Stonehenge's Altar Stone Origin Mystery Deepens as Scottish Source Theory Is Ruled Out

A six-tonne slab of sandstone lies at the heart of Stonehenge in Wiltshire, and no one can yet say where it came from. A new geological study has crossed one more candidate off the list: the Orcadian Basin in north-east Scotland, which until now was the leading suggestion.

The Altar Stone has puzzled archaeologists for a long time. For more than a century, the prevailing view was that it came from Wales. A more recent theory overturned that assumption and pointed instead to the Orcadian Basin, some 435 miles from Stonehenge.

An international team has now put that Scottish theory to a test. Researchers from institutions across the UK, Canada, Italy and Sweden combined a wide range of analytical techniques on the stone itself with rock testing in the Orcadian Basin. They did not find any location in the region with the same characteristics as the Altar Stone.

Professor Duncan Pirrie, emeritus professor at the University of South Wales, led the work. He described it as the most comprehensive geological investigation of the Altar Stone ever undertaken, and said it has produced the clearest picture yet of the stone's composition. He was plain about the result: so far, none of the locations examined share the stone's full set of characteristics. In his words, the search is not over, but researchers are now much better equipped to focus on the areas most likely to give an answer.

The stone matters because of how Stonehenge was built. Academics understand the monument went up in several phases between around 3000 and 1600 BC, with stones hauled in from far away. The smaller bluestones travelled roughly 225 km from west Wales to Wiltshire. The Altar Stone is regarded as unique among the stones at the site.

Professor Richard Bevins, honorary professor in the department of Geography and Earth Sciences at Aberystwyth University, noted that for almost a century the Welsh origin was the accepted view. His team's earlier research overturned it and favoured Scotland. He said the latest study sharpens the hunt by identifying the specific geological characteristics future investigations need to look for, so they can be better targeted. Each new piece of evidence, he said, brings researchers closer to solving one of Stonehenge's most enduring mysteries.

Professor Nick Pearce, professor of geochemistry at Aberystwyth University, framed the elimination as progress in its own right. In a geological detective story, he said, ruling out potential source areas matters as much as identifying new ones, since every location eliminated narrows the search. Tracing the source, he added, would give valuable insight into the stone's journey to Salisbury Plain thousands of years ago.

For now the origin remains unknown. Wales and the Orcadian Basin have both been tested against the rock evidence, and the next step is for researchers to use the newly defined traits to guide where they look next.