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...