Almost forgot, look into mallet fast, which is hidden in the lightbox folder in zbrush. You can create rough gouges in rock surface that you can then go back over with the surface smoothing brushes I mentioned.
I think you should work on creating more convincing rock forms, because right now if you took away the rock alphas this would kind of look like whipped cream. You can easily create much better rocks with just a bit more practice and the right tools. For Z brushes, try dam standard, trim smooth border, orb's crack brush for…