The Manson Impact Structure (MIS) in north-central Iowa, with a diameter of 35 km (22 mi), is the largest impact structure known in the United States. ⁴⁰Ar/³⁹Ar isotope studies of microcline grains from the MIS yielded an age of 65.7 ± 1.0 Ma, an age that is indistinguishable from the age of the K-T boundary impact layer in western United States exposures (Kunk et al., 1989). Izett (1990) and Sharpton et al. (1990) compared rock and mineral grains recovered from the K-T impact layer in exposures around the world with rocks recovered from the Manson structure and concluded that the MIS is the probable source of these grains. Stratigraphic studies of rock samples from water wells in the structure led to the discovery of a terrane of down-dropped blocks within the ring-graben complex, immediately inside the crater’s perimeter. Within this complex, an extensive area of thick Upper Cretaceous marine rocks were identified, apparently structurally preserved during the formation of the crater. These include rocks tentatively identified as Graneros Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, and Pierre Shale, and, when combined with the Dakota Formation rocks also encountered, total a thickness of at least 189 m (630 ft) of Upper Cretaceous strata. The marine units are not present in the immediate region outside of the MIS, having been eroded back to their present-day outcrop limits, as much as 300 km (185 mi) west of the MIS. Studies of these structurally preserved rocks provide valuable information about the extent and nature of Upper Cretaceous marine units near their eastern depositional limits. Additionally, preliminary studies indicate that in some areas of the MIS these units are overlain by impact ejecta deposits, implying the “instantaneous” burial of the Late Cretaceous surface and the preservation of an environmental record that has been eroded elsewhere along the eastern margin areas of the Western Interior.