Excavations at two ancient quarry sites in western Wales suggest how ancient people probably quarried some of the stones now standing at Stonehenge.

The 42 stones in question are some of the smaller parts at Stonehenge, relatively speaking: they still weigh two to four tons each. They're called the bluestones, and they came all the way from western Wales. Chemical analysis has even matched some of them to two particular quarries on the northern slopes of the Preseli Hills.

One, an outcrop called Carn Goedog, seems to have supplied most of the bluish-gray, white-speckled dolerite at Stonehenge. And another outcrop in the valley below, Craig Rhos-y-felin, supplied most of the rhyolite. University College London archaeologist Michael Parker Pearson and his colleagues have spent the last eight years excavating the ancient quarry sites, and that work has revealed some new information about the origins of Stonehenge.

Kind of like string cheese

Both craggy rock outcrops are the petrified remains of a long-past volcanic eruption. The lava cracked as it cooled and hardened, like mud in a dry lakebed, and those cracks reached well down into the deposit, so the cooled lava ended up in a cluster of vertical pillars. (The Giant's Causeway in Ireland is another well-known example of that process.)

That geological coincidence made the work of quarrying much easier than in Egypt. There, at around the same time, workers were chiseling blocks out of solid bedrock. In Wales, the quarry workers just had to wedge a column apart from its neighbors at the joints (it's difficult not to imagine peeling a really big piece of string cheese, honestly).

Pearson and his colleagues found some evidence of how they did it, thanks in part to the almost-universal human tendency to drop things and forget to pick them up again. The ancient quarry-workers left behind mudstone wedges and stone hammers, which they would have driven into the cracks between the pillars to carefully pry them apart. The wedges were precision tools for delicate work; mudstone is considerably softer than the rhyolite and dolerite columns, and Pearson says that made a difference.

"An engineering colleague has suggested that hammering in a hard wedge could have created stress fractures, causing the thin pillars to crack," he said in a statement to the press. "Using a soft wedge means that, if anything were to break, it would be the wedge and not the pillar."

Once they'd pried a pillar out of the formation, the workers would have carefully lowered it to a platform built of stone and earth, positioned at the foot of the outcrop. At both sites, the human-made platforms stand about a meter (3.28 feet) off the ground. Pearson and his colleagues say ancient quarry workers would have used them as loading docks, where they would have lowered the newly-quarried columns onto wooden sledges for transport.

Adam Stanford of Aerial-Cam Ltd

Stefan Kühn / Wikimedia

Dating the quarries

In the soft soil of a sledge track at Craig Rhos-y-felin, Pearson and his colleagues found a seemingly innocuous object that would turn out to be a vital clue: a small piece of charcoal. Another hunk of charcoal turned up on the platform at Carn Goedog. The archaeologists radiocarbon-dated both pieces, which turned out to have been dropped around 3,000 BCE. That matches radiocarbon dates from the cremated remains of people once interred beneath the bluestones, which range from 3,180 BCE to 2,380 BCE. The timing suggests that whoever dropped the charcoal may have had a hand in quarrying the stone for the distant stone circle.

People apparently buried their dead (after cremation) at Stonehenge for several centuries, in the circle of pits now called the Aubrey Holes. But the standing arrangement of vertical and horizontal sandstone slabs that most people recognize today (called sarsen stones) weren't erected until around 500 years after the bluestones. Stonehenge's form (and maybe its purpose) changed several times over the centuries, and archaeologists are still trying to piece together the details of its story and the stories of the people who built it and gathered in its circles.

Investigating the Welsh connection

One enduring mystery of the megalithic monument is why people hauled four-ton stones nearly 290 km (180 miles) from western Wales to southern England to build it. "Every other Neolithic monument in Europe was built of megaliths brought from no more than 10 miles away," said Pearson.

Last year, a team of archaeologists investigated the cremated remains of people buried at Stonehenge. The archaeologists discovered that the ratios of different isotopes of the element strontium in at least 10 of the cremated bodies matched the hills of western Wales, not the chalk plains of southern England. That suggested that the prehistoric deceased spent the last decade or so of their lives nearly 180 miles from Stonehenge—but maybe not far at all from the bluestone quarries (although it's also possible that they came from Scotland, Ireland, or continental Europe). Some of them may even have been carried to their final resting places after death and cremation in leather pouches. And the authors of that study say they could have come traveled with the bluestones.

But archaeologists still aren't sure what connected these distant locations.

"We're now looking to find out just what was so special about the Preseli Hills 5,000 years ago and whether there were any important stone circles here, built before the bluestones were moved to Stonehenge," said Pearson. He's suggested before that the bluestones could have formed part of a local monument in Wales before their journey to Stonehenge—although he's not yet sure what would have prompted the move.

Antiquity, 2019. DOI: 10.15184/aqy.2018.111