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.
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.
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.
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.
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"..........
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.
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