Constraining the provenance of the Stonehenge ‘Altar Stone’: Evidence from automated mineralogy and U–Pb zircon age dating (original) (raw)
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13 December 2024, 2024
The most enigmatic "foreign" stone at Stonehenge is its Altar Stone. Long thought to have been sourced from the Old Red Sandstone beds of South Wales, new studies have focused on its origin in the Orcadian Basin of northeastern Scotland, either in the Caithness region or in the Orkney Isles. Despite one paper (Bevins et al, 2024) providing compelling evidence that the Altar Stone does not exactly match the composition of Old Red Sandstone beds on the Orkney Mainland, those responsible for transporting the stone to Stonehenge were almost certainly its original builders, the Grooved Ware culture, who first emerged on Orkney during the Late Neolithic. We look at everything known about the Altar Stone and how its presence at Stonehenge might relate to its construction. We also go in search of its lost companion, and examine where both these huge monoliths might have stood within the monument, and how all this might relate to the monument's underlying geometry.
Stone 80, the recumbent Altar Stone, is the largest of the Stonehenge foreign "bluestones", mainly igneous rocks forming the inner Stonehenge circle. The Altar Stone's anomalous lithology, a sandstone of continental origin, led to the previous suggestion of a provenance from the Old Red Sandstone (ORS) of west Wales, close to where the majority of the bluestones have been sourced (viz. the Mynydd Preseli area in west Wales) some 225 km west of Stonehenge. Building upon earlier investigations we have examined new samples from the Old Red Sandstone (ORS) within the Anglo-Welsh Basin (covering south Wales, the Welsh Borderland, the West Midlands and Somerset) using traditional optical petrography but additionally portable XRF, automated SEM-EDS and Raman Spectroscopic techniques. One of the key characteristics of the Altar Stone is its unusually high Ba content (all except one of 106 analyses have Ba > 1025 ppm), reflecting high modal baryte. Of the 58 ORS samples analysed to date from the Anglo-Welsh Basin, only four show analyses where Ba exceeds 1000 ppm, similar to the lower range of the Altar Stone composition. However, because of their contrasting mineralogies, combined with data collected from new automated SEM-EDS and Raman Spectroscopic analyses these four samples must be discounted as being from the source of the Altar Stone. It now seems ever more likely that the Altar Stone was not derived from the ORS of the Anglo-Welsh Basin, and therefore it is time to broaden our horizons, both geographically and stratigraphically into northern Britain and also to consider continental sandstones of a younger age. There is no doubt that considering the Altar Stone as a 'bluestone' has influenced thinking regarding the long-held view to a source in Wales. We therefore propose that the Altar Stone should be 'declassified' as a bluestone, breaking a link to the essentially Mynydd Preseli-derived bluestones.
Non-invasive survey in the Stonehenge 'Triangle', Amesbury, Wiltshire, has highlighted a number of features that have a significant bearing on the interpretation of the site. Geophysical anomalies may signal the position of buried stones adding to the possibility of former stone arrangements, while laser scanning has provided detail on the manner in which the stones have been dressed; some subsequently carved with dagger and axe symbols. The probability that a lintelled bluestone trilithon formed an entrance in the north-east is highlighted. This work has added detail that allows discussion on whether the sarsen circle was a completed structure, although it is by no means conclusive in this respect. Instead, it is suggested that it was built as a facade, with other parts of the circuit added and with an entrance in the south.
New U–Pb zircon ages from rhyolite samples of the Fishguard Volcanic Group, SW Wales, confirm a Middle Ordovician (Darriwilian) age for the group. One of the samples is from Craig Rhos-y-felin, which has recently been identified on petrological and geochemical grounds as the source of much of the debitage (struck flakes) at Stonehenge. Analysis of a Stonehenge rhyolite fragment yields an age comparable with that of the Craig Rhos-y-felin sample. Another Stonehenge fragment, thought to come from orthostat (standing stone) 48 and on petrographical grounds to be derived from the Fishguard Volcanic Group (but not Craig Rhos-y-felin), yields an age also consistent with a Fishguard Volcanic Group source.
Linking derived debitage to the Stonehenge Altar Stone using portable X-ray fluorescence analysis
The Altar Stone at Stonehenge in Wiltshire, UK, is enigmatic in that it differs markedly from the other bluestones. It is a grey-green, micaceous sandstone and has been considered to be derived from the Old Red Sandstone sequences of South Wales. Previous studies, however, have been based on presumed derived fragments (debitage) that have been identified visually as coming from the Altar Stone. Portable Xray fluorescence (pXRF) analyses were conducted on these fragments (ex situ) as well as on the Altar Stone (in situ). Light elements (Z<37) in the Altar Stone analyses, performed after a night of heavy rain, were affected by surface and pore water that attenuate low energy X-rays, however the dry analyses of debitage fragments produced data for a full suite of elements. High Z elements, including Zr, Nb, Sr, Pb, Th and U, all occupy the same compositional space in the Altar Stone and debitage fragments, and are statistically indistinguishable, indicating the fragments are derived from the Altar Stone. Barium compares very closely between the debitage and Altar Stone, with differences being related to variable baryte distribution in the Altar Stone, limited accessibility of its surface for analysis, and probably to surface weathering. A notable feature of the Altar Stone sandstone is the presence of baryte (up to 0.8 modal%), manifest as relatively high Ba in both the debitage and the Altar Stone. These high Ba contents are in marked contrast with those in a small set of Old Red Sandstone field samples, analysed alongside the Altar Stone and debitage fragments, raising the possibility that the Altar Stone may not have been sourced from the Old Red Sandstone sequences of Wales. This high Ba 'fingerprint', related to the presence of baryte, may provide a rapid test using pXRF in the search for the source of the Stonehenge Altar Stone.
Archaeology and legend: investigating Stonehenge
Archaeology International
Stonehenge is one of the world’s most famous prehistoric monuments, built 4,500–5,000 years ago during the Neolithic in a time long before written history. The recent dramatic discovery of a dismantled stone circle near the sources of some of Stonehenge’s stones in southwest Wales raises the fascinating possibility that an ancient story about Stonehenge’s origin, written down 900 years ago and subsequently dismissed as pure invention, might contain a grain of truth. This article explores the pros and cons of comparing the legend with the archaeological evidence.