On the significance -- or otherwise -- of >99.9%

Amid all of the recent media frenzy relating to the Rhosyfelin paper (Ixer and Bevins 2011), Rob has encouraged us to concentrate on the paper itself rather than on the hype and nonsense appearing in sections of the media.  That's fair enough, although as I have pointed out some of the hype and misunderstandings must be down to ambiguities in the press release which flagged up the importance of the paper to an outside world which has never been particularly well informed.  So as a service to our faithful blogging community, not all of whom have access to the paper itself, I have escaped from the Christmas mayhem for a little while and -- while others snooze -- have had a good look through the article.  I have copied a few key extracts at the base of this post.  Some of the main points to come from the article are as follows:

1.   At the Pembrokeshire end, 39 samples were investigated from the Pont Saeson - Brynberian - Crosswell area.  Within this total, 12 were from Pont Saeson - Craig Rhosyfelin, collected in June 2011.  A further 20 samples were collected in 1978. A further 7 samples were collected in 2010 by BJ, MPP and others.  To be more specific, 19 out of the 39 samples were from Craig Rhosyfelin, and seven from Pont Saeson -- the area around SN 1158 3599 9 (near the bend in the river, about 200m SSE of the Rhosyfelin rocky spur ).

2.  At the Stonehenge end, samples (an unspecified number) were analysed from excavations in the Stonehenge Avenue, Trench 44 and 45, the Aubrey Holes, from around the Heelstone, and the Darvill - Wainwright 2008 Stonehenge excavations.  We don't know how many fragments from the Stonehenge layer were collected and how many were rejected.  Neither do we know what proportion of the fragments were sarsen, or flint, or dolerite, or spotted dolerite, or other rock types, or rhyolite.  We do not know how typical of the Stonehenge Layer the sampled areas actually are.  So there is a strong bias in the collection of the samples.  All we do know is that within the collection of RHYOLITE fragments from the sampled locations the link between those chips and flakes and the Craig Rhosyfelin outcrops seems to be strong.

3.   The samples taken from near Pont Saeson appear to match very closely a rhyolite sample taken from the Cursus Field (Ixer and Bevins 2010).

4.  The samples taken from Locality 8 near the tip of the Rhosyfelin rocky spur match very closely with two Stonehenge samples -- one fragment from near the Heelstone, and another from the Darvill-Wainwright 2008 excavation.

5.  The four standing Stonehenge rhyolite bluestones numbered  SH38, SH40, SH46 and SH48 do not match any of the rhyolites found at Pont Saeson or Rhosyfelin.  However, some rhyolite fragments at Stonehenge do match standing stone SH48.  Source unknown.  Work is ongoing on these other rhyolites.

6.  In the Pont Saeson - Craig Rhosyfelin area, the peculiar foliated rhyolites described in this paper seem to occur over an area of "a few hundred square metres."

7.  There is a suggestion that orthostats SH46 and SH48 might match with lithic-crystal tuff outcrops lower down the valley and about 300m NW of Craig Rhosyfelin.

8.  According to the authors, in excess of 99.9% of of the Stonehenge rhyolitic ‘debitage’ can be petrographically matched to the rhyolitic rocks found around Craig Rhosyfelin and Pont Saeson.  It is largely because of that statement that the media have become obsessed with the idea that the source of the bluestones has now been found.  However, as I have pointed out before on this blog, the >99.9% figure is meaningless, since we have no idea how many rhyolite  fragments have been examined, how they were distributed in the Stonehenge Layer,  how close together the collection points were, and what degree of selection was employed in the collection of samples during the respective digs.  Percentage figures like this should never be used without a full presentation of the numerical data from which they are supposedly derived.   In any case, we have no idea which rhyolites occur in the Stonehenge Layer in those large segments of Stonehenge that have not been excavated, and in the soils of the area around Stonehenge, and what their frequency may be.

9.  There seems to be an assumption that the stump of orthostat SH32e could be an exact match with the samples from Locality 8 in the Rhosyfelin geological sampling exercise.  However, we should bear in mind that no samples from that stump have been examined, and that the suggestion is based entirely upon an examination of the photographs in the Atkinson collection.  Clearly some real science is needed on this matter........

10.  I take issue with the authors over their suggestion that the narrowing down of rhyolite sources to a very small area, and in one case to within a few square metres,  "is perhaps suggestive of human agency" in the transport of the bluestones.    They argue that "a large number of different rock types from disparate areas" would suggest a glacial or "natural transport mechanism" -- but they imply that their research is not helpful to those who argue for glacial transport.  They also seem to think that the "transport problem" now needs to be solved by archaeologists -- inevitably through a search for Neolithic quarries at Craig Rhosyfelin.   I disagree with that too, as I have told Rob many times.  In my view the problem needs to be solved by reference to glaciology and geomorphology.

My reading of the evidence presented?  It looks as if this is a classic case of glacial entrainment, with rhyolitic and related rocks picked up by overriding ice from a stretch of the Afon Brynberian valley which is about 500m long and maybe 100m wide, from north to south.  That's an area of 50,000 sq m.  The provenancing of one or two of these rocks to a very small area on the ridge of Craig Rhosyfelin is an excellent and exciting geological development.  It also appears that the stones from this locality (which may or may not have been orthostats or pillars) have been largely destroyed and redistributed as flakes or fragments -- although it may be that stump 32e is all that is left of one of them.  In my view none of this work has any archaeological significance -- and it has no bearing on the bluestone transport debate.  I can understand why Rob has reiterated several times on this blog that the transport issue is one that he treats with sublime indifference -- and assume that the hints about human agency were inserted into the paper in deference to the fact that it was published by an archaeological journal!

====================

SOME EXTRACTS

The local geology:

In the Pont Saeson area the Fishguard Volcanic Group
comprises a strongly foliated to foliated and lensoidal
rhyolitic rock the like of which is not seen elsewhere in
the outcrop of the group across the 32 km of strike section
from Pen Caer in the west to Crymych in the east. These
very distinctive rhyolitic rocks can be traced for no more
than 150 m from the northeasternmost end of Craig Rhosy-
felin. The foliation strikes quite consistently at between
040-050°, dipping to the northwest at between 40-80°
(Figure 2). Whilst the fabric is macroscopically typically
planar in thin section it can be seen to be lensoidal where
there are included, typically flattened, ovoid lithic clasts
which are usually 2 – 3 cm in maximum length (Figure
3), although larger elongate clasts up to 5 cm can be
located. These clasts are microtonalite. The main rock
is commonly traversed by thin (mm scale) quartz veins
which are tightly folded, with their folds being axial
planar to the foliation.


Field sampling:

Twelve accurately located in situ samples from Pont
Saeson (and especially Craig Rhos-y-felin) were collected
in June 2011. The sampling locations are shown in Figure
1. .... Polished sections were made of each of the twelve samples. These samples
augmented the original twenty in situ samples collected
by Bevins in 1978, during a reconnaissance study of the
Ordovician rocks of the Pont Saeson area. Due to this
the 1978 samples were not collected with the degree of
accuracy of the 2011 samples. These older samples are
provenanced therefore more generally at two locations
with grid references centred on SN 1166 3615 and SN
1158 3599. Additional samples from Craig Rhos-y-felin
collected by Mike Parker Pearson (2010) and Brian John
(2010) were also studied in order to be certain that the
full range of lithologies had been sampled.

In total thirty-nine lithics were investigated including nineteen from
Craig Rhos-y-felin and seven from the rhyolitic outcrops
centred on SN 1158 3599. Following a thorough
macroscopical description, detailed ‘total petrography’
as defined by Ixer (1994) and Ixer et al (2004) was
undertaken on the polished thin sections using both
transmitted and reflected light.

at the Stonehenge end:

These were compared with polished thin sections
of macroscopically similar rocks (groups A-D) from
the excavations of the Stonehenge Avenue, Trench 44
and 45, Aubrey Holes and the April 2008 Stonehenge
excavations.


----------------

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.

NB this statement:

One sub-crop, however, namely Craig Rhos-y-felin
Locality 8 appears to be the unique origin of the highly
distinctive Jovian fabric (an extreme combination of a
foliation with a strongly associated lensoidal fabric) seen
within some of the Stonehenge rhyolitic ‘debitage’. The
outcrop is situated at the extreme northeastern end of
Craig Rhos-y-felin.

Two Stonehenge samples have been matched with Craig Rhosyfelin Locality 8

Stonehenge Heel Stone area 1979 SH 79+520
M/L2 31.5.79 Excavated by Pitts (1982).
Rhyolite with fabric. 37.0gms after sectioning.

Stonehenge Excavation STH08 Context 3
G/2 Find No. 738 Excavated by Darvill and
Wainwright 2008.
Rhyolite with good planar fabric. Weight 72gms A
classical example of Group C.

(The 3 samples were deemed to have a common origin.) 

Conclusions

Detailed collecting and identification of in situ rocks
from Pont Saeson and comparison with Stonehenge
material allows for a number of important conclusions.
The overwhelming majority of the Stonehenge rhyolitic
‘debitage’, namely that belonging to Groups A-C, can be
sourced from the Pont Saeson area and perhaps entirely
from Craig Rhos-y-felin, but from more than one site on
the crags.

The four standing Stonehenge orthostats SH38, SH40,
SH46 and SH48 offer no petrographical match for any
rhyolitic lithology at Pont Saeson.

In essence therefore Pont Saeson area may be the
dominant origin for the rhyolitic ‘debitage’ (Groups A-C)
but is eliminated as the source of any of the four standing
Stonehenge orthostats (SH38, SH40, SH46 and SH48).
It may be possible to go further. In 2011 Ixer and
Bevins stated that buried orthostat SH32e could also
originate from the Pont Saeson area and if that were so
“it would be the first non-preselite bluestone orthostat
to be precisely provenanced”. Examination of the Craig
Rhos-y-felin outcrops and comparison with photographs
of SH32e in excavation strengthens this belief and,
sometime when SH32e is re-examined and its foliation
described in detail, it should be possible to exactly match
the artefact to sub-crops at Craig Rhos-y-felin.

As importantly, the three macroscopically different
rocks (Groups A-C), originally assumed to have different
geographical provenances, are now shown to have a single
geographical origin. The number of discrete geographical
sources for the Stonehenge bluestones is of much
significance in the nature versus human transport debate,
with a large number of different rock types from disparate
areas supporting a natural transporting mechanism
whereas a restricted number of geographical origins is
perhaps suggestive of a human agency.

Ixer and Bevins (2010) declared that “The discovery of
the stilpnomelane lensoidal rhyolites is the first time that
a non-dolerite bluestone has been clearly provenanced, it
will not be the last”. It should be noted that the envisioned
provenance area was several hundreds of square metres.
This statement has proved to be correct and can now be
strengthened to state that ‘This is the first time that any
lithics from Stonehenge have been unequivocally assigned
to an area of a few square metres, namely to within a very
small single outcrop or couple of outcrops; it may not be
the last’.

It is now possible to attempt a focussed archaeological
examination of these restricted areas with some scientific
justification. These areas should include Locality 3 but
especially Locality 8 at the termination of Craig Rhosy-
felin.