Aber, J. S. (1991). "The Glaciation of Northeastern Kansas". Boreas. 20 (4): 297–314. डीओआई:10.1111/j.1502-3885.1991.tb00282.x. (contains a summary of how and why the Nebraskan, Aftonian, Kansan, and Yarmouthian stages were abandoned by modern stratigraphers).