"Dino Paravano, Ron Wilson with Tim Robson 1983"<
Near the Jukskei River
South Africa

Robson was an educator - using lemon juice (an acid) he could carve drawings out of an ultramarine wash.
And he explained values in his unique pointillist style. Here he shows how we can create texture in rocks:
There's wrinkle in his notes but basically he says half-lid your eyes to see the effect in the distance.