The mystery of MIS-3 and MIS-4

Wogan Cave, beneath Pembroke Castle.  Still being studied -- what will it tell us about the Devensian changes in local climate?

Climatic oscillations over the past 150,000 years.  Based on the Sidestone Press diagram.

In Britain it is quite challenging to match up the archaeological / anthropological records with the Marine Isotope Stages given the numbering MIS3- MIS5.  There are some mis-matches and some awkward fits.  Probably as we should expect.  It is widely believed now that during the Ipswichian Interglacial there was virtually no humanoid presence in the British Isles -- maybe because at the beginning of the interglacial sea-level rise was so fast that the North Sea and English Channel were flooded catastrophically before westward migrations could take place.

Fifty years ago it was widely assumed that the Ipswichian Interglacial ended about 70,000 years ago.  Nowadays that termination date has been pushed back to 116,000 years ago, and there is an acceptance of a full interglacial  (MIS5e) followed by a long "intermediate phase (116,000 - 71,000 yrs BP) of oscillating climate, with several substages including the Chelford Interstadial around 100,000 yrs BP. 

In the following paper by Robert Dinnis most of the emphasis is on human occupation of the British Isles in MIS3, with discussion of the importance of 6 Welsh bone caves, 4 in Pembrokeshire and 2 in the Gower.   In the fascinating Wogan cave, beneath Pembroke Castle, there is some evidence of Ipswichian bone remains, but the main sedimentary sequence speaks of intermittent, low density occupation by humans in MIS3.    There do not appear to be any signs of a full glacial episode in the Early Devensian (MIS4) or mid Devensian (MIS3) which might back up the theory of an ice cover extending as far as Lundy Island at this time (see below).

However, there is still the possibility that the Wogan, like other bone caves in West Wales, was sealed by overriding ice during the LGM.  Robert Dinnis and his colleagues have not as yet expressed a view on this.  

The Early Upper Palaeolithic in British caves: problems and potential
Robert Dinnis
Jnl of the Royal Anthropological Institute
First published: 20 August 2025, pp 1-17
https://doi.org/10.1111/1467-9655.14313

Abstract
Recent years have seen landmark progress in our understanding of early Homo sapienso ccupation of Europe, owing to new excavations and the application of new analytical methods. Research on British sites, however, continues to lag. This is because of limitations inherent in existing cave collections, and limited options for new fieldwork at known sites. Some of these limitations are described here. In the light of this, recent work at the new Early Upper Palaeolithic site of Wogan Cavern (Pembrokeshire) is outlined. Initial observations indicate a significant quantity of intact sediments and high-quality archaeological deposits amenable to modern research methods.

See also:

https://www.wogancavern.org/uploads/1/3/3/0/133004851/dinnis_et_al_2023_cks_wc22.pdf

In Britain, the boundary dates for Marine Isotope Stages (MIS) 3, 4, and 5 align with the standard European Quaternary chronology, though local stages like the Devensian and Ipswichian are used to describe these periods:

MIS 3 (c. 57,000 – 29,000 years ago): A complex, relatively mild (interstadial) period within the Last Glacial. In Britain, this includes the Upton Warren Interstadial (c. 44,000 – 42,000 years ago), a brief warmer phase marked by evidence of Neanderthal reoccupation.

MIS 4 (c. 71,000 – 57,000 years ago): A major cold, glacial and periglacial phase. During this stage, Britain saw significant ice expansion, with evidence suggesting the British and Scandinavian ice sheets may have merged in the North Sea. Further south, permafrost prevailed.

MIS 5 (c. 130,000 – 71,000 years ago): This long stage (almost 60K years) includes the last full interglacial and subsequent cooling phases:

MIS 5e (c. 130,000 – 116,000 years ago): Known as the Ipswichian Interglacial in Britain, the last time the climate was as warm as or warmer than today.

MIS 5d–5a (c. 116,000 – 71,000 years ago): Transitional phases leading into the main glacial. Notable British substages include the Chelford Interstadial (MIS 5c, c. 100,000 years ago) and the Brimpton Interstadial (MIS 5a, c. 80,000 years ago).

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

So how do we move forward in our understanding of the various phases of the Devensian?  One exciting line of research involves the dating of speleothems (mineral precitates including stalagmites and stalactites, associated with constantly running or dripping water).  Most caves linked with running water hold speleothems; over time the process of precipitation and growth can be switched on and switched off as a result of climate change.  The assumption is that during episodes of continuous permafrost  -- for example at the peaks of a glacial episodes -- all water is frozen and immobilised.  Speleothem layers oir periods of rapid accumulation can be dated with considerable accuracy using the Uranium / Thorium (U/Th) method.

The evidence from Welsh caves generally supports ice-free conditions in the early Devensian, though this period was likely a cold, non-glacial tundra environment rather than a warm one.  While the Late Devensian glaciation (approx. 26,000–19,000 years ago) is well-documented as having obliterated much surface evidence, speleothem and sediment records within caves provide a more continuous timeline for the preceding stages.

Key Speleothem Evidence for Ice-Free Early Devensian in Wales:

• Continuous but Low Growth: Uranium-series dating of speleothems in the British Isles shows a period of "low but finite" growth between 90 ka and 45 ka (MIS 4 and MIS 3). This suggests that while it was too cold for the lush growth seen in interglacials, liquid water was still moving through the systems, which would be impossible under a permanent ice sheet or continuous permafrost.

• Cave Sequences as Refugia: In South Wales, coastal cave sequences such as Long Hole and Bacon Hole in the Gower contain Early and Middle Devensian deposits that are not found elsewhere because they were protected from later glacial erosion.

• Interstadial Peaks: Specific peaks in growth at approximately 76 ka, 57 ka, and 50 ka have been identified as indicators of Devensian interstadials (brief warmer pulses). These peaks confirm that ice had not yet advanced to cover these cave-bearing regions during the early part of the last cold stage.

• Ice-Free Uplands: While some geologists previously hypothesised an early Devensian ice cap in the Welsh mountains, the presence of speleothems and lack of unequivocal early glacial deposits suggest that these areas remained largely ice-free until the Late Devensian.

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

The paper by Rolfe et al 2012:

https://brian-mountainman.blogspot.com/2014/07/lundy-island-and-lgm-dilemma.html

The cosmogenic dating results presented by Rolfe et al in 2012:

"Paired 26Al and 10Be exposure ages from Lundy: new evidence for the extent and timing of Devensian glaciation in the southern British Isles"
C.J. Rolfe, P.D. Hughes, C.R. Fenton, C. Schnabel, S. Xu, A.G. Brown
Quaternary Science Reviews 43 (2012) 61e73
https://www.researchgate.net/profile/Christopher_Rolfe/publications

This paper has been hotly disputed by John Hiemstra, Simon Carr and others:

Landscape evolution of Lundy Island: challenging the proposed MIS 3 glaciation of SW Britain
Simon J. Carr, John F. Hiemstra, Geraint Owen, Proceedings of the Geologists’ Association 128 (2017) 722–741

http://dx.doi.org/10.1016/j.pgeola.2017.06.005

A B S T R A C T

Lundy Island, in the Bristol Channel of south-west Britain, holds a pivotal place in understanding theextent and timing of Quaternary glaciations in southern Britain, in particular the timing, extent anddynamics of the Irish Sea Ice Stream during the Devensian glaciation. New geomorphological observations and revised interpretations of geomorphological and cosmogenic exposure data lead to the conclusion that Lundy was not covered by ice in the last (Devensian) glaciation. Geomorphological features are related to surface lowering by means of granite weathering under mainly periglacial and cool-temperate conditions. Previously reported cosmogenic ages are re-interpreted to reflect a dynamic equilibrium of cosmogenic nuclide production and surface lowering during a prolonged period of subaerial granite weathering. This re-evaluation of the geomorphology of Lundy Island challenges recently proposed interpretations of early glacial cover of Lundy (MIS 4-3) and for cold-based ice cover at the Last Glacial Maximum (MIS 2), and instead supports existing regional ice sheet reconstructions. This study demonstrates that a robust, coherent geomorphological framework is fundamentally important to support the validity of detailed geochronological and stratigraphic investigations.

https://brian-mountainman.blogspot.com/2017/12/more-fun-and-games-on-lundy-island.html

In short, the jury is still out on a range of issues...........

Cosmogenic nuclide dating of rock surfaces in West Wales

BRITICE map showing the suggested exposure time since ice sheet inundation across the UK.  Note that in Pembrokeshire, it is suggested that north Pembrokeshire was glaciated during the LGM, but south Pembrokeshire was not.  That is the hypothesis that needs to be tested.

It's good to be a part of this research group.  This is Alissa's short presentation for a recent geomorphology conference in New Zealand.  Alissa, Prof John Hiemstra, Prof Iain Robertson and I collected numerous rock samples in early June which are currently being processed prior to analysis.

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

Using multiple cosmogenic nuclides to constrain the British-Irish Ice Sheet in Pembrokeshire (Wales) during the Last Glacial Maximum (LGM) 

Dr Alissa Flatley, Dr John Hiemstra, Dr David Fink, Dr Keith Fifield, Dr Brian John, Dr Reka Hajnalka Fülöp


Dr Alissa Flatley, February 5, 2026


There is considerable debate surrounding the southern limit of the British Irish Ice Sheet (BIIS) particularly in Pembrokeshire, Wales. This is partly due to differing interpretations of the sedimentary and geomorphic record, but a lack of detailed geochronology of regional LGM ice extent is also a major barrier. In Pembrokeshire, proposed LGM ice limits trace the northern flanks of the Preseli Hills suggesting that southern Pembrokeshire may have remained ice-free. However, significant uncertainty remains regarding the interaction between the Irish Sea Ice Stream and the Celtic Sea lobe, and the maximum extent of the Welsh Ice Cap. This project addresses these uncertainties by using multiple in-situ cosmogenic nuclides 10Be, 36Cl and 14C in lithologically varied samples across Pembrokeshire to further our understanding of the region's deglaciation history. 

Cosmogenic nuclides can determine the maximum time a bedrock surface or glacial erratic has been exposed. However, it is possible that exposure may have been intermittent over this time (e.g. complex exposure history) or that the previous exposure signal has been retained (i.e. it has not been removed through erosion under the most recent ice advance), an effect called ‘inheritance’. The issue of nuclide inheritance within samples can yield anomalously ‘old’ exposure ages common in more erosion resistant lithologies prevalent across the Pembrokeshire coastline and therefore it acts as a key scientific constraint surrounding LGM ice sheet reconstruction. Cosmogenic nuclides with a short half-life (e.g. in situ 14C) will be integrated into the project to date short timeframe changes in ice dynamics. For glaciations < ~ 20-30ka, in situ 14C, with a half-life of 5.7 kyr, can be used in conjunction with longer-lived nuclides such as 10Be to identify inherited signatures in samples. This project will provide a much-needed understanding of ice sheet dynamics across the region, advancing understanding of complex landscape processes.

Book of abstracts: 3ce3bf9f40984a6ab3bfc62b2a194cd5.

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

Iain, John and Alissa sampling bedrock at one of the north Pembrokeshire sites.

Dr Iain Robertson -- about to take a pXRF reading on the Abermawr erratic boulder

The other scenario.  In this BRITICE model (from Clark et al, 2022) LGM ice fills the Bristol Channel and affects the coasts of Cornwall, Devon and Somerset.  Note that this reconstruction applies to the Late Devensian only -- it does not preclude a more extensive ice cover during earlier glacial episodes. 

The proposed ice-free corridor at the time of the LGM -- as featured in numerous publications.  This is what the new project seeks to test through a substantial sampling programme.

The ice-free corridor as proposed in some of the early BRITICE work..........

A more realistic proposal for the extent of LGM glaciation in the South Wales / Bristol Channel arena.  Here the inner reaches of the Bristol Channel are essentially ice-free, with the depression occupied instead by a large pro-glacial lake..........

For a detailed study of the problem, see this paper from 2023:

https://www.qra.org.uk/mp-files/qn158_1_late-devensian-ice-free-corridor-in-pembrokshire.pdf/

John, B.S. 2023. Was there a Late Devensian ice-free corridor in Pembrokeshire? Quaternary Newsletter 158, pp 5-16.

Above pics, thanks to Alissa.  John and Iain at work at Maiden Castle, Trefgarn Gorge, in June 2025.

Me at the Roch Castle rhyolite crag.  Thanks to Alissa for the pic.  Don't worry --we had permission.......!!

The Meaden Cobble -- probably from Somerset?

The Meaden Cobble -- a clast of dark-coloured Carboniferous Limestone, probably from one of the Black Rock Limestone outcrops in Somerset

Further to my earlier post, we now have more information on the "Meaden Cobble" found near the West Kennet long barrow. 

https://brian-mountainman.blogspot.com/2026/01/a-black-limestone-cobble-from-west.html

In the opinion of my colleague John Downes: 

 "I think that it is likely to be a bit of Black Rock Limestone from the Mendip region. It clearly has a substantial proportion of carbonate mud and hand lens examination shows a grain supported structure which suggests a packstone according to the Dunham classification. Possibly fragmented crinoids and shell fragments. The network of calcite micro veining shows the rock has been minutely fractured during orogenesis and subsequently calcite has been precipitated. I scraped a small vein with a penknife which produced a powder. Hence this is not quartz which would have been too hard to scrape. I could not get much reaction with white vinegar but then the specimen is muddy and the vinegar was probably not strong enough to produce effervescence. A thin section examination might be more revealing." 

THE BACKGROUND


• Black Rock Limestone: well-bedded, poorly sorted, dark grey to almost black crinoidal limestone with chert nodules. Clifton Down Group: calcitic and dolomitic, oolitic and bioclastic sediments. Hotwells Group: massive fine-grained, smooth-textured calcitic muddy limestone with corals, ooliths, and brachiopods.

Possible locations: Burrington Coombe, Avon Gorge, Cheddar Gorge

It occurs in Pembs too:

Pembrokeshire, the Black Rock Limestone (technically the Black Rock Limestone Subgroup) occurs primarily in the southernmost part of the county. It is part of the broader Pembroke Limestone Group and is well-exposed along several key coastal areas:
• South Coastal Cliffs: It forms the base of the spectacular cliffs between Tenby and Frainslake Sands. You can see it at major landmarks like St Govan’s Head, Stackpole Head, and Lydstep Point.
• Tenby Area: It is famously associated with the Blackrock Quarries near Tenby(located near what is now Kiln Park), which historically supplied limestone to local lime kilns.
• West Angle Bay: The rocks here include fine-grained grey and dark limestones (~350 million years old) that are part of this succession, containing abundant fossils of crinoids and corals.
• The "North Crop" Outcrop: A narrow, intensely quarried band of this limestone runs inland from Carmarthen Bay through areas like Ludchurch and Templeton, eventually narrowing and dying out south of Haverfordwest.
• Caldey Island: The northern half of Caldey Island is a geological extension of the limestone bands found on the mainland near Tenby and Penally.
Identifying it in the field:
In Pembrokeshire, while the weathered surface of these cliffs often appears light grey, the "fresh" or broken surfaces are typically much darker or black due to their organic content. If you visit the Castlemartin Firing Range (home to the Green Bridge of Wales), you are walking directly on these Carboniferous formations.

Gower too…….

In Gower, the Black Rock Limestone (specifically the Black Rock Limestone Subgroup) is exceptionally well-exposed along the south coast, particularly where the oldest Carboniferous rocks are brought to the surface by geological folds.
It occurs in several prominent locations:
• Rhossili and Worm's Head: The jagged rocks on the beach below the Coastguard Lookout and the causeway leading to Worm's Head are made of this limestone. Here, you can see the strata dipping steeply as they form the "core" of the local anticlines (upward folds).
• Three Cliffs Bay: It forms the striking, sea-dipping slabs on the western side of the bay. This area is a type section for the Penmaen Burrows Limestone Group, which is the local name for the sequence that includes the Black Rock Limestone.
• Mewslade and Fall Bay: While the upper cliffs are often younger formations (like the Gully Oolite), the base of the cliffs and the surrounding foreshore between Mewslade Bay and Fall Bay contain exposures of this subgroup.
• East Gower: It crops out along the limbs of the Cefn Bryn Anticline and forms much of the dramatic coastline from Oxwich Point through to Mumbles Head.
Notable Features in Gower:
• Fossils: These rocks are famously bioclastic, containing abundant remains of crinoids and brachiopods like Chonetes.
• Calcite Veins: At Rhossili, the dark limestone is often cut by bright white calcite veins and red haematite infills, creating a high-contrast "striped" appearance on the rock face.
• Superficial Colouring: Interestingly, while the rock is naturally dark grey/black, researchers note that the intense black appearance on some Gower cliff faces is often enhanced by a bio-film of black lichen or cyanobacteria.

Pembs differences — deeper water and so more detrital material / darker colour?

The fossil assemblages in the Black Rock Limestone of Pembrokeshire and Somerset are remarkably similar because both regions were part of the same shallow, tropical marine shelf during the Lower Carboniferous period.
However, there are subtle differences in preservation and diversity due to the specific local environments (facies) at the time of deposition:

The Core Similarities
In both locations, the limestone is bioclastic, meaning it is primarily composed of the broken remains of marine organisms. You will find:
• Crinoids: These "sea lilies" are the most abundant fossil in both areas, often occurring in such high volumes that they form "crinoidal packstones".
• Brachiopods: Rugose and chonetid brachiopods (like Rugosochonetes vaughani) are standard index fossils for this layer in both regions.
• Zaphrentoid Corals: Small, solitary horn corals (e.g., Zaphrentites delanouei) are characteristic of the lower parts of the succession in both Somerset and Pembrokeshire.

So for the moment we have a pretty robust suggestion from two geologists as to the possible origin of the cobble. The fossils may hold the key. We seem to be looking at impressions rather than actual organic or mineralised remains, but I will see what assistance we can get from experts in the field……….

Could this be a glacially transported clast?  Yes, it is possible.  Long ago Kellaway claimed that he had seen ancient glacial deposits near Bath, and that he had also seen a degraded diamicton on the edge of one of the stone quarries near Nunney and Frome.  If the Meaden Cobble really is an erratic, it could have been transported by ice travelling west - east, or SW to NE.  

The West Kennet granidiorite boulder (or boulders) could have travelled in the same direction -- but this would go against the speculation by Ixer and Bevins that it has travelled southwards from a source in NE England.    They and other colleagues assume that the boulder has come from Cunyan Crags. but that is based on the questionable assumption that erratics always come from upstanding craggy outcrops.  That goes against the current thinking in glacial geomorphology.......

There is certainly a puzzle here, largely because of the difficulty of discerning ancient ice movement directions in the field., where things are dominated by evidence from the LGM.

Watch this space.........

The Kokelaar Guide to erratic pebbles

The info below is extracted from the following excellent resource: 

https://kokelaargower.com/towards-stonehenge-the-anglian-glaciation-of-gower/

The pebbles illustrated below have all come from Gower raised beaches -- derived from many different places.  I'm particularly struck by the Ramsey Island connection -- as flagged up here by somebody who knows the igneous rocks of that island particularly well.  Some of these look very familiar -- I have seen them in abundance on the Pembrokeshire beaches, and also on Flat Holm in the middle of the Severn Estuary.

Silicic volcanic and shallow-intrusive rocks -- probably mostly from Ramsey Island

Mafic intrusive and extrusive rocks -- mostly from NW Pembrokeshire and maybe Skomer island

Diverse igneous erratics -- some from North Pembrokshire coast, Skomer and possibly Scotland

Silicic and intermediate intrusive rocks -- from Ailsa Craig and other distant sources

For further details on the provenances and rock types, please consult Peter's blog -- link as above.

For comparison, here are some of the erratic pebbles collected on Flat Holm in 2014:

The most abundant beach pebbles on the island are made of Carboniferous limestone and ORS, but igneous pebbles are also found on every beach on the island

The Contentious Reach

 As mentioned in a previous post, I really like an idea put forward by Prof Peter Kokelaar in his blog -- namely the idea of the "contentious reach".  This is the ill-defined area to the east of the Bristol Channel coasts of Cornwall, Devon and Somerset in which the evidence of Quaternary environmental change is quite difficult to interpret.  It is also the area which is now crucial to the debate about the transport of the bluestones that are present in the stone setting of Stonehenge.  I have discussed the evidence from this area on many occasions in this blog, for example in this post:

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

Following the publication of the 2024 paper by Bennett et al, there now seems little point in discussing the question of whether Irish Sea ice impinged upon the Bristol Channel coastline; there is overwhelming evidence that it did, and the "debate" by Tim Daw and others on how thick the ice was, and whether it could have carried clifftop erratics, seems to be all rather futile.  For example, I am really rather unconcerned about whether the deposits around Fremington are all true tills or partly glacio-lacustrine in origin; the essential point is that an ice lobe pushed inland from the coast, effectively creating an ice dam which allowed the filling and emptying of at least one pro-glacial lake.  Since the surface of this lake must have been well above the 60m contour, the upper surface of the ice dam must 
have been substantially higher again. Did it lie at +80m? Or perhaps at +100m? Who cares.........

Bennett, J. A., Cullingford, R. A., Gibbard, P. L., Hughes, P. D., & Murton, J. B. (2024). The Quaternary Geology of Devon. Proceedings of the Ussher Society, 15, 84-130.
https://ussher.org.uk/wp- content/uploads/benettetal1584130v2.pdf

So let's accept that the edge of the Irish Sea Ice Stream, on at least one occasion, more or less coincided with the cliffline of the western coasts of Cornwall, Devon and Somerset. This is a perfectly straightforward scenario, given that the cliffline is a substantial natural barrier and given that on at least one occasion the Scilly Islands were covered with glacier ice:

https://www.researchgate.net/publication/328413421_Evidence_for_extensive_ice_cover_on_the_Isles_of_Scilly 

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

Let's for the moment forget about the debate surrounding the extent of Late Devensian (LGM) Irish Sea Ice Stream ice.......  and let's forget the claim (based on zero evidence) that the large erratics found in places on the coast have been ice-rafted at times of high sea level.

So what do we know about the Quaternary record inside the "contentious reach"?  Well, the evidence seems pretty convincing that there were at times small local ice caps on Dartmoor, Exmoor and Bodmin Moor, and possibly small localised snow domes on some of the smaller hill masses.  Candidates would be the Quantocks, Brendon Hills and Blackdown Hills. Stephan Harrison has suggested that there were a number of small "plateau icefields" which supported thin and more or less immobile ice in a landscape of extensive snowfields and permafrost.

https://dartmoorsociety.com/pastevent/the-glaciation-of-dartmoor/#:~:text=The%20Irish%20ice%20cap%20did,Southwest%20England%20was%20not%20glaciated.  

The Mendips might be looked on as a special case -- and I have examined (in previous blog posts) some of the signs that suggest dramatic meltwater flow and a periodic ice cover.

https://brian-mountainman.blogspot.com/2018/10/on-significance-of-mendip-for.html

Then we come to the position of an ice edge coinciding with the Somerset coast of today. It is highly unlikely that the ice edge ever sat in this position for an extended period, since during glacial episodes the coastline was removed far to the west, somewhere near the -120m seabed contour.  Dynamic or active ice will always seek to fill pre-existingt depressions in preference to seeking to surmount inconvenient obstacles. I will hazard a guess that no glaciologist would argue with the point that if glacier ice reached Kenn, Gordano Valley and other locations near Bristol up to 25 km inland from the coast, it must also have filled the Somerset Levels depression.  We know that a diamicton widely interpreted as a till exists at Greylake, c 15 km inland of the present coast, and it is in my view inevitable that ancient tills will in due course be discovered beneath thick peat and other sediments in many other west Somerset locations.

In other words, the famous Gilbertson and Hawkins map of 1978 is not far wide of the mark..........

  

What about areas to the north -- around Oxford and the Thames valley?  Well, glacial clay deposits have been identified in the "wooded plateau" area of the Chilterns: isolated plateau areas to the north-west of the Chilterns Escarpment around Brightwell, Grove and Oakley Wood.  

https://owls.oxfordshire.gov.uk/wps/wcm/connect/occ/OWLS/Home/Oxfordshire+Landscape+Types/Wooded+Plateau/

The "plateau drift" or "northern drift" also occurs quite widely on old hill summits and plateau remnants to the NW of Oxford, in the Evenlode Valley, and also in isolated patches to the south of the city and to the south of the Thames river.  The evidence is disputed, but I recall being taken on a field trip by Dr Kenneth Sandford way back in the 1960's -- on which I was very convinced that the "plateau drift" is (at least in part) a genuine in situ glacial deposit which can only have been dumped by ice flowing down from the north or north-west.  Others interpret the plateau drift as a pre-Anglian fluvial deposit extending as far south as southern Berkshire and marking a very early course of the River Thames.  In a comprehensive study of the "plateau drift" in 1980, Shotton et al concluded that it is a complex deposit of great age, including reworked and fluvially redistributed gravels, some of which have been "decalcified" and subjected to mechanical and chemical alteration over many millennia.  But crucially they determined that some of the deposits analysed could not be explained other than by glaciation of the Oxford region.

https://www.researchgate.net/publication/247604991_Cromerian_Interglacial_Deposits_at_Sugworth_Near_Oxford_England_and_Their_Relation_to_the_Plateau_Drift_of_the_Cotswolds_and_the_Terrace_Sequence_of_the_Upper_and_Middle_Thames#:~:text=are%20drawn:-,1.,Terrace).

In their consideration of the "Ardleigh erratic" found in river gravels not far from Colchester, Rose et al (2010) were in no doubt that there was at least one glaciation of the upper Thames catchment in the region of the Cotswold Hills.  The glaciation was associated with the Bruern Till, and must have involved ice that originated in North Wales.  Bridgland and others who have also studied the Upper Thames terraces have also agreed that there are many locations where the high erratic content in the gravels indicates derivation from destroyed or undiscovered early glacial till deposits -- in other words, from the "northern drift" or "plateau drift"..........

https://nora.nerc.ac.uk/id/eprint/11416/1/Ardleigh.pdf

https://brian-mountainman.blogspot.com/2022/02/a-very-erratic-essex-erratic.html

If we wish, therefore, to take the idea of the "contentious reach" seriously, we would include within it the Upper Thames Basin, the Cotswolds, the Mendips, the Severn Esuary and lower Severn Valley, the Newport - Gloucester area, the  chalk downs of Wiltshire and Salisbury Plain, and much of Somerset, Devon and Cornwall.

Based on the Gibson and Gibbard map, I have designated the area within the dashed blue line as "the Contentious Reach"...............

All in all, it is certain that there are many incontrovertible traces of glaciation within the Contentious Reach.  The size of this disputed area will inevitably shrink as more and more evidence is assembled.

Updated marine isotope stages (MIS sequence)

In some of the recent articles I have been reading I have seen a few anomalies in the use of the MIS labels.  This makes things somewhat confusing.  So I asked the Google AI search facility for an updated listing of the MIS sequence as used in recent publications,  with British labels attached.  Worth sharing.  Here it is.

I am particularly intrigued by MIS 10 -- a glacial episode which might be represented in the South Midlands.   But it seems to have no name.   More on this in due course........

Erratic paradise

 

One of the great joys of our holidays in the Stockholm Archipelago is the exploration of small islands and skerries by kayak, discovering erratic conglomerations like the one above.  In places there are exposed, smoothed and striated rock outcrops visible along the shoreline, and in places the shoreline is covered with accumulations of erratics that have come from multiple locations.  There are very few morainic constructional features that one might refer to as terminal or recessional moraines.  The litter is everywhere -- made very visible and prominent because the whole landscape is "washed" as a result of ongoing isostatic uplift or recovery.  Many of the low skerries have literally emerged from the sea within the last few centuries, as evideenced by intermittent raised shorelines of cobbles and gravels.

This whole archipelago was deeply submerged beneath the ice of the Scandinavian Ice Sheet on several occasions during the Quaternary.  There was a great deal of areal scouring and very little erosion by identifiable ice streams -- although it is clear that basal ice did move more rapidly in places in response to topographic irregularities on the bed.  For example, the deep channels between the islands of Blidö and Yxlan must have enouraged streaming flow and must have led to localised variations in the direction of basal ice flow -- which was overall directly north to south.

Satellite image of part of the Stockhgolm Archipelago.  Ice movement here was north to south, but on a small scale there were variations in ice flow directiuons influenced by old valleys, hills and other topographic details

In all of my coastal observations I have never been able to trace a true earratic train for more than a few tens of metres.  The abiding impression is that every boulder and cobble has a unique history -- sometimes moving, sometimes stuck, sometimes travelling directly southwards and sometimes in zigzag fashion dependent on topographic controls and oscillations in basal ice temperatires and flow characteristics.  And while, during any given glaciation, there must have been entrainment of fresh blocks on the glacier bed, many blocks such as those in the photo above must have been moved, embedded, and moved again over a series of glacial episodes.  

It seems to me that this process of entrainment and debris recycling goes on regardless of where the glacier snout is positioned.  This is an important point, since it means that in any given glaciation of the Bristol Channel old erratics can be stuck or dumped virtually anywhere, and new erratics can be entrained almost anywhere, right up to the ice edge as long as the ice continues to be active.  There are a number of different mechanisms.  For further details, see the standard texts............

In response to this constant process of entrainment, dumping, recycling and renewal, the majority of erratics on any ice sheet bed at any given time will be rather local, some will have travelled modest distances (tens of kilometres) and a few will be far-travelled (maybe hundreds of km).

This generalisation confirms what I have observed in the Quaternary sites around the Pembrokeshire coast.  It's rather futile to do a numerical / statistical analysis on this, because local circumstances vary so much, but in till (or on storm beaches such as Abermawr, where almost everything is derived from destroyed glacial deposits) MOST of the clasts are local, SOME of the clasts have come from more distant sources (such as Ramsey Island), and A FEW have travelled really long distances (such as Ailsa Craig).  

Newgale storm beach, where the great majority of stones have come from destroyed glacial deposits during the inexorable advance of the sea across the land surface.

As for the shapes of boulders and smaller stones in these three categories, they are not greatly dependent upon distance travelled.  A rockfall boulder deposited on a glacier surface will retain its angular edges for maybe hundreds of kilometres so long as it is not incorporated into the glacier mass and subjected to abrasional and other processes.  In contrast, a boulder entrained near a glacier snout can be heaily abraded if it is stuck on the glacier bed for decades or centuries with ice flowing over and around it.  Great care is needed in the interpretation of ALL clasts, including sharp-edges boulders and those that are heavily abraded and striated.........