The Hunt for the Altar Stone Source - the video

The Youtube video

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

Searching for the origin of the Stonehenge Altar Stone

A paper now published in the Journal of the Geological Society reopens the question of where the Stonehenge Altar Stone came from. Pirrie, Bevins, Ixer and ten further co-authors give the most detailed description of the stone to date: sedimentology, petrography, heavy minerals, clays, geochemistry and isotopes. Their conclusion is that no outcrop of Old Red Sandstone (ORS) examined so far matches it on every count, and that its provenance "has yet to be determined with confidence".

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
Data: figshare doi:10.6084/m9.figshare.33993488.v1

Fig. 1. (a) Map of the UK showing the location of Stonehenge and the distribution of non- marine sediments of the Old Red Sandstone. (b) Plan showing the position of the Altar Stone within the Stonehenge monument (c) Photograph looking towards the SW; the Altar Stone (arrowed AS) is lying mostly underneath the two large sarsen stones. (c)
Source: photograph by Adam Stanford.

Where the question stood

The Altar Stone is a block of about six tonnes (4.9 × 1 × 0.5 m) lying flat beneath fallen sarsens 55b and 156. For a century it was assigned to the Cosheston beds of West Wales, following H.H. Thomas in 1923. Bevins and colleagues refuted that in 2020, and in 2023 they excluded the Anglo-Welsh Basin as a whole: of 58 samples, all but four failed on geochemistry and the remaining four on mineralogy. In 2024 Clarke et al. (Nature) dated zircon, apatite and rutile grains from Altar Stone debitage and proposed the Orcadian Basin of NE Scotland. Clarke et al. (2026) later suggested Caithness or the Inverness–Black Isle area as the most likely sources.

What the new paper adds

The study combines previously published in-situ pXRF on the three exposed areas of the stone with new laboratory work on three museum fragments (MS1–MS3): automated SEM-EDS mineralogy, X-ray powder diffraction of bulk rock and clay fractions, cathodoluminescence, C–O isotopes and colour. Sr–Nd isotopes were measured on MS1 and MS2. The characteristics that emerge are:

• A bedding plane. Low-angle illumination in 2024 showed climbing ripples and slumped foresets on the exposed surface. If it is a bedding plane, the bed was at least 0.5 m thick and extended at least 4.9 m. There is no cleavage, and no petrographic sign of palaeosols or near-surface soil formation.
• Chrome spinel. The chrome spinel:zircon index (CZi) is 35.9–38.6 in the three fragments. In ORS comparison data it averages 1.3 on Orkney (n = 35), 1.4 on the Tarbat peninsula, Inner Moray Firth (n = 42), 0.4 on Foula and Shetland (n = 13) and 0.2 in the Lower ORS of Pembrokeshire (n = 32).
• Clay minerals. The clay fraction contains illite, mixed-layer illite/smectite, kaolinite, chlorite and tosudite. The illite/smectite is about 20% expandable in all three fragments. Expandability falls with burial and heating, and the ORS of Caithness and Orkney is below 10%, which excludes them. The clays alone would not exclude parts of the Moray Firth.
• Barium. Dispersed baryte cement gives pXRF Ba of about 1000–5500 ppm (mean 2758 ppm), linearly related to Sr. This separates the stone from ORS analyses in Wales and Orkney.
• Sr–Nd isotopes. ⁸⁷Sr/⁸⁶Sr is 0.7163–0.7169 and ¹⁴³Nd/¹⁴⁴Nd 0.5119–0.5121. Published Carboniferous sandstones (Leng et al. 1999) have either higher Nd or higher Sr isotope ratios, so there is little overlap. The Nd range of the ORS overlaps closely, but no ORS Sr isotope data exist, so the comparison cannot be taken further. The authors conclude that an ORS source is more likely than a Carboniferous one.
• Diagenesis. Pore-filling kaolinite pre-dates significant compaction; baryte post-dates compaction; both pre-date the dominant poikilotopic burial calcite cement.

What is excluded

• West Wales and the Anglo-Welsh Basin: excluded on earlier geochemistry and mineralogy, and in this paper by the absence of chrome spinel in West and South Wales and Somerset. Welsh Borderland tosudite occurrences are excluded on Ba.
• Orkney and Caithness: excluded on clay expandability. Chrome spinel is also scarce in the Orkney, Tarbat and Shetland samples; Caithness itself was not sampled for it.
• Moray Firth / Tarbat: compatible on clay, excluded on chrome spinel.
• Midland Valley ORS: excluded on abundant lithic grains (Phillips 2007). In the NE Midland Valley, the Stonehaven and Arbuthnott groups have too little I/S expandability, and the zircon populations of the Dunnottar–Crawton to Strathmore groups differ. The Strathmore Group clays are comparable, but its zircons and the texture of its tosudite are not.
• Southern Uplands: chrome spinel is common in the Lower Palaeozoic sandstones, but the Altar Stone contains kaolinite, which indicates it has not undergone low-grade metamorphism.
• Late Carboniferous–Triassic of south-central Britain: no Variscan-age zircons in the Altar Stone.

What remains open

• The ORS between the Midland Valley and the Welsh Borders, where modern heavy-mineral and zircon data are virtually absent.
• Areas with Ba anomalies in stream-sediment geochemistry, including the Lanark Basin and the Fintona Basin of Northern Ireland. The paper reports no chrome spinel, zircon or clay data for the Lanark Basin.
• Carboniferous sandstones recycled from Caledonide sources, such as the chrome-spinel-rich Clifton Rock of the Pennine Basin. Its zircon pattern differs (a large Early Palaeozoic peak, no Neoproterozoic group) and the Sr–Nd data favour the ORS, but the authors do not exclude it.

The diagnostic toggle map

These tests can be switched on and off in my Altar Stone diagnostic toggle map. Nothing to do with the paper or authors, I have just updated it with their data.  Each test is a layer: Ba/Rb screening, K-feldspar, zircon, clay and thermal maturity, strontium (L10, L11), and the chrome spinel (U1) and clay expandability (U2) results of Pirrie et al. The Pirrie layers start off; the preset "CZi + I/S ON (Pirrie et al. 2026)" switches both on. A place survives only if it fails none of the layers switched on, and untested is not the same as failed. Leave-one-out presets show how much each test contributes. The areas listed above as open, and the Sr–Nd result, are not scored on the map.

Relation to the NE Scotland proposal

The paper accepts the zircon, rutile and apatite ages of Clarke et al. and a Laurentian source for the heavy minerals. The argument is that the same age pattern can be shared by several basins, so ages alone cannot identify an outcrop, and that once chrome spinel, clays and barium are added the Orcadian candidates fail. Bevins and Ixer co-authored the 2024 Nature paper, so this is in part a team revising its earlier interpretation. The authors also regard the glauconite reported in one fragment by Clarke et al. (2026) as probably a misinterpretation of SEM-EDS spectra, and caution against relying on a single youngest zircon age.

Caveats

• Three fragments. The new analyses are on three small museum fragments, and the Sr–Nd data on two. Their attribution to the Altar Stone rests on earlier pXRF and petrographic comparison, because the stone itself cannot be sampled.
• Comparison data. The Pembrokeshire CZi value is from unpublished data, and the Orkney, Tarbat and Shetland values come from earlier compilations.
• Elimination, not identification. "No match found" covers only the ground that has been sampled. Large areas of ORS have no modern heavy-mineral or zircon data.
• Coarse Ba screening. National stream-sediment Ba maps are low resolution and can be distorted by historic mining.
• Sr/Ca. Surface pXRF Ca on the in-situ stone is lower than on debitage, which the authors attribute to weathering of the calcite cement. Sr/Ca from the surface pXRF data is therefore biased upward and should be treated with caution.

Why it matters

The origin of the Altar Stone determines the journey and the contacts it implies. A Moray Firth or Caithness source suggests a long coastal voyage; a Lanark Basin source, routes via the Clyde and the Irish Sea; Fintona, a crossing from Ulster. Until a source is identified, the origin is a variable in route modelling, not a fixed point.

The characteristics reported in the paper define the tests a candidate source must pass: high Ba with baryte cement, CZi of about 36–39, illite/smectite about 20% expandable with tosudite, plagioclase far exceeding K-feldspar, and an ORS-type Nd isotope signature. Those are the tests for the next field campaign.

Figures and short quotations are from Pirrie et al. (2026); see the paper and its supplementary tables for the data.

Four Young Officers, Stonehenge 1938

This post is the companion to the short film Four Young Officers, made for Remembrance Sunday 2026: the four Royal Artillery subalterns who painted Stonehenge green in June 1938, and what became of them.

The night of 16 June 1938

By the summer of 1938 the four were serving with anti-aircraft and coast-defence units — Sherrard and Skinner at the School of Anti-Aircraft Defence, Biggin Hill; Peirce with the 22nd Anti-Aircraft Battery at the Royal Artillery experimental camp at Watchet; and Shearme at the Coast Artillery School, Shoeburyness — and had gathered at the School of Artillery, Larkhill, three miles north of Stonehenge, for a gunnery course. On the night of Thursday 16 June, after a mess guest night, they took green line-marking paint from the Larkhill tennis courts and daubed the monument, several of the upright sarsens and the Heel Stone among them, together with road signs nearby. The damage was found on Saturday 18 June. The Times of 20 June reported that Stonehenge “was found on Saturday to have been daubed with green paint” and that “signposts in the vicinity were similarly daubed”. Mike Pitts assembled the fuller sequence from the contemporary press.

Summons and sentence

The four were identified and summoned. The Times reported in July that “four Army officers were summoned for damaging Stonehenge on June 16” and that the hearing “was adjourned until August 9”. On 9 August they pleaded guilty at Salisbury County Petty Sessions. Each was fined £1 and ordered to contribute towards the repair and costs, and the General Officer Commanding, 3rd Division, admonished them. The American weekly TIME, reporting the case on 22 August under the heading “Druidical Sacrilege”, quoted the prosecutor's warning that “It may take 1,000 years for Stonehenge to regain its old weathered appearance”.

Four young officers

They had been commissioned as second lieutenants in the Royal Regiment of Artillery from the Royal Military Academy, with seniority dated 27 January 1938 (London Gazette 34477, p. 587). The Gazette gives the surnames as Peirce and Shearme, where the 1938 newspapers printed Pierce and Shearne:

• William Laurence Sherrard
• William Howard Skinner
• John Edward Passingham Peirce
• John Lambert Shearme

They stood among fifty-two gunner cadets in a single passing-out list. Sixteenth on it was Robin Horace Walford Dunn, winner of the Academy's Sword of Honour, who would be evacuated from Dunkirk, fight in the Western Desert and Normandy, be wounded three times and appointed MC, and in civilian life become a Lord Justice of Appeal; his memoir Sword and Wig records the war into which the cohort was commissioned. Dunn and Sherrard had both come to Woolwich from Wellington College.

What became of them

Captain William Laurence Sherrard served with 12 Battery, 6th Heavy Anti-Aircraft Regiment, Royal Artillery. In 1942 the regiment was among the anti-aircraft units defending the oil refineries at Palembang, in southern Sumatra. On 14 February 1942 Japanese paratroops were dropped on the refineries and the nearby airfield, and the gunners, the only regular troops in the area, fought them at close range. Sherrard was killed that day at the Pladjoe refinery. He was 24. He was mentioned in despatches, is buried in Jakarta War Cemetery, and is named on the war memorial at Ditchling, Sussex, where his parents lived. His batman wrote to his mother that his son “gave his life at a place called Pladjoe … taking on odds of fifty to one” (Ditchling History Project).

The other three survived the war. The London Gazette records that Peirce, Shearme and Skinner were all promoted major on 27 January 1951, thirteen years to the day after their commission (London Gazette 39132). Shearme was placed on retired pay on 29 September 1958 (41508). Peirce rose to lieutenant-colonel and was appointed OBE in the New Year Honours of 1962 (42552, 1 January 1962). Their wartime postings have not yet been traced.

Notes on the film

The newspaper reports are retyped in the film rather than shown. The photographs of the Heel Stone and the 1938 snapshot (photographer unknown) are from the author's collection; the map of Sumatra is drawn from the NOAA GLOBE land mask. The music is original.

Sources

The London Gazette, issue 34477, 28 January 1938, p. 587 (commission); issue 39132, 23 January 1951 (promotions to major); issue 41508, 30 September 1958 (Shearme's retirement); issue 42552, 1 January 1962 (Peirce OBE).

The Times, 20 June 1938, p. 16, “Stonehenge daubed with green paint”; The Times, July 1938, “Officers charged with Stonehenge damage”.

TIME, 22 August 1938, “Foreign News: Druidical Sacrilege”. time.com

“A night out” (London, 11 August 1938), Waipawa Mail, 14 September 1938, p. 1. Papers Past

Commonwealth War Graves Commission, casualty record of Captain W. L. Sherrard, 12 Bty, 6 HAA Regt RA.

Ditchling History Project, “For the Fallen”. ditchlinghistoryproject.org

Dunn, R. 1993. Sword and Wig: Memoirs of a Lord Justice. Quiller Press. (Dunn's place in the commission list: London Gazette 34477, p. 587.)

Moody, F. 2024. Bygone Salisbury: History of protest at Stonehenge. Salisbury Journal, June. salisburyjournal.co.uk

Pitts, M. 2016. How the Magpie Musicians came to stand for free Stonehenge. Digging Deeper, 5 April. mikepitts.wordpress.com

LiDAR Lens V.2

 

Durrington Walls and Woodhenge through the lens, with the What's here? card showing the nearby records and photos.

I have been building LiDAR Lens for a while now, starting with Wiltshire. It now covers the whole of England and Wales, and it is live at lidarlens.sarsen.org. It is free, there is no login, and it works on a phone. The code is open source on GitHub, so you check it and build your own.

What it does

A circular lens slides over ordinary aerial imagery. Inside the lens you see the bare ground from LiDAR, with the trees and buildings taken away, so banks, ditches, barrows and old field systems stand out. You can switch between relief views such as hillshade and sky-view (on a phone these are the mode buttons 1 to 8), turn the sun round and change its height, add vertical exaggeration, pick a colour ramp, or let auto-contrast do the work.

You can search by place name, postcode or grid reference, or start typing the name of one of about 24,000 scheduled monuments from Historic England and Cadw and pick it from the list.

What's here?

Click anywhere and the What's here? card shows:

• nearby Historic England or Cadw records;
• a measurement of the shape from the LiDAR: its size, height, profile and slope;
• nearby Geograph photos, with their photographers credited;
• a Look here up button, which opens Google AI Mode to ask about the archaeology and history of the spot.

There is also an elevation profile tool, and every view has a link you can share, so someone else sees exactly what you are looking at.

One caution: the measurements come from the terrain model, not from anyone standing in the field. They tell you the shape of the ground, not what it is.

The data

England uses the Environment Agency LiDAR Composite DTM at 1 m, and Wales uses Welsh Government LiDAR, both under the Open Government Licence. My thanks to both for making it open.

County lenses

The county versions are still there too, including the Wiltshire LiDAR Lens, along with others.

The code is open source on GitHub. If you find something interesting, or something that does not work, do let me know in the comments.