There are tons of articles out there on how to make rock castings, as well as many books. This article doesn't really present a unique technique, but it might give you some ideas.

I needed to create some thin rock castings for a particular part of my N-scale layout where the track was a bit too close to the edge of a cliff. The commercial rubber molds I had purchased in the past work well, but most of them are rather deep. To use them to make thin rock castings means pouring in a thin layer of plaster. Removing the castings from the mold can break them quite easily because they are so fragile. I am sure I have read this somewhere before, and it may not be a new idea, but what I came up with was to take a piece of aluminum foil, crumple it up, and use that as the mold.

I then unwrapped the ball of foil. This takes a bit of patience as the foil can tear very easily. After mixing up the plaster (in this case "Plaster of Paris"), I poured it over the foil and let it dry overnight.

When the plaster was dry, the next step was to remove the casting from the foil. Because of the deep ridges in the foil, some of the foil was hard to remove. The results are shown below, left. Some of the pieces were still a bit too thick for my layout, so I tried another approach. I crumbled the foil up again, and then spread it out on the workbench. I smoothed it out a lot more.

As an experiment I decided to add a bit of acrylic Burnt Umber to the plaster as I was mixing it. It colored the whole batch nicely, however, I did notice that the plaster started to set really fast. I quickly poured it on my new mold and spread it out. Within about 30 minutes I could remove it from the foil. This time the foil came off easily and it didn't tear, so I could use it again.

This last photo shows the various castings broken up and glued in place. I used Liquid Nails glue.