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Thursday, 28 September 2017

The Late Devensian Glaciation in Wales and SW England

Late Devensian Glaciation of South-western Britain and the Celtic Sea

Here is my latest attempt to summarise what is currently known about the extent of ice in the Late Devensian, around 25,000 years ago (see Note).  The maximum ice positions may have occurred at different times in different parts of the glaciated area, as a result of surges or pulsing behaviour.  Also, the boundary between Irish Sea ice and ice from the Welsh and Irish Ice Caps must have oscillated in a fashion not currently fully established.
Note the following:

1.   The ice movement directions shown for the Celtic Sea Piedmont Glacier are approx 90 degrees away from the directions assumed by most glacial geomorphologists in their maps.  This is because ice marginal deposits at more or less the same levels (ie just a few metres above present sea level)  in western Pembrokeshire, on Caldey Island and in the Isles of Scilly cannot be explained by a glacier with a gradient running from NE towards SW.  Ice must always move perpendicular to the ice edge except in troughs and other tightly constrained situations.  That means the ice gradient must have run from NW towards SE, as shown on the map, driven from a dynamic and vary large ice dome over Ireland. 

2.  If, as I believe,  glacier ice over-rode Lundy Island and impinged against the coastline of Devon and Cornwall, there is a possibility of a large meltwater lake being trapped within the Bristol Channel.

3.  I have shown local ice caps over the uplands of Devon and Cornwall, and assume that in the Late Devensian they did not come into contact with ice coming in from the NE.

Note added 29.9.2017:  recent papers are suggesting that the glacial maximum occurred around 25,000 years ago -- this pushes the date back by about 5,000 as compared with the widespread assumptions of a few years ago.  Partly this adjustment is related to the advent of new dating techniques, which have supplemented -- and sometimes -- supplanted  evidence based solely on radiocarbon dates on organivc materials.

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