I bought some polystyrene from this little bit of antenocitis workshop. If anyone knows a cheaper scource of this stuff please tell me. Its not extortionate, but there must be a way of buying bulk from a home insulation wholesalers or something (surely). This tile took six A4 (a British paper size) chunks of 50mm thick styrofoam to make it, four for the base and two for the rocks. I threw on a few real stones from the beach for added detail.
To form the rocks I just hacked at the stuff with a semi-blunt kitchen knife. Something about the way it responds to the jagged violence of the knife leaves effects akin to weathering, crude Orcish tool marks and the shearing power of tectonic plate movement.
I added GW Mordeheim Tufts and some tall grass from a model railway shop around the bases of the rock formations, figuring the water would run off them, hydrating the soil below and welcoming the grass seed that collected there. The last picture was taken before I put the tufts in place.