The Devonian period marks the beginning of extensive land colonization by plants. With large herbivorous land-animals not yet being present, large forests could grow and shape the landscape.
The Devonian Period began about 416 million years ago and ended about 359 million years ago. Devonian rocks crop out in a north-south band through the central part of the state and then eastward along the Lake Erie shore from Sandusky to Ashtabula. They are also present in northwestern Ohio although outcrops are few due to a thick cover of Pleistocene Ice Age sediments. In addition to being present in the subsurface of eastern Ohio, Devonian rocks crop out in an isolated area near Bellefontaine, in Logan County. This feature is known to geologists as the Bellefontaine Outlier because these Devonian rocks are about 30 miles from the nearest outcrop of rocks of similar age.
Ohio was in equatorial latitudes during the Devonian. The outcropping rock record representing earliest Devonian time is absent, with one minor exception, in Ohio as much of the state was apparently above sea level. Early Devonian rocks are present in the subsurface of eastern Ohio, indicating that the sea was nearby. The lone exception to the absent Lower Devonian rock record is a small lens of dark-gray shale that was discovered in Holland Quarry, in Lucas County west of Toledo, in the 1920