The Tully formation is a thin wedge of sediments lying disconformably on the Hamilton formation. At its type locality, Tully, Onon‐daga County, New York, the Tully is 30 feet thick, composed chiefly of limestone thinning westward to disappearance on the east side of Canandaigua Lake. Eastward it thins to eighteen feet in the vicinity of Smyrna, Chenango County, and then thickens to 150 feet in Otego Valley. The exact thickness is unknown east of Otego Valley, because of the disappearance of certain fossils used as horizon markers.
At the type section the formation is divisible into three members. The middle member is characterized by Hypothyridina and is traceable to the western limit of the formation and eastward to Unadilla Valley. The uppermost member is continuous from Seneca Lake (west side) to Otego Valley.
In eastern New York between Unadilla and Schoharie valleys the Tully is composed . . .