The name Sonyea is applied to the combined Middlesex and Cashaqua, which are a natural unit, exposed on Cashaqua Creek, near Portage, NY. Unit is overlain by Attica (combined Rhinestreet and Hatch), and included in Nunda group.

Source: GNU records (USGS DDS-6; Reston GNULEX).

Sonyea Formation (as its Middlesex Shale Member) extended from southwestern NY and western PA into eastern OH, western MD, and WV.

Source: GNU records (USGS DDS-6; Reston GNULEX).

In outcrop in western NY, the Sonyea consists of a basal black shale, the Middlesex Shale Member, and an upper gray shale with limestone nodules, the Cashaqua Shale Member. Members grade eastward into sequence of siltstone and silty shale, which is part of the general turbidite facies of the Catskill delta. The Middlesex is about 65 ft thick in the reference section 0.8 mi south of the village of Middlesex, Yates Co., NY. Thickens eastward to 75 ft at Chidsey Point Gully. Thins westward to 6 ft in Erie Co. Occurs in subsurface of PA, OH, WV, and MD, but is apparently is not present in VA. Makes up most of the Sonyea in the subsurface. The Cashaqua is thin, but recognizable from western NY, through PA and Garret Co., MD, to southwestern WV and has also been identified in southeastern OH. Age is Late Devonian.

Source: GNU records (USGS DDS-6; Reston GNULEX).