Côr y Cewri: Bluestones glacier theory is now frozen out

Erthygl o'r Times gan Norman Hammond, Archaeology correspondent – pryd??

The Welsh quarry which provided the 'bluestones' at Stonehenge, the smaller stones in the foreground,

has been identified to within a few square metres.

The long-running debate about the origin of the Stonehenge "bluestones" and how they got to Salisbury Plain some four millennia ago has taken another turn: a precise quarry source for much of the Stonehenge rock has been pinned down to a few square metres in southwestern Wales. This supports the notion that the bluestones were taken by human agency all the way from Pembrokeshire to Wiltshire, rather than helped along their way in the Ice Age by glacier transport.

"The glacial theory is frozen out by this new evidence," Dr Rob Ixer of Leicester University told The Times. If the stones had been transported east of the Bristol Channel by glacial action, a much wider range of sources would be expected. The pinpoint sourcing that has now been done argues strongly for human quarrying and transport of the bluestones, whatever the motivation and precise route employed.

Stonehenge's "bluestones" are not the enormous sarsen trilithons which form the bulk of the visible monument, but relatively short, slender, single shafts which were used in an earlier version of the stone circle and then repositioned within the final layout.

Three major rock types and two minor ones can be identified within the "bluestone" range using both the entire stones and waste chips known as debitage which result from trimming the slabs on site at Stonehenge. The three major groups, originally thought to be from different geographical sources, can now be shown to be from the same locale.

The area of the new find lies at Pont Saeson on the northern flank of the Preseli mountains, long known as the general source of the bluestones, some 6.5 kilometres (four miles) from Newport in north Pembrokeshire. The discovery follows the use of zircons included in the rocks to identify an area near Pont Saeson as one likely source of Stonehenge material by Dr Ixer and his colleague Dr Richard Bevins of the National Museum of Wales.

"Almost all – 99.9 per cent – of the Stonehenge rhyolitic 'debitage' can be petrographically matched to rhyolitic rocks found within a few hundred square metres at Pont Saeson and especially to those found at Craig Rhosyfelin.

"However, it is possible in a few cases, where the petrography of these Welsh in-situ rocks is so distinctive, to suggest an even finer provenance to within square metres, namely to individual outcrops," Ixer and Bevins report in Archaeology in Wales.

They have pinned down the source of rhyolite rock fragments, found at Stonehenge more than 60 years ago and stored in a shoebox for decades, to a specific outcrop at Craig Rhosyfelin, part of the Pont Saeson outcropping. "These very distinctive rhyolitic rocks can be traced for no more than 150 metres from the northernmost end of Craig Rhosyfelin," they say.

The outcrop itself is some 70 metres long and has many tall, narrow slabs up to two metres (6.5ft) high as the dominant feature, splitting off from the parent rock and reminiscent of the Stonehenge bluestones. One of the Stonehenge shafts, known as SH32e, can be matched very closely to this outcrop, and must have been quarried there, not transported by a glacier.

"I have always wanted to tell this story under the tabloid heading 'Old shoebox held key to Stonghenge mystery," Ixer said. "The work stems from an old box in the Salisbury Museum holding stones collected in 1947.

"The overwhelming majority of the Stonehenge rhyolitic 'debitage' can be sourced from the Pont Saeson area and perhaps entirely from Craig Rhosyfelin, but from more than one site on the crags," Ixer and Bevins conclude. The dispute over natural versus human transportation for these elements of an early and important phase of Stonehenge now seems to be settled – as Ixer says, the glacial theory is out cold.

Archaeology in Wales Vol. 50 pp 21-31.