1936. Geological Survey (U.S.). Contour interval 10 feet;"Surveyed in 1931-1932";(W 88⁰00ʹ00ʺ--W 87⁰45ʹ00ʺ/N 40⁰30ʹ00ʺ--N 40⁰15ʹ00ʺ);Relief shown by contours and spot heights.;Sho... Geological Survey (U.S.)|Geological Survey Division (Urbana, Ill).
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1915. Geological Survey (U.S.). Contour interval 20 feet.;"Surveyed in 1911-1913.";"Surveyed in cooperation with the State of Illinois."; (W 87⁰45ʹ00ʺ--W 87⁰30ʹ00ʺ/N 38⁰45ʹ00ʺ--N ... Geological Survey (U.S.)|Geological Survey Division (Urbana, Ill).