In this month’s Science Notes, we turn to one of the most immediately recognisable monuments in the world – Stonehenge – examining how the origin of its bluestones was taken for granted for so long, and how it shows why research is ever evolving, and never absolute.
The bluestones make up the monument’s smaller inner circle and inner horseshoe, and they are thought to have been erected on Salisbury Plain some time before the larger sarsen stones were raised to create its outer circle. While these latter components are made of a hard, durable sandstone that likely comes from the local area (probably the Marlborough Downs), the bluestones represent a range of different stone types – including dolerites, rhyolitic tuffs, argillaceous tuffs (or ‘calcareous ash’), and sandstones – and they are thought to have been brought from west Wales.
The exact origins of the bluestones were first hypothesised by H H Thomas in his seminal paper, The source of the stones of Stonehenge, published in 1923. He asserted that the dolerite stones probably came from Carn Meini and Cerrig Marchogion, the rhyolitic ones from the nearby outcrop of Carn Alw, and the ‘calcareous ash’ ones from the northern slopes of what is now called Foel Drygarn – all sites within the Mynydd Preseli region of north Pembrokeshire. These results remained unquestioned for over 80 years, until new research called Thomas’ work into question (see CA 311).
A paper by Richard Bevins and Rob Ixer, recently published in Antiquity (https://doi.org/10.15184/aqy.2018.10), examines how Thomas came to his conclusions, re-examining many of the samples he used. In terms of the rhyolitic samples, it appears that they did not come from in situ bluestones and instead from fragments collected in the nearby area. Re-examination using more modern techniques, including analysing the petrographical and mineralogical characteristics of the stones using both transmitted and reflected light microscopy as well as U-Pb dating, shows that these samples, as well as the stones themselves, likely came from Craig Rhos-y-felin, and not from Carn Alw as stated by Thomas.
Regarding the ‘calcareous ash’, Thomas attributes it to an outcrop north of Foel Drygarn, but modern analysis shows that this type of bluestone lacks any defining features and can, at the moment, only be broadly attributed to the north Pembrokeshire area.
In the case of the dolerites, Thomas’ misattribution is perhaps more understandable. The Stonehenge dolerites contain distinctive white spots – characteristics that are found at Carn Meini, which is the location that Thomas assigned them to. But Carn Meini is not the only outcrop in the region to sport such spots and, in fact, Thomas should have known this as site reports show that he visited and collected samples from Carn Goedog, which has the same characteristics.
Even if Thomas had discovered and acknowledged this possibility, however, it would have been difficult to discern between the two using the techniques available to him. It was only through geochemically analysing major and trace elements present within the bluestones that it could be established that the majority of the Stonehenge dolerites came from Carn Goedog and not Carn Meini, with a few stones potentially coming from other nearby outcrops. While Thomas was an excellent geologist, he had only conducted one field survey in the Mynydd Preseli area, lacked more modern tools of analysis, and had limited access to samples of comparable Welsh rocks. Knowing this, it becomes easy to understand how and why Thomas erred.
Antiquarians like Thomas did the best they could with what they had, but methodologies have improved. It is not just antiquarians we should be questioning, though. No paper or finding is ever absolute, and it is the role of the inquisitive academic to continually question and re-examine. In academia today, too often grants and other sources of funding are only given to research that offers to examine ‘new’ questions. Proposals that hope to replicate research, providing increased surety in these findings, remain underfunded. But, as Bevins and Ixer’s work proves, this type of research – in any discipline – is essential in maintaining academic rigour.
Recent excavations at Craig Rhos-y-Felin and Carn Goedog have uncovered exciting evidence for the location of Neolithic bluestone quarries, as described by Mike Parker Pearson in CA 311. We will bring you the latest update from this research in the next issue of CA.
This article appeared in CA 343.