Kevin says, "A simple method for making hills and mountains is to crumple up newspapers or supermarket paper bags into basic shapes and cover that whole thing with small pieces of paper towels soaked in watery patching plaster (powder form). After the basic shapes have hardened, apply another coat of plaster using a 1" wide brush. When that's dry, a final plaster coat (very thick) can be "glopped" on vertical areas using the same brush. Let the final coat dry for a several hours and when the plaster has become somewhat hard, but still somewhat pliable, begin to carve cracks, strata lines, etc. into the surface, all going mostly in the same direction. When done carving, let it fully harden."
You can see examples of using this method on the model railroad site where Ryan explains the steps in making hardshell scenery which is what this method of plaster of paper towels is often called.