The Geography of Income in the USA

Source: Wikipedia

There are very large differences in median household income in different counties in the United States. The east coast and the west coast have large green areas on the map, as expected, and there are large swaths of white in the south and four corners area. Some of the other affluent areas are harder to explain -- perhaps the energy boom in North Dakota, Denver in Colorado are intuitively affluent, but how about the big area in Wyoming?