The western half of New York has become the classic reference area for the Upper Devonian series of North America during the past 120 years. Although studied by many geologists, the stratigraphy of the area has long been the subject of differences of opinion because of the complex intertonguing of the many different types of fine-grained sedimentary rocks that occur in cyclic repetition in the Upper Devonian sequence. The Upper Devonian rocks of western New York have been studied and mapped since 1947 by the writers and by other members of the U.S. Geological Survey. Many exposures were measured in detail by plane-table methods, and key beds, members, and formations were mapped in the area between Lake Erie and Tully, Onondaga County. The strata were subdivided into several mappable formations, and the intertonguing relation of many cyclically recurring facies was worked out. The results of this study make it necessary to revise much of the Upper Devonian nomenclature in the western part of the state. The Genesee group of Chadwick is here called the Genesee formation. The Genesee formation includes all the rocks between the top of the Tully limestone or the top of the Moscow shale of the Hamilton group where the Tully limestone is absent - and the base the Middlesex shale member of the Sonyea formation in the western half of New York. The Genesee formation is composed of 7 intertonguing facies which are designated as members. In the vicinity of Canandaigua Lake and the Genesee River valley the Genesee formation consists, in ascending order, of the following members: the Geneseo shale, Penn Yan shale, Genundewa limestone, and West River shale members. At Cayuga Lake, about 30 mi. E, the writers recognize the following members in the Genesee formation: the Geneseo shale, Penn Yan shale, Sherburne flagstone, Renwick shale, Ithaca, and West River shale members. The top of the formation is about 600 ft. higher stratigraphically than the top of the Genesee group of previous workers. The Sonyea and West Falls formations, which overlie the Genesee formation in ascending order throughout western New York, are also composed of intertonguing cyclically recurring facies similar to the facies that constitute the Genesee formation.