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Friday, 21 October 2016

New dates confirm Mid Ordovician age for FVG


 Fishguard Volcanic Group outcrops are shown with the grey stipple.  Source:  Phillips, Kerr and Bevins 2016

There's an interesting new paper by Richard Bevins and colleagues, presenting new U-Pb zircon ages for rock samples from the Fishguard Volcanic Series and from the Stonehenge collection of bits and pieces.

Details:

"U–Pb zircon age constraints for the Ordovician Fishguard Volcanic Group and further evidence for the provenance of the Stonehenge bluestones."  2016. Richard Bevins, Nicola Atkinson, Rob Ixer and
Jane Evans.  Journal of the Geol Soc. 2016 (online)
doi:10.1144/jgs2016-042

Abstract:  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.

This is essentially a paper aimed at geologists, but there are broader and very interesting points in it too.  This is a nice summary of the state of play on FVG identifications and the Stonehenge link:

........Craig Rhos-y-felin has been shown to be the major source of  rhyolitic debitage (struck flakes) in the Stonehenge Landscape (Ixer  & Bevins 2011a). Paradoxically, however, Craig Rhos-y-felin is not the source for any of the four rhyolitic and dacitic orthostats (stones  38, 40, 46 and 48) currently exposed at Stonehenge (Ixer & Bevins   2011b), although Bevins et al. (2012) have suggested, on petrographical grounds, that they too most probably have a source somewhere amongst the outcrops of the Fishguard Volcanic Group exposed in the north Pembrokeshire area. Specifically, these recent studies have shown that Carn Alw, Carn Llwyd and Carn Clust-y-Ci are not sources of Stonehenge rhyolitic or dacitic bluestones (either extant orthostats or debitage).

I keep on telling the geologists this, but they should not keep on saying that Rhosyfelin is THE major source of Rhyolitic debitage in the Stonehenge landscape.  They do not have enough evidence to support that contention, since most of the Stonehenge landscape has never been investigated.  What they should say is this:  Craig Rhos-y-felin has been shown to be the major source of  rhyolitic debitage (struck flakes) in that part of the Stonehenge Landscape which has thus far been investigated.  It may sound like a small point, but it is a very important one.

Five rock samples were analysed:  one rhyolite from Craig Rhosyfelin, two rhyolites from Fishguard Old Harbour, one ‘rhyolite with planar fabric’ debitage sample from the 2008 excavations at Stonehenge, and one debitage fragment, a blocky rhyolitic ash-flow tuff sample from the 2008 excavation at the Stonehenge Avenue.   The Rhosyfelin sample was dated to 462 million years, and the Stonehenge debitage sample was more or less the same.  The Avenue debitage sample came out as approx 464 million years, and the Fishguard Old Harbor samples came out at c 465 million years, meaning that they came from lower in the volcanic sequence.

The dates confirm that all of these samples have come from the Llanvirn part of the Ordovician volcanic sequence -- as anticipated.  (The relevant part of the stratigraphic column is now referred to as Darriwilian)  So there is a nice match between the petrology work and the dating work reported in this paper.

It's a pity that the paper is spoiled towards the end, in the discussion about possible sources for orthostats 38, 40 and 46,  by this statement:  "Nevertheless, this region (ie the lower land to the north of the Preseli ridge) provides an obvious target to search for further Neolithic quarry sites to add to those identified most recently by Parker Pearson et al. (2015)."    Let the archaeologists talk about Neolithic quarries if they want to, but this geological work is devalued because the authors have allowed themselves to be sucked, yet again, into this area of unsupported speculation and story telling.

My final gripe is about this statement:  Ages for samples SW52 and SH08 offer strong support, in terms of overlapping ‘high-precision’ zircon U–Pb dates and Th/Uzircon values, for the contention that the majority of the rhyolitic debitage in the Stonehenge Landscape is from Craig Rhos-y-felin, in the eastern part of the Mynydd Preseli area, in sympathy with other geological and archaeological evidence.  Yes, there is other convincing geological evidence, but sure as eggs there is not any archaeological evidence that stands up to scrutiny.

1 comment:

TonyH said...

Hubris, hubris, all is vanity and hubris.

Show me a geologist working in an archaeologically - "hot" environment, rich is monuments and myths, e.g. Preseli, and I'll show you a guy who's prepared to go right Over The Top in his clamour to become Famous for Fifteen Minutes, Andy Warhol - style.