Extremely heavy and incredibly far; or the saga of the strangest stone of Stonehenge

Most visitors to Stonehenge come to admire the site’s most striking stones: the huge, upright sarsen blocks that form the circle’s pillars, as well as the scattered bluestones that take on a bluish tinge after rain. Yet one of the strangest bluestones, lying at the heart of the huddled monument, has a different appearance and origin. This six-tonne slab of red sandstone, the Altar Stone, lies flat on the ground, almost obscured by toppled pillars that tumbled onto the sandstone long ago. “The Altar Stone might escape most people’s notice today,” says Amanda Chadburn, a visiting lecturer at Bournemouth University and a trustee of World Heritage UK. Yet to the attentive visitor, the stone peeks out, decidedly grey-purple and darker than the stones above it.

The altar stone was named in 1620 by Inigo Jones, the court architect of King James I, who surveyed, measured and drew the monument. Stonehenge contains two concentric arrangements of stones. The inner horseshoe contained five trilithons: two vertical stones topped by a horizontal lintel stone, which were fitted into one another. Two of the trilithons have partially fallen over, and the altar stone was placed in a key location directly in front of one of the tallest trilithons, “which is probably the focal point of the monument and through which you can view the setting sun at the winter solstice,” Chadburn said.

Stonehenge was built over a period of about 1,500 years, with the first large stones placed around 2500 BC. In recent years, researchers have traced the origins of various stones to a handful of locations, some many miles from Stonehenge. Archaeologists suspected and later confirmed that the large sarsen stones, each weighing about 25 tons, came from the Marlborough Downs area about 20 miles away. In 2015, researchers confirmed that the smaller bluestones came from a site 180 miles away in southwest Wales – perhaps an even more impressive transportation feat for the Neolithic people who built Stonehenge. The altar stone was thought to have come from south Wales until researchers debunked the theory in 2023.

Now scientists have found that the altar stone actually comes from northeast Scotland. The 5.5-ton, 4.8-meter-high boulder had to be hauled at least 745 kilometers over land or an even longer sea route. It appears to be the stone that has traveled the furthest to get to Stonehenge, and its journey took place before the invention of the wheel. “I was delighted and excited by the discovery, but honestly not that surprised,” said Chadburn, who was not involved in the new study. She noted that a famous polished club head The monument found at Stonehenge was made from a metamorphic rock called gneiss, which almost certainly came from the Isle of Lewis in Scotland, showing “a connection of some sort with Scotland during the early phase of the monument,” she added.

The authors of the new study, based in Australia, Wales and England, tracked down the altar stone by examining the age and chemical fingerprint of its fragments. In particular, they analyzed grains of zircon, apatite and rutile that contained uranium, which allowed them to determine the stone’s age through radiometric dating. “Given its Scottish origin, the results raise intriguing questions about how such a massive stone could have been transported over long distances around 2600 BC, given the technological limitations of the Neolithic period,” Anthony Clarke, a doctoral student at Curtin University in Perth, Australia, and author of the new study, said in a opinion.

It is certainly a fascinating question: How on earth could prehistoric people move this massive boulder nearly 800 kilometers, especially without the wheel? The apparent improbability of such a journey prompted one researcher suggest that the other bluestones of Stonehenge were not transported by prehistoric people, but were deposited by a glacier that pushed them over from West Wales and deposited them on the field. Archaeologists overwhelmingly disagree with this hypothesis.

Could the Altar Stone have been a boulder? For some, the new paper has re-opened this old question, but the authors of the new paper dismiss this theory as geologically impossible. Glaciers in northeast Scotland have been moving north over the past two ages, and there is little evidence of Scottish boulders around Stonehenge. Chadburn says she is also completely convinced, pointing out that the sarsen stones came from 25 miles away. “These can only have been transported by humans, I see no reason why a much smaller stone could not have been transported by humans as well,” she said. In the eyes of Mike Pitts, the author of How to build Stonehenge, The insistence on the glacier theory betrays a certain modern superiority complex. “Even if it were possible, why this reluctance to attribute abilities or imagination to Neolithic people?” Pitts tweeted.

The most fascinating and eternal question is perhaps not how, but Why People would have gone to such lengths to build Stonehenge or any of the hundreds of Neolithic stone circles in the British Isles. This question has been around for centuries: When the prolific English diarist Samuel Pepys visited Stonehenge in 1668, wrote of the stones: “I went there and found that they were as amazing as any story I had ever heard about them, and that it was worth making the journey to see them. God knows what they were used for.”

Although Stonehenge was built around the same time as the Sphinx and the Great Pyramid of Giza, much less is known about the British monument. “My best guess is that Stonehenge was some kind of religious-funeral monument, or a temple if you like,” said Chadburn, who recently co-authored a book with Clive Ruggles called Stonehenge: The sun spotted, which explores the site’s relationship with the sky and early astronomy. Built to align with the sun at the solstice, Stonehenge was clearly one of the most important monuments of its time to the people who built and visited it, Chadburn said.

The Altar Stone remains a particular mystery, as researchers do not know whether the stone was originally placed standing or lying down, or exactly when it arrived on the site. The authors of the new paper argue that people would have likely transported it by sea rather than taking a rough overland route. Pitts, on the other hand, argues for an overland route, stressing the dangers that can arise if such a valuable cargo is lost on a boat. And an overland route would also have touched many villages along the way, some of which had their own henges. “Stonehenge was not just a great ceremonial or religious monument, but also a social structure: it was designed to impress and bring together far-flung communities, so transportation and construction were important parts of its purpose and identity,” Pitts wrote in his personal blog.

However the altar stone was brought to the heart of Stonehenge, it strengthens the links between Neolithic societies across the British Isles. Stonehenge was not just a geological collaboration, but a human one too. “It seems that it could have had significance for many people across the island of Britain, as the building materials appear to have been sourced from various places on the island,” Chadburn said. “What it meant exactly is very difficult to decipher.”