North Boulder group (MTpCAnb;0)

North Boulder group: greenish-gray coarse conglomerate at base, with arkose, conglomerate, and sandy and silty strata above. Locally includes strata resembling the Greyson and Spokane shales, thought to rest directly on the pre-Belt complex of metamorphic rocks. Some areas shown as Spokane shale, and possibly other units may include rocks of the North Boulder group. Distinguished from the vicinity of North Boulder River eastward along and north of Jefferson River and in the Bridger Range.

State	Montana
Name	North Boulder group
Geologic age	Proterozoic \| Mesoproterozoic
Original map label	pCnb
Primary rock type	meta-conglomerate
Secondary rock type	quartzite
Other rock types	meta-argillite; limestone
Lithologic constituents	Major Metamorphic > Metasedimentary > Metaclastic > Quartzite (Greenschist)Weakly metamorphosed sandstones of mainly arkosic content. Metamorphic > Metasedimentary > Metaclastic > Metaconglomerate (Greenschist)meta-conglomerate Minor Metamorphic > Metasedimentary > Metaclastic > Argillite (Greenschist) Incidental Sedimentary > Carbonate (Bed)Weakly metamorphosed limestones.
Map references	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.
Unit references	Ruppel, E.T., O'Neill, J.M., and Lopez, D.A., 1993, Geologic map of the Dillon 1o x 2o quadrangle, Idaho and Montana: U.S. Geological Survey Miscellaneous Investigations Series Map I-1803-H, scale 1:250,000. [http://pubs.er.usgs.gov/publication/i1803H] Chadwick, R.A., 1978, Geochronology of post-Eocene rhyolitic and basaltic volcanism in south-western Montana: Isochron/West, no. 22, p. 25-28. Wilson, A.B., and Elliott, J.E., 1997, Geologic maps of western and northern parts of Gallatin National Forest, south-central Montana: U.S. Geological Survey Geologic Investigations Series Map I-2584, 2 sheets, scale 1:126,720. [http://pubs.er.usgs.gov/publication/i2584] Robinson, G.D. and Barnett, H.F., 1963, Geology of the Three Forks quadrangle, Montana, with sections on petrography of igneous rocks by H.F. Barnett: U.S. Geological Survey Professional Paper 370, 143 p. [http://pubs.er.usgs.gov/publication/pp370] McMannis, W.J., 1955, Geology of the Bridger Range, Montana: Geological Society of America Bulletin, v. 66, no. 11, p. 1385-1430. McMannis, W.J., 1963, LaHood Formation--A coarse facies of the Belt Series in southwestern Montana: Geological Society of America Bulletin, v. 74, no. 4, p. 407-436. Skipp, Betty, and Peterson, A.D., 1965, Geologic map of the Maudlow quadrangle, southwestern Montana: U.S. Geological Survey Miscellaneous Investigations Series Map I-452, 2 sheets, scale 1:24,000. [http://pubs.er.usgs.gov/publication/i452]
Geographic coverage	Gallatin - Jefferson - Madison - Silver Bow