| 2. |  | Illinois, Sterling quadrangle : 15-minute series | Maps; Topographic maps; Topography; Illinois; Sterling (Ill.); Rock Falls (Ill.); Whiteside County (Ill.); Carroll County (Ill.) | 1 map : col.; Relief shown by contours and spot heights.; "Mapped by the Geological Survey, 1946"; "Contour interval 20 feet." |