An investigation of the geology and ground-water resources of the Pine Ridge Indian Reservation, South Dakota, was made at the request of the Bureau of Indian Affairs as part of the U.S. Department of the Interior's program for the development of the Missouri River basin.  The area of investigation (about 3,930 square miles) includes all the land within the original boundaries of the Reservation, except for that part of Bennett County south of the U.S. Highway 18, where there are only small tracts of Indian land.

Emphasis in the study was placed on mapping major rock units and on determining their potential as sources of water for domestic and stock use.  The geologic map on sheet 1 shows the areal extent of the major rock units, and a summary of the stratigraphy and physical characteristics of the exposed geologic formations is given in the table on this sheet.

The writers appreciate the cooperation of the personnel of the South Dakota State Geological Survey, who provided most of the well data for Bennett County, information about the stratigraphy and geology of selected areas, and copies of published and unpublished geologic maps.  (See map showing areas mapped prior to this investigation.) Also, they appreciate the cooperation of ranch owners and tenants, who permitted access to their lands, and the personnel of the Bureau of Indian affairs, who supplied soils mps and other data.

Data on which this report is based, including logs of wells and test holes, chemical analyses of water and records of wells and springs, have been summarized by the authors in a basic-data report published jointly by the South Dakota Geological Survey and South Dakota Water Resources Commission (Water Resources Report 4, Basic hydrogeologic data - Pine Ridge Indian Reservation, South Dakota).  A selected bibliography of reports pertaining to the geology of the area has been included in the basic-data report.  This atlas will be more useful if studied in conjunction with a copy of the basic-data report.