Alternatively, you can use the method which I used on Vaniya which is to place your mountain brushes on about 3 or 4 different layers making sure you change layers every time you place a new mountain that overlaps another one. This way you can go through the layers and delete the bits of mountain that are below any others. It's a bit of a ballache method but if you stick with it it works. The clone stamp method works great but has the downside that you will need to use a multiply blend mode to get rid of the white background that cloning will give the brushes. I personally find this too limiting.