I use plaster bandage, (the kind doctors use to make casts of broken limbs) over a base of carved styrofoam blocks, (usually taken from packaging materials) and then cover the bandage material with a thin coat of plaster of paris or water-thinned spackle paste. Then sculpt the smaller terrain features and while it is still wet, dust the entire surface with dry plaster of paris shaken through a kitchen strainer. This leaves the whole surface with a fine, grainy texture as the dusted plaster sticks and hardens to the wet plaster underneath. When dry, I paint it with diluted acrylics, swirling different colors together to creat various effects, then follow with grasses, bushes, trees, etc.