New research is calling into question the origin and function of the Altar Stone at Stonehenge.
A century ago, Herbert Thomas announced that the non-sarsens, dolerites, and volcanic tuffs making up the Stonehenge bluestones had been transported to Wiltshire from outcrops in the Preseli Hills in west Wales. He knew, however, that the Altar Stone – unique among the bluestones in size, geological age, and composition (it is a grey-green sandstone of Old Red Sandstone age) – was from further east, and believed it had probably been picked up along the transport route by the builders of Stonehenge. For decades, the favoured origin (now totally discounted) for the Altar Stone was Milford Haven, but it has been more recently linked with Bannau Brycheiniog (the Brecon Beacons).
For the last 17 years, an ever-enlarging and now international team of geologists, petrographers, mineralogists, and geochemists have been trying to find the correct provenance for this mammoth stone. Each new technique/area of expertise has brought unexpected results – most notably the discovery that the sandstone has anomalously high concentrations of barium, in the form of baryte, and that it carries an unusual clay mineralogy.
In a recent paper published in the Journal of Archaeological Science: Reports (https://doi.org/10.1016/j.jasrep.2023.104215), the team – led by Aberystwyth University – have taken these results and tried to match them with samples collected throughout the Old Red Sandstone from south Wales, Somerset, and the English Marches. Very few samples show a similar barium concentration to the Altar Stone, with their detailed mineralogy showing that they were unrelated – meaning that a Welsh origin for the Altar Stone seems increasingly unlikely.
This was a total surprise, and has spurred the team to rethink their research three times. First, they have determined not to restrict themselves to Old Red Sandstone age rocks, but to include Permo-Triassic and perhaps even Tertiary sediments, and, second, to find new places with suitable barium-rich, thickly bedded, terrestrial sandstone outcrops, especially those coupled with Neolithic activity. All possible areas suggested, though, are a great deal further from Wessex than south Wales, including the Cheshire Basin, the Eden Valley, Orkney, the Isle of Arran, around the Moray Firth, and Aberdeenshire. Third, and perhaps more radically, they have decided to resignify the Altar Stone – forever the elephant in the circle – and reassess its relationship with/to the bluestones. Unlike the Preseli bluestones, there is no evidence for when it arrived in Wiltshire, and if the Altar Stone can be shown not to be associated with their transport route, as the now accumulating data suggest, can it still be considered a bluestone? Was it moved with the Preseli bluestones, or brought by different people at a different time? The team are now fully prepared for the unexpected.
Text: Rob Ixer / Photo: Nick Pearce