Not an easy answer…if you look at this Node Tree…and examine the Snow-Rock amount Frame…you can get the basic idea…
Basically you are taking the Snow texture and combining with the Rock texture and using a vector to apply only in the Z direction, controlled with a Color-Ramp.
end result is something like this…
You should be able to adapt it to your needs…you will need to join the landscape with the rocks…and make sure to apply all modifiers before joining…