I've visited Glencoe Quarry twice. We discovered it quite by accident the first time. Back then, as we entered the quarry we experienced a feeling almost like that which Lucy and her siblings must have felt as they stumbled through the wardrobe into Narnia.
The quarry was super-quiet inside and surrounded by sheer rockfaces - there were huge boulders on the ground, patches of neat grass, and a handful of rock climbers practicing their skill quite high up against the rockface. It was quite different to the hiking trail that we'd just been on!
When we arrived this time I discovered a large metal door in my path and this slightly-defaced sign indicating that the quarry had been closed to the public. I also happened to notice a small pathway leading up and around the quarry to a spot that rock climbers (including the one I spotted descending from the pathway) seem still to make use of.
It's sad that the area had to be restricted. I'm guessing that it could have been closed off because the quarry was so secluded that it may have been dangerous for people to spend time inside. It's sad, but the unfortunate reality of having that kind of area near on the perimeter of a large city.