Painting rocks would be similar to painting geometric cubes. The tops of the rocks would be facing the sky. Similar to the ground, the top would be darker than the sky (value). One side of the rocks could be the local colour in sunlight and the other side might be the form shadow. I didn’t like the sharp contrast between the light value top and the side with the darker form shadow. Therefore, I made the edge of the rock between the side’s form shadow and the top, a value closer to the side’s form shadow but not as dark. The contrast between the light value top and the side with the form shadow was reduced. I was then able to lighten the rock’s top but kept this away from the edge.