A retrospective detailing 21st century Altar Stone research is attached below; it highlights the incremental ‘journey’ taken by researchers and the stone as they progressively moved away from west Wales.
By the end of the 20th century it was generally accepted that the bluestones of Stonehenge had been moved manually from the Preseli Hills in west Wales to Wessex. This was essentially based on Thomas’ work who in 1923 recognised the geographical origin of the spotted dolerites, a main component of the bluestones, suggesting they were collected from outcrops on the main ridge of the hills including Carn Meini. He also recognised that the Altar Stone was not from the Preseli Hills but proposed that it was an extra, collected along the transport route taken by the collected bluestones. This suggested route rafted the bluestones from Milford Haven along the south Wales coastline to Somerset and Wessex.
2006: Ixer and Turner using transmitted light petrography and a manual point counter described the Altar Stone (using a standard thin section namely Wiltshire 61 #277 from the Southwest Group of Museums and Galleries Implement Petrology Committee Collections, labelled Altar Stone in Clough and Cummins 1988) in detail, contrasting it with examples of the Stonehenge Lower Palaeozoic Sandstone. They concurred that the Altar Stone was probably from the Old Red Sandstone Senni Beds in South Wales. Thin section Wilts 277 was believed to be the only labelled and so only ‘known’ Altar Stone sample. Other Victorian thin sections labelled ‘Altar Stone’ included samples of sarsen and volcanic ash. Victorian and Edwardian descriptions of Altar Stone debris collected from excavations and hence recorded in the literature were treated with caution.
2007: Ixer casts doubt on a sea route for the Stonehenge bluestones along the Severn Estuary from Milford Haven to the Somerset coast, showing petrographically that the Steep Holm ‘bluestone’ metamorphic rocks were unlike any from the Preseli Hills or from the Stonehenge Landscape. They are erratics or even ship’s ballast rather than ‘lost’ Stonehenge bluestones.
2013: Ixer and Bevins using transmitted and reflected light petrography described three additional Altar Stone samples excavated from Stonehenge. Approximately 20 small Altar Stone samples from 21st century excavations were recognised as debitage knocked from the Altar Stone. Both the Altar Stone and the Lower Palaeozoic Sandstone were described as originating from Milford Haven by Atkinson in his 1970 Stonehenge book. In situ collecting of Old Red Sandstone sandstones from Milford Haven included the proposed site for the Lower Palaeozoic Sandstone at Mill Bay as described by Sir Kingsley Dunham in the late 1940s.
2017: Ixer, Turner, Molyneux and Bevins using palynology plus detailed transmitted and reflected light petrography showed that the non-Altar Stone sandstone debitage found in the Stonehenge Landscape, but said to be Old Red Sandstone in age and from Mill Bay (Milford Haven), was older than the Devonian Old Red Sandstone and probably Ordovician in age. The sandstone was formally designated (within the Stonehenge Landscape) as the Lower Palaeozoic Sandstone. Its age suggested a geographical origin to the north and northeast of The Preseli Hills.
2018: Bevins and Ixer reviewed Thomas’ 1923 classical work on the Preseli Bluestones and their origins showing that his detailed provenances were unsound and probably not based on examination of newly collected samples from the field but on Victorian collected Stonehenge debitage. Earlier papers had shown this was also true of Thomas’ discussion of the Altar Stone.
2019: Ixer, Bevins, Turner, Power and Pirrie using automated scanning electron microscopy (SEM-EDS) and linked energy dispersive analysis (QEMSCAN) on the newly described Altar Stone samples compared Wilts 277 to other Altar Stone debitage. Quantitatively they confirmed that all samples are the same lithology and in addition recognised the presence of an early cementing kaolinite, quartz and baryte in addition to the main carbonate cement; they also noted the lack of potassium feldspar. The presence of significant baryte and lack of potassium feldspar became pivotal parameters in the subsequent search for the origin of the Altar Stone. They suggested that the origin should be sought within the eastern end of the Old Red Sandstone Senni outcrop in eastern Wales.
2020: Ixer, Bevins, Pirrie, Turner and Power in a complementary paper and using the same techniques and methodology fully disassociated the Ordovician Lower Palaeozoic Sandstone from the Devonian Old Red Sandstone Altar Stone.
2020: Bevins, Pirrie, Ixer, O’Brien, Parker Pearson, Power and Shail 2020 using petrography, modal analysis, age dating accessory zircons and initial clay mineralogy continued to fully differentiate between the Lower Palaeozoic Sandstone, Old Red Sandstone Cosheston sandstone at Mill Bay and Altar Stone samples. Clay mineralogy suggest an origin for the Altar Stone within southeast rather than southwest Wales. Some of the zircons gave very old dates.
A recap.
In 2020 it was still thought that the Altar Stone is most likely to be from eastern Wales or the Welsh Marches and the Lower Palaeozoic Sandstone from west or central Wales, north and east of the Preseli Hills; neither of these two Stonehenge-related sandstones is from Mill Bay, Milford Haven as had been suggested. Showing the Mill Bay Old Red Sandstone rocks (as suggested by Dunham) to have no association with Stonehenge removes the main ‘proof’ for Milford Haven as an exit port for the Preseli Hills bluestones. Numerous other Stonehenge bluestones studies by the group showed that the various igneous bluestones came from the northern slopes of the Preseli Hills and not from the main Preseli ridge and that Carn Meini on the southern slopes could be discounted as a source. This also suggested that the bluestones were transported along a land route rather than sea.
2021: Bevins enquired about Wilts 277 and it was discovered that a hand specimen of the Altar Stone collected in the 19th century had been curated in the Salisbury Museum collections, complete with label. Thin section Wilts 277, originally described in 2006 had been taken from this specimen, that itself had been directly collected from the underside of the Altar Stone. Wilts 277 and other investigated and suggested examples of the Altar Stone were now shown to be from the Altar Stone. All earlier work and attributions were confirmed as correct.
2021-2022: Pearce and Bevins in 2021and 2022 performed portable X-Ray Fluorescence (pXRF) analysis on the in situ Altar Stone at Stonehenge in order to compare with Altar Stone debitage samples from recent excavations and with the newly rediscovered hand specimen parent of Wilts 277.
2022: Bevins, Pearce, Ixer, Hillier, Pirrie and Turner using portable X-Ray Florescence (pXRF) linked Altar Stone debitage fragments to the Altar Stone based on the in situ analyses of the recumbent stone. The high Ba content present as cementing baryte in all the material was recognised as unusual and in marked contrast to Old Red Sandstone from Wales hence raising the possibility it may not be Welsh. It was noted that the Ba content might be a determining characteristic in the search for a geographical origin for the Altar Stone.
2022: Ixer, Bevins, Pearce and Dawson published a popular account of the re-finding of the Altar Stone hand specimen (Salisbury Museum 2010K 240) concentrating on its Victorian label as a guide to authenticity but also giving some initial pXRF analysis obtained from the specimen and from the Altar Stone.
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.
2023: Bevins, Pearce, Pirrie, Ixer, Hillier, Turner and Power using petrography, automated SEM-EDS analysis and pXRF analysis compared in situ pXRF data from the Altar Stone and with petrography of the newly re-found hand specimen (Salisbury Museum 2010K 240, the source of thin section Wilts 277) and to other and earlier described Altar Stone thin sectioned samples. The data showed them all to be the same, confirming the attribution of 2010K 240 as an Altar Stone fragment (and Wilts 277 its thin section also). Hence ratifying all the earlier Altar Stone studies.
2023: Bevins, Pearce, Ixer, Pirrie, Ando, Hillier, Turner and Power using pXRF, automated SEM-EDS analysis and Raman Spectroscopic analysis compared the Altar Stone and numerous Old Red Sandstone sandstones from south Wales, Somerset and Welsh Marches. Data show their mineralogy and geochemistry to be dissimilar and that the Altar Stone was not from the Devonian age Anglo-Welsh Basin. Welsh Marshes samples with a high Ba content are rare and display a different mineralogy to the Altar Stone and therefore can be discounted. They suggested looking further afield including northern England and Scotland, as well as considering younger terrestrial sandstone sources within Permian strata.
2022-2023: Ixer, Bevins and Pearce (unpublished) examined a large collection of lithics collected by Cunnington in the 19th century and curated in the Salisbury Museum. Many sandstones were described as Altar Stone. Macroscopic identification confirmed by pXRF showed a number to be Altar Stone. However, others were shown to be Lower Palaeozoic Sandstone or Sarsen. The number of confirmed Altar Stone samples, although still few, but potentially available for investigation has been substantially increased.
The search is ongoing both in the laboratory and the field.
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