yo jirobi when sketching that big your better off using single lines for shading instead of zig-zag lines i see here and there. Zig-zag shading like that always give off the feeling that you don't really know what your doing and make it look way to rushed. Even in sketching and thumbnailing you got to take your time to think about why you are putting something on paper. so next time when doing rocks just think of your hand being a printer needle and just fill it up with nice smooth lines.
But keep it up you will get the hang of it
also freebie