You can't be in theatre long before a show comes along that takes place in a setting full of rocks. While there are many ways to create rocks (and I have a few examples of methods I have used), today's entry is a few sites where you can buy rocks - or rather fake rocks.
This site sells pre-made fake rocks in several materials. many of their designs are created to hide something visually unappealing though they have other types as well (pet houses for example). They have a few tree stumps as well and they are able to custom make rocks for your unique purposes. While the rocks aren't inexpensive, depending on how close the audience is, and your financial and labor situations, this could be a possible solution.
Much bigger scale for the rocks on this site, and I am sure the costs reflect that. Since they handle costs through quotes, I imagine that this would be a pricey option. However, the selection of what is available on this site is pretty interesting. What intrigued me the most was the tree and the flex bark that they sell. While rocks can be difficult, they usually turn out well in the long run. Realistic bark is difficult to make.
This company id most similar to the first, specializing in smaller rocks to hide eyesores. They are based in Wisconsin, though you can search for a local distributor (multiple came up for my location).
So, for the next show that comes along that has a pile of rocks, add rock shopping to your list of possible technical solutions.