From Ohio History Central
The Delaware Limestone is of Middle Devonian age and was named for exposures near Delaware, Delaware County. It overlies the Columbus Limestone and has been quarried for many years. The Delaware Limestone was called the “Blue Limestone” by early geologists because of its bluish-gray color, which contrasts with the white-to-buff colored Columbus Limestone. This color difference reflects the greater silt content of the Delaware and represents the beginning of the rise of the Acadian Mountains due to continental collision far to the east. The silt was carried out into the Ohio sea by streams eroding the Acadian Mountains. Fossils are abundant in the Delaware.