The altar stones from Stonehenge were carried to Scotland for transportation.

A study of the six-tonne altar stone at the heart of Stonehenge has revealed that it was almost certainly brought from northeast Scotland, much further away than any of the other stones in the megalithic structure.

“We were all in shock, we couldn't believe it,” the geologist said. Anthony Clark Curtin University, Perth, Australia.

It's unclear how the altar stone got from Scotland to southern England, but it was probably by sea, Clark said, because there is evidence people at the time traveled by sea.

Stonehenge is thought to have been begun about 5,100 years ago and constructed over a period of about 1,500 years. The outer circle is made of large stones called sarsens, weighing about 25 tons, while the inner circle and altar are made of small stones called bluestones, weighing about 3 tons. Bluestones are any rock that is not a sarsen. Bluestones are made of many different types of rock.

“What's unique about Stonehenge is the distance the stones were transported,” the geologist says. Richard Bevins Bevins, a researcher at Aberystwyth University in the UK, said most of the stone circles were made from rocks found within one kilometre of the site.

But the sarsens' source has been identified as West Woods in Wiltshire, about 15 miles (25 km) from the site, and Bevins' team has found that almost all of the bluestones came from the Preseli Hills in Wales, about 175 miles (280 km) away. One theory is that they were part of an even older Welsh stone monument that had been moved.

Stonehenge's Altar Stone is different to other bluestones: “By the end of 2021, we had concluded that the Altar Stone does not match any known geology in Wales,” team members said. Nick Piercealso at Aberystwyth University.

The five-metre-long stone is set into the ground with only one side exposed and partially covered by two other stones. It is thought to have been placed there around 4,500 years ago.

Clark is currently analyzing samples of the altar stone using sophisticated equipment commonly used in the mining industry. The altar stone is made of sandstone, which means grains of rock that were deposited on the floor of an ancient sea eroded away and eventually stuck together to form new rock. The age of each grain varies depending on when the eroded rocks first formed, so each sandstone is a mix of grains of different ages.

Clark analyzed the zircon, apatite, and rutile crystals in the rock sample. These minerals contain uranium, which slowly decays into lead, so the ratio of uranium to lead can be used to determine the age of the rock. For example, the zircon in the rock is between 500 million and 3 billion years old.

The dating pattern indicates with more than 95 percent certainty that the altar stone is made from ancient red sandstone from the Auckland Basin in northeast Scotland, team members say. Chris Kirkland Located at Curtin University, the basin was once a huge ancient body of water called Lake Orcadie.

The nearest older red sandstone sites to Stonehenge are near Inverness, 750 kilometres (470 miles) away, and the furthest are in the Shetland Islands, up to 1,000 kilometres (620 miles) away, so the team believes the altar stone was probably transported by sea.

Glaciers can carry rocks long distances, and there's evidence that during the last ice age, ice in the Orkney region flowed north rather than south, Kirkland said.

So why was the altar stone transported so far? “That's a big question that's impossible to answer,” Clark says. “All we know is that it's a six-tonne rock that was transported from 750 kilometres away. That alone tells us an enormous amount about Neolithic societies and their connections.”

“What they did was pretty rigorous.” David Nash A team from the University of Brighton in the UK has pinpointed the exact source of Wiltshire sarsens: “This is really solid research.”

Nash said pinpointing the source of the altar stone more precisely would be difficult because the Orkney Basin spans a vast area and is up to five miles deep. “It's a huge task, because there's a huge amount of old red sandstone in the north of Scotland.”

In contrast, finding the exact source of the sarsens was easier because there were fewer possible sources, he said.

Genetic studies have shown that the people responsible for much of Stonehenge's construction were largely replaced by new waves of immigrants by about 4,000 years ago, likely after a major epidemic wiped out much of Europe's population.