The Bristol Channel: lost in a fog of MIS-information

The Irish Sea Ice Stream "assault" on the southern coasts of the Bristol Channel, modified after Gilbertson and Hawkins.  The caption refers to the Anglian Glaciation, but it now looks as if this was actually the situation during the Late Wolstonian Glaciation.

We are all lost in the fog, looking down into the murky depths of the Bristol Channel and trying to navigate around ice floes bearing giant erratics..........

Things are very murky indeed, and I suspect that all of us who have been involved in the discussions about the Bristol Channel (James Scourse, Nick Pearce, Rob Ixer, Chris Rolfe, Richard Bevins, Phil Gibbard, Olwen Williams-Thorpe, John Hiemstra, Danny McCarroll, Ed Lockhart and myself, to mention just a few) are more than a little confused, because we don't really know what to make of the evidence.   So in the mix we find field observations, assumptions and speculations relating to ice rafting, the positions of giant erratics, isostatic and eustatic interactions, tectonic uplift, ice edge positions, glaciological and climatic factors, the reliability of OSL and other dating methods, and much else besides.  

Part of the difficulty that we all face arises from the conflation of a number of different issues.  So arguments about ice rafting and relative sea level (RSL) positions and the interpretations of sea floor sediments  have got tangled up in arguments about the likelihood of expansive glaciation in the Early and Mid Devensian (MIS 3 - MIS 4).  Yes, these things are interrelated, but the "Devensian debate" is a hugely complex one, and it is made much more complicated than it needs to be by the obesssion that some people seem to have with the ice rafting of big erratics and the necessity of working out a mechanism for big dirty icebergs to afflict the Devon and Cornwall coasts with RSL more or less where it is today.

The pink erratic at Saunton, sealed beneath sandrock and tied into the stratigraphic sequence. This boulder (reputedly from Gruinard Bay, Wester Ross, in the north western highlands of Scotland) was probably present on the rock platform prior to the Last Interglacial. Other "freestanding" erratics might have been carried into this locality during the Late Devensian glaciation.

This, if I may say so, is a sterile debate because it is completely unnecessary unless you are intent on denying that glacier ice has, on one occasion or more, come into contact with the southern shores of the Bristol Channel.  Following the publication of the latest comprehensive study by Bennett et al (2024), it is incontrovertible that the ice of the Irish Sea Ice Stream has come into contact with the cliffline and pressed inland into embayments, all the way from the Bristol Avon to Land's End.  As James Scourse has said, some of the field evidence is unequivocal and some is equivocal.  That's no big deal, and it can also be said of all of the coasts of Wales.  Large stretches of the coastline of western Pembrokeshire are apparently devoid of evidence of glaciation, and the best evidence of Quaternary sedimentation is found in embayments and coastal valleys protected from the effects of coastal erosion and cliffline retreat in a high energy storm wave environment.  This does not mean that just a few isolated parts of the coastline were overridden by glacier ice or affected by lobes of ice  projecting beyond the position of the glacier front.  Lobes only exist where there are channels and topographical constraints.

Common sense (and glaciology) dictates that if Lundy and Flatholm were overridden by streaming ice, and if the Isles of Scilly were similarly affected, the west-facing and north-facing clifflines of Cornwall, Devon and Somerset must have presented a massive barrier which prevented the ice from moving far inland.  Of course, at the time this coastal tract was far above RSL, and the cliffs must have been partly buried by banks of scree and landslide debris -- but they must still have been high enough to hinder ice ingress towards Exmoor and the uplands of Cornwall.

The cliff rampart on the North Devon coast, near Woody Bay

The simplest explanation of the "giant erratics" resting on the coastal rock platform and also at higher altitudes is that they are glacially transported and that they are exposed by washing processes up to, and some way above, high water spring tides (HWST).  At higher altitudes (maybe up to 100m in places) the erratics are sometimes exposed at the surface but are still, for the most part, buried in till and slope deposits.  This is exactly the situation in the Fremington - Croyde - Saunton area.

Some of the boulders are sealed beneath possible Ipswichian and probable Devensian deposits, and so they must have been transported and emplaced by Wolstonian or earlier glaciers.  Other boulders which have no stratigraphic context (like the Limeslade boulder or the Flatholm pink granite boulder) might just have been carried by Devensian ice........

If one sees some merit in Occam's Razor and accepts the above narrative, discussions about tectonics, isostatic loading effects and RSL positions become immaterial, and we can move on.

My latest reconstruction of the Late Devensian glaciation of South Wales and the Bristol Channel region.  I would not be at all surprised if some of the deposits of the Fremington - Croyde area tirn out to be of Late Devensian age.

=========

So -- what about an Early and Mid Devensian glaciation?  Ah, that is another matter entirely, which deserves another post.....

The myth of ice-rafted coastal erratics

Features seen on Höga Kusten, Sweden, around and beneath the 260m marine limit.  Kalottberg hills are "capped" with moraine and glacial and fluvioglacial deposits, providing excellent conditions for forest cover to survive.  Beneath the highest strandline surfaces are washed -- which means that most of the fines have been washed out during isostatic uplift.

Beneath the highest shoreline in the Höga Kusten district, the finer materials in the morainic cover are washed out, leaving massive accumulations of boulders and smaller stones. The term "Klapperfält" is used to describe them.. Most of the boulders are local, but scattered among them are far-travelled erratics, coloured orange in this diagram.  In areas such as this, stillstands or pauses in the rate of uplift may result in the formation of storm-beach ridges.

Erratics of many different lithologies on washed rock surfaces in the Stockholm Archipelago.

Washed surfaces, kalottberg and klapperfält features can be found anywhere within the dark coloured area on this map.

It's intriguing that ice-rafted erratic boulders are still being talked about in the context of the large erratics found on the shorelines of southern Britain.  Ice rafting seems to be the "process of preference" among those who -- for whatever reason -- do not wish to accept that full glacial processes have substantially affected the Bristol Channel coasts of Cornwall, Devon and Somerset.  

About a year ago I posted these items on the blog: 

https://brian-mountainman.blogspot.com/2023/12/coastal-geomorphology-in-devensian.html

https://brian-mountainman.blogspot.com/2023/12/the-big-erratic-boulders-of-devon-and.html

........... and I have discussed the matter in many other posts as well.  Use the search box to find them.

The latest spat is with James Scourse, who argued in a very strange paper that the big coastal erratics were emplaced by floating ice at a time of high relative sea level (RSL) and cold climate.

https://brian-mountainman.blogspot.com/2024/03/new-scourse-paper-on-giant-erratics-is.html

That of course flies in the face of our understanding of eustatic / isostatic relationships during the Devensian and earlier glacial episodes.  In the latest rather crude attack on me by Pearce, Ixer, Bevins and Scourse they argue again that the big coastal erratics (like the Limeslade erratic) on the Bristol Channel coasts are "ice rafted"  -- and that there was "asynchrony between Early and Middle Devensian regional ice sheet development and global sea level".   I don't have a clue what that means, and no evidence in support of the contention is provided.  I see no sign at all of Early and Middle Devensian glacial activity in the Quaternary stratigraphy of West Wales, and I see no evidence of substantial isostatic depression at the time, or tectonic tilting.  Bennett et al, in the new paper on the Quaternary in Devon, say with respect to the coastal erratics: "...........another view is that the coastal platform erratics were delivered by icebergs calved from glacier ice farther north, an interpretation that is consistent with the widespread distribution of erratics on shore platforms on the Bristol Channel and English Channel coasts."

Even if there was a substantial ice mass in Wales during the Early and Mid Devensian, global sea level at that time was around -70m, and I can see no mechanism for a sinking of the Devon and Cornwall coast by an equivalent amount, thereby creating conditions for ice rafted debris to be dumped on the coastal rock platform.  On the contrary, when one looks at known isostatic adjustment mechanisms, there would have been a hinge line along the Bristol Channel and a rise in the relative altitude of the South-West Peninsula, making giant erratic emplacement from floating ice even less likely.

Icebergs do not, in general, deliver erratics to distant coastal platforms. Dirty icebergs moving away from floating ice fronts almost always operate in deep water, dropping their debris load as a consequence of bottom melting -- ending up as glacio-marine sediments with added dropstones. There are vast thicknesses of these materials in the Celtic Sea and in the outer Bristol Channel. (They have received impressive attention during the BRITICE studies.)  Icebergs do get stranded in embayments in the intertidal zone, and they do break up into bergy bits and release big boulders onto mud flats.  But most of the ice with "rafting" characteristics is sea ice which develops as flat sheets and then gets broken up by lateral pressure, and broken ice floes are almost always clean.  Where there is an ice foot on a polar coastline,  it is almost always a mixture of sea ice, frozen spray and detached chunks of winter snowbanks.  It sometimes contains erratic boulders, but these are generally taken from pre-existing coastal materials including beach accumulations, till and scree.  They are seldom carried in from the sea; they are more likely to be carried OUT from the coastline to be dropped in deeper water offshore. 

As I see it, the big erratic boulders scattered along the coasts of the Bristol Channel are NOT all concentrated in the current intertidal zone.  They occur at altitudes up to at least 80m.  They have to be residuals or remnants of ancient glacial deposits scattered along the Bristol Channel coasts.  As I have said before, the "concentration" of these erratics on the intertidal rock platform is more apparent than real, since these are washed surfaces similar to those associated with "kalottberg" hills around the marine limit in Sweden.  Nobody ever suggests that the boulders found at the marine limit are unique in any way;  they almost always also occur above and below the limit.  So the position of the marine limit -- like RSL on the coast of the Bristol Channel -- is interesting but not geomorphologically significant.

https://brian-mountainman.blogspot.com/2018/07/kalottberg-features-in-northern-sweden.html

It seems to me highly likely that the erratic boulders on the Bristol Channel coasts are simply the remnants of ancient glacial deposits that have been almost entirely removed on the coast by marine, periglacial and other processes, possibly over several glacial / interglacial cycles. Further inland, away from the coast, there may well be in situ glacial deposits (with giant erratics!) still awaiting discovery.

 The boulders themselves may date from a number of different glacial phases, including the Anglian, Late Wolstonian and Late Devensian, in which Irish Sea ice from the N and NW crossed the coastline and pressed inland via convenient depressions and wide valleys.   Just a few of the boulders (such as the famous pink erratic at Saunton) can be tied into the stratigraphic sequence,  sealed by sandrock or slope breccia.  At Whitesands in North Pembrokeshire  the boulder bed (which I think is related to some of the erratics on coastal rock platforms) appears to consist of boulders emplaced prior to the last interglacial and then modified by wave action.  It is not visibly associated with the Ipswichian raised beach. 

Some of the boulders of the Whitesands boulder bed, held in cemented slope breccia at the base of the Devensian sediment sequence.  The boulders were probably emplaced during an early glaciation and rounded by wave action during the Ipswichian interglacial.  This narrative probably applies to many of the Bristol Channel "giant erratics"........

Looking again at the ice-rafting hypothesis, I cannot see any tectonic or other evidence which suggests that floating ice was involved in the emplacement of any of the big erratics on the Bristol Channel coast.  On those occasions when there were dirty icebergs in the Celtic Sea, sea level was probably at least 100m lower than today, and the coastline was at least 10 km further to the west than it is now.  

More support for a big Wolstonian glaciation

 

I have been looking again at the big article by Bennett et al (2024) on the Quaternary in Devon.  As indicated on the table above, they believe that the one glacial episode that substantially affected the North Devon coast occurred during the Late Wolstonian (MIS 6).  They consider that the glaciation of the Bristol Channel during the Late Devensian (MIS 2) was less extensive, although they accept that the evidence for that time demonstrates a local ice cap on Dartmoor. (There must, of course, also have been a Dartmoor ice cap in the Late Wolstonian, but the evidence for it is not easy to find.........)

As for the Anglian (MIS 12), the authors are cautious.  They suggest that the evidence for a big glaciation at that time is not clear in and around Devon, and therefore they prefer to stick with a "periglacial" climate label for that period. 

Commendable work, with great relevance for other parts of SW Britain. 

Quote:

The most recent research on glaciation in the Bristol
Channel was published by Gibbard et al. (2017, 2022). This
included a synthesis of new publicly available borehole and
bathymetric data, combined with a wealth of other existing
disparate data sources. Sediment boreholes throughout
the Bristol Channel confirm the area was glaciated in the
Pleistocene. Till is present below marine deposits and, in some
areas, is visible morphologically as submerged moraines (Fig.
9). In the central and eastern Bristol Channel the submerged
valley course of the palaeo-Severn is very clear in new highresolution
bathymetric surveys. This former river course and
associated tributaries cross-cut through glacial sediments in
the Bristol Channel. At least three phases of glaciation are
recorded in the Bristol Channel, one related to the southern
limits of a Late Devensian Substage (~MIS 2) Welsh Ice Cap
which reached into Swansea Bay, an earlier Devensian (MIS
4 to 3) glaciation associated with Irish Sea ice, and another
older glaciation that is associated with ice that filled the entire
outer and central Bristol Channel (Fig. 1). The age of the older
Bristol Channel glaciation is still open, although it predates
the Devensian (Late Pleistocene) and must date to the Middle
Pleistocene, and as noted above is likely to correlate with MIS
6 (i.e., the Late Wolstonian). This has implications for the age
of the glacial deposits present on land in the Barnstaple area,
which have traditionally been correlated with the Anglian Stage
(MIS 12) (e.g., Croot et al. 1996). It is more likely, however,
that these deposits correlate with the oldest glaciation of the
Bristol Channel immediately to the north, i.e., with MIS 6 and
the Late Wolstonian Substage (potentially the Moreton Stadial
of Gibson et al., 2022).

I am unconvinced about the Early Devensian  (MIS 4-3) glaciation associated with Irish Sea ice, since the evidence is just not there -- and  I suspect this idea will be dumped before too long.......

The Quaternary Geology of Devon

Glacial deposits and erratics in the Fremington area.  Note that the till cover extends over the 60m contour.  It is therefore not surprising that "high level erratics" are found at even greater altitudes elsewhere on the Devon coasts.

Been waiting for this for some time.  A very comprehensive and interesting summary.  I'm intrigued by the  suggestion that glacier ice "approached" the North Devon coast on at least three occasions from the Bristol Channel, and probably crossed it more than once.  The authors are in no doubt about the presence of glacial deposits in the Fremington area.  I would have liked a more careful consideration of the ice rafting hypothesis in the section where they talk about erratic boulders.  They say that the rivers that cut the deep river valleys were graded to low, cold period sea levels, but if that was the case you cannot have high sea levels with ice rafting of erratics when conditions were of "full glacial" status.  You can't have your cake and eat it.

Anyway, I'll give this useful article greater consideraton on another occasion.

===========

Bennett, J.A., Cullingford, R.A., Gibbard, P.L., Hughes, P.D. and Murton, J.B. 2024. The Quaternary Geology of Devon. Proceedings of the Ussher Society, 15, 84-130.

https://ussher.org.uk/journal/catalogue/volume-15-part-3-2024

Abstract

Throughout the 2.6 My of the Quaternary, Devon has occupied a critical position with respect to the evolution of Britain in that it lies close to the North Atlantic Ocean between the southern coast of the Bristol Channel and the northern coast of the English Channel. This setting results in the area being highly sensitive to climatic and environmental change. Although the county lies beyond the general limit of the major glaciations of the last 0.5 My, it was impacted by glacial ice and its associated meltwater that approached the north coast on at least three occasions. Glaciers also left deposits on the Bristol Channel floor and potentially locally on Dartmoor and possibly Exmoor. Ice-rafting of erratic rocks also occurred. Nevertheless, the whole region has been repeatedly subjected to severe cold-climate, periglacial conditions for much of Quaternary time. Under cold periods frost- dominated climates have driven the formation of a thick carapaces of slope-derived debris (head deposits), with wind erosion shaping the tors characteristic of the high moorlands. At the same time deep river valleys have been carved by continual severe seasonal snow meltwater, the streams transporting weathered rock derived from the steep slopes. The rivers that cut these valleys were graded to low, cold period sea levels, and their valleys frequently include multiple terrace accumulations, the highest of which date from the Middle Pleistocene. Apart from the Late-glacial open vegetation and Holocene blanket peatland, evidence of earlier warm-climate conditions also occur, but are limited in comparison to the cold-climate accumulations. Past and present interglacial (temperate) deposits are also known from the county. They include not only river deposits, but also cave infillings and high sea-level, fossil beaches, often including fossil evidence. Coastal erosional landforms such as wave-cut platforms and cliff formations are commonly found, and submerged offshore examples are also known, particularly from the English Channel coast. Despite all this evidence, its fragmentary nature means that the record of events in the region is both limited and intriguing. The advent of numerical age determination and other modern analytical methods have improved knowledge of the timing of some events and provenance of materials in the region, but much more work is required to fill in the substantial gaps in current knowledge.

Pearce et al: Much ado about nothing much

The map published by Scourse et al, 2021. The labels used for the glaciers are no more "correct" than the labels that might be used by others on their own maps.  The ice edge positions shown are also matters of opinion, as are the directions of ice flow........ and I happen to disagree with several of the key features on this map.

With regard to that petty and mean-spirited ad hominem attack the other day from Pearce, Ixer, Bevins and Scourse, one of the things that most intrigued me was the obsession with silly little details.  

One comment (with the use of "sic") related to my use of the term "Irish Sea Glacier" somewhere in the text, whereas I should apparently have used the term "Irish Sea Ice Stream" as defined by Scourse and many other colleagues in 2021.  Big deal.  Well yes, that is the term used for many years now to describe the ice mass flowing in the Celtic Sea arena and into the Bristol Channel.  For many years I have used the term "ice stream" myself, in this blog and in publications.  But the term "Irish Sea Glacier" is and has been used widely across the literature for many years, and is widely understood as the ice mass that extended all the way out to the Celtic Sea shelf edge.  An ice stream is simply a big glacier, flowing fast and flanked by ice that is stagnant or flowing much more slowly.  A glacier might be described as an ice stream channelled in a trough or bounded by topographic highlands.  Pearce et al apparently want the rest of us to restrict the term "Irish Sea Glacier" to the ice that flowed into the Cheshire Basin, flanked by the uplands of North Wales and the southern Pennines.  But it was no more constrained by topography than the "Irish Sea Ice Stream"  that flowed through St Georges Channel -- and I think that the term "ice lobe" better describes the characteristics of the ice that flowed into Cheshire and the north Midlands.  

It may also be argued that the use of the term "Irish Sea Ice Stream" for the ice occupying the Celtic Sea is inadequate, since the feature (possibly during several glaciations) had many of the characteristics of a piedmont glacier, as I pointed out in one of my earliest glaciology articles, in 1968:

https://brian-mountainman.blogspot.com/2011/02/glaciological-dilemma.html

https://brian-mountainman.blogspot.com/2016/07/ice-in-celtic-sea-piedmont-glacier-or.html

https://www.researchgate.net/publication/319922666_Short_Notes_Directions_of_Ice_Movement_in_the_Southern_Irish_Sea_Basin_During_the_Last_Major_Glaciation_An_Hypothesis

Thus there is no "correct" terminology in any of this.  It is disingenuous of Pearce et al to pretend that there is, and that others are "in error" if they use terms that do not conform to somebody else's labelling system.

Reference:

https://brian-mountainman.blogspot.com/2021/05/new-dating-for-lgm-irish-sea-ice-stream.html

Maximum extent and readvance dynamics of the Irish Sea Ice Stream and Irish Sea Glacier since the Last Glacial Maximum
J. D. Scourse, R. C. Chiverrell, R. K. Smedley, D. Small, M. J. Burke, M. Saher, K. J. J. Van Landeghem, G. A. T. Duller, C. Ó Cofaigh, M. D. Bateman, S. Benetti, S. Bradley, L. Callard, D. J. A. Evans, D. Fabel, G. T. H. Jenkins, S. McCarron, A. Medialdea, S. Moreton, X. Ou, D. Praeg, D. H. Roberts, H. M. Roberts, C. D. Clark
Jnl of Quaternary Science, 7 May 2021 (special issue article)

https://onlinelibrary.wiley.com/doi/10.1002/jqs.3313?af=R

https://doi.org/10.1002/jqs.3313

This all reminds me of a spat I had with James Scourse five years ago, following the publication of my QN article on the glaciation of the Isles of Scilly.  Scourse attacked me, using very intemperate language, partly because I had the temerity to disagree with a part of his sedimentary stratigraphic labelling system.  In his eyes, I dare say, his labels were 100% reliable, and not open to negotiation by anyone.  Anyway, after some insulting bluster and questioning of my competence, he had to grudgingly agree that the Devensian ice limit as drawn by me did actually have some merit..........

https://brian-mountainman.blogspot.com/2019/03/scourse-versus-john-rather-scilly-spat.html

Then there is the matter of the Anglian Glaciation, about which I have written at length on this blog.  In their QN piece, Pearce et al have a silly dig at my "contention" that the ice advance responsible for bluestone transport was during the Anglian Glaciation, and claim that there is no geochronological or other evidence to support this.  It's true that when I wrote my article I did mention the Anglian episode (MIS 12), but in doing that I was representing an almost unanimous view across the research community that this was the most extensive pre-Devensian glaciation across much of the British Isles.  

LEE, J R, ROSE, J, HAMBLIN, R J, MOORLOCK, B S, RIDING, J B, PHILLIPS, E, BARENDREGT, R W, AND CANDY, I. 2011. The Glacial History of the British Isles during the Early and Middle Pleistocene: Implications for the long-term development of the British Ice Sheet. 59-74 in Quaternary Glaciations–Extent and Chronology, A Closer look. Developments in Quaternary Science. EHLERS, J, GIBBARD, P L, AND HUGHES, P D (editors). 15. (Amsterdam: Elsevier.)

I am quite unconcerned about whether the Anglian was or was not the largest British glaciation, and if Pearce et al had bothered to look at my published output they would have seen that I have discussed at length the possibility of a very large Wolstonian glaciation -- and that I am rather convinced by the latest suggestions of Phil Gibbard and others that it was more important than the Anglian for landscape transformation (and sediment transport) in the Celtic Sea arena. 

https://brian-mountainman.blogspot.com/2024/08/towards-late-wolstonian-mis-6-ice-limit.html  

More to follow.......

The Berry House boulder, north of Shebbear

 

From Google streetview -- this is a hefty boulder, on the roadside verge at the entrance to Berry House

More on the Shebbear erratic boulder

Found in the NDAS Newsletter, Autumn 2017
http://www.ndas.org.uk/NDAS%20Newsletter%20Autumn%202017.pdf


This is very interesting. More on Shebbear........... John Bradbeer is quite certain that the Shebbear and Berry House stones are both sarsens, related to those on Salisbury Plain and other parts of southern England. He suggests that the two stones have come from destroyed Tertiary deposits, and have been "let down" onto the present land surface.  Until further evidence is forthcoming, this sounds like a reasonable explanation.  More research please, from those who live over that way......

The Devil’s Stone at Shebbear: A Landscape Enigma

John Bradbeer

Members will probably be aware of the Devil’s Stone, which lies at the west end of the square, just outside the churchyard in Shebbear. It represents an enigma taking in archaeology, geology and geomorphology (the study of landforms). The stone itself is at the centre of much folklore, culminating in a ceremony every 5th November, when the stone is turned. This is to flush the Devil out from his
possible hiding place under the stone and failure to do so puts next year’s crops in jeopardy. Another element to the story is that the Devil was escaping from Northlew, where he was in danger of catching his death of cold and to hasten his progress, he dropped the stone in Shebbear. We can probably explain the November timing of the turning of the stone with reference to the Celtic year in which the
first of November, Samhain (pronounced sawin) marks the start of winter. The shift to 5th November almost certainly came about after the Gunpowder Plot of 1605.

The stone itself is around 500 to 700 kilograms in weight and sub-angular rather than round in shape. It is described geologically as a conglomerate orthoquartzite, and to the untrained eye, the shiny quartz grains resemble the quartz crystals in granite. It is in fact a sedimentary rock and current thinking suggests that it is Tertiary in age and thus comparable with the sarsen stones (or ‘grey wethers’) that are found on Salisbury Plain and which, of course, were famously used at Avebury and Stonehenge. It is generally accepted that sea levels were very much higher in the Tertiary period, roughly 5 to 50 million years ago, and many of the succession of erosion surfaces (from c 50 to c 350 metres OD) that give such flat skylines across much of the county were cut at this time. Most of the presumed Tertiary cover of South West England has long since been eroded away, but Tertiary deposits are preserved on the top of Haldon Hill, south west of Exeter and in the down-faulted Bovey Basin in South Devon and here in North Devon in the Petrockstow Basin and the off- shore Stanley Basin near Lundy. Orleigh in Buckland Brewer has a flint gravel deposit presumed to be of Tertiary age and derived from a former cover of chalk. The Tithe Apportionment of 1841 records some fields as ‘Flint Hill’. So geologists can offer a plausible origin for the Devil’s Stone, but what happened to the other survivors from this former Tertiary cover remains an enigma.

In central southern England, besides the sarsens used at sites like Avebury and Stonehenge and incorporated in some of the barrows, there are clusters such as those found in a dry valley on
Fyfield Down, just north of Pewsey in Wiltshire. Geomorphologists can explain such a cluster by reference to solifluction flow during the very cold periods in the Quaternary when southern England was effectively tundra, lying just to the south of the great ice sheets and the summer thaw delivered sufficient water to move soil and sarsens stones down slope. The river terrace gravels along the Solent also contain many smaller fragments of sarsen stone, brought down by the rivers that drain much of Salisbury Plain. But where are the other sarsen stones from North Devon? On Salisbury Plain it is plausible to speculate that early humans found and moved many of the suitably large stones to incorporate in monuments, but in North Devon, there are no megaliths formed of sarsens. North
Devon’s river gravel terraces, of which there may be at least four or five, have never been exploited so no sarsens have been exposed from these. Perhaps there never were as many sarsens here and most were quite small and thus readily transported or fragmented into yet smaller pieces.

The Devil’s stone at Shebbear 

However, the enigma has another twist. For about 750 metres from the Devil’s Stone is another sarsen, rather larger at an estimated 1,500 to 2,000 kilograms, now on the verge in front of Berry House and one that looks far more like potential megalith material in shape. That two such stones should survive so close to each other yet no others appear to have survived anywhere in North Devon requires explanation. Clearly human agency has to be invoked in the survival and folklore attached, especially to the Devil’s Stone, but archaeology and geology have no real explanation as to why there should be just these two sarsens and no others known in North Devon.  (Note from BJ:  could these stones have been carried from a source area near the coast, by an ice tongue pressing inland from the Fremington - Barnstaple area?)

The Berry House erratic, more than twice the size of the Shebbear stone, and weighing in 

at almost 2 tonnes.........

A very silly rant from the pet rock boys

 

Well, this is rather entertaining -- and more than a little pathetic.  Pearce, Bevins, Ixer and Scourse have put together a furious synthetic rant designed to question my competence and destroy my credibility.  It's just been published in Quaternary Newsletter:

Pearce, N., Bevins, R., Ixer, R. and Scourse, J.  2024.  Comment on "An igneous erratic at Limeslade, Gower, and the Glaciation of the Bristol Channel" by Brian John.  Quaternary Newsletter 163, pp 15 - 20.

Also very entertaining is Tim Daw's instant report on his blog.  Ah, the faithful retainer can always be relied on to help his muckers when they are in a spot of bother.  His report, with carefully selected quotes, is flagged up as "the professional response" to my Limeslade article !!  It's quite touching to see such blind loyalty from an amateur.

Maybe you shouldn't be too surprised.  I'm not surprised at all.........

As readers of this blog will know, my short article published earlier this year presented some preliminary information on the Limeslade boulder, including pXRF data kindly provided by the late Prof Tim Darvill, and assessed its importance in the debate about the glaciation of the Bristol Channel.  In the article I recognised the shortcomings of just three readings from one sample from the boulder, and looked forward to seeing more intensive and detailed analyses of other samples from the boulder by other researchers.  I said: "There is inadequate data for the creation of scatter diagrams or bivariate graphs involving the Limeslade boulder ppm readings. So it is not possible at present to say that the pXRF readings occupy a different visualised “compositional space” for trace elements than the readings for the Preseli tors."

Not everybody has access to research funds and top class laboratory facilities, and when Prof Tim Darvill and Dr Steve Parry offered to help in obtaining pXRF readings, I was grateful for their involvement. 

Instead of accepting this preliminary work with good grace as a starting point for future research, Pearce et al  have subjected it to detailed -- and it has to be said, obsessively aggressive and petulant -- scrutiny, while in the process questioning the competence of other geologists whose notes I reproduced word for word.  I am at a loss as to why these four academics have allowed themselves to be sucked into this absurd spat.  They cannot possibly come out of it with any credit.

In the second part of their article Pearce et al accuse me of  "a polemic against the advocates of human transport (e.g. Parker Pearson et al., 2021)".  I strongly refute that.  My assessment of the human transport thesis (on pp 10 and 11) is carefully phrased, and constitutes a straightforward review of the narrative developed over the last decade by Parker Pearson, Ixer, Bevins and others.  Indeed, my comments are supported by the dramatic retreats made by these authors from the spectacular claims they were making just a few years ago.  These retreats (for example on Waun Mawn) are well known to all who read the literature.

The latter part of the Pearce et al article relates to the glaciation / sea ice transport issue, and I take issue with almost everything that they say.  I will revisit that in a later post.  I will not accept snide comments from people who have apparently never done any field work in West Wales relating to the Quaternary stratigraphic sequence.  Nor will I accept a "holier than thou" attitude from geologists who have, over the last decade, refused to cite "inconvenient literature" or to accept that any of their ideas are questioned or disputed by anybody else.

As for their parting shot:  "This article merely represents a disingenuous cover to justify a rehearsal
of the now well-worn and increasingly tedious debate concerning transport of the Stonehenge bluestones."  That really is beneath contempt.  The article I published was fashioned in part by the constructive comments of the journal editor and referees. The "tedious debate" to which Pearce et al  refer has been fuelled and perpetrated by an endless stream of journal and popular science magazine articles which they themselves have written, many of them recycling the same basic data, designed to promote the strange fantasy that the Preseli bluestones at Stonehenge were targetted, quarried and transported by our Neolithic ancestors.

Watch this space........

Details:

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

The article is freely accessible, and can be downloaded here:

https://www.qra.org.uk/quaternary-newsletter/quaternary-newsletter-current/

It is also on Researchgate, and can be accessed here:

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

The Shebbear erratic

https://www.facebook.com/groups/2003881253140331/posts/turning-of-the-devils-stone-shebbear-2024/2559590600902724/

Thanks to Tony for drawing attention to this.  The famous Shebbear erratic lies at an altitude of c 146m and is c 15 km from the Devon coast.  It is likely to have been carried into position by an ice stream travelling from the N or NW.  Over most of its surface it appears to be deeply weathered and abraded, but some breakages are apparent, and these might be of different ages.  They look like glacial facets, since the fracture scars are themselves abraded.  Some damage might of course have been done down through the centuries because of the village obsession of tipping it over every year on November 5th............

There is no reason at all to think that the Devils Stone might have been collected up somewhere on the coast and carried into its present position by the villagers or their ancestors.......  However, there is one tradition that the stone came from Henscott, near Bradford,  to the SW of Shebbear and to the west of the River Torridge.  LIDAR images show that there were a number of small quarries in the area, no doubt used for local building purposes.

At both Shebbear and Henscott the local rocks are fine grained sedimentary rocks of Carboniferous age -- mostly mudstones, siltstones and sandstones.

There are still people who argue that the large glacial erratics in Devon and Cornwall are concentrated in the intertidal zone along the present coast.   This is just not true, as I have pointed out on many occasions.  On at least one occasion there were Quaternary ice caps on Dartmoor and Exmoor, and we know from other evidence that glacier ice from the Irish Sea Ice Stream pressed inland from the Bristol Channel;  but we still do not know where the boundary between local ice and "invasive ice" might have been.

The stone is said not to be local, and it looks to me as if it might be a coarse conglomerate or igneous rock.  Some say it is a gravelly sarsen stone or an "arenaceous conglomerate".  Others report it as being made of "quartz" and others claim it is made of pinkish granite.  It has dimensions c 6 ft x 4 ft and is reputed to weigh about a tonne.  It was clearly once a great deal bigger than it is today-- the facets are substantial, both on the top and bottom of the boulder.

The turning of the stone in 1946

In one of the reports on the stone, it says that there are other similar erratics in the vicinity.  One stone is mentioned c 750m to the north of the village, where the surface altitude is 173m.   I am intrigued............

25th Anniversary -- Kindle celebration

 

This has nothing to do with Stonehenge or the Ice Age, but one of the joys of being a blogger is that one can very occasionally post something quite irrelevant, in the hope that some people out there might be interested.  Anyway, forgive me dear reader -- but here's the deal........

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

For five days, starting on Monday 4th November, we will have a free "anniversary" promotion on the Amazon web site, with the Kindle versions of the first three books in the saga.  You can download the digital or "electronic" versions of the three novels completely free of change.

https://www.amazon.co.uk/dp/B074CFGRJ6?binding=kindle_edition&qid=1730659108&sr=1-1&ref=dbs_dp_rwt_sb_pc_tkin

Just click on the Kindle edition of each book to order your free download.

Enjoy!!

Here's the story:

In 1999 my wife and I flew to Gran Canaria on holiday, and en route I was struck down by aerotoxic syndrome. I felt sick before we landed, but then I experienced classic flu-like symptoms. I went straight to bed when we arrived at the apartment, and spent the night wide awake, feeling very ill indeed. One hears about "a fevered imagination" and now I know what it means. Anyway, a story came into my head - of a feisty woman called Martha Morgan. It was "narrated" -- I can still recall the female voice. Dates, places, characters, and a storyline covering the greater part of her life from 1796 to 1855. Individual episodes came into my head, and I even "heard" key conversations.

In the morning, not having slept a wink, I felt better, but the story was fixed firmly inside my head. I told my wife about this strange experience, and she said “Well then, you’d better start writing!” So I did..... and I knew that the story had to be told in Martha's own words, with immediacy, through diary entries.

Twenty-one years later, I still do not know what to make of that episode. l think that the story was “given” to me, and that in order to keep faith I had -- in the beginning -- to try and put into words the emotions and experiences of a pregnant, suicidal 18-year-old female who lived more than 200 years ago.

The narrative extended, eventually, across eight novels; and I can honestly say that the only one of those that involved the "invention"of characters and narrative was "Guardian Angel". The reasons for that will be obvious to the loyal readers of the Saga.