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McNamara, Missoula Co., Montana, USAi
Regional Level Types
Missoula Co.County

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Latitude & Longitude (WGS84): 46° 54' 38'' North , 113° 40' 35'' West
Latitude & Longitude (decimal): 46.91056,-113.67639
GeoHash:G#: c2r4v5z3u
Locality type:Formation
Köppen climate type:Dfb : Warm-summer humid continental climate

The locality of McNamara is several miles east of the town of Bonner, Missoula County, Montana. The geographic name refers to a historical settlement that has only a few remaining buildings on the north side of the Blackfoot River in the vicinity of the “McNamara Bridge.” Montana Highway 200 (“MT-200") follows the river from Bonner to McNamara Bridge. East of the bridge, MT-200 separates from Blackfoot River, following instead Union Creek that flows over more open terrain. An unpaved road continues in the canyon of Blackfoot River, allowing access to additional outcrops. The west end of McNamara Bridge is indicated by the latitude and longitude values provided for the locality.

The site of McNamara is the type locality of the McNamara Formation, and the formation is exposed along much of this stretch of the Blackfoot River and MT-200. The McNamara Formation is part of the Missoula Group in the Belt Supergroup of Mesoproterozoic age (1,600-1,000 million years). Argillite, siltstone, and quartzite (metamorphosed sandstone) are dominant lithologies in the McNamara. The McNamara here also contains dark-colored chips and spherules of siliceous argillite grading to chert, as well as apparently transported grains of light gray to pink chert, some including sparry calcite partially replaced by chert. This largely detrital interval also contains thin siliceous, phosphoritic, and calcareous layers. Karstic features indicate presence of some substantial carbonate intervals, but their stratigraphic relationships are not well known, as is typical for the Missoula Group and adjacent portions of the Belt Supergroup throughout western Montana (e.g., see Ross, 1959). Gabbro and diorite sills of younger Precambrian age are also exposed in the area.

Regions containing this locality

Rocky Mountains, North America

Mountain Range - 2,070 mineral species & varietal names listed

Select Mineral List Type

Standard Detailed Strunz Dana Chemical Elements

Mineral List

8 valid minerals.

Detailed Mineral List:

Formula: CaSO4
Reference: Norman King Collection
Formula: CaCO3
Reference: Norman King: acid test for calcite; thin sections for chalcedony
Formula: CaMg(CO3)2
Reference: Norman King Collection
Formula: α-Fe3+O(OH)
Reference: Norman King Collection
Formula: Fe2O3
Reference: Norman King Collection
Formula: KAl2(AlSi3O10)(OH)2
Reference: Norman King Collection
Formula: FeS2
Reference: Norman King Collection
Formula: SiO2
Reference: Norman King: acid test for calcite; thin sections for chalcedony
Quartz var: Chalcedony
Formula: SiO2
Reference: Norman King: acid test for calcite; thin sections for chalcedony

List of minerals arranged by Strunz 10th Edition classification

Group 2 - Sulphides and Sulfosalts
Group 4 - Oxides and Hydroxides
var: Chalcedony4.DA.05SiO2
Group 5 - Nitrates and Carbonates
Group 7 - Sulphates, Chromates, Molybdates and Tungstates
Group 9 - Silicates

List of minerals arranged by Dana 8th Edition classification

Group 2 - SULFIDES
AmBnXp, with (m+n):p = 1:2
Group 71 - PHYLLOSILICATES Sheets of Six-Membered Rings
Sheets of 6-membered rings with 2:1 layers
Group 75 - TECTOSILICATES Si Tetrahedral Frameworks
Si Tetrahedral Frameworks - SiO2 with [4] coordinated Si
Unclassified Minerals, Rocks, etc.
var: Chalcedony-SiO2

List of minerals for each chemical element

H Goethiteα-Fe3+O(OH)
H MuscoviteKAl2(AlSi3O10)(OH)2
C CalciteCaCO3
C DolomiteCaMg(CO3)2
O AnhydriteCaSO4
O CalciteCaCO3
O Quartz (var: Chalcedony)SiO2
O DolomiteCaMg(CO3)2
O Goethiteα-Fe3+O(OH)
O HematiteFe2O3
O MuscoviteKAl2(AlSi3O10)(OH)2
O QuartzSiO2
Mg DolomiteCaMg(CO3)2
Al MuscoviteKAl2(AlSi3O10)(OH)2
Si Quartz (var: Chalcedony)SiO2
Si MuscoviteKAl2(AlSi3O10)(OH)2
Si QuartzSiO2
S AnhydriteCaSO4
S PyriteFeS2
K MuscoviteKAl2(AlSi3O10)(OH)2
Ca AnhydriteCaSO4
Ca CalciteCaCO3
Ca DolomiteCaMg(CO3)2
Fe Goethiteα-Fe3+O(OH)
Fe HematiteFe2O3
Fe PyriteFeS2

Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on

Holocene - Pleistocene
0 - 2.588 Ma

ID: 2070418
Surficial deposits

Age: Pleistocene (0 - 2.588 Ma)

Description: alluvium, fan gravel, terrace gravel, landslide deposits, travertine, till, glacial lake deposits, outwash deposits

Lithology: Sand, gravel

Reference: Zientek, M.L., P.D. Derkey, R.J. Miller, J.D. Causet et al. Spatial Databases for the Geology of the Northern Rocky Mountains - Idaho, Montana, and Washington. U.S. Geological Survey Open-File Report 2005-1235 Version 1.0. [25]

1000 - 1600 Ma

ID: 3186914
Mesoproterozoic sedimentary and volcanic rocks

Age: Mesoproterozoic (1000 - 1600 Ma)

Stratigraphic Name: Purcell Group

Comments: Omineca Belt

Lithology: Mafic-intermediate volcanic rocks; limestone,shale,argillite,quartzite,dolomite,siltstone; turbidite

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

1000 - 1600 Ma

ID: 2839687
Upper Missoula Group: McNamara, Bonner, and Mount Shields Formations; locally includes lower Libby Formation in northwestern Montana

Age: Mesoproterozoic (1000 - 1600 Ma)

Stratigraphic Name: McNamara Formation; Bonner Quartzite; Mount Shields Formation; Libby Formation

Description: McNamara Formation: Dense green and red siltite and argillite in mudcracked couplets containing diagnostic chert beds and rip-up clasts. Thickness as much as 1,650 m (5,413 ft). Bonner Formation: Pink, cross-bedded, feldspathic, medium- to coarse grained quartzite. Thickness as much as 580 m (1,903 ft). Mount Shields Formation: Upper part: red quartzite, siltite, and argillite in mud-cracked couples and couplets with abundant salt casts. Lower part: light gray, flat-laminated, feldspathic, fine-grained quartzite. Thickness as much as 2,000 m (6,562 ft). In northwestern Montana - Libby Formation:Light to dark gray and greenish gray siltite and argillite with subordinate quartzite in mud-cracked couplets. Thickness as much as 2,300 m (7,546 ft).

Comments: Original map source: Vuke, S.M., Porter, K.W., Lonn, J.D., and Lopez, D.A., 2007, Geologic Map of Montana - Compact Disc: Montana Bureau of Mines and Geology: Geologic Map 62-C, 73 p., 2 sheets, scale 1:500,000. This map was digitized in 2012 as a result of a contract betwe

Lithology: Major:{metasiltstone,argillite,quartzite}, Incidental:{chert, salt}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Data and map coding provided by, used under Creative Commons Attribution 4.0 License


Sort by

Year (asc) Year (desc) Author (A-Z) Author (Z-A)
Ross, C. P. (1959) Geology of Glacier National Park and the Flathead region, northwestern Montana. U.S. Geological Survey Professional Paper 296, 125 pages.
Nelson, W.H., Dobell, J.P. (1961) Geology of the Bonner Quadrangle, Montana. U.S. Geological Survey Bulletin 1111-F, 189-235 (see p. 203).
Berg, R.B. (2006) Geologic Map of the Upper Clark Fork Valley between Bearmouth and Missoula, southwestern Montana. Montana Bureau of Mines and Geology, Open-File Report MBMG 505 (see p. 15).
Lonn, J.D., McDonald, C., Sears, J.W., Smith, L.N. (2010) Geologic Map of the Missoula East 30' x 60' Quadrangle, Western Montana. Montana Bureau of Mines and Geology Open File 593, plates 1 and 2.

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