Stonehenge’s altar stone was brought all the way from Scotland

A study of the 6-tonne altar stone at the heart of Stonehenge has shown that it was almost certainly brought there from north-east Scotland, much further than any other stone in the megalithic structure.

“All of us were stunned. We couldn’t believe it,” says geologist Anthony Clarke at Curtin University in Perth, Australia.

How the altar stone was transported all the way from Scotland to the south of England isn’t known, but it is most likely to have been brought by sea, says Clarke. There is evidence that people at this time were making sea journeys, he says.

Stonehenge is thought to have been built over about 1500 years, starting around 5100 years ago. It consists of an outer circle of large stones weighing around 25 tonnes each, known as sarsens, and an inner ring and altar made of smaller stones generally of around 3 tonnes, known as bluestones. The term bluestone just means any rock that isn’t a sarsen – the bluestones are made of various kinds of rock.

“The thing that’s unique about Stonehenge is the distance that stones have been transported,” says geologist Richard Bevins at Aberystwyth University, UK. Most stone circles are made from rocks found within a kilometre of the site, says Bevins.

The source of the sarsens, however, has been traced to the West Woods of Wiltshire, around 25 kilometres from the site. And Bevins’s team has shown that almost all the bluestones come from the Preseli hills in Wales, about 280 kilometres away. One idea is that they were part of an even older Welsh stone monument that was moved.

The altar stone at Stonehenge is different to the other bluestones. “By the end of 2021, we’d come to the conclusion that the altar stone didn’t match any of the geology that we knew in Wales,” says team member Nick Pearce, also at Aberystwyth University.

This 5-metre-long stone is embedded in the ground with only one surface showing and is partly covered by two other stones. It is thought to have been put in place about 4500 years ago.

Now, Clarke has taken sophisticated equipment usually used in the mining industry and analysed samples of the altar stone. It is a sandstone, which means it is made of eroded grains of rock that piled up at the bottom of an ancient ocean and eventually stuck together to form a new rock. The age of each grain varies depending on when the rock it eroded from first formed, so different sandstones have a distinctive mix of grains of different ages.

Clarke analysed individual crystals of the minerals zircon, apatite and rutile within samples of the stone. These minerals contain uranium, which very slowly decays to lead, allowing them to be dated from the ratio of uranium to lead. For instance, the zircon in the stone is between 500 million and 3 billion years old.

The pattern of ages shows with greater than 95 per cent certainty that the altar stone is Old Red Sandstone from the Orcadian basin in north-east Scotland, says team member Chris Kirkland at Curtin University. This basin was once a massive ancient water body called Lake Orcadie.

The nearest matching Old Red Sandstone to Stonehenge is 750 kilometres away in the vicinity of Inverness, and the furthest is in the Shetland Islands up to 1000 kilometres away – hence why the team thinks the altar stone was probably transported by sea.

Glaciers can carry boulders long distances, but the evidence is that during the last glacial period, the flow of ice in the Orcadian region was northwards rather than southwards, says Kirkland.

So why was the altar stone brought such a long distance? “That is the great unanswerable question,” says Clarke. “All we know is it’s a 6-tonne piece of rock that’s come from 750 kilometres away. That, by itself, tells us an awful lot about the Neolithic society and its connectivity.”

“What they’ve done is pretty rigorous,” says David Nash at the University of Brighton, UK, whose team identified the precise source of the sarsens in Wiltshire. “It’s a really sound piece of work.”

Pinning down the source of the altar stone more precisely will be difficult because the Orcadian basin extends over a vast area and is up to 8 kilometres deep, says Nash. “That’s a big, big job, because there’s an awful lot of Old Red Sandstone in northern Scotland.”

With the sarsens, by contrast, there were fewer potential sources, so locating the exact one was easier, he says.

Genetic studies have shown that the people who did most of the construction of Stonehenge were largely replaced by a new wave of migrants by about 4000 years ago. This could be because a plague pandemic wiped out a large proportion of Europe’s inhabitants around this time.