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........