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Tuesday, 7 February 2017

Rhosyfelin entrainment area



 Oblique air photos from Toby Driver (RCAHMW), from the Coflein web site.  I have added approximate ice directions on each photo, and on the lower one we see the approximate outline of outcropping foliated rhyolite. Click to enlarge.

One might have some gripes with the updating of records on the Coflein (RCHAMW) web site, but we have cause to be thankful for the amazing collection of oblique air photos by Toby Driver  -- which keeps on growing!  These two excellent photos have been added to the Coflein Rhosyfelin record, giving us a new perspective on the likely rhyolite entrainment area at the time of the Anglian glaciation.

As we can see from the lower photo in particular, there is a relatively large area of outcropping foliated rhyolite here.  It is quite possible that overriding ice during the Anglian glaciation could have picked up or entrained slabs, pillars or rhyolitic debris from within and indeed beyond the area enclosed by the white line.  As we have said many times before on this blog, the geologists have not demonstrated beyond reasonable doubt that some of the material in the Stonehenge debitage came from within a few square metres of the tip of the Rhosyfelin spur, and we can assume that prior to the Anglian glaciation (ie three glaciations ago) the upstanding rhyolite crags here must have been much more prominent than they are today.  Where has all the material gone?  Well, much of it is littered across the landscape downglacier from here, and some of it appears to be at Stonehenge.......

Postscript:

Since I have been accused (rather aggressively) of getting this whole issue all screwed up, of failing to do proper fieldwork, and totally misunderstanding the local geology, I went back to source.  I found that, according to Ixer and Bevins, the foliated rhyolite rocks are indeed exposed in the Pont Saeson - Rhosyfelin area in pretty well the exact area shown on the lower photo, ie within a distance of c 150m from the tip of the spur.  I am unaware of any published evidence that contradicts this.

Bevins et al, 2012
Journal of Archaeological Science 39 (2012) 1005e1019
Provenancing the rhyolitic and dacitic components of the Stonehenge landscape bluestone lithology: new petrographical and geochemical evidence
Richard E. Bevins, Rob A. Ixer, Peter C. Webb, John S. Watson
Department of Geology, National Museum of Wales, Cathays Park, Cardiff, Wales CF10 3NP, UK

Quote:
We conclude that currently the only dacitic or rhyolitic lithology which can be matched with any degree of confidence between the Stonehenge landscape and a specific source area is the so-called ‘rhyolite with fabric’ lithology, which matches with foliated rhyolitic rocks exposed in the Pont Saeson area of north Pembrokeshire, in particular including those from Craig Rhos-y-felin. This is now correlated with some confidence in terms of petrography, whole-rock geochemistry and mineral (zircon) chemistry.

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Rob Ixer and Richard Bevins. 2011. Craig Rhos-y-felin, Pont Saeson is the dominant source of the Stonehenge rhyolitic ‘debitage’. Archaeology in Wales, 50, 21-31.

Quotes:
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 Rhos- y-felin.
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.
Although the degree of foliation is strong to very strong, the associated lensoidal fabric varies from weak but pervasive to very strong.......

3 comments:

Myris of Alexandria said...

No that is NOT the approximate outcrop of the foliated rhyolites almost ALL of the area outlined/invented by you is rhyolitic tuffs plus some microtonalite.
The foliated rhyolite is restricted to the 70m of the quarry face and the other side of the small Craig Rhosyfelin outcrop. If you go to the original I and B paper you will see that your outcrop includes lithologies that are not foliated rhyolite.
Lookat the photos you can see the Craig Rhosyfelin is a spur that is different from the rest of the area.

Do TRY to stick to the data and not use your own alternative data. Why not draw a line about Dyfed and use that.

Better still why not go into the field and map the area and then produce an accurate outcrop map.

The ice would have to dip into the quarry face to take the rhyolites.

This is another of your attempts (like the "hundreds" of different stones at Stonehenge)to defend the indefensible in the latter case the unique glacial dumping of approximately 80 worked orthostats.

Bloody do some proper field work.
M

BRIAN JOHN said...

Myris -- it's all very well you getting furious, but hold your hat! I'm sure some of your papers show a much wider extent for the foliated rhyolite. You even sampled it further up the valley, and found a fabric remarkably similar to that at site 8. If I am wrong, I will correct the text. Instead of telling us your version of the truth, let's see the colour of the evidence. Show, don't tell! Where is your field map? Has it been published? If so, where? And which original paper are you talking about?

"Hundreds" of different stones at Stonehenge? Where on earth does that come from? Not from me. I still think there might be as many as 30 different rock types represented in the bluestone assemblage of blocks, slabs, stumps, chips and packing stones -- if you just want to count monoliths, I think it's illogical, but that's your privilege.

BRIAN JOHN said...

Myris, by the way, I suspect I have done rather more fieldwork in the Rhosyfelin area than either you or Richard....... and I consider mine to have been entirely proper.