"Stratigraphic Classification of Coals and Coal-Bearing Sediments" Hollis D. Hedberg, Geological Society of America Bulletin, V. 72, p. 1081-1088, July 1961, Box: 10, Folder: 33. Hollis Hedberg papers, RH MS 546. University of Kansas. Kenneth Spencer Research Library. https://archives.lib.ku.edu/repositories/3/archival_objects/149255 Accessed May 18, 2024.