My own map of the ice edge is rather different from theirs, but that is largely because I have put in much more detail, whereas theirs is very generalised indeed, using a straight line for something that would in nature have been highly irregular. Ice sheets do not have straight edges, except perhaps on very flat terrain.
Glacial Lake Brynberian may have looked like this -- mountains on one side, a widening strip of water, and retreating ice on the other side. Click to enlarge.
The illustration above, showing an ice-dammed lake on Axel Heiberg Island, gives us a pretty good idea what this lake might have looked like when the ice was close to its maximum extent, and beginning to retreat. There must have been a thin sliver of meltwater along the edge of the mountain, held up by the retreating ice edge. As the ice retreated, the lake will have widened and deepened until at last (according to the Etienne et al scenario) if flowed over a col at 220m and was drained via the Rhosddu Channel. That needs to be verified by fieldwork, as indeed does the very existence of this supposed lake.
As I have said before, I'm not aware of any varved or other lake deposits in the Brynberian - Crosswell area -- and a programme of drilling is needed in order to find them if they are there. One of the other interesting things about this is that Craig Rhosyfelin, the site of the "rhyolite bluestone quarry" studied by MPP and colleagues, would have been slap in the middle of this lake, and deeply submerged by meltwater c 20,000 years ago. Also, it's possible that the valley of the Afon Brynberian, in which the rocky spur is located, was dramatically refashioned by meltwater as the lake was being drained.
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