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Geologic units in Teton county, Montana
[Additional scientific data in this geographic area]
- Horsethief sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers < 0.1 % of this area
Horsethief sandstone: shaly sandstone grading upward into massive brownish cliff-forming sandstone with local concentrations of magnetite in beds near top.
- Telegraph Creek formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.5 % of this area
Telegraph Creek formation: buff mainly soft, fissile sandy shale with subordinate amounts of concretionary sandstone.
- Mississippian, undifferentiated (Phanerozoic | Paleozoic | Carboniferous Mississippian) at surface, covers 6 % 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.
- Terrace deposits (Phanerozoic | Cenozoic | Tertiary Quaternary) at surface, covers 9 % of this area
Terrace deposits: gravel, sand, and silt of terrace remnants.
- Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 3 % 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.
- Two Medicine formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 16 % 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.
- Eagle sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 4 % 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.
- Tertiary sedimentary rocks, undifferentiated (Phanerozoic | Cenozoic | Tertiary) at surface, covers 3 % 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 8 % 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
- Kootenai formation and associated rocks (Phanerozoic | Mesozoic | Jurassic Cretaceous-Early) at surface, covers 5 % 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.
- Jurassic, undifferentiated (Phanerozoic | Mesozoic | Jurassic) at surface, covers 0.9 % of this area
Jurassic, undifferentiated: calcareous shale and sandstone; includes the Morrison formation, the Ellis group, Sundance formation, and other rocks of Jurassic age.
- Montana group, undifferentiated (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.8 % of this area
Montana group, undifferentiated: mainly sandstone and shale, soft and in part poorly consolidated; some coal; shown undivided only in areas where detailed data are lacking and in these areas some rocks not of Montana age may have been included. In particular, rocks designated Km in southwestern Montana may include some Tertiary strata.
- Diorite and gabbro (Phanerozoic | Mesozoic | Cretaceous(?)) at surface, covers < 0.1 % of this area
Diorite and gabbro: dark, moderately fine grained rocks in sills, dikes, and irregular bodies; not readily distinguished from similar rocks of other ages. Hence in the less well-known areas errors in age assignment may remain. These rocks are assigned to the Cretaceous(?).
- Devonian, undifferentiated (Phanerozoic | Paleozoic | Devonian) at surface, covers 2 % of this area
Devonian, undifferentiated: comprises Three Forks formation consisting of carbonaceous and calcareous shale with some sandstone and limestone, Jefferson limestone, and unnamed units of Devonian age.
- Colorado shale (Phanerozoic | Mesozoic | Cretaceous-Early Cretaceous-Middle(?) Cretaceous-Late) at surface, covers 41 % 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.
- Cambrian, undifferentiated (Phanerozoic | Paleozoic | Cambrian) at surface, covers 0.6 % of this area
Cambrian, undifferentiated: comprises Deadwood formation, in south-central Montana, and Red Lion formation, Dry Creek shale, Hasmark formation, Pilgrim limestone, Silver Hill formation, Park shale, Meagher limestone, Wolsey shale, Flathead quartzite, and other units. In a few places quartzite of Cambrian age may be mapped with the Belt series or quartzite of Belt age with the Cambrian rocks.