Lower Middle Devonian strata (Eifelian to lower Givetian stages) of New York are identified under the names Onondaga and Marcellus. As has been New York practice for over 80 years, they represent time-significant allostratigraphic units, which to some degree cut across lithologic boundaries. The Onondaga Formation is a relatively tabular, limestone-dominated unit throughout New York. Strata thin from both east and west into more basinward facies in the central part of the state. In contrast, the (revised) “Marcellus subgroup” forms an eastward-thickening and coarsening wedge of siliciclastic-dominated facies. Marcellus-equivalent strata range in thickness from less than seven meters in the western New York subsurface to an estimated maximum thickness of 580 meters in the Hudson Valley, eastern New York.