Named as a member near top of Trout Peak Trachyandesite of Sunlight Group (new) of Absaroka Volcanic Supergroup. Is named for Pacific Creek, the type area, near Two Ocean Pass, Yellowstone National Park. Extends south into Teton Co. Both localities are in Yellowstone province. Is an extensive, though discontinuous, pale-brown to yellow-brown, poorly resistant, partially welded ash-flow tuff that contains abundant phenocrysts of anorthoclase and biotite. Is 80 ft thick at type and 300 ft in east-central part of Yellowstone Park. Has an age of 48.0 +/-1.3 m.y. (K-Ar on biotite), or middle Eocene.

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