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Thursday, 1 December 2016

Rhosyfelin - More glacial misunderstandings

There is another post on Mike Pitts's blog, relating to the latest paper on the dating of the Fishguard Volcanic Group rocks.  As we have said before, this paper is not really of any relevance to the Rhosyfelin quarrying debate,  since it simply confirms that the FVG rocks are indeed in the right place in the Ordovician stratigraphic column.  Pure geology.

Mike accepts that the new paper is "not dramatic",  but then he says: 

"But it confirms the direction of current work which suggests that many of the Welsh bluestones came from north of the Preseli hills, rather than the top or to the south as traditionally believed (HH Thomas identified Carn Alw as a source for these particular stones, see map). The significance of this, as argued earlier by Richard Bevins and Rob Ixer, is that while Carn Alw might allow for ice transport of stones at least part of the way to Wiltshire, the nearer the sources move downslope towards the coast, the less supportable that becomes. And Mike Parker Pearson may have another quarry to look for."

Where on earth does this strange and convoluted line of thinking come from?  Whoever said that a Carn Alw provenance for stones at Stonehenge might suggest glacial transport, but that provenances from the northern slopes of Preseli do not?  That is complete nonsense, as I have tried to explain many times on this blog.  Precisely the reverse is true.  Because ice crossing Preseli must have been compressing as it crossed the rising land between the north coast and the ridge of Mynydd Preseli, conditions for glacial entrainment would have been significantly enhanced.  Why do people keep on expressing these views when it is clear that they know nothing whatsoever about glaciology or glacier behaviour?

MPP also demonstrated his lack of understanding the other day by stating (so we are reliably informed) that glaciers never flow uphill......... that's almost as bad as his statement of a few years ago that there is no evidence of ice ever having travelled eastwards in the UK........ oh dear oh dear.......

Here, Mike Pitts gets more sensible, in referring to the newly dated rock samples:  "The other two, from the coast, are a little older, at around 464 and 465 million years old. Interestingly, the archaeological sample (which can be linked to stones 38, 40 and 46 at Stonehenge as well as 48) is also around 464 million years old. Not from Craig Rhos-y-felin, then, but, say the scientists, probably from Fishguard outcrops exposed across the low ground north of Mynydd Preseli. “This region”, they conclude, “provides an obvious target to search for further Neolithic quarry sites.”

Forget about the geologists' obsession with quarry sites -- if we want to put any interpretation on this work, it is to say that fragments from the FVG found at Stonehenge have come from several different localities in North Pembrokeshire -- and that that significantly increases the likelihood that they have been entrained and transported by glacier ice.  Nice and simple.


TonyH said...

Are you going to communicate with Mike Pitts and put him straight?

BRIAN JOHN said...

Yes! Done......

MoA said...

Apart from the geological importance of having now established an independent and absolute (non-fossil based) age for the FVS, the remarkable age co-incidence between the Stonehenge debitage sample (a semi-Jovian textured sample) and the in situ sample taken from the quarry face next to site 8 and close to the proposed orthostat extraction site is of archaeological importance.
The dating technique is so precise that other locality along the Craig Rhosyfelin ridge (for example the initial sampling site of Ixer and Bevins) that does not lie along the quarry face (they all lie along the same strike line) would/should give a slightly but measurably different age to the quarry face. In particular site 10 that Brian has proposed as being a possible candidate for the Stonehenge debitage would give an age that is as different form the quarry face as is possible to get from any sample site at Craig Rhosyfelin. Indeed the very worst choice.
Without knowing and understanding the ‘dip’ and strike of the rhyolite sequence, as any first year u/g who is correctly aught will know, it is poor guess work.

BRIAN JOHN said...

You are getting in a frightful tangle here, Myris. Everybody who visits the site can see where the dip and the strike run -- you have described it all in your paper, and nobody is disagreeing. The provenancing of some of the rhyolitic debitage at Stonehenge to somewhere around Rhosyfelin is of no archaeological importance whatsoever, since there is no archaeological evidence of quarrying. I have never said that site 10 is on the same strike or is from the same foliation layer as site 8 -- it's probably 30m off, so it's related but not identical. All I'm saying is that a sample from site 10 looks very similar to a sample from Stonehenge. Are you saying to me as a geologist that those two samples (OU18 and PS10) are completely different and are therefore unrelated? Come on -- stop flannelling and tell us what the score is.

By the way, I have never claimed that site 10 is a candidate for "the Stonehenge debitage". What I am suggesting is that it is a candidate for SOME of the Stonehenge foliated rhyolite debitage. Would you disagree with that?

MoA said...

Yes I think I would. Site 10 is a very small 'outcrop' with no sign of any quarry face, extraction point etc. It is of course a member of the CRyf A-C Quarry rhyolites in the Ixer and Bevins classification but nothing more.
The debitage at Stonehenge is not random glacial scatter as much as you wish it but from debitage from worked orthostat(s)taken from the quarry face.
So site 10 is of no importance other than to show the extent of the Cryf rhyolites.
So not in the quarry face so no not a candidate for SH rhyolite debitage.

BRIAN JOHN said...

Who said you need a quarry face at sampling point 10? For the umpteenth time, glaciers can pick up stones from almost anywhere. No "face' is needed. Read a text-book or two, Myris, and try to forget what MPP tells you about quarries. He seems to have moved on himself. Time for you to move on too?

So you disagree with my suggestion that OU18 and PS10 could be related? On the basis of petrology and geochemistry? Really? And you a geologist?

Most of the debitage has come from the working of erratic boulders found at or near Stonehenge? No problem with that.