SCM Draft or Starforge, copy and paste tiles and blend them together to make it look good.
http://www.staredit.net/index.php?showtopic=17806For cliffs.
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... How informative. You don't actually just "copy and paste", it's more of a matter that you have to set up the dirt to cliff piece, then the secondary above it, then the "extenders" and so on. As explained in the tutorial.