From the Newberry Library, a wonderful research tool for genealogists researching U.S. locations: the Atlas of Historical County Boundaries at Newberry.org.
A project of the William M. Scholl Center for American History and Culture at The Newberry Library in Chicago, the Atlas of Historical County Boundaries is a powerful historical research and reference tool in electronic form. The Atlas presents in maps and text complete data about the creation and all subsequent changes (dated to the day) in the size, shape, and location of every county in the fifty United States and the District of Columbia. It also includes non-county areas, unsuccessful authorizations for new counties, changes in county names and organization, and the temporary attachments of non-county areas and unorganized counties to fully functioning counties. The principal sources for these data are the most authoritative available: the session laws of the colonies, territories, and states that created and changed the counties.
The Atlas is designed to be as comprehensive as possible, leaving no gaps in either space or time. The historical scope covers every day, starting in the early 1600s and extending through the end of the year 2000. Geographically, the range for each state includes all the territory within its bounds in 2000, regardless of what government created or altered a county there, plus any other territory that may have been within the state’s jurisdiction at an earlier time.
The Atlas of Historical County Boundaries at Newberry.org is keeper for U.S. research.