Usage:
Ellis Group (MT*,ND*,WY*,Canada)
Ellis Formation (MT*,WY*)
Subunits:
GROUP STATUS (alphabetical): Piper Formation (MT*,ND*,WY*) or Piper Limestone (MT*), Rierdon Formation (MT,ND*,WY) or Rierdon Limestone (MT*), Sawtooth Formation (MT*,ND*), Swift Formation (MT*,ND*,WY) or Swift Sandstone (MT*).
Geologic age:
Middle to Late Jurassic (Bajocian to Oxfordian)*
Type section, locality, area and/or origin of name:
Named from Fort Ellis, an old military post in Livingston quadrangle, to east of Bozeman, MT, near which the formation is mapped in USGS Geol. Atlas of the US, Livingston folio, no. 1 (US geologic names lexicon, USGS Bull. 896).
Type section: on north side of U.S. Highway 10, north side of Rocky Creek Canyon, about 3.7 mi southeast of site of Fort Ellis or 7 mi southeast of Bozeman Court House, in sec. 19, T. 2 S., R. 7 E., Gallatin Co., MT (Cobban and others, 1945).
AAPG geologic province:
Central Montana uplift*
Montana folded belt*
Sweetgrass arch*
Williston basin*
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 ([ ]).