For the rock outcroppings I decided to try Cripple Bush Casting rubber rocks. These are great - no mess like you have with plaster, no need to color them (unless you want to) and the things are flexible so you can run them behind a curved cut without any issues. The only negative I can think of is the price - they're not cheap, but you can get a lot of rock outcroppings out of one large sheet.
I start by cutting them to shape and hot gluing them in place.
Then I blend the edges in with earth-colored Sculptamold - just enough to blend the rocks into the hillsides.
Once the texture is added and the trees are placed the scene is starting to shape up.
Then I carved the hillsides behind the track in Everett and painted the first layer of the background hills.
With the trees on the hillside behind Everett.