In all of the shots I can find of that scene, the canyon is clearly a quarry with blocks cut out by wire saw (filmed in a quarry in Vermont). I guess the histor

In all of the shots I can find of that scene, the canyon is clearly a quarry with blocks cut out by wire saw (filmed in a quarry in Vermont). I guess the historic quarries in Iowa are filled in with water to create artificial lakes by now, but that's something that only Iowans would really know.