I have been looking at these quarries again today -- and I have found another one, on the eastern flank of Banc Du, to the north of Gernos Fach Farm. Like the others, this is a meta-mudstone quarry.
When I put up a post a few weeks ago, suggesting that the quarries are modern, opened up to provide meta-mudstone slabs for building purposes, I was challenged on another Facebook site as follows: "Your hypothesis. You carry the burden of proof. Now prove it!" Perfectly fair and reasonable, since that's what I say to our friend MPP and to various others!
I gave some supporting evidence in my earlier post:
So we have slabs used in abundance at the ruined cottage called Ffos-felen. Today I looked at the ruined farmhouse at Tafarn y Bwlch, and this is what I found: