I remember reading one L&C fic long ago that took place in Kansas, but a very hilly area. I think it was a canyon within a state park, and L&C got chased by bad guys, maybe Bureau 39. It seems like maybe Clark had been exposed to Kryptonite, too, because I keep thinking that Lois had to practically haul him out of there. Anyone know what fic that is?

That sounds like A Question of Trust, by Raconteur. You can find it here: http://lcfanfic.com/stories/2001/qoftrust.txt

That's it! Thanks loads.