Hartley Fort State Preserve in northeastern Iowa protects a rare fortified Native American effigy mound settlement on a bluff-edge terrace above the Upper Iowa River in the Driftless Area. The two-acre site preserves earthworks and ruins linked to mound-building peoples of the Woodland period, with connections to Oneota and Cahokia cultural traditions. Near the river’s junction with the Upper Mississippi, it is significant for the archaeological record it retains rather than for recreation or scenic use. Though designated as a state preserve, it remains privately owned and is not open to the public.