I'm not sure exactly what location you start out at in America but a lot of it just looks like some area in Colorado before you get to the mountains. It doesnt really look like any of the midwest which is mostly flat, hills, bluffs (along the Mississippi), or forests. Not a lot of rocks and boulders. They seem to be everywhere in this game though.
There's an in-game and lore explanation for why the US looks the way it does in the game which Kojima has spoken about before and related to the Time Fall phenomena.