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Geology of the Upper Missouri River Breaks National Monument

Eagle Sandstone Formation
This formation is a light gray to buff colored coarse grained sandstone with ferruginous concretions. It was deposited as beach and barrier facies during regression of an inland sea that covered the central area of the North American Continent during the Cretaceous Period. The formation derives its name from, and is located on, Eagle Creek at its confluence with the Missouri River. It weathers to form statuesque features, arches and hoodoos. Some of the natural features carved from this formation are Eye of the Needle, Hole in the Wall, Steamboat Rock and Seven Sisters.

Judith River Formation
This gray to yellowish massive sandstone is interbedded with silty mudstones and lignites and contains a wide variety of fossil flora and fauna. It formed as a lagoonal deposit when there were many river deltas and tidal flats on the edge of the transgressing Bearpaw sea during late Cretaceous time. It is an abundant source of petrified wood and invertebrate fossils for rock hounds, and extensive vertebrate bone beds also exist. Some duck bill dinosaur finds from this formation are on display at the Museum of the rockies in Bozeman, Montana.

Bearpaw Formation
This is a dark gray to black thinly bedded shale with calcareous concretions. It was deposited in the deep water environment of the cretaceous sea. It is a source of marine shellfish fossils known as amonites and bauculites. Marine reptiles called plieseosaurss and mosasaurs have also been found.

Hell Creek/Lance Formation
This is a dark gray to red and green sandstones, siltstones, carbonaceous shales and lignites are present. They were deposited in a lowland area after the last regression of the Cretaceous age Bearpaw sea. These are the latest Cretaceous-aged rocks exposed in the sequence of fossilized beds and are the source of the T-Rex specimens on display at the Museum of the Rockies.

Alkalic Intrusions 
These are fine-grained igneous rocks, dominated by dark-colored minerals occur as dikes, sills and stocks injected into fractures in the cretaceous age sandstones and shales. They range in age from Tertiary to late Cretaceous. They are more resistant to weathering than the enclosing sedimentary rocks causing them to from promontory features in the surrounding terrain. Some of these that have been named along the river are Dark Butte, LaBarge Rock, Citadel Rock, Pilot Rock and Steamboat Rock. North of the river, some of the natural features are Eagle Buttes, Birdtail Butte and Chimney Rock.  

Baker Monument
This is an erosional remnant of a volcanic vent rising about 180 feet above the surrounding terrain on state lands. Geologically it is one of about 35 features described as diatremes that occur between the Highwood Mountains to the west and the Little Rocky Mountains to the east. The mineral composition of the igneous centers are similar to diamond bearing kimberlites that produce diamonds in other parts of the world. Although diamonds have yet to be found, these features constitute the emphasis of mining exploration in the Missouri Breaks area.  

Saskatchewan Butte
This is an erosional remnant of a volcanic vent rising about 200 feet above the surrounding terrain located on federal land. The Butte is about 10 acres in size and has potential for gemstone occurrence. It is typical of other features described as the Missouri Breaks Diatremes in numerous professional papers and mineral reports prepared by the U.S. Geological Survey and Bureau of Mines.








Last updated: 06-22-2015