Oh right, I've never noticed them before!
To be honest, it's rare I ever look up at the enclosure, normally I choose to use the above viewing than risk the glare and noise of the dodgems etc...
That is a bit of natural rock strata. Possibly the only bit in the whole enclosure maybe, but as a background it makes the photo look very naturalistic. I know the actual location isn't the best though.
It's a rectangular piece of hillside with this rock outcrop and grass, not particularly large but not small. No mixed species but still very effective but not a groundbreaking exhibit!