Pg. 620, 622, 641. Anderson phyllite. Largely recrystallized dark-gray and black slaty rocks, with some calcareous and a few sandy strata; much pyrite. Thickness 1,600 feet. Intergrades with overlying Ranger marble and with underlying Nash marble series. Age is pre-Cambrian (early Algonkian).
Named from old Anderson mining prospect, on Libby Creek, Medicine Bow Mountains, southeastern WY, near contact of this formation with Nash marble series.
[Libby Creek in T. 16 N., R. 78 and 79 W., Centennial and Medicine Bow Peak 7.5-min quadrangles, Albany Co., southeastern WY.]

Source: US geologic names lexicon (USGS Bull. 896, p. 51).

A three-fold unit consisting of Nash Marble (oldest), Anderson Phyllite, and Ranger Marble (youngest) of Blackwelder (1926) is combined into one map unit named the Nash Fork Formation of the Libby Creek Group. Anderson Phyllite is a lenticular unit and cannot be traced beyond the Anderson mining prospect on Libby Creek, Albany Co, WY. Phyllites, which were the only lithology used to separate Nash from Ranger, occur at different stratigraphic levels in the Nash of Blackwelder. Locally, where marble and phyllite can be distinguished on the geologic map of this report, phyllite is shown as separate lithology and as part of the Nash Fork. Blackwelder's Nash, Anderson, and Ranger not used.

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