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:
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:
See also this post on Rosemergy niche glacier:
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.