Aug 2, 2020 9:00 AM

Archaeologists Have Found the Source of Stonehenge’s Boulders

Modern scholars have only been able to speculate about where the huge stones came from—until now.

The huge slabs of stone that make up the most iconic structures at Stonehenge came from about 25 kilometers away, according to chemical analysis. Since the 1500s, most Stonehenge scholars have assumed the 6- to 7-meter-tall, 20-metric-ton sarsen stones came from nearby Marlborough Downs, and a recent study by University of Brighton archaeologist David Nash and his colleagues has now confirmed that.

Recent studies have traced Stonehenge’s bluestones to quarries in the Preseli Hills of western Wales, about 300 kilometers (200 miles) away. When another group of archaeologists studied the chemical isotope ratios in the cremated remains of people once buried beneath the bluestones, those researchers found that many of those people may have come from the same part of Wales between 3100 and 2400 BC. Ancient builders set up the sarsen stones a few centuries after the arrival of the bluestones. Modern scholars have only been able to speculate about where the huge boulders came from—until now.

Sarsen, also called silcrete, is a sedimentary rock mostly made up of quartz sand cemented by silica (quartz is just silica in crystal form), formed in layers of sandy sediment. Thanks to erosion, sarsen boulders are now scattered in clumps all over southern England. Prehistoric Britons built monuments like Stonehenge and Avebury with sarsen boulders, Roman settlers used sarsen bricks to build their villas, and medieval people built sarsen churches and farm buildings. But the largest sarsen boulders we know of in Britain today are the ones at Stonehenge.

About 99 percent of the average sarsen boulder is silica, but the other 1 percent contains trace amounts of other elements, like aluminum, calcium, iron, potassium, magnesium, and others. That extra material is different in different sarsen sources, as it depends on the minerals in the ground where the rock formed. Nash and his colleagues used those trace elements as a geochemical fingerprint to match the Stonehenge sarsens to their most likely source.

The largest concentration of sarsen in the UK is at Marlborough Downs, an area of round, grassy hills 25 to 30 kilometers (17 miles) north of Stonehenge. Centuries of archaeologists and antiquarians have assumed the Stonehenge sarsens came from the Marlborough Downs, mostly because the area is nearby and full of the right material. But that idea hadn’t been scientifically tested, and the bluestones demonstrate that the Neolithic people who built Stonehenge had a far-flung and complex supply network—and their own reasons for doing things, often inscrutable to modern researchers.

To track down the source of the sarsens, archaeologists first had to solve a more recent mystery: What happened to three missing chunks of Stonehenge?

One of the trilithons (arch-like structures made of two upright stones supporting a horizontal lintel stone) in the central horseshoe fell down in 1797. A century and a half later, in 1958, a restoration project set the massive stones in place again—but one of the uprights, called Stone 58, had cracked along its length. To help hold the cracked stone together so it could stand and support its half of the lintel stone, restorers drilled three holes through the stone and inserted metal ties. After the project, the three stone cores they’d drilled out seemed to vanish into thin air.

In 2018 one of the restorers, Robert Phillips, returned a broken but complete core from Stone 58 to the UK. Part of a second core turned up in the Salisbury Museum in 2019, but one and a half of the stone cores are still out there somewhere. Samples from the Phillips core gave Nash and his colleagues the chance to compare the chemical makeup of Stone 58 to sarsen boulders from sites all over Britain.

The match turned out to be exactly what various researchers had assumed for the last 500 years. The only boulders that matched Stone 58 came from one site in the southeastern Marlborough Downs: West Woods, in Wiltshire, about 25 kilometers (16 miles) north of Stonehenge and just 3 kilometers (2 miles) south of where most studies had looked for Neolithic sarsen quarries. West Woods is a 6-square-kilometer (4-square-mile) plateau, partially wooded and dotted with large sarsen boulders and pits from millennia of quarrying.

“West Woods lies within a concentration of Early Neolithic activity, being close to Avebury, numerous long barrows, and the causewayed enclosures at Knapp Hill,” wrote Nash and his colleagues. Thousands of years ago, many of the woods were open farmland, which probably fed the same people who built Stonehenge and Avebury and feasted at nearby Durrington Walls. From West Woods, ancient builders probably hauled the sarsens down into the Vale of Pewsey and along the River Avon to Stonehenge, where the bluestones had already stood for centuries.

Nash and his colleagues used portable x-ray fluorescence to check the chemical makeup of all 52 sarsens at Stonehenge (the only survivors of the 80 sarsens that once stood at the site). Each element emits a slightly different wavelength of light when hit by x-rays, and by measuring those emissions, researchers can map the composition of an object without damaging it. Fifty of the sarsens shared very similar chemical fingerprints, which means they probably all came from the same place. And since those 50 sarsens included Stone 58, it’s likely that they all came from West Woods.

The other two surviving sarsens came from two different places, which archaeologists haven’t pinpointed yet. Nash and his colleagues say different communities with access to different stone sources may have contributed stones, and those communities may have brought some of the sarsens now missing from the monument. “But we will probably never know,” they wrote.

Missing stones notwithstanding (pun intended), the 50 sarsens made up different sections of Stonehenge, but they all came from the same place. That means that people probably built the outer Sarsen Circle, the Central Horseshoe, and the individual stones like the Slaughter Stone and Station Stones at around the same time, probably during the second phase of construction around 2500 BC.

The study also raises some new questions, such as why Neolithic people, who had so many sarsen sources to select from, would go to one site in particular. “Why, in a region with the greatest density of extant sarsen stones in Britain, West Wood was selected as the primary source for the Stonehenge sarsens is unclear,” Nash and his colleagues wrote. They suggest that West Woods may have offered larger or better-quality stones than other sites, or that its location may have made it easier to transport the boulders to Stonehenge.

As always, there’s more work to be done and more evidence to look for. “Archaeological investigations and further detailed sampling of sarsens from West Woods and surrounding areas are now required to more tightly constrain the precise source areas and identify prehistoric sarsen extraction pits,” wrote Nash and his colleagues.

Science Advances, 2020 DOI: 10.1126/sciadv.abc0133; (About DOIs).

This story originally appeared on Ars Technica.