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Geologic units in Yellowstone county, Montana

Additional scientific data in this geographic area

Claggett formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 17 % of this area

Claggett formation: chiefly dark-gray shale with iron-stained concretions; locally sandstone present; numerous bentonite beds near base.

Lithology: shale; sandstone; bentonite

Judith River formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 15 % of this area

Judith River formation: light-colored sandstone at top; lower third somber-gray siltstone and sandy shale; greenish-gray clay and some lignite beds; includes the Parkman sandstone member of south-central Montana.

Lithology: sandstone; mudstone; coal

Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 15 % 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.

Bearpaw shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 12 % 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

Hell Creek formation (Phanerozoic | Mesozoic Cenozoic | Cretaceous-Late Tertiary | Paleocene) at surface, covers 10 % of this area

Hell Creek formation: somber-gray sandstone and greenish shaly clay and mudstone containing dinosaur bones; a few thin lignite and subbituminous coal beds.

Lithology: sandstone; mudstone; coal

Colorado shale (Phanerozoic | Mesozoic | Cretaceous-Early Cretaceous-Middle(?) Cretaceous-Late) at surface, covers 9 % 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.

Frontier formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 7 % of this area

Frontier formation: mainly gray sandy shale; locally Torchlight sandstone member constitutes upper third and thinner Peay sandstone member is at base; contains some thick beds of bentonite.

Telegraph Creek formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 6 % of this area

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

Lithology: mudstone; sandstone

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.

Lithology: sandstone; mudstone; coal

Mowry shale (Phanerozoic | Mesozoic | Cretaceous-Early) at surface, covers 3 % of this area

Mowry shale: chiefly light-gray silicified shale and claystone with minor amounts of sandy shale and sandstone; contains some thick beds of bentonite.

Kootenai formation and associated rocks (Phanerozoic | Mesozoic | Jurassic Cretaceous-Early) at surface, covers 3 % 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.

Fort Union formation (Phanerozoic | Cenozoic | Tertiary Cretaceous-Late | Paleocene) at surface, covers 0.9 % of this area

Fort Union formation: Clay shale, siltstone, and sandstone; local lenses of impure limestone, and numerous lignitic beds; contains Tertiary plant and animal fossils but no dinosaurs; base generally placed at the lowest of the succession of lignite beds within it; includes the Tongue River member, Lebo shale member, and Tullock member.

Lennep sandstone (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.3 % of this area

Lennep sandstone: mainly dark-brown andesitic sandstone with intercalated shale; locally contains thin coal beds.

Lithology: sandstone; mudstone; coal

Thermopolis shale (Phanerozoic | Mesozoic | Cretaceous-Early) at surface, covers 0.1 % of this area

Thermopolis shale: dark-gray shale with some sandstone. The subsurface consists of Muddy sandstone member or Newcastle sandstone member at top, Skull Creek shale member in middle, and Fall River sandstone or First Cat Creek sand of drillers at base.