Calico: I would not necessarily call it a terrain really. It is more just a field of massive scale fake stones that have been displaced. I started at the compute terrain node and worked my way up. Mostly just random displacements and variations to certain heights/slopes by warping, twisting and redirecting. Sorry it is so messy but I don't really work systematically, I just keep adding or deleting nodes till something looks nice! Besides that, it isn't really the terrain that makes the picture, but more the fact that a single surfacing layer can give so much variation in tone.