Frank has developed an excellent process for making terrain walls. You can see it here in the first picture. The diorama is called "Mail Call" and you can check it out here: The Mail Call Diorama

In this tutorial he shows us how he does it.

Here is what the wall work looks like in a diorama.

There are four steps in the process of making a rock face like this:

This next picture shows a slab of foam. It shows the four steps from left to right.

1. Cut your foam to the desired size and shape

2. Carve the foam to get the rock face looking the way you want it.

3. Spray it with a promer that will melt the foam a little bit. Go easy with this step. Too much primer can eat all the way through the foam. Experiment with a test piece of foam.

4. Paint it as a rock face with various colors including black, green and brown. Then add terrain textures like tufts of grass.