There are a number of roof-falls in Wrysgan. This chamber that I'm standing in has many large boulders lying around that have fallen from the roof, high above.
It's hard to get an idea of perspective, but that boulder I'm looking at is about sixty feet away, and is bigger than my house. It was by no means the biggest one I found, either.
(Photo of Wrysgan Slate Quarry uploaded by Penitent Miner. Please do not copy without permission).