A systematic investigation documents the prehistoric earthen mounds, embankments, and fortified enclosures across the Mississippi Valley, providing site descriptions, measurements, plans, and detailed plates. It classifies monument types, reports burial contexts and associated artifacts, and maps geographic distribution. Field observation is combined with interpretive discussion about the builders and their practices, and attention is given to preservation concerns and directions for further inquiry. The result is both meticulous archaeological documentation and broader reflections on the origins and functions of the region's ancient earthworks.