Named Middlesex black shale for town of Middlesex, Middlesex Valley, Yates Co., NY. Unit is basal part of Naples beds in Ontario Co. It is overlain by Cashaqua shale and rests on Genesee shale and is considered of Late Devonian age.

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

Middlesex Shale Member of Sonyea Formation extended from southwestern NY into western PA and into subsurface in eastern OH, WV, and western MD.

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 apparently is not present in VA. Makes up most of the Sonyea in the subsurface. Age is Late Devonian.

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