This picture is an exercise in creating the perception of depth.

The image is in three parts. The stones in the right section appear to be the closest because they are the largest. The stones in the left section appear to be in a plane behind the light middle section because they are darker. The appearance of being farthest back is accentuated because the stones are also smaller than the largest stones on the right and visually relate to it by being similarly dark. Actually, the stones are all in one plane.