In The Silmarillion it is said that only Eru possesses the Flame Imperishable, and thus only he can create life. For example, when Aulë tries to create the dwarves, these result in a kind of marionettes without life of their own. It's only when Eru intervenes, when dwarves are given a true life. But then it's also said that forces of evil (particularly Morgoth) are sterile and can't create, but only mock and corrupt others' creations. I don't remember if something about this is said in the actual book, but at least is said in some of Tolkien's late essays (if someone could point to some passage in the book, it would be greatly appreciated).
Then, if this is so, what's the difference between Morgoth and the other Valar? The Valar can create things like mountains, just as Morgoth, but they can't create life either. Wouldn't they be "sterile" as well?