Stone 62 - Spotting Its Source

Ciborowski and Nash’s reply to Pearce et al. includes a worked example on Stone 62. It does not propose a new source for the stone. It raises a methods question: how a geochemical similarity ranking should be read when it disagrees with a first-order petrographic observation, namely whether the rock is spotted.

The arithmetic-method discussion is in The Numbers Ain’t Enough and The Numbers Talk Back. This note is only the Stone 62 example, and only the spotting.

Stone 62

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 showed that the Waun Mawn dolerites go to Cerrig Lladron, not to Stone 62.

The published source, as of 2022, is therefore an unspotted outcrop at the eastern end of the ridge. Carn Goedog is the spotted-dolerite source proposed for a different group of Stonehenge stones. The two outcrops are not interchangeable.

The tors

Carn Goedog sits on the north side of the ridge. The unspotted Group 2 sources used in the 2014 classification 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.

The papers already treat these as separate tors. A similarity ranking that places Carn Goedog first for an unspotted pillar is matching a different outcrop, not a lateral change of texture on the same crag.

Click to embiggen

• 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.

The similarity ranking

Ciborowski and Nash took Pearce et al.’s Preseli pXRF numbers and applied 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 agrees with Pearce et al. (2022). The core ranking does not. That is the result Pearce et al. (2026) discuss.

Petrography

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.

Whether a Preseli dolerite is spotted is a field observation. The white spots have been used as a discriminant since Thomas. Spotting can be uneven within a sill, so a sparsely spotted face is not automatically a different magma. Stone 62, however, is unspotted, and Garn Ddu Fach is an unspotted outcrop to which it has already been matched on chemistry and thin section.

Ciborowski and Nash state that they did not assign a source. They ranked a worked example. That is how the paper is framed. The core ranking still places Carn Goedog first for an unspotted pillar. If the method is used as a provenance tool, that ranking needs to be read against the petrography, which is the limit they state in general.

Source

Stone 62’s published source remains Garn Ddu Fach (Pearce et al. 2022). Ciborowski and Nash did not overturn that, and they say they were not trying to. On the in situ data their method already agreed with Garn Ddu Fach. The core ranking is the result that sits at odds with the spotting. Pearce et al. (2026) treat that as a methods point: similarity calculated from the geochemistry should not be taken in isolation from the petrography.

The arithmetic method can still be a useful ranking tool if it is used alongside those observations rather than instead of them.

References

• Bevins, R. E., Ixer, R. A. and Pearce, N. J. G. 2014. “Carn Goedog is the likely major source of Stonehenge doleritic bluestones: evidence based on compatible element geochemistry and Principal Component Analysis.” Journal of Archaeological Science 42: 179–193.
• Ciborowski, T. J. R. and Nash, D. J. 2026. “Defining similarity: An arithmetic method for archaeological source provenance targeting using geochemical data.” Journal of Archaeological Science: Reports 69: 105513. https://doi.org/10.1016/j.jasrep.2025.105513
• Ciborowski, T. J. R. and Nash, D. J. 2026. “Arithmetic methods for exploring archaeological source provenance using geochemistry – A reply to Pearce et al.” Journal of Archaeological Science: Reports 75: 106012. https://doi.org/10.1016/j.jasrep.2026.106012
• Pearce, N. J. G., Bevins, R. E. and Ixer, R. A. 2022. “Portable XRF investigation of Stonehenge Stone 62 and potential source dolerite outcrops in the Mynydd Preseli, west Wales.” Journal of Archaeological Science: Reports 44: 103525. https://doi.org/10.1016/j.jasrep.2022.103525
• Pearce, N. J. G., Bevins, R. E., Ixer, R. A. and Pirrie, D. 2026. “Arithmetic approaches alone are inadequate in defining similarity: A comment on Ciborowski and Nash 2026.” Journal of Archaeological Science: Reports: 105874. https://doi.org/10.1016/j.jasrep.2026.105874