Thanks Doug. 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. Here is another picture from earlier in the process to help give you a better idea of what I did. Some of the hydrocal castings are stacked up under the bridge.
