Stonehenge is a place where human history and Earth’s history meet. Neolithic people in Great Britain used plant-fiber ropes, smaller rocks, pulleys, logs, and antlers to carve, set, and configure the rings of standing stones.
“There’s always been this one extra stone which has been a bit of a mystery. And it’s a really important stone in the position of the monument, because it’s right in the center.”
But Stonehenge’s story doesn’t start in 3000 BCE—around when scholars believe the first parts of the megalith were arranged. Its central megalith, the Altar Stone, tells a tale spanning millions of years and hundreds of kilometers, according to a new study.
Stonehenge is composed of two types of rocks. The upright sarsen stones were sourced locally. Prehistoric Britons gathered and hauled these boulders—each weighing roughly 20 metric tons (44,000 pounds)—from 25 kilometers (16 miles) away. Many of the smaller rocks in the configuration, known as bluestones, originated from a quarry in Wales, 220 kilometers (140 miles) away.
“There’s always been this one extra stone which has been a bit of a mystery,” said archaeologist Susan Greaney from the University of Exeter, who was not part of the study. “And it’s a really important stone in the position of the monument, because it’s right in the center.” The Altar Stone, a recumbent 6-metric-ton rectangular monolith, tells a different story than its counterparts at Stonehenge.
The new study, published in Nature, chips away at some of the geological puzzles about the Altar Stone, suggesting that the slab originated 750 kilometers (470 miles) away in Scotland’s Orcadian Basin. The finding implies that ancient British civilizations were much more connected by culture—and geology—than previously known.
Tracing the Genetic Heritage of Sandstone
Other Stonehenge bluestones are igneous rock, formed from magma that cooled at or near the surface. “The Altar Stone is sandstone, which means it’s made of tiny, microscopic grains,” said Anthony Clarke, first author of the study and a Ph.D. student in geology at Curtin University. Each grain has a unique age and set of chemical characteristics.
Sandstone, a sedimentary rock, is created through erosion. When a mountain or area of high ground is worn away by wind, rain, rivers, and glaciers, bits of rock are transported and accumulate in layers. Over time, these layers thicken, stick together, and compact, forming sandstone.
“For a geologist, the interest in sandstones is that they contain a wide range of datable minerals,” said Chris Kirkland, a geochronologist also from Curtin University and one of the study’s coauthors. Dating the individual mineral grains can provide information about the origin of the sediment—the rock’s unique fingerprint and homeland. “It means we can compare quite a detailed sequence of ages to a database of previously published ages from other sandstones to arrive at a correlation,” Kirkland explained.
The researchers examined a small sliver of the greenish-gray Altar Stone gathered during a previous archaeological dig. “It’s got a sheen to it,” said Kirkland of the Altar Stone. “It has a texture different than the other rocks.”
Using uranium-lead dating, which combines the measured ratio of uranium to lead isotopes with the known rate of uranium decay, the researchers calculated when three types of minerals contained in the Altar Stone sample (zircon, apatite, and rutile) formed.
Some of these mineral grains were exceptionally ancient—from 1 billion to 2 billion years old. Others were a much more youthful 450 million years old. With this information, the researchers could compare the rock with other sandstone samples and find a match.
The results of this DNA-like test suggested that the Altar Stone’s birthplace was in the Old Red Sandstone of northeastern Scotland’s Orcadian Basin, about 750 kilometers (470 miles) away from Stonehenge.
“Being able to narrow down to a specific area of Old Red Sandstone is quite impressive,” said Anna Bird, a geologist from the University of Hull who was not involved in the study.
Ancient Scottish Roots
Roughly 470 million years ago, the Iapetus Ocean closed, connecting the landmasses of what are now Scotland and England and creating the Himalayan-scale Caledonian mountain range. The range extended through landscapes that now include east Greenland, Scandinavia, the Orkney and Shetland islands, the Scottish Highlands, and the East Coast of North America.
“But once mountains get high, they start to erode,” Bird said.
The Orcadian Basin is located on the tip of northeastern Scotland and the islands to the north. The area is composed of Old Red Sandstone formed as the prehistoric Caledonian mountains shed their rocky DNA into a vast, low-lying basin where sediment could collect for millions of years.
“In Scotland, the rocks are very ancient—some of the oldest rocks in the world. The underlying bedrock is distinctly older [than that of] southern Britain,” Clarke said.
“Two hundred and fifty kilometers is quite extraordinary. Seven hundred and fifty is ridiculous.”
Having tentatively identified the source of the Altar Stone, scientists are now trying to identify the specific outcrop or quarry from which the Altar Stone was extracted and transported south at about 2600 BCE. Some of the researchers have ruled out the Orkney Islands by examining the chemical composition of sandstones used in similar stone circles as well as in other natural rock formations there.
The ancient inhabitants of what are now the Scottish Highlands must have gone through a lot of work to deliver a stone the size of several cars to a site nearly 750 kilometers away. Theories abound about how they managed such a task, because the land route covers hilly, boggy, and heavily forested terrain. Most likely, the Altar Stone was transported by sea, both Clarke and Kirkland suggested.
“Two hundred and fifty kilometers is quite extraordinary. Seven hundred and fifty is ridiculous,” Greaney said. “It connects Stonehenge with that part of northeast Scotland, where we knew there were connections before. There’s very similar shared pottery at that time. But we haven’t had a direct link. So this, for the first time, provides that direct link.”
—Rebecca Owen (@beccapox), Science Writer