Natural rock is expensive in our region, and I am faced with the problem of providing an old-fashioned rock garden. Is there an easy way of casting "rocks" to look as natural as possible? It occurred to me that this might be a good way of getting rid of batch ends, and discard concrete.
The easiest way is simply to dig a hole in the ground, ram the bottom and sides to an irregular shape and dump the mix into it. On hardening dig the castings out and place them with the part that was buried exposed to view. A more refined approach is to dump into a hollow in a bed of coarse aggregate, or to prepare a hollow in a sand bed within which stones to be embedded in the surface of the "rock" have been pressed. Note that if stones are to be exposed on the surface of the rock they should be of a type that are sound and durable, particularly with respect to repeated freeze-thaw exposure.