From introduction This report describes large-scale depositional units referred to as architectural elements, interpreted from outcrops of three distinct fluvial sandstone units of the Lakota Formation in the southwestern Black Hills area of Custer and Fall River Counties, South Dakota. These fluvial deposits formed near the end of an episode of continental deposition that began when the Jurassic Sundance sea regressed from the region and ended when the Early Cretaceous Skull Creek sea encroached from the north in Albian time.