The mystery of Stonehenge’s ‘bluestones’

A new paper published in the Journal of the Geological Society has lent support to recent theories about the origin of the mysterious ‘bluestones’ of Stonehenge.

It has long been known that some of the rocks that make up Stonehenge must have travelled a long distance before becoming part of the monument. Whilst the larger sandstone blocks (‘sarsen’ stones) that make up its outer circle are thought to have a local origin from the Marlborough Downs area, the smaller ‘bluestones’ are exotic to the region.

The term ‘bluestone’ in relation to Stonehenge encompasses around twenty different rock types, including rhyolites, dolerites and ‘calcereous ashes.’

In 2011, a megalithic bluestone quarry was discovered at a site in South West Wales known as Craig Rhos-y-felin. Since then, it has been suggested as the source of the Stonehenge bluestones, bolstering the theory that the bluestones were taken to the site by humans. A competing theory is that the stones were brought closer to the site as glacial erratics. Key to solving the mystery is establishing as precisely as possible where the bluestones originated from.

The new study uses uranium-lead dating techniques to compare the ages of samples from Stonehenge with rocks from the Fishguard Volcanic Group in South West Wales, including a sample from Craig Rhos-y-felin. The research was designed to narrow down the age of the Fishguard Volcanic Group and compare with the ages of the Stonehenge samples, testing recent theories for the provenance of the bluestones.

The results of the U-Pb dating support previous arguments that the Fishguard Volcanic Group dates back to the middle Darriwilian – between 467.3 – 458.4 million years ago. Zircon crystals from the Stonehenge samples derived an age of 462.20 +/- 0.26 Ma. In particular, overlapping dates from the sample from Craig Rhos-y-felin and a sample from Stonehenge strengthen the argument that much of the rhyolitic debitage at Stonehenge was quarried at Craig Rhos-y-felin.

As the authors of the new paper state, ‘The precise location of the sources is critical for focusing archaeological investigations in order to take forward the long-running debate as to whether the Stonehenge bluestones were transported to Salisbury Plain by the actions of humans or by ice, and if the former, by what route.’

Journal reference:

• Richard Bevins, Nicola Atkinson, Rob Ixer and Jane Evans, U–Pb zircon age constraints for the Ordovician Fishguard Volcanic Group and further evidence for the provenance of the Stonehenge bluestones, Journal of the Geological Society, first published on November 3, 2016, doi:10.1144/jgs2016-042

Further reading:

• UCL News, 7 December 2015, ‘Stonehenge ‘bluestone’ quarries confirmed 140 miles away in Wales’
• Wales Online, 13 December 2015, ‘Why Stonehenge’s bluestones ‘were moved from Wales by glaciers NOT prehistoric people’
• The Telegraph, 28 October 2016, ‘Massive 25 ton stones of Stonehenge may have come from further afield’