 It was named in honor of the Marquis de LaFayette, French hero of the American Revolutionary War.According to the U.S. Census Bureau, the county has a total area of , of which is land and (1.2%) is water.In recent years, average temperatures in the county seat of Vandalia have ranged from a low of in January to a high of in July, although a record low of was recorded in January 1985 and a record high of was recorded in July 1980.