Stonehenge’s Altar Stone Still Has No Home, Geologists Say | Sci.News

A new analysis by University of South Wales Professor Duncan Pirrie and his colleagues undercuts the leading theory that the 6-ton slab of sandstone came from northeastern Scotland, though the geologists stop short of ruling it out entirely.

The Altar Stone, a roughly 4.9-m slab weighing an estimated 6 tons, 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 Curtin University researcher Anthony Clarke, using the ages of tiny mineral grains in fragments of the stone, concluded it came from the Orcadian Basin of northeastern Scotland, a journey of some 700 km or more.

According to Professor Pirrie and co-authors, the picture is far less settled.

They do not dispute the earlier dating work, which points to a Laurentian origin, meaning ancient crust associated with the Scottish Highlands, for the heavy minerals in the stone. But age data alone, they argue, are not enough.

“This is the most comprehensive geological investigation of the Altar Stone ever undertaken,” Professor Pirrie said.

“By combining a wide range of analytical techniques, we have built the clearest picture yet of the stone’s composition and tested the leading theories about its origins.”

“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.”

Because the Altar Stone itself cannot be sampled, the researchers worked from eight museum fragments confidently linked to it, along with portable X-ray measurements of the three small patches of the stone that are exposed at Stonehenge.

The stone contains a distinctive scatter of baryte, a barium mineral that cements the sandstone’s grains, which gives the stone 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, and shows no sign of the low-grade metamorphism found in the Southern Uplands of Scotland, which rules the region out.

The stone’s ripple-laminated structure is consistent with deposition by flowing water, and that its calcite cement formed during burial rather than near the surface.

Other candidates, West Wales and the wider Anglo-Welsh Basin, were excluded earlier.

The Midland Valley of Scotland is ruled out by its abundance of rock fragments.

Carboniferous sandstones from southern and central Britain are a poorer isotopic match than the Old Red Sandstone, although the Old Red Sandstone’s isotope record is incomplete because strontium data are missing.

“For almost a century, the prevailing view was that the Altar Stone came from west Wales,” said Aberystwyth University’s Professor Richard Bevins.

“Our previous research overturned that assumption and pointed instead to Scotland as the most likely source.”

“This latest study significantly refines the search by identifying the specific geological characteristics we need to look for, allowing future investigations to be targeted more effectively.”

“With each new piece of evidence, we move closer to solving one of Stonehenge’s most enduring mysteries.”

“In a geological detective story such as this, ruling out potential source areas is just as important as identifying new ones,” added Professor Nick Pearce, also from Aberystwyth University.

“Every location eliminated narrows the search and brings us closer to discovering where the Altar Stone originated.”

“The stone is unique among those at Stonehenge, and tracing its source would provide valuable new insights into its remarkable journey to Salisbury Plain thousands of years ago.”

The study was published today in the Journal of the Geological Society.

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Duncan Pirrie 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; doi: 10.1144/jgs2026-090