Flying on the West Coast, I am used to just about everywhere being considered mountainous except for portions of California’s Central Valley. I had a picture of the map in my mind, but couldn’t remember where I had seen it.
After a little digging, I found two sources:
The AIM 5−6−5. ADIZ Boundaries and Designated Mountainous Areas also has a map.
The MEA and MOCA on airways in mountainous areas provide 2,000′ of obstacle clearance while in non-mountainous areas they only provide 1,000′.