This chart divides the nation’s adult population into four equal groups each decade. The 1st quartile (green line) are the quarter of U.S. adults in the counties with the highest percentage of those over 25 with college degrees. The 4th quarter are those adults living in the counties with the smallest percentage of people with college degrees. You can see that since 1970 the gap between the most and least educated counties has widened significantly as the nation has sorted educationally.