I think the problem with your rocks is the same I had with my sword; you got excited with your progress and posted it here before you could really finish it. Those rocks look great so far! Just keep the bum in the seat until you distribute one or two more shades... Maybe that's not it though. Putting more thought into it, your rocks may be suffering from the repeated pattern of, what looks like a boxing glove. Unless those are million year old water rocks, Rocks tend to be randomly shat out by volcanoes. You either have a subconscience interest in Fighting, or butts, hahaha(it's probably just me) ! Finding rocks with lines running down the middle would be like finding 4 leaf clovers... Anyways, expirment with shading misshapen trapezoids, penta deca octagons, instead of lines for rocks. I hope that makes sense.
Back in my youth, I was obsessive with symmetry. It showed in a number of ways: I would level my skills evenly in RPG's, Lego creations were symmetrical. It's eased a bit with age, but it's still there.
I understand where your coming from, shading the trees evenly from left to right. Unfortunately, only certain things in nature are naturally, uniformly shaded.
Tims reply is spot on when it comes to shading. If you're still having trouble, try looking at how games in the past.did their trees.