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Monday, 7 November 2016

Outcropping layers and foliations

I came across these wonderful images of landscapes created by some crazy sculptor who likes to carve landscapes out of collections of old bound volumes.   I suppose the motive is to demonstrate that all the best novels have a strong sense of place.......

Putting frivolity to one side, they do illustrate exactly the point which I and my geomorphology colleagues have been making about outcropping foliations and layers in near-vertical strata appearing across large tracts of countryside.

In their big paper on Rhosyfelin, Ixer and Bevins say this about "locality 8" -- before going on to say that they had provenanced most of the Stonehenge foliated rhyolite debitage to "within a few square metres":

The majority of the lithic comprises alternating fine-grained quartz and slightly coarser grained chlorite-bearing layers. In thin section the rock is banded with a pronounced planar foliation and a lensoidal fabric giving the ‘Jovian’ fabric. Very fine-grained quartz layers that look streaky alternate with slightly coarser grained quartz/feldspar-chlorite-rich layers. The fine-grained quartz carries rare, small quartz/feldspar-chlorite lenses......... 

OK -- that's all very fine.  Now, let's imagine that locality 8 is exposed on one of the dark-coloured hard covers of one of the books shown on the right hand side of the top image.  Can the geologists give us any information that might serve to disprove the thesis that anything found at locality 8 might well also outcrop at multiple locations cross-country, just as you can trace the individual book jackets across country in the lower image?

If they try to tell us that the fabric or "signature" revealed in thin section at locality 8 is completely unique to that locality, I will not believe them, since they do not have a dense enough grid of sampling points to support that contention.


Myris of Alexandria said...

One of the problem is that without the correct nomenclature it is difficult to see what you ask
Taking the bottom diagram (The sculptor would only need a couple of the Tolstoy SH volumes to recreate the sculpture -wonder as it is.).The pet rock boys sampled along the sequence and only found the jovian texture at site 8 the rhs cover in the lower diagram. Take a look at Cryf before any of the veg was removed and there was no hint of the quarry face (the rhs cover in the diagram). So the rather remarkable feat of petrography was correct in saying that they had provenanced the jovian debitage to within a few metres. Now this is where the nomenclature is helpful. The jovian texture will occur, of course, everywhere along its strike(by definition). What is the strike of the fabric at site 8 - the 'plane'(strikes are imaginary lines on a geological map) that is the rhs cover aka the Cryf quarry face.
Imagine the sculture covered in cloth so only the front surface is visible (the original work of the pet rock boys) and then the cloth is removed and the planar face lying along the strike of the jovian texture is revealed (the MPP excavations)
The jovian textured debitage has come from somewhere along this "strike-plane".

MPP and team have on archeological grounds determine from whence the orthostat was removed within that "strike-plane".

This could all be done in three sentences using the correct nomenclature.

So the realisation that the debitage was provenanced to within metres stands proud and true. Within the whole of the Fishguard Volcanic Sequence the debitage has been provenance to a few metres. Quite breath-taking really.

No man and no blog is an island, there are lots of blogs within the over Arching! (See)Brian blog many of we smaller fleas are there to bite him. Or is this to poetically obscure.

Like other media don't like, don't indulge.

BRIAN JOHN said...

Sorry Myris. Don't accept any of that. Forget about imaginary quarry faces and imaginary orthostats, and forget that archaeologists have ever been near the place. Think geology and geomorphology. You have done nothing here to demonstrate that you have provenanced anything to within a few square metres. In fact, in admitting that the jovian texture will occur anywhere along the strike of the sampled layer, and approximately along the rock face (as I have pointed out over and again) you are admitting that roughly the same texture or signature could occur for many km across country, along the strike, and that the similar (NOT identical) debitage fragments at Stonehenge could have come from hundreds or even thousands of potential source outcrops.

Breath-taking?! Breathtaking chutzpah, not breathtaking science..........

Steve Potter (of Tegryn, Pembrokeshire) said...

Oh dear. Whatever anyone may think about Brian's position on the glacial transport hypothesis and his sometimes-trenchant presentation of it, it has to be acknowledged that that presentation is in beautifully lucid English, employing such old-fashioned delights as punctuation and an understanding of grammar.

Compare any of Brian's contributions to this blog with any from Mavis of Ixerlandia. This one will do as a recent example:

"No man and no blog is an island, there are lots of blogs within the over Arching! (See)Brian blog many of we smaller fleas are there to bite him. Or is this to poetically obscure.

Like other media don't like, don't indulge."

Mavis, old love: in my experience (and I'm old) sloppy writing is a sure indicator of sloppy thinking. If you think you have something to contribute to this blog, back off, calm down and write in English. Then your contribution might be of some value. And note that Brian contributes under his own name. Couldn't we all do that, if we have some belief in the value of our contributions?

Steve Potter (of Tegryn, Pembrokeshire)

TonyH said...

Blimey, Steve, if that is your real name why haven't you contributed before, are you a friend of Ian Hislop of Private Eye and do you have a good lawyer?

BRIAN JOHN said...

Oh, it's not THAT dangerous being a participant in our amiable discussions. The seriously dangerous ones have long since been cast into outer darkness. Myris is a cuddly, friendly fellow really, doing his best in difficult circumstances to defend the indefensible........ without much help from anybody else.

Myris,, of,, Alexandria!! said...

Oh Brian did you learn so little field craft at Durham? The fabric is present metre tens of metres perhaps hundreds of metres below ground as well as your "kilometres along the countryside"
You forgot a few things namely the size of rhyolitic outcrops -these are not ignimbrites after all- you must remember why calc-alkaline volcanoes are explosive.
Second even looking at the google map you so favour you can see that CRyf is fault bounded.
Every serious outcrop in the areas has been examined -it was Dr Bevins' PhD after all to look at the FVS. No other outcrop has shown the texture otherwise the pet rock boys would have noted that CRyf was one of a number etc. They did not because there is not.

Unless there is outcrop it matter not one jot the full distribution of the jovian texture.
No within the FVS the jovian texture and by extension the debitage and its parent SH orthostat have been uniquely identified.

Some geological realism please we allow (but do not forgive) the fanciful Preseli stones on Salisbury Plain but mighty Jove's hand throughout Wales is an image too far.

You can imagine how distraught I am that some obscure but named and shamed OAP is worried about the split infinitive in "to poetically obscure". To imagine that it was a typo is just too (sic) dull and pedestrian. Here are some ,,,,,,,,, for you please distribute as the whim takes you.

Did give me a smile and reminded me of messages written prior to Richard the second's death.

Oh thank you Brian for your kind remarks.


BRIAN JOHN said...

Of course the Rhosyfelin rhyolites are fault-bounded. There are faults everywhere, as is very clear from the geological maps. Where the bounding faults are in this case is a matter of opinion, since the sedimentary overlay makes it very difficult to find them. Too much till and slope materials! Just as the fabric goes down into the depths, it was also once upon a time going up into the skies, until erosive processes brought the crag down to its present level. After all the patient geomorphological informing I have been doing on this blog, do you really think that ice only entrains erratics from exposures that are currently visible? Take it from me that ice picks up materials from thousands of entrainment sites that are later buried beneath till and fluvioglacial materials. Richard no doubt did fine work in examining all the outcrops he could find -- but he sure as eggs did not examine all the bedrock outcrops that are now buried beneath regolith.

In any case, as I have said before, you appear to have nothing in the Stonehenge thin section collection that exactly matches the thin section from location 8. Similar, yes. Identical, no.

TonyH said...

Are you the 'Mine Host' at the Butcher's Arms, Tegryn, Steve?

BRIAN JOHN said...

Myris, take another look at this post.
I am trying to be really helpful here! I do not see anything, in any of the slides you have published, to say that the sample from Rhosyfelin Point 8 matches anything from Stonehenge.

What I might find more convincing is an argument that sample OU 18 from the Stonehenge Avenue matches pretty well sample PS10 from Pont Saeson.

Looking at the whole mass of your published data, and all the slides showing a Jovian fabric with lensoidal structures in it, all you can say is that some of the foliated rhyolites at Stonehenge have probably come from the Rhosyfelin - Pont Saeson area. That's enough to be going on with.

BRIAN JOHN said...

Thanks for your post, Steve. Until now, one of our many "hidden"readers! I try to hold standards up, while appreciating that some contributors bash things off on their mobile phones at great speed, with arthritic fingers and too much going on in the background.......

Steve Potter said...

In response to TonyH, heavens, no. The landlord of the Butcher's Arms is famously grumpy. I'm not.

The Butcher's Arms Landlord said...

Potter, you're now banned from my pub!!!

BRIAN JOHN said...

Aw, go on, Mr Potter. Give him a pint -- he was only joking!