Forgive the picture quality as this was my first layout about 20 years ago. In the picture below, look to the upper middle to right hand side. You will see where I covered the sides of mountains with green scenic material. On the advice of the forum I painted everything a soil color before placing the scenic material. If you are going to do something similar please DO NOT paint the surface a soil color. No matter how tightly I glued the scenic material clumps next to each other there were small gaps which showed up as annoying and terrible looking brown spots when I took photos. To the naked eye it looked perfect but the camera catches EVERYTHING. If you are planning to do something similar I recommend using a dark green color.
You can see some of the brown spots in these photos, especially at the bottom on the one on the right (and yes, my son was young and into Godzilla and Military figures):