The altar stone of Stonehenge was brought all the way from Scotland

An examination of the six-ton ​​altar stone at the heart of Stonehenge has shown that it was almost certainly brought there from northeast Scotland, a much longer distance than all the other stones in the megalithic structure.

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

How the altar stone was transported all the way from Scotland to the south of England is unknown, but it was most likely brought by sea, says Curtin. There is evidence that people were making sea journeys at the time, he says.

Construction of Stonehenge is thought to have taken place over a period of about 1500 years, beginning about 5100 years ago. It consists of an outer circle of large stones, each weighing about 25 tons, called sarsen stones, and an inner ring and altar of smaller stones, generally weighing about 3 tons, called bluestones. The term bluestone simply refers to any stone that is not a sarsen – the bluestones are made up of several different types of stone.

“What’s unique about Stonehenge is the distance the stones were transported,” says geologist Richard Bevins of Aberystwyth University in the UK. Most stone circles are made from stones found within a one-kilometer radius of the site, Bevins says.

However, the origin of the sarsen stones could be traced back to the West Woods of Wiltshire, around 25 kilometres from the site. And Bevins’ team was able to prove that almost all of the bluestones came from the Preseli Hills in Wales, around 280 kilometres away. One theory is that they were part of an even older Welsh stone monument that was relocated.

The altar stone at the heart of Stonehenge is different from the other bluestones. “At the end of 2021, we concluded that the altar stone did not match any of the geology we knew in Wales,” says team member Nick Pearce, also from Aberystwyth University.

This 5-metre-long stone is set so deep into the ground that only one side is visible. It is partially covered by two other stones. It is believed to have been erected around 4,500 years ago.

Clarke has now used sophisticated equipment normally used in the mining industry and analyzed samples of the Altar Stone. It is a sandstone, meaning it is made up of eroded rock grains that piled up at the bottom of a primordial ocean and eventually stuck together to form a new stone. The age of each grain varies depending on when the stone it eroded from formed, so different sandstones have a distinctive mix of grains of different ages.

Clarke analyzed individual crystals of the minerals zircon, apatite and rutile in samples of the Altar Stone. These minerals contain uranium, which decays very slowly to lead, so they can be dated based on the ratio of uranium to lead. The zircon in the Altar Stone, for example, is between 500 million and 3 billion years old.

The age pattern shows with over 95 percent certainty that the Altar Stone sandstone is Old Red Sandstone from the Orcadian Basin in northeast Scotland, says team member Chris Kirkland of Curtin University. The Orcadian Basin was once a huge ancient lake called Lake Orcadia.

The closest Old Red sandstone to Stonehenge is 750 kilometres away near Inverness, and the furthest is in the Shetland Islands, up to 1,000 kilometres away, so the team believes the altar stone was probably transported by sea.

Glaciers can carry rocks long distances, but there is evidence that during the last ice age, ice in the Orkney region flowed north rather than south, Kirkland says.

So why was the altar stone transported such a long distance? “That’s the big question that has no answer,” says Clarke. “All we know is that it’s a six-ton ​​boulder that was transported here from 750 kilometers away. That alone tells us a lot about Neolithic society and its interconnectedness.”

“They’ve done a very meticulous job,” says David Nash of Britain’s University of Brighton, whose team has determined the exact origin of the Wiltshire sarsens. “It’s really solid work.”

Determining the exact origin of the altar stone will be difficult because the Orkney Basin covers a huge area and is up to 8 kilometers deep, says Nash. “It’s a huge task because there is a lot of Old Red Sandstone in the north of Scotland.”

In the case of the sarsen stones, however, there were fewer possible sources, making it easier to determine the exact source, 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 immigrants around 4,000 years ago. This may be because a plague epidemic wiped out a large part of the European population at that time.