Read about Aaron Copland and the American Western. I'll just note here that there are actually two alternative models for the "American West" sound.
One is the Magnificent Seven/Bonanza, optimistic, idealistic, heroic kind of sound (I mean, come on, Magnificent Seven's got Steve McQueen). There is actually a melodic theme to this kind of sound...but all heros need themes, right?
The other is the "anti-hero," Good, Bad, and the Ugly kind of Western, that draws on the electric guitar, whistling, and the plaintive "panpipes" (not sure what the technical term is...I am guessing the instrument might be Native American, but I don't really know for sure). Melodically, the music is fragmentary: there IS a melody, but it is quite often broken up by interfering instruments. And so I call it the "post-modern" West.