Another bullet-shaped glacial erratic clast

I was mending one of my stone walls yesterday when I happened upon a rather fine small boulder which was virtually a mirror image of the famous "Newall Boulder" which sits in Salisbury Museum.  It's almost spooky -- but this one has not had chunks knocked off it, and it has not lost large slices to geological sampling.....

This one is made of dolerite, not rhyolite, and its dimensions are c 27 cm x 15 cm x 10 cm.  It weighs 5 kg, which is approx what the Newall Boulder will have weighed before human beings started messing about with it.  It has a weathered surface, with some facets more weathered than others.  It has 8 main facets and 4 smaller ones -- some flat and others curved or undulating, and there are variations in surface micro-morphology as well.  There are no striations (dolerite surfaces seldom hold striations because of the coarse graining) but there are several small but distinct pressure fractures.  There are no calcium carbonate concretions or nodules (this is an acid water environment), but parts of the boulder surface are covered with patches of whitish lichen.

I am in do doubt at all that this is a glacially transported clast, found in Devensian glacial deposits in the immediate vicinity and used in the construction of my stone wall.  We'll call it the "Trefelin Boulder". It has probably travelled 2 km at the most, from outcrops to the N or NW.  But boulders like this can be carried hundreds of kilometres -- sometimes with ongoing modification but sometimes unchanged after carriage in a "protective environment."  The interesting thing is that each facet has a different micro-morphology and therefore a different history.

I will describe this boulder in more detail in another post, which I hope will be instructive for those who think that the Newall Boulder is just the broken off tip of a monolith transported by our heroic Neolithic ancestors from the so-called quarry at Craig Rhosyfelin........

"Side on" view of the boulder, showing 7 of the 12 facets created by subglacial or 

englacial fracturing and abrasion.

For comparison, here are a couple of photos of the Newall Boulder:

Suggested original shape of the boulder -- before prehistoric and modern modification

The extent of the damage on the lee (blunt) end and on the flank

In the case of the Newall Boulder, the nose (pointed end, up-glacier) has been broken or sheared off, as has the blunt end (down-glacier), leaving us with perhaps 70% of what was there originally.......

Banded rhyolite, Newport, Pembs

Three huge chunks of banded rhyolite have been used on the Parrog in Newport as a memorial for Brian Watts, who died in 2005.  They are pretty spectacular -- the larger of the three probably weighs ten tonnes.  I don't think they are glacial erratics -- from their sharp edges, I think they have been quarried from a rock outcrop somewhere in the neighbourhood.  There are a number of rhyolite outcrops on the northern flank of Carningli.   I am trying to discover the provenance........

Text-book examples of contorted flow banding, created by flowing lava and picked out by layers of quartz (?) crystals......  Click to enlarge.

PS.  The provenance will remain a mystery.  Apparently Glyn Rees, the landlord of the Golden Lion in Newport, "obtained" it from a secret location in the year 2000, as a Millennium commemoration stone, and never divulged where it came from.  So the secret went to the grave with him when he died.......

New Altar Stone paper -- professional scrutiny from Dr Richard Thomas

Exposed beneath a big fallen sarsen stone -- the Altar Stone. From Orkney?  

From the sublime to the absurd?

Scientific papers are worthless without scrutiny, as I have said many times on this blog. I have already expressed my reservations about the paper that purportedly demonstrates that "The Altar Stone has not come from Wales".  I was not convinced:

https://brian-mountainman.blogspot.com/2023/09/when-is-bluestone-not-bluestone.html

Anyway, I am not alone in having serious reservations.  Here is another professional opinion.  Grateful thanks to Dr Richard Thomas, who knows a thing or two about the ORS:

Altar Stone paper
"The Stonehenge Altar Stone was probably not sourced from the Old Red Sandstone of the Anglo-Welsh Basin: Time to broaden our geographic and stratigraphic horizons?” by Richard E. Bevins, Nick J.G. Pearce, Rob A. Ixer, Duncan Pirrie, Sergio Ando, Stephen Hillier, Peter Turner, Matthew Power.
Journal of Archaeological Science: Reports 51 (2023) 104215

https://www.sciencedirect.com/science/article/abs/pii/S2352409X23003905?via%3Dihub

Comments from Dr Richard Thomas

Background:

In 1923, H.H. Thomas (p. 245) wrote: “From general considerations, however, the type of heavy [mineral] residue and the lithology of the rock as a whole are sufficient to make the identification of the altar-stone with the Old Red Sandstone of South Wales almost a matter of certainty.” It’s been 50 years since I knelt by the Altar Stone to take some photos and was greatly amused to hear two ladies behind me say: “what’s ee doin’?” … “ee’s takin’ a picture of a ten-pence piece.” My immediate impression at that time was to wholeheartedly agree with Thomas’s assessment of the Altar Stone’s probable provenance, and nothing in this latest publication leads me to change my opinion.

Comments:

1. The presentation, superficially at least, appears to be authoritative. The element concentrations are what they are. Using ternary plots of 3 elements or element ratios is a well-established and effective procedure for discriminating between, or correlating, igneous units such as lavas and volcanic ashes. The problem I have with them using such an approach for sandstones is that there are many variables that contribute to the initial composition of a fluvial sand body when deposited, and many more that affect its diagenetic history and hence, its ultimate composition (e.g., mineral dissolution and replacement, etc.). It would be interesting to compare and contrast other detailed studies attempting to correlate sandstones on the basis of their geochemical composition.

2. Another issue I have is that the heavier elements they use in their diagrams (e.g., Nb, Th and Zr) are concentrated within the sandstones' heavy mineral fraction (such as in zircons, tourmalines and rutiles = ZTR). The more mature your sandstone is (i.e., well sorted and composed of more resistant minerals), the more likely it is that the heavy mineral fraction will be ZTR dominated. As a result, you have to compare apples with apples -- i.e., sandstones with similar grain sizes and levels of textural and mineralogical maturity. Have the authors done this? I don't know because I haven't seen their sample site table, but I suspect not.

3. As far as I know, there has been no recent sampling of the Altar Stone itself – apparently, English Heritage will not permit it. In my view, unless such sampling occurs (e.g., a 25cm x 2.5 cm diameter core drilled from the underside of the Altar Stone), the provenance of the Altar Stone can never be conclusively proven. This lack of unequivocal Altar Stone samples is one reason for Bevins et al.’s heavy reliance upon high-tech analytical methods.

4. Unfortunately, I don’t have access to their Supplementary Table 1 which lists the site details for the 58 samples they analysed from the ORS of the Anglo-Welsh Basin, nor the 2006 paper by S. Hillier et al. whose sample set they draw upon extensively.

5. Bevins et al. frequently state they have “proven” (or “confirmed”) that various debitage samples were derived from the Altar Stone. Key amongst such samples is ‘Wilts 277’ (a.k.a. 2010 K 240) which they state is a thin section collected from the underside of the Altar Stone in 1844. They base this “proof” on comparisons between their portable XRF analyses from the weathered surface of the Altar Stone with SEM-EDS data from Wilts 277. They place great emphasis on the ‘high’ Ba content of the Altar Stone (to which we’ll return).

6. Rob Ixer has told me that he and Richard B do not think H.H. Thomas examined thin sections taken from the Altar Stone. I do not know the reason they believe this but, from my reading of the latter’s paper, I disagree.

7. Thomas (1923, p. 244) states that “the Altar Stone is exceedingly rich in garnet of small dimensions”. In addition, the upper surface of the Altar Stone, which is small-scale trough cross-laminated, is highly micaceous (with muscovite the dominant species). In 2010, I examined thin section Wilts 277 and told Rob and Richard that I thought it couldn’t be a specimen of the Altar Stone because it did not contain sufficient muscovite or garnet.

8. In my opinion, since sandstone composition (especially in fluvial sandstone bodies) varies both laterally and vertically, and with distance from source, due to textural differences and the influence of hydraulic equivalence, the use of whole-rock geochemical results (and primarily, in this case, the concentrations of one element) is not a viable provenance matching method for most sandstones.

9. The use of SEM-EDS analysis to generate modal compositional data for sandstones is also problematic because a “stepping interval” of 10 microns (as used by Bevins et al.) means that you sample individual grains within rock fragments and inclusions within mineral grains, etc.

10. Subjectively, I feel that Bevins et al. are tackling the analysis of sandstones almost as if they were igneous rocks (e.g., page 4: [Zr, Nb and Th] “will reside in accessory phases in the sandstones” – presumably they mean within heavy minerals.

11. Muscovite abundance is one of the Altar Stone’s definitive features, and yet Bevins et al. (page 5) state that it is not “a critical characteristic discriminator in terms of comparison between known Altar Stone and questioned ORS samples.”

12. We need more information about the Anglo-Welsh ORS – after all, most ORS sandstones are red – and the authors have not stressed the importance of the (primary) grey-green colour of the Altar Stone from a provenance perspective.

13. For example, they have invested a great deal of effort in comparing Wilts 277 with their WM 6 sample – because of the latter’s ‘elevated’ Ba content. Surely, in sandstone provenance studies the idea is to compare like with like, whereas WM 6 is red and coarser grained than Wilts 277 and so it’s hardly a surprise that they are not a match.

14. Based on their previous work, Bevins et al. have started their Altar Stone provenance study at the microscopic level. For example, they base their heavy mineral analysis of sample MS-1 on 0.1973g of material – which seems unlikely to constitute a statistically representative sample.

15. I believe such a study should instead begin with the ‘big picture’ (basin-wide outcrop) level before homing in on candidate sandstones’ microscopic characteristics. In terms of the latter, thin sections offer a wealth of provenance-matching information that Bevins et al. ignore or barely mention.

16. I haven’t kept up with the literature, but I don’t think there have been any modern, detailed studies of the petrography and diagenetic histories of grey-green L. ORS sandstones across the A-W Basin, have there?

17. From personal observations I know that grey-green sandstones within the Senni (Beds) Fm. vary considerably from west to east in terms of their detrital compositions and dominant cements (cf. Heol Senni, Brecon Beacons and Primrose Hill, etc. quarries).

18. While Bevins et al.’s discovery of relatively ‘high’ baryte concentrations in the Altar Stone is certainly interesting, it should be regarded as the ‘icing on the cake’ (to confirm or dismiss potential provenance matches for the Altar Stone) rather than the primary investigative tool.

19. Bevins et al. also make a big deal about the relative lack of high Ba concentrations in stream sediments on the A-W ORS. I don’t find this to be a concern. What was the spacing between samples? Look at the relative sizes of the outcrop areas of the ‘Red Marls’ and Brownstones versus that of the Senni (Beds) Fm.

20. I think Bevins et al. are barking up the wrong tree(s) but support them in their desire to solve the mystery.

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

The EH version of Stonehenge geology

Here we go again.  Somewhat belatedly, the Independent has picked up on the latest press release from Bevins, Ixer & Co on the origin of the Altar Stone:

New mystery over origins of Stonehenge after remarkable discovery

For the past 100 years, the Altar Stone at Stonehenge was thought to come from south Wales - but new research provided a new theory 

Alex Ross

Remarkable discovery? Well, however Richard Bevins spins it in his quote for the media, the geologists have NOT discovered that the Altar Stone did not come from Wales, as I have explained in my scrutiny of the paper:

https://brian-mountainman.blogspot.com/2023/09/when-is-bluestone-not-bluestone.html 

They have hypothesised that the Altar Stone might not have come from any of the locations that they happen to have sampled -- while acknowledging that there are huge variations in the ORS strata which mean that even sampled locations my not be reliably represented in the data base. Sophisticated analytical techniques and impressive diagrams in learned papers cannot compensate for sampling shortcomings.  They may or may not be right in what they say, but this is a speculation, not a discovery.

Then we see this in the press report:

“It’s broadened our horizons,” said Dr Jennifer Wexler, from English Heritage. “We’ve gone from believing we had two types of stone [bluestone and sarsen], now we have three from different places. This opens up a whole new exciting look at the origins of Stonehenge and possibly new connections to other regions of Britain.

“During the late Neolithic age people were coming from places, some a long distance away, and were bringing things from places which were important to them. Now we are looking at a new area people brought stones to Stonehenge from.

“The new study offers a ‘fingerprint’ from the Altar Stone which teams will now look to match with somewhere in the country, it is like a big detective job.”

The stones brought to Stonehenge were believed to have been pulled over sledges and trackways probably sing large teams of people, or potentially even using animals such as oxen and cattle, said Dr Wexler.

Even by the debased standards of the printed media, this is pretty appalling nonsense.  There are not two "types of stone" from two different places. According to the geologists, even the sarsens are quite variable, having almost certainly come from several different locations.  And as for the bluestones, no matter whether you are a geological "lumper" or "splitter", you have to agree that they have come from around 46 different locations (if you count all the foreign clasts of all sizes from the Stonehenge landscape) or around 22 different locations (if you count just the monoliths and the related debris). 

And the stones "brought to Stonehenge" ??  Oh dear oh dear -- don't get me going on that one.......... and don't get me going either on the question "When is a bluestone not a bluestone?"..........

More on the Nevern Estuary washed till

 I have discussed (on several occasions) the exposure of till in the Nevern Estuary -- sometimes nicely exposed in the inside of the coastal dune belt.  I have also speculated on the possibility that the last active ice in this area might have been Welsh Ice, not Irish Sea Ice.........

On our walk today I noticed a distinct gradation in the washed till / pebble accumulations on the tidal shoe on the inside of the dune belt.  This is all very unscientific, since I have done no pebble collecting or counting, but I think there is a far greater range of rock types represented on the washed till surface towards the southern end of the exposure, and a far greater percentage of grey and brown gritstones, sandstones and shales (typical of the rock types exposed in Ceredigion) towards the north.  If we take igneous pebbles as a proxy for debris derived from Irish Sea till, they are far more numerous in the south and diminish in frequency northwards over a distance of c 100m.

So was there a contact zone between Irish Sea Ice and Welsh Ice during the Late Devensian here in the Nevern Valley?  It's quite possible.......

Southern exposure of pebbles washed out of till -- with abundant quartz and igneous materials including lavas and tuffs 

Northern exposure with a much greater concentration of grits, sandstones and shales (from the Ceredigion coastal strip?)

Biological processes and rockface destruction

 

Just came across this on Facebook, in quite another context.  It struck me as particularly apt, given that some people persist, in spite of everything, in believing that Craig Rhosyfelin is the site of a Neolithic bluestone monolith quarry.  

https://brian-mountainman.blogspot.com/2018/05/biological-processes-at-rhosyfelin.html

https://brian-mountainman.blogspot.com/2020/09/biomechanical-processes-at-rhosyfelin.html

https://brian-mountainman.blogspot.com/2015/03/the-rhosyfelin-shrubbery.html

"Follow the science" is a meaningless mantra

I've gone on about this before, but it was good to see Tory MP Esther McVey having a go at the phrases "follow the science" and "follow the evidence" in a recent speech in the house about Covid vaccines.   I have no wish to talk here about either Covid or the vaccines, but to hear somebody like McVey addressing the issue of scientific practice and reliability is actually quite refreshing.  I agree with her.  There is no such thing as THE science, and anybody who claims that is either a fool or a charlatan.  Yet we heard the phrase over and again -- almost on a daily basis -- in the TV briefings from Johnson, Hancock and their tame "scientific experts" during the course of the pandemic.  Just to re-iterate.  There is strong evidence and there is weak evidence -- some of it withstands scrutiny, and some doesn't. There MUST be scrutiny.  And there is good science and bad science, some of it produced by good scientists and some by people who are not scientists at all but who use technology and pretend that they are being scientific.  Science is all about knowledgeable questioning and about debate, and those scientists who pretend that their views are representations of the truth, or that their views are undisputed by others, are not just deluded but dangerous. 

 https://www.youtube.com/watch?v=gdeYzaTZsOA