Making scenery using blue foam board - Model Railroader Magazine

What you would use depends on what type of terrain and texture you wish to convey.  If you are talking about bald-butt prairie, then splash on some light grey-brown latex and pour some light green, dry-looking ground foam...add some sage bushes here and there, maybe.  A few pines.

If you carve up the foam at all, to get some topographical relief, then your method of covering the seams, if they show, will depend on what you want the cuts to look like.  A ditch is easy, and needs nothing more than to clean up the soring from the wire brush marks, or the nicks from the blade, and you can line it with a thin layer of patching plaster or hydrocal. Paint it up to look muddy and dank, then pour some two-part epoxy here and there to simulate pooled stagnant water.

For larger, deeper edges and cuts, including cliffs and tunnel entrances that are not cemented or stoned with conventional portals, slather on some hydrocal or wall patching plaster, let it set up a bit, and then press and carve it with a thin blade to look like limestone strata or like harder granites and metamorphic stone.

This topic is well served in various books, none of which I have.  I do my research here on the web.