Stonehenge -- did the rhyolite fragments come from bluestone monoliths?

(Thanks to Rob for drawing our attention to this article from 2013.  It reports that Ixer and Bevins have now been involved in a "lumping" exercise by combining the three rhyolite groups A-C into one single category which has quite a lot of variation within it.  Makes a change from the "splitting" which geologists usually do.  Doing a lumping exercise has the merit of simplicity, since we are all confused enough as it is, but there is also a danger of creating complacency and causing non-geologists to assume uniformity where there is none.  I am even more confused because the authors now seem to assume that all three types of foliated rhyolite from Rhosyfelin were to be found on a single monolith at Stonehenge which has now been destroyed.........  we will no doubt have that explained in due course.  See also this post:
http://brian-mountainman.blogspot.co.uk/2013/01/the-rhyolitic-debitage-around-stonehenge.html
But what is especially noteworthy in this article is the contribution from Mike Pitts, who makes exactly the same point as I am making below.  A link to the paper is added at the end of the original post.)

The rhyolite outcrops at Rhosyfelin. A good place for making Mesolithic microliths and Neolithic axes?  That's a question worth asking.......

 Weights and specimen numbers from the Stonehenge Layer.  These graphs have no statistical significance, but they are interesting.  These are just some of the total number of fragments examined from all contexts

Let's agree with Rob Ixer and Richard Bevins that some of the rhyolite debitage in the Stonehenge landscape has come from the Pont Saeson area near Brynberian --  and more particularly from the Rhosyfelin locality.  Of the 4000 or so "bluestone" fragments examined, over 1200 (approaching 30% by number) of those have been made of foliated rhyolite similar to those identified around Rhosyfelin.  That's  about 19% of the total, by weight -- and surprisingly, that figure is greater than the 16% for spotted dolerite. Bear in mind that there are far more spotted dolerite orthostats than there are rhyolite ones -- so as Rob and Richard have pointed out, that means that any rhyolites in the Stonehenge area have been bashed about in preference to bashing about the spotted dolerites.  Maybe that's simply because it was easier to do, since flaky and fractured rhyolites are easily broken up, and dolerites are far harder.......   Interestingly, there is no segment on the diagram above for unspotted dolerite.

Given that there is huge sampling bias here (archaeologists have only examined a few parts of the Stonehenge debitage / soil layer and have automatically homed in on "interesting" samples while rejecting "boring" ones) we can say certain things about the material that HAS been collected and examined.

1.  We are talking about very small weights here.  What is the total weight of all the rhyolite fragments examined?  Perhaps Rob will tell us, but it may be no more than a few kilos.  The weight of a few small stones or axes?  I take the point that Rhosyfelin was not an established source of axes, and that there is no known "axe group" identified by the Implement Petrology Group with the right petrological characteristics, but that does not mean that Rhosyfelin was not used by some individuals as a source of material for the making of axes.  If that happened, those axes (which may not have been very good ones) could also have been traded, brought to Stonehenge, and then destroyed because they were unfit for purpose.  (Forgive me.  I feel a story coming on.......)

2.  I'm not sure why it should be assumed that because there are "Rhosyfelin rhyolites" in the debitage at Stonehenge, they must have come from one or more destroyed orthostats.  That's a reasonable hypothesis, given those enigmatic stumps that look as if they are made of the right material (32d and 32e, if we have the numbering right) -- but it's also a perfectly reasonable hypothesis to suggest that the fragments might have come from "inconvenient" pebbles, chunks or boulders that happened to be lying around in the Stonehenge landscape and which were simply smashed up because they were in the way or were of no use.

3. There are three different rhyolite fabrics in the frame here.  These are, as defined by Ixer and Bevins in 2011:

A. Dark/black, sharp, flinty rhyolite ± joint planes. Rare, pale-coloured, flinty rhyolite is probably weathered dark rhyolite.
B. Rhyolite with a planar fabric. Rare, extreme examples of this group initially were classed as ‘slate/phyllite’.
C. Rhyolite with a pronounced planar and lensoidal fabric ± joint planes.

Forgetting about Groups D and E for the moment, they say "the other three groups are significant in percentage terms both by weight and number."

I accept that there is considerable variation in the characteristics of the rhyolites across the rock face exposed at Rhosyfelin.  But do all three types occur on this single rock face?  From my careful reading of the paper, I am not sure that this has been established.  So even if some very accurate provenancing can be demonstrated (ie to within a few metres) the possibility remains that substantial amounts of the Stonehenge rhyolitic debitage have come from other sampled or unsampled locations across the Pont Saeson area either from currently exposed rock outcrops or from others that are currently invisible.  (Even if you look at the published thin sections showing the "Jovian fabric" about which all the fuss has been made, the samples from Stonehenge and Locality 8 are similar but NOT identical, as the authors recognize.)

Top image -- Jovian fabric from Locality 8 at Rhosyfelin.  Bottom image -- Jovian fabric from one of the Stonehenge samples. The fabrics are similar but not identical, as Ixer and Bevins point out.

My point is that no matter how good the geological provenancing of these rhyolite fragments may be, the orthostat obsession has led to the quarry obsession, and that in turn has led to all sorts of skulduggery in the interpretation of perfectly natural features observed during the 2011-2014 digs.........  Ah, geology as the root of all evil!

Quotes from the Ixer / Bevins papers called "Chips off the Old Block" and "CRAIG RHOS-Y-FELIN, PONT SAESON IS THE DOMINANT SOURCE OF THE STONEHENGE RHYOLITIC ‘DEBITAGE":

** Since 2008 there has been a systematic lithological investigation of debitage found within the Stonehenge Landscape; most of these lithic fragments were re-examined in April 2013. Just over 4000 bluestone debitage fragments weighing between 0.1 and 8500g have been lithologically classified macroscopically, the majority being from the April 2008 Darvill and Wainwright excavation within the Stonehenge stone circle (3657 fragments) but also including lithics from the Heelstone Ditch excavations (171) (Pitts, 1982; Ixer and Bevins, 2013), the Stonehenge Avenue including Trenches 44 (20) and 45 (71), Aubrey Hole 7 (54), and from surface finds and test pits in the area close to the western end of the Stonehenge Greater Cursus (31) (Ixer and Bevins, 2010). ............  In addition small numbers of bluestone fragments have been identified from locations in the vicinity of the Stonehenge Landscape although no bluestones have been identified from West Amesbury Henge. In addition over 100 thin sections, polished blocks and polished thin sections have been described for the major non-dolerite classes of debitage and these have been compared with polished thin sections from all the sampled bluestone orthostats.

**  Although there are subtle but distinct differences between different rhyolitic outcrops at Pont Saeson, including those on Craig Rhos-y-felin, they share a distinctive petrography that is unrecognised from elsewhere in the Fishguard Volcanic Group. Texturally all are foliated usually with an associated lensoidal fabric where deformed lithic clasts occur and carry a similar dominant mineralogy, although, in addition, some outcrops have rare, unusual minerals.

**  Almost all (>99.9%) of the Stonehenge rhyolitic ‘debitage’ can be petrographically matched to rhyolitic rocks found within a few hundred square metres at Pont Saeson and especially to those found at Craig Rhos-y-felin. However, it is possible in a few cases, where the petrography of these Welsh in situ rocks is so distinctive, to suggest an even finer provenance to within square metres, namely to individual outcrops.

**  Well over 1200 rhyolitic lithics belonging to Groups A – C have been identified and their distribution is widespread throughout the Stonehenge Landscape including close to the Stonehenge Greater Cursus area but no parent orthostat has been securely identified although hitherto unsampled buried stones SH32d or SH32e are strong candidates. The majority of the rhyolite lithics are small struck flakes but at least one large rough out weighing 190g (April 2008 excavation context 12/10; Roman) has been recognised.  It is interesting that it does not belong to any of the established axe groups established by the Implement Petrology Group (Clough and Cummins, 1979, 1988). 

 ===========================

Addendum

A re-examination of rhyolitic bluestone ‘debitage’ from the Heelstone and other areas within the Stonehenge Landscape
by Rob A. Ixer and Richard E. Bevins, with a contribution from Mike Pitts
Wilts Arch & Nat Hist Mag 106 (2013), pp 1-15.
https://www.academia.edu/4036994/Lithics_from_Heelstone_Ditch_area_Stonehenge

ABSTRACT

Recently it has been proposed that the Stonehenge rhyolitic debitage can be distributed into five petrographical groups (A-E) (and that at least three of them (A-C) are from rocks cropping out at Craig Rhos-y-felin). This supersedes an earlier classification scheme of this important category of Stonehenge material. The earlier 1980s scheme, based on lithics found close to the Heelstone, divided the rhyolites into two groups (A and B) and sub-divided the larger into two further
sub-groups (Bi and Bii). Re-examination of this earlier material together with other Stonehenge rhyolites has allowed the two schemes to be compared and integrated.
The original 1980s Group A lithics are identical to the present Group B, (both are small groups). This group is described in detail so completing the petrographical descriptions of the Stonehenge rhyolitic debitage. Despite bearing feldspar megacrysts this group shares sufficient petrographical characteristics with rocks from Craig Rhos-y-felin to support the view that that location is the geographical origin of the group.
Lithics belonging to the 1980’s groups Bi and Bii, however, are randomly distributed amongst the present A and C groups and there are no strict correspondences. The designation Bi and Bii should therefore be abandoned.
Using the new scheme it should now be possible to map more precisely the distribution of the rhyolitic debitage in the Stonehenge landscape to inform such questions as to the number of rhyolite orthostats originally present and their fate.


Extract from the contribution by Mike Pitts

Such issues of context are extremely important if
we are to understand the significance of the different
types of stone used at Stonehenge. Context has been
conspicuously absent from most debate, which has
focussed on whether the smaller megaliths were
brought to Stonehenge from Wales, or found lying
on Salisbury Plain. Debris at Stonehenge or nearby
is typically treated as if it were a proxy for megaliths.
Distinctions between different types of bluestone are
assumed to matter only to modern scientists, not
to the people who built or used Stonehenge. None
of this is helpful (archaeologists and geologists are
equally at fault).

It is notable that all the samples matched in this
study to Craig Rhos-y-felin come from debitage and
not from megaliths (although Ixer and Bevins (2011a
and b) have suggested that buried megalith SH32e
may also come from Craig Rhos-y-felin). One of the
distinctive features of the rhyolitic rocks is that they
are flinty – they have a good conchoidal fracture.
That makes them relatively easy to break up, if they
are standing as monoliths at Stonehenge. But it also
makes them suitable for making portable artefacts.
There are flaked bluestone ‘tools’ from Stonehenge
(including some from the stone floor). Which of
these are made from debris created when stones
were dressed on site? Which are made from broken
up megaliths? And which were made in Wales and
brought to Stonehenge by people visiting, perhaps
on a pilgrimage of some kind? Clearly the distinction
has important implications for how we understand
Stonehenge.

These are questions that future research can
approach through excavation in Wales and at
Stonehenge and study of the debris. Very limited
excavation at Stonehenge would allow modern
identification of the stumps and other pieces of
megalith at the site, and perhaps the matching of
some to Ixer and Bevins’ rhyolitic groups. The
dressed dolerite fragment and apparent tools from
the 1979–80 excavations suggest that re-excavation of
Hawley’s stone dumps would be productive, despite
the lack of context. The search for sarsen quarries,
too, is critical.

A good start, as Ixer and Bevins suggest, would
be to re-examine all previously excavated rhyolitic
debitage. As they say this needs to reach beyond
Stonehenge. Over 30 years ago I compiled a list
of Stonehenge-type rocks found away from the
monument, noting that there had been ‘surprisingly
little consideration of the stones imported to the site
for use as megaliths’ (Pitts 1982, 124–126). Perhaps
now this might change.


The point about Rhosyfelin rhyolites being used for tool making is a good one, and needs to be answered.  Mike also wonders why we should always assume that the debitage at Stonehenge has come from monoliths rather than from other stone debris or from broken up tools which were imported onto the site by traders or visitors.  In my view this is much more likely than the "smashed monolith" hypothesis.  After all, why would the Stonehenge builders go to the trouble of carrying a rhyolite monolith (or several of them) all the way to Stonehenge in the knowledge that it was no use at all as a standing stone, and then to smash it up on site?  It would have been far simpler, if they were more interested in rhyolite tools from Rhosyfelin, to do the work at Rhosyfelin itself, leaving all the debris behind.  Back to my point -- was it the tools that were smashed up, rather than the monoliths?