Explaining prehistoric mound development requires both anthropological and geoarchaeological perspectives. Illinois Hopewell (Middle Woodland) mounds are remarkable for the range of earthen materials used in their construction. Adding to this variety we document the presence of upturned sod blocks in a mound at the Mound House site. There and at other Illinois sites the sods have dark, 3-10-cm-thick A horizons with minimal or no evidence of B horizon development They required no more than a few decades to form and did so under a grass cover. Humans probably created the conditions that enabled sods to form, but the sod blocks were not cut from soils adjacent to the mounds (unless from another mound surface nearby) or from soils in habitation areas. In some respects, sod blocks would have been a superior earthen building material, appropriately chosen, for instance, to construct stable, near-vertical walls of above-ground tombs. Their selection and use, however, cannot be explained solely according to principles of sound and efficient mound construction. We argue that sod blocks and other kinds of earth for Illinois Hopewell mounds surely had important symbolic dimensions in addition to their structural properties.