The mysterious origin of Stonehenge’s altar stone might have been solved

After more than a century of searching, researchers may be closing in on the source of the altar stone that lies in the center of Stonehenge. The age and chemistry of minerals that make up the sandstone block point to an area in Scotland—some 466 miles away from the monument, researchers report today in Nature.  

“It’s amazing,” says Susan Greaney, an archaeologist at the University of Exeter in England who was not part of the work. It’s “really exciting,” she says, that the team identified a location in the far northeast of Scotland—possibly even Orkney, which seems to have been a hotspot of Neolithic culture and activity. Meanwhile, Stonehenge stands at Salisbury Plain in Southern England, and its construction began around 5,000 years ago in the same timeframe. “It underlines links between those two areas that have been, up to now, a kind of hypothesis.”

Questions of why and how ancient people built the stone circle have long perplexed researchers, including where the stones were sourced. Recent detective work tracked the sarsen stones that make up Stonehenge’s iconic outer ring to about 16 miles north of Salisbury Plain. The monument’s bluestones, or rocks that aren’t local, have been linked to Wales going back to the 1920s. Bevins and colleagues have traced some of those stones to outcrops in southwest Wales, about 140 miles from Stonehenge. (See National Geographic’s first photo of Stonehenge from 1922.)

But the so-called altar stone has remained an enigma, despite efforts to pinpoint its origin since the 1870s and 1880s, says Richard Bevins, an earth scientist at Aberystwyth University in Wales and one of the study’s authors. The stone’s true use remains unknown, but its placement evokes an altar, hence the name. “This stone is different from the bluestones in terms of its weight, its size, the type of rock it is, its position in the monument,” he says.

Sourcing the stone

Bevins has been searching for the source of the altar stone for 15 years. By comparing the chemistry of the altar stone with outcrops across Wales and areas of England, his team has ruled out dozens of potential sites. Now they’ve finally found a match. “It’s quite remarkable,” he says. “You have to pinch yourself occasionally.”   

This time, Bevins teamed up with Anthony Clarke, an earth science graduate student at Curtin University in Perth, Australia, to borrow techniques from geology. Working with a fragment removed from the altar stone in 1844 and verified as a match to the block’s chemical makeup, the researchers identified the ages of the different minerals that had cemented together to form the sandstone. They compared these results from the altar stone with data reported for outcrops of sedimentary rocks across Great Britain and Ireland.

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“The only area where there was a match was northeast Scotland,” says study coauthor Nick Pearce, a geologist and geochemist also at Aberystwyth University. “These fingerprints are very characteristic,” he says. “I don’t think there’ll be anywhere else where it would match.”

The locale, called the Orcadian Basin, covers several thousands of square kilometers, and spans up to around five miles thick in some places. The exact spot the stone came from remains unknown.

By land or by sea?

It’s also not clear how Neolithic people would have transported the over 6-ton, 16-foot-long rock across this vast distance. Some have posited that glaciers could have carried the monument’s stones. Based on what’s known about the movement of glaciers across the British Isles, “there’s no way, pretty much, that a block of sandstone that size would have been transported from northern Scotland to Stonehenge by ice,” says David Nash, a geomorphologist at the University of Brighton in England who wasn’t involved with the study. Though a glacier could have dragged it part of the way, he says.

Perhaps ancient builders could have shuttled the altar stone over land—also unlikely. Scotland is “incredibly mountainous,” and Britain was heavily forested back then, Greaney says.

Instead, the authors suggest, ancient people may have shipped the block by sea. They probably navigated along the coastline, and then maybe inland by river before carrying the stone over land to its resting place, says Jim Leary, an archaeologist at the University of York in England who wasn’t involved with the work. The exact timing of the block’s passage and when it arrived at Stonehenge is unknown. But there’s evidence that people were moving other heavy things by sea around this time, including cattle, and that they had seafaring boats to travel between islands.

The stone must have had profound importance because of how far Neolithic people moved it, Leary notes. It’s possible the megalith was used in other stone circles or monuments along the way, he says. “You can imagine that maybe that stone has had a very long, circuitous route over perhaps a period of 100 plus years before ending up at Stonehenge.” (It’s not just Stonehenge. Read about Great Britain’s other megalithic monuments.)

The altar stone’s long journey highlights the connections between different communities across what are now the British Isles, Greaney says. Archaeologists have traced links during the Neolithic between places across Scotland, Wales, England and Ireland through similarities in tools, pottery and monuments. Evidence of prehistoric pig roasts at sites around Stonehenge also suggests the area brought disparate communities together.

People seem to have traveled between these places taking ideas and ritual practices with them, and they probably shared religious beliefs and had similarities in their language and how they organized their societies, Greaney says. Even the altar stone’s central position at Stonehenge speaks to the importance of these ties. “It’s one unusual stone, and it’s right flat back in the center, and the rest of the monument is arranged around it,” she says. These connections, she says, “must have been really important to the people building Stonehenge, otherwise they wouldn’t have put it in such a prominent position.”