I used the same method I show in my battlemap tutorial. Step 3 discusses the brush I used, and 4 the layer effects. Basical I use a dynamic brush with scattering and a lot of randomness to place rocks where I think they go in a vague way, and then use some bevel and drop shadow to make them look a tad more 3D. If that doesn't answer your question let me know and I'll try to be more specific.