From Ohio History Central
An ODNR map showing the Bellefontaine Outlier in Logan and northern Champaign Counties.
The flat, nearly featureless glaciated till plains of western Ohio are abruptly interrupted by a hilly area in Logan County and parts of adjacent Champaign County. One of these hills, Campbell Hill, marks the highest point in Ohio at 1,549 feet above sea level. It was noted long ago by geologists that this area in the vicinity of Bellefontaine, Ohio is underlain by the Devonian-age Columbus Limestone and Ohio Shale but is surrounded on all sides by older Silurian rocks. The nearest continuous outcrop of Devonian rocks is more than 30 miles to the east; thus, the Devonian rocks in Logan County are a geologic island, or outlier.
It was not until information became available about the structure of rocks deep beneath the area that the reason for the outlier became apparent. The Devonian rocks of the outlier consist of a series of fault blocks that were probably dropped downward and protected from extensive erosion that removed other Devonian rocks across western Ohio. Gradually, erosion exposed the buried fault blocks, which were more resistant to erosion than the Silurian limestones and dolomites surrounding them. Eventually, the Devonian rocks stood out as an elevated area.