Be sure to check out the "Maps of Illinois" page at Map Of US.org! Here you can find a link to a list of facts about Illinois counties, view old atlases and map images, and check out an interactive timeline of Illinois county formation changes from 1790 through 1869. This interactive map is especially useful for family historians who are researching early Illinois pioneer ancestors, as it can help you determine if your ancestor lived in an county that underwent changes, and help determine which county holds your ancestor's records.
This site is a great reference resource for Illinois county research!
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