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Wednesday, 10 April 2013

Carn Alw and spherulites


One of the Stonehenge samples examined by Thomas -- note the spherulitic structure on the bottom part of the slide and the transition to a more flaky structure at the top.  An indication  of a sedimentary origin?

 A new sample from Stonehenge, from the 2008 Darvill / Wainwright dig.  Again there is a transition from a spherulitic structure to a flaky structure.  Does this transition absolutely rule out Carn Alw as a source locality?  Not on the evidence thus far presented......

I have been looking again at the new paper called "Carn Alw as a source of the rhyolitic component of the Stonehenge bluestones: a critical reappraisal of the petrographical account of H.H. Thomas" by Richard Bevins and Rob Ixer ----- and I have to say I am a bit less convinced than I was the other day.....

The authors tell us that the 4 rhyolite orthostats at Stonehenge (numbered 38, 40, 46 and 48) are ash-flow tuffs  with pumice, lithic and crystal fragments and in one case well-preserved glass shards. In contrast, the Carn Alw and Stonehenge samples described by HH Thomas and now re-investigated are "recrystallized, typically spherulitic rhyolitic lavas which might have originally been part of a thick lava flow or dome."  If the orthostats are classified correctly, they do not match any of the five slides examined in this new paper -- or indeed the slides originally examined by Thomas in his 1923 paper.  We have to take that on trust, because the authors do not reproduce photomicrographs from samples from any of the 4 Stonehenge orthostats -- so we do not know how different they actually are. 

So what we are talking about here is this terrible stuff called the "debitage".  Was Thomas at fault in saying that the two thin sections of rocks found at Stonehenge actually came from Carn Alw?  Richard Bevins and Rob Ixer say he was at fault, and that these two samples (Thomas's Figs 1 and 3) are different from those which they have examined from Carn Alw.  When we look closely, they do not seem to me to be all that different, but the reader does have a few problems in sorting this out, because both HHT and the new authors reproduce bits and pieces of their thin section slides, sometime this way up and other times that way up, sometimes under crossed polars and other times under plane-polarised light, and at different scales.  So real comparisons are notoriously difficult for Joe Public to make.  

We have to take it on trust that the Carn Alw rhyolites are actually very different from anything found at Stonehenge, but on looking through my references and my own field notes, I see that some of the Carn Alw rhyolites are spherulitic and others are not, and that there is great variation  in rock textures from one part of this large craggy outcrop to another.  We see from the paper that 6 samples from Carn Alw have been analysed, but we do not see the photomicrographs reproduced in the paper.   So here is a request to the authors.  Please can we see them? I promise to publish them on this site as an aid to discussion.

A lot of the discussion in the paper relates to the supposed origin of different spherulite structures.  The authors suggest that on Carn Alw the rhyolites are genuine volcanic rocks which have not been redeposited -- whereas other rhyolites may have been deposited in water, as shown by the transitions between spherulite structures and argillacious transitions to flaky or platy structures -- typical of fine clay particles settling out in waterlain sediments.

Quote:  "We consider that the spherules in sample E1993 (Figure 3 of Thomas and Figs 8a and 8b of this paper) and in sample SH08-390, collected during the 2008 Stonehenge excavations (see Fig 9) might be accretionary lapilli rather than spherules generated as a result of recrystallization of rhyolitic volcanic (sic).  Accretionary lapilli develop in volcanic ash clouds as a result of collision of liquid-coated particles with binding of the ash particles as a result of surface tension. The variable supply of ash particles of different sizes results in a concentric growth pattern in the lapilli. Accretionary lapilli falling into the sea will accumulate in the sedimentary sequence, forming an horizon related to the supply of the lapilli. The end of the supply of the lapilli will see a switch back to the accumulation of normal sediment which in the case of an environment offshore would typically be mud-dominated (ie argillitic). However, accretionary lapilli typically range in diameter between 3-4 mm but reaching up to 10 mm, considerably larger than the spherules examined here, although interestingly Lowman and Bloxam (1981) described air fall tuffs containing accretionary lapilli from the Fishguard Volcanic Group west of Ty Canol Wood, some 3 km WNW of Craig Rhos-y-felin." 

So the question is this:  have all rock samples showing "accretionary lapilli" come from unknown areas (like the area to the west of Tycanol Wood) or could some of them have come from Carn Alw?

Another question is this:  how unique are the banded rhyolites and rhyolites with spherules described by Thomas and reexamined in this paper?  The authors assign one of Thomas's slides to Rhosyfelin on the basis of similarities, but how secure is that assignation?  In other words, are there any other outcrops within or outside the mapped outcrops of the Fishguard Volcanic Group where similar banded rhyolites occur?  Or are they using Rhosyfelin as a probable source simply because they know it well -- in other words, could the assignation be down to sampling bias?

And again:  how certain are the authors that they know about all of the variations that occur within the crags of Carn Alw?

And again:  how certain are the authors that there are not other areas where rhyolites with spherules can be found?  Indeed, to their credit they suggest that there might be, and suggest that at least three source areas for the rhyolitic debitage at Stonehenge are still to be found.  That's in addition to the source areas that still have to be found for the four Stonehenge rhyolite orthostats.

There will be many more twists in this story before it is done....... and thank goodness that we have two very experienced geologists on the case.


21 comments:

Constantinos Ragazas said...

Brian,

I fail to see the relevance of all this to Stonehenge.

So what if some “debitage” fragments came from Carn Alw or from Rhosyfelin or elsewhere? What is relevant is: why some of the “debitage” stone fragments do not match any of the megaliths at Stonehenge.

This begs the questions: “why these stone fragments are found at Stonehenge and how they got there?”

Correct me if I am wrong, but nothing in this paper or any other papers answers this. Just more expert distractions from seeking truth.

Kostas

GCU.Intwominds said...

I am informed that the main reason that Ixer and Bevins publish their
Petrography in the ferret club mag aka Wiltshire Arch and Nat Hist Soc Magazine aka Wiltshire Studies is that they allow detailed descriptions nd lots and lots of photomicrographs.
SO READ the papers. I and B 2012 on the orthostats shows many pics. Get hard copies of the papers,find a flat surface and put the pages side by side.
This blog is fun but is not a journal
That is the correct place to present data and I believe only place.
Yes in the final analysis you have accept that the opinion of the experts is probably correct.
You have not understood the significance of the Clastic nature of the Stonehenge debitage cf Carn Alw non clastic rhyolites. You have fallen for the sleigh of hand of Thomas.
M

BRIAN JOHN said...

Myris -- I looked at the I+B 2012 paper yesterday -- and looked at the photomicrographs. All I am saying is that if a case is to be made for uniqueness / differences of rhyolites from different locations, with respect to matches or non-matches with debris from Stonehenge, the case needs to be made! I am making exactly the points which I would have made if the paper had been submitted to me for refereeing.

BRIAN JOHN said...

Kostas -- this is all highly relevant to Stonehenge. We all want to know what the characteristics are of the debitage and orthostats at Stonehenge (why doesn't EH sanction a proper sampling programme for all of the 43 bluestone orthostats and stumps?) We also all want to know -- I assume -- exactly where these rocks and bits of rock have actually come from. Many sources or few? HHT thought there were few sources. Rob Ixer and Richard Bevins are gradually showing us that the sources are probably many and that the Carn Meini / Carn Alw area was not the "unique" or "special" place from which the stones came. The question of how the stones got from their source locations to the Stonehenge area is another question.

BRIAN JOHN said...

Myris -- Quote: "....the Clastic nature of the Stonehenge debitage cf Carn Alw non clastic rhyolites."

Are you saying that all of the rhyolites at Stonehenge are clastic - sedimentary in nature, whereas the Carn Alw rhyolites are all non-clastic or unmodified volcanic rocks?

GCU:Intwominds said...

NO I am saying that the clastic debitage is clastic the section you show is designated as a 'volcanic with non-planar fabric' in the main Ixer scheme of things. This major class of SH debitage has not been described in detail in the literature and has nothing to do with CRyf.
Please read what I write not what you think I write. I take enormous care in my publications -they are my children -please do not spoil them.
Now the CRyf rhyolites may be rhyolites or they may be rhyolitic tuffs. 0ne is clastic and one is non-clastic but none is 'volcanic with sub-planar fabric' Go back and read the first I and B 2010 to see a snapshot of the full range of SH-style debitage.
This is why the journals are the place to publish not this blog.
Kostas soon time for Easter.

BRIAN JOHN said...

I'm not belittling your work here, Myris. Indeed, I think it is of the highest importance. I'm just looking for clarification and explanation on the basis that certain things are not clear to me. Partly that may be because I am stupid, and partly because you have not explained things in the paper as clearly as they might have been explained. That's no reflection on you -- I have written scores of papers myself, and not one of them was perfect. In fact, some of them make me cringe when I look back at them.....

There is nothing about a journal publication that makes it better or more worthy than comments which we might make on a blog. Some journals publish crap, as we all know -- and some are not even properly refereed. Let's measure the quality of a debate by the worthiness of the points raised, and not by the medium in which it is conducted.

BRIAN JOHN said...

Clastic debitage is "..... a major class of SH debitage has not been described in detail in the literature and has nothing to do with CRyf." Now that's rather interesting -- presumably we are talking here about sandstones and other clastic rocks that might include igeneous material which has settled onto the sea bed?

Constantinos Ragazas said...

Brian,

you write, “Let's measure the quality of a debate by the worthiness of the points raised, and not by the medium in which it is conducted.”

I completely agree! And add, “nor the proponent of the points”.

Kostas

Constantinos Ragazas said...

Myris you write, “Kostas soon time for Easter.”

Yet another resurrection of the same old narrative a la MPP and Stonehenge.

Kostas

Constantinos Ragazas said...

Brian,

you write, “why doesn't EH sanction a proper sampling programme for all of the 43 bluestone orthostats and stumps?”

Yet another enigma of Stonehenge! Were they to do this, it would put the exceptional talents of Dr Ixer et al to lasting use.

Kostas

GCU.intwominds said...

No the problem is the complexity of the material, classes of material and the need to be an excellent/obsessive petrographer. Dr Ixer has been looking at the material since 1988 and has spent 1000s of hours looking at the rocks. That is what is needed or rather the experience that those hours give. It is trying to keep all the data alive and kicking.
No paper is an island. The truth is the papers are being written to remain correct through the centuries to come as well as now and have to be read in their historical sequence.
Journals are the correct place for depositing the primary data not blogs however lovely.
I repeat I and B 2010 Cursus paper introduces all the non sandstone classes of debitage

Jon Morris said...

Yet another enigma of Stonehenge!

The Crown owns Stonehenge. EH just administer the site. I imagine they might have the power to sample the stones, but it's quite possible that they do not.

EH are a heritage organisation charged with getting the best out of heritage and preserving it for future generations. Finding out what happened in the past appears not to be a defined part of their mission statement: I imagine there must be a special brief for Stonehenge.

Helen said...

Be patient for the BBC will explain everything next week - http://www.bbc.co.uk/programmes/b01s1ll4

chris johnson said...

@Helen.
Thanks for the link. I hope you or someone can give a little summary as it is not shown in my area.

Helen said...

@chris johnson:

I'm not entirely sure if it'll be shown in my area either, although if it really does contain "exciting new evidence about Stonehenge [...explaining] why the monument is where it is, and reveal[ing] how long ago it was occupied by people" then perhaps it will receive wider coverage soon enough?

Also, Stonehenge "occupied by people"? That part if the programme will certainly be interesting to see! I wonder if anyone's told MPP...

geocur said...

Helen , there is the possibility of a house/shelter post dating the first phase at Stonehenge ,(as it built over an Aubrey Hole) under the south "barrow " .Doubt how it could be seen from the air though .Have to say I don't hold out much hope for the prog fulfilling it's promises .

GCU:Intwominds said...

Sky at Night kept mentioning the glaciers on Salisbury Plain.
Brian pick up the ball please!!!.
Red-planet Pillinger was saying this.
M

BRIAN JOHN said...

I'd better have a look at this Sky at Night prog, by the sound of it.... not to mention this other one this evening. how that will be on iPlayer...... Out at the cinema this evening, to see "Chasing Ice" -- which is mightily impressive -- and depressing -- by all accounts.

geocur said...

Anything different from this ?
http://www.bbc.co.uk/news/uk-england-wiltshire-16986737

BRIAN JOHN said...

Geo -- no, it was the same story as in the BBC writeup. Will do a post about it.