Oolitic Limestone slabs used in West Kennet Long Barrow

 

Here is some info:

https://reports.cotswoldarchaeology.co.uk/content/uploads/2018/01/Binder1.pdf

I have in the past cast serious doubt on some of the more excitable claims made for the transport of raw materials used in the construction of Newgrange -- and I have noted that the somewhat imaginative reconstruction of said monument caused a considerable stir at the time.

There is a similar issue at West Kennet -- and of particular interest is the discovery of about a tonne of small Oolitic Limestone slabs, assumed to have come either from near Calne or near Frome.  Apparently the slabs were very rotten when they were exposed during the digs by Piggott and others, and were replaced with a tonne of nice shiny new slabs in the 1950's by those who wanted to preserve the site for posterity. The work included the installation of a nice skylight.

Apparently the barrow was rebuilt significantly above its original level using new (imported) and relocated stones. Concerns at the time included a lack of clear guidance from the original excavators, the omission of the original stone numbering system, and questions about whether the stones were returned to their true original positions, raising doubts about the accuracy and integrity of the restored structure.

Now here is a question.  How long does it take for a slab of Oolitic Limestone to rot away and lose its coherence?

Here is the CoralJackz video about the site, making some very interesting points:

https://www.youtube.com/watch?v=LFCQ4ctN8hU&t=173s

Very spotty spotted dolerite

 

 

This is the memorial stone to WR Evans, located at Glynsaethmaen, on the south side of the main Preseli ridge.  It has been bashed about and sand blasted, so we can't say anything about its shape,  except that it has been used as a gatepost before being donated for use as a memorial stone.  Note the weathering patina on the lower part of the front.  This may have formed over recent decades, assisted by the presence of a rusty broken gate hanging bolt......... 

The stone is VERY spotty -- and it is most likely to have come from Carn Meini -- this tor was used in historic time as a nice source for gateposts.

 

Thanks to Hugh Thomas for the pic of a broken metal pin or chisel in a crack up on Carn Meini.  Rock -- spotted dolerite.   Presumably a trace of an attempt by a local farmer to extract a nice handy gatepost, back in the day..........

Bevins et al, 2025: more about that very silly rant

To continue where I left off in my short programme of correcting the "corrections".............

8.  Salisbury Plain and the distribution of bluestone fragments 

Bevins et al claim that bluestone has not been found on Sallisbury Plain, except within 4 km of Stonehenge.  They are being very economical with the truth.  For a start, we must question what they mean by "bluestone", noting that they choose to ignore any fragments which they consider inconvenient and which have no known relationship with bluestone monoliths.  As noted by Kellaway and Thorpe et al, many years ago, there are many finds of small fragments more than 4 km from Stonehenge.  Ixer, Bevins and others have themselves investigated the fragments of a granidiorite boulder (or several?) found near West Kennet, Avebury.  and that's not all.  Watch this space for future postings. 

It may well be that no large bluestone boulders have been found away from Stonehenge on Salisbury Plain,  but it is disingenuous to compare them with the relativce frequency of sarsen boulder occurrences, since the latter are essentially indigenous, having been let down onto the chalklands from a broken silcrete crust over a period of many millions of years.

The Boles Barrow bluestone boulder, found close to the edge of Salisbury Plain, does in my view count as a broken glacial erratic in view of  its shape, it weathering characteristics and its find location.  But I acknowledge that this view is hotly contested!  The boulder's origins and status have been debated for decades, and this is not the place for further argument.  Suffice to say that some authorities believe that the boulder was used more or less where found, near Heytesbury, while others think it came from Stonehenge and has been mislabelled and misinterpreted by one generation after another.....  Use the search box on this blog to find out more.

https://brian-mountainman.blogspot.com/2022/06/the-boles-barrow-bluestone-too.html

As regards the field walking survey by McOmish and others inside the firing ranges, it is a moot point whether we should consider these to be "investigations" as the term is normally understood.  During a field walking exercise in grass covered terrain, and given the close similarities in appearance between dolerite cobbles and sarsen cobbles, I defy anybody to claim that there are "no bluestone fragments or stones inside the firing ranges." 

I dispute the claim that "no erratics occur within the Pleistocene gravels of the rivers draining Salisbury Plain".  Bevins et al cannot say that.  True, Christopher Green made that claim, and it has been repeated by Scourse and Parker Pearson, but I do not believe that the research data collected backs up the claim, as outlined in a number of posts on this blog.

https://brian-mountainman.blogspot.com/2010/04/gravels-pebbles-and-bluestones.html

I have never doubted that in the vicinity of the stone monument at Stonehenge there are abundant fragments and chippings derived from the shaping and destruction of bluestone monoliths,  and it is disingenuous of Bevins et al to make accusations against me on that score.   I maintain my view that SOME of the fragments do not appear to be fresh and angular, and that these MIGHT have been affected by glacial transport.  To claim that ALL of the fragments "undoubtedly" derive from human shaping and breaking of monoliths is patently absurd.  Further, I am accused of not knowing the difference between a Neolithic and a Palaeolithic axe.  I simply referred in my 2024 article to the possible shaping of the Newall Boulder by "an axe maker", and any sensible reader would have understood that I was talking Palaeolithic, not Neolithic.  The silly rant continues to be very foolish indeed.........

The sterile string of accusations goes on and on, with abundant speculations from Bevins et al dressed up as facts. Their text on page 11 is complacent in the extreme. The degree of certainty claimed by the authors of the rant is almost touching in its naivety,  and they seem to be incapable of recognizing that the date of use of the bluestonhes at Stonehenge is not necessarily the same as the date of arrival of the stones on Salisbury Plain.  They also fail to recognise that the labelling of layers exposed in Stonehenge excavations incorporates the assumption that no bluestone fragments can possibly exist in layers that are deemed to be older than the Bronze age. In Cleal et al (1995) there are a number of interesting and "anomalous" occurrences which suggest circular reasoning.  In my view there are  bluestone fragment occurrences in layers that pre-date the supposed date of bluestone arrival.........

Finally in this section, Bevins et al seek to deny my contention that the majority of bluestones at Stonehenge are clearly not quarried.  Some of them, especially those in the bluestone horseshoe,  have been shaped by human beings -- but most are in my view typical faceted and abraded glacial erratics picked up and used without modification.  Bevins et al, as usual with this group of researchers, simply slide away and fail to confront this issue.  Then they make another obvious mistake when they claim that the sarsens are also weathered with rounded edges -- claiming that in this respect they are no different from the bluestones.  There is in fact a huge difference.  The sarsens have been exposed to the elements for millions of years, whereas the bluestones have been exposed for maybe 200,000 - 300,000 years. Maybe much less.  To be determined.  To pretend that the sarsens and the bluestones have acquired their weathering crusts and smoothed edges within the last 5,000 years or so is to completely misunderstand the operation of natural processes in the environment.

9.  The archaeological context.

This is where it gets really interesting, as we enter a fantasy world.    Contrary to the claim made by Bevins et al, I have never argued that the bluestones were used at Stonehenge without being transported by humans.  It is self-evident that they were collected and carried to the places where they were incorporated into the stone monument.  My argument has always been that human beings were vanishingly unlikely ever to have transported the stones from West Wales to Stonehenge, and that they were collected up and used having been found a short and convenient distance away.  I also maintain my argument that the discovery of the bluestones in a cluster might have determined the location of the monument -- just as many authors have suggested for other megelithic structures elsewhere in the British isles.

The authors of the rant argue that the finds of "foreign" fragments and larger stones at West Kennet and elsewhere confirm "a tradition of far-flung human contact" which is also confirmed by the trading of axe-heads etc. That is one view.  Another, proposed by Thorpe et al back in 1991 and by others since, is that Stonbehenge itself was used as a quarry for raw materials in the manufacture of implements.  As for the altar Stone and its supposed origin in the Orcadian Basin of Scotland, the least we can say is that the jury is still out.........

Bevins et al do recognize that many of the stones used in British megalithic structures might be glacial erratics.  I accept that in Brittany and elsewhere very large monoliths appear to have been moved  several km from their supposed places of origin, but I do not accept the argument that "because they did wonderful things there, they also did them here."  That is supposition, not evidence.  We need the latter.

The claim that there was long-distance transport of "large and small stones" from as much as 80 km away for the building of the passage tombs of Newgrange and Knowth looks very shaky when the evidence is carefully scrutinised.  And when we come to the "very strong" evidence of quarrying of bluesone monoliths at Rhosyfelin and Carn Goedog, I fundamentally disagree with all of the fanciful identifications of "engineering features."  They are all natural features, misinterpreted by a team of people who have needed to find quarries in furtherance of their Stonehenge bluestone narrative.  Bevins et al cite Parker Pearson et al (2019 and 2022), but they fail to cite the following articles in peer-reviewed journals which comprehensively dismiss the quarrying narrative:

Brian John, Dyfed Elis-Gruffydd and John Downes (2015). "Quaternary Events at Craig Rhosyfelin, Pembrokeshire." Quaternary Newsletter, October 2015 (No 137), pp 16-32.
https://www.researchgate.net/publication/283643851_QUATERNARY_EVENTS_AT_CRAIG_RHOSYFELIN_PEMBROKESHIRE

Brian John, Dyfed Elis-Gruffydd and John Downes (2015). "Observations on the supposed "Neolithic bluestone quarry" at Craig Rhosyfelin, Pembrokeshire". Archaeology in Wales 54, pp 139-148. (Publication 14th December 2015)

https://www.researchgate.net/publication/286775899_OBSERVATIONS_ON_THE_SUPPOSED_NEOLITHIC_BLUESTONE_QUARRY_AT_CRAIG_RHOSYFELIN_PEMBROKESHIRE


To claim that heavy capstones on cromlechs or portal dolmens in West Wales provide evidence of "haulage technology" is patently absurd, since the ability to raise a large stone with levers and blocks does nothing to support the hypothesis of long-distance haulage over rough and difficult terrain.  In fact, cromlechs like Pentre Ifan, Coetan Arthur and Carreg Samson demonstrate that Bevins et al are fundamentally at fault since the capstones, as far as I can see, were all obtained from the immediate neighbourhood of the chosen site.

In the convoluted argument on p 13 about the trading networks of Britain in Neolithic times, I fundamentally disagree with the claim that the transport of 80 or so bluestones from West Wales "special sites" to Stonehenge would have been relatively easy.  Further, I can see no evidence at all that Stonehenge was a special destination for anything -- and as Gordon Barclay and Kenny Brophy have argued, the idea that Stonehenge was a centre of political unification has more to do with a fantasy narrative than it has to do with hard evidence on the ground.

Contrary to the claim of Bevins et al, I have never argued that bluestones from assorted dolerite and rhyolite outcrops have never been used in local megalithic structures. What I have said is that I can see no evidence that rhyolite and dolerite from particular places were ever considered to be "special" -- and of course this has a bearing on the quarrying hypothesis.  If rhyolite was used in the Early Bronze Age kerb cairn at Pensarn, that does not mean it was special or highly valued.  It simply means that it was local and abundant.

Bedd yr Afanc -- and a jolly variety of lithologies

As regards Bedd yr Afanc tomb, the fact that it lies close to Rhosyfelin and spotted dolerite outcrops is of no significance whatsoever.  It was built of a mottley collection of stones, all simply picked up from a local erratic scatter. They were not "brought" from anywhere significant, and to suggest otherwise is simply to add yet another fanciful component to an already extraordinary and eccentric narrative.

The final part of the article by Bevins et al expands the fanciful narrative even further, based upon the fantasy that the Preseli area was either a "marshalling area" for bluestones intended for Stonehenge, or for the construction of multiple monuments (including several stone circles) such as the "giant stone circle" at Waun Mawn or a "dismantled stone circle" near Crosswell.  Waun Mawn has been comprehensively dismissed as a site of any Neolithic importance, and there is no evidence of any link with Stonehenge.  Bevins et al fail to address any of the issues raised in my Waun Mawn article published in 2024:

John, B.S. 2024. The Stonehenge bluestones did not come from Waun Mawn in West Wales. The Holocene, 20 March 2024 (published online), 13 pp.

https://www.researchgate.net/publication/379121966_The_Stonehenge_bluestones_did_not_come_from_Waun_Mawn_in_West_Wales

10.  Conclusions

In thjis final section the authors simply reiterate some of the assertions made in their paper.  As stated above, I disagree with almost all of the claims made by the authors.  We should all agree that the debate about the glacial transport / human transport mechanisms for the movement of the Stonehenge bluestones is by no means resolved.  There are as yet no "killer facts" which utterly destroy the case made by one side or the other,  and if Bevins et al claim that the matter is now closed, they need to do a serious reality check.  Too many of their claims for their human transport narrative are based on the flimsiest of evidence which does not withstand scrutiny.  It is pleasing, as far as I am concerned, that there have been substantial retreats from some of the wilder narrative components that have been irresponsibly promoted to the media by Parker Pearson and his associates.  Perhaps I can claim some of the credit for that........... but as we see in this article by Bevins et al, reputations are being furiously defended by people who are scrabbling about on very thin ice, and it is not a pretty sight.  But never fear, dear reader. Truth will out.

Final points.  If the Newall Boulder turns out to be just a chunk of a larger rock which is either missing or present at Stonehenge, I will be perfectly willing to accept that.   But at present the evidence is just not strong enough to be acceptable.  Also, if the foliated rhyolite fragments at Stonehenge do turn out to have come from the Rhosyfelin rock face, that too will be of no concern to me.  These matters fit perfectly simply into a glacial transport scenario.  But I remain adamant that the evidence for bluestone quarrying at Rhosyfelin and Carn Goedog is so flimsy that it can be dismissed.  And it is a tragedy that the curse of Stonehenge hangs over this whole debate.  The archaeology of West Wales is quite interesting enough without any mention of the old ruin on Salisbury Plain.  But as somebody said long ago, Stonehenge makes everybody mad.......

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

These are the two papers that are obsessively cited in the article by Bevins et al:

John, B. S. 2024. A bluestone boulder at Stonehenge: implications for the glacial transport theory. E&G Quaternary Sci. Journal 73, 5 June 2024, pp 117–134,

https://www.researchgate.net/publication/381205542_A_bluestone_boulder_at_Stonehenge_implications_for_the_glacial_transport_theory

 John, B.S. 2024. An Igneous Erratic at Limeslade, Gower & the Glaciation of the Bristol Channel. Quaternary Newsletter 162, 28 June 2024, pp 4 - 14.

https://www.researchgate.net/publication/381775577_Quaternary_Newsletter_Article_AN_IGNEOUS_ERRATIC_AT_LIMESLADE_GOWER_AND_THE_GLACIATION_OF_THE_BRISTOL_CHANNEL

Bevins et al, 2025: a very silly rant

Did they really think that their vicious ad hominem attack on me in the Journal of Archaeological Science would cause me to shut up and go away?  If so, they are seriously deluded.

Three months after publication, I am still amazed that they thought they could "shut down" the debate about bluestone transport by forming a "gang of eleven" to write what they supposed would be the definitive and reputable opinion on glacial transport. Juggernauts and steamrollers come to mind. Articles like this, especially if they contain personal vilification,  are almost always counter-productive.

I have never been so heavily cited in my life -- 44 citations in a single article, at a quick count.  I am very flattered..........

This is the article:

Richard E. Bevins, Nick J.G. Pearce, Rob A. Ixer, James Scourse, Tim Daw, Mike Parker Pearson, Mike Pitts, David Field, Duncan Pirrie, Ian Saunders, Matthew Power, The enigmatic ‘Newall boulder’ excavated at Stonehenge in 1924: New data and correcting the record,
Journal of Archaeological Science: Reports, Volume 66, 2025, 105303,
ISSN 2352-409X,
https://doi.org/10.1016/j.jasrep.2025.105303.

What a fuss about a little piece of stone!!  Lots of technology, but not much science.  What there is is so biased and selective that we can call it pseudo-science.

Richard Bevins, lead author

  

Anyway, here are my thoughts. I refer to the sections in the examined paper.

2.  The Newall Boulder

This is really a rather petty and sterile discussion about who did what, and when, in the history of the Newall Boulder investigations.  I thank the authors for clarifying the sampling history of the stone, but some of the criticisms of Bevins et al seem to be directed at me for using the term "brief examination" when I should have used the word "investigation".   I am also criticised for not referring to studies that had not been written or published when I wrote my article that was published in E&G Quaternary Science Journal in 2024.  Quite extraordinary. (I may have several talents, but  reviewing articles that are not yet written is not one of them.)   Attention is drawn to an article by Ixer et al which was published in  August 2022 -- but not in a scientific journal.  I was not impressed.  See my assessment:  https://brian-mountainman.blogspot.com/2022/09/the-newall-boulder-with-thee-samples.html

Another source of huge confusion introduced by Bevins et al is the use of the term "areas" with reference to six parts of the boulder.  These "areas" are shown on Figure 1 and itemised in Table 1.  They are not areas at all -- they are sampling locations.  In my article of 2024 I tried objectively to describe and analyse the surface characteristics of the various "facets" of the boulder, and Bevins et al should have used the same terminology instead of making the waters very muddy indeed. 

3.  Testing a Craig Rhosyfelin source for the Newall Boulder

I am criticised for stating that there is no evidence in support of the claim that the rhyolitic tuff in the Newall boulder "joint block" has "all the key characteristics needed to assign it to Rhyolite Group C from Craig Rhosyfelin".  Bevins et al go on at great length about the petrographic evidence and indeed introduce new data.  I should have said "no CONVINCING evidence -- since the word "evidence" covers a multitude of sins.  Much of the new data introduced here is interesting, but it still does not demonstrate that the Newall Boulder came from Rhosyfelin.  Bevins et al do not seem capable of recognising that their analyses reveal strong similarities between some Stonehenge samples and some samples from Rhosyfelin, but there are no identical matches, and they do not have a sufficient density of sampling points in the Rhosyfelin - Pont Saeson area to demonstrate that the Stonehenge samples cannot have come from anywhere else.  They are obsessed with precision provenancing.  If they want us to be convinced by their glorification of Rhosyfelin, they have to demonstrate that all of the other foliated rhyolite outcrops within a few km of their supposed quarrying site are substantially different.  And they need to go back and do some more fieldwork, instead of relying largely on bits of rock collected decades ago and found in cardboard boxes in museums..

4.  Clarification of Craig Rhosyfelin rhyolite at Stonehenge

This section cites me as commenting on the confusing labelling and mis-labelling of bluestones 32c, 32d and 32e at Stonehenge.  That's fine, and they accept that I am not the source of the confusion.  There has been mis-labelling by Thorpe, Atkinson, Cleal, Ixer and others over the years, and things are not much better today.  In the caption to Fig 5 Bevins et al claim that 32d is a foliated rhyolite "almost certainly from Craig Rhosyfelin".  They do not know that, and it is disingenuous to make the claim.  The stone has not been sampled or petrographically described, and the "identification" is based on the interpretation  of a very old photograph.  Stone 32d should certainly NOT be used in any way to reinforce the impression that there is a link between Stonehenge and the site which they refer to constantly and mistakenly as a Neolithic monolith quarry.

5.  Morphology of the Newall Boulder

This section contains a good deal of condescending nonsense, supported by highly selective citations of evidence and ignoring anything inconvenient.  In my article in 2024 I examined the claim made by Parker Pearson and others that there was a "monolith extraction point" at Rhosyfelin, which coincides with the exposure shown in Figure 6.  I am perfectly aware that the shape of the proposed monolith would have been a tapered pillar; and I dispute the claim that tapering pillar shapes are common at Rhosyfelin. I have looked in detail at the fracture patterns on the rock face and in the upper slopes, and these patterns are sometimes rectilinear and sometimes chaotic. I have made a number of blog posts on this matter.   Many fractures are localised and discontinuous. Bevins et al claim that the base of the imagined extracted monolith had similar dimensions to the stump of stone 32d at Stonehenge.  In an extraordinary flight of fancy, they imply that said stump is actually the bottom section of the broken monolith.  Having examined the "monolith extraction point" so celebrated in the media, I am quite convinced, from a careful examination of the fracture scars on the face, that they are of several different ages, demonstrating that during the evolution of the rock face a number of small blocks have fallen away.  Now and then, over a long period of time.  This is NOT the scar of a single removed monolith.

https://brian-mountainman.blogspot.com/2018/04/that-famous-monolith-extraction-point.html

https://brian-mountainman.blogspot.com/2021/12/message-to-mpp-please-pay-attention.html

Bevins et al make great play of the fact that blunt bullet-shaped clasts are common at Rhosyfelin, and indeed they are, but they are common in many areas where glacial and fluvioglacial deposits are displayed, and it is difficult to discern what point they are trying to make.  But might the Newall Boulder be the broken-off end of a larger block of rhyolite?  Yes, of course that is quite possible.  No problem with that.  The breakage might well have occured during or following glacial transport.

6.  So is the Newall Boulder a glacial erratic?

In their discussion of this issue, Bevins et al claim that the seven "diagnostic features" itemised in my 2024 paper "could be simply generated by surface weathering exploiting internal discontinuities".  I dispute that contention, which arises from a clear lack of appreciation of glacial processes.  To claim that my conclusion relating to glacial transport "has no basis in evidence" and that it is presented as "fact" is frankly absurd.  It is an hypothesis based on my reading of the evidence before me, based on decades of professional experience -- and in my view substantially more worthy than the bluestone hypotheses presented by Bevins and his colleagues over the years.

In the convoluted arguments presented by Bevins et al on glaciation and ice sheet modelling, there are a number of fundamental errors.  The presence of far-travelled erratics on the shores of the Bristol Channel is dismissed as having no bearing on whether Salisbury Plain might have been glaciated. On the contrary, these erratics (and associated glacial deposits) demonstrate that a powerful ice stream from the west was capable -- on at least one occasion -- of reaching the coasts of Devon and Somerset  and pushing into the Somerset Lowland. If the ice reached Greylake (as is commonly acknowledged) it could also have reached the chalk escarpment further east -- and Salisbury Plain comes into the frame.

Bevins et al state that many of the erratics relate to ice rafting and "glacimarine transport".  I refute that, and insist that there is not a shred of hard evidence to support this contention, whatever James Scourse may say about the matter. 

On the possible dating of one or more  ancient glaciation that might have been more extensive than the Late Devensian glaciation, I am not so stupid as to mix up evidence with speculation.  Of course I have speculated about the extent of Anglian and Wolstonian ice in the British Isles, as have scores of other authors over the years.  I have changed my mind frequently as to the most likely scenarios, in response to emerging evidence.   But the statement "there is no evidence to support the reconstruction of Anglian ice limits across SW England" is nonsensical, given that abundant evidence has been cited in innumerable publications by many researchers across a century or more of field research.

Of course models are not a substitute for hard evidence on the ground -- or "ground truthing".  I refute the charge that I have used ice sheet models as primary evidence for past glaciations. And  I will take no lessons from Bevins et al on the use and misuse of such models, having been one of the first glacial geomorphologists (together with my friend David Sugden) to use models extensively in our textbook called "Glaciers and Landscape" back in 1976.  Of course the older models of Devensian ice sheet growth and decay are unreliable to some extent, but they were nonetheless immensely valuable in assessing the interactions between multiple parameters.  The BRITICE-CHRONO models are excellent but of limited use for the reconstruction of ancient glaciations, since they specifically seek to represent the events of the LGM.  And they too have their errors, as the researchers acknowledge.

https://brian-mountainman.blogspot.com/2025/07/was-there-bristol-channel-ice-stream.html

As regards the evidence of erratics on p 10 of the paper by Bevins et al, it is true that there are thus far no known or recorded erratics of spotted dolerite on the inner shores of the Bristol Channel -- but that does not mean that they do not exist. After all, much of the erratic transport route between Pembrokeshire and Salisbury Plain is currently under water, and rather difficult to examine.  The authors of the article are mistaken when they claim that the BGS surveyors found no spotted dolerite erratics south of Narberth.  In one of the memoirs there is a record of just such an erratic near Pendine.  As for the "evidence" of Neolithic stone extraction at Rhosyfelin and Carn Goedog, Bevins et al know full well that that evidence is hotly disputed, and has been since 2015.  Until now they have chosen to completely ignore the "inconvenient" data and conclusions contained in two peer-reviewed articles by Dyfed Elis-Gruffydd, John Downes and myself -- a matter which demonstrates their obsession with Stonehenge and with ruling hypothesis confirmation.  There are no Neolithic quarries at the named locations.  That is my considered opinion and I am sticking with it.

7.  The bluestone assemblage

This discussion of lithologies is as sterile as ever, particularly with regard to the number of lithologies represented at Stonehenge.  The debate is immediately compromised because I insist that ALL lithologies (including hammer stones, packing stones, monolith knockoffs, etc) are relevant for our understanding of the stone monument at Stonehenge, whereas Ixer and Bevins insist that only stones which they deem to be "bluestones" are relevant. That introduces an unacceptable bias.  So it is no surprise when I refer to around 46 different lithologies and provenances, while they refer to just 12 - 15.  They fail to point out that within each of their named lithologies there are samples with such substantial petrographic or geochemical variations that they must have come from different locations.  And they themselves have compounded confusion by periodically changing the names of the lithologies and rock types that they have examined over and again over the past 15 years.  Of course I recognize that old labels have caused some confusion, with numerous redundant labels being used for rhyolites, sandstones and tuffs, for example  -- but the range of rock types represented at Stonehenge is so great that it must represent a glacially transported erratic assemblage.  As pointed out by Thorpe et al (1991)  and Kellaway (1970) many of the stones are "rubbish stones" that can have had no value as monoliths or slabs in the context of a stone monument.  Are Bevins et al seriously suggesting that there were 12 -15 neolithic quarries in Pembrokeshitre, all deemed to be special or sacred and linbked somehow to Stonehenge?

It is thus disingenuous of Bevins et al to claim that the "foreign" stones at Stonehenge have come from a "small number of specific localities", selected and transported by human beings.

No so long ago Ixer and Bevins tried to argue that because there were so many provenances represented in the foreign stone collection at Stonehenge, that proves that vast numbers of tribal groups must have been involved in the collection and transport of "tribute stones" (if that's what they were), thereby demonstrating the vast political power of the Stonehenge builders............ Now they appear to have reveresed their opinion on that.

In the face of such inconsistency, it's difficult to know what Bevins et al really believe.

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

That's not all.  To be continued...........

It is the fate of all steamrollers to end up as scrap metal........

Glacial deposits and lead isotope signatures

In the worthy attempts to create a "lead isoscape map" for Great Britain, a map such as this one (from Booth et al, 2015) would be of much greater use than the bedrock geology map in suggesting where the contours between adjacent "isoscape regions" should actually be drawn.  The map is inaccurate in many respects, but it it is nonetheless very useful.

Average thickness of superficial deposits (BGS).  There is no key in the relevant BGS publication, but the range appears to be from a few metres (blue) up to 250m (orange).

Map showing the extent of superficial deposits in Great Britain (BGS).  

Over the last week or two I have been in touch with quite a few colleagues from assorted university departments in a number of disciplines, and am gratified that they are broadly supportive of the points I have been making following my scrutiny of the "cattle tooth" paper.  So I am grateful to them for their comments.  

One of the most perceptive comments comes from Prof David Evans from my old Geography Dept (in Durham University).  David points out that the lead isoscape map (or any other isoscape map, for that matter) cannot be deemed to be at all accurate unless it is based on the analyses of  samples taken from superficial deposits such as till or glaciofluvial sands and gravels.  As far as we can see, the samples used in the creation of the "lead isoscape map" have been taken exclusively or largely from lead ores found in quarries, mines or surface spoil, or from bedrock outcrops.  The texts of the key papers hardly mention superficial deposits at all, and the underlying source for the map is the solid geology map of Great Britain.  Hence we have mentions of assorted geological provinces that coincide with Mesozoic, Upper Palaeozoic and Lower Palaeozoic domains......

This is a very strange state of affairs, since on the other side of the North Sea related studies are concerned not with solid geology and rock outcrops but with the distribution of superficial deposits. If you are trying to trace where grazing animals might have spent part of their lives, you have to accept that the prime isotopic signature features will have come not from bedrock but from the nature and thickness of the superficial deposits on the grazing route.

In a study of the grazing animals associated with the Viking settlement of Birka in Sweden, the researchers used a regional map of superficial deposits in seeking to find signature matches and animal provenances. They discovered that some animals had travelled from grazing areas c 180 km away. 

Walking commodities: A multi-isotopic approach (δ13C, δ15N, δ34S, 14C and 87/86Sr) to trace the animal economy of the Viking Age town of Birka
Nicoline Schjerven et al
https://www.sciencedirect.com/science/article/pii/S2352409X24001718?via%3Dihub
Journal of Archaeological Science: Reports
Volume 56, June 2024, 104543

ln a study of strontium and other isotope signatures in the Netherlands, the authors ignored the bedrock geology map and concentrated entirely on the isotope signatures of the main groups of superficial deposits including glacial till. They stated that because much of the Netherlands is composed of relatively young glacial and river deposits, the strontium isotope ratios are determined primarily by surface sediments, not the underlying ancient bedrock.

It is strange that the British paper by Evans et al makes no mention of superficial deposits, since the lead author (Prof Jane Evans) was a co-author of several of the key papers relating to other countries that have been heavily glaciated.........

At this point it needs to be pointed out that for lead isotope studies it can be accepted that grazing animals will consume (deliberately or accidentally) small quantities of soil, mud and dust that are derived largely from underlying sediments.  By and large, these materials will not have come from bedrock which may be buried beneath sediments many metres thick.  It would be disingenuous to pretend that "ingested mineral materials" should be discounted because they might be contaminated as a result of industrialisation over the past few centuries.  If they are good enough to be used in studies in Sweden, the Netherlands and elsewhere, they are good enough for the British Isles.

So the map that should. have been used as the base map for the lead isoscape studies is th one above, showing generlised ice movement directions and the distribution of glacial sediments.  Both till and glaciofluvial sediments are made of rock debris derived from areas overridden by glacier ice.  By looking at the established maps of iceflow it is possible to draw conclusions on what "inherited signatures" there might be. Thus the glacial tills around the Bristol Channel should carry an inherited isotopic signature from Lower Palaeozoic rocks situated upstream.  The tills of the English West Midlands will contain inherited isotopic signatures from the Welsh uplands and from the Pennines, and perhaps from even further afield.  The tills of the Oxford region were generated by "Northern ice" and will contain inherited signatures from the Pennines.  The tills of the East Midlands will contain inherited signatures from NE England and maybe even further afield.

To summarise, the geological maps of superficial deposits show that the published contour lines between adjacent "isoscape" regions have almost everywhere been drawn in the wrong positions.  Broadly, ice movement directions show that bedrock-derived debris has been moved southwards, leading to "inherited signatures" some distance from the bedrock lead sources.  The bio-availability of isotopes is therefore misrepresented in maps such as this:

It follows that there may be many more potential source areas for the "Stonehenge cattle tooth" than those considered by the authors of the published article.  The suggestion that the cow that owned the tooth came from SW Wales is completely unsupported by hard evidence.  It is much more likely that it came from the Cotswolds or the Thames Valley.

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

See also

https://pmc.ncbi.nlm.nih.gov/articles/PMC12173513/

Strontium isoscapes for provenance, mobility and migration: the way forward
Maximilian J Spies et al
R Soc Open Sci. 2025 Jun 18;12(6):250283

“Quaternary Provinces and Domains – a quantitative and qualitative description of British landscape types”
Proceedings of the Geologists' Association, Volume 126, Issues 4–5, October 2015, Pages 608-632
Steve Booth, Jon Merritt, James Rose

BGS superficial deposits study:

https://www.bgs.ac.uk/datasets/superficial-thickness-model-1-km-hex-grid/

The Mynydd Melyn moraine?

 

 

Out doing a bit of exploring today -- and I think I may have found the traces of a morainic ridge at around 900 ft on the southern flank of the Mynydd Melyn summit.  It's not very obvious on the ground, but there is a distinct linear accumulation of erratic boulders in a field adjacent to the road.  There is a "gravel pit" marked on the map, but it cannot have been uised for gravel -- for stone building materials, mopre likely......

I will go back and explore the feature in more detail when the weather is more auspicious.

Now that Mynydd Dinas is our local mountain, rather than Carningli, I have a lot of exploring to do.  I am more and more convinced, from the appearance of the ground surface and the wide rolling plateau surface, that this was a perfect location for the establishment of a small ice cap at various stages during the Quaternary.

Just to the south of the map reproduced above are the famous Russia Stones, which I have featured in earlier posts.

How reliable is the BGS lead isoscape map?

 

The BGS lead "isoscape map" -- a crude sketch map dressed up as a piece of high precision cartography?

Quite apart from the issue of proposing --  on the basis of extremely thin evidence -- that the famous cattle tooth found at Stonehenge came from Pembrokeshire, there is the question of the reliability of the lead "isoscape" map published by BGS, used by Evans et al, and assumed to be accurate.  But how accurate is it?  

It comes from an article by Jane Evans and others published in PLOS ONE journal in 2022.  Here is a quote from the recent tooth article:

The variation in Pb isotopes across Britain is based on lead mineralization and the isotope composition is related to the timing of major tectonic events. This provides a broad-brush subdivision of Britain into three major Pb tectonic zones with the addition of Chalk/Limestone as a fourth zone (Evans et al., 2022b). The applicability of these rock and mineral zones to biological tracking is in its infancy and factors such as seawater/rainwater contributions are not yet assessed.

https://www.bgs.ac.uk/news/new-lead-isoscape-map-for-archaeological-provenance-studies-in-great-britain/

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0274831

Evans JA, Pashley V, Mee K, Wagner D, Parker Pearson M, Fremondeau D, et al. (2022) 

Applying lead (Pb) isotopes to explore mobility in humans and animals. 

PLoS ONE 17(10): e0274831. https://doi.org/10.1371/journal.pone.0274831

So there is an admission that the division of Britain into four "major Pb tectonic zones" is rather broad-brush and "in its infancy".  In other words, the method is rather crude and untested. And yet it is being used by Evans et al (2025) in a piece of "high precision provenancing" to link one cattle tooth to Mynydd Preseli in Pembrokeshire..!!  

So how was the lead isoscape map produced?  The first thing that needs to be said is that it based on a remarkably small number of samples or data points (total 633?).  It appears that there are none in Pembrokeshire, only three in South Wales and only three in the English Midlands.

Many of the data points and signature values come from an article by Blichert-Toft et al in 2016.......

We don't have any names for the four sites in South Wales, and neither do we have any of the "signature" values.  I have tried to track them down, but we don't even know which article they came from (multiple citations are not very helpful when it comes to finding raw data).  And some of the potential source articles are behind paywalls -- thus  inaccessible.

And how were the contour lines between areas of differing lead isotope signatures drawn?  According to the relevant figure caption in the PLOS ONE  article, contouring is based on Inverse Distance Weighting (IDW) with ten natural break intervals.  What does this mean?  Well, as I understand it each data point is used as the centre of a circle, and a determination is made regarding the circumferance within which values are probably similar.  If there is a high density of sampling points with similar signatures, then the map can with a reasonable degree of confidence be given in the appropriate colour.  

The precise positioning of your contour can I suppose be inserted by your computer.  But major errors can occur if a sampling point -- or many sampling points -- are located close to the edge of a significant geological outcrop such as that marking the edge of British Lower Palaeozoic rocks.  In that case it will clearly be foolish to assume that the rocks within a circle centred on any particular data point will have the same signature.  Some of them might, but most of them might not.........

Since there are no data points in Pembrokeshire, the colouring of the map must be based on the signature of rocks either in Ceredigion or Carmarthenshire. But in those counties the rocks are mostly sedimentaries and are very different indeed..........

On the lead isoscape map (at the head of this post) South Pembrokeshire is given a different value from North Pembrokeshire.  The line between the two zones roughly coincides with the boundary between the Lower Palaeozoic and Upper Palaeozoic rocks in the county, and the line must have been inserted by hand, based on an assumption of two different signatures.  That is not very scientific..........

All in all, I have a bad feeling about this map, and I have serious concerns about its scientific value.   On checking up on the literature, I found the following points:   

1.  An IDW map is highly unreliable in areas with a low density of sample points. The method's core assumption—that local influence is the dominant factor—breaks down when there aren't enough local points to provide a meaningful average.

2.  In areas with sparse data, such as South Wales, the prediction is essentially a guess, and you have no way of knowing how much you can trust it.

3.  The IDW method cannot predict values higher or lower than the maximum and minimum measured values. How many of your sampling points were in "characteristic" locations?  Many of them might have been in "exceptional" locations, for a variety of reasons.   In sparsely sampled areas, this leads to an underestimation of high values and an overestimation of low values, as the interpolated value gets pulled toward the average of all available points.

4. Spatial autocorrelation is an issue.   IDW assumes that nearby data points are more similar than distant ones. While this is useful for some isotope systems, it can be a problem for isotope systems that vary sharply or are heavily influenced by discrete geological features rather than gradual spatial changes, which can be the case for lead.

------------------------

To summarise:  since the lead "isoscape" map used by Evans et al in the provenancing of a Stonehenge cattle tooth to Mynydd Preseli has no data points in Pembrokeshire, and uses highly questionable cartographic techniques, no precision is possible.  So the provenancing is essentially worthless.

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

PS.  In contrast to the very dodgy piece of mapping discussed above, here is an example of a good map.  It is based on many thousands of data points, many of them described in the literature on multiple occasions over more than 200 years.  This version is quite old, and there are frequent minor corrections, as there should always be in cartography.

The Doniford gravels and the curse of the LGM bias

 

"Churned" deposits at the base of the gravel sequence at Doniford.  At the base are Lias shale and limestone beds.  therev isb evidence here of a permafrost environment, with the operation of both fluvial and poeriglacial processes?  But are there any signs here of glacial action?

One of the great curses of modern stratigraphic studies is the abundance of "recent" sediments as compared with those which are ancient.  Recent events (such as glacial episodes) tend to wipe out most of the evidence of preceding similar events -- and we must be very careful that we do not automatically assume that the LGM glacial episode (MIS- 2) was longer and more powerful than its predecessors simply because LGM sediments and erodional traces are more frequently discovered and examined.  So was the Late Devensian glacial episode the most important one in the Quaternary record in the British Isles?  There are many who think that it was -- especially in the western parts of the glaciated or ice-affected area.    In the Midlands and in the east, the evidence is more equivocal, and the evidence seems to suggest that pre-Devensian glacial episodes were far more powerful and prolonged that that of the LGM.

The principle of LGM bias is one that we have to think about carefully when we examine Quaternary sites where all the evidence seems to suggest ba simple straigraphic sequence in which all the deposits seem to ralate to a single glacial cycle.  This is what I found over and again, in site after site, when I did my doctorate research on the Pembrokeshire coast in 1962-1965.  The overall sequence was this:

7.  Modern soil horizon

6.  Recent colluvium -- Holocene

5.  Fossil ice wedges and "churned" deposits relating to Late Glacial climate changes

4.  Fluvioglacial deposits and flowtills etc -- ice wastage phase

3.  Irish Sea till -- LGM glacial episode (MIS-2)

2.  Periglacial deposits including slope breccia and colluvium -- Early and Middle Devensian? MIS-4 and MIS-3)

1.  Raised beach deposits -- assumed to be Ipswichian in age (MIS-5)

-------------------------------------

The known sequence of marine isotope stages:

MIS 2 – 29,000 yrs BP Late Devensian or LGM -- peak glaciation. COLD

MIS 3 – 57,000 mid-Devensian.  COOL

MIS 4 – 71,000 Onset of Last Glacial Period / Weichselian / Devensian / Wisconsin in North America.  COLD

MIS 5 – 130,000 Eemian interglacial, or Ipswichian in Britain -- many substages. WARM

MIS 6 – 191,000 Illinoian glacial in North America, Saalian in northern Europe and Late Wolstonian in Britain)  COLD

MIS 7 – 243,000 (Aveley Interglacial in Britain) WARM

MIS 8 – 300,000 Early Wolstonian in Britain. COLD

MIS 9 – 337,000 (Purfleet Interglacial in Britain) WARM

MIS 10 – 374,000 COLD

MIS 11 – 424,000 Hoxnian Interglacial in Britain WARM

MIS 12 – 478,000 Anglian Glacial in Britain, Elster glaciation in northern Europe. COLD

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

Only in a very few locations in Pembrokeshire have I found direct evidence of ancient glacial deposits, heavily stained and cemented.  The key sites are Black Mixen near Lydstep, Ceibwr near Moylgrove, Witches Cauldron near Moylgrove, and Traeth Mawr, Newport.  There are other cemented deposits, but I'm uncertain about their significance and stratigraphic relationships.  We need some proper cosmogenic dating.........

Anyway, switching rapidly to the Somerset coast, one of the most intruguing sites is Doniford,  near Watchet on the coast of Bridgwater Bay.  Here there is an intriguing set of gravels resting on a rock platform made of Liassic shale and limestone beds.  The basal layer is referred to as the "cobbly gravel bed", about 3m thick.  Since glacier ice has indubitably affected the Devon coast near Saunton, the Somerset coast near Bristol and the Somerset lowlands at leat 15 km inland, one would expect to find traces of glaciation on both the north-facing and west-facing segments of the Bridgwater Bay coast.  If only things were that simple..........

The gravels in the Doniford - Watchet area have been studied for many years, most notably by the Wedlakes and by Gilbertson and Mottershead in 1975.  See this:  https://cdn.fieldstudiescouncil.net/fsj/vol4.2_100.pdf

Campbell et al (in the GCR series; SW England volume) described the exposures in 1998, and there is now a new study by Basell et al, published in August 2025:

Resolving the late Pleistocene (MIS3–1) sedimentary sequence from Doniford, UK: Implications for British-Irish ice sheet extent, megafaunal history and hominin occupation
L.S. Basell et al, 2025
Quaternary Science Reviews 368 (2025) 109509

https://doi.org/10.1016/j.quascirev.2025.109509

ABSTRACT

We present a new Optically Stimulated Luminescence (OSL) chronology, detailed sedimentological evidence, new palynological data and a new Palaeolithic artefact from a classic site known for over 100 years. The flu-vioperiglacial sedimentary sequence and chronology irrevocably indicates that the BIIS did not reach the north shore of the SW peninsular of the British Isles in MIS 3–2. Both the sedimentology and palynology suggests cool–cold steppic conditions rather than polar desert. The discovery during this project of a new unrolled bout coup´e biface is significant since it adds another westerly example of this characteristic Middle Palaeolithic form associated with Neanderthals, that is relatively common in Britain (compared to Europe). The chronology and sedimentology also confirm the likely reworking of the cold fauna and Acheulian artefacts from regional floodplains prior to MIS 3. The site highlights the archaeological potential of actively eroding cliffs for expanding knowledge of hominin occupations of south–western Britain, near to an ‘edge’ of the Middle Palaeolithic world.

Much of the paper is devoted to the archaeological finds from the gravel exposures, but there are a number of points that have a bearing on our understanding of the Quaternary climate oscillations that have affected this part of the coast.  Quote from the text:

Our model has clear glacial implications as the latest modelling suggests the BIIS reached its maximum extent (LGM) at 26 ka (Hughes and Gibbard, 2015; Scourse et al. 2021; Clark et al., 2022). Interestingly, the maximum modelled LGM ice cover incorporates Lundy and reaches the north coast of Devon and Cornwall BP (Hughes and Gibbard, 2015; Clark et al., 2022, Fig. 8). GL05062 (25 ± 3 ka) which dates DM–Unit 3n, covers this period, but it is most likely that DM–Unit 3 was deposited either prior to the LGM and/or after this as the ice began to retreat and water flow reactivated. With the ice margins so close both to the west and the north of Doniford, at the LGM the landscape would have been frozen and inactive (as is seen in other UK fluvial systems e.g. Rivers Axe (Dorset/Somerset/Devon) and Trent)). The solifluction deposit DM–Unit 4 is consistent with post LGM climatic amelioration but being undated can only be said to be Late Devensian. 

Another quote: 

The Bristol Channel including Lundy Island provides evidence of early Devensian ice dynamics (Rolfe et al. 2012; Gibbard et al., 2017) though this is debated (Carr et al., 2017). The basal date and the erosive boundary at the base of the Doniford Member supports such an early glacial ice advance. The Doniford Member, Helwell Bay Member and potentially the Swill Member are consistent with a proximal ice margin. This is evidenced on the ground and via modelling. The gravels record the repeated periglacial processes combined with pulses of sediment deposition during periods of glacial retreat or seasonal thawing.

The  OSL dates used in the paper are difficult to interpret, and the authors clearly have difficulty in incorporating into their modelled or proposed chronology the proposed Early Devensian glaciation of Lundy Island (see other posts on this).  But what we can say with reasonable confidence is that the evidence from Doniford confirms that glacier ice did not reach this part of the Somerest coast during the Devensian LGM.  But it was not far away, with an ice edge somewhere in the Bristol Channel........

As for the date of the glacial incursion which DID affect the Somerset coast, having read the article, we are not much wiser. There is a possibility that at the base of the Doniford sequence the gravels incorporate cobbles and boulders that are derived from older glacial deposits.  Although no far-travelled erratics have been identified, the wide range of stone shapes is suggestive of the presence at some stage of glacier ice.  The basal gravels are poorly bedded and poorly sorted, as noted by Gilbertson and Mottershead in 1975 as they pondered on a partly glacial origin.   I do not see any evidence here that there was an Early Devensian incursion of ice across the coast; and the most likely scenario still seems to involve an extensive glacial episode in Late Wolstonian time, at the time of MIS-6, maybe around 200,000 years ago.  

Evidence from Eastern England now suggests two Wolstonian glacial episodes, one in MIS-8 and the other in MIS-6.  The evidence in the field is confusing and sometimes contradictory -- and it remains to be seen whether these two episodes can be discerned in the record for Western Britain.

Rock surface exposure age dating using cosmogenic nuclides

Sample 1 (dolerite) --  showing three of the four faces

Sample 1 -- reverse side, showing the smooth and iron-stained "secondary weathering surface"

This is the latest technique, now pretty well tried and tested, which will have to be used on a substantial scale if we are ever to sort out the length of time over which the Stonehenge bluestones have been exposed to the atmosphere.  There is a vast reference list, but the introductions written by Bethan Davies are very useful.  Here is one of them:

https://www.antarcticglaciers.org/glacial-geology/dating-glacial-sediments-2/cryospheric-geomorphology-dating-glacial-landforms/cosmogenic-nuclide-dating-cryospheric-geomorphology/exposure-age-sampling-methodologies/

Whether one ius sampling a glaciated rock surface or a glacially-transported boulder, one should be aware that even within one small area there may be very different "nuclide accumulation" histories, as illustrated here:

This illustration comes from a paper about avalanches and landslides, but it can equally well be applied to glacial landscapes where block entrainment and prolonged abrasional processes have altered surface profiles.  Erratics or fallen blocks on a moraine or in a glacial trail may have very different TCN concentrations and hence different apparent "cosmogenic exposure ages".  A sloping or vertical surface will also have a different exposure to incoming radiation than a horizontal surface, and will therefore have a younger "exposure age."

Getting reliable dates from samples is no simple matter, and over and again in the literature we see results that have to be rejected simply because the samples taken have included surfaces with very different surface exposure ages and also buried quartz fragments with inherited ages.  In some of the samples I have been taking (with permission!) and looking at, there are 4 or 5 surfaces, each with a different thickness of weathering crust and different degrees of iron staining.  This is because when you try to take a sample from a horizontal surface (as recommended by the experts) the rock fractures in complex ways because of the stress exerted at the impact point of the hammer or chisel. Fractures will occur on pre-existing planes of weakness such as joints and faults. If any of these surfaces have been exposed to cosmic bombardment, the exposure age will be altered.  

Also, because cosmic bombardment can influence a rock surface to a depth of 10 m or more, if abrasion and surface fracturing on an erratic or bedrock exposure during a glacial episode only extends to a depth of 5 cm (for example) then the "new" surface will carry with it an inherited age.  This will lead to an over-estimation of the time that has elapsed since the last glacial episode.

Note also that cold ice protects, and warm ice erodes -- so it is possible for a prolonged glaciation to have very little effect on the subglacial rock surface, with little or no abrasion or plucking and the preservation of the old TCN signal......

Two examples.  

In sample 1 (unspotted dolerite), at the head of this post, the sample obtained from a good bash with the hammer has four faces.  Obviously, if one is trying to obtain a reliable age for the oldest weathered face (with a good crust and some lichen traces) it is not a good idea to incorporate in the processed sample a sizeable part of the secondary weathered and iron-stained face, because the nuclide signature here will be different;  and so the aggregate date will be younger than it should be.  But here comes the problem.  If the rock you are dealing with does not contain much quarz, and you just shave off the primary weathered surface to a depth of 10mm and throw the rest away, you may not get a large enough sample for the lab to deliver a reliable result.  For some rock types they may need a sample weighing 1 kg or more -- so you know in advance that your delivered date will be an aggregate date, and not very reliable.

In sample 2 (spotted dolerite), there are 5 surfaces, two of which are fresh and unweathered and 3 are weathered to varying degrees.  Again, if the laboratory sample is shaved off to a depth of maybe 10mm to incorporate just the rough primary weathered surface, a reasonably accurate result will be obtained.  On the other hand, if the whole sample chunk is needed in order to provide adequate quartz for the lab analysis, then the weathered (and possibly exposed) secondary and tertiary surfaces will have to be incorporated, introducing more errors and compromising the accuracy of the end result.  Even bigger errors will be introduced if the rock beneath the surface carries an inherited age from earler exposure to the atmosphere and cosmic bombardment.

Two images of sample 2 (spotted dolerite)

In general, dolerite is not a very good rock for cosmogenic nuclide dating, because it is not rich in quartz.    Granites and sandstones are apparently much more suitable........

From Google searches:

Inherited ages, or inheritance, in rock surface dating refer to an older-than-actual age recorded by a rock surface due to a previous exposure to cosmic rays that wasn't completely "zeroed" by sufficient erosion or signal resetting. This is a problem in cosmogenic nuclide (CN) dating, where the accumulation of nuclides is interrupted but not fully reset. Inheritance can lead to an overestimation of the rock's last true exposure or burial event. 

Rock surface exposure age inheritance is when a rock contains an older-than-actual surface age, often because cosmogenic nuclides from previous periods of surface exposure were not fully removed by subsequent erosion or burial. This "inherited component" complicates accurate dating, especially in glacial settings where cold-based ice can shield rocks from erosion, leading to apparent ages that are too old. Geologists account for inheritance by sampling multiple boulders, looking for ages that are tightly clustered, and identifying outlier ages as likely having inherited components.

Carling on erratic block shapes and evolution

Shap Pink Quarry -- in the centre of the original cliff exposure which appears to have been the main source for Shap Granite erratics.  This is on the down-glacier (plucked) face of a large roche moutonnee which seems to coincide with the granite pluton.

This is an interesting article from Paul Carling, giving us further insights into the entrainment and transport of erratic blocks -- here concentrating on the famous Shap Granite.  Very distinct erratics of this granite -- assumed to have come from Wasdale Crag -- are found widely across northern England, associated with a variety of ice flows during the Devensian and earlier glaciations.

The source of the erratics is a little way to the west of the M6 motorway, and c 10 kn south of Shap; and the erratics are dispersed to the south, south-east and east of the source.  Some of them have travelled hundreds of kilometres.

This is interesting, from thr Abstract:  The degree of edge rounding for individual blocks increases in a punctuated fashion with the distance from the outcrop as blocks fracture repeatedly to introduce new fresh unrounded edges. In contrast, block shape is conservative, with parent blocks fracturing to produce self-similar “child” shapes with distance.

This matches with my own observations on far-travelled erratics, indicating that the "joint controlled blocks" which are entrained as a result of tensile stress on the glacier bed are very rapidly modified, assuming characteristics which are largely the result of the processes operating on the glacier bed.  In other words, geomorphology takes over from geology...........

And as I have observed, large parent blocks break down into smaller "child" blocks, large slabs break down into smaller slabs, and large pillars tend to break down into smaller pillars.

I would have liked to see more consideration of the processes operating in englacial erratic transport, as distinct from subglacial transport.   It is highly likely that many of the Shap erratics have spent at least a part of their time in transport within the body of a flowing ice stream, rather than on the bed, given the complexities of ice flow in an area of shifting ice divides.  Over and again, dominant ice streams must have ovgerwhelmed weaker ice streams, flowing over more sluggish  ice masses and inducing striking changes in ice flow directions.  Indeed. this is demonstrated over and again by references to "basal ice flow directions" and "surface ice flow directions" or "high level ice".  Nevertheless, this work is very relevant to our understanding of the shapes and surface characteristics of the Stonehenge bluestones -- a subject to which I shall no doubt return.......

Thanks to Carling et al (2023) and some of his sources for this map, showing the multiple zigzag pathways that Shap erratics must have followed during the late Devensian glaciation prior to final emplacement

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

The conclusions:  

The hypothesis that granite blocks would display an increase in edge rounding with distance from the source outcrop is confirmed, whilst the hypothesis that shape would evolve with distance is refuted. Although the increase in the mean radius of edge rounding for the whole block population increases
exponentially with distance, edge rounding on individual blocks is an irregular function mediated by block fracture mechanics, as block size is reduced (with shapes fluctuating between cuboids, slabs, and rods) with distance and new sharp edges are provided to partially edge-rounded blocks. Thus, edge rounding and shape coevolve as block size is reduced. Fracture transverse to block orientation is in
accord with the application of tensile stress, which controls the process by which block form is conserved as block size is reduced. Consideration of the orientation of the tensile fractures on blocks in the field might be used to approximate the direction of ice flow at the time of fracture.

Overall, the results indicate that edge rounding is unlikely to be advanced if blocks continue to fracture. Wellrounded blocks must represent blocks that have resisted splitting. In the case of exceptionally large, rounded blocks, the rock mass likely is unusually homogeneous, lacking potential fracture lines. However, smaller blocks are less likely to contain potential fracture lines, so fracture should become less prevalent as blocks are reduced in size, which then promotes edge rounding.

Future work should consider developing mathematical models that represent the function of edge rounding as predicated by a model (e.g. silver ratio) describing block size reduction. Similar studies considering other lithologies (e.g. stratified sedimentary rocks) would likely find different shape evolution patterns in contrast to the cuboid central tendency displayed by the homogeneous granite, with concomitant implications for edge rounding trends with time and distance.

------------------------

Coevolving edge rounding and shape of glacial erratics: the case of Shap granite, UK

Paul A. Carling

Earth Surface. Dynamics 12, pp 381–397, 2024

https://doi.org/10.5194/esurf-12-381-2024
Published: 26 February 2024

Abstract.  

The size distributions and the shapes of detrital rock clasts can shed light on the environmental history
of the clast assemblages and the processes responsible for clast comminution. For example, mechanical fracture ue to the stresses imposed on a basal rock surface by a body of flowing glacial ice releases initial “parent” shapes of large blocks of rock from an outcrop, which then are modified by the mechanics of abrasion and fracture during subglacial transport. The latter processes produce subsequent generations of shapes, possibly distinct in form from the parent blocks. A complete understanding of both the processes responsible for block shape changes and the trends in shape adjustment with time and distance away from the source outcrop is lacking. Field data on edge rounding and shape changes of Shap granite blocks (dispersed by Devensian ice eastwards from the outcrop) are used herein to explore the systematic changes in block form with distance from the outcrop. The degree of edge rounding for individual blocks increases in a punctuated fashion with the distance from the outcrop as blocks fracture repeatedly to introduce new fresh unrounded edges. In contrast, block shape is conservative, with parent blocks fracturing to produce self-similar “child” shapes with distance. Measured block shapes evolve in accord with two well-known models for block fracture mechanics – (1) stochastic and (2) silver ratio models – towards one or the other of these two attractor states. Progressive reduction in block size, in accord with fracture mechanics, reflects the fact that most blocks were transported at the sole of the ice mass and were subject to the compressive and tensile forces of the ice acting on the stoss surfaces of blocks lying against a bedrock or till surface. The interpretations might apply to a range of homogeneous hard rock lithologies.

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See also:

https://doi.org/10.1016/j.pgeola.2023.01.002

P.A. Carling, T. Su and L.Meshkova, Distribution of Devensian glacial erratics and related evidence elucidate complex ice flow changes across a former ice divide: Northern England

Proceedings of the Geologists' Association, 2023.

https://doi.org/10.1016/j.pgeola.2023.01.002

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