The search for the source of the Stonehenge bluestones has been a long time in the making. Unlike the monument’s iconic sarsens – the largest of its standing stones, including the site’s immediately recognisable trilithons – which are thought to have been quarried relatively locally, probably in the Marlborough Downs to the north of Stonehenge – the bluestones represent a variety of rock types, all exotic to the area.
The first detailed account of their lithologies was published almost a century ago, by H H Thomas in 1923. He proposed a precise location for where the stones might have been quarried, at the eastern end of the Mynydd Preseli hills in west Wales. The majority of the stones, the dolerites, came from Carn Menyn (now more commonly known as Carn Meini), he wrote, while the rhyolites and tuffaceous rocks could be traced to Carn Alw and the northern slopes of Foel Trigarn (Drygarn) respectively. This remained considered opinion for over 85 years – but recent findings are now changing this picture.
The bluestones have been our focus of study since 2008, when we first examined a collection of rock fragments recovered from close to the Stonehenge Cursus – now held by Salisbury Museum – and a surprising find leaped out at us. The collection was thought to mainly represent rocks from the Ordovician Fishguard Volcanic Group sequences of north Pembrokeshire, but among its contents was a strongly foliated volcanic kind of rock that Richard did not recognise, despite having worked with material from this area for over 30 years.
Intrigued, Richard revisited his research, and realised that he had a set of samples from the Brynberian area of this region that he had never studied microscopically. Might these shed some light on the matter? To investigate, he had thin sections made from them, and discovered that the mystery lithology was a near perfect petrographical match with the Brynberian samples, particularly with those taken from outcrops at a place called Craig Rhos-y-Felin.
It was a fascinating but unexpected result, as Craig Rhos-y-Felin is not a location traditionally associated with the Stonehenge bluestones. To explore this link further, we teamed up with geochemists Nick Pearce from Aberystwyth University, and Peter Webb and the late John Watson from the Open University, to see if we could establish close matches – both in mineral (zircon) and whole-rock chemistry – between the Cursus debitage and samples from Craig Rhos-y-Felin. To add to this picture, we also examined debitage that had been collected from the Stonehenge landscape on other occasions, including during Tim Darvill and Geoff Wainwright’s excavation in 2008 (CA 252), and Mike Parker Pearson’s Stonehenge Riverside Project, and we also re-examined material from Mike Pitts’ 1979-1980 Heel Stone excavations. The more we investigated, the more it became clear that most of the rhyolitic debitage from around Stonehenge came from Craig Rhos-y-Felin.
Interestingly, however, although quantities of waste flakes can be traced conclusively to that outcrop, the four bluestone orthostats that are known to be dacitic and rhyolitic (stones 38, 40, 46, and 48) cannot. For now, their source remains obscure, though we are now focusing on outcrops on lower ground to the north of Mynydd Preseli as possible candidates – but we can say that Carn Alw, the location suggested by H H Thomas in 1923, is not the correct answer.
If Craig Rhos-y-Felin is the source of much of the Stonehenge debitage, however, why is it not represented among the standing stones themselves? The answer may well lie buried beneath the surface, in stumps of broken stones including numbers 32c, 32d, 33e, 33f, 40c, and 41d. None of these have been sampled to-date, but a black-and-white photograph, taken during Atkinson’s 1954 excavation, provides tantalising clues that at least one, stone 32d (and possibly, though less likely, another, 32e), might have the right attributes in terms of foliation and fracturing characteristics to match Craig Rhos-y-Felin – and perhaps to represent the source of the debitage we have been examining.
Spot the difference
More recently, we have turned our attention to the dolerites, a relatively common type of igneous rock that makes up the majority of the bluestones, and an important component of the Stonehenge debitage. For all that dolerite is a fairly widespread rock type, however, the variety seen at Stonehenge is very distinctive, with a scattering of white spots that are also seen in many of the Preseli outcrops – something originally noticed by H H Thomas, and a key factor in his theories about Carn Meini. Our research with Nick Pearce has revealed that the truth is not so simple, however.
By analysing what are known as ‘compatible elements’ – nickel, chromium, magnesium, and iron – we were able to identify three discrete groups within the Stonehenge dolerites, and a key finding was that the monument’s stones 33, 37, 49, 65, and 67, along with a number of the debitage samples, closely matched dolerite from Carn Goedog – and none directly matched dolerite from Carn Meini. This is surprising, given that Tim Darvill and Geoff Wainwright have suggested evidence for Bronze Age quarrying at Carn Meini – but wherever these stones ended up, they were not among those that have been sampled from Stonehenge.
So what next? Our ultimate aim is to create a comprehensive description of all the bluestone lithologies – ideally with matching source locations in Wales – and to pin down once and for all quite how many types of bluestone lithology there actually are, estimations of which currently vary widely. To that end, we have recently published a detailed account of the volcanic material, and will next turn our attention to the sandstones, of which there are two main types. One most likely comes from the Ordovician sequences of north Pembrokeshire, like the other lithologies we have been examining; but the second variety, which is seen in the Altar Stone – the largest and most enigmatic of any of the Stonehenge orthostats – and a scatter of debitage fragments, appears to belong to a younger sequence, and may yet prove to be the exception to the bluestone norm. There is clearly still much to learn here, but as work on the potential Preseli sources continues, we hope that our detailed petrology will help to resolve the ongoing debate about how the bluestones arrived at Stonehenge, and from exactly where.
Rob Ixer is Honorary Senior Research Associate at UCL’s Institute of Archaeology and the University of Leicester, and Richard Bevins is Keeper of Natural Sciences at Amhueddfa Cymru-National Museum Wales.