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Mineral Resources > Online Spatial Data > Geology > by state > Montana Geology

Geologic units in Glacier county, Montana

[Additional scientific data in this geographic area]

Two Medicine formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 25 % of this area

Two Medicine formation: greenish-gray clay with local nodular limestone and crossbedded sandstone; locally some coal in lower part. Rocks equivalent to Judith River formation, Claggett formation, and upper part of Eagle sandstone are included in this unit.

Lithology: clastic; volcanic rock (aphanitic); limestone; bentonite

Eagle sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.6 % of this area

Eagle sandstone: sandstone and shaly sandstone with lignite beds in basal part of upper unit (Keu). The Virgelle sandstone member (Kvi) at base is distinguished where possible. Near Yellowstone National Park rocks incorrectly called Laramide in early reports and now regarded as roughly equivalent to the Eagle sandstone are tentatively mapped as Eagle sandstone. Gray to buff massive cliff-forming sandstone with iron stained concretions in the upper part. In western Montana the Virgelle is a formation and in central Montana it is the basal member of the Eagle sandstone.

Lithology: sandstone; siltstone

Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 1 % of this area

Alluvium: mainly valley fill consisting of silt, sand, and gravel; includes some terrace deposits and glacial drift of Pleistocene age in some areas; locally includes hot spring tufa. The older part of the alluvium, where present, is probably of Pliocene age.

Lithology: alluvium; glacial drift; carbonate

Terrace deposits (Phanerozoic | Cenozoic | Tertiary Quaternary) at surface, covers 1 % of this area

Terrace deposits: gravel, sand, and silt of terrace remnants.

Lithology: alluvial terrace

Jurassic, undifferentiated (Phanerozoic | Mesozoic | Jurassic) at surface, covers < 0.1 % of this area

Jurassic, undifferentiated: calcareous shale and sandstone; includes the Morrison formation, the Ellis group, Sundance formation, and other rocks of Jurassic age.

Lithology: fine-grained mixed clastic; limestone; evaporite

Glacial drift (Phanerozoic | Cenozoic | Quaternary) at surface, covers 18 % of this area

Glacial drift: morainal and outwash plain deposits of mountain glaciers; mainly ill-sorted and poorly rounded boulders, cobbles, pebbles, and sand; may include alluvium in places

Lithology: glacial drift; alluvium

Tertiary sedimentary rocks, undifferentiated (Phanerozoic | Cenozoic | Tertiary) at surface, covers 0.7 % of this area

Tertiary sedimentary rocks, undifferentiated: clastic deposits in western Montana, mostly in valleys, and in most places not divided into formations; mostly poorly consolidated gravel, sand, silt, and clay; includes some tuffaceous material and locally lenses of lignite and bentonite; a little hot spring tufa; and in areas not yet mapped in detail, lava may be included. These rocks were in part laid down in lakes but a large part was formed in streams and alluvial fans. These rocks are Tertiary in age and as now mapped may even include some beds of Cretaceous age. Some late Tertiary terrace deposits may be included.

Lithology: medium-grained mixed clastic; coarse-grained mixed clastic; lava flow; ash-flow tuff; tuff; coal; bentonite; carbonate

Kootenai formation and associated rocks (Phanerozoic | Mesozoic | Jurassic Cretaceous-Early) at surface, covers 2 % of this area

Kootenai formation and associated rocks: conglomerate, sandstone, shale, and mudstone; purplish and green beds are common; mainly the Kootenai; in southern Montana includes strata that have been mapped as Cloverly formation. Includes Second Cat Creek and Third Cat Creek sands of drillers in central part of State; Sunburst sand of drillers in north-central part; and Cut Bank sand of drillers in western part. As here mapped, may locally include thin units of Jurassic age.

Lithology: medium-grained mixed clastic; mudstone; limestone

Willow Creek formation (Phanerozoic | Cenozoic | Cretaceous-Late) at surface, covers 5 % of this area

Willow Creek formation: Variegated clay and soft sandstone, chiefly maroon to chocolate brown; local lenses of purple-gray nodular limestone.

Lithology: mudstone; sandstone; limestone; conglomerate

Horsethief sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 3 % of this area

Horsethief sandstone: shaly sandstone grading upward into massive brownish cliff-forming sandstone with local concentrations of magnetite in beds near top.

Lithology: sandstone; conglomerate; shale

Mississippian, undifferentiated (Phanerozoic | Paleozoic | Carboniferous Mississippian) at surface, covers < 0.1 % of this area

Mississippian, undifferentiated: sandstone, shale, and limestone, in part dolomitic, with chert nodules, some quartzite; includes Big Snowy group in central part of State, Madison group in central and southwestern parts; and Hannan and Brazer limestones in the northwestern part; may include small amounts of Pennsylvanian rocks in areas where stratigraphic studies are incomplete.

Lithology: carbonate; fine-grained mixed clastic; medium-grained mixed clastic; chert

Bearpaw shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 4 % of this area

Bearpaw shale: Dark-gray and brownish clay shale; thick units of nonfissile bentonitic shale; calcareous and ferruginous concretions throughout; contains some thick bentonite beds.

Lithology: shale; sandstone; bentonite

Telegraph Creek formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers < 0.1 % of this area

Telegraph Creek formation: buff mainly soft, fissile sandy shale with subordinate amounts of concretionary sandstone.

Lithology: mudstone; sandstone

Siyeh limestone (Proterozoic | Mesoproterozoic) at surface, covers 4 % of this area

Siyeh limestone: crystalline limestone of varying impurity; mostly thick bedded or massive, but with thin wavy banding on fresh fractures within the beds; dusky blue or greenish where fresh, weathering orange and brownish; "molar tooth", similar structures, and stromatolites are common.

Lithology: carbonate; quartzite; meta-argillite

St. Mary River formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 7 % of this area

St. Mary River formation: Greenish-gray clay with local nodular limestone and crossbedded sandstone.

Lithology: mudstone; sandstone; limestone

Missoula group (Proterozoic | Mesoproterozoic) at surface, covers 2 % of this area

Missoula group: chiefly red, maroon, or purple argillite; sandy or quartzitic argillite, and generally impure quartzite and limestone. The larger limestone masses are similar to the Siyeh limestone of the Piegan group. The Missoula group includes numerous named formations, most of which cannot be traced with confidence far from their type localities. Among these are the Marsh shale in the Helena region, the Striped Peak and Libby formations in northwestern Montana, five near Missoula, and others in and south of Glacier National Park.

Lithology: quartzite; meta-argillite; carbonate; meta-conglomerate

Cretaceous, undifferentiated (Phanerozoic | Mesozoic | Cretaceous) at surface, covers 17 % of this area

Cretaceous, undifferentiated: used in only a few areas, such as the Disturbed Belt east of Glacier National Park, where subdivision in difficult.

Lithology: mudstone; sandstone; limestone

Appekunny argillite (Proterozoic | Mesoproterozoic) at surface, covers 3 % of this area

Appekunny argillite: gray quartzitic argillite and quartizite.

Lithology: meta-argillite; quartzite

Grinnell argillite (Proterozoic | Mesoproterozoic) at surface, covers 3 % of this area

Grinnell argillite: reddish-purple and green argillite, quartzitic argillite, and some quartzite; generally contains some feldspar and carbonates.

Lithology: meta-argillite; quartzite; meta-conglomerate

Altyn limestone (Proterozoic | Mesoproterozoic) at surface, covers 2 % of this area

Altyn limestone: dominantly dolomite and magnesian limestone that weathers to a distinctive yellowish brown; some feldspathic quartzite and conglomerate. Correlation with the Ravalli group is provisional.

Lithology: carbonate; quartzite; meta-argillite

Colorado shale (Phanerozoic | Mesozoic | Cretaceous-Early Cretaceous-Middle(?) Cretaceous-Late) at surface, covers 0.6 % of this area

Colorado shale: dark-gray shale and siltstone with many concretions and sandy units. Includes equivalents of Fall River, Skull Creek, Newcastle, Mowry, Belle Fourche, Greenhorn, Carlile, and Niobrara formations, and locally Telegraph Creek formation. In the less well-known areas beds of other ages may have been included.

Lithology: fine-grained mixed clastic