Named as one of the upper tongues of Green River Formation for exposures along Middle Fork of Stewart Gulch, Rio Blanco Co, CO in the northern and central parts of the Piceance basin. Type section selected at SW1/4 NE1/4 NW1/4 sec 3, T4S, R96W, where it is mostly a brown-gray to light-gray marlstone with beds of gray marly siltstone, light-brown, fine- to medium-grained sandstone, and light-brown, marly shale 50 ft thick. Extends 6-7 mi north of the Rio Blanco-Garfield Co Line. Probably joins the main body of Parachute Creek Member 11-12 mi south of the Garfield Co Line. Ranges in thickness from 10-55 ft. Is predominantly a marlstone throughout its outcrop area. Overlies and underlies Uinta Formation. Is younger than the Coughs Creek Tongue. Assigned to the Eocene.

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