As Robert Phillips neared his 90th birthday, the former diamond cutter decided to return a priceless piece of history to the United Kingdom: a 91-centimeter-long cylinder of rock from the heart of Stonehenge. Now, archaeologists working with the so-called Phillips core have all but conclusively shown that the famed monument’s largest building blocks came from a forest about 25 kilometers away, confirming a long-standing hypothesis.
It’s “a quality piece of work,” says Richard Bevins, a geologist and Stonehenge researcher at the National Museum Cardiff. Knowing the stones’ origin could reveal future excavation sites in the region, shedding light on the ancient stoneworkers who constructed these mysterious monuments.
Beginning around 3000 B.C.E., Stonehenge was built up over hundreds of years as a ceremonial spot by people hailing from what today is Wales. The monument includes 52 massive, 25-ton silica stones known as sarsens. For centuries, most researchers have assumed the stones came from the closest major boulder field, some 25 to 30 kilometers north of the site in a region called the Marlborough Downs.
Others have pointed out that the monument’s mysterious masons weren’t necessarily sticklers for convenience. After all, Stonehenge’s smaller “bluestones”—which lie at its center and have been much more intensively studied than the sarsens—traveled some 150 kilometers from various sites around Wales.
Enter the Phillips core. In 1958, Phillips was part of a crew contracted to re-erect three massive blocks that had toppled more than 100 years earlier at Stonehenge. When the workers lifted one of them, Stone 58, they realized it was cracked. So they cut a hole through it and pinned it with a metal bolt to reinforce it. Phillips took the drilled-out core as a souvenir; it hung in his U.K. office for years before he took it with him when he retired to Florida.
When the Phillips core re-emerged in 2018, David Nash, an archaeologist and geographer at the University of Brighton, and colleagues knew they finally had the missing piece they needed to pinpoint the sarsens’ origins. Today, destroying intact pieces of Stonehenge is forbidden, Nash says, but because it has already been removed, the Phillips core gave them a unique opportunity for analysis.
Nash and University of Brighton geologist Jake Ciborowski used a portable x-ray spectrometer—which Nash says “looks a bit like a ray gun from an old sci-fi movie”—to take nondestructive surface readings of all 52 sarsens’ chemical compositions. Despite being more than 99% silica, the stones also contained traces of other elements, including aluminum, carbon, iron, potassium, and magnesium. Critically, 50 of the 52 sarsens—including the Phillips core’s parent—had a practically identical chemical makeup, suggesting the stones all came from a single site.
Nash’s team then pulverized half of the Phillips core and ran it through a gantlet of chemical analyses that return a much higher resolution signature than x-ray spectrometry can provide.
When the researchers compared the core’s detailed chemical signature to samples taken from 20 boulder fields from southern and eastern England, it matched with nearly 100% confidence to boulders from a forest called West Woods. Located in the southeast of the Marlborough Downs, that’s precisely where most experts had long assumed the sarsens originated, the researchers report today in Science Advances.
“This really does fill in that gap with a lot more confidence,” says Richard Madgwick, an archaeologist at Cardiff University who studies Stonehenge. He stops short of calling the case closed, however, adding that the team will need to sample far more than 20 sites before he’s convinced they’ve ruled out all other candidates.
Robert Ixer, a geologist at the University College London Institute of Archaeology, agrees that more sites should be sampled. But after decades of bluestone-focused research, he’s happy to see sarsens getting their due. “It’s the sarsens’ time, really,” he says. “They have been for too long neglected.”