Demo with your artwork, very fast mockup... see how now some of the trees on the highest level clip over the rocks to hide some of them from view, instead of all of the rocks splaying out at an even inclination? This implies that that spot is steeper, and the cliff might be crumbling. This kind of overlap can give an extra layer of dimension, though obviously what I did here is not a high quality example and if you drew it you'd probably break up the branches a bit, exposing pieces of the rock underneath. Much easier to do that digitally, making a tree layer and a cliff layer, and drawing in more of the cliff than will actually be seen in the end, so you can have bits of it exposed through the branches. I'm just showing how if you moved it a bit you get a greater illusion of depth. I think the cliff shading itself is very nice.
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