The ceiling tile looks very good, near prototypical, and as somebody who has spent almost his entire life surrounded by the bare geology of the desert Southwest, I have a certain eye for these things.
Depending on the resources you have available, you may give either of these two a try:
1) Sandblast the ceiling tile formations, varying the angle of the sandblasting. Obviously, you would want to do this prior to placing the formation on the layout. You can also do this with straight foam formations as well.
2) Again, before you place your geologic formation on the layout, you could blast it with very focused pressure washing. This you would want to do with either plaster or foam formations, but not with anything that will become waterlogged.
Note that these are simply speculative suggestions, as I've not had the opportunity to try either. Being erosion based shaping methods, theoretically either should give you good results. The devil may be in the details though.