The names Genesee, Portage and Chemung are indelibly linked to the early Upper Devonian stratigraphy of the Catskill delta in western New York. For over a century generations of stratigraphers struggled in the attempt to correlate these groups eastward from the Genesee Valley into sections around the delta-front at Cayuga Lake. Rapid thickening and intertonguing of facies were the chief difficulties, but the misinterpretation of ammonoids also played a major role in obscuring the facies relationships. Manticoceras sinuosum (Hall, 1843), the index fossil of the Portage Group, was mistaken by James Hall and J. M. Clarke and later workers for species of Ponticeras and Koenenites from horizons in the older Genesee Formation east of Canandaigua Lake.

Some of these Genesee ammonoid horizons have been traced from the pelagic facies in the basin into the benthic facies of the delta-front. One of these horizons in the Upper Penn Yan and Middle Ithaca Members may record an interval of transgression or standstill in the progradation of the delta. The ammonoid horizons in the Genesee Formation at Cayuga Lake are stratigraphic markers which should aid in the taxonomic and biostratigraphic revision of the benthic groups and in the refinement of the paleoecological associations in the delta-front facies.