I can't stress it enough: No black! Rocks don't do well with black shadows. Much too dark. Most textured brushes will do fine, and you can just take a picture of the pavement in front of your house to get a decent texture. You can also use the steps shown as guide to how much detail you need in the distance.
"Ranarh's tiny tips" is a collection of iddy-biddy tutorials, each containing small bites of advice anyone can chew, aimed at beginner's level artists. They are not software-dependent. If there's a problem you have, something you feel someone should finally explain, or are just curious what I have to say on a particular artistic issue, feel free to tell me.
You'll find the whole Tiny Tips Collection, plus some full-grown walkthroughs in my tutorial section. Check out my blog and website for exclusive tutorials, walkthroughs, and tips.