A wonderful photo with which to celebrate the arrival of 2014 -- this is actually Jokulsarlon in Iceland, a glacial lake filled with floating ice debris, mostly derived from the nearby glacier snout. (It's full of dark-coloured morainic debris, and looks very different from lake or sea ice, which tends to be clean.) Anyway, it gives us a good impression of what Glacial Lake Brynberian might have looked like c 20,000 years ago. You can see a further consideration of this lake and its effects here:
Just to put things in focus, here is my impression of what the lake might have looked like from the air, and a presnt-day image of the likely site of the lake:
On the upper of these photo (a flipped photo of a glacier-dammed lake on Axel Heiberg Island) you can imagine the hills as being Mynydd Preseli. For purposes of orientation, you can imagine Craig Rhosyfelin as being deeply inundated some way off the left edge of the lower photo.
Happy New Year!!