Archaeologists have solved a longstanding mystery about Stonehenge: the origin of the monument's iconic sandstone boulders

The origin story of Stonehenge has baffled archaeologists for centuries.

The mysterious monument, erected in two waves of flurried construction 5,000 and 4,500 years ago, on the UK's Salisbury Plain features two distinct types of stone slabs in half circles.

Researchers traced one stone type, the smaller bluestones, to a site in Wales. But the origin of Stonehenge's 30-foot (9-meter) sandstone boulders, called sarsens, remained an unsolved puzzle until now. 

According to a study published Wednesday in the journal Science Advances, Stonehenge's builders dragged most of these 50,000-pound (22,700-kilogram) sarsens from a woodland area in Wiltshire.

The area, called West Woods, is more than 15 miles (25 kilometers) from the monument — "which is insane really if you think about it," David Nash, the lead author of the study, told Business Insider.

He added, "Our results suggest that most of the sarsens at Stonehenge share a common chemistry, which is why we're saying they come from the same area."

The findings could help archaeologists figure out how the builders transported the giant stones south.

Stonehenge's sarsens came from woodlands 15 miles away

Stonehenge originally had 80 sarsens erected in square-shaped archways, but only 52 remain.

According to Nash's team's analysis of elements present in the rocks, 50 of those 52 sarsens share the same chemical makeup.

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Armed with that chemical signature, the team searched for other sarsens across the southern UK and compared those boulders with the ones at Stonehenge. They found their match in West Woods, about 15 miles north of the monument.

Before this discovery, archaeologists had speculated the sarsens came from a nearby region called Marlborough Downs, since "there were big grey stones at Stonehenge, and the sarsens in Marlborough Downs were big and grey," Nash said.

The West Woods are part of that region, but researchers hadn't ever searched that particular spot for clues because most of the sarsens there were hidden under vegetation. 

Plus, Nash said, the distant origin of Stonehenge's bluestones offered evidence that builders didn't necessarily cull rocks from the most convenient areas.

"Given that the builders bothered to bring bluestones from Wales to Stonehenge, why would they bother bringing sarsens from the closest place?" Nash said. At least four dozen 2- to 5-ton bluestones came from Wales' Preseli Hills, about 150 miles away. 

"The people who built Stonehenge wouldn't have been bothered about distance," Nash added.

The reason the builders used sarsens from West Woods is still unclear, but the study authors suggested it likely had to do with "the size and quality of the stones present there."

All the sarsens may have been moved at the same time

The new finding doesn't confirm what Stonehenge was used for — Nash said theories include a burial and cremating ground and a place of ancient healing. But knowing where the sarsens came from could at least help experts figure out how the monument's builders erected it and the route they took to transport their building materials.

Nash said it was likely Stonehenge's builders used some sort of roller or dragged the sarsens on a slippery surface like vegetation or frosty ground.

"There's no evidence they used animals to do it, but we don't know," he said.

The new study also supports the idea that the builders carved and raised all the sarsens into their standing positions in Stonehenge's rock circle at the same time, about 2,500 BC, after transporting them en masse.

"For me, it kind of confirms the idea that all the stones were moved in one go, at same time," Nash said. "It's an astonishing thought: How many people would need be involved dragging enormous boulders as part of one big project?"