Out, damned spot! Out, I say! — Macbeth, Act V, scene 1
Ciborowski and Nash’s reply to Pearce et al. has a second, quieter row in it, about Stone 62. It is not a new source for the stone. It is an argument about whether a geochemical similarity score is allowed to outrank looking at the rock.
The arithmetic-method dispute is covered in The Numbers Ain’t Enough and The Numbers Talk Back. This post is only the Stone 62 example, and only the spotting.
What Stone 62 is
Stone 62 is a shaped, unspotted dolerite pillar in the bluestone circle. Pearce, Bevins and Ixer (2022) analysed it by portable XRF, compared it with Preseli dolerite outcrops, and matched it petrographically to sample PGDF24 from Garn Ddu Fach. Carn Ddafad-las is the same intrusion and a looser chemical envelope. That paper also closed the Waun Mawn “key in a lock” claim: the Waun Mawn dolerites go to Cerrig Lladron, not to Stone 62.
So the published source, as of 2022, is an unspotted outcrop at the eastern end of the ridge. Carn Goedog is the spotted-dolerite source for a different group of Stonehenge stones. The two are not interchangeable.
The tors are not one place
A map is the shortest way to say it. Carn Goedog sits on the north side of the ridge. The unspotted Group 2 sources are other named outcrops: Cerrigmarchogion 1.7 km to the west (Stone 45), and Garn Ddu Fach 2.3 km to the east (Stone 62), with Carn Ddafad-las 460 m west of Garn Ddu Fach on the same intrusion. Craig Talfynydd, the other 2014 Group 2 candidate, is south of Carn Goedog. Waun Mawn is 6.9 km west of Garn Ddu Fach; its dolerites go to Cerrig Lladron, not to Stone 62.
You do not need to walk the ridge to use that geography. The papers already treat these as separate tors. A similarity score that puts Carn Goedog first for an unspotted pillar is not discovering that the spots run out at the west end of the same crag. It is matching a different outcrop.
- Named dolerite tors on Mynydd Preseli.
- Red: Group 1 spotted (Carn Goedog).
- Orange filled: Group 3 spotted (Carn Breseb, Carn Gyfrwy, vicinity of Carn Alw).
- Orange open: Carn Menyn, spotted dolerite but not a 2014 Group 3 outcrop.
- Blue filled squares: Group 2 unspotted (Cerrigmarchogion, Craig Talfynydd, Garn Ddu Fach, Carn Ddafad-las).
- Blue open square: Cerrig Lladron, the Waun Mawn source, not in the 2014 Group 2 list.
- Carn Goedog to Garn Ddu Fach is 2.3 km; Carn Goedog to Cerrigmarchogion is 1.7 km. Craig Talfynydd* is placed in OS kilometre square SN1231.
- Basemap OpenStreetMap / OpenTopoMap. Groups 1–3 after Bevins, Ixer and Pearce 2014; Stone 62 after Pearce et al. 2022.
What Ciborowski and Nash did with it
They took Pearce et al.’s own Preseli pXRF numbers and ran them through their arithmetic similarity method as a worked example. The ranking depended on which Stone 62 dataset they used:
| Stone 62 dataset | 1st | 2nd | 3rd |
|---|---|---|---|
| In situ analyses | Garn Ddu Fach | — | Carn Ddafad-las |
| 1980s core | Carn Goedog | Carn Ddafad-las | Garn Ddu Fach |
The in situ ranking is Pearce’s answer. The core ranking is not. That is the result Pearce et al. (2026) went for.
The Pearce objection
Ciborowski and Nash number Pearce et al.’s conclusions and answer them in order. Their point 7 is this sentence, given as Pearce’s:
The calculation approach fails to take into account even the most basic petrographic observations, which in the case of Stone 62 at Stonehenge shows that it is a non-spotted dolerite, hence cannot be sourced from a spotted dolerite source such as Carn Goedog.
That is local observation. You can see whether a Preseli dolerite is spotted. The white spots are a first-order field discriminant, used as such since Thomas. A similarity score that puts a spotted-type tor first for an unspotted pillar is showing you what chemistry-without-petrography can do. It is not showing you a source.
Ciborowski and Nash’s defence is that they did not assign a source. They ranked a worked example, and they said so. That is true of the framing. It is also a bit thin, given that the paper is about archaeological source provenance and that Carn Goedog came out on top for the core. If the method is being offered as a tool for that job, a ranking that contradicts the spotting is a problem for the tool, not a problem for the rock.
The tu quoque
They also point at Bevins, Ixer and Pearce (2014). That paper’s Group 1 is eleven samples: spotted monoliths SH33, 37, 49, 65 and 67, plus debitage OU1, OU8, OU11, OU12, OU14 and OU19A, assigned to Carn Goedog on compatible-element chemistry and PCA. Ciborowski and Nash name OU8, OU12 and “potentially” OU15 as non-spotted chips so assigned. OU15 is not in that 2014 eleven-sample list. It is listed as Group 1 in Bevins, Pearce and Ixer (2021). Whether any of those chips are petrographically unspotted is their claim. If it holds, it is a real inconsistency in the 2014 practice. It does not make the Stone 62 core ranking a source. It makes the “never chemistry without petrography” rule one that has to apply to both sides. The chips should be checked against Thorpe et al. (1991) before the point is used as more than a caution.
Why looking at the stone still wins
Spotting is a texture. It can be uneven within a sill; a sparsely spotted face is not automatically a different magma. That is a reason to sample properly, not a reason to ignore the texture when it is unambiguous. Stone 62 is unspotted. Garn Ddu Fach is an unspotted outcrop to which it has already been matched on chemistry and thin section. Carn Goedog is the spotted source for a different group. The arithmetic method, on the in situ data, already agreed with Garn Ddu Fach. The result that needed explaining was the core, and the explanation is the one Pearce gave: the ranking omitted the observation you make standing in front of the stone.
Local observation is not a romantic alternative to geochemistry. It is the first filter the geochemistry is supposed to survive. When it does not, you do not promote the score. You go back to the rock.
What this does not change
Stone 62’s published source remains Garn Ddu Fach (Pearce et al. 2022). It is not Carn Goedog. It is not Waun Mawn. Ciborowski and Nash did not overturn that, and they say they were not trying to. Pearce et al. did not need a new analysis of Stone 62 to make their methods point. They needed the spotting, which was already in the 2022 paper.
The arithmetic method can still be a useful ranking tool if it is not asked to overrule petrography. That is the same limit Ciborowski and Nash state in general, and then bump into on Stone 62. Out, damned spot: it does not wash out in a geometric mean.
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