Dan, I have used several of the methods described above for contours/walls/mountains, including Sculptamold and plaster cloth. Both are very effective for their purposes.
Depending upon your purpose, I would also recommend Shaper Sheets, a product from Woodland Scenics (https://woodlandscenics.woodla....com/show/item/C1178). It can be easily cut to desired dimension, moldable, and can be configured into many shapes. Lightweight, but also can be made rigid with plaster or plaster cloth. Can be painted. Can attach rock molds, but I have not done that.
I am still learning skills on scenery, so always try to create practice sections before going to the layout. I attached several photos of different applications on my layout, which show the practice items first.
My first use was for the edge of a lighthouse scene. Here are photos of practice using small pieces of Shaper Sheet for rock contours, grandson assisting in application of plaster over the Sheet, and final product. (Handles are to swing the unit up on hinges to allow access to interior of round-the-room layout.)
Here are photos of sledding slope for Christmas/winter scene. For this one, I used plaster cloth over the Sheet to create a smooth slope.
Here are photos of mountain backdrop for Lionel Rocket Launcher platform, currently under construction. For this one, I used spray paint directly over the Sheet, then various acrylic colors and grass items for highlights.
Hope these help to show another useful product for mountains and rock walls.
Enjoy,
Michael
