Taking a cue from my classmate Barry who had palette knived a lovely streetscape, I attacked it with brilliant colours, moulding gel and palette knife.
As it turned out, the inexact nature of marks left by the knife was perfect for the rocks. As I didn't clean it in between colour changes there was a lot of serendipitous mixes. After the foreground was done, it looked like two different paintings as the cliff face was still brush painted. I had to introduce some palette knife marks on that in a paler colour to make the painting look more cohesive. But now I'm satisfied with the results.