Pg. 6 (fig. 1), 115-120, pls. 2, 4. Au Train formation. Dominantly medium- to fine-grained dolomitic sandstone. Comprises two unnamed members. Lower member, approximately 100 feet thick, characterized by abundant glauconite, which occurs as disseminated grains in dolomitic sand and concentrated in thin dark-green beds. Thin beds of glauconite more abundant near base of section at 3- to 12-foot intervals. Bedding in glauconitic member thin and undulatory; color buff to brownish gray, except for more glauconitic areas where color is speckled green or solid dark green. Some of more dolomitic beds blue to bluish gray. Much of weathered surface a definite brown. In upper member, glauconite completely absent and thin sandstone lenses numerous. Sandstone beds, more than 10 feet thick, also present. Sandstone characteristically medium- to fine-grained and contains few thin lenses of blue to greenish shale. Maximum thickness of formation, indicated in core samples, slightly more then 300 feet but only lower 125 feet exposed at type locality. Overlies Miners Castle member (member) of Munising formation with unconformity. [Hamblin, following Grabau (1906), proposes that Au Train formation be used for these rocks, which had been included under Hermansville limestone.]
Type locality: Type locality: Au Train Falls, in Alger Co., northern MI, where rocks are best exposed and sections are thickest.

Source: US geologic names lexicon (USGS Bull. 1200, p. 174-175).

U.S. Geological Survey currently [ca. 1966] designates the age of the Au Train as Early and Middle Ordovician on the basis of a study now in progress.

Source: US geologic names lexicon (USGS Bull. 1200, p. 174-175).