After mooching about recently in the area around Tafarn y Bwlch I took the chance today to have a wander about -- over 20 deg on a November day, not a cloud in the sky, and not a breath of wind. And no rugby World Cup matches on the telly.......
I'm now quite convinced that there is a massive terminal moraine here, just to the south of the proposed new Visitor Centre. Here is an old OS map of the area:
On the map, you can see the position of the col between the Brynberian and Cwm Gwaun catchments (purple rectangle) just to the west of the old farm and near a pair of standing stones. Note the contours. To the west of that, the ground (very soggy) slopes gradually down towards the position of the Gernos morainic accumulations, and all of the stream cuttings show that this is a till plain littered with sub-angular and sub-rounded boulders, some of them over 3m in diameter. I had expected to see here an extensive sheet of fluvio-glacial materials -- an outwash plain or sandur -- but that is not what I found. There is continuous till at the surface all the way from Tafarn-y-bwlch to Gernos, and then further down the valley towards Gelli-fawr -- a distance of around 2 km.
To the south of Tafarn-y-bwlch, the land surface rises gradually and when we pass the edge of the cultivated land the slope steepens considerably. On the 1:25000 map it is referred to as "Banc Llwydlos". The bank, about 50m high, is too steep and prominent to have been cut by river action or spring sapping, and the only reasonable interpretation has to be that it is an ice-contact slope associated with a lobe of ice sitting on Brynberian Moor. This is exactly what is shown on the landform / ice limit map shown above. So we should not be surprised. I explored along this slope today, and there are boulder accumulations all over the place -- I hesitate to call them "erratics" because most of them are rather local dolerites, gabbros and ashes. These are typical:
I estimate that the morainic accumulation here is at least 40m thick -- and that must represent a very considerable stillstand of the ice margin. Of course, this lobe of ice must also have been responsible for the till that we now find on Brynberian Moor and at Rhosyfelin.
More work needs to be done on tracing the Late Devensian ice edge on the north face of Mynydd Preseli.
Finally, it's worth mentioning that there is no trace of a meltwater overspill on the col at Tafarn-y-bwlch. There is no channel which might be associated with a "Lake Brynberian" -- and I am more and more convinced that there never was a sizeable meltwater lake here either before or after the LGM.