Thanks for the comments everyone.
Bob,
Here are some more details about how I did the rocks:
I used latex molds from Bragdon Enterprises to make hydrocal castings. I placed these as close together as possible and filled in the seams with carving plaster from Woodland Scenics. Then I used a small metal scraper to rough up the carving plaster to make it blend in with the hydrocal castings. Finally, I painted the rocks using the "leopard spot" technique that is described in this video from Woodland Scenics.
This is a picture of the latex molds. I made some smaller castings to help fill in the gaps.
This shows some rock castings in place but before the seams are filled in.
This is about half way through the process.