after 1892
Geological Survey (U.S.)
1967
Willman, H. B.
January 1 1961
Illinois State Geological Survey
circa 1935
1985
Dahlberg, Richard E.
1945
Weller, J. Marvin
1956
Bier, James A.
1996
Abert, Curtis C., Illinois Department of Natural Resources
1993
1979
1922 – 1954