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Monday, 27 August 2012

British Glacial Limits

 Proposed glacial limits for the Anglian and Devensian Glaciations in Southern Britain 
(BSJ, Aug 2012)

Here we are.  This is my latest attempt at portraying the British Glacial Limits for the Devensian and Anglian glaciations -- along the southern margins of the British and Irish Ice Sheet.  I think it accords pretty closely with the evidence on the ground and with the glacial modelling which I have covered quite extensively on this blog over the past couple of years.  Current dating puts the Anglian at around 450,000 years ago, and the Devensian at about 20,000 years ago.  There may well have been another glacial episode between these two -- currently referred to as either the Saalian or Wolstonian Glaciation.  Many authors have attempted to map the limit of that glacial episode -- broadly, it seems to have been more extensive than the Devensian glaciation, and less extensive than the Anglian.  However, the line drawn by Gibbard and Clark is so unsatisfactory, in so many ways, that I have left it off my reconstruction above.  Let's just say for the moment that the events -- and the deposits -- of that episode are complicating factors, and that they will one day get sorted out........

Let's concentrate on South Wales and Southern England.  Note that on the map I have shown the Anglian and Devensian ice edges in more or less the same position on the north coasts of Devon and Cornwall, assuming that the cliff barrier was sufficient to prevent any great ice incursion inland during either of the glacial episodes.  In both glaciations the Irish Sea Glacier affected the Bristol Channel; in the earlier one the ice was so powerful that it pushed all the way eastwards into Somerset and maybe into Wiltshire, deeply inundating the whole of Pembrokeshire in the process; but in the Devensian the ice was more limited in extent, affecting Carmarthen Bay but maybe failing to penetrate further east than the Gower Peninsula.

Map showing the proposed extent of the Anglian Glaciation in Southern Britain, with approximate flowlines.  On this map the small ice caps and perennially snow-covered terrain of SW England are shown as being incorporated into the glaciated area.  In reality the margin between Irish Sea ice and locally generated ice caps would have been very difficult to discern.  The ice caps over Exmoor, Bodmin Moor, Dartmoor, the Blackdown Hills, the Mendips and the Cotswolds are shown with the dotted symbol and identified by letters.

One or two points relating to the Anglian.  The ice direction arrows are now pretty reliable, having been thoroughly established over many years by reference to striae, erratic transport, and glaciological modelling.  That having been said, there must have been a very complex contact zone in the South Wales coastlands between Welsh ice streaming southwards from the Welsh Ice cap and Irish Sea ice pressing in with great force from the north-west and west. The evidence for pushing the glacier into Wiltshire?  Well, glacial deposits on the Somerset coast, into the Somerset Levels and on the flanks of the Mendips, and glacial erratics on Salisbury Plain.  This very extensive ice cover is also needed to provide a satisfactory explanation for the giant erratics (apparently ice rafted) on the English Channel coasts. It's all to do with isostatic depression. See many earlier blog entries on this.   If anybody doesn't like this reconstruction, please give me a better explanation of these large coastal erratics......

One feature which is crucial to my reconstruction is the difference between the western and eastern pro-glacial zones -- ie the areas beyond the Irish Sea Glacier edge.  I think that in the west, in the counties of Cornwall, Devon and Somerset, there was considerable snow and ice accumulation during each glacial episode, giving rise to many small local ice caps and perennial snowfields.  David Evans, Stephan Harrison and colleagues have given us a good examination of one of these -- the Dartmoor Ice Cap.  Further to the east, the air temperatures and ground temperatures were lower, and there was not so much precipitation -- so the landscape was essentially one affected by frozen ground or periglacial processes.  The boundary between these two zones was probably in Wiltshire and Dorset.

A reconstruction of the Celtic Sea Piedmont Glacier during the Devensian maximum.  The map excludes the complex contact zone between Welsh Ice and Irish Sea Ice in South and West Wales, and the ice cap / perennial snowfield zone of Devon and Cornwall.

One of the key features of my Devensian map is the ice margin in the outer part of the Bristol Channel.  I think that the outer edges of the Pembrokeshire Peninsula were glaciated in the Devensian, since we find fresh glacial deposits near Dale, at West Angle Bay, and on the island of Caldey.  On Caldey the ice was moving pretty well west to east.  Maybe the ice margin was a little further eastwards, in Carmarthen Bay.  There is still doubt about the events on Gower -- I would appreciate input from others on this.  So was there effectively an ice dam across the Bristol Channel?  As the ice started to melt, could there have been a great glacial lake here?  There might have been -- and a reexamination of all of the submarine materials in the channel might give us some clues on this.  The Devensian ice also reached the Scilly Isles -- as established by James Scourse and others, with the aid of radiocarbon dating evidence.  However, we have to explain the very similar altitudes of the ice marginal deposits on all of these coasts, from St David's Head to the Scillies -- and if you reconstruct the surface contours of the Irish Sea / Celtic Sea ice mass, the contours must run broadly parallel with the ice edge.  That means that the ice must have been moving predominantly from the NW towards the SE.  I part company with most other geomorphologists on this -- they want ice in the Celtic Sea to be moving from NE towards SW, and they also want an ice surface gradient which is so low that it seems to me to defy the laws of physics.

So there we are, boys and girls.  That's my current best explanation of the situation on the ground.  Now let's test it to destruction........

12 comments:

chris johnson said...

Very interesting posts in recent days. So SH is a few miles east of the BGL in the Anglian or is the black square supposed to represent SH? I wonder if the periglacial area is capable of transporting 4-5 ton stones?

From some of your iceland pictures it looks like glaciers can have ragged edges.

MPP seems quite convinced that the avenue stripes are periglacial... Has he convinced you?

BRIAN JOHN said...

Thanks Chris. I'm not too bothered if Stonehenge is inside the glacial limit, on it, or outside -- there is still a requirement for erratics to be gathered up over a certain radius, and brought to the site. So I do not deny the "collecting ability" of the builders of Stonehenge. Indeed that has always been a part of my working hypothesis.

Glaciers can indeed have very ragged edges -- it depends on the terrain.

Periglacial slope processes can indeed move big stones, but only straight down a slope under the influence of gravity.

MPP may well be right -- but I reserve judgment until I have seen some evidence. Charlie French presumably has it, but it has never been published, to the best of my knowledge.

Myris of Alexandria said...

Very nice Brian.
One worry the incursion onto Salisbury Plain is based on 'erratics found on Salisbury Plain'-are these your erratics namely the SH stones or other people's data.
Glacial deposits in N Somerset/Mendips/Scillies are attested by others?

I have looked at the temper from the pots beneath Trefael- nothing from any lithology from SH (I wanted 'sacred Rhos-y-felin rhyolite') but perhaps erratic material. Not a lithology I have seen before but may be local litharenite. Your erratics not yet prepared.
Rob

BRIAN JOHN said...

Myris -- yes, I am citing the erratic assemblage in the Stonehenge area as evidence of glaciation. Why should I be defensive about this? The evidence is staring us all in the face -- and as I have often said, the assemblage consists of so many rock types flung together in a relatively small area that the "human transport" hypothesis looks absurd 9at least from where I stand.) Also, the Stonehenge area is incredibly intensely studied -- and it is perfectly reasonable to think that there could be similarly interesting and similarly dense concentrations of foreign material in other locations nearby. That having been said, of course I think that the builders of Stonehenge did scour the countryside for usable boulders, and brought them to their building site -- up to the point where all the "nice" boulders were used up, or the cost-benefir tatio on going further afield did not make sense any longer.

As for the glacial evidence from Devon, Cornwall,Somerset and the Mendips, there is a huge literature. Many researchers over many years. Just look at the big Geological Conservation Review volume -- it's a good synthesis of the evidence on the ground and under the surface. But it's very cautious and -- dare I say it -- somewhat dated in that there was very little consideration by the authors of glaciology.

Trefael -- ah yes, I must try to pay a visit....

Myris of Alexandria said...

I am not asking you to be defensive-you are most entitled to draw your lines anywhere across your map!!
I was hoping that someother inland Wessex glacial deposits were known. It would have made the argument seem slightly less circular.
However, I finally now belief the big dropstones as I can see the glaciers reach76 the Western Approaches.
Do visit! There is an interesting story in the Nevern Valley.
M.

BRIAN JOHN said...

Of course I was talking to myself rather than to you, Myris! And yes, it would be very nice if we could find unequivocal glacial deposits elsewhere in Wessex. The evidence gap -- we would all like just a bit more than we actually have ........ such is the nature of ongoing science.

Tony H said...

MPP said at the dig near Marlboro' that there will be an article appearing in the Antiquaries' Journal [if I've given it the wrong name I apologise -Myris of Alexetcetc will correct me - not a good day for me today]. Remains to be seen HOW MUCH detailed info it contains. There are supposed to be various Papers appearing on individual subjects (such as periglacial processes near Heel Stone & Avenue) as time goes on - Myris, what say you?]

Myris of Alexandria said...

I am still trying to complete my portions of the MPP monographs for the Riverside project.
I do not think that I am involved in other things.
There is suposed to be a PPS paper with all involved.
Close enough 'The Antiquaries Journal' Not certain they include the Saxon Genitive (but should.
No I cannot help here.
M

Tony H said...

CHARLY FRENCH [as he likes to spell his Christian name] was one of the two members of the SRP MPP team given the credit for identifying the periglacial stripes (see Chris & Brian's comments at top).

I've just attempted to check out Charly French: still linked to University of Cambridge Archaeology, but, although I traced a list of his publications, latest 2010, no mention there of this revelatory discovery on Stonehenge's Avenue.He has worldwide geoarchaeolology interests it seems.

We did discuss him a year or so ago and this can be brought up via "Search" easily enough.

There seems to be a website which requires you to sign up for a Password. This website may reveal more about Monsieur French.I think I've succeeded doing this earlier.

Incidentally, he got his first Archaeology Degree at Cardiff. Josh Pollard and MPP also have past links with Cambridge; Josh Pollard has also been at Univ. of Wales, Newport.

BRIAN JOHN said...

From Lionel Jackson, who is having problems getting comments onto the blog:

With regard to past ice on the Salisbury Plain, I am still troubled by the lack of lithologies exotic to the Avon basin in the pebbles of its river terraces. The terraces go back to the middle Pleistocene but do not appear to have been proglacial based upon their content of pebbles that reflect the present boundaries of that basin. How do you account for this?

I was heartened recently to learn of the evidence for the past glaciation of Dartmoor. It certainly supports your overall thesis of more extensive past glaciation.

BRIAN JOHN said...

Lionel -- we have discussed the terrace gravels of the Rover Avon and other catchments already on this blog, in some detail. See here:
http://brian-mountainman.blogspot.co.uk/2010/04/gravels-pebbles-and-bluestones.html

I also suggest you do a search on the blog, entering "Chris Green" and "terrace gravels".

Frankly, I don't accept Chris's terrace gravel interpretations, since they seem to be based upon a search for white quartz pebbles as indicators of "foreign" material on Salisbury Plain. Small igneous fragments or shales or sandstones (for example) would be very difficult to pick up in a sample of 50,000 pebbles -- I know. I have counted pebbles too in Pembrokeshire, and I would never dream about making a definitive statement about the presence or absence of something on the basis of such work.

BRIAN JOHN said...

Another from Lionel Jackson: It sounds like the terraces would make a good thesis project for a student with an eye to testing the hypothesis. The cost in this cash-strapped times would be food, lodging and gas.

Another thought: LIDAR immagery would be a great tool for bringing out older glacial features. I suspect that much of England likely has been scanned.