Correcting Brian John's Silly Mistake

Last year Brian John published a paper in Archaeology in Wales Vol 63 arguing that “Carn Goedog on Mynydd Preseli Was Not the Site of a Bluestone Megalith Quarry" Unfortunately, his argument was built on false bedrock. The journal has published a correction.

Pearce, N.J.G., Bevins, R.E. and Ixer, R.A. (2026) ‘Carn Goedog is a 10m thick, northward dipping sill: a comment correcting geological errors in “Carn Goedog on Mynydd Preseli was not the site of a bluestone megalith quarry” by Brian John (2023)’, Archaeology in Wales, 64.

In the 2023 paper, John noted that dolerite crops out on the north face of Carn Goedog from roughly 305 m down to 235 m elevation — a vertical drop of about 70–75 metres. He declared: “If all of the rocks of Carn Goedog belong to the same sill, it must be at least 75 m thick.”

A sill is a flat, sheet-like body of igneous rock (here, spotted dolerite) intruded between older layers. Its true thickness is the shortest distance measured perpendicular to its top and bottom surfaces. It only the same as the vertical drop if the sill is perfectly horizontal. The Carn Goedog sill is not. It dips gently northward at about 23°.

This is the geological equivalent of looking at my soughdough heel and announcing that the inserted slice is 2 inches thick because the bread is 2 inches tall.

John used his 75 m “thickness” as the foundation for three key claims:

• the sill must be internally chemically differentiated (like much thicker sills elsewhere), • the geologists’ sampling was inadequate, • the geochemical provenancing to Carn Goedog was unreliable.

Once you correct the thickness to ~10 m and recognise it as a thin, simply dipping sheet the entire geological critique collapses like an old quarry face.

Cross-section from the highest part of Carn Goedog (approx. 300m AOD) to an elevation of about 215m with the position of the Carn Goedog sill marked (shaded red). Horizontal and vertical scales are the same. - From https://www.academia.edu/164795472/Carn_Goedog_reply  

This was not a subtle difference of interpretation — it was a basic first-year structural geology error. The 2025 correction paper by Pearce, Bevins & Ixer politely but firmly points this out with clear cross-sections, structure contours and LiDAR. They also note, with admirable restraint, that John misunderstands how analytical geochemistry works: no two samples from the same outcrop are ever perfectly identical because of natural heterogeneity and analytical precision.

John also asserted that the Carn Goedog sill forms one vast, kilometre-scale continuous outcrop stretching 3 km west and 2 km east across the Preseli ridge, implying the geologists had under-sampled a gigantic body. In reality, as Pearce et al. show the exposures are nothing more than isolated crags; the supposed “continuous sill” is an illusion created by joining up unrelated dots on the BGS map. Carn Goedog and Carn Breseb, for example, sit 75 m apart stratigraphically and belong to completely different geochemical groups. John’s claim that columnar jointing covers less than 10 % of the outcrop is equally wide of the mark — the entire 10 m sill is columnarly jointed, and the jumble of blocks on the slope is simply the weathered, disaggregated result. All of this fed his central complaint that the geologists’ sampling was “inadequate”; once you realise they had sampled the full 7–10 m thickness of each thin, northward-dipping sheet and analysed continuous vertical sections, that complaint evaporates too.

I couldn't resist calling it a "silly" mistake, but it is actually rather more than that, his whole paper is built on a fundamental geological misunderstanding, he needs to withdraw.