This coverage contains information about the western limit of glaciation within the Standing Rock Indian Reservation, Sioux County, North Dakota, and Corson County, South Dakota. 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 The geology for Corson County was based on soil maps prepared by the U.S. Bureau of Indian Affairs (1959) and unpublished maps of the U.S Soil Conservation Service, modified by test drilling and field reconnaissance.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 is from the description of the surficial geology by Howells (1982).

The surface geology, like the topography, has been strongly influenced by continental glaciation and by Pleistocene erosion on a land surface underlain by soft unconsolidated deposits of continental and marine shale and sandstone. The Standing Rock Indian Reservation is on the western margin of the midwestern area that was invaded by great ice sheets during the last million years. Though at most only 60 percent of the reservation apparently was covered by glacial ice, the effects of the glaciers were pervasive: not only did the ice sheets grind away the land surface in the areas that they invaded, but they also changed the courses of rivers and created a new river--the Missouri. In addition, changes in weather patterns associated with glaciation profoundly influenced streamflow and erosion in the area not reached by the ice sheets.

Because the Standing Rock Indian Reservation was on the border of the glaciated region, much of the area is free of glacial deposits and most of the glacial deposits present are thin, discontinuous, and of negligible hydrologic importance.