Altar Stone's mineralogy and chemistry fit poorly with northeastern Scotland

Geologists led by Professor Duncan Pirrie of the University of South Wales report that the Altar Stone's mineralogy and chemistry fit poorly with the leading candidate source, northeastern Scotland. The study appears in the Journal of the Geological Society (DOI 10.1144/jgs2026-090).

The Altar Stone is a slab about 4.9 m long, weighing an estimated 6 tons. It lies mostly buried beneath two fallen sarsen stones at the center of the monument. For decades it was thought to have come from West Wales, like most of the smaller bluestones. In 2020, geologists showed this was wrong. In 2024, a team led by Anthony Clarke of Curtin University dated tiny mineral grains in fragments of the stone. They concluded it came from the Orcadian Basin of northeastern Scotland, a journey of some 700 km.

Pirrie and his co-authors do not dispute that dating work. It points to a Laurentian origin, meaning ancient crust associated with the Scottish Highlands, for the heavy minerals in the stone. They argue, though, that age data alone are not enough.

The stone itself cannot be sampled. The team therefore worked from eight museum fragments confidently linked to it, along with portable X-ray measurements of the three small patches exposed at Stonehenge. They compared the stone's mineralogy, chemistry, texture and isotopes with candidate source regions.

The stone contains a distinctive scatter of baryte, a barium mineral that cements the sandstone's grains and gives unusually high barium readings. It also holds chrome spinel, a heavy mineral that is scarce in Old Red Sandstone from much of the Orcadian Basin, including Orkney, Caithness, the Moray Firth and Shetland.

The stone's mixed-layer illite/smectite clay has about 20% expandable layers, a sign it has not been deeply buried or heated. Rocks in Caithness and Orkney show values below 10%, which would exclude those areas. The stone also contains tosudite, a rare clay mineral. It shows no sign of the low-grade metamorphism found in the Southern Uplands of Scotland, which rules that region out. The Midland Valley of Scotland is ruled out by its abundance of rock fragments. West Wales and the wider Anglo-Welsh Basin were excluded earlier.

"This is the most comprehensive geological investigation of the Altar Stone ever undertaken," Pirrie said. He added: "So far, none of the locations examined have the same set of characteristics as the Altar Stone. That tells us the search is not over, but we are now far better equipped to focus on the areas most likely to yield an answer."

Professor Richard Bevins of Aberystwyth University said that for almost a century the prevailing view was a west Wales source. His colleague Professor Nick Pearce said the stone is unique among those at Stonehenge. He added that tracing its source would provide valuable new insights into its journey to Salisbury Plain thousands of years ago.

Carboniferous sandstones from southern and central Britain are a poorer isotopic match than the Old Red Sandstone. The study's authors note that the Old Red Sandstone's own isotope record is incomplete because strontium data are missing.