Usage:
Fortress Mountain Member of Sepulcher Formation of Washburn Group of Absaroka Volcanic Supergroup (MT*,WY*)
Geologic age:
early Tertiary (Eocene)*
Type section, locality, area and/or origin of name:
Type area: Bighorn Peak, at northern boundary of Yellowstone National Park, Gallatin Co., southwestern MT. Named from Fortress Mountain [about 5 mi northwest of type area], Gallatin Co., southwestern MT (Smedes and Prostka, 1972).
AAPG geologic province:
Montana folded belt*
Yellowstone province*
For more information, please contact Nancy Stamm, Geologic Names Committee Secretary.
Asterisk (*) indicates published by U.S. Geological Survey authors.
"No current usage" (†) implies that a name has been abandoned or has fallen into disuse. Former usage and, if known, replacement name given in parentheses ( ).
Slash (/) indicates name conflicts with nomenclatural guidelines (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005, 2021). May be explained within brackets ([ ]).