|Geologic age||Phanerozoic | Cenozoic | Tertiary Cretaceous|
|Comments||Unit as mapped includes large area surrounding the Crazy Mountains that is now known to consist of Fort Union Formation (Paleocene).|
|Primary rock type||volcanic rock (aphanitic)|
|Secondary rock type||fine-grained mixed clastic|
|Other rock types||medium-grained mixed clastic|
Igneous > VolcanicThe unit as mapped includes a wide variety of igneous rock types and forms, including intrusive and extrusive varieties. Rock types are mainly andesitic, but range from felsic to intermediate to felsic. Forms include domes, plugs, sills, dikes, tuffs, lava flows and a lot of agglomerate (lahars). These are interlayered with true clastic sedimentary rocks that were reworked from the stratavolcanoes.
Sedimentary > Clastic > Mixed-clastic > Sandstone-Mudstone (Bed)
Sedimentary > Clastic > Mixed-clastic > Conglomerate-sandstone (Bed)
Ross, P.R., Andrews, D.A., and Witkind, I.J., 1955, Geologic map of Montana: U.S. Geological Survey, 2 sheets, scale 1:500,000.
Roberts, A.E., 1972, Cretaceous and early Tertiary depositional and tectonic history of the Livingston area, southwestern Montana, in Geology of the Livingston area, southwestern Montana: U.S. Geological Survey Professional Paper 526-C, p. C1
|Counties||Gallatin - Meagher - Park - Sweet Grass|