Great Plains and other physiographic provinces
(Source: USGS A Tapestry of Time and Terrain)
You can divide the United States into physiographic provinces, to identify regions such as the Great Plains with common physical characteristics. Between the Mississippi River and the Sierra Nevada/Cascades are three major physiographic regions:
An even-simpler way to look at the United States is to focus on the 100th Meridian, that line of longitude 100 degrees west of Greenwich, England. (To orient yourself on maps that show just state boundaries, note that the 100th Meridian defines a north-south portion of the Oklahoma/Texas border.)
East of the 100th Meridian, the landscape is wet, crowded, and flat. West of the 100th Meridian:
- average precipitation (mostly in the form of rain and snow) is less than 20 inches per year
- average population density is less that 20 people per square mile
- average elevation is higher than 2,000 feet
