chelle said:
...mortar and rock wall... but I'm not too sure that I'll be fond of the non-natural form.
...slate, but...also using field rock and/or faux rock?
...a dry wall stack using slate for the height might be my best bet. If so, how deep does the earthworm gravel barrier need to be, or would I need it at all for a flat stack?
I wouldn't suggest a mortared rock wall unless you actually have a use for one. If the wall idea is just to create height, making it in a dry pack style would allow the wall itself to be planted though. But it sounds to me like what you really want is to create something that looks reasonably like a natural outcrop (unless I'm misinterpreting), so why not go for that?
I think a mix of rock type could certainly be made to look good; it would take more thought though.
I'm not familiar with the concept of an "earthworm barrier" so couldn't say.
I'll post some references on rock garden construction later...
