Scientists say they have solved the mystery of the 6-tonne rock at the centre of Stonehenge. The discovery deepens the mystery of the site.
The “altar stone,” the sandstone boulder at the center of the ancient monument, likely came from modern-day Scotland, according to a study published Wednesday in the journal Nature. That means it was transported more than 450 miles to southern England, significantly farther than any other stone at Stonehenge of known origin.
The findings raise profound questions. Researchers suspect the altar stone was placed about 4,500 years ago, meaning Neolithic people were able to move the rock hundreds of miles — long before lightweight, spoked wheels were invented.
The findings also suggest that cultural and social structures in the British Isles at the time were more intertwined than previously thought, and that Neolithic people were able to carry out complex projects with relatively simple tools.
The discovery was based on an analysis of the age of mineral grains in the sandstone rock. After profiling the ages of the grains, the researchers were able to compare the altar stone’s age “fingerprint” to a database of sandstone samples from nearby sites in Britain and Brittany, France.
“We can actually relate the age spectrums to a very specific site in north-east Scotland with greater than 95% statistical certainty,” said Chris Kirkland, a co-author of the study and professor of Earth and Planetary Sciences at Curtin University in Australia.
The Orcadian basin, the region Kirkland is referring to, also includes the Orkney Islands, known for their elaborate stone circles.
“We can’t directly answer the specific reason for the transport of this material,” Kirkland said. “All we know is that it’s a 6.5-ton boulder from 450 miles away. That in itself tells us a lot about the Neolithic society and its affiliation.”
Stonehenge—a UNESCO World Heritage site and one of the best-preserved prehistoric megalithic monuments—is surrounded by large sandstone tablets called “sarsens.” These in turn support stone lintels—horizontal openings in the rock, some of which are interlocked by joints. Inside the sarsen outline is an inner circle of “bluestones,” which have an additional horseshoe shape inside that circle.
It is thought that the sarsens were brought from about 24 kilometres north of the Stonehenge site, while the bluestones were brought from Wales, about 225 kilometres away.
The new study concerns the central altar stone: a tablet-shaped boulder about 16 feet tall, showing evidence of being shaped with human tools. Other Stonehenge tablets now sit on top of the altar stone, apparently having been toppled over over time.
“For whatever reason, it’s a special stone,” says David Nash, a professor of physical geography at the University of Brighton who has studied Stonehenge but was not involved in the new research. “It’s a stone that’s very different from any other stone in the monument.”
Kirkland and his fellow researchers looked at three ways the altar stone could have traveled from Scotland to Stonehenge. One is that it was transported by glacial ice during the Ice Age, but the study authors think this is not the most likely explanation. The second is that humans transported it by land, but the team suspects the forested terrain would have been too challenging.
The third option, which they think is most likely, is that the stone was transported by boat. There is evidence of maritime transport during this period, when modern-day England had a different coastline.
Nash said the authors reached “robust conclusions” about the origin of the altar stone.
“What they’ve done is really interesting,” he said, adding that their findings support evidence that Neolithic people travelled across the British Isles and were part of a broader social structure. “There are clearly social structures, there are connections, there’s a place where ideas are communicated very clearly.”
Stonehenge is one of about 1,300 ancient stone circles in existence, according to the British Museum. Researchers think the site’s stones were shaped with hand tools and erected using winches and pulley systems. They are aligned with the movements of the sun and the summer and winter solstices.
Experts suspect that Neolithic people used these areas for ceremonies or rituals, but details are lost to time. It is unclear what specifically made the altar stone special.
“Today millionaires decorate their mansions with Carrara marble from Italy and I will never understand why they do it, so it’s just a mystery,” says Anthony Clarke, lead author of the new study and a PhD student at Curtin University. “Humans have always had a fascination with finding the perfect rock, and perhaps the Neolithic Britons were the same, so their motivation has been lost over time.”
The researchers said they next hope to identify exactly where the rock originated, but the fundamental puzzle is unlikely to be solved.
“They placed a huge value on bringing this rock 700, 800, 900 kilometres,” said Nick Pearce, another co-author of the study and professor of geography and earth sciences at Aberystwyth University in Wales. “However they brought it, it meant something to them. What did it mean? Why was it so important to them? It gives us all something to think about.”
This article was originally published on NBCNews.com