Another thing I might mention...don't know if this would so much apply to a mountain and ground plane, but it does apply to multiple terrains sharing the same material. If you're using, say, the Grass and Rock material on one terrain, and you want it on a second terrain as well, rename the second one to Grass and Rock 2 or something, as long as it's a different name. That will eliminate all the weirdness you might see in the materials getting "confused", particularly where altitude dependence is being used. Just a tip. :)