Grüger, Eberhard (1972): Pollen and seed studies of Wisconsinan vegetation in Illinois, U.S.A. Geological Society of America Bulletin, 83(9), 2715-2734, doi:10.1130/0016-7606(1972)83[2715:PASSOW]2.0.CO;2
Grüger, Eberhard (1972): Pollen and seeds from Hickory Ridge Basin, Illinois, U.S.A. North American Pollen Database, National Oceanic and Atmospheric Administration (NOAA), ftp://ftp.ncdc.noaa.gov/pub/data/paleo/pollen/asciifiles/fossil/ascfiles/gpd/hickory.txt