Very well done Meleri! I thought that it was called the Bryn Hall Quarry as the warning signs called it that, but I then noticed on-line that it is also called Bryn Hafod-y-Wern quarry.
The "sandwich filler" was the nearby Moel "Wnion", the road that could be "paper" was the A5, the "hall on a hill" was Bryn Hall which is right above it, the "slate parish" was the nearby village of Llechwedd which is right below it, and they can all be seen from the zip-wire in Penrhyn Quarry on the other side of the valley. Bryn Hall quarry is over 100ft deep and regarded as very dangerous to dive in as it is full of barbed wire and various other obstacles.
I remember a man trying to commit suicide with his two children and then changing his mind once they were in the water having to be rescued there a few years ago. I walked past it on Thursday while descending from Moel Wnion and I'll post some details of the walk and some pics on the "walking" thread tomorrow.