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.

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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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Over 4 million, and climbing fast

 

I have only just noticed, but this blog has now gone past 4 million page views, with almost 100,000 page views on 28th August alone.  That is really weird -- why so many page views on that particular day?  On digging deeper into the Blogger statistics, I see that the viewings were not just of the latest posts, but of all sorts of posts across a wide range of topics, some of them from many years ago.  There was a similar spike on 11th August (102,000 views) and another on 21 July (75,000 views).   If anybody has any insights into the reasons for these strange spikes, I will be interested to hear what you have to say.

Anyway, many thanks to my faithful readers -- your support is greatly appreciated.

One of the things that I think might have happened over the last couple of years is that the algorithms used by Google and other search engines have picked up on the popularity of the blog in the fields of geomorphology, glaciology, archaeology and geology and have as a consequence started placing it high in the lists of articles that come up in response to searches.  

If there really is a sort of geometric progression going on, we should hit the 5 million mark in no time at all.........

PS.  I have been doing some homework, and have discovered that sometimes you can get "an invasion of the bots" -- God knows what they are, but they are not human.  Maybe I have had a few invasions?  I could dig very deeply by using Google Analytics, but I don't want to do that because you have to open yourself up to a tremendous invasion of privacy by Google, which knows too much already.

I don't do SEO (search engine optimisation) on this blog because then you get preoccupied with keywords and marketing / PR tactics and so forth.  Life is too short for all that sort of stuff.  In any case, I am not a commercial blog, unlike many others, where the blog is essentially a shop front for a range of products.  I have just one product, namely my book called "The Stonehenge Bluestones" -- but I don't get many sales through the blog.  So I will go on as I started -- putting up posts on topics that interest me as and when I can find the time..........

PPS.  Done yet more research, and have discovered very high levels of page views from Vietnam and Brazil!  Apparently this is down to robots or "bots" which crawl the internet for content (for example relating to "Stonehenge") which is then redistributed via other search engines, web-sites and blogs.  No doubt there is a lot of AI translation into other languages going on too.  Most of the consumers or customers are not in Vietnam and Brazil!  So in one sense this is piracy, or theft of intellectual property.  No point in getting upset about that -- we all know that once something is published on the web, it is out there, and nobody can call it back again or claim breaches of copyright.  But on the up-side, it means that many thousands of people who might not have been aware of my work on this blog will now have individual posts brought to their attention, in their own language, probably on page one of a search listing.  So educational materials and research that might have otherwise been hidden in the dark are now out there, in the open, in the light of day!