Decals of this nature seems like ~30 minutes of fiddling in PS with a couple custom brushes (that you could probably make from scratch pretty quickly), that will net you some brushes you can use forever into the future, and easy high-res source files to tweak as needed. AI is good at what it's seen a lot of. Examples…
Also you could add a volume of rocks that surround the potential hole and using volume select and delete mesh modifiers create an animation of rocks being crunched up.
Love sushi, there's a great spot in Chinatown, across from the Grant Plaza Hotel, called Sushi Rock. http://www.yelp.com/biz/sushi-rock-san-francisco-2
Dinosaurs could happen :P Here's the start of some rock wall textures. These rock walls are going to be pretty small. Maybe 1 or 2 players tall. :poly142:
Turns out after having another look at the rocks, figured they are pretty rubbish, so I remade 3 different rocks and have started to recreate the outside of the cave.
I don't feel like that type of rock was used in medieval castle construction. It looks like sheet rock on the side of a cliff. In the reference the stone is cut into linear blocks.