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Monday, 7 September 2015

Rhosyfelin - some geological questions

Jovian texture shown in thin section from a sample from site 8 near the tip of the Rhosyfelin spur (photo: Ixer and Bevins)


 Prof Mike Parker Pearson pointing to the exact site from which a rhyolite orthostat was taken away and transported to Stonehenge (photo: The Duke).  According to the photographer, "Even evidence of cutouts for wedges to be driven in to split the stone away from the face are visible. Awesome site."  So there we are then.

 In recent weeks I have visited Rhosyfelin in the company of more than a dozen glacial geomorphologists and have had some rather interesting discussions.  Most of them have raised one question which is worth sharing:  "What is the actual evidence which enables the geologists to claim that they have provenanced much of the rhyolite debitage found at Stonehenge to the very place where Prof MPP is standing in the above photo?"  That question relates in particular to the fact that there is not a particularly dense cover of sampling points across the landscape, and that great tracts of the Stonehenge landscape debitage haven't been sampled either.   Some of the visitors have read the literature, and they are intrigued by the sheer bravado of statements like these: 

"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."

"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."

CRAIG RHOS-Y-FELIN, PONT SAESON IS THE DOMINANT SOURCE OF THE STONEHENGE RHYOLITIC ‘DEBITAGE’
Rob Ixer and Richard Bevins, Archaeology in Wales 50, pp 21-31

"The ‘rhyolite with fabric’ (almost all is Craig Rhos-y-felin rhyolitic tuff) and the ‘volcanics with sub-planar texture’ debitage are universally distributed within the Stonehenge Landscape." (from "Chips off the old block" by Ixer and Bevins)

".......... matching the distinctive ‘rhyolite with fabric’ debitage (first seen in Stone’s stones) from Stonehenge to very detailed sampling along the Welsh outcrop showed that rocks from the extreme north- east of Craig Rhos-y-felin (‘site 9’) were identical to Stonehenge rhyolites showing the 'Jovian’ texture. This suggests an almost impossible provenance of ten squares metres. The archaeologists were told where to dig. In September 2011, Professor Mike Parker-
Pearson of Sheffield University and his team cleared the vegetation from the northern end
of Craig Rhos-y-felin and excavated. They found, just a few metres from site 9, a large
proto-orthostat, a large joint block set for its journey to Salisbury Plain......."
(As pointed out before, that should be "site 8".....)


Rob Ixer, Digging into Stonehenge’s past.  Mineral Planning, issue 143 / October 2012, p 13

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As I have indicated before on this blog, I too have some doubts about the  confidence with which the geologists can cite the Rhosyfelin crag as "the dominant source" for rhyolite fragments found in the Stonehenge area.  So some more questions:

1.  Of the 1200 or so rhyolite fragments found in the Stonehenge landscape and looked at by the geologists, how many have been used for thin section analysis?

2.  What assurance do we have that the thin sections published are actually typical of the whole assemblage?

3.  How much of the Stonehenge landscape has been excavated (5% at most?), and how much of the Stonehenge monument itself has been excavated (50% at most?).

4.  How many rhyolite sites in the Pont Saeson -- Rhosyfelin -- Brynberian area have now been sampled, and how many of those have been used for the preparation of thin sections?

5.  Are there not other localities in the Pont Saeson area which have provided thin sections with "Jovian fabric" which are virtually identical with the one from locality 8 on the crag itself?

6.  Is there any evidence that suggests that all or most of the rhyolite debitage at Stonehenge comes from one or two destroyed orthostats?

7.  Have you considered the possibility that the "rhyolite with fabric" fragments found in some parts of the Stonehenge landscape (I use those words advisedly) have come from parts of the crags in the Rhosyfelin - Pont Saeson area that no longer exist, having been entrained by overriding glacier ice?

8.  How much variation is there in the fabric of the rhyolites across the 50 m or so of the exposed rock face?

9.  How much variation is there in the fabric as you go deeper into the cliff face and examine some of the the foliated rhyolites exposed side-on where cliff face sections have sheared away? 

10.  Have you sampled the "proto-orthostat", and can you identify exactly where on the crag it has come from?

None of this detracts from the very smart provenancing work that has been done.  It's just that several of us share concerns about some of the statements made and conclusions drawn.........  Apologies if some of these questions are already answered in some of the papers published!  Can't keep track of everything -- but repeat answers will be appreciated.

To find further discussions on this issue,  just put "Jovian fabric" into the search box.



18 comments:

Myris of Alexandria said...

Some good and valid questions, quite a few are in the literature.Read the introductions to the most recent pet rock boys papers.
Of course there might be another rhyolite outcrop yet unrecognised,if and when that is found there will be revision.
I must assume you are not impuning the integrity or skill of the boys but the over enthusiasm in interpreting their data.
Number of sections are in the literature, Cryf is probably one of the most sectioned outcrops in Wales but yes there are areas not covered BUT the data are on this blog, sketch the orientation of the quarry face, the foliation plane and say what do we happens along strike.
May I suggest you write these important questions in a letter somewhere so requiring an answer allowing the pet rock boys the chance of putting these supplementary data in press.
Citing this blog is too ephemeral.
Some good questions, some repeats, some how long is a piece of string.
Think of Cryf as being south Wales answer to the Marquis of Anglesey outcrop.
M
And I have not even had my croissant yet.

BRIAN JOHN said...

Thanks Myris -- far be it from me to question the integrity or skill of the good doctors Ixer and Bevins! I have far too much respect for that.....

I agree that a letter to the journal would be better than raising issues on this blog. Might just do that........ then as you say, extra info can go into the public domain, in print as it were.

All that having been said, can you please give us a few answers to the questions?

Myris of Alexandria said...

The other three groups can be matched to the Pont Saeson area both macroscopically and microscopically.

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. They were sourced at the accuracy of ten figure grid references by using a software application (http://wtp2.appspot.com/wheresthepath.htm) which combines a satellite image in a split screen with an Ordnance Survey map, along with a grid reference for the location of the screen cursor to ten figures, allowing extremely accurate sample site locations to be determined. 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.

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.

XXXXXXXXXXXX

Since then, the quarry face has been systematically sampled and sectioned don't know the number >>dozen plus into the quarry face ie perpendicular to the strike of the foliation dozen.

So probably between 40 -60 sections. Note cost of preparation alone is approx. £2000- £2500 many of the original ones paid for by Dr Ixer from his consulting monies. NO public grant but his generosity and curiosity.

Note recommended sampling for this sort of area would be 15 samples and that would be considered rather good!!

They are remarkably similar most are semi/sub jovian but locally Jovian.

CRyf debitage would take me a couple of hours to list I would have to go to my original paper copies and would be a waste of my time. It will be given in the Darvill and Wainwright monograph. Probably 20/25. But new ones from new locations still being done.


You know how we breate Kostas for not reading the lit and asking the same questions ......well think obsidian.

M


BRIAN JOHN said...

Thanks Myris -- some questions answered, and some not. Yes, familiar with that quote from the article.

There you go again -- referring to a "quarry face." A strict referee would put a red line straight through that expression, until somebody has proved something by the citation of hard evidence. What were we saying about over-interpretation? The term "rock face" is a perfectly respectable one, which carries no baggage with it.

chris johnson said...

I am still puzzled exactly how unique the rhosyfelin rock crop is in the wider landscape. So what are the chances of the debitage coming from elsewhere in the hills. It seemed a year or so ago that little sampling had been done elsewhere.

Perhaps this is a stupid question for a non-geologist but that is what I am - not a geologist :)

Myris of Alexandria said...

Brian you have drunk from the fount but others have not so I offered a little dew drop.
I shall answer more questions a little at a time but some question that appear superficially cogent aren't. The correct answer to the question how many ts does it take to recognise that thousands of bits of debitage come from the Cryf quarry is enough, when every suspected Cryf sample turns out to be Cryf. It is the same as asking how many straie in a glaciated surface were minutely examined before being pronounced glacial straie, the answer is enough. By chance some of the debitage is highly distinctive for example SH48 knock off, about a dozen it is in the primary lit, were recognised from the thousands of bits. Everyone was sectioned and all but one were indeed SH48, the rogue bit was SH 38. All the debitage has been macroscopically examined twice and the difference between the results was slight. Mainly between very small bits of Lower Palaeozoic sandstone and Volcanics A. Neither has been correctly described in the lit yet.
Another question answered.
But you will ask this again, it is the Kostas-curse and I am damned to be van Helsing, would far prefer Vlad himself Ah the children of the night.What beautiful music they make. Second best lines in cinema after Bestest Rutti of course I have seen....tears in the rain.
Genius pure pure pure genius.
M

Myris of Alexandria said...

Dr Richard Bevins's Ph.D thesis 1970/80s was to look at every outcrop in the Fishguard Volcanics, including CRYf, by chance he stopped sectioning all his outcrops JUST before CRyf it was the next to be done(Ph.D time constraints) and was not done until the pet rock boys joined up read the appended conclusions to their first joint Stonehenge related paper. Geologically nothing exciting was expected from another rhyolitic outcrop.
So CRyf could have been known as the quarry source 30+ years ago (just think how the 1991 OU paper would have been different if they had known of a quarry site) but for chance and getting a Ph.D within 3 years.
The key was the presence of stilpnomelane in the debitage, without Richard recognising that its Welsh source was still missing none of this joy would have been ours even Kostas would be stripped of his "rounded glacial pebbles".
Kostas to write that you do not read the technical literature (I guess facts do get in the way)is so so foolish, firstly not to read it and secondly to say so in print.
Now I must cease to be your champion and ask that you retire from the lists!
M

chris johnson said...

When I understand correctly there IS now a solid link between cRyf and an orthostat at Stonehenge?

BRIAN JOHN said...

Not as far as I know -- all we have is the matching up of samples from Rhosyfelin and the surrounding countryside and the rhyolite debitage in those parts of the Stonehenge environs from which samples have been collected. Same situation as before.

Myris of Alexandria said...

That is my understanding too
M

chris johnson said...

The more I look at the second picture with Mike in the foreground, the more I am reminded of pareidolia. It seems there are several images of birds, animals, faces looking out from the rocks.

I wonder if anybody else is seeing this, or is just me again?

BRIAN JOHN said...

Beware -- we know that Stonehenge makes all men mad, but maybe Rhosyfelin does the same?

Myris of Alexandria said...

Google Gary Denke, ark of the covenant, heelstone.
M

TonyH said...

Then there's Physics Doctor and Tornado Meteorologist Terence Meaden and the images he's photographed at Avebury and Stonehenge. Some are even the subject of one of his books....Also, Black Sabbath lover Dennis Price of the Eternal Idol blogsite, who tells us what image his daughter perceived....a hound, wasn't it? It weren't nuthin but a Hound Dog, apparently, may have been a prehistoric Guardian of the Aurochsen? You never know, release you mind and put on a Donovan record, happy trippin'.But I say, bring on the Formation of Geomorphologists Over Rhosyfelin, they'll sort it ALL out!!

Myris of Alexandria said...

And the row of Blick Mead pink flint ducklings, they must be in the firing line.
Stonehenge sane ones, the raised finger of one hand.
M

chris johnson said...

Finally googled Gary Denke and got the message. I saw a very convincing photo of the heelstone on Dennis Price's site a while back - when it was still working properly. Some kind of animal head is very clearly visible.

I sent Brian a rather convincing photo I took myself a while back at Carn Menyn but rightly sceptical that he is he has not published it (yet). Then again there is the photo of the beaver/afanc taken of the outcrop above Pentre Ifan.

The Ark of the Covenant is a bit far fetched, although there ARE all those stories about the lost tribes of Israel turning up in Britain, and where better to bury something very important.

TonyH said...

Talking of images as we have been, I cannot help seeing a familiarity between this Post's coloured photographic image of Prof MPP and another larger - than - life academic - turned - celebrity, Professor of Botany etc, David Bellamy.

The stance of Prof MPP as well as his facial expression is very reminiscent of that other former television personality.

Somewhere I have got one of "Doctor Brian's" guides to Pembrokeshire dating from the 1980s, which has an introduction by his old Durham colleague, David.

TonyH said...

Hold your hand up, Myris, how many fingers can you count? (only Joshing).

...........talking of Josh Pollard, HAS he got a Licence to Drive that famous mini - digger up on Rhosyfelin for another "steaming gun" (or was it smoking?) moment as the good Prof Parker Pearson infamously, possibly impetuously, and, probably, soon to be regrettably - in - retrospect uttered back in 2012.

Famous for Fifteen Minutes? I reckon MPP has milked it all for as long as he possibly could. Cometh next the Formation of Glacial Geomorphologists, Mike.