Hi David
The mountain in my display is made from fibreglass. I actually bought mine from a garden shop and then using a jigsaw cut it in half so I could have 2 mountains.
Originally I tried using papier-mache over a chicken wire cage. However it just wasn't going to look as good as what I wanted and when I saw the mountain in the nursery on the way to work every day I just had to have it as nothing else I could make would look as good. Unfortunately I didn't know that the factory that makes them was about a 4 minute drive away from the nursery and I could of had one designed to suit my needs with little ledges for houses and other scenery.
Other ideas are using styrofoam from packaging to shape the mountan and then painting it. Some people use plaster as well.
In the old display I actually used large rocks that I found on the side of the road as nothing looks like real rocks than.....well real rocks. I then painted them the same colour so they looked the mountain display I had. The old mountain display was actually made from 2 boogie boards. I cut them in half so I had 2 lots each and then carved the edging in. See pic below.
If you have any other questions about them I will try my best to answer them or if I missed anything let me know.
Dyar