Eagle Formation

Gray or grayish brown, thick-bedded sandstone, sandy shale, and thin interbedded coal. Chert-pebble lag deposit common at top of formation. Virgelle Member (lower part of formation in parts of central Montana): very light gray, gray, or brown sandstone that coarsens upward. Root zone and thin carbonaceous beds at top of member. Delta, coastal plain, tidal flat, and marine shoreface and foreshore. Thickness as much as 150 m (492 ft).
State Montana
Name Eagle Formation
Geologic age Cretaceous
Lithologic constituents
Major
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Clastic > Mixed-clastic > Sandstone-mudstone (Bed)sandy shale
Minor
Sedimentary > Coal (Bed)
Incidental
Unconsolidated > Coarse-detritalChert-pebble lag deposit common at top of formation
Sedimentary > Carbonate (Bed)thin carbonaceous beds at top of member
Sedimentary > Chemical > Chert (Bed)Chert-pebble lag deposit common at top of formation
References

Vuke, S.M., Porter, K.W., Lonn, J.D., and Lopez, D.A., 2007, Geologic Map of Montana - Compact Disc: Montana Bureau of Mines and Geology: Geologic Map 62-C, 73 p., 2 sheets, scale 1:500,000. This map was digitized in 2012 as a result of a contract between the U.S. Geological Survey and the Montana Bureau of Mines and Geology.

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