The Preseli trimline

You should never go for a walk in the country and come back empty-handed.  Metaphorically speaking, of course.   We went for a quick ramble yesterday up around the parking area at Bwlch-gwynt (the col on the Preseli ridge used by the main road).  In the brief pause between fronts, we splashed and squelched our way around on the sodden grasslands for a while, and got rather wet -- but I discovered two things.  

One, that the little tor called Carn Fach, at about 390m, is distinctly ice-moulded, and looks as if it might have been affected by overriding Late Devensian glacier ice.  

Carn Fach -- heavily denuded, with many signs of ice moulding.

And two, that when you walk the footpath eastwards from the car park and along the edge of the forestry land, till is exposed on the path up to an altitude of c 420m -- at which point it suddenly stops, to be replaced by broken rock debris with virtually no trace of unusual stone shapes, stone types and sizes.  A clay-rich matrix is also replaced by something more sandy and silty as you approach the sharp corner in the fence line.  

In my book, that means that there is till in the col up to an altitude of c 420m, meaning that ice must have passed over the col while leaving the higher slopes of Foelcwmcerwyn and Foel Eryr unaffected.

In previous posts on the Preseli trimline I have suggested that it lies at a lower altitude -- around 340m -- in line with evidence further to the east, around Mynydd Bach and Carn Goedog.   We must assume that the ice which left the till in the col of Bwlch-gwynt was the ice of the Irish Sea Ice Stream, coming from the N or NW at the time of the LGM.

https://brian-mountainman.blogspot.com/2016/11/the-south-pembrokeshire-problem.html

http://brian-mountainman.blogspot.co.uk/2016/08/the-devensian-preseli-ice-cap.html

https://brian-mountainman.blogspot.com/2020/09/the-late-devensian-trimline-on-north.html

https://brian-mountainman.blogspot.com/2017/01/devensian-ice-edge-in-pembrokeshire.html

Some answers and yet more questions.  Does the trim line (either at 420m or 340m) represent the highest Late Devensian ice surface elevation in this part of west Wales?  Or was the ice surface initially higher, with the trimline traces belonging to later oscillations or readvances of the ice front with different glaciological conditions?  And what were the interactions with the Preseli ice cap?

Another trip up to Bwlch-gwynt is clearly required.