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.
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
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.
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