I made some rocks out of fondant recently....they were really easy to do. Just swirled a few different earthy colors of fondant together, then molded them by hand & kind of squashed them against the counter to get some flat edges. I let them sit overnight to dry. They're in my photos on the Mater & McQueen cake.
I did what pancake did and they are the talk of the town (Ok, well at least my office). I made enough to go around the entire 10" cake in about 10 minutes. They were so easy and the only part of the cake I'm proud of. I just colored pieces of fondant either grey or grey with a slight bit of brown and then swirled in some black for the marbly effect. Mine were small so I just pinched them a bit to make them look more "rocky".
Here's a link http://www.sugarcraft.com/catalog/flowers/sprinkles.htm
Sugarcraft sells them, they are listed under misc. sprinkles middle of the page. Hope this helps. 
You could make Seafoam candy boulders. They bash onto nice rough pieces that could look like they were dug up by the dozer...
http://www.cooksrecipes.com/candy/old-fashioned-sea-foam-candy-recipe.html
Quote by @%username% on %date%
%body%