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Thursday, 28 July 2016

Volcanics at Stonehenge: two more types, two more provenances

One of the figures from the paper, showing the stumps that might be candidates as sources for the volcanic rock fragments.


There is a new paper from our prolific pet rock boys.  The details are below. It now appears that volcanic fragments found in the Stonehenge debitage are of two different types, meaning that they have in all probability come from two different locations on the northern flank of Mynydd Preseli.   TRhe Volcanic Group A rocks are argillaceous lithic tuffs, and the Group B rocks are very hard, rather rare, and contain graphitising carbon.   There are currently no orthostats or stumps that can be identified as the source of the scattered fragments.

Note that these rocks are different from the rhyolites that have come from the Rhosyfelin area or elsewhere.  In addition, in the debitage and in the orthostat collection there are spotted dolerites and unspotted dolerites, and assorted sandstones including the Altar Stone.

There is no new fieldwork here, either in Pembrokeshire or in the Stonehenge area.  The two authors have examined samples already collected from various contexts.

The rock types examined do not speak of wonderful orthostats or tools.  Indeed, the two geologists suggest that the rock type was not a great deal of use for anything.  Rubbish rock?  This suggests to me that the source of the Volcanic Groups A and B materials might well have been a nondescript erratic boulder, rather like many of those which still exist in the bluestone circle.

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Rob A. Ixer and Richard E. Bevins (2016), "Volcanic Group A Debitage: its Description and Distribution within the Stonehenge Landscape". 
Wilts Arch and Nat Hist Mag., vol 109 (2016), pp 1-14


Abstract


The three major groups of debitage found in the Stonehenge Landscape are dolerites, rhyolitic tuffs (almost exclusively from Craig Rhosyfelin) and ‘volcanics with sub-planar texture’. This last group comprises two separate lithological sub- groups namely, Volcanic Group A, friable rocks with abundant white mica and a strong metamorphic fabric, and very rare Volcanic Group B hard rocks that are characterised partially by an unusual mineralogy including graphitising carbon.
Petrography, whole rock X-ray diffraction and whole rock geochemistry suggest that Volcanic Group A is a coherent group representing a single lithology, namely an argillaceous lithic tuff and that it is quite different from Volcanic Group B (including Stonehenge orthostat SH38) and from Stonehenge orthostat SH40.
Spatially, as with the other major debitage groups, Volcanic Group A lithics are widely and randomly distributed throughout the Stonehenge Landscape; temporally, almost none of the debitage has a secure Neolithic context. The debitage cannot be matched to any above-ground Stonehenge orthostat but may be from one or more of five buried and, as yet, unsampled stumps.
The lithology is believed to be from an unrecognised source on the northern slopes of the Preseli Hills but perhaps not from Foel Drygarn as has been suggested.

Conclusions


Although not totally resolved, petrography, whole rock XRD and geochemical analysis strongly suggest that Volcanic Group A is a single but quite variable lithology rather than two separate but related lithologies, namely calcite-bearing and non-calcite- bearing. Evans’s (1945) description of the same/ similar rock types in the (north Pembrokeshire) field suggests that, although they vary in terms of their primary and secondary petrography, they are a single geological unit.

Petrography, whole rock XRD, and preliminary whole rock geochemistry confirm that Volcanic Groups A and B are distinct from each other and also that Volcanic Group A is not related to Stonehenge orthostat SH40.

The use of the terms ‘basic tuffs’ by Howard (in Pitts 1982) and Ixer and Bevins (2010) and ‘calcareous ash or tuff ’ by Thomas (1923) to describe Volcanic Group A (and B) debitage are misleading and poorly describe many of the samples, especially the muscovite-rich ones. Bevins and Ixer (2013) called one such sample an ‘argillic rock with accretionary lapilli’ recognising its phyllosilicate-rich nature. Evans (1945) (from outcrops not archaeological specimens) called similar lithologies vitro-lithic tuffs and his description of them would encompass all the Stonehenge ‘volcanics with sub-planar texture’. It is suggested that the term argillaceous lithic tuff is better for these samples and indeed for all Volcanic Group A samples.

Unlike SH38 and its associated Volcanic Group B samples and SH48 and associated Rhyolite Group E, where debitage can be matched petrographically to a single standing orthostat, none of the potential parent orthostats for Volcanic Group A (SH32c, 33e, 33f, 40c and 41d) have been petrographically examined, making it impossible to relate this debitage to any (or all) of the buried stones.
The temporal and spatial distributions of Volcanic Group A lithics mirror that of the other major non-dolerite debitage, namely Rhyolite A–C (Craig Rhosyfelin). Both have a widespread distribution and are very numerous (Volcanic Group A ~45% and Craig Rhosyfelin rhyolite ~30% by number of all the bluestone debitage) within the immediate Stonehenge Landscape and both occur further away at the Stonehenge Greater Cursus area. The temporal distribution of both is very similar to that for the rest of the examined debitage in that most pieces are found from post-Neolithic contexts, but have been found on the Stone Floor close to the Heelstone area. There appears to be no clear systematic change in size distribution with context/ time for any of the debitage about Stonehenge.

So far, trying to find this lithology in the Foel Drygarn as proposed by Thomas (1923) has been unsuccessful (as has trying to locate and exam the original Part thin section that Thomas relied upon). Hence, it is safer to be guarded about attributing this lithology specifically to the northern slopes of Foel Drygarn. However, on present knowledge the origin(s) of the Volcanic Group A lithics is still expected to be found within the Ordovician volcanic sequences in the north Pembrokeshire area on the northern side of the Mynydd Preseli range probably amongst those outcrops examined by Evans (1945).

Relevant Bevins and Ixer References:

BEVINS, R.E and IXER, R.A., 2013. 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. Journal of Archaeological Science 40, 3293–3301
IXER, R.A., 1990, Atlas of Opaque and Ore Minerals in their Associations. Milton Keynes: Open University Press
IXER, R.A., 1994. ‘Does ore petrography have a practical role in the finger-printing of rocks?’ in N. Ashton and A. David (eds), Stories in Stone. Proceedings 10th Anniversary Conference. Oxford 1993. Lithic Studies Occasional Paper 4. 10–23. London: Lithic Studies Society
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, R.E., 2011. The detailed petrography of six orthostats from the Bluestone Circle, Stonehenge. WANHM 104, 1–14
IXER, R.A. and BEVINS, R.E., 2013. A re-examination of rhyolitic bluestone ‘debitage’ from the Heelstone and other areas with the Stonehenge Landscape. WANHM 106, 1–15
IXER, R.A. and BEVINS, R.E., in press (title?), in Parker Pearson, M., Pollard, J., Richards, C., Thomas, J., Tilley, C. and Welham, K., Stonehenge for the Ancestors. Prehistoric Society monograph. Oxford: Oxbow
IXER, R.A., BEVINS, R.E. and GIZE A.P., 2015. Hard ‘Volcanics with sub-planar texture’ in the Stonehenge Landscape. WANHM 108, 1–14
IXER, R.A., WILLIAMS-THORPE, O., BEVINS, R.E. and CHAMBERS A.C., 2004, ‘A comparison between ‘total petrography’ and geochemistry using portable X-ray fluorescence as provenancing tools for some Midlands axeheads’, in E.A.Walker, F.Wenban-Smith and F.Healy (eds), Lithics in Action. Lithics Studies Society Occasional Paper 8, 105–15. Oxford: Oxbow Books

14 comments:

TonyH said...

All in all, Another Brick in the Wall, another bleak finding for the Rhosyfelin Man- Made Quarrying Boys & Girls from England........a triumph of prosaic Science over romantic Fantasy....the evidence, for those with eyes to see and not blinkers to hinder sight, continues to stack up. When will they ever learn?

BRIAN JOHN said...

Oh, the quarrying brigade will think of something. No matter how many provenances there are for the bluestones and fragments found at Stonehenge, they will probably say that each one of those provenances was associated with one or another of the ancient ancestors -- great grandpa Dafydd came from Carngoedog, great-great grandpa Emrys came from Rhosyfelin, great aunt Myfanwy came from Carn Alw, and so on and so on. And of course great grandma Blodwen came from somewhere up near Llandeilo, where all those Senni Beds are located......... I can see it now -- the next article for Antiquity is probably already in the preparatory stage.......

TonyH said...

Please could you expand on "somewhere up near Llandeilo, where all those Senni Beds are located....". Are we talking Altar Stone, now?

But, yeah, we have to always remember that more and more of the Archaeology Departments are now calling themselves Archaeology AND Anthropology Departments, eg Bristol, so all these extended family linkages amongst out revered and much - travelled Ancestors turn out to be of the utmost significance.

Neither must we ever forget, as Winston might have reminded us were he still here, puffing on his cigar, that many of these illustious Archaeologists [Pollard, Colin Richards, and Parker Pearson, etc] have been to Rapa Nui, which is Easter Island [with its moveable
statues on the cliffs] to the mere likes of you and me. That makes them Wise Men.

TonyH said...

Then again, Brian, Lloyd George was my father, my father knew Lloyd George (but you knew that!).

David H. Jones said...

Tony,
What are you rambling on about?

BRIAN JOHN said...

As for the Senni Beds of the ORS series, yes, we are talking Altar Stone. It is after all included as a bluestone, so whatever hypothesis is invoked in order to explain the presence of bluestones at Stonehenge has to find space for that one too.....

As for Lloyd George, he probably didn't know my father.

MoA said...

And not just the Devonian Altar Stone.

For some reason "the Senni Beds of the ORS" reminded me of Howard Keel introducing himself as "Gaylord Ravenell of the Tennessee Ravenells". (Showboat the very best musical of all time)perhaps on the same steamer that Dave Robicheaux sees.

Just finishing Pryor's detective novel did not like it to start with, the archy stuff was intrusive, but really quite good. I suspect that some of the characters are quite like 'real' people especially some of the women specialists.
M of the Alexandrian Ms

BRIAN JOHN said...

Gosh, have I missed a trick here? "Sex among the sarsens" and so on.........?

MoA said...

That is only too well trodden a path in fiction, (there I some truly dreadful SH fiction around and films) and I expect in fact.
M

TonyH said...

Hear's an amusing anecdote, from last weeks's Radio Times, no less. We all, at least those of us old enough to remember the predecessor to Wet, Wet, Wet's version of "Love Is All Around". That's right, it was written by Reg Presley of the Troggs back in the '60's, who also wrote "Wild Thing". Well, alright, I know the link between the Troggs and Blue Stones is rather faint, but we've had Elvis on here a number of times linked to the Preseli Hills....

Another tenous link: Reg Presley hailed from Andover, just down the road from Amesbury & Stonehenge, "As well as being Andover's major contribution to rock culture, he was an expert on crop circles. In an intriguing sideline, he also invented and patented a bird scarer for runways. The Trogg Tapes, in which his band were secretly recorded in a hilarious assessment of their career while trying to record a new single, is said to have inspired 'This Is Spinal Tap'[beloved by Geo Cur on this Blog, amogst others]."

"But my favourite Reg story concerns the time he was working at the same studios as Bob Dylan. As Bob headed for lunch, he passed a studio where Reg was sitting astride an amp, struggling to master the bass guitar, tunelessly. 'Been Playing that long?' asked Bob. 'All bloody a'ternoon', said Reg in broadest Hampshire."

Myris Of Alexandria said...

We played the Trogg's tape often at the end of a night/early morning in The Bell at Banwell. Throughout the 70s.
Probably still have the cassette -no player of course.
The sound of the door slamming as people walked out and the recording engineer going slowly demented whilst the drummer still could not get the rhythm. "split your 'ands". Bom-bom bom-bom BOM, yeees, NOOO.
M

TonyH said...

Since this particular Post is about Rock Provenances, it should also be remembered that the leader singer and songwriter of the Moody Blues, Justin Hayward, was born just up the road from Avebury and not that far either from Stonehenge, at Swindon, Wiltshire.

Mike Parker Pearson has declared that his favourite song is "Driftwood", which contains the line:

"Don't let me be driftwood on the shore"

I don't think there's much chance of that, Mike.

BRIAN JOHN said...

Oh, I don't know -- there comes a time when we are all washed up, stranded and ignored.......

TonyH said...

"Washed up, stranded, and ignored....)

Sounds like a line from George Harrison R.I.P., "Handle me with care": come to think of it, that's another great title for an archaeologist to choose for one of his Desert Island Discs.