A three-fold unit consisting of the 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. Nash Fork is the fifth formation from base of eight formations. The newly named Nash Fork is named for exposures near Nash Fork Creek, T16N, Rs78 and 79W, Albany Co, WY, Northern Rocky Mountain region. No type locality designated. The Anderson Phyllite is a lenticular unit which cannot be traced beyond Anderson mining prospect on Libby Creek, Albany Co, WY. Phyllites, which were the only lithology useful in separating Nash from Ranger, occur at different stratigraphic levels in the Nash Fork. Locally, where marble and phyllite can be distinguished, the phyllite is shown separately on geologic map. Blackwelder's Nash, Anderson, and Ranger not used. Overlies Sugarloaf Quartzite (formerly Sugarloaf Metaquartzite) and underlies Towner Greenstone, both formations of Libby Creek Group. Geologic map; stratigraphic table. Of medial Precambrian age.

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

Revised in that Nash Fork Formation is assigned as the basal formation (of 3) to the upper part of the Libby Creek Group of the Snowy Pass Supergroup (first used) in the Medicine Bow Mountains of south-central WY in the Northern Rocky Mountain region. Is separated from the underlying Sugarloaf Quartzite of the lower part of the Libby Creek by a thrust fault. Is structurally conformable with the overlying Towner Greenstone of the Libby Creek. Consists predominantly of tan metadolomite with thick lenses of black phyllite. Has some thin beds of quartzite, metachert, flat-pebble conglomerate, and iron-formation. Stromatolitic bioherms (about 150 mapped by earlier authors) present in variety of shapes and sizes in the metadolomite. The metadolomite is mostly dolomite and quartz with varying amounts of plagioclase, muscovite, phlogopite, tremolite, talc, apatite, and opaque minerals. Minerals present in phyllite listed. Considered to be an intertidal flat, shallow marine deposit of Proterozoic age. Geologic map; stratigraphic chart.

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

Assigned as the lower formation of the upper part of the Libby Creek Group of the newly defined Snowy Pass Supergroup in the Medicine Bow Mountains, WY in the Northern Rocky Mountain region. Is 1,580 m thick. Separated from the older Sugarloaf Quartzite of the lower part of the Libby Creek by a fault. Overlain by Towner Greenstone of upper part of Libby Creek Group. Correlated with most of Slaughterhouse Formation of the Snowy Pass Group in the Sierra Madre. Geologic map; correlation chart. Of Early Proterozoic age.

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