- and planar-laminated intervals of the Moray succession are the
lacustrine flag intervals of Section 3.1–3.2. Texturally these are comparable
to the East Caithness flagstones — they are the same quiet-water facies — but
here they are subordinate to marginal fluvial red sandstone and are laterally
impersistent.
●
Colour and composition. The succession is dominated by red, grey and
yellow calcareous sandstones with cornstone palaeosols; a clean, grey-green,
K-feldspar-poor sandstone of the Altar Stone type is not specifically reported.
This oxidised, pedogenically modified, marginal character is a real point of
difference from the reduced, deep-water grey flagstone facies — though the grey
lacustrine flags of Section 3.1 show the reduced facies is locally present.
●
Monolith potential. The fine intervals are thin, impersistent and
interbedded with coarser and nodule-rich beds — a poorer prospect for a
coherent monolith of the required dimensions than the thick, laterally
persistent Caithness flag sequences.
●
Diagnostic diagenesis (baryte, tosudite). No published clay or
cement data exist for the Nairn–Moray flags. On the screen, the belt returned
only a weak and poorly bedrock-verified barium signal — the proxy for the Altar
Stone’s pervasive baryte cement — in contrast to the strong, well-verified signal
at East Caithness. This is the most substantive point against the belt, and it
is a proxy, not a measurement.
The
provisional dismissal rested on the observation that every fine lacustrine
facies in the Moray belt is fish-bearing. That observation is correct but
non-discriminating, because the East Caithness ground on which the whole
enquiry rests is itself the fish-bearing flag system. The Lower Caithness
Flagstone Group is built from the Clyth and Lybster subgroups above the Sarclet
Group, with the Achanarras Fish Bed within it; the quiet-water fish-bed facies
of the Clyth and Lybster subgroups are locally carbonate-rich, approaching
dolomitic limestone; and the basin’s fish beds carry bituminous residues from
oil generation. The Achanarras assemblage that marks the Moray fish beds is the
same marker found at the Niandt Limestone of east Caithness, the Sandwick Fish
Bed of Orkney and the Cromarty and Edderton fish beds of Easter Ross. The fish
beds are the correlatable quiet-water phase of a single lacustrine system, not
a property that distinguishes one part of it from another.
It follows
that fossil content cannot exclude the Moray margin without also excluding the
preferred East Caithness lead — an argument that proves too much. The Altar
Stone is barren of macrofossils simply because it derives from the barren
sandstone phase of a depositional cycle rather than from the thin fish-bed
phase at the cycle base; both phases are present in every cycle, in Caithness
and in the Moray margin alike, and a monolith is by definition drawn from the
sandy phase. If anything, the presence of the Achanarras-assemblage flags on
the Moray margin is evidence that the correct lacustrine facies family is
developed there — a point of similarity, not the decisive difference the
provisional dismissal took it to be. The criterion is therefore withdrawn.
The honest
position is narrower than a lithofacies closure but is sufficient for the
enquiry’s purposes. The Nairn–southern Moray Firth belt is a marginal, more
oxidised, more heterogeneous expression of the same Lake Orcadie lacustrine
system that reaches its deep-water optimum in East Caithness. Its fine
lacustrine flags are the right facies family and cannot be excluded on facies
or fossils; what places the belt well below East Caithness is the combination
of a marginal and oxidised overall character, subordinate and impersistent flag
development with poor monolith potential, and — most substantively — a weak,
poorly bedrock-verified barium signal where the Altar Stone’s defining baryte
cement should produce a strong one. The diagnostic baryte–tosudite assemblage
is untested here, exactly as it remains under active validation at East
Caithness.
The belt is
therefore recorded as deprioritised and untested, not eliminated. What
would resolve it is the same test that will confirm or refute East Caithness:
direct sampling of a fine grey lacustrine flag interval — clay XRD for
tosudite, modal K-feldspar, baryte-cement habit and rock geochemistry —
benchmarked like-for-like against the Altar Stone. On present evidence the
prior is low and the enquiry’s effort is better spent on the stronger East
Caithness lead; but recording the Moray margin as a proven facies exclusion
would overstate the evidence, and would rest on an argument that also excludes
the lead.
A systematic
lithofacies audit of the Nairn–Findhorn–Elgin margin does not reproduce the
provisional “too many fossils” dismissal; it replaces it. The fine lacustrine
flag intervals of the belt are the same fish-bearing quiet-water facies as the
East Caithness flagstones, so fossil content cannot discriminate between them.
The belt is instead deprioritised on its marginal and oxidised character, its
subordinate and impersistent flag development, and its weak barium signal, with
the diagnostic clay and cement assemblage untested. This distinguishes the
Moray margin, within the enquiry’s ledger, from the Midland Valley and Orkney:
those were screened out; the Moray margin is present but
deprioritised and untested. With that distinction stated honestly, the
enquiry’s signal remains concentrated on the East Caithness flagstone coast —
the only ground where a strong, well-verified barium signal, a compatible and
thickly developed flag facies, and independent detrital-mineral geochronology
converge, and where the diagnostic baryte–tosudite assemblage is under active
validation. The next phase is detailed target refinement within that East
Caithness fairway.
Status
Nairn /
southern Moray Firth Devonian ORS strand: DEPRIORITISED and untested — a marginal expression
of the same lacustrine system as East Caithness; the fine flag facies is
present and shares the Achanarras fish assemblage, so it is not a facies or
fossil exclusion; deprioritised on marginality, poor monolith potential and a
weak barium signal, with the diagnostic baryte–tosudite clays untested. Not
eliminated. New Red Sandstone (Hopeman aeolian, Burghead fluvial) excluded on
age. Enquiry focus remains on East Caithness (Sarclet–Lybster–Clyth)
refinement.
Selected references
Bevins, R.E. et al.
(2024). Was the Stonehenge Altar Stone from Orkney? Journal of Archaeological
Science: Reports, 58, 104738.
British Geological
Survey. Devonian, Grampian Highlands; Middle Old Red Sandstone, Northern
Highlands of Scotland; Bedrock Geology UK North — the Old Red Sandstone
Supergroup. BGS Earthwise.
British Geological
Survey. Fortrose and eastern Inverness (Sheet 84W), memoir for the 1:50 000
geological map.
Clarke, A.J.I. et al.
(2024). A Scottish provenance for the Altar Stone of Stonehenge. Nature.
Clarke, A.J.I. et al.
(2026). From Highlands to Henge. Journal of Quaternary Science.
Daw, T. (2026). The
Stonehenge Altar Stone: Screening the Orcadian Basin. sarsen.org.
Dineley, D.L. &
Metcalf, S.J. (1999). Fossil Fishes of Great Britain. Geological Conservation
Review Series 16 (incl. Tynet Burn).
Trewin, N.H. &
Thirlwall, M.F. (2002). The Old Red Sandstone of Scotland (in The Geology of
Scotland, ed. Trewin).
Re-examining Orkney: a barium–rubidium and lithofacies screen of the Eday Group and outer islands as a candidate source for the Stonehenge Altar Stone
Third
in a sequence applying the Screening the Orcadian Basin method (Daw 2026) to
candidate ground beyond the primary study area
sarsen.org Altar Stone Sourcing Enquiry ·
working paper · 12 July 2026
Abstract
Bevins et al. (2024) excluded Mainland
Orkney as the source of the Stonehenge Altar Stone on the basis of a limited
set of Stromness- and Rousay-Flagstone field samples, leaving the outer islands
and the Eday Group untested. We re-examine Orkney comprehensively, combining
the barium–rubidium stream-sediment screen of Daw (2026) with a lithofacies
analysis of the Eday Group and a re-reading of the one published Eday
clay-mineral datum, and we test the specific fault-controlled high-barium
corridors on Sanday that a parallel desktop analysis proposed as candidate
ground. Orkney returns essentially no signal on the primary screen: a 0.03%
barium-floor hit rate against 10.8% for the mainland Orcadian outcrop — a
single cell, at Yesnaby, independently attributable to vein baryte. The
proposed Sanday corridors fall below both the barium floor and the ratio
threshold and do not register. On lithofacies, Sanday and the coarse Eday
Sandstone are disqualified: the only Altar-Stone-compatible facies, the Eday
Flags, thins northward from about 150 m at Deerness and South Ronaldsay to
roughly 10 m of flaggy sediment on Sanday and the Calf of Eday, where the
succession is dominated by coarse, pebbly, cross-bedded sandstone. The sole
facies-plausible residual — the Eday Flags of Deerness and northern South
Ronaldsay — is not a screen hit, shows no elevated barium consistent with the
Altar Stone’s diagnostic baryte cement, lacks tosudite (absent from every
Orkney sample measured), and remains gated on an untested Eday-Group detrital-zircon
comparison. We conclude that the screening process does not identify Orkney as
a candidate source. The result is independent of, and consistent with, the
Bevins mineralogical exclusion, and extends it to the outer-island gap that
direct sampling had not reached.
Clarke et al.
(2024, 2026) established, from detrital-zircon and apatite/rutile U–Pb
geochronology, that the Altar Stone’s detritus derives from the Orcadian Basin
of northern Scotland. Bevins et al. (2024) then tested Mainland Orkney
directly, using portable XRF, automated SEM-EDS mineralogy, and clay X-ray
diffraction on field samples of the Stromness and Rousay Flagstone formations,
and concluded those units do not match the Altar Stone — principally on their
abundant detrital K-feldspar, the absence of the pervasive diagenetic baryte
cement that characterises the stone, and the absence of its diagnostic tosudite
clay. That is a well-evidenced exclusion of the sampled units. It does not, by
itself, speak to the Orkney ground that was not sampled: the outer islands, and
in particular the Eday Group, which crops out most fully on Eday and Sanday and
only marginally on the Mainland localities Bevins examined.
This paper
closes that gap. It is the third in a sequence that applies the desk-based
barium–rubidium screen of Daw (2026) — developed to rank the Orcadian Basin,
and extended nationally in that paper’s Appendix C — to candidate ground the
primary study set aside. The first established East Caithness (near Sarclet,
and the Lybster–Clyth flagstone coast) as the strongest candidate,
independently corroborated by the Clarke zircon match. The second closed the
Midland Valley of Scotland. Here the method is turned back on Orkney, prompted
by a parallel desktop analysis that proposed a set of fault-controlled,
high-barium corridors on Sanday — in the Upper and Middle Eday Sandstone, near
the North Scapa Fault and the Cata Sand system — as a previously untested Orkney
target. We assess those corridors, and Orkney more generally, on the same two
axes the method rests on: stream-sediment geochemistry, and lithofacies.
The
geochemical screen is that of Daw (2026): a basin-relative composite condition
on the BGS G-BASE 500 m kriged stream-sediment grids — a barium floor (1025
ppm) combined with a barium/rubidium ratio threshold (the 95th percentile, ≈
14) — with each surviving cell bedrock-verified by point-in-polygon join
against the BGS Geology 625k map. Rubidium substitutes for potassium and so
tracks K-feldspar and mica; the ratio isolates the Altar Stone’s distinctive
combination of high barium (baryte cement) with a deficit of K-feldspar. Two
limits of the method are load-bearing here and are stated at the outset. First,
stream-sediment values are not rock values, and cannot be compared directly
against the Altar Stone’s measured rock geochemistry; the screen is calibrated
against stream-sediment thresholds only. Second, formation identity is not
facies identity: a cell on genuine Old Red Sandstone bedrock may still be the
wrong lithofacies, so a geochemical hit is a necessary but not sufficient
condition and must be read together with the sedimentology.
The
lithofacies analysis draws on the regional survey of Mykura (1976), after the
sedimentological work of Fannin (1970) and Ridgway (1974), which maps the Eday
Group and its internal thickness variation along the 58 km north–south outcrop.
The clay-mineral comparison uses Table 4 of Bevins et al. (2024); we re-read
the single Eday Group entry (sample 5514) directly from the published table
image, because a flattened transcription of that row misassigns its values
(Section 4.4).
On the barium
floor alone, the Orkney archipelago returns a 0.03% hit rate against valid grid
cells — a single cell, at Yesnaby, itself independently attributable in the
literature to vein-hosted baryte rather than diagenetic cement — against 10.8%
for the mainland Orcadian outcrop (Caithness, Sutherland, Moray, Black Isle)
and 2.8% for Shetland: a roughly 350-fold contrast between Orkney and the
mainland basin. Under the full composite condition, no Orkney cluster survives.
The Deerness area of East Mainland and the island of South Ronaldsay — which,
as Section 3.3 shows, carry the only Altar-Stone-relevant facies — both fall
within this near-zero population. There is no Orkney screen hit to rank.
The parallel
analysis reported its strongest Sanday barium pixels at roughly 670–675 ppm
with barium/rubidium ratios of about 8.5–8.9. Both figures sit below the
screen’s thresholds — the barium floor of 1025 ppm and the ratio threshold of
about 14 — by a clear margin. On the method’s own terms these are not
anomalies; they are ordinary background, and they do not register. The
corridors were interpreted as fault-controlled secondary baryte along the North
Scapa Fault trend, which is the same vein-baryte association that makes the
single Yesnaby cell a documented false positive rather than a candidate. We
note one mitigating caveat, developed in Section 4.3: Orkney is extensively
covered by blown sand and till, and its G-BASE coverage is sparser than the mainland,
so a stream-sediment null over Orkney is softer evidence than a null over open
mainland ground.
The Eday
Group is roughly 1,000 m of dominantly fluvial sandstone (Lower, Middle and
Upper Eday Sandstone), with red marls and two finer intervals; of its
formations, only the Eday Flags contain the lacustrine, finely laminated, grey
‘quiescent-water’ facies that resembles the fine, well-sorted, ripple-laminated
Altar Stone. Mykura (1976) maps the thickness of the Eday Flags along the whole
outcrop, and the gradient is monotonic: thick in the south, effectively gone in
the north.
The proposed
Sanday corridors sit in the bottom row, and in the coarse Middle/Upper Eday
Sandstone rather than the Eday Flags. On Sanday the Middle Eday Sandstone alone
reaches some 400 m of reddish-purple, trough-cross-bedded, pebbly gritty
sandstone, with conglomerates at Hegglie Ber — a fundamental lithofacies
mismatch to the Altar Stone. The one fine interval on the island, at roughly 10
m, is too thin and too sandy to source a coherent monolith of the required
dimensions, and is untested. Sanday, the Calf of Eday, and the fault-proximal
Eday ground (where, along the North Scapa Fault, the Flags horizon passes
entirely into sandstone) are therefore closed on facies.
The
lithofacies analysis leaves a single facies-plausible residual: the Eday Flags
where they are thick and best developed, at Deerness and northern South
Ronaldsay. This ground is not, however, a product of the screen — it carries no
elevated barium (Section 3.1). Since the Altar Stone’s defining diagnostic
feature is its pervasive baryte cement, which is precisely what generates a
high-barium signal, the absence of any barium anomaly over the fairway is at
best neutral and arguably mildly counter-indicative: it suggests the baryte
cement is not developed in these Eday Flags. Bevins et al. (2024) found no
diagenetic baryte in any Orkney sample, consistent with that reading.
The only
published Eday Group clay analysis is Bevins et al. (2024) sample 5514, from an
undivided Eday Group exposure at Bu 1 on southern Mainland — not from the
Sanday corridors, and not from the Deerness/South Ronaldsay fairway. Read
directly from the table, its <2 µm assemblage is about 10% illite, 49%
R1-ordered mixed-layer illite/smectite (expandability ~25%), and 41% kaolinite,
with no tosudite and no dioctahedral chlorite. Against the Altar Stone (illite
14–19%, dioctahedral chlorite 12–13%, tosudite 15–21%, R1 I/S 26–33%, kaolinite
16–25%), this is a partial match: it shares the kaolinite and the R1-ordered
illite/smectite — the closest any Orkney sample comes to the stone — but lacks
the tosudite and dioctahedral chlorite that are the Altar Stone’s diagnostic
phases. Tosudite is absent from every Orkney sample Bevins et al. measured.
It is worth
being explicit about why Orkney closes, because the parallel analysis reached a
superficially similar conclusion by arguments that do not hold. It compared a
Sanday stream-sediment barium/rubidium ratio (≈8.6) against an Altar Stone
rock-level ratio (≈105), an invalid cross-medium comparison that would equally
‘exclude’ the corroborated East Caithness lead (stream-sediment ratio 18.2); it
invoked a basin-wide thermal ceiling to argue tosudite could not survive, an
argument that would also exclude the Caithness flagstones, where tosudite is
likewise unreported; and it rested partly on a clay transcription that
misassigned sample 5514’s values (Section 4.4). None of those arguments is
needed, and none is sound. The defensible closure is simpler and independent of
them: Orkney produces no screen signal (Section 3.1–3.2), and the coarse Eday
facies that the barium corridors actually sample is the wrong rock, while the
only compatible facies has pinched out in the north (Section 3.3).
Intellectual
honesty requires that the Deerness/South Ronaldsay Eday Flags be recorded as a
residual rather than a closed case. It is the one Orkney ground that is the
right facies and carries a partial clay match. But it is a facies-and-clay
inference the geochemistry does not support, and it is gated on a
detrital-zircon question that has not been answered. The Eday Group is
stratigraphically higher than the Stromness/Rousay flags on which Clarke’s
Orcadian match was established, and it was fed from the south and south-west
carrying rhyolitic and volcanic detritus, with contemporaneous Middle-Devonian
volcanism at the base of the Eday Flags. Its provenance therefore cannot be
assumed to reproduce the Altar Stone’s signature merely because it lies within the
basin — the same reasoning that governed the Midland Valley Upper Old Red
Sandstone. A young or distinct volcanic zircon population, if present, would be
inconsistent with the Altar Stone’s youngest concordant grain of ~498 Ma.
Resolving whether any Eday Group detrital-zircon dataset exists, and how it
compares, is the one desk check that could either close this residual outright
or promote it to a field target; until then it is untested, not eliminated.
A
stream-sediment null over Orkney is weaker evidence than the same null over
open mainland ground. Orkney carries extensive blown sand and till — Sanday
especially, with the Cata Sand system — and its G-BASE sampling is
correspondingly sparser, so the grid may in places be sampling superficial
cover rather than bedrock. This is a genuine reason not to treat the Orkney
null as a hard exclusion. It is not, however, a reason to read the null as
encouragement: the absence of a barium signal remains consistent with the
mineralogical absence of baryte cement that Bevins et al. measured directly,
and the lithofacies closure of Sanday (Section 3.3) does not depend on the
geochemistry at all.
This
re-examination is independent of the Bevins exclusion — it uses different data
(national stream-sediment geochemistry and regional lithofacies rather than
local field mineralogy) — and reaches a consistent result by a different route.
Where Bevins et al. excluded the sampled Mainland flagstone units on
mineralogy, the present screen finds no Orkney signal at all, and the
lithofacies analysis accounts for the outer-island Eday Group that direct
sampling had not reached. The two lines together close the outer-island gap
that the original Orkney rejection had left open, with the single, explicitly
flagged residual of Section 4.2.
Applied
comprehensively to Orkney — including the previously untested outer islands and
the specific fault-controlled corridors proposed on Sanday — the screening
process does not identify a candidate source for the Altar Stone. Orkney is
effectively silent on the barium–rubidium screen (a single vein-baryte cell
against a 350-fold-higher mainland hit rate), the Sanday corridors fall below
threshold, and Sanday together with the coarse Eday Sandstone is disqualified
on lithofacies, the only Altar-Stone-compatible facies having thinned to a
marginal remnant in the north. The sole residual, the Eday Flags of Deerness
and northern South Ronaldsay, is facies-plausible but unsupported by the
geochemistry, without the Altar Stone’s baryte cement or tosudite, and gated on
an untested Eday-Group zircon comparison; it is recorded as untested rather
than eliminated. This is a negative result, and a useful one: it removes Orkney
from contention on the method’s own terms and, with the Anglo-Welsh Basin and
the Midland Valley, leaves the enquiry’s signal concentrated where the screen,
the lithofacies, and the independent zircon evidence agree — the East Caithness
coast of the mainland Orcadian Basin.
Selected references
Bevins, R.E. et al.
(2024). Was the Stonehenge Altar Stone from Orkney? Investigating the
mineralogy and geochemistry of Orcadian Old Red sandstones and Neolithic circle
monuments. Journal of Archaeological Science: Reports, 58, 104738.
Clarke, A.J.I. et al.
(2024). A Scottish provenance for the Altar Stone of Stonehenge. Nature.
Clarke, A.J.I. et al.
(2026). From Highlands to Henge: Refining the Provenance and Transport Pathways
of Stonehenge’s Altar Stone. Journal of Quaternary Science.
Daw, T. (2026). The
Stonehenge Altar Stone: Screening the Orcadian Basin. sarsen.org / repository.
Fannin, N.G.T. (1970).
The sedimentary environment of the Old Red Sandstone of western Orkney. PhD
thesis, University of Reading (unpublished).
Hillier, S., Wilson,
M.J. & Merriman, R.J. (2006). Clay mineralogy of the Old Red Sandstone and
Devonian sedimentary rocks of Wales, Scotland and England. Clay Minerals, 41,
433–471.
Mykura, W. (1976).
British Regional Geology: Orkney and Shetland. HMSO, Edinburgh.
Ridgway, J.M. (1974).
The Sedimentology and Palaeogeography of the Eday Group, Middle Old Red
Sandstone, Orkney. PhD thesis, University of London (unpublished).
Project record · Midland Valley strand closure · second in the sequence applying the barium–rubidium screen to candidate ground beyond the primary Orcadian study area
Clarke
et al. (2026) excluded the Midland Valley terrane on the basis of a
distinctive mid-Palaeozoic and Neoproterozoic detrital-zircon signature carried
by its Lower Old Red Sandstone successions — a signature the Altar Stone lacks.
A national barium–rubidium stream-sediment screen, however, also identified
clusters within the Upper Old Red Sandstone Stratheden Group on the Clyde and
Ayrshire coast. These lie stratigraphically above the measured Lower ORS
datasets and were therefore untested by Clarke. This paper applies a four-test
veto framework — direct zircon data search and statistical comparison if
available, source-terrane geometry and recycling assessment if no data exist,
facies/lithology comparison to the Altar Stone’s defining characteristics, and
clay mineralogy focused on the diagnostic tosudite phase — to close that gap
independently. No published detrital-zircon U–Pb dataset exists for the
Stratheden Group or the Upper ORS of the northern Midland Valley. The
source-terrane geometry and recycling analysis returns an unfavourable prior:
even the northern-flank clusters carry forward the forbidden grain populations
via demonstrable reworking of Lower ORS detritus. Lithofacies comparison
reveals a clear mismatch — the Stratheden Group is coarsening-upward,
conglomeratic, aeolian-influenced and locally volcaniclastic, whereas the Altar
Stone is fine-grained, well-sorted, ripple-laminated and non-volcaniclastic.
Clay mineralogy shows that the diagnostic tosudite phase of the Altar Stone is
documented in the Midland Valley only within the Lower ORS (Strathmore Group)
and is absent from the Stratheden Group. Three independent lines therefore
converge on exclusion of the Stratheden clusters at high confidence, although
the closure remains indirect because no zircon or clay measurement has been
performed directly on the Ba/Rb cluster ground itself. The Lower ORS clusters
are excluded by direct measurement. With the Midland Valley strand closed, the
enquiry returns to the Orcadian Basin for refined search within the East
Caithness candidate ground.
Clarke et al. (2024, 2026) established from detrital-zircon and
apatite/rutile U–Pb geochronology that the Altar Stone’s detritus derives from
the Orcadian Basin, and further showed that the Midland Valley of Scotland
carries a mid-Palaeozoic and Neoproterozoic zircon signature the Altar Stone
conspicuously lacks. Their methods statement records that the comparison
datasets “encompass all currently published detrital zircon U–Pb ages from
Scottish Old Red Sandstone successions.” That compilation rests on McKellar et
al. (2020, 2021) for the northern Midland Valley (Strathmore Basin) Lower
ORS and Phillips et al. (2009) for the southern (Lanark Basin) pre-ORS
Silurian. Both are Lower ORS or older; no Upper ORS data entered the
discriminant.
The
national barium–rubidium screen reported in Appendix C of Daw (2026)
nevertheless fired on genuine Old Red Sandstone clusters both inside and
outside the Orcadian Basin. Within the Midland Valley these resolved into two
stratigraphic populations: Lower ORS clusters (Arbuthnott–Garvock Group,
Strathmore Group, and the Silurian–basal Lanark ground) that sit squarely
inside Clarke’s measured material, and Upper ORS Stratheden Group clusters on
the Clyde and Ayrshire coast (17.8, 13.2 and 9.5 km² at 74–81 % bedrock purity)
that do not. The Lower ORS clusters are therefore already excluded by direct
measurement. The Stratheden Group clusters survived contact with the published
zircon evidence only because the evidence was never pointed at them.
To
decide whether the Stratheden Group clusters warranted further masking or
fieldwork, a four-test veto framework was applied in cheapest-and-most-lethal
order. Any one decisive failure shuts the strand; it remains open only if it
passes or ties on all four. The tests are: (A) search for any published
detrital-zircon U–Pb dataset for the Stratheden Group and, if grain-level ages
are recoverable, run a two-sample KS test against the 56 concordant Altar Stone
grains; (B) if no zircon data exist, assess source-terrane geometry and
recycling for the specific Ayrshire–Clyde clusters (Highland/Grampian versus
Southern Uplands routing, and whether Lower ORS detritus is reworked upward);
(C) compare Stratheden lithofacies directly against the Altar Stone’s defining
sedimentological characteristics; (D) test whether the diagnostic
tosudite/aluminous kaolinite clay assemblage is documented in the Stratheden
Group itself. The present paper executes those tests and records the closure.
It is the second in the sequence that applies the desk-based barium–rubidium
screen of Daw (2026) and its associated veto methods to candidate ground beyond
the primary Orcadian study area. The first established East Caithness
(Sarclet–Lybster–Clyth flagstone coast) as the strongest national candidate,
independently corroborated by Clarke’s zircon match at Sarclet. This paper
closes the Midland Valley strand. The third turns the same method back on
Orkney to close the outer-island and Eday Group gap left by Bevins et
al. (2024).
The
Stratheden Group was initially a legitimate lead for three reasons. Its
Famennian age is compatible with the Altar Stone’s youngest concordant detrital
zircon (~498 Ma Cambrian); the zircons impose no maximum depositional age that
forbids an Upper ORS host. The volcanic axis (Ochil–Sidlaw) that supplies
Clarke’s Midland Valley discriminant was extinct by Stratheden time, so the
direct arc input might not extend upward. And a clay-mineral observation
(Wilson 1971, reported in Hillier et al. 2006) appeared to point the right
way: the Upper ORS of the Midland Valley was described as mineralogically
similar to the Upper ORS of the Orcadian Basin and the Middle ORS around the
Moray Firth — the very ground already implicated by the Altar Stone’s aluminous
kaolinite–tosudite assemblage.
Set
against these considerations was a mechanistic prior drawn from source-terrane
geometry and palaeodrainage. The Altar Stone’s zircon load is Mesoproterozoic
and Archaean (Laurentian, overprinted by Grampian ~460 Ma magmatism) and lacks
both a pronounced Neoproterozoic population and a mid-Palaeozoic
(Ordovician–Silurian) one. Across Britain and Ireland the Upper ORS is
characterised by precisely those missing populations, recycled from Southern
Uplands–Longford–Down sources or introduced by contemporaneous volcanism. For
the documented Borders–Solway outcrop of the Stratheden Group, granitic clasts
and palaeocurrents indicate a Galloway Hills / Southern Uplands source and
internal drainage to the Jedburgh area — a southern routing opposite to the
Altar Stone’s northern, Grampian-overprinted character. Even for the
northern-flank (Ayrshire–Clyde) clusters that survived the Ba/Rb screen, the
Stratheden sits unconformably on older Midland Valley ORS and demonstrably
reworks it, so the forbidden grain populations can be carried forward
regardless of the extinct volcanic axis. The honest prior was therefore
unfavourable, but it remained a prediction from terrane models rather than a
measurement on the candidate ground itself. The project exists because a confident
stratigraphic prediction (“Middle ORS”) turned out to be unverified; the same
discipline requires that an unverified exclusion also be tested rather than
assumed.
This
paper therefore executes the four tests, documents each result, records one
clarification to the initial clay reading for the Stratheden lead, and hands
the enquiry back to the Orcadian Basin with every screened Midland Valley
candidate now accounted for.
The four-test veto framework applied here is as follows. Four tests
are applied in order; any decisive failure shuts the strand.
Test A — Direct zircon (if data exist). Trace
any recoverable Stratheden Group or northern Midland Valley Upper ORS U–Pb
detrital-zircon dataset. If grain-level ages are recoverable in the form
provided by Strachan et al. (2021) for Orcadian localities, run a
two-sample Kolmogorov–Smirnov test against the 56 concordant Altar Stone grains
exactly as performed for Sarclet/Braemore. Verdict rule: p > 0.05 and no
Neoproterozoic or mid-Palaeozoic mode → OPEN strongly. Any failure → SHUT
decisively. This test dominates all others if data can be found.
Test B — Source-terrane geometry (if no data). Establish the sediment routing of the Ayrshire–Clyde Stratheden
specifically: Highland/Grampian source versus Southern Uplands source, using
palaeocurrents, clast petrography and heavy-mineral data. Unavoidably
Southern-Uplands-sourced or demonstrably recycling-dominated → SHUT on
prediction (flagged as such). Genuinely ambiguous or northern-sourced without
recycling carry-over → stays OPEN pending sampling.
Test C — Facies / lithology. Compare the
Stratheden Group against the Altar Stone’s defining characteristics:
grey-green, fine-grained, well-sorted, K-feldspar-poor,
calcite/baryte-cemented, non-volcaniclastic, with quiescent-water ripple
lamination. Coarsening-upward, aeolian-influenced, conglomeratic or locally
volcaniclastic character → can SHUT the strand on mismatch alone.
Test D — Clay mineralogy. Check whether
tosudite and the associated aluminous kaolinite assemblage are documented in
the Stratheden Group specifically (distinct from the Lower ORS
Strathmore/Stonehaven Groups where Hillier et al. 2006 place it). Match on
the diagnostic phase → corroborates OPEN. Absence → weakly negative, not
decisive by itself.
Data sources comprise the published zircon compilations already used
by Clarke et al. (McKellar et al. 2020, 2021; Phillips et
al. 2009; Strachan et al. 2021), the BGS Geology 625k and 1:50k maps
for cluster verification and formation attribution, the regional
sedimentological and provenance syntheses of Bluck (1978, 1980), Paterson et
al. (1990) and Mykura (1991), the clay-mineral compilation of Hillier et
al. (2006) including the Wilson (1971) observations, and the BGS Earthwise
Old Red Sandstone reviews together with the relevant Midland Valley memoirs
(Greenock district and regional geology). The Ba/Rb screen thresholds and
cluster statistics are those of Daw (2026). No new field or laboratory data
were generated; the exercise is entirely a desk-based audit and synthesis of
existing published and archival sources.
Intellectual hygiene requires that exclusions be stated only at the
stratigraphic and geographic level the evidence actually supports, that
prediction and measurement be kept in separate columns, and that anything
asserted as closed must be closed by evidence pointed at the thing itself — not
at its neighbour, its parent unit, or its terrane label.
The
national screen flagged Lower ORS clusters within the Arbuthnott–Garvock Group
(94.5 % and 77.5 % bedrock purity), small Strathmore Group clusters, and the
Silurian–basal Lanark ground. These sit squarely inside the material Clarke et
al. characterised. McKellar et al. (2020) provide ten zircon samples
across a 9 km succession in the northern Midland Valley (Stonehaven,
Arbuthnott–Garvock and Strathmore Groups) with a proximal easterly /
SW-Baltican source. Phillips et al. (2009) cover the southern pre-ORS
Silurian of the Lanark Basin. Both datasets contain the diagnostic
mid-Palaeozoic (~490–420 Ma) and Neoproterozoic grain populations shed from the
Ochil–Sidlaw volcanic axis and proximal sources — populations the Altar Stone’s
56 concordant grains lack (youngest ~498 Ma). The Lower ORS Midland Valley
candidates are therefore excluded by direct measurement on the precise
stratigraphic units concerned. No further masking or fieldwork is required for
them.
Following
Clarke et al.’s own methods statement, an exhaustive search was made for any
published or archived U–Pb detrital-zircon dataset for the Stratheden Group or
for Upper ORS successions of the northern Midland Valley (Fife, Firth of Tay,
Ayrshire–Clyde coast). None was found. The gap is real; it is not an omission
by Clarke but a reflection of the published record at the time their
compilation was assembled. Consequently Test A cannot strongly open the strand
by direct statistical comparison, nor can it shut it on a failed KS test. The
result of Test A is therefore recorded as “no recoverable data exist.” The
strand proceeds to Test B.
The
mechanistic prior for the Stratheden Group as a whole is unfavourable. The
Altar Stone lacks both a pronounced Neoproterozoic (~1000–540 Ma) population
and a mid-Palaeozoic (Ordovician–Silurian, ~490–420 Ma) one. The Upper ORS
across Britain and Ireland characteristically carries both, either recycled
from Southern Uplands–Longford–Down volcanism and metasediments or introduced
by contemporaneous arc input. For the documented Borders–Solway outcrop, BGS
provenance work and clast petrography indicate granitic detritus from the
Galloway Hills / Southern Uplands and drainage into internal basins in the
Jedburgh area — a southern routing geometrically opposite to the Altar Stone’s
Grampian-overprinted northern signal. This geometry alone predicts the introduction
of the missing populations.
For the specific
northern-flank clusters (Clyde and Ayrshire coast) that survived the Ba/Rb
screen, the picture is not provenance-monolithic; a Highland/Grampian source
remains conceivable in principle and would remove the Neoproterozoic and
mid-Palaeozoic objection at a stroke. However, the escape hatch is closed by
recycling. The Stratheden Group sits unconformably on the older Midland Valley
ORS succession and demonstrably reworks it. The very mid-Palaeozoic and
Neoproterozoic grains that Clarke used as the Midland Valley discriminant can
therefore be carried forward into the Upper ORS regardless of whether the
Ochil–Sidlaw volcanic axis was extinct by Famennian time. No published
palaeocurrent, clast or heavy-mineral dataset for the Ayrshire–Clyde Stratheden
overturns this carry-over. Test B therefore returns an unfavourable verdict on
prediction, reinforced by demonstrated reworking. The strand is shut at this
point on the strength of source geometry and recycling; direct zircon
measurement on the cluster ground would be required to overturn it, and none
exists.
Even if
the zircon prior had been favourable, Test C would shut the strand on its own.
The Altar Stone is a fine-grained (dominantly fine sand), well-sorted,
grey-green, ripple-laminated sandstone with pervasive diagenetic baryte and
calcite cement and negligible detrital K-feldspar — a quiescent-water
lacustrine or marginal facies. The Stratheden Group (the lower unit of the
Scottish “Upper Old Red Sandstone”, Famennian) is a coarsening-upward sequence
of red sandstone and conglomerate with aeolian influence, trough cross-bedding,
pebbly gritty horizons and local volcaniclastic input. On the Clyde–Ayrshire
coast the Middle and Upper Eday-equivalent sandstones reach hundreds of metres
of reddish-purple, trough-cross-bedded, pebbly gritty sandstone with
conglomerates; the finer intervals are thin and sandy. None of this matches the
Altar Stone’s defining sedimentological characteristics. A clear facies mismatch
exists. Test C alone is sufficient to exclude the Stratheden clusters.
Re-examination
of the Wilson (1971) observations as synthesised in Hillier et al. (2006)
shows that the noted mineralogical similarity between the Upper ORS of the
Midland Valley and the Upper ORS of the Orcadian Basin / Middle ORS of the
Moray Firth is limited to the kaolinite + illite/smectite component. The
diagnostic tosudite (and associated aluminous phases) that characterise the
Altar Stone’s <2 µm assemblage are documented in the Midland Valley only
within the Lower ORS Strathmore Group; they are not reported from the
Stratheden Group or other Upper ORS successions. On the Altar Stone’s defining
clay phase, therefore, the proxy is a mismatch rather than a match. This
reading corrects an initial interpretation in the opening analysis of the
Stratheden lead, which had taken the Wilson (1971) similarity as more
favourable than the full published evidence supports. The similarity is real
but partial and does not extend to the diagnostic assemblage required by the
Altar Stone.
Convergence. Tests B, C and D each return negative independently and on
different axes (provenance/recycling, facies, clay). Test A could not strongly
open the strand because no data exist. No axis rescues the candidate. The
Stratheden Group clusters of the Clyde and Ayrshire coast are therefore closed
by convergent inference at high confidence. The closure is strong but remains
indirect: no detrital-zircon U–Pb or matched clay separation has been performed
on the specific Ba/Rb cluster ground itself.
The unscreened
Stratheden outcrops (Fife, Arran, Kintyre) produced no Ba/Rb hit and are
excluded by null result, subject to the same 500 m-resolution and
superficial-cover caveats noted for other null areas in the national screen.
The
temptation to state a clean universal negative (“nowhere in the Midland Valley
ORS could match the Altar Stone”) is resisted. That phrasing would repeat, in
the opposite direction, the error the enquiry was built to catch — a confident
claim outrunning the measurement that licenses it. What is actually supported
is more precise and remains defensible:
•
Lower ORS and Silurian
successions: excluded by direct measurement (Clarke et al.’s zircon signatures
obtained on those exact Groups).
•
Upper ORS Stratheden clusters
(Clyde/Ayrshire): excluded by convergent inference — a direct facies mismatch,
a recycling-reinforced unfavourable zircon prior, and a clay proxy that fails
on tosudite. High confidence, but no zircon or clay measurement performed on
the cluster ground itself.
•
Every screened Midland Valley
candidate is now accounted for; the unscreened Stratheden outcrops are excluded
by null Ba/Rb result.
What
would convert inferred to measured is straightforward: a single direct sample
from one Clyde/Ayrshire Stratheden cluster, with U–Pb detrital zircon run
against the 56 concordant Altar Stone grains (KS test) plus a matched clay
separation tested for tosudite. Nothing found in this investigation suggests
such measurement would reverse the verdict, but it has not been done and the
paper records that fact explicitly.
It
would be inaccurate to record that “Clarke was right without doing the full
analysis.” Clarke closed the terrane on Lower ORS data and, by their own
methods statement, had no Upper ORS zircon to test; the Stratheden Group was
the genuine gap in their reasoning, not something they had already closed. What
this strand supplied was the missing independent work — facies comparison,
recycling audit and clay re-reading — and arrived at the same terrane-level
verdict Clarke had assumed but could not yet support for the Upper ORS. The
correct record is therefore that Clarke reached the right answer for the
terrane on the data available to them; the work that makes the answer right for
the Upper ORS is work they had not done and that is now complete.
As
noted under Test D, an initial reading of the Wilson (1971) observations (as
synthesised in Hillier et al. 2006) had logged the mineralogical
similarity between Midland Valley Upper ORS and Orcadian/Moray Firth ORS as a
favourable clay signal for the Stratheden lead. That reading was too generous.
The similarity covers only the kaolinite + illite/smectite component; it does
not extend to tosudite, which in the Midland Valley is a Lower ORS Strathmore
phase and is not documented in the Upper ORS. On the Altar Stone’s diagnostic
phase the clay proxy is therefore a mismatch, not a match. The same
intellectual-hygiene rule applied throughout the enquiry — that conclusions
must not outrun the measurements that license them — requires this
clarification here.
“Closed”
commits the project only to the following: every screened Midland Valley
candidate is accounted for; no axis favours a match; the single residual
(direct measurement on cluster ground) has not been performed but is not
expected to alter the outcome on present evidence. No further field sampling or
laboratory allocation is warranted for the Midland Valley strand. Stated at
this register, the conclusion is reproducible and defensible in review.
The
Midland Valley episode illustrates the same discipline applied in reverse to
the Middle ORS episode that prompted the enquiry. In both cases an unverified
label (first “Middle ORS is established fact”, later “Midland Valley is ruled
out”) threatened to do more work than the measurements licensed. The rule is
identical in both directions: state exclusions at the level the data support
(Lower ORS: excluded by measurement; Stratheden: untested until this strand,
now closed by convergent inference), keep prediction and measurement in
separate columns, and require that anything asserted as closed be closed by
evidence pointed at the thing itself.
The
Midland Valley of Scotland is closed as a source for the Altar Stone. The
Silurian and Lower Old Red Sandstone successions are excluded by Clarke et
al.’s directly measured detrital-zircon signatures. The Upper Old Red Sandstone
Stratheden Group clusters of the Clyde and Ayrshire coast — the only Midland
Valley candidates that survived contact with the published zircon evidence —
are excluded by convergent inference from three independent lines: a clear
facies mismatch, an unfavourable zircon prior reinforced by demonstrated
Lower-ORS recycling, and failure of the clay proxy on the diagnostic tosudite
phase. Every screened candidate is accounted for. The Stratheden closure is
high-confidence but indirect; no zircon or clay measurement has been made on the
cluster ground itself, and that limitation is stated explicitly.
With
the Midland Valley strand closed, the enquiry returns to the Orcadian Basin for
the more detailed search within the East Caithness (Sarclet–Lybster–Clyth)
candidate ground that both the original barium–rubidium screen and Clarke’s
independent zircon corroboration identify as the strongest remaining lead.
Status: Midland Valley strand CLOSED. Lower ORS — excluded by measurement
(Clarke et al. 2024, 2026). Upper ORS Stratheden clusters — excluded by
convergent inference (facies + recycling-reinforced zircon prior + clay proxy),
high confidence; direct measurement on cluster ground not performed. Key
sources: Clarke et al. 2024, 2026; Daw 2026; McKellar et al. 2020,
2021; Phillips et al. 2009; Strachan et al. 2021; Hillier et
al. 2006 (Wilson 1971); Bluck 1978/1980; Paterson et al. 1990; Mykura
1991; BGS memoirs and Earthwise ORS reviews. Next: detailed Orcadian Basin
search within East Caithness candidate ground.
Bevins, R.E. et al. (2024). Was the Stonehenge Altar Stone from
Orkney? Investigating the mineralogy and geochemistry of Orcadian Old Red
sandstones and Neolithic circle monuments. Journal of Archaeological
Science: Reports, 58, 104738.
Bluck, B.J. (1978). Sedimentation in a late orogenic basin: the Old
Red Sandstone of the Midland Valley of Scotland. In: Tectonic evolution of
the Caledonides. Special Publication of the Geological Society of London.
Bluck, B.J. (1980). Evolution of a strike-slip fault-controlled
basin, Upper Old Red Sandstone, Scotland. In: Sedimentation at oblique-slip
margins. Special Publication of the International Association of
Sedimentologists.
Clarke, A.J.I. et al. (2024). A Scottish provenance for the
Altar Stone of Stonehenge. Nature.
Clarke, A.J.I. et al. (2026). From Highlands to Henge: Refining
the Provenance and Transport Pathways of Stonehenge’s Altar Stone. Journal
of Quaternary Science.
Daw, T. (2026). The Stonehenge Altar Stone: Screening the Orcadian
Basin. sarsen.org / repository. (Includes national Ba/Rb run in Appendix C.)
Hillier, S., Wilson, M.J. & Merriman, R.J. (2006). Clay
mineralogy of the Old Red Sandstone and Devonian sedimentary rocks of Wales,
Scotland and England. Clay Minerals, 41, 433–471.
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