Usage:
Montana Group (KS,MT*,ND,SD)
Subunits:
(alphabetical): Bearpaw Shale (MT*), Big Skunk Formation (MT), Claggett Formation (MT*,WY*) or Claggett Shale (MT*,WY*), Eagle Formation (ND) or Eagle Sandstone (MT*), Fox Hills Formation (ND,MT*), Gammon Shale (MT*), Golden Spike Formation (MT*), Hell Creek Formation (MT), Horsethief Sandstone (MT*), Judith River Formation (MT*), Lennep Sandstone (MT*), Parkman Sandstone (MT*), Pierre Shale (ND,SD,MT*), Telegraph Creek Formation (MT*), Two Medicine Formation (MT*), Virgelle Sandstone (MT*).
Geologic age:
Late Cretaceous (Santonian to Maastrichtian)*
Type section, locality, area and/or origin of name:
Named from extensive development in Montana, especially in Upper Missouri River region (US geologic names lexicons, USGS Bull. 896, 1200).
AAPG geologic province:
Montana folded belt*
Central Montana uplift*
Sweetgrass arch*
Powder River basin*
Williston basin*
Green River 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 ([ ]).