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Thursday, 28 September 2017

The Greatest British Glaciation in Wales and SW England

The extent of the "Greatest British Glaciation" in the south-western quadrant of the Celtic Ice Sheet (sometimes called the British and Irish Ice Sheet)

This is my latest attempt to map the greatest-ever Pleistocene extent of ice in this area, bearing in mind evidence from a wide variety of sources.  I'll be happy to discuss modifications if anybody has any doubts about what the evidence on the ground actually shows.

When was this?  The jury is still out. The most common assumption nowadays is that the glaciation was very ancient indeed  -- the Anglian Glaciation (MIS 12) conventionally dated to c 450,000 years ago.  However, Phil Gibbard is suggesting nowadays that maybe this was the Wolstonian, which used to be called the Riss or Saalian glaciation on the continent, and which occurred around 200,000 years ago (MIS 6).  Maybe the ice edge shown is a composite one, but this will need to be established by cosmogenic dating such as that undertaken by the BRITICE project over the past few years.

Please note the following:

1.  The ice direction arrows shown are approximate only -- one must bear in mind that there are considerable swings as ice masses wax and wane.  For example, before the Irish Sea Glacier became prominent, the Welsh Ice Cap would have been more extensive, pushing ice further out from the core of the Welsh uplands.  Individual outlet glaciers at such a time would also have been more topographically controlled.  Something similar might well have happened at the end of the glaciation, as the Irish Sea Glacier waned and retreated from its maximim position.  We do not know at present how many phases or pulses of glacier expansion there may have been -- the evidence on the ground is very patchy.

2.     South-central England is shown as a permafrost zone, but there must have bee extensive areas of perennial or semi-permanent snowfields.

3.  It is assumed that the ice extended to the south of the Isles of Scilly and that there may well have been a large calving bay at the ice grounding position, in the Celtiv=c Sea and off the bottom left corner of the map.

4.  I'm assuming that moving ice pressed onto the Cotswolds, into the Bristol and Bath area, over the Mendips, and across most of the Somerset Levels.  I believe that there us reasonable evidence to support the position of the ice limit as shown.

5.  In Devon and Cornwall, it is reasonable to assume that there were small  discrete ice caps over Exmoor, Dartmoor, Bodmin Moor and some of the other uplands.   These ice caps might have coalesced, and they would have been in contact with the Irish sea ice somewhere near the position of the present coastline.  

6.  Note that the coastline is shown here for guidance only.  In reality, at the time of this very large glaciation, sea-level must have been at least 120m lower than it is today. 




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