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Sunday, 16 August 2015

Periglacial Preseli


 The Preseli winter landscape.  This image (not very good definition) shows a scattered snow-cover on Foel Drygarn (in the foreground) and eastwards along the Preseli ridge.  Carningli and the sea can be seen in the far distance.

Chris was asking the other day what Preseli might have looked like during the "missing millennia" between 18,000 yrs BP (when we can rather safely assume that the last of the glacier ice in the region had melted away) and the Neolithic, around 5,000 yrs BP (when we can safely assume that the resident population was large enough for the human impacts on landscape, through burning, animal grazing etc were big enough to make a real impact). 

Well, for most of the time a periglacial climate must have dominated, as suggested in my post of 3 January 2013:
    
http://brian-mountainman.blogspot.co.uk/2013/01/ten-thousand-missing-years.html

A bleak tundra landscape would not have looked all that different from the Preseli winter landscape of today, except that "high arctic" vegetation would have been more prominent than the grassland - moorland which we see today.

The only major change which we can assume from the climate records is that of the Younger Dryas, around 13,000 years ago, when for a thousand years or so temperatures plummeted, snow-cover increased and there was a "mini Ice Age."  Chris asked whether glaciers might have been re-formed on Preseli at this time, and whether glacier ice could have affected Rhosyfelin.  I don't see any evidence that suggests this -- but it is quite possible that for some centuries there were permanent snow patches and neve fields, and possibly one or two "niche glaciers" in places like Cwm Cerwyn, which left very subtle traces when they finally melted.

See this post:

http://brian-mountainman.blogspot.co.uk/2012/02/pembrokeshires-last-glacier.html

See also this post on Rosemergy niche glacier:

http://brian-mountainman.blogspot.co.uk/2015/05/rosemergy-niche-glacier-cornwall.html

You will also see in that 2012 post that I date the Younger Dryas at around 10,500 BP.  A few years ago it was still common to refer to Zones 1-3 and the Older Dryas (cold), Allerod interstadial (warmer), and Younger Dryas (cold) phases.  That sequence can be picked up in some sedimentary sequences on the continent, but it is difficult to recognise in Britain, and so nowadays geomorphologists and palaeobotanists tend to refer to a singe "cold snap" that might have had  oscillations within it which were not necessary in phase all over the British Isles.

Baby Glacier on Axel Heiberg Island, a typical niche glacier.  This has real glacier ice at its core, showing that it is probably rather old.  The Younger Dryas niche glaciers in Wales were much more ephemeral.

Summer landscape near Honningsvag, Norway.  Upland Pembrokeshire might have looked like this during the Younger Dryas, with a  mosaic of temporary (seasonal) snowpatches, perennial snowpatches, neve fields and niche glaciers.

1 comment:

Hugh Thomas said...

After the final retreat of the ice , would there have been a period where bare rock would have been more widespread until soil had a chance to build up ? Am just trying to picture the scene.... ;) Also just to the east of Carn Goeddog on the northern slope there is a large scar in the slope like a cwm, would this type of feature been caused by ice ?