Enhancing the glacial transport theory

Ice flow reconstructions for the Anglian Glaciation of the Bristol Channel region. 

 After Williams-Thorpe, Kellaway and others.

The other day, on one of those endless social media discussion threads, somebody complained that I was a miserable old git unable to come to terms with the "fact" that the glacial transport theory has been comprehensively dismissed.  Assorted senior professors keep on saying the same thing:  and they have been saying it for the last 20 years.  Anyway, I asked said complainant who had done the dismissing, and he failed to reply.  Have I missed something?  The last serious attempt to dismiss the glacial transport theory, as far as I know, was in 1997, when James Scourse was having a go at Geoffrey Kellaway and citing glaciological theory.  That was 24 years ago.  Since then, nothing.  I suppose the principle is that if you keep on saying "the glacial transport theory has been comprehensively dismissed or disproven" enough times, people will come to think it's true, and you don't even have to cite any sources.

https://brian-mountainman.blogspot.com/2019/05/salisbury-plain-and-impossible-glacial.html

The unfortunate thing about James Scourse's 1997 book chapter was that he was seeking first and foremost to dissect Geoffrey Kellaway's ideas in detail -- some of which were unconventional to say the least. But with respect to glaciation as far east as Salisbury Plain, he did use the word "impossible" -- and that is never a good idea, as Olwen Williams-Thorpe and others have pointed out. 

Since the turning of the century a lot has changed, and a vast amount of research has been published, to transform our knowledge of what went on in the Ice Age.  I have summarised and analysed scores of important articles in this blog, dealing with the geology, geomorphology and glaciology of western Britain and its glaciers.  So what's the state of play?

1.  Evidence of erratics and coherent deposits from Flatholm, the Bristol area, the hills around Bath, the Mendip fringes and the Somerset Levels confirms that ice from the west has penetrated far inland from the Bristol Channel on at least one occasion -- and probably several times. None of this was disputed by Scourse in 1997, although he argued that all of the glacial traces to the east of the Bristol Channel coast were "ice-marginal".   We still do not know where the eastern limit of the greatest glaciation was, but glaciological theory suggests it might be somewhere near the chalk escarpment on the western edge of Salisbury Plain.

2.  Computer modelling by Alan Hubbard and colleagues, James Patton and others have indicated that it is perfectly feasible that ice from the Irish Sea Ice Stream (ISIS) actually reached the Stonehenge area.  They do NOT support Scourse's contention that this was "impossible".  

One of the glaciation models -- this is from Edwards et al, 2017.  Patton et al (2016) ran a whole series of models for the growth and decay of the LGM ice sheet.  Modelling work is still in progress.

3.  The size of the Celtic Sea ice lobe is now known to have been far greater than previously appreciated.  Not so long ago, it was thought that the ice edge rested on the Isles of Scilly, but my own fieldwork chimes with that of the BRITICE team to show that it extended far to the south at the time of maximum glaciation -- all the way out to the shelf edge.  If the ice flowed further to the south than previously thought, it must also have spread further to the east.  Again, that chimes with my own recent work in south Pembrokeshire.

My attempt to portray the LGM extent of ice in the SW part of the British Isles.  In a previous glaciation, we know from the distribution of glacial deposits that the ice was more extensive than this in the inner reaches of the Bristol Channel and elsewhere.

4.  From the work of Prof Dave Evans and others, we now know that Dartmoor supported its own ice cap on at least one occasion during the Ice Age, and if there was glacier ice there, climatic conditions must have been quite suitable for glacier ice to reach Salisbury Plain.  There are abundant erratics (which must be glacial in origin) across SW England, and especially on the coasts of Devon and Cornwall, as itemised in my book "The Stonehenge Bluestones".

5.  There can be little doubt that the bulk of the 43 Stonehenge bluestones have most of the characteristics of glacial erratic boulders and slabs.  Only a few of them can be described as pillars. Apart from a few that have been worked, they are faceted and heavily abraded, and carry thick weathering crusts. They have none of the characteristics of quarried stone.  This matter was not considered by Scourse (1997) when he argued for the human transport of the bluestones.

6.  The fact that most of the Stonehenge bluestones have come from the north slope of Mynydd Preseli in Pembrokeshire chimes with glaciological theory, which predicts that there must have been a "zone of enhanced erosion and entrainment" just in those areas where bluestone provenances have been established by Ixer and Bevins. Research in northern Finland also shows that where overriding ice encounters a hill barrier such as Ounastunturi (or indeed Preseli) erosive capacity is enhanced on the up-glacier side.

7.  It is still true that no "free boulders" or monoliths classified as bluestones have been found across the Stonehenge landscape, except at Stonehenge.  However, there is the Boles Barrow bluestone anomaly which suggests that at least one bluestone boulder was present on Salisbury Plain  long before Stonehenge was built.  There are also thousands of bluestone "fragments" scattered across the landscape, and as pointed out by Olwen Williams-Thorpe and others, it is perhaps too convenient to refer to all of these as "fragments of destroyed monoliths".  Some of the fragments deserve to be referred to as pebbles, and some appear to be in Mesolithic or early Neolithic contexts.  Analyses of these fragments in the past have been bedevilled by assumptions that there was a "bluestone arrival date" ( if not several), and by dubious assignments of fragments to secondary or tertiary sediments or fills, for example in the Stonehenge ditch and in the Aubrey Holes.

8.  Ongoing research into the "clay-with-flints" and other sediments on Salisbury Plain suggests the incorporation of several different  materials, some of which have the characteristics of degraded glacial tills.

9.  Recent work on the glaciation of the Celtic Sea shelf by the Irish Sea Ice Stream has shown that the Devensian ice surface gradient was exceptionally shallow, dropping from +750m in the Irish Channel to +400m in St Georges Channel to -200m at the southernmost ice edge about 500 km away. There must have been substantial lateral spreading and gradient reduction on the ice surface once it was clear of the constriction between North Pembrokeshire and the coast of SE Ireland.  It was proposed by Prof Geoff Boulton many years ago that this was because of the soft and saturated sediment bed on what had been the sea floor, facilitating a high rate of bed deformation, lubrication and basal sliding.  This has been confirmed by subsequent work by James Scourse and the BRITICE team.  So could the ice from the west have surmounted the chalk escarpment on the edge of Salisbury Plain? Since the Plain has a surface altitude of 200m-250m, an ice front capable of surmounting the chalk scarp must have had a surface altitude of c 300m.  That could not have happened in the late Devensian glaciation, but it would have been perfectly feasible in the Anglian, at a time when the whole of Preseli must have been deeply inundated by ice, with a glacier surface at c 750m. The soft sediments of the Somerset Levels would have facilitated ice movement eastwards from the Bristol Channel.

I am not aware of any research (published or unpublished) within the last 24 years that conteracts or contradicts the above points, and I believe that the case for a glacial incursion onto Salisbury Plain (by ice carrying North Pembrokeshire erratics) is stronger now than it ever has been. 

Reproduced from "Stonehenge for the Ancestors" (2020).  The Boles Barrow boulder (or piece of it) now stored in Salisbury Museum.  The argument rages on about whether it ever was at Stonehenge, or ever was in a Neolithic long barrow near the western edge of the chalklands..........

 

The rhyolite pebble that may or may not have come from a Mesolithic context at Durrington

Links:

https://en-gb.topographic-map.com/maps/ddp/

https://brian-mountainman.blogspot.com/2020/10/on-lateral-spreading.html

http://publications.thebritishacademy.ac.uk/pubs/proc/files/92p271.pdf