Near the center of the roughly 5,000-year-old circular monument known as Stonehenge is a six-ton, rectangular chunk of red sandstone. In Arthurian legend, the so-called Altar Stone was part of the ring of giant rocks that the wizard Merlin magically transported from Mount Killaurus, in Ireland, to Salisbury Plain, a chalk plateau in southern England — a journey chronicled around 1136 by a Welsh cleric, Geoffrey of Monmouth, in his “Historia Regum Britanniae.”

Since then, the accepted provenance of the Altar Stone has shifted, spanning a range of possible sites from east Wales and the Marches to northern England. On Wednesday, a study in the journal Nature reroutes the megalith’s odyssey more definitively, proposing a path much longer than scientists had thought possible.

The researchers analyzed the chemical composition and the ages of mineral grains in two microscopic fragments of the Altar Stone. This pinpointed the stone’s source to the Orcadian Basin in northeast Scotland, an area that spans Inverness, the Orkney Islands and Shetland. To reach the archaeological site in Wiltshire, the megalith would have traveled at least 465 miles by land or more than 620 miles along the present-day coastline if it came by sea.

“This is a genuinely shocking result,” said Rob Ixer, a retired mineralogist and research fellow at University College London who collaborated on the project. “The work prompts two important questions: How and why did the stone travel the length of Britain?”

Stonehenge features two kinds of rocks: larger sarsens and smaller bluestones. The sarsens are sandstone slabs found naturally in southern England. They weigh 20 tons on average and were erected in two concentric arrangements. The inner ring is a horseshoe of five trilithons (two uprights capped by a horizontal lintel), of which three complete ones still stand.