What you want to do is gonna be a pain in the ass, because you basically want a 3D cliff.
There is only one way to do this, your going to have to use boulders and rocks and glue them up to make an overpass, and then still have it so players could walk underneath. I recommend that you edit the map generation scripts, so that mountains randomly spawn boulders on top of them every once in a while, so players get used to the fact that boulders spawn on mountains. In fact, I'm thinking of stealing this idea now myself.
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