The Genesee group, traced eastward from Lake Erie, changes facies in the central Finger Lakes region, New York. Black shales of western New York interfinger gradually with coarser Sherburne beds deposited on a marginal marine plain, indicating shallow water deposition for the former. Because of a temporary extension of the marginal marine plain, beds of Sherburne lithology (Starkey tongue) divide the West River shales into the Penn Yan tongue below and the Milo tongue above.
The isolithic boundary, or zone of facies change, may be extended southward to join with similar belts of change in other areas. The trend of this extension is a product primarily of the height, proximity, and trend of the highland which supplied the sediments.
In conjunction with the field study, rocks of different facies were examined in thin section for accurate determinations of rock types. Rocks are classified on the basis of size of grain and composition. Although the size of individual quartz particles ranges widely, the facies as a whole changes only one size grade, and the coarsening is due primarily to an increase in the amount rather than in the size of the grains.
A principal stratigraphic limit exists at the base of the Geneseo. There is no direct evidence that this relates to the boundary between the Middle and Upper Devonian.