General George H. Thomas gained the nickname "The Rock of Chickamauga" when he stood his ground at that battle. Chicago's largest post of the G.A.R. took his name and, in 1894, honored General Thomas by erecting a twelve-ton boulder taken from the cliffs at Chickamauga National Battlefield. Seventy-two veterans were interred in the section surrounding this monument. [Wendell]
Note: the battlefield is usually spelled "Chickamauga", but the alternate spelling "Chicamauga" is carved into the monument. General Thomas himself is buried elsewhere.