I have speculated at length about these old grassed-over quarries, and have done a number of posts:
I have been up to have another look at them today, and although I think that some small parts of the diggings might actually be prehistoric, I am now convinced that the main pits date, for the most part, from the last 300 years. The quarries have been used here on the principle that it is better to carry stuff downhill rather than uphill; and because the local meta-mudstones break up naturally into thick slabs with flat faces and flat edges. So the purpose has been nothing to do with cutting and scraping tools, or proto-Stonehenges, but with the provision of building materials for houses, cottages, farm outbuildings and stone walls.
The clues which led me to this conclusion are all to be found in the ruin called Ffos-felen at SN 086345. Maybe it was once a windmill? Anyway, there is not much left of it apart from a gable end with a large window space in it. But the crumbling walls give us lots of clues. The builders have used glacially-transported and shaped dolerite boulders of many different sizes, but they are so difficult to use in a building of this type (rounded or irregular rather than rectangular) that meta-mudstone slabs have been needed in almost every course to maintain levels and to give the wall stability. We can see this in the following photos.
You can see clearly how dolerite boulders and blocks -- all sourced locally by picking up in the surrounding countryside -- are used with the smallish slabs of meta-mudstone. The latter have got good flat outer edges, and they are used everywhere to level up the courses and add stability. I have no idea how old the above building is, but I would hazard a guess that it was put up some time in the 1700's.
It's possible that some of the thinner slabs were used for roofing purposes as well -- but I have no evidence on that at the moment.
This is how the local stone walls and hedges are also built, using meta-mudstone slabs in a zig-zag pattern -- they are no doubt also derived from the quarries:
All I need to do now is to check out other old buildings in the vicinity, like Tafarn y Bwlch, Penylan-fach and Trebwlch, to see if they support the latest working hypothesis! That's for another day..........
Always happy to change my mind if that is what the evidence tells me to do......
PS (added 17th Sept 2018)
This photo is from the highest of the ruined Garfeth cottages, to the NW of the summit quarries and only about 1 km away. It confirms that in this cottage, at least, meta-mudstone slabs were used in construction on quite a large scale. Quite easy to carry the slabs downhill by horse and cart from the quarrying site. I'll hazard a guess and suggest that this cottage started as a Ty Unnos cottage and that it was probably built between 1810 and 1830...........