Article

Carn Alw as a source of the rhyolitic component of the Stonehenge bluestones: A critical re-appraisal of the petrographical account of H.H. Thomas

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Abstract

The source of the Stonehenge bluestones was first determined in the early 1920s by H.H. Thomas who was an officer with the Geological Survey of England and Wales. He determined that the so-called ‘spotted dolerites’ could be petrographically matched to a small number of outcrops in the Mynydd Preseli district in south-west Wales. The bluestones, however, comprise a number of additional lithologies, including rhyolite and ‘calcareous ash’, as well as various sandstones. Thomas was convinced that the volcanic lithologies in the bluestone assemblage were all sourced from a small area at the eastern end of the Mynydd Preseli, with the rhyolites originating from the prominent outcrop known as Carn Alw. Recently, provenancing of these rhyolites to Carn Alw has been questioned on the evidence of whole-rock geochemistry. This raised concerns over the original petrographical attribution. Accordingly a re-investigation was undertaken of the rhyolite petrography by re-examining the original specimens used by Thomas. Three of the original four thin sections studied by Thomas were re-examined, along with a newly made thin section from the fourth of Thomas' rock samples as the original thin section could not be located. The new petrographical evidence demonstrates convincingly that the two pairs of thin sections from the Preseli and Stonehenge as examined by Thomas do not match despite his contention and argues strongly that Carn Alw is not the source of the Stonehenge rhyolites which Thomas described. This reinforces the geochemical evidence presented recently and supports the contention that Craig Rhos-y-felin, to the north of Mynydd Preseli, is an important source of rhyolitic debris in the Stonehenge Landscape. Nevertheless, there remain uncertainties over the provenance of other Stonehenge rhyolites (and dacites), including four of the orthostats themselves.

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Poster
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The long-distance transport of the Stonehenge bluestones from the Mynydd Preseli area of north Pembrokeshire was first proposed by geologist H.H. Thomas in 1923. For over 80 years, his work on the provenancing of the Stonehenge bluestones from locations in Mynydd Preseli in south Wales has been accepted at face value. New analytical techniques, alongside transmitted and reflected light microscopy, have recently prompted renewed scrutiny of Thomas's work. While respectable for its time, the results of these new analyses, combined with a thorough checking of the archived samples consulted by Thomas, reveal that key locations long believed to be sources for the Stonehenge bluestones can be discounted in favour of newly identified locations at Craig-Rhos-y-felin and Carn Goedog.
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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.
Article
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Article
The source of the bluestone component found in the Stonehenge landscape has long been the subject of great interest and considerable debate. The bluestones are a mix of lithologies, the standing orthostats being predominantly dolerites, variably ‘spotted’, with only four of them being of dacitic and rhyolitic composition and the Altar Stone being sandstone. However in the 1920s the spotted dolerites were sourced to outcrops which comprise tors in the summit regions of the Mynydd Preseli in north Pembrokeshire, west Wales. There were also speculations about the possible sources of the dacitic and rhyolitic components, ideas which were elaborated on in the early 1990s when the original petrological provenancing was supplemented by whole-rock geochemical analysis. Most recently, new petrographical investigations have been combined with zircon geochemical data to determine the possible source of one type of rhyolite, the so-called ‘rhyolite with fabric’, found abundantly as débitage in the Stonehenge landscape (but not composing the four orthostats) to outcrops in the vicinity of Pont Saeson, especially a large craggy outcrop called Craig Rhos-y-felin, located in low ground to the north of the Mynydd Preseli. In order to test this provenance whole-rock geochemical analysis has been undertaken on samples of débitage from the Stonehenge landscape and from the Pont Saeson area, including Craig Rhos-y-felin. These data are then compared with other new and existing geochemical data for dacitic and rhyolitic lithologies recovered from the Stonehenge landscape, including the four orthostats, as well as geochemical data from outcrops of the same lithologies from the two main volcanic horizons exposed across north Pembrokeshire, namely the Fishguard Volcanic Group and the Sealyham Volcanic Formation, both of Ordovician age. This study concludes that previous, 20th century, attributions of provenance to a number of dacitic and rhyolitic outcrops in the north Pembrokeshire have been in error whilst the new data for the Pont Saeson rhyolite accords well with elemental contents recorded in the ‘rhyolite with fabric’ lithology from the Stonehenge landscape débitage. This study therefore endorses the proposal that the Pont Saeson area is indeed the source of the ‘rhyolite with fabric’ lithology recovered from numerous sites in the Stonehenge landscape, and is the only reliable provenance for any of the dacitic and rhyolitic bluestone material collected to date. It also serves to endorse the use of zircon chemistry as a provenancing tool in archaeopetrological investigations.
Article
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Article
The Ordovician rocks of this district range from the Arenig to the Bala, with the succession generally ascending northwards. The rocks usually possess a Caledonoid strike and are disposed about two eastwardly pitching anticlines and a complementary syncline, which are dislocated by faults and overthrusts to the south-east. The Sealyham Volcanic Series is composed of rhyolites and tuffs in the western part of the area, but in the east the formation is almost entirely represented by trachytes. The Didymograptus bifidus beds occupy nearly the whole of the Prescelly Hills and they are mainly composed of shales and mudstones. The Fishguard Volcanic Series consists of fine-grained spherulitic and flow-brecciated rhyolites with thick beds of tuffs and local developments of ashy shales and mudstones. Sills of quartz-dolerites (among them the “spotted dolerite” which has been described as the analogue of the blue-stones of Stonehenge) and a single intrusion of a keratophyria rock occur in the Arenig and Llanvirn rocks but do not penetrate the shales, mudstones and sandstones of the Llandeilo and Bala formations. Petrographic descriptions of the igneous rocks are given, as well as a figure illustrating the stratigraphical correlation of the beds with those of other areas.
Article
The petrology of the Fishguard Volcanic Group and associated rocks is described from a remapped area of Cambrian and Ordovician rocks between Fishguard and Newport, N Pembrokeshire (Dyfed). The volcanic rocks are mainly rhyolitic in composition and consist of pyroclastic flows, falls, lavas, epiclastic deposits, and alloclastic intrusions; welded pyroclastic flows comprise the major component. A small area of basaltic pillow lavas and hyaloclastites is associated with the rhyolitic rocks. The volcanic rocks are accompanied by numerous sheets of dolerite and gabbro, together with diorites and microgranitic intrusions. The sequence at Lower Fishguard Harbour shows an intimate association of welded and non-welded pyroclastic flows with graptolitic shales and other marine sediments, which suggests that welding may have occurred subaqueously. The cooling rate of submarine pyroclastic flows and lavas is thought to be significantly reduced by the formation of a steam jacket and a quenched marginal crust, both providing low thermal conductivity. The hydrostatic pressure of seawater maintains cohesion and retains heat and volatiles within the flow. In addition, water saturation of the rhyolitic melt throughout its cooling maintains mobility and is conducive to subsequent welding.
Article
Extract It may be seen, on referring to Mr. Greenough’s Geological Map of England, and to the map annexed to the paper of Mr. Webster on the Basins of London and Hampshire; that the tertiary formations which occupy the basin of London, become gradually contracted in their progress westward through Berkshire, until they terminate in a point at Savernake Forest, between Hungerford and Marlborough. The strata of chalk, on which these formations repose, dip inwardly from the circumference towards the axis of the basin, and sink, nearly on all sides, beneath overlying beds of the Plastic and London clays. A remarkable exception, however, to this arrangement occurs near the south-western extremity of the basin, a few miles to the south of Newbury, at Kingsclere and Highclere. It is the chief object of the present communication to describe the appearances attending this exception, and to point out some general conclusions with respect to the origin of certain valleys from the elevation of the strata that now inclose them. The observations on which this paper is founded, were made principally during a visit in the summer of 1819 to the Earl of Carnarvon at Highclere. I had then an opportunity of carefully surveying the ground in question, which I had viewed hastily some months before on a journey through the district in company with Mr. Greenough and Mr. Warburton. But for the more minute details of the limits of the several formations, as they wind along the south-western border of the basin, I
On the road to Stonehenge: report on investigations beside the A34 in 1968
  • M W Pitts
Pitts, M.W., 1982. On the road to Stonehenge: report on investigations beside the A34 in 1968, 1979 and 1980. Proceedings of the Prehistoric Society 48,
2 is a photomicrograph of thin section E13868 in the British Geological Survey Collection
  • Fig
Fig. 2 is a photomicrograph of thin section E13868 in the British Geological Survey Collection;