I'm building some cliffy bits. An armature, covered with papier mache, with plaster cast rock moulds pressed in while the mache is still soft, and then the gaps filled with papier mache and brushed in with a stencil brush. I think this is going to work quite well, it looks a lot like the small earth sections you get in real cliff walls, and the papier mache is flexible so should cope with expansion better than the plaster.
But that was an idea I made up on the spot, what do others do?
Incidentally, art mache (the ready mixed paper powder stuff) is expensive but good, you can work it well and texture the surface with a wet hand, a paintbrush or a spatula for various effects.
Guy