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After 5,000 years, scholars are beginning to unlock one of the greatest mysteries surrounding Stonehenge.

Geologists long ago traced dozens of the Neolithic monument’s smaller “bluestones” to the Preseli Hills in western Wales, some 180 miles away from Stonehenge itself. Now, a new study goes further than ever, revealing the exact location of two of the bluestone quarries, as well as when and how the stones might have been removed.

The massive sandstone slabs, or sarsens, used to construct Stonehenge on southern England’s Salisbury Plain seem likely to have come from nearby quarries, or may even have been scattered on the ground when construction began. But the origin of the monument’s 80 bluestones (around 40 of which remain today) was a harder puzzle to solve.

While the geologist Herbert Henry Thomas first traced dozens of Stonehenge’s smaller “bluestones” to the Preseli Hills back in 1923, a team of scientists more recently pinpointed their source to a different outcrop of rocks than Thomas had originally identified.

Carn Goedog

Carn Goedog, the source of rhyolite rocks found at Stonehenge.

The new study, published in Antiquity Journal, draws on eight years of excavations by a team of archaeologists and geologists at that outcrop, Carn Goedog, as well as another, smaller outcrop in the valley below, known as Craig Rhos-y-felin. They found that at least five of Stonehenge’s spotted dolerite bluestones—which are named for the white spots that appear in the igneous blue rock—came from Carn Goedog, and identified Craig Rhos-y-felin as the source of one of the types of rhyolite (another kind of igneous rock) found at Stonehenge.

Perhaps even more significantly, the researchers were able to date quarrying at the two outcrops to around 3000 B.C., after uncovering pieces of charcoal of that age at both sites. They now believe that Stonehenge began as a circle of rough bluestone pillars in pits known as the Aubrey Holes, while the sarsens were added around 500 years later.

“What's really exciting about these discoveries is that they take us a step closer to unlocking Stonehenge's greatest mystery—why its stones came from so far away," the team’s leader, Mike Parker Pearson of University College London, said in a statement. “Every other Neolithic monument in Europe was built of megaliths brought from no more than 10 miles away."

According to Pearson and his colleagues, the bluestone outcrops naturally formed vertical pillars. This means that Neolithic quarry workers could have separated these pillars from the rock face relatively easily, by pushing wedges into the ready-made vertical joints between them. This would have taken far less effort than was required in ancient Egyptian stone quarries, where workers had to carve obelisks out of solid rock.

The two-ton stones would then have been lowered onto wooden sledges, the researchers say, and dragged or carried the nearly 200 miles to their present location, on the Salisbury Plain. The study adds to the growing body of evidence against the theory that the stones were transported by sea down the Welsh coast, which became popular after Thomas first suggested their distant origin in the 1920s.

“Some people think that the bluestones were taken southwards and placed on rafts or slung between boats and then paddled up the Bristol Channel and along the Bristol Avon towards Salisbury Plain,” study co-author Kate Welham pointed out. “But these quarries are on the north side of the Preseli hills so the megaliths could have simply gone overland all the way to Salisbury Plain.”

It’s still unclear, however, why the prehistoric builders of Stonehenge chose such exotic stones to build their massive monument. In fact, another group of scientists have argued in the journal Archaeology in Wales that evidence of quarrying operations in the Preseli hills is lacking, and the stones and rocks were far more likely to have been transported via glacial movement rather than human actions.

According to Parker Pearson, his team is far from finished with their investigations: “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.”

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