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.