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Geology and Physiography of Fort Belknap

This page was written by Erin Klauk as part of the DLESE Community Services Project: Integrating Research in Education.

Figure 1: Cross Section of Fort Belknap Reservation. Details

Located at approximately 48 degrees N and 108 degrees W, most of the reservation is located on the northern Great Plains (more info) . The southern portion of Fort Belknap is located in the Little Rocky Mountains (Fort Belknap Reservation (more info) ). Elevations within the reservation range from 2,300 ft. to 5,000 ft. (Fort Belknap Reservation (more info) ). The Fort Belknap Reservation covers alluvial bottom land, glacial till plains, and the Bearpaw and Little Rocky Mountain ranges. Most of the reservation drains into the Milk River (northern boundary of reservation) and consists of flat treeless glacier plains and alluvial bottom lands. The southern part of Fort Belknap drains into the Missouri River and consists of rolling grassland, river "breaks", and the Bearpaw and Little Rocky Mountain ranges which reach an elevation of approximately 6,000 feet (Fort Belknap Indian Community: Community Environmental Profile (more info) ). The western boundary of the reservation is dominated by three buttes; Twin Buttes, Wild Horse Butte and Snake Butte (Fort Belknap Reservation (more info) ).

The Little Rocky Mountains are part of the Central Montana Alkalic Province that extends from Yellowstone National Park to the northeast Canadian border. The Little Rockies are a dissected domal structure due to the laccolithic intrusion of phosphycitic rock (Landusky and Zortman Historic Context (more info) ). This intrusive area consists mainly of stocks and laccoliths ranging from the late Cretaceous to the mid-Tertiary ([Hastings, 1988] ). During the Tertiary, igneous activity domed the Little Rocky Mountains and formed the laccolithic intrusion, dikes and sills where gold-silver mineralization occurred (Status of Mineral Resource Information for the Fort Belknap Indian Reservation, Montana ( This site may be offline. ) ).

The oldest sedimentary rocks in the Little Rocky Mountains are Precambrian amphibolite grade gneiss and schist which are overlain by Cambrian sandstone and shale, Ordovician dolomite, Devonian to Mississippian shale and limestone, and Mesozoic sandstone, shale and conglomerate. ([Hastings, 1988] ). For more detail of rock units, refer to Figure 2.

Figure 2: Detailed stratigraphic column of the Fort Belknap area (after Peterson et al, 1987). Details

To further investigate the geology of Fort Belknap, follow the links below.

Geology of Fort Belknap

Resources containing information about the geology of Fort Belknap:

  • Fort Belknap Reservation. This resource contains general background information about the Fort Belknap Reservation, as well as regional geology, structural geology and geologic history information. Also included are descriptions, detailed tables, and figures illustrating conventional and unconventional play types. These include shallow Cretaceous biogenetic gas, northern and Great Plains medium and low potential biogenic gas, Jurassic and Cretaceous sandstones, fractured/folded anticlines in Mississippian carbonates, Mississippian and Devonian carbonates, fractured Bakken, and Cambrian sands. Other features of interest include a stratigraphic column and detailed structural and time-interval cross-sections of the Fort Belknap area and Rocky Mountain region. (more info)
  • Status of Mineral Resource Information for the Fort Belknap Indian Reservation, Montana. This U.S. Bureau of Mines report provides information about the geology and mineral resources on the Fort Belknap Indian Reservation. (more info)
  • Gold Deposits of Zortman-Landusky, Little Rocky Mountains, Montana. [Hastings, 1988] This paper discusses the history and development of the gold deposits of the Zortman-Landusky mine. Geologic information is provided about the regional setting, structural geology, sedimentary rocks, igneous rocks, and ore deposits of the area. Many figures accompany the text, including a geologic map and stratigraphic column of the Little Rocky Mountains. (citation and description)
  • The Geology of the Little Rocky Mountains. [Weed and Pirsson, 1896] This article describes the geology of the Little Rocky Mountains in north central Montana. The piece describes geologic structures, sedimentary rocks, fossils, igneous rocks, and ore deposits of the area in detail. (citation and description)

Geologic Maps of Fort Belknap

Resources containing geologic map information for Fort Belknap:

To further investigate the physiography of Fort Belknap, follow the links below.

Physiography of Fort Belknap

Resources containing information about the physiography of Fort Belknap.

For ideas on how to use these webpages in a classroom, a Study Guide is provided.

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