For Stonehenge’s Altar Stone, an Improbably Long Ancient Journey

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.

The bluestones, mainly two- to four-ton rocks made of sandstone and igneous material, derive their name from the bluish-gray hue they acquire when wet or freshly broken. The smaller bluestones were lugged to Stonehenge from as far as 140 miles away and arranged in a double arc between the sarsens. Some of the remaining bluestones have fallen; others are mere stumps. The largest bluestone, 16 feet long and near the center, is the Altar Stone.

Archaeologists speculate that the Altar Stone was installed at Stonehenge during the second construction phase, around 2620 B.C. to 2480 B.C. Back in the Stone Age, during the winter solstice, the sun would have set in a narrow slot between the tallest trilithon and dropped down over the Altar Stone, which was placed across the solstice axis. The effect is no longer visible: Half of the trilithon has collapsed and today lies in a jumble atop the Altar Stone.

Last year a research team led by Richard Bevins, a geologist at Aberystwyth University in Wales and an author of the new study, published a paper demonstrating that the Altar Stone was not Welsh but most likely from the north. The researchers were intrigued by the presence of a few extremely old zircons, chemically stable minerals that are highly resistant to weathering and heat. They enlisted colleagues in Western Australia, who had access to an array of instruments used in the mining industry for research and exploration.

From shards of the Altar Stone, the Australian researchers analyzed grains of zircon, apatite and rutile. “All three minerals contained uranium, which effectively made them miniature atomic clocks,” said Anthony Clarke, a doctoral candidate in geology at Curtin University in Perth, who conducted the inquiry. “You can determine age by measuring the ratio of uranium to lead and using the known rate of uranium decay.”

This kind of radiometric dating led Mr. Clarke and his team to conclude that the sources of the zircon were largely from the Mesoproterozoic Era (1,600 million to 1,000 million years ago) and the Archaean Eon (4 billion to 2.5 billion years ago), while the apatite and rutile were mid-Ordovician (470 million to 458 million years ago). “We squished the ages together to create a fingerprint of the mineral grains’ source,” he said.

The “fingerprint” was compared to those of sedimentary deposits in thousands of sandstone outcrops in Britain, Ireland and northern Europe. “Putting the data together revealed a striking similarity to the sandstone of the Orcadian Basin,” Mr. Clarke said. “The Altar Stone has a Scottish signature statistically distinct from terrains in the south.”

Nick Pearce, a geochemist at Aberystwyth University who also helped with the new study, said that the new research scuttled the theory that the Altar Stone was a glacial erratic that reached southern England on an ice floe. “During the last two ice ages, the ice directions in northeast Scotland were toward the north,” he said. “The idea that the Altar Stone arrived by glacial transport is almost impossible.”

So how did the Altar Stone get to Salisbury Plain? “There are two options: overland or maritime,” Dr. Pearce said. “Each has merits and huge issues as well.”

Marine transport would have entailed putting a six-ton rock on a Neolithic boat, he said. The land route presented rivers to cross, mountains to go around and dense forests to thwack through. “I’ll leave that question to the archaeologists,” Mr. Clarke said.

Tim Daw, a Wiltshire farmer who once worked at Stonehenge for English Heritage, a charity organization, said the new study brought us closer to understanding our Stone Age past. “How the discovery informs us as to the societal organization of Neolithic people is the start of a great debate,” he said.