I don't think there's any cliffs in Iowa anywhere near as steep as the one shown in the beginning car chase. We have some little hills, but we're pretty flat. Unless anybody knows somewhere I haven't heard of? I figure that it could have been some sort of result of World War III or the Eugenic War, though.
I have driven through Iowa, and you don't know flat until you come here, to Oklahoma
Well, being from Iowa, I must say it's not
Oklahoma flat, or central Illinois flat, for that matter (I swear you can see the Sears Tower from Normal).

The landscape can mostly be likened to a rolling sea, as exaggerated by Grant Wood in his landscapes. To be fair, there are pool table flat spots, though. And, rather big bluffs around the Missouri and Mississippi rivers. And, the area around Riverside is a flat part of the state until you get up by Iowa City.
Having grown up only about two miles from a quarry, I never gave any thought to the quarry being in the movie (though it was nowhere that big).