1916. Geological Survey (U.S.). Contour interval 20 feet.;"Surveyed in 1913.";"Surveyed in cooperation with the State of Illinois.";Relief shown by contours and spot heights.;1 ma... Geological Survey (U.S.) and State Geological Survey (Urbana, Ill.).
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1916. Geological Survey (U.S.). Contour interval 20 feet.;"Surveyed in 1913.";"Surveyed in cooperation with the State of Illinois.";Relief shown by contours and spot heights.;1 ma... Geological Survey (U.S.) and State Geological Survey (Urbana, Ill.).