I just finished reading volume 2 of the 1922 book THE CENTENNIAL HISTORY OF ILLINOIS. It has a series of 3 population maps, that show Illinois was first settled in the southern portion of the state(because people could come via the Ohio River).
Chicago started to expand outwards. The last region settled was the Livingston County area. Probably because the Indians were not removed from this area until 1832.
Note the legend is backwards on the 1840 map.....compared to the first 2 maps. This is a mistake in the book. As the population density increases, the map areas should go from plain to dark.