Many years ago Model Railroader magazine asked me to build a project layout. They wanted something that would fit in a 4' by 8' space and be HO scale. I came up with the Cactus Valley. A railroad based in the southwest desert of the USA. It was fictional but the scenery was modeled after parts of Arizona and New Mexico.
The scenery was made entirely from insulating foam applied using spray cans. After the foam was blow in place and expanded, I carved it to the landforms and shapes I wanted using a serrated knife. The foam was coated and sealed with thinned dry wall mud and colored using acrylic paints.