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Wednesday, 5 August 2020

A mysterious Stonehenge bluestone slide



I have been sent some photos relating to British geological sites by a gentleman who recently acquired them.  He wants to remain anonymous, but is happy for the images to be reproduced here.  They seem to have been made by a professional slide maker called AT Davies, but it is a mystery whether he is the same "Davies" who seems to have worked with Teall and Cunnington around 1870 - 1890 in the collecting and examination of igneous fragments from Stonehenge.

The thin section reproduced here, labelled as a diabase from Stonehenge, is at two different magnifications and seen through crossed polars.  It is probably not an "official" slide" in that it has no Geological Survey or Natural History Museum number.

In their paper about HH Thomas in 2018, Richard Bevins and Rob Ixer say as follows:

"In looking at these collections of specimens in more detail, we can see that Thomas examined almost entirely (if not exclusively) debris material from Stonehenge. The Cunnington samples were clearly all debris material. The survey daybook records E1987– 99 as being specimens collected and presented to the Museum of Practical Geology, Jermyn Street, by W. Cunnington, FGS, and adjacent entries suggest that they were donated in 1893 (falling between the entries for 20 March and 5 June 1893). This appears to be a subset of Cunnington’s samples, a more complete set of which were examined and described by Maskelyne (1878), Cunnington (1884) and Teall (1894), and which Harrison et al. (1979) recorded as being collected from excavations at Stonehenge between 1876 and 1881. Curiously, Thomas records some of the Cunnington samples (specifically E1994 and E1993; his pl. XXII, fig. 3 and pl. XXIII, fig. 3, respectively) as being fragments collected by W. Gowland. This is clearly in error, as is his reference to an axiolitic-type rhyolite as being sample E1977 (1923: 251), the latter being a Geological Survey sample collected from the Isle of Man. We think that this should read E1993. Gowland excavated at Stonehenge in 1901, and debris material from Gowland’s excavations was acquired by the Natural History Museum in 1914 (sample numbers BM1914.1–1914.37), so it is probable that Thomas also had access to this as well. Interestingly, however, Thomas makes no reference to these samples."

Maybe it is a different Davies, since the owner of the slide says that AT Davies became a fellow of the RMS in 1916 -- so that would date the slide to that date at the earliest.  But it is possible that this slide will have been seen by HH Thomas while he was working on his Stonehenge bluestones paper which was published in 1923.

Alfred Thomas Davies lived from 1850 to 1934.  The slide is labelled "diabase", and I assume it is unspotted dolerite with an ophitic texture -- but I stand to be corrected.  (When I studied optical mineralogy I hated it with a passion, and absorbed virtually no information!  Ixer and Bevins would be appalled at my lack of judgment....!!)  But that was a very long time ago.


Here is a picture of the little cabinet of slides -- the labelling is somewhat erratic, but there is some interesting material in there!

Friday, 31 July 2020

Sarsen sources and instrumental /analytical uncertainty


In the midst of all the media celebrations about the solving of the great sarsen stone mystery, I'm quite intrigued by some of the comments from quite senior academics in Twitter conversations with David Nash. There seems to be more or less universal praise for the research, which suggests to me that nobody has actually read the article properly, and that academics have simply been reacting rather lazily to the press coverage and the items broadcast on the BBC, ITV,  and Sky.

I am really the only person concerned about the following?

Ref:  Nash et al, 2020: https://advances.sciencemag.org/content/6/31/eabc0133

1.  No attempt is made to explore the possibility that the Stonehenge sarsens were all picked up from the chalklands of Salisbury Plain.  (Instead, for some reason, areas like the North Downs, the East Anglian Heights, and Dorset were treated as higher priorities.)

2.  The conclusion that 50 of the 52 Stonehenge sarsens are from one common source is not adequately supported by Figure 2 of the paper, since only some of the 260 data points (5 readings were taken from each stone) are identified, and there are many "outliers" which are simply ignored.  I have been unable to access the full data sets and supplementary materials -- I don't know why. (I don't have a problem with most of the Stonehenge sarsens having a shared geochemistry -- that's what one would expect if the stones were picked up locally --  but it would be good to know just how wide the variability is, rather than just being told that 50 of the sarsens have come from a single source.)  Here is the link, if anybody else wants to try: http://advances.sciencemag.org/cgi/ content/full/6/30/eabc0133/DC1

3.  The "matching" of the Stonehenge sarsens to the sarsens of West Woods is dependent upon the examination of just ONE sarsen from West Woods and just ONE sarsen from Stonehenge.  This is completely unsatisfactory, as the authors of the paper must know.  And yet they have claimed that "most of the sarsens at Stonehenge have come from West Woods."

4.  I'm worried that the plotting of the trace element data for stone 58 and for the 20 sampled sarsen sites does not involve a direct "like for like" comparison.  This is because the core samples were done on rock taken from deep within the sarsen stone, while the other measurements were from non-invasive surface readings.  I would like to know how this might have affected the information plotted.

5.  The plot of trace element ratio data for West Woods is used by the authors of the paper as the "killer fact" that establishes beyond all reasonable doubt that the Stonehenge sarsens came from this site.  And yet, when we look at it carefully, its value is very limited indeed.

 

On this plot, the black line plots the ratios measured from the "missing but now found" core from stone 58.  The little cross bars show the range of uncertainty in the readings.  The pink band shows the range of instrumental / analytical uncertainty for the same trace elements at the selected site.  But note that the scale of this uncertainty here is the third highest of all the sarsen stone readings (only Monkton Down and Totterdown Wood have greater uncertainties).  This means that the validity of the main conclusion (namely that West Woods provides the best match) has to be questioned.  If the scale of the uncertainty had been just a little less, then the pink band would have become a thin pink line, and so many readings would have been adrift that West Woods would not have been a favoured candidate source at all.  Just look at the figure showing all 20 sites:


With less uncertainty (ie a thin pink line instead of a broad band) at West Woods, the best candidates for the "sarsen stone source area" would be the samples numbered 3, 4, 7, 11, 12, 13, 14, 17 and 20.  That is, nine other sites would have had equivalent or better fits...............

Sarsens in West Woods (Marlborough News)

I repeat -- the conclusion that the Stonehenge sarsens came from West Woods is not adequately supported by the data.

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

Note added 7th August 2020:  Since this post is still getting lots of hits (590 so far, and counting) and since most readers will not have the stamina to read more than 50 discussion contributions, this is the state of play:

Point 1.  This is still a matter of concern
Point 2.  Lead author David Nash is adamant that 50 of the Stonehenge sarsens are from a common source, and that the "scatter" seen on Fig 2 of the paper is down to "natural variation" within samples.    I'm still not convinced.  Further research will no doubt reveal the truth.
Point 3.  I'm happy to correct this.  In fact THREE stones from West Woods were sampled.  I was misled by an error in the paper (now acknowledged by David) which referred to "20 field samples" (ie one from each of 20 sampled field locations) when it should have referred to "60 field samples" (ie three sampled boulders in each location).  I should have checked this out in the Supplementary Data files, but initially had problems accessing them.  Anyway, apologies for that mistake.
Point 4.  Again, my mistake.  I was misled by some ambiguous wording in the paper to believe that ALL of the sampling was non-invasive.  In fact the non-invasive sampling was done just at Stonehenge (the stones are deemed too precious to harm in any way), while the stones from other sites were investigated via rock samples taken to the lab.  I should have read the paper more carefully.  There was a "like for like" comparison of the stone 58 (Stonehenge) core with the 60 or so samples taken from other locations.
Point 5.  David claims that the West Woods trace elements ratio graphic is as close to a perfect match with the data from core 58 as it is possible to get.  He also says that a perfect match would only be proposed if ALL of the element ratios overlap on the graphic.  To me, it looks as if one of them (Uranium?) doesn't overlap, but that may be down to my fuzzy computer screen.......  At any rate, I haven't changed my view that the best we can currently say is that "Of the 20 potential sarsen provenencing sites so far investigated, West Woods provides the best match, and on current evidence it is the most likely source for the bulk of the Stonehenge monoliths."  That is very different from saying "We now know where the Stonehenge sarsens came from."


Wednesday, 29 July 2020

The Stonehenge sarsens -- did they come from Overton Down / West Woods? On this evidence, probably not.

The main map from the article, showing the sarsen sites sampled.

Oh dear oh dear.  The hype has already started, on the BBC -- and tomorrow there will be a flood of similar stuff from the press:

Mystery of origin of Stonehenge megaliths solved

https://www.bbc.co.uk/news/uk-england-wiltshire-53580339

So -- another Stonehenge technical paper (I hesitate to call it a "scientific paper" because it is not really very scientific) which looks impressive until you read it properly.  Of course, it is now being given the full media hype as another "groundbreaking study" which supposedly solves yet another of the great Stonehenge mysteries, but it is in reality yet another piece of assumptive research, filled with exactly the sort of interpretative inflation that Gordon Barclay and Kenneth Brophy were so concerned about, a few weeks ago.

Gordon J. Barclay & Kenneth Brophy (2020): ‘A veritable chauvinism of prehistory’: nationalist prehistories and the ‘British’ late Neolithic mythos, Archaeological Journal,

DOI: 10.1080/00665983.2020.1769399

Here are the details about the eagerly anticipated new paper:

D. J. Nash, T. J. R. Ciborowski, J. S. Ullyott, M. P. Pearson, T. Darvill, S. Greaney, G. Maniatis, K. A. Whitaker, Origins of the sarsen megaliths at Stonehenge. Sci. Adv. 6, eabc0133 (2020).

ABSTRACT

The sources of the stone used to construct Stonehenge around 2500 BCE have been debated for over four centuries. The smaller “bluestones” near the center of the monument have been traced to Wales, but the origins of the sarsen (silcrete) megaliths that form the primary architecture of Stonehenge remain unknown. Here, we use geochemical data to show that 50 of the 52 sarsens at the monument share a consistent chemistry and, by inference, originated from a common source area. We then compare the geochemical signature of a core extracted from Stone 58 at Stonehenge with equivalent data for sarsens from across southern Britain. From this, we identify West Woods, Wiltshire, 25 km north of Stonehenge, as the most probable source area for the majority of sarsens at the monument.

https://advances.sciencemag.org/content/6/31/eabc0133

Underlying assumptions:

1.  The sarsens were "sourced" (ie collected from) a single provenance area at some distance from the monument.

2.  There were originally c 80 sarsens at Stonehenge, of which only 52 remain.

3.  There was some ritual or political motivation for the sourcing of the sarsens which "overrode" or rendered undesirable the collection of monoliths from within the Stonehenge landscape.

4.  The sarsens could not possibly have been moved by any other mechanism than that involving human effort and ingenuity.

5.  Since the sarsens must have come from one source locality, all "inconvenient" research anomalies and outliers on graphic plots should be ignored.

These assumptions should have been scrutinised by the authors of this article, but they are not.  Even more extraordinary, the fundamental question "Could the Stonehenge sarsens have been sourced locally?" is not even asked, let alone answered.  The paper is hugely devalued as a result.

So what have the researchers actually done?  Well, first they made non-intrusive or non-destructive PXRF chemical analyses of all 52 sampled Stonehenge sarsens.  Quote:  Ten of the PXRF analyses at the monument record anomalously low Si (see Materials and Methods), which most likely indicates that nonquartz accessory mineral grains were excited by the x-ray beam during data acquisition. These readings are excluded from subsequent statistical investigations.  I'm always suspicious when inconvenient sampling results are rejected without adequate explanation; if there were non-quartz accessory mineral grains in ten samples, might that not indicate that they were chemically different from the others, and from a different source area?  Next, linear discriminant analysis (LDA) and Bayesian principal component analysis (BPCA) were used to analyze the PXRF data. BPCA was chosen over standard principal component analysis (PCA) for a number of technical reasons which I did not find entirely convincing.  The LDA results (in Fig 2A) show quite a wide scatter of points within and outside the 95% confidence ellipsoid, but only three stones (26, 156 and 160) are identified as being chemically distinct from the rest of the monument.  They are certainly the outliers on the diagram, but there are many other points that are chemically less distinct but which have differences assumed to be insignificant.  They might also point to different source areas.  The point clustering shown in Fig 2B (for the BPCA analyses) is stronger, but again there are many outliers that the authors have chosen to ignore.  It's a bit difficult to see what is going on here -- some figures and tables are only presented in the "supplementary information" which can only be accessed via another hyperlink.........  At any rate, the overall conclusion is that 50 of the 52 analysed Stonehenge sarsens belong to one geochemically similar group, and that only stones 26 and 160 are "distinctly different."


Next, material from stone 58 (the one that was cored in 1958, only for the core to be lost and then found again) was studied in more detail, leading to the conclusion that it was "chemically representative of the majority of sarsens at Stonehenge."

https://www.archaeology.co.uk/articles/getting-to-the-core-of-stonehenge.htm

Back to Nash et al.  Extended Quote:

For there to be a permissible match between the immobile trace element signature for Stone 58 and a potential source area, we argue that all the trace element ratios for the Phillips’ Core must lie within the limits of instrumental uncertainty of that area. As shown in Fig. 3, the geochemical signature for the Phillips’ Core exhibits a poor match for all sites beyond the Marlborough Downs (sites 7 to 20 on Fig. 1), with disparities evident for two or more of the 21 trace element ratios calculated for each site. It is therefore highly unlikely that Stone 58 was sourced from these areas. On the same basis, we can discount five of the six sampling localities within the Marlborough Downs (sites 1 to 5) as potential sources; this includes Piggledene, identified previously as an unlikely source region on the basis of heavy mineral analyses (11).

The remaining site, West Woods, in the southeast Marlborough Downs, yields permissible matches for all median immobile trace element ratios from the Phillips’ Core; this includes Pr/Zr, U/Zr, and La/Zr, which fall within instrumental uncertainty. We can therefore conclude that, based on our data, Stone 58 and, hence, the majority of the sarsens used to construct Stonehenge were most likely sourced from the vicinity of West Woods. Archaeological investigations and further detailed sampling of sarsens from West Woods and surrounding areas are now required to more tightly constrain the precise source area(s) and identify prehistoric sarsen extraction pits.


In other words, when the measurements for the "Phillips core" of stone 58 (note that we are talking about just one stone chosen to represent all of the Stonehenge sarsens) are set against the measurements for 20 sarsen stones from other parts of southern England-- including South Downs, North Downs, East Anglia and Dorset -- the best match is one stone sampled at West Woods.  Even on the West Woods graph not all of the stone 58 points fall within the "pink zone" of instrumental uncertainty.  One might argue that the "pink" zone is unusually large for this sample, possibly showing more internal variability than in some of the other stones sampled.  One of the Essex samples (Gestingthorpe 1) is almost as good a fit, sample 7 from Mutters Moor in Devon is close, and sample 11 (from Bramdean, Hants) is pretty close too.


The West Woods representation, showing (in the pink zone) an unusual instrumental uncertainty in the measurements taken.  In other words, the pink band is wider.  This of course makes it more likely that there will be an overlap with the Stone 58 representation, shown by the points on the black line.

I appreciate that this sort of sampling is sophisticated and difficult, and that the statistical analyses are also complex,  but if the authors had taken maybe half a dozen of the Stonehenge graphs and shown them as a shaded data range (instead of using just stone 58 and deeming it to be "typical") the overall results would have looked very different.

As it is, the gigantic conclusion that the Stonehenge sarsens probably came from West Woods is based upon a matching of just two stones -- stone 58 at Stonehenge (assumed to be "typical" of all of the sarsens there) and one sampled stone in West Woods (also assumed, without any associated research, to be "typical" of the sarsens there.  Not sure what a statistician would make of that, but I venture to suggest that he would not be impressed........

Rolling on towards the end of the paper, the discussion following the presentation of the research results is best forgotten about, since it is so replete with assumptions and fantasies (for example concerning motivations, extraction options and haulage transport routes) that it devalues what was, up to this point, quite a serious academic paper.  It's another classic piece of myth promotion or interpretative inflation, or however you want to put it.

CONCLUSION

It's very useful to have this research for the "other" sarsen areas and to see how these compare with the Marlborough Downs sarsens, and so from that point of view the paper is worthwhile.  But I don't like the  assumption that the outliers in the Fig 2 diagrams are "insignificant"; I don't like the authors' interpretation that 50 of the Stonehenge sarsens have a common provenance;  I don't like the assumption that stone 58 is the best representative of the whole group of Stonehenge sarsens; I don't like the graphical representation method chosen; and I certainly don't agree with the authors that West Woods is the "most likely" site from which the great majority of Stonehenge sarsens was sourced, since the evidence is just not strong enough to say that.

In summary, an interesting paper spoiled by its own assumptions and leaving us not much better off than we were before on the origins of the Stonehenge sarsens.  And why on earth did the researchers not even bother to investigate whether the sarsens could have come from within the Stonehenge landscape?

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

We can already see on the press coverage that the authors' "most likely" statement has been transformed into something quite definitive -- and no doubt this will have been encouraged in the press release that has come from the University of Brighton.  So we see the Stonehenge myth machine in top gear, with interpretative inflation completely unchecked.   Sad -- but did we really expect anything different?

https://www.independent.co.uk/news/science/archaeology/stonehenge-stones-sarsen-archaeology-a9644436.html

https://www.theguardian.com/uk-news/2020/jul/29/archaeologists-discover-source-stonehenge-giant-sarsen-stones

There is a short popular article by Nash and Darvill here:

https://theconversation.com/stonehenge-how-we-revealed-the-original-source-of-the-biggest-stones-143564

Nothing subtle about this -- for the purposes of public consumption, the origin of the sarsens is now sorted, and to hell with academic rigour or caution.........










Tuesday, 28 July 2020

Thank you for the birthday greetings!


Nothing to do with Stonehenge or the Ice Age -- just old age!  But herewith some grateful thanks for the cheery greetings I received yesterday, from assorted old friends of this blog, on reaching the ripe old age of 80.  Much appreciated, folks!  We had a quiet day, with just our son Martin over from Somerset -- but we did enjoy an excellent rainbow trout for supper, a rather fine zoom session with our nearest and dearest from assorted parts of the globe, together with many video greetings from old friends.  These modern whizzo gadgets are sometimes rather useful.

Today, back to normal, with house cleaning, gardening, repairing the paving in the back yard, and showing some potential buyers around the house and grounds.  So life goes on........

Saturday, 25 July 2020

Another aurochs horn


The aurochs horn found recently in the Severn Estuary.  It's a nice one, but it's not quite as big as it looks in the photo.  A lucky find -- it was embedded in the mud and is very well preserved.  Pic: Martin Morgan.  Further info here:

Thursday, 23 July 2020

Geological Notes on the Altar Stone paper


An EH photo of the surface of the Altar Stone, visible between fallen sarsens 156 and 55b (trilithon 55 is broken into two portions of almost equal size.)  There are some doubts about its dimensions -- most references quote approx 4.9m x 1.5m x .5m -- but one EH publication refers to it as being 3m wide, which means it should be referred to as a slab and not a pillar.


An extract from Anthony Johnson's excellent plan of the Stonehenge stones.  The Altar Stone is stone 80, with two sarsens lying across it.  

Geological Notes on the Altar Stone paper


It’s good to get some comments from another geologist who has actually studied the Altar Stone and the Devonian sandstones of West Wales. I’ve been in correspondence with Richard Thomas, who now lives in Newfoundland, for some years. Last time he visited Wales, he managed to fit in a visit to Rhosyfelin as reported here: https://brian-mountainman.blogspot.com/2018/04/another-rhosyfelin-sceptic.html

Key stratigraphic and lithological information is here:
http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=COB

This is the paper being discussed:

Constraining the provenance of the Stonehenge ‘Altar Stone’: Evidence from automated mineralogy and U–Pb zircon age dating.
Richard E. Bevins, Duncan Pirrie, Rob A. Ixer, Hugh O’Brien, Mike Parker Pearson, Matthew R. Power, Robin K. Shail
Journal of Archaeological Science 120 (2020), 105188

Here are some of Richard’s points:

I found the paper very interesting, and we should commend Rob Ixer and Richard Bevins for all the hard work they have put into the provenancing of the Altar Stone.

I have suggested to the authors that there are two fundamental provenancing issues:

(1) the epic task of unequivocally identifying the parent formation and source location of the Altar Stone (Stonehenge Stone 80);

(2) proving whether or not the sandstones comprising the Altar Stone and the so-called "Altar Stone debitage" are indeed one and the same -- i.e., were derived from a single, lithostratigraphic source unit. Neither issue can be completely resolved unless fresh samples are obtained for analysis from the Altar Stone itself.

I'm under the impression that no indisputably genuine thin sections of the Altar Stone are known to exist. As summarised by Ixer et al. (2019, p.1): "It is not certain that the Altar Stone has been directly sampled, certainly not in the last two centuries, and all samples labelled Altar Stone in museum collections appear to be from loose fragments/debitage and designated as 'Altar Stone' by 19th- and 20th-century excavators." However, in an earlier paper, Ixer and Turner (2006, pp. 2-3) state that: "...the thin section labelled '277 Altar Stone Stonehenge' is ....the only piece of the monolith available for investigation." With respect to issue (2), the various Altar Stone-related papers (including Bevins et al., 2020), contain references to the parentage of the samples described that I find somewhat confusing and, at times, contradictory. For example, on p.1: " .... mineralogical data from proposed Stonehenge Altar Stone debris..."; p.3: "..three debris samples, also thought to be derived from the Altar Stone."; p.3: "..the second type of sandstone, found as rare debris at Stonehenge as well as comprising the Altar Stone..."; p.3: "...six of which have been identified petrographically as being derived from the Altar Stone..."; p.5 (Table 2 caption): ".....samples analysed in this study from the Stonehenge Altar Stone.."; p.7: "...the samples interpreted as derived from the Altar Stone..."; p.7: "...the three Altar Stone samples..."; p.11 "...the so-called Altar Stone sandstone..."; p.12: "...study of the clay mineralogy of the Altar Stone sandstone...". In a similar vein, from Ixer and Bevins (2013, p.14): "...potential Altar Stone debitage ..."; "Three typical Altar Stone debitage lithics.."; "...it is now possible to provide a 'standard' petrographical description for the Altar Stone...”.

This seems to me to be a case of mixed messaging. Ixer and Bevins are trying to prove that sandstone samples such as SH 08, HM 13 and FN 573 are identical to, and/or derived from, the Altar Stone, but how is that possible without having an absolutely bona fide Altar Stone sample for comparison? To eliminate any ambiguity, I think all supposed Altar Stone sandstone samples should have qualifiers such as: 'suspected', 'potential', 'purported', 'possible', 'so-called', etc. added to their descriptions. Unless, of course, 277 is the real McCoy (see below).

One technical point: it's the Mill Bay Formation that is the bio- and lithostratigraphic equivalent of the Senni Formation of the Brecon Beacons area, and not the entire Cosheston Subgroup.

The key point of the paper: Ixer and Bevins have confirmed in the new paper that the analysed micaceous sandstones from the Stonehenge Landscape were not derived from the Mill Bay Fm. section at Mill Bay. The underlying, 435-550m-thick Llanstadwell Fm. also contains numerous green, highly micaceous sandstone units although personally, I don't think they're a match for the Altar Stone either.

From the location information in Ixer et al., 2020 (p.10), it's clear that sample MB3 collected by Brian John actually came from the lower Lawrenny Cliff Fm. -- which explains its compositional differences from the other Mill Bay samples. According to my old field notes, the junction between the two formations (underlying Mill Bay Fm and overlying Lawrenny Cliff Fm) crosses the HWM about 60m south-west of the Whalecwm slipway.

Regarding Ixer et al., 2019 (p.4), my macroscopic description of the Altar Stone (included in Thorpe et al., 1991) was based on me crawling over it and taking photographs (back in January, 1973). I never looked at the samples described by Huggett (1993).

QEMSCAN is a great tool -- especially when used to complement thin section optical mineralogy. The latter will still be essential for determining the Altar Stone's provenance. Using it you can quickly rule out potential source rocks. For example, it only took me a minute to realise that OU9 (444) was not a Cosheston sandstone. Likewise, as you know, verifying whether (p.7) "...quartz and albite cementation, or grain boundary dissolution may also have occurred..." would be simple in thin section. In my view, the fact that QEMSCAN 'reports' rock fragments as their constituent minerals is a major shortcoming, particularly since intraformational and extraformational lithoclasts are such a common component of most Anglo-Welsh ORS sandstones. I suspect that thin section analysis of the diagenetic histories of possible source sandstones will play an important role in unravelling the Altar Stone's provenance. Carbonate cements (including poikilotopic calcites) are common in the Senni Fm. and tend to preferentially attack certain minerals or rock fragments (e.g., K-feldspars), meaning that modal analyses can't give you a true picture of the original composition of the framework-grain fraction. Thin section work, in tandem with QEMSCAN, will be the key to identifying any given sandstone's diagnostic, compositional and textural 'signature' for comparison with those of potential source units.

Quote from P.7: "Clay minerals in sandstones may occur as a detrital matrix or as ......diagenetic cements." They also, of course, occur as claystone or fine siltstone intraclasts, and as alteration products.

Could there be any other cryptic "species" of sandstones among the Stonehenge Landscape debitage?

Is the famous slide 277 really a thin section of the Altar Stone? When I looked at this thin section, I felt that it didn't match what I'd expect a genuine Altar Stone specimen to show. This sandstone is finer grained (very fine vs. fine sand grade) and a lot less micaceous than the Altar Stone, and only contained a couple of garnets. I think it unlikely that it is a genuine Altar Stone sample.

I have no problem agreeing that the "Altar Stone" samples (Table 2; Figs. 2-5 and 8) -- especially FN 573, HM 13 and SH 08 -- share the same parentage. Again, the key question is whether they share that parentage with the Altar Stone. From an inspection of Table 2 and Fig. 2, I'm sceptical that the six numbered samples were derived either from the Altar Stone or its parent unit. Two compositional issues bother me:

(1) Thomas (1923, p.244) describes the Altar Stone's heavy mineral suite as being "exceedingly rich in garnet" which I interpret as being on a par with garnetiferous Llanstadwell Fm. and (especially) Mill Bay Fm. sandstones. However, the analyses indicate that the garnet content of the two Mill Bay Fm. sandstones is an order of magnitude greater than that of the supposed "Altar Stone" samples;

(2) the Altar Stone (from my field inspection) contains abundant muscovite -- certainly equal to the muscovite content of most 'lower' Cosheston Subgroup sandstones. From Table 2, Mill Bay 1a and 1b (average = 4.88%) contain close to double the amount of muscovite present in the six "Altar Stone" samples (average = 2.48%). I consider these differences to be significant grounds for suspecting that the latter are not of genuine Altar Stone derivation. However, this question can only be resolved when/if English Heritage allows new Altar Stone samples to be collected.

Burl (2007, p.281) gives the length, width and thickness of the Altar Stone as 5.0m, 1.1m and 0.53m, respectively. If I remember correctly, the latter's small-scale, trough cross-lamination indicates it is lying right way up stratigraphically. I'd suggest that two samples of the Altar Stone be taken: one from near the stratigraphic top of the unit and one near its base. Both sample sites would be hidden from public view (beneath the turf) because the top sample would have to be taken below the upper portion of the Stone that has been damaged/mineralogically altered by the fires lit upon its exposed surface.

Most Senni Fm. in-channel sandstone bodies that I have observed, generally fine upward. When sampling potential Altar Stone source units, it will be extremely important to ensure that the grain-size distributions of the sandstones collected match that of the Altar Stone as closely as possible, so that compositional differences related to hydraulic equivalence are minimised.

Senni Fm. sandstones vary in colour, provenance, distance from sediment source(s) and diagenetic history from West to East across the Formation's outcrop. I agree with Ixer and Bevins that the eastern portions of its outcrop are the most promising in terms of potential Altar Stone source units. It may be worthwhile checking for any fracture pattern differences across the outcrop, in order to locate joint systems capable of "releasing" sandstone blocks up to 6m long and up to 0.6m in thickness.

The zircon images and date ranges published in the new paper are fascinating! P.8: "A simple interpretation is that the Altar Stone grains reflect a more mature sedimentary environment.....". I suspect that provenance differences are a more likely explanation for the observed 'disparity' in zircon morphologies. The range in zircon morphology and their ages furnishes additional evidence of the prevalence of multicycle detritus within the Cosheston Subgroup sequence (cf. Thomas et al., 2006). The greater range of ages represented by the"Altar Stone" micaceous sandstone zircons is very interesting and suggests a geologically complex source area for these rocks and/or the availability of an abundance of second- and multi-cycle detritus.

I am a strong proponent of the glacial transport of the bluestones to a 'greater Stonehenge' catchment area rather than human transport, and consider the archaeologists' 'new' land route proposal (apparently signed up to by Ixer and Bevins) to be completely lacking in credibility. Also, during my visit to Craig Rhos-y-felin I could find no evidence in support of quarrying at the site.

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Most of Richard's observations are of a technical nature,  but it's interesting that, like me, he takes issue with the assumption that all of the supposed "Altar Stone samples" have actually come from the Altar Stone,  and like me he feels that the authors of the paper should have been much more circumspect in their wording.   We also share concerns about the promotion (yet again!) of the human transport hypothesis, and about the presumption that there is a man-made Neolithic quarry at Rhosyfelin.  On the other hand we are both happy that the "Altar Stone" samples probably do come from a common source; that the sampled debris did not come from the Mill Bay Formation;  and that the real provenance of the "Altar Stone" fragments probably lies somewhere in the eastern part of the Senni Beds outcrop, far away from Preseli.  As I have said before, ice from that area flowed on more than one occasion southwards and south-eastwards towards the Severn Estuary,  so this new work does nothing at all to dent the glacial transport hypothesis.

Wednesday, 22 July 2020

The Mill Bay samples


I just wanted to put on the record the approx positions of the sampling sites I visited some years ago when I gathered rock samples for Rob Ixer and Richard Bevins.  The above map is Fig 1 reproduced from the seminal paper by Richard Thomas and others:

THOMAS, R G, BARCLAY, W J, MORRISSEY, L, WILLIAMS, B P J, and ALLEN, K C. 2006. Enigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south-central Pembrokeshire, Wales. Geological Journal, Vol. 41, 481-536.

I have marked two sampling points on the map.

MB 1 and MB2 came from Mill Bay, close to the bend in the coast. So there is no doubt that they represent one of the facies of the Mill Bay Formation.  The other point shows a sampling point (MB3) near Whalecomb jetty.  Richard Thomas thinks that the junction between the older Mill Bay Formation and the newer Lawrenny Cliff Formation lies about 60m south of the jetty.  So I think that when I took that sample I inadvertently sampled the Lawrenny Cliff Formation sandstone near its base -- this explains the slight differences between this sample and Samples MB1 and MB2.

Nonetheless, it's fairly clear that the 3 samples are all closely related, and that they are very different from the supposed "Altar Stone" samples analysed by Ixer and Bevins and reported in their recent paper.