Proposed the name Penn Yan tongue for lower shales of West River shale. Lower part consists of interbedded black and blue-black shales, blue-gray shales and thin limy argillaceous beds; unfossiliferous flags interbedded in dark shales. In upper part, shales are generally lighter colored and more fossiliferous. Concretions are plentiful; marcasite nodules present. Unit is about 140 ft thick in type area but thins eastward. Contact with underlying Geneseo shale is difficult to place but is placed at base of sequence of dark limy shales and interbedded shaly limestone layers. Top of formation is placed arbitrarily at base of Crosby lentil of Starkey tongue, although Starkey-type flags begin lower.

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

Penn Yan Shale Member of Genesee Formation extended from southwestern NY into western PA. West of Canandaigua Lake in western NY, the boundary between the Middle and the Upper Devonian lies within the lower part of the Penn Yan. Unit consists mainly of dark- to medium-gray shale or mudrock and some beds of black shale, abundant limestone nodules, and a few thin beds of dark-gray siltstone. The Penn Yan is overstepped to the southwest along the Middle Devonian unconformity by the West River Shale Member. To the east of Canandaigua Lake, the Penn Yan and the West River grad laterally into the Sherburne Flagstone and Ithaca Members.

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