IT WAS Herbert Henry Thomas, a British geologist, who first declared that the “foreign stones” of Stonehenge—those that did not come from the vicinity of the prehistoric monument and whose raison d’être was therefore most shrouded in mystery—had been hewed from rocky outcrops in west Wales. In 1923 he pointed to the Preseli Hills of Pembrokeshire. Nearly a century later he has been found to be nearly, but not quite, right.

Thomas thought Carn Menyn the most likely source of Stonehenge’s spotted dolerite bluestones, so-called because of the pale flecks that dapple their blue-green tint, and many teams have since focused on that location. But in 2013 geologists redirected the search. By observing the molecular signatures of nine of Stonehenge’s bluestones, they found that Carn Goedog (pictured), a little more than a kilometre to the north-west of Carn Menyn, was the best match.

Archaeologists moved in, hot on the heels of the geochemists. They were led by Mike Parker Pearson of University College, London. Dr Parker Pearson says that on their first visit to the site he and his colleagues soon discovered large gaps in the cluster of rocky pillars that rise above the heather. These seemed to be places from which stones had been removed. Here, they spotted a small mouth-like hollow that had been made in a natural crack in the rock. Then they found the clincher: some stone wedges that would have been inserted into hollows like the one they had discovered, in order to prise cracks open and remove blocks of stone.

In a paper just published in Antiquities, the team describe, based on what they found at Carn Goedog and studies of modern hunter-gatherers in south-east Asia, how the ancient stone workers must have quarried the bluestones. Wedges of various sizes, some of which show signs of battering on their butt-ends, would have been hammered into narrow joins between the pillars to loosen them from the rock face. A system of opposing ropes would then have been used to lay the vertical pillars on their side: ropes lassoed around the top of a pillar would pull it away from the rock face, while others held by workers standing at the summit of the outcrop would steady its descent.

In this way, pillars between one and four metres tall could be laid down on a large rocky platform that sits in front of the quarried face. From there, they could have been lowered to a wooden sled and dragged away along a hollowed-out track.

There are 27 spotted dolerites at Stonehenge. Nine of these have been sampled for geochemical analysis, which showed that at least five of them were removed from Carn Goedog. Other bluestones, made of a rock called rhyolite, which has a different composition from dolerite, have been traced to the nearby but smaller quarry at Carn Rhos-y-felin. Burnt wood at both sites suggests they were in use just before the oldest dates that have been obtained for Stonehenge, around 3000BC.

How and why these monoliths, which weigh several tonnes each, were carried hundreds of kilometres to Stonehenge is still a mystery. The location of Carn Goedog and Carn Rhos-y-felin, on the north side of the Preseli hills, does not favour a previous theory that they were transported to the south and then by sea.

Instead, Dr Parker Pearson suggests, they may have been dragged over land, perhaps to form part of one or more other monuments first, before being taken on to Stonehenge. He and his colleagues are eyeing up a partial stone circle at Waun Mawn, 3km west of Carn Goedog. They remain tight-lipped about their findings. But if they do have evidence that the bluestones made a pit-stop there, that would mean Thomas’s work was truly inspired: back in 1923 he speculated that the bluestones may have formed part of a “venerated stone circle” in the Preseli hills before they reached their final resting place, in Stonehenge.