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Geologic units in Lake county, Montana
[Additional scientific data in this geographic area]
- Glacial lake deposits (Phanerozoic | Cenozoic | Quaternary) at surface, covers 15 % of this area
Glacial lake deposits: mainly silt; believed to have been deposited in lakes formed behind temporary dams of ice or morainal deposits
- Grinnell argillite (Proterozoic | Mesoproterozoic) at surface, covers 7 % of this area
Grinnell argillite: reddish-purple and green argillite, quartzitic argillite, and some quartzite; generally contains some feldspar and carbonates.
- Appekunny argillite (Proterozoic | Mesoproterozoic) at surface, covers 9 % of this area
Appekunny argillite: gray quartzitic argillite and quartizite.
- Missoula group (Proterozoic | Mesoproterozoic) at surface, covers 0.2 % 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.
- Piegan group (Proterozoic | Mesoproterozoic) at surface, covers 15 % of this area
Piegan group: most of the Piegan group is subdivided into formations that are in part equivalent to each other. Where correlations are relatively doubtful the group designation is retained. Carbonate-bearing rocks predominate in the group, but the proportions and character of the impurities in these rocks and the relations to non-carbonate-bearing rocks vary from place to place.
- Wallace formation (Proterozoic | Mesoproterozoic) at surface, covers < 0.1 % of this area
Wallace formation: A heterogeneous unit that includes dark-gray argillite, arenaceous and argillaceous limestone, in part dolomitic, and gray limy quartzite, with shale and sandstone in large areas. The argillaceous and sandy or quartzitic rocks are commonly slightly calcareous. The dominantly carbonate-rich rocks contain "molar tooth" or segregation structures. Commonly characterized by thin laminae. Locally red rocks near the top may represent a transition into the Missoula group.
- Prichard formation (Proterozoic | Mesoproterozoic) at surface, covers < 0.1 % of this area
Prichard formation: dark-gray, generally argillaceous rocks, locally sandy or quartzitic; locally metamorphosed to schist.
- Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 12 % 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.
- Ravalli group (Proterozoic | Mesoproterozoic) at surface, covers 20 % of this area
Ravalli group: a diverse assemblage with numerous subdivisions, only a few of which have been recognized over large enough areas to be distinguished here. Near Idaho the rocks are light colored and siliceous, ranging from pure white quartzite to siliceous shale, mostly in subdued tones of gray, green, purple, and red. Near Missoula the rocks are dark-gray quartzite and quartzitic argillite. In and near Glacier National Park the Grinnell, Appekunny, and Altyn formations are distinguished.
- Tertiary sedimentary rocks, undifferentiated (Phanerozoic | Cenozoic | Tertiary) at surface, covers < 0.1 % 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.
- Glacial drift (Phanerozoic | Cenozoic | Quaternary) at surface, covers 12 % 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