Polygons of US state boundaries for the period 1960--2000. Subset from Siczewicz (2011). Licensed under CC BY-NC-SA 2.5 by the Atlas of Historical County Boundaries.
us_statesA data.frame object.
Siczewicz, P. (2011) U.S. Historical States and Territories (Generalized 0.01 deg). Dataset. Atlas of Historical County Boundaries, edited by Long, J. H. . Chicago: The Newberry Library, 2011. Available online from https://publications.newberry.org/ahcbp/.