Enormous standing stones at Stonehenge are of sarsen, a local sandstone, but the smaller ones, known as bluestones, came from two prehistoric quarries in Wales, according to an international team of scientists led by University College London archaeologist Prof. Mike Parker Pearson.

The Stonehenge bluestones are of volcanic and igneous rocks, the most common of which are called dolerite and rhyolite.

Where precisely these stones were quarried, when they were extracted and how they were transported has long been a subject of speculation, experiment and controversy.

The discovery of a megalithic bluestone quarry at a site called Craig Rhos-y-Felin, in north Pembrokeshire, Wales, four years ago marked a turning point in this research.

Now, Prof. Parker Pearson and his colleagues have identified the Craig Rhos-y-Felin quarry as a source for one of Stonehenge’s rhyolite bluestones and another quarry called Carn Goedog as a source of spotted dolerite bluestones.

“The special formation of the rock, which forms natural pillars at these outcrops, allowed the prehistoric quarry-workers to detach each megalith with a minimum of effort,” explained team member Dr Joshua Pollard, an archaeologist with the University of Southampton.

“They only had to insert wooden wedges into the cracks between the pillars and then let the Welsh rain do the rest by swelling the wood to ease each pillar off the rock face,” he said.

“The quarry-workers then lowered the thin pillars onto platforms of earth and stone, a sort of loading bay from where the huge stones could be dragged away along trackways leading out of each quarry.”

Radiocarbon-dating of burnt hazelnuts and charcoal from the quarry-workers’ camp fires reveals that there were several occurrences of megalith-quarrying at Craig Rhos-y-Felin and Carn Goedog.

“Both of the quarries were exploited in the Neolithic, and Craig Rhos-y-Felin was also quarried in the Bronze Age, around 4,000 years ago,” the archaeologists said.

“We have dates of around 3400 BC for Craig Rhos-y-Felin and 3200 BC for Carn Goedog, which is intriguing because the bluestones didn’t get put up at Stonehenge until around 2900 BC,” Prof. Parker Pearson said.

“It could have taken those Neolithic stone-draggers nearly 500 years to get them to Stonehenge, but that’s pretty improbable in my view. It’s more likely that the stones were first used in a local monument, somewhere near the quarries, that was then dismantled and dragged off to Wiltshire.”

“The ruins of any dismantled monument are likely to lie somewhere between the two megalith quarries,” added team member Prof. Kate Welham, of Bournemouth University, UK.

The findings, published this week in the journal Antiquity, may also help to understand why Stonehenge was built, according to the scientists. They believe that the bluestones were erected at Stonehenge around 2900 BC, long before the enormous sarsens were put up around 2500 BC.

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Mike Parker Pearson et al. 2015. Craig Rhos-y-Felin: a Welsh bluestone megalith quarry for Stonehenge. Antiquity, vol. 89, no. 348, pp. 1331-1352; doi: 10.15184/aqy.2015.177