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Wednesday, 14 May 2014

Bluestones - "possible" sources in Wales


 Carn Goedog -- now favoured as the most likely source of most of the dolerite 
orthstats at Stonehenge

 This is a nice summary (from STONECHAT magazine) of the recent geological work.  Note the use of the word "possible" by the geologists -- it would be nice if that word was used rather more often by the archaeologists too......

Richard Bevins, Rob Ixer and Nick Pearce:
Three recent papers linking the petrology of Stonehenge Bluestones to possible sources in SW Wales.
http://implementpetrology.org/wp-content/uploads/2014/04/2014-03-stonechat.pdf
STONECHAT 1: March 2014

Two recent papers by Dr Rob Ixer (Institute of Archaeology UCL) and Dr Richard Bevins (Amgueddfa Cymru, National Museum Wales) and Professor Nick Pearce (University of Aberystwyth) were tabled for information and discussion at the 18th IPG Meeting, York 11-12 Jan 2014). The papers reported the results of their investigations into the geographical provenance for the bluestones of Stonehenge and the meaning –if any- of the debris/debitage that is associated with the Stonehenge Landscape. Their earlier papers (2010-2013) had concentrated on describing and determining the origin of the silica-rich (rhyolitic) bluestones and their debris. This work contributed to the discovery of a possible quarry source at Craig Rhosyfelin.

In their first paper (Bevins, Ixer & Pearce: 2013), focus on the spotted dolerites, considered to be the most abundant of the bluestones. Using ‘total petrography’ (a methodology synonymous with Rob, which consists of the full petrographical description of a rock in both transmitted and reflected light), Ixer and an Open University team first examined these rocks in the mid-1990s. More recently however, Bevins, Ixer and Pearce examined the rocks using a more robust methodology achieved by combining petrography and whole rock geochemistry; they concentrated on the compatible elements
which, they believed, provided a clearer discrimination between sets of samples from Stonehenge and the Preseli and revealed hitherto undetected geochemical groupings. They concluded that although there are at least two geographical sources in the Preseli Hills and perhaps more, Carn Goedog is the source of the numerically largest group of dolerite orthostats, and is the likely major source of Stonehenge doleritic bluestones. Their evidence, based on compatible element geochemistry and Principal Component Analysis, reused the Open University’s geochemical data, earlier petrographical analyses and their own previously unpublished data. To date, they have been unable to match any of the Stonehenge bluestones to the spotted dolerite bluestone quarry site at the Preseli Hills.

In the second paper (Ixer and Bevins: 11-22 ), the focus is on the relative position of the standing stones and their debris within Stonehenge and its immediate environs. It is the first paper to discuss in any detail the bluestone debitage material and to try to relate its distribution to the standing, lying and buried orthostats. For the non-dolerite lithologies there appears to be an antipathetic relationship between the debitage and the surviving orthostats. They predict that attributing the spotted dolerite debris to a named orthostat may be difficult as many dolerite orthostats cannot be distinguished from each other by either petrographical or geochemical means.

An interesting finding was that the Rhyolite Groups A-C (rhyolite with fabric) debitage, identified by Ixer and Bevins as coming from Craig Rhosyfelin, appears not to be associated with any above-ground orthostat but may be from buried orthostat SH32d or SH32e. This debitage is common in secure and non-secure prehistoric contexts. Work on the spotted dolerites including the paper above suggest that other bluestone lithologies have yet to be identified and fully described.

REFERENCES

Bevins, R.E, Pearce, N.J.P, and Ixer, R.A Pearce (2011) Stonehenge rhyolitic bluestone sources and the application of zircon chemistry as a new tool for provenancing rhyolitic lithics. Journal of Archaeological Sciences, 38, 605-622.

Ixer, R.A, Williams-Thorpe, O, Bevins, R.E, and Chambers, A.D (2004) A comparison between ‘total petrography’ and geochemistry using portable X-ray fluorescence as provenancing tools for some Midlands axe-heads; in E.A Walker, F. Wenban-Smith and F. Healy (eds), Lithics in Action. 105-115: Oxford; Oxbow/Lithics Studies Society Occasional Publication 8.

Ixer, R.A and Bevins, R.E (2010) The petrography, affinity and provenance of lithics from the Cursus Field, Stonehenge. WANHM 103, 1-15

Ixer, R.A and Bevins E.R (2011) The detailed petrography of six orthostats from the bluestone circle, Stonehenge; in Wiltshire Archaeological & Natural History Magazine 104, 1-14.

Ixer, R.A and Bevins, E.R (2013) Craig Rhos-y-Felin, Pont Saeson is the dominant source of the Stonehange rhyolitic ‘debitage’; in Archaeology in Wales 50, 21-31

Ixer, R.A and Bevins, R.E (2013) Chips off the old block: the Stonehenge debitage dilemma. Archaeology in Wales 52, 11-22.

2 comments:

Constantinos Ragazas said...

Brian,

Quoting directly from the article in your post,

An interesting finding was that the Rhyolite Groups A-C (rhyolite with fabric) debitage, identified by Ixer and Bevins as coming from Craig Rhosyfelin, appears not to be associated with any above-ground orthostat but may be from buried orthostat SH32d or SH32e. This debitage is common in secure and non-secure prehistoric contexts.

If EH is such stickler for certainty why don't they permit Dr Ixer and his team to sample ” buried orthostat SH32d or SH32e”?

Kostas

Dave Maynard said...

I walked up to Carn Goedog the other day. Very interesting, especially all the stuff downhill from the rocks.

What is very apparent is that there are plenty of megalith shaped rocks lying around the place, no need to quarry them out of the ground as might have happened at the other place to the north west.

Dave