A new geological study led by Professor Duncan Pirie of the University of South Wales challenges the prevailing view that Stonehenge’s six-tonne altar stone came from north-east Scotland. The researchers have identified several characteristics that do not match known source areas, although they say Scotland has not been completely ruled out and that the search for the stone’s origin must continue.
The altar stone is a slab approximately 4.9 metres long and estimated to weigh six tonnes. It is largely buried beneath two fallen sarsen stones at the centre of Stonehenge.
For decades, researchers thought the stone, like other small bluestones at the monument, had come from west Wales. In 2020, geologists showed that this interpretation was incorrect.
In 2024, a team led by Curtin University researcher Anthony Clarke analysed the ages of tiny mineral particles in fragments of the stone. The researchers concluded that it had travelled more than 700 kilometres from the Orcadian Basin in north-east Scotland.
More than age data is needed
Pirie and his co-authors do not dispute earlier dating studies indicating that heavy minerals in the stone have a Laurentian origin, meaning they are associated with ancient crust linked to the Scottish Highlands. However, they argue that age data alone cannot identify the stone’s source.
“This is the most comprehensive geological survey of the altar stone ever undertaken,” Professor Pirie said.
“By combining a wide range of analytical techniques, we have built the clearest picture yet of the stone’s composition and tested leading theories about its origin.”
“So far, none of the sites investigated have the same set of characteristics as the altar stone.”
“This shows that the search is not over yet, but we are now much better equipped to focus on the areas that are most likely to yield answers.”
Because the altar stone itself cannot be sampled, the team examined eight museum fragments confidently associated with it. The researchers also used portable X-ray measurements from three small sections of stone exposed at Stonehenge.
The stone is unusual because it contains barite, a barium mineral that hardens sandstone grains and gives the altar stone unusually high barium readings.
Geological clues narrow the search
Old Red Sandstone, which occurs across much of the Orcadian Basin—including the Orkney Islands, Caithness, the Moray Firth and the Shetland Islands—also contains the rare heavy mineral chromium spinel.
However, the stone’s mixed-layer illite/smectite clay contains about 20% expansion layers, suggesting that it was not deeply buried or heated. Rocks from Caithness and the Orkney Islands have values below 10%, leading the researchers to exclude those areas.
The altar stone also contains the rare clay mineral tosudite and shows no evidence of the low-grade metamorphism found in Scotland’s southern Highlands, excluding that region as well.
Its rippled, laminated structure is consistent with deposition by running water. The stone’s calcite cement appears to have formed during burial rather than near the surface.
West Wales and the wider Anglo-Welsh Basin had already been ruled out as possible sources. The researchers also say that the required geological setting is absent from Scotland’s Midland Valleys, where rock fragments are abundant.
Carboniferous sandstones in southern and central Britain have a weaker isotopic identity than Old Red Sandstones. However, the isotopic record of Old Red Sandstones remains incomplete because strontium data are missing.
The search for the source continues
Professor Richard Bevins of Aberystwyth University said the findings build on earlier work that shifted the focus from Wales to Scotland.
“For almost a century the prevailing view was that the altar stone came from west Wales,” Bevins said.
“Our previous research overturned that assumption and instead pointed to Scotland as the most likely source.”
“This latest study significantly improves our survey by identifying specific geological features we need to look for, allowing us to conduct future surveys more effectively.”
“With each new piece of evidence, we get closer to solving one of Stonehenge’s most enduring mysteries.”
“In geological mystery novels like this, ruling out potential source areas is just as important as identifying new source areas,” added Professor Nick Pearce, also from Aberystwyth University.
“Each time you remove a location, you narrow your search and get closer to discovering the origin of the altar stone.”
“This stone is unique among the stones at Stonehenge, and tracing its source will provide valuable new insight into the remarkable journey to Salisbury Plain thousands of years ago.”
The study was published in the Journal of the Geological Society.
Duncan Pirie et al. 2026. “Exploring the origin of Stonehenge’s altar stones: new sedimentological, petrographic, mineralogical, geochemical, and isotopic constraints.” Journal of the Geological Society 183. doi: 10.1144/jgs2026-090.
Source: www.sci.news
