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In the ’30s — the 1130s — Welsh cleric Geoffrey of Monmouth created the impression that Stonehenge was built as a memorial to British nobles slain by the Saxons, with the wizard Merlin enlisted to move a ring of giant mystical stones from Ireland to southern England.

In the ’50s — the 1950s — a chunk of rock went missing. A 3½-foot core had been drilled out of one of the site’s sarsen stones during repairs and taken home by an employee of the firm that carried out the work.

The core, repatriated after 60 years, turned out to be pivotal, helping pinpoint the source of the sarsens — a mystery that has bedeviled geologists and archaeologists.

Two kinds of stones make up the 5,000-year-old monument. A small inner horseshoe consists of blocks of varied geology, called bluestone.

The sarsens, sandstone slabs that weigh 20 tons on average, form the central horseshoe, the uprights and lintels of the ragged outer circle, as well as the outlying Heel Stone, Slaughter Stone and Station Stones. Conventional wisdom holds that they derived from Marlborough Downs, 18 miles north.

David Nash, a geomorphologist at the University of Brighton and lead author on the study published in the journal Science Advances, said the idea that the slabs hailed from the Downs dates to the writings of William Lambarde, a 16th-century antiquarian. “Lambarde came to that conclusion based on little more than the appearance of the stones,” Nash said. “This idea has stuck around for more than 400 years but has never been tested.”

Nash traced the source of almost all the sarsens to West Woods, several miles closer to Stonehenge. His team analyzed the geochemical fingerprint of the 52 sarsens that remain in situ at the ancient site.

The breakthrough came last summer when the long-lost core from Stone 58 was returned to English Heritage, the charity that manages Stonehenge. The sarsen cylinder offered Nash the opportunity to analyze a sample unaffected by surface weathering. Drilling through the ancient stones is now discouraged.

“There are literally thousands of pieces of sarsen sitting in museums across Britain,” he said. “However, to my knowledge, the core from 58 is the only piece where we can identify precisely which stone it came from.”