Named for exposures on divide between Hyndman Creek and East Fork of the Big Wood River, Blaine Co, ID in the Snake River basin. Lies in a belt in east-central part of quad. Extends into Custer Co in the Idaho Mountains province. No type locality designated. Has a minimum thickness of 1,500 ft. Divisible into lower limestone, quartzite, and upper limestone members. The lower limestone consists of nearly pure, buff to cream limestone that alternates with beds less than 1 inch to 2 ft thick that contain diopside. The quartzite member is massive, vitreous with rounded grains and crystals of pyrite and has a maximum thickness of 350 ft. The upper limestone is thinly banded, blue-gray to gray-white, coarsely crystalline 805+ ft thick. Geologic maps. Measured sections. Columnar section. Is younger than Hyndman formation (new name) and older than unnamed Ordovician rocks. Algonkian age assigned.

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

Age changed from Precambrian? to Middle and (or) Late? Ordovician. Divided into three members (E-G). Occurs in thrust slices chiefly in northwestern half of metasedimentary belt, Custer Co, ID in Idaho Mountains province. Member E thought to correlate with Ella Dolomite of early Middle Ordovician age. Member F is unfossiliferous. Member G has trilobite fragments and is thought to correlate with the Saturday Mountain Formation of Middle and Late Ordovician age.

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