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Friday, 24 February 2012

The Early Devensian Welsh Ice Cap


I have been having a little discussion with Henry Patton about the likelihood of an Early Devensian Welsh Ice Cap growing sufficiently to extend beyond the present coastline.  Henry kindly sent me the above graph, which represents one of his early attempts to model the volume and extent of Welsh ice, plotted against the GISP2 oxygen isotope data for Greenland.

On the graph, the Last Glacial Maximum (LGM) shows up very clearly, with a very rapid ice expansion after a long period (10,000 years or so) of small-scale glacier expansion and contraction in the uplands.  the remarkable speed of ice cap expansion and then collapse is very impressive -- shown on this graph as falling in a period of around 5,000 years and centred on 24,000 yrs BP.  One might reasonably conclude that a long period of cold and snowy conditions is a prerequisite for ice expansion on a substantial scale.......  but the oxygen isotope data do not adequately explain the sudden increase in ice volumes and the sudden expansion of the glaciated area.  Other very important factors must have come into play......

As for the Early Devensian, we can assume that there was a Welsh Ice Cap in existence between 60,000 and 70,000 years BP, and maybe for  a few millennia before and after -- but it does not seem to have been more extensive than about 400 sq km (as compared with 30,000 sq km in the LGM).  What is interesting from the O18 graph is the huge climatic oscillations that seem to have occurred at this time, with temperatures apparently plummeting to a lower level than at any time during the LGM -- balanced by very rapid warming phases.  Maybe this sort of climatic instability was just not right for the gradual buildup of ice over the Welsh uplands..... 

Around 50,000 years ago there does seem to be evidence of a warmer period -- albeit with cold snaps now and then -- and this seems to me to accord with the evidence for the weathering of Early Devensian periglacial slope deposits around the coast.  Many other authors in other parts of the UK have brought forward evidence for one or more "Middle Devensian Interstadials."

To return to the Early Devensian Welsh Ice Cap, and the apparent mis-match between my field evidence from New Quay and the glacier modelling done by Henry, we still have to find out whether there are deficiencies in the model or misinterpretations of the field evidence.  Probably a bit of both......

Then we have the interesting question of the "Paviland Moraine" in western Gower, assumed by Prof David Bowen to be of Anglian age.  I'm not all that convinced by the dating of that feature -- could it be of Early (or even Late) Devensian age?  Watch this space.....

3 comments:

Henry Patton said...

Exposure dating on the boulder by Phillips et al 1994 came out at ~23ka ±2ka, so presumably moved there during the LGM. It was one of the few timing constraints I had on the WIC modelling

BRIAN JOHN said...

Don't have a copy of that paper -- but I recall being rather unconvinced by it! What controls were there to indicate that the date was more or less reliable? Do we know whether the sampled surface was on the top, bottom or side of the boulder? If (as I imagine) they sampled the top, how do we know which way up the boulder was to start with? Were the authors assuming that the surface was a freshly eroded face? And what evidence do they invoke to support that supposition? Don't expect you to know the answers to all of that, but dating the surfaces of boulders built into Neolithic monuments is a process fraught with difficulties........

Jon Morris said...

This programme may interest you Brian : Seems from the blurb to be covering the transportation issue

http://tvmag.lefigaro.fr/programme-tv/fiche/arte/documentaire/51518674/de-nouvelles-revelations-sur-stonehenge.html

De nouvelles révélations sur Stonehenge (this evening).