A new study of Stonehenge bluestone is out. It’s short and densely written, and not dramatic. But it confirms the direction of current work which suggests that many of the Welsh bluestones came from north of the Preseli hills, rather than the top or to the south as traditionally believed (HH Thomas identified Carn Alw as a source for these particular stones, see map). The significance of this, as argued earlier by Richard Bevins and Rob Ixer, is that while Carn Alw might allow for ice transport of stones at least part of the way to Wiltshire, the nearer the sources move downslope towards the coast, the less supportable that becomes. And Mike Parker Pearson may have another quarry to look for.
Bevins, Nicola Atkinson, Ixer and Jane Evans describe their work in the Journal of the Geological Society. Before, the Fishguard Volcanic Group in north Pembrokeshire had been dated geologically only by fossils. They have now dated two types of rock by uranium-lead analysis of zircon crystals (like radiocarbon dating, but for much older stuff). They compare these with dates similarly obtained from pieces of stone from Stonehenge.
One rhyolite sample came from the Craig Rhos-y-felin outcrop, known as a possible quarry for Stonehenge bluestones, and two from outcrops at Fishguard Old Harbour on the coast. The Stonehenge samples were both excavated in 2008, one (previously thought to come from Craig Rhos-y-felin) by Tim Darvill and Geoff Wainwright, and one (petrographically identical to bluestone 48, and previously thought to have come from an unknown location in the Fishguard Volcanic Group) by Mike Parker Pearson’s team.
The Craig Rhos-y-felin sample came out at 462 million years old, the same age as the rhyolite sample from Darvill and Wainwright’s trench, confirming the archaeological identification.
The other two, from the coast, are a little older, at around 464 and 465 million years old. Interestingly, the archaeological sample (which can be linked to stones 38, 40 and 46 at Stonehenge as well as 48) is also around 464 million years old. Not from Craig Rhos-y-felin, then, but, say the scientists, probably from Fishguard outcrops exposed across the low ground north of Mynydd Preseli. “This region”, they conclude, “provides an obvious target to search for further Neolithic quarry sites.”
See “U–Pb zircon age constraints for the Ordovician Fishguard Volcanic Group and further evidence for the provenance of the Stonehenge bluestones,” by R Bevins, N Atkinson, R Ixer & J Evans, Journal of the Geological Society 2016.