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Tuesday, 22 April 2014

On the significance of Rhosyfelin Locality 8

 The Craig Rhosyfelin area, showing the grid of sampled points referred to by Ixer and Bevins

Having been out to look at Rhosyfelin again yesterday, it's not a bad idea to look again at this statement:

Quote from Ixer and Bevins 2011
"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......"
Article: "CRAIG RHOS-Y-FELIN, PONT SAESON IS THE DOMINANT SOURCE OF THE STONEHENGE RHYOLITIC ‘DEBITAGE’
Rob Ixer and Richard Bevins, Archaeology in Wales 50, 2011, pp 21-31
http://www.academia.edu/3990426/Craig_Rhos-y-felin_is_the_dominant_source_of_the

This implies that Locality 8 on the air photo is actually the source of the "rhyolite with fabric" found in the Stonehenge debitage.  It also explains why Mike Parker Pearson and his colleagues have become so obsessed with the idea of the "Rhosyfelin Quarry" -- in the conviction that the geologists have given them the "all clear."

However, the geology is not that simple, and I have previously questioned the reliability of  that statement made by Rob Ixer and Richard Bevins.
http://brian-mountainman.blogspot.co.uk/2011/12/on-significance-or-otherwise-of-999.html
I have argued that without a rather dense sampling grid, it would be a mistake to argue that the "foliated rhyolite with Jovian fabric" matched up with Stonehenge samples is restricted to the one site (locality 8) at the NE tip of the Rhosyfelin rocky ridge.  The geologists don't know where else this particular rock type might outcrop -- or if they do know they haven't given the evidence to support the contention.

 Strong planar foliation shown on the side of a detached block in the Rhosyfelin "litter" beneath the rock face.  Some fractures run parallel with this foliation, and others cut across it

One problem I have with the evidence as presented is that we do not see (in the Archaeology in Wales paper) the thin section slides for the other localities, so we have nothing to compare with locality 8 or with the samples from Stonehenge.  What we do have are these 3 illustrations:

 Jovian fabric -- locality 8

 Jovian fabric -- Stonehenge debitage

 Jovian fabric -- Stonehenge debitage near Heelstone

 Bear in mind that one of these illustrations is at a slighly different level of magnification than the other two.  In the Heelstone sample, the black blobs look like little fishes or tadpoles -- and they do not look like this in the other slides.  In the other Stonehenge sample there are fewer black blobs and they are less elongated, and the other components in the slide look almost liquid rather than crystalline.  The locality 8 sample is different again, with a greater density of black blobs and with much more irregular shapes.  To my untrained eye, these three fabrics do not look identical, but they are similar, suggesting that they are part of a continuum, and in my view it is quite possible that the Stonehenge samples have NOT come from the few square metres around locality number 8.

I would be more convinced if we had at our disposal the thin section slides showing us the fabric at all the other sampling points identified on the air photo.  How different, or how similar, are they to the three slides shown above?  At one point I recall Rob referring to locality 9 as the possible source of some of the Stonehenge debitage.......... was that a mistake, or was that at one time a working hypothesis, later changed when the slide from locality 8 had been examined?

So if the area of foliated rhyolite with Jovian fabric is more extensive, where might other outcrops be located?  Here is a photo of the rock face:

The Rhosyfelin rock face, after clearing by the archaeology team.  According to the grid of geological sampling points, no samples have been taken from anywhere along this face.

Rob tells us that the crack featured in my last post is 90 degrees away from the direction of the foliations.  We have to work in three dimensions here -- and this means that either the foliated band  sampled at point 8 runs as a very thin band all the way along this face and might outcrop in other localities in the Rhosyfelin - Pont Saeson area, or else it runs more or less parallel with the rock face, either along the cliffline itself, or some distance outside it or some distance within it.  Such a "sheet" (we cannot call it a bed) could run deep into the ground and could have been exposed much higher up than the current crest of the ridge.  In other words, bits and pieces of it could have been carted away from this general area by overriding ice at a time when the landscape had a rather different appearance from the Rhosyfelin of today.

We need more geological info here, chaps........ is it in the publication pipeline?

POSTSCRIPT

I have just found this thin section slide from one of the other geology papers by Rob and Richard.  It's from Point 10, quite a way to the south of the rock ridge where the archaeologists have been working and over 200m from Point 8.  Is this also a part of the same "rhyolite with fabric" continuum?  There are strong similarities with the slides shown above, except for the absence of the prominent black blobs.





 

8 comments:

TonyH said...

By Jove! Myris must be off on his hols again, tripping his way up the Blue Nile perhaps, judging from the lack of any reaction, chemical or otherwise, to this Post. We wait with bated breath....

BRIAN JOHN said...

All comments gratefully received -- we are all in search of the truth here, and everything I say is constructive, designed to encourage the writers of learned papers to SHOW us why they have reached their conclusions. Same principle applies in all branches of science... "Show me, don't tell me, and then we'll see whether we agree...." as my old college tutor Ernest Paget used to say.

Dave Maynard said...

Out of interest, how much size reduction could be expected on a rock transported by ice from west Wales to Salisbury plain?

I imagine it is impossible to say. Some might be delivered intact, while others are ground to dust.

Also if a rock is transported, I assume some might have no signs of glaciation, while others are badly mauled with clear indications.

Has anyone looked through the Stonehenge debris for this type of classification?

Dave

BRIAN JOHN said...

Good questions, Dave. There is a lot on this blog already about erratic shapes etc. Do a search! Here is one entry:
http://brian-mountainman.blogspot.co.uk/2011/10/on-elongated-erratics.html
The trouble is that when an erratic is discovered, you have no idea what it looked like when it was picked up or entrained, or what size it originally was. The degree of attrition or rounding or breakage depends on where a rock is transported -- on or in the glacier. If it is carried on the surface for scores or hundred of miles, it can be largely unmodified, as seems to have been the case with the Big Rocks at Okotoks, carried in the Foothills erratic train.

I'm not aware that anybody has done a proper shape analysis of the bluestones at Stonehenge -- or an analysis of surface features that might have glacial origins. Seriously needed......but the stones are highly variable in shape, size and surface morphology, as I have often pointed out.

Constantinos Ragazas said...

Brian,

The Rhosyfelin rhyolite fragments found at Stonehenge with no sensible explanation how these got to be at Stonehenge will need to be included in such study as Dave asks.

Kostas

Dave Maynard said...

I was thinking that perhaps the Stonehenge debris layer might be the right size to come from the alcove. Perhaps this line of thought is moving away from your main thesis?

Yes I'll have to do the searches and read up more on the debris.

Dave

BRIAN JOHN said...

The famous alcove is entirely irrelevant, as Myris has pointed out. There wasn't even a geological sample from there -- so it's a bit of a mystery why there should be such focus on it. There are, after all, plenty of other cracks too, along the rock face! But it is realistic to wonder whether a bucket or two of small fragments from Rhosyfelin might have been taken to Stonehenge for purposes that we are at a loss to understand.

BRIAN JOHN said...

Kostas -- I agree that the debitage at Stonehenge needs to be given due consideration. I have always said that ALL of the foreign material at Stonehenge needs due attention, because it all got there somehow, and from where I stand it all contributes to a mottley collection of bits and pieces making up an erratic assemblage and supporting the glacial transport hypothesis.

When it comes to counting up rock types, Myris and I part company. He wants to concentrate on the orthostats, while I want to include orthostats, stumps, hammerstones, fragments and anything else lying around.