The Newall boulder -- how not to analyse a glacial erratic

The Newall Boulder with the thee samples placed in their correct positions. 

(Acknowledgement:  BGS photograph)

"When you are in a deep hole and can't get out, for God's sake stop digging!"  One would have thought that piece of sound advice would be heeded by geologists Ixer, Bevins and Pearce --  but no -- they are determined to keep on shovelling away, assisted this time by museum director David Dawson and blogger Tim Daw.  Actually Tim is not listed as a co-author of the latest piece, but he provides enthusiastic back-up support in the blogosphere..........

This is the latest article:

https://the-past.com/feature/victorian-gifts-new-insights-into-the-stonehenge-bluestones/

Victorian gifts: new insights into the Stonehenge bluestones
Rob Ixer, Richard Bevins, Nick Pearce, and David Dawson explain more.
CURRENT ARCHAEOLOGY, AUGUST 29, 2022, 5 pp

Actually, the picture of geologists digging a hole is not particularly apt in this instance, because what we have is yet another study of ancient slides and samples found in  museums, as a substitute for fieldwork.  As a result, the arguments about the geology of the bluestones get even more tangled than they were before, with the geologists making yet more claims that are unsupported by their own evidence.

It's a strange article, published in a popular glossy magazine without peer review.  So it doesn't make any attempt to be a "scientific article" -- it is simply intended to promote an opinion, or a set of opinions.  The hypothesis of bluestone quarries is mentioned in the very first sentence, presented as a fact -- so we all know where the article is heading. The first part of the article is about the discovery (in the Wiltshire Museum) of slides made from William Cunnington's 1876-1881 Stonehenge collections.  There is a strange mention of sarsens being "quarried locally" -- the authors mean "collected locally", but they just love the idea of quarrying.  Let that pass.  Then they get onto rock names and have a go at me:  "The plethora of rock names in existence has been abused by some to suggest that the range of bluestone rock types is far wider than it really is, a misdirection made in order to promote the belief that they are random erratics rather than having been quarried and transported by humans."  This is a bit rich, coming from researchers who have constantly added new groups to the list of Stonehenge rock types (and changed their own terminology) over the past decade, so that we now have various tuffs or rhyolites, various unspotted dolerites and various spotted dolerites, andesites, dacites and sandstones, with assorted fragments that seem to be unique, from provenances unknown.  With every article they write they demonstrate greater bluestone variability, while claiming the opposite.  Some of the sandstones are called bluestones, and one (made of greenstone) is referred to as a "non-bluestone".  When, I wonder , does a non-bluestone become a "true bluestone"?   All very strange.......... and very unscientific.

And don't get me going on stump 32c, which has been referred to in paper after paper as a Rhosyfelin foliated rhyolite (and sometimes as an altered volcanic ash) and is now referred to as the source of  "much of the andesite group A debitage".  Confused?  So you should be.  And I make no apologies far gently asking the geologists to sort out their own nomenclature shambles before telling the rest of us what to think about anything.

https://brian-mountainman.blogspot.com/2022/02/the-cunnington-slides-and-some-andesite.html

https://brian-mountainman.blogspot.com/2021/08/the-absurdity-of-quarrying-hypothesis.html

https://brian-mountainman.blogspot.com/2022/06/dacites-and-fictional-orthostat.html

Back to the latest article.  One section deals with the Altar Stone.  The authors suggest that thin section "Wilts 277" (which has been enthusiastically discussed on this blog and in many other contexts) came directly from a lump knocked off the bottom of the Altar Stone in 1844 by "Mr Brown of Amesbury".  The key sample of the Altar Stone is now held in the Salisbury Museum, with (museum label 2010K 240.  That's interesting, but of course the new find does nothing to bring the discovery of the real source of the Altar Stone any closer.  It is still a mystery.

The lump of dark coloured sandstone found in Salisbury Museum and now claimed to be the source of thin section 277.

Now to the part of the article in which the authors really are out dancing with the fairies.  It's entitled "Cunnington and Kellaway" and it deals with the "Newall boulder" labelled RSN18 ENQ 2305 which is held in Salisbury Museum and which I examined in June in the company of Tony Hinchliffe.  Ixer et al refer to the boulder as "a broken joint block" -- a completely weird term, since virtually any pebble, cobble, boulder or lump of natural rock found anywhere in the world could be given the same label, on the basis that rocks tend to fracture or break along joints.  

Anyway, Ixer and his colleagues home in on the supposed striations on the stone and on Kellaway's suggestion that it is a glacial erratic that might have come from North Wales.  They state baldly:  "We have recently reunited and examined the joint block and all its offcuts and associated thin sections, and the rhyolitic tuff shows all the key characteristics needed to assign it to Rhyolite Group C from Craig Rhos-y-Felin."  They provide no evidence to support that assertion.  However, they do mention the confusion over the OU analysis of a sample that may, or may not, have come from the boulder.  I have discussed this elsewhere:

https://brian-mountainman.blogspot.com/2022/06/sample-ou2-was-not-from-newalls-boulder.html

Then they say:

".......very recent analysis by pXRF on all pieces of the joint block plus two other visually similar rhyolitic tuffs from the same Newell collection (RSN9 ENQ 2295 and RSN10 ENQ 2296) clearly show that these fragments are compositionally Rhyolite Group C, confirming the petrographic identifications."

However many samples have been taken and analysed, and even if they all belong to "Rhyolite Group C", this does NOT demonstrate that the boulder was a quarried lump of rock that came from Rhosyfelin.    There are substantial differences, in hand samples, between the Newall Boulder and the foliated rhyolites from Rhosyfelin, as we can see below:

Close-up of the fresh face of the Newall Boulder, referred to by by Newall and Kellaway as an “ignimbritic rhyolite” and by Harrison as a "dark blue / blackish flinty welded tuff" with intersecting fractures.

Close-up of a sample from Rhosyfelin, a light blue-grey foliated rhyolite which breaks into 

thin slabs or plates.

Harrison's description (1971):
RSN 18 ignimbritic tuff-lava
“This large, dark blue-grey, hard, flinty (? partly worked artifact) shows a white weathered crust up to 5 mm thick. The thin section shows a complex structure of very finely banded welded tuff (compressed foliated shards cemented by fine silica) with composite quartz grains and strings of dusty leucoxene, separated by patches of much finer grained, finely fluxioned glassy lava with patches of granular quartz, This specimen appears to represent a complex of originally viscous glassy lava and welded vitric tuff, all presumably of rhyolitic composition.”

That does not sound to me like a description that would fit Rhosyfelin. 

This is the evidence used by the authors of the article to demonstrate that the Newall Boulder samples match those from Rhosyfelin.  It's a geochemical plot of ppm of selected minerals on A and B axes, for a large number of "Craig Rhosyfelin" analyses (well over a hundred)  compared with analyses for three Newall samples numbered RSN18, RSN9 and RSN10.  We have no idea whether the Rhosyfelin X-ray analyses were all conducted on rock faces at Rhosyfelin, or whether some (or even most) of the analyses were conducted on samples found at Stonehenge but deemed to have come from Rhosyfelin.  The "field" covered by the Rhosyfelin samples is a very big one, and plots from many other Stonehenge rhyolites and dolerites would fall within it if they had been added.  So the diagram itself, impressive as it might appear, tells us nothing whatsoever about the source of the Newall boulder.

Plots of "geochemical signatures" of groups of samples are useless without much more detailed information about sampling methods and locations.

All that having been said, I am actually not that bothered where the Newall boulder has come from.  If it really is a North Pembrokeshire rhyolite, that's fine by me.  I have no reason whatsoever to wish that it might have come from North Wales!

I have a much more serious issue with the cavalier and slapdash manner in which this boulder has been examined by Ixer and his colleagues.  They completely ignore my detailed analysis of it which was published on Researchgate in June of this year (updated on 15 July):

https://www.researchgate.net/publication/361939331_A_glacially-transported_clast_at_Stonehenge

They fail to mention that I identified and described the slickensiding features on one of the boulder faces, and that I distinguished between these and the faint striations on other parts of the surface.  They fail to mention the weathering characteristics of the boulder, or its glacial facets, or its edge abrasion features, or the pressure fractures that clearly have nothing whatsoever to do with human interference. They fail to mention the tufa deposits or other evidence that this boulder has had a long and complex history, and that it has clearly NOT been quarried at Rhosyfelin and carted off to Stonehenge for no apparent reason by our heroic ancestors.  As Kellaway said when he examined the boulder, why would anybody gather up a useless small stone of this size, carry it all the way to Stonehenge, knock a few bits off it, and then throw it away?  It just does not make any sense.

Ixer et al are clearly intent on proving that the Newall boulder is a quarried lump of foliated rhyolite from Rhosyfelin which has been carried to Stonehenge, or else that it is a lump knocked off a larger rhyolite orthostat.  The evidence on the surface of the boulder itself is all that is needed to disprove that hypothesis, and I begin to wonder whether they have really looked at it at all.

No marks out of ten.  They must do better,  by paying more attention to the literature and listening to the advice given to them by others.

===========================

PS. Prof Peter Kokelaar very kindly looked at my photos of the Newall Boulder and said this:

"Problem is that lavas commonly have various vesiculation bands that collapse and look vitroclastic and welded. I’d buy into the petrographer’s interpretation of welded igneous rock. One pic appears to contain a xenolith so that the streaky fabric perhaps on balance is welded pyroclastic, but then lavas contain xenoliths too.' 

".......... no question in my mind with the data available to me that the Newall stone can have originated in North Pembs. I’ve seen so much Ramsey erratic rock on Gower that Ramsey is a prime candidate, but there are numerous other Ordovician rhyolite sheet possibilities. Given that we know for certain Pembs supplied volcanic (and dolerite, and Skomer, and St Davids Head) erratics at least as far as Gower, it becomes an exceptional plea to have it come from Snowdonia."

Regarding the slickensiding:  "In my book the surface shows quartz-mineral ribbon/fibre growth slickensiding. Top right the transverse bands are growth increments, where movement is missing-part down to left. The ribbons have stepped ends down-left where missing counterpart broke away."

Another photo "shows layers of fibres with some changes of growth direction. Beneath capping vein (white patch), part the fibres on the left are NW-SE, then middle W-E, then NW-SE again. Evidently part of a fault zone..."