Just found this site which may be of interest
http://www.cwmorthin.com/9._the_people.html In regards to the underground workings, the following paragraph taken from the "people of Cwmorthin" page, caught my attention.
Carwenna Williams recorded a little about her life in the valley during a BBC Cymru
programme in which she states the following -
"My Uncle would get up at 5 in the morning in order to walk over the tops to the Oakley
quarry, although there was a tunnel that went straight through from Cwmorthin."
Wonder which underground connection betwen the quarries is referred to here? No clear date is given which might make it a bit easier to try and guess which route this might have been. I'm intrigued!
There is also a section on geology of the valley which makes interesting reading.
'I wonder how many breakfasts, and other meals we have missed inside that nasty clockless, timeless hole?'
'The Hobbit'
J R R Tolkien.