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Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
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Friday, 10 February 2012

Even more about the Stonehenge rhyolites



Rob Ixer has kindly sent me a PDF of the latest paper in the series of geological publications homing in on the rhyolites in the Stonehenge debitage. 

This paper examines the evidence for rhyolite fragments that might have come from sites other than Craig Rhosygfelin, using petrography and "whole-rock geochemistry."  The abstract is below.  The main conclusions of the research are that there are no traces thus far of any fragments from the Sealyham Volcanic Formation (which outcrops to the south and west of the Preseli Hills; and that of those fragments which have come from the Fishguard Volcanic Series the Craig Rhosyfelin outcrops are by far the most important sources.  Other sites thought in the past to have been source sites for rhyolite fragments -- namely Carn Clust-y-ci and Carn Lwyd near Newport -- are not now considered to have provided anything to either the orthostats at Stonehenge or to the debitage.  Then the authors say:  "Nevertheless, the possibility still exists that all of the dacitic and rhyolitic bluestone lithologies can be sourced in outcrops of the Fishguard Volcanic Group, in the tract of country between Fishguard and Crymych. If this indeed proves to be the case it will undoubtedly influence the location of future archaeological excavations to determine whether there exists evidence of anthropogenic activity in working particular outcrops or, in the absence of such evidence, whether entrainment and
transport by ice remains a viable alternative for the transport of the bluestones to the Stonehenge landscape.
"

This is all very interesting to those of us who have an interest in the geology -- but we do need to bear in mind that there is a strong sampling bias here, both at the Stonehenge end and in North Pembrokeshire.  Attention is focussing on the Fishguard Volcanic Series, and other sources for orthostats, stumps and fragments may well be found as work continues. These might even be from the North Pembrokeshire coast or on the Pen Caer Peninsula.  As the authors point out, their conclusions are still based upon a relatively small data base.  Much material still remains to be provenanced.  More publications are promised -- watch this space....  and when the rhyolites are sorted out, there is a great deal more to be done on the dolerites and sandstones.

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Provenancing the rhyolitic and dacitic components of the Stonehenge landscape bluestone lithology: new petrographical and geochemical evidence

by Richard E. Bevins, Rob A. Ixer, Peter C. Webb, John S. Watson
Journal of Archaeological Science 39 (2012) 1005-1019

ABSTRACT
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.

1 comment:

Tony H said...

Aberystwyth lecture from Bevins very soon, yes?