This coverage contains information about the surficial geology for the area within the Standing Rock Indian Reservation, Sioux County, North Dakota, and Corson County, South Dakota. Identified units include: 1) alluvium, terrace, outwash, colluvium, eolian deposits, and buried-valley fill; 2) glacial till; 3) Fort Union Formation; 4) Hell Creek Formation; 5) Fox Hills Formation; and 6) Pierre Shale. The digital data were produced by the U.S. Geological Survey (USGS) in cooperation with the U.S. Environmental Protection Agency.

Figure 5 in Howells (1982) was scanned and digitized on-screen to create this coverage. See cross reference information for more detail.

According to the map credit for figure 5, the geology for Sioux County was based on soil maps prepared by the U.S. Bureau of Indian Affairs (1959), data collected by Randich (1975), and a geologic map by Carlson (1978). The geology for Corson County was based on soil maps prepared by the U.S. Bureau of Indian Affairs (1959) and unpublished maps of the U.S. Soil Conservation Service, modified by test drilling and field reconnaissance.

The following definitions were used for figure 5.

Alluvium, terrace, outwash, colluvium, eolian deposits, and buried-valley fill-- The small scale prevents differentiating these deposits on this map. Alluvium is water-laid material deposited on flood plains and valleys of rivers and streams. Most of the material is clay, silt, and fine sand but there is some coarser material. Terraces are older alluvial deposits that now are above the level of the flood plain because the stream has eroded its valley deeper since deposition of the sediment. Outwash was deposited in streams and lakes formed by melting glaciers. Maximum thickness of alluvium or outwash is less than 120 feet; of terraces, more than 50 feet. Colluvium is landslide and slumpage detritus, commonly deposited at the foot of the steep slopes. The material usually is fragments of shale, silt, and sand, but may include gravel or sandstone cobbles from terraces or sandstone beds capping higher benches or buttes. Deposits may exceed 100 feet in thickness. Eolian deposits are windblown material, mostly silt and fine sand, but include some clay and, in some areas, much medium sand. Deposits are as much as 20 feet thick, but a thin coating, as little as a fraction of an inch thick, overlies most of the area. Buried valley fill is mostly glacial stream and outwash deposits but includes some till. The material is mostly moderately-sorted gravel, sand, silt, and clay. Maximum thickness locally exceeds 300 feet. Quaternary Period.

Glacial till -- Deposited beneath or at the margins of the continental glaciers. The material is a heterogeneous mixture of sizes from clay to boulders. Maximum thickness probably is less than 40 feet. Quaternary Period.

Fort Union Formation -- Gray to buff interbedded very fine to medium-grained sandstone, siltstone, claystone, silty clay, and shale; thin carbonaceous or lignitic beds near the base. Contains three widely persistent sandstone beds. Maximum thickness is greater than 400 feet. Tertiary Period.

Hell Creek Formation -- Somber-colored, soft clay shale and buff to gray, weakly-cemented, coarse- to fine-grained sandstone and siltstone. Contains lignitic lenses and thin, black carbonaceous shale beds. Maximum thickness may be greater than 400 feet. Cretaceous Period.

Fox Hills Formation -- Dark- to light-gray silty and sandy clay, clayey silt, and very fine-grained sandstone; overlain, in Corson County, by dark-gray to yellowish-orange, weakly-cemented, very fine-grained sandstone. The upper part of the formation is a thinly-bedded sequence of clay, silt, and sand, that contains discontinuous beds of silica-cemented very fine-grained sandstone. Contacts between the Hell Creek and Pierre Shale are gradational. Maximum thickness is about 400 feet. Cretaceous Period.

Pierre Shale -- Gray to brown, tough, gummy to friable shale, noncalcareous to highly calcareous, contains widely persistent zones of bentonite and of iron manganese or limestone concretions. Where exposed at the surface, the top few feet commonly is weathered. Maximum thickness is about 1,400 feet. Cretaceous Period.