The South Dakota Geological Survey website provides some good base data–imagery, DRGs, DEMs, and DLGs.  There is a searchable database of core cuttings and the state data server has bedrock data for the eastern portion of the state and a statewide layer, presumably surficial geology, called geology.  Quad-based geological maps are also available (PDF format) in the Publications and Maps section.  The data I downloaded was in shapefile format with minimal attribution.