Altar Stone diagnostic toggle map

The Altar Stone diagnostic toggle map is an interactive screen of candidate source areas for the Stonehenge Altar Stone. Each published diagnostic is a layer that can be switched on or off, so the effect of any one test on the list of surviving candidates can be seen directly. The code and data are in the GitHub repository.

What it plots

The map covers 85 candidate places: Ba/Rb geochemical clusters across Britain and Ireland, plus named sites with published measurements. An Old Red Sandstone underlay from the BGS Geology 625k dataset shows where the outcrop lies. Fourteen layers score each place:

• L0–L9: the baseline screen, covering barium and Ba/Rb geochemistry, mineralogy, zircon ages, thermal maturity and related published criteria.
• L10 and L11: a strontium ceiling and the Sr/Ca ratio, applied only where rock Sr has been measured.
• U1 and U2: chrome spinel (CZi) and illite/smectite expandability, from Pirrie et al. (2026). Both start off.

How to read it

A place is a match only if it fails none of the layers switched on. Each layer has strict and relaxed settings where the evidence allows. Untested is not failed: a place with no data for a layer stays open on that layer. Presets switch one test or group off at a time (leave-one-out), which shows how much each test contributes, and the preset "CZi + I/S ON (Pirrie et al. 2026)" applies the new chrome spinel and clay results.

Several things are deliberately not scored: the ORS between the Midland Valley and the Welsh Borders, the Lanark Basin, Carboniferous sandstones such as the Clifton Rock, and the Sr–Nd isotope result, because the evidence is missing or has no comparison data. The Latheron–Niandt cool sliver is drawn as schematic only.

Limits

The map is a screening tool. A place that survives is untested or consistent with the evidence so far, not identified as the source, and exclusion by elimination covers only the ground that has been sampled. Corrections and additional data are welcome.

The Pirrie, Bevins, Ixer et al paper that prompted the latest update is discussed in a separate post.

Paper: Pirrie, D., Bevins, R.E., Ixer, R.A., Pearce, N.J.G., Turner, P., Power, M.R. et al. 2026. Searching for the origin of the Stonehenge Altar Stone: new sedimentological, petrographical, mineralogical, geochemical and isotopic constraints. Journal of the Geological Society 183, jgs2026-090. doi:10.1144/jgs2026-090