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.) and Geological Survey Division (Urbana, Ill).