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In the hope of solving one of the persistent mysteries surrounding the iconic monolith, geologists have moved the search for the origins of Stonehenge's central altar stone to Scotland, after recent research moved the search elsewhere from Wales.
But in a surprising twist, new analysis suggests the stone did not come from Orkney, an archipelago off the northeast coast of Scotland that is home to 5,000-year-old Neolithic sites.
The findings, published Sept. 5 in the Journal of Archaeological Science: Reports, follow a study published in August that pointed to northeastern Scotland as the likely origin of the Altar Stone. That research, published in the journal Nature, dismissed a centuries-old idea that the stone came from what is now Wales.
The Altar Stone, the largest of the bluestones used to build Stonehenge, lies at the heart of the ancient monument in southern England. The 13,227-pound (6 metric ton) block was likely transported 435 miles (700 kilometers) from northeast Scotland, possibly by sea, nearly 5,000 years ago. But the stone's purpose remains a mystery.
A geological team, including many of the same authors as the Nature study, examined Neolithic stones at two well-known ancient sites on Mainland, Orkney's largest island, that were similar in size and rock type to the Altar Stone. But the analysis found no connection.
"The mystery of where the stone came from is becoming increasingly clear as we begin to rule out specific areas in north-east Scotland," lead researcher Richard Bevins, an honorary professor in the department of geography and earth sciences at Britain's University of Aberystwyth, said in a statement.
"This research radically changes the way we think about the origins of the Altar Stone. It is exciting to know that our chemical analysis and dating work is slowly unraveling this great mystery."
Putting together an age-old puzzle
Previous research suggests that construction of Stonehenge began as early as 3000 BC and occurred in several phases. It is thought that the altar stone was placed in the central horseshoe during the second phase of construction, sometime between 2620 and 2480 BC.
Extensive research has been conducted into the types of stone used over the years to assemble the iconic circle in Wiltshire, England. Previous analyses have shown that bluestones, a type of fine-grained sandstone, and silicified sandstone blocks called sarsens were used in the construction of the monument. The monument stands on the southern edge of Salisbury Plain, which was inhabited between 5,000 and 6,000 years ago.
The sarsens came from the West Woods near Marlborough, about 25 kilometres away. Some of the bluestones came from the Preseli Hills area in West Wales and are thought to be the first stones to be placed at this site.
"The Altar Stone is different in many ways from both the bluestones and the sarsens at Stonehenge," Bevins said. "It is in an unusual position at the monument, in a near-central location and not within the Bluestone Circle or the Bluestone Horseshoe. It is also a grey-green sandstone, unlike any other bluestone."
Bevins has been researching Stonehenge for 15 years and his quest to find the stones' origins has taken him across England and Wales. But the discovery that Wales was probably not the original site for the Altar Stone was a game changer, Bevins said.
"Suddenly, we couldn't think about where the Altar Stone might have come from, which opened up entirely new possibilities for research and exploration," Bevins said via email.
Looking for answers in Scotland
The search led the researchers to Mainland in June 2023 to conduct fieldwork.
"Evidence for long-distance connections between Orkney and Stonehenge is partly related to the exchange of particular styles of material culture," Bevins said.
The longstanding link between Stonehenge and Orkney includes the styles of decorated pottery and similar architecture of ancient dwellings found at both sites, Bevins said.
The team brought portable X-ray machines to Orkney to conduct chemical analysis of minerals in exposed rocks at the Stones of Stenness and the Ring of Brodgar, two ancient remains of stone monuments. The Ring of Brodgar is a huge ceremonial stone circle dating back to the third millennium BC, while the Stones of Stenness were once a circle of 12 stones with a central hearth built more than 5,000 years ago, one of the earliest monuments in the British Isles.
The researchers compared their findings with samples of the Altar Stone and found no mineralogical relationship between the rocks.
But the study suggests it is still likely that the Altar Stone came from elsewhere in northeastern Scotland, providing a large search area for the team to investigate in the future. The stone's origins could be anywhere between "Orkney and Shetland, through parts of Caithness and Sutherland, to Inverness and then east into Aberdeenshire," Bevins said.
Identifying the source of the altar stone could help researchers gain more insight into Britain's Neolithic history, the people who built Stonehenge and how advanced they were.
"As an academic, I have been fascinated by Stonehenge for decades," Bevins said. "Myself and my other colleagues in the team will continue to try to find out exactly where in north-east Scotland the Altar Stone came from."
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