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

Geologic units in Toole county, Montana

[Additional scientific data in this geographic area]

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

Judith River formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 1.0 % 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

Colorado shale (Phanerozoic | Mesozoic | Cretaceous-Early Cretaceous-Middle(?) Cretaceous-Late) at surface, covers 74 % 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

playa (Phanerozoic | Cenozoic | Quaternary) at surface, covers 0.1 % of this area

playa

Lithology: playa

Tertiary coarse-grained intrusive rocks (Phanerozoic | Cenozoic | Tertiary) at surface, covers 0.7 % of this area

Tertiary coarse-grained rocks: Some of these rocks have the composition of quartz diorite, monzonite, and similar rocks but most are alkalic syenite, leucite-rich rocks, and other alkalic varieties. Some of the rocks that are associated areally with the Cretaceous volcanic rocks may be Cretaceous in age. Includes the larger intrusive masses in the eastern part of western Montana and the western part of eastern Montana, most of which have domed upper surfaces. Many of these masses have been regarded as laccoliths, but generally without adequate proof. The Boulder batholith and related stocks are mapped separately.

Lithology: alkalic intrusive rock

Two Medicine formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 4 % 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 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

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

Claggett formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 4 % 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

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

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

Lithology: mudstone; sandstone

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