These will be the portions of the rocks that the light illuminates the most. Now using the Hard Round brush, start by adding some random organic shapes to define the tops of all the rock faces. By doing this you can begin to quickly apply paint to the new layer without worrying about cleaning it up later. Any paint applied to the new layer will now only be visible where the silhouette layer underneath it has opaque pixels.

If you create a new layer above the layer with a silhouette on it, you can alt+left click the space between the layers and clip the new layer to the existing layer. Here, Photoshop's clipping layers will be helpful. The next step can easily be the most time-consuming: we need to find the details in each layer of the composition.

Photoshop’s clipping layers will help find the details in each layer of the composition