By Will Dunham
(Reuters) – At the centre of Stonehenge lies the Altar Stone, a massive slab of sandstone whose origins and purpose are among the enduring mysteries of the famous megalithic monument. A new analysis has revealed that this rectangular colossus has undergone a remarkable journey to become part of one of humanity’s ancient wonders.
Its geochemical fingerprint matches perfectly with bedrock found in northeast Scotland, researchers said Wednesday, suggesting the altar stone – weighing an estimated six tons – was transported by the builders of Stonehenge about 700 to 750 kilometers (435 to 465 miles) to Salisbury Plain in southern England.
The results stunned researchers. Stones from other monuments from this period are not known to have been transported over such a distance.
“We couldn’t believe it,” said Anthony Clarke, a doctoral student in geology at Curtin University in Australia and lead author of the study published in the journal Nature.
For the past century it was widely believed that it came from Wales, like several other major Stonehenge components. The Altar Stone, which is recumbent rather than standing, is 4.9 metres (16 feet) long, one metre (3-1/4 feet) wide and half a metre (1-2/3 feet) thick. It is made of grey-green sandstone, although its undulating and weathered surface now has a reddish-brown tinge.
Transport over such a distance – possibly by both land and sea – suggests a level of social organisation among the Neolithic communities of Britain that is unexpected for the time of the movement, which is thought to have occurred between 4,600 and 2,500 years ago, roughly contemporary with the great pyramids of Ancient Egypt.
“This discovery fundamentally refines our understanding of prehistoric contexts and technology in Neolithic Britain,” said Chris Kirkland, professor of isotope geology at Curtin University and co-author of the study.
Previous analysis had identified two main types of stones used to build the monument’s stone circle: some came from around Marlborough, about 25 km (15 miles) away, and others from the Pembrokeshire area of Wales, 250 km (150 miles) away.
Sandstone contains grains of various minerals. The researchers analyzed the age and chemistry of zircon, apatite and rutile grains in two altar stone fragments. The zircon was mainly dated to one to two billion years old. The apatite and rutile were dated to about 450 million years old.
The chemical properties of the fragments were consistent with Scottish bedrock from the Orkney Basin region, which extends from Inverness to the northeastern tip of Scotland and beyond.
“It’s a microanalysis technique,” said geologist and study co-author Nick Pearce of Aberystwyth University in Wales.
The researchers used a laser beam to vaporize a selected mineral in the rock. They then analyzed the gas vapor and measured the ratio of isotopes – variants of a particular element – of uranium and lead to determine the age of the mineral.
“And the match of this kind of age imprint that we got from these minerals is identical to the Orkney Basin in Scotland, not Wales,” Pearce said. “So it’s suddenly gone from about 250 kilometers (155 miles) to 700 kilometers from Stonehenge.”
Partially covered by two other fallen stones, the Altar Stone is the only large stone in the monument that is not part of its circular design. Although the exact location is unknown, getting such a large load from Scotland to Wiltshire must have been a challenge, highlighting the importance of the Altar Stone to the builders of Stonehenge.
“How it was moved is unclear, but sea is a reasonable assumption given the mountainous and forested terrain of possible land routes,” said Aberystwyth University geologist and study co-author Richard Bevins.
The researchers say that the simple boats of the time could have been used for transport along the coast.
The impressive name of the altar stone goes back to an interpretation by the 17th century English architect Inigo Jones.
“It gets its name from the fact that it’s flat like a table, but we don’t know if it’s always been that way, nor do we know what it was used for exactly. All we know is that it’s illuminated by the setting sun at the solstice,” Kirkland said.
“That’s really unique about this monument,” Bevins said. “There’s a lot of uncertainty about what the altar stone means.”
Built in several phases over a period of about 500 years starting around 3000 BC, Stonehenge remains a fascinating site that attracts tourists from all over the world.
“It creates a connection to our ancestors who created something that has stood the test of time,” Kirkland said.
(Reporting by Will Dunham in Washington; additional reporting by Matthew Stock in London; editing by Rosalba O’Brien)
