Thanks for the comments everybody. The lighting was a bit hit-and-miss but obviously I posted the pictures at the "hit" end of the scale.
I now realise that the workers must have cut the top face of the sandstone they wanted to extract and then packed it with the slate-like material, when I was down there I assumed that was the natural layers.
Now I think of it the dressed stone was probably standard practice, you would do a layer of blocks, then dress the face ready for another layer. The curved one might have been for a millstone, or there might have been an order for curved blocks.
These woods have at least ten quarries marked on maps from the early 20th century, mostly described as disused. I believe there may be more holes somewhere and will definitely be looking for them. I've seen pictures of another one labelled as Nydie, which is a bit to the North of this, so I'll be looking for that too.