Grove Karl Gilbert (1843–1918) began his work in professional geology in 1869 as “local assistant” on the newly established Second Geological Survey of Ohio (the “Newberry Survey”). He worked without salary but received $50 per month for expenses. Gilbert investigated Williams, Fulton, and Lucas Counties, the three most northwestern counties adjacent to the Michigan State line. The area extended from Indiana on the west to Toledo at the western end of Lake Erie. His originality, analytical power, and clear verbal and graphic exposition, which were the distinguishing characteristics of his work throughout his career, are completely exhibited in this his...