Thanks for the comments.
I will try my best to explain the "rock." In summary, it is a cement coating over expanding foam. I got the idea from various sources all over the web, including these forums. I didn't really follow any single guide.
First, I drew up a basic design. I then used egg-crate material and various pots, recycled containers, etc to fashion a frame and give the whole thing a basic shape. I used the egg crate to mark out the areas reserved for plumbing, but I wish I had thought to just drop some 6 inch pvc pipes and elbows in.
I then used outdoor expanding foam for pond-use. There are a couple brands, and the foam comes out black. I like this better than the usual expanding foam because it seems to set denser.
I used one can at a time, very spread out, and let it set over night. Then I would carve or sculpt as I saw fit. I used 7 or 8 cans, but had I framed things out better, I could have used far less. I really want to try this again with more fore-thought on the plumbing and framing.
I then did my final sculpting, and I used a rasp to really rough up the foam. This is a critical step. In places where I did not rough up the foam, I had to reapply cement after it flaked off.
To coat the expanding foam, I used portland cement mixed with a charcoal cement dye and quikcrete acrylic fortifier. I am not positive the latter is necessary, but saw it suggested on one guide. I used three coats of cement. I made a small error working with the cement in that I added it to the expanding foam without wetting the areas first, and I let it set too quickly. Once I started planting the ledge, and adding more humidity, things shifted just enough to cause some cracks in the final coat. Nothing the mosses aren't hiding.
That was about all there was to it. Fairly simple in theory, but working with the foam and cement can be tricky in the technique (wear gloves!). I would recommend practicing making a faux rock on a piece of plywood or something similiar first.