Once we came down out of the Rockies into Denver everything started to change almost instantly.
After a peak of around 9,000 ft we came down into Denver at about 5,000 ft. Over night by the time we reached Iowa we had dropped to 2,000 ft.
Now into Iowa it sits around 900 ft, so we're closer to the normal altitudes we are used to in Australia. This countryside is much like OZ, undulating pasturelands. The only thing that tells you it ain't OZ is the architecture, completely different style of house construction.