Mormon Times provided a link today to a new genealogy resource: the Atlas of Historical County Boundaries. This interactive map provides extensive historical data about the boundaries and county jurisdictions of each of the states. It is provided as a service of the Newberry Library's Dr. William M. Scholl Center for American History and Culture together with the National Endowment for the Humanities.
The site has maps to each of the states and territories and interative maps of each of the states. Some of them are quite curious: in 1852 Sanpete County stretched well into Colorado and was about half the size of the entire state of Utah. Iron County stretched from the western border of Nevada to the middle of Colorado.
A randomly selected date of July 4, 1900, shows the counties of Utah looking much like they do today.