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Monday, 31 December 2012

Wales Devensian Map revised

This is a high definition map -- click to enlarge...

This is a recent revision of the Wales glaciation map, from many sources.  The valley glaciers are shown in blue -- at the maxima of the glaciations these would of course have been submerged or incorporated into the ice of the Welsh ice cap -- although there would have been some streaming within individual troughs at various times.  The arrows are reasonably good representations of ice movement directions, both for Welsh Ice and for Irish Sea ice.  That having been said, I am still working on the story for the N Pembs coast, and as indicated on earlier posts, I think the ice directions need revision here.

 This is an attempt by Patton et al (2012) to portray the thickness, area and volume of the Welsh Ice Cap during the Devensian.  The "peak" of the glaciation is assumed to be around 23,000 years ago, but because of the complexities of glacier dynamics there is often a lag, and it seems that the ice on the south Wales coasts might have reached its maximum extent around 20,000 BP.  After that, as we can see from the graph, there was a very rapid phase of deglaciation, with huge volumes of ice melting away over the course of a few centuries.

2 comments:

Dave Maynard said...

Brian,

Do the other parts of the country outside of North Pembs have as many questions to be answered about the effects of the glaciation? Say for example, the extent of the Devensian margin south of Shrewsbury, is the evidence less easily seen here than in North Pembs? Not forgetting that it is easier for you to see it here.

You mentioned about a year ago the effects of the Anglian glaciation, being harder to see. Are there undoubted examples in the UK of the effects of the Anglian?

Blwyddyn Newydd Dda

Dave Maynard

BRIAN JOHN said...

Dave -- the most frequently cited actual deposits which appear to date to the Anglian are in the Midlands and in East Anglia -- but many of them are degraded and reworked, and there is still a lot of debate about what is Anglian, what is Wolstonian and what is Devensian. One complicating factor is that some of the areas where Anglian till -- from Scandinavia, and later from the British Ice Sheet -- is found have large expanses of fluvio-glacial deposits well to the south of the Devensian ice margin. Work in progress, shall we say.......

Elsewhere, the evidence is fragmentary. David Bowen and others think that there is an Anglian moraine -- called the Paviland Moraine -- on Gower, but I am not convinced by that designation. There are a few patches of till -- for example at Pencoed, Lydstep and possibly at West Angle -- and elsewhere all we have are erratic boulders, either lying free on the surface or incorporated into newer sediments including raised beach deposits and periglacial slope accumulations -- as at Whitesands, Ogof Golchfa, Abermawr and elsewhere.

So if, at Stonehenge, all we have to tell us of an ancient glaciation are erratics scattered on the land surface, that would not be at all unusual.