stiffening up mountain

i use stack pieces of foam and cover that with plaster cloth.  i then spray the plaster cloth with water so it will harden.  while it's wet, i sprinkle on most of the terrain.  it will hold the trees that i poke in to it.  sometimes the cloth isn't terribly solid so you have to make sure your base is long enough to get to the foam. 

David - I had a lot of success using plaster cloth.  I'm sure you are familiar with this product.  I would wet the plaster cloth then drape it over cardboard strips that are weaved in the shape of a slope or mountain.  After the cloth dries I hot glue plaster rocks made from molds onto the cloth.   When it dries I fill in the spaces with Sculptomold.  

Paul

If you already have your mountain shape and just want it to have a thicker, stiffer surface, I'd put on rubber gloves, mix up a big mash of Sculptamold and slather it on about 1/4" thick all over.  While you're at it you can fill in or re-sculpt any areas you'd like to change the shape of.  Sculptamold leaves a great lumpy texture, ready to paint or groundcover, and ready to drill holes for trees etc.

Maybe you could grab a cheap bucket of wall mud/plaster at the hardware store and spackle it on?  You cannot "poke" trees into it but you can drill a small hole and then drop it in.  If you want even more hold, dab a little hot glue on the end before putting it in and viola!  It works well for me!

I just use foam board, shape it, carve it and paint it. No plaster is necessary.