Manson crater
In 1991 and 1992 the U.S. Geological Survey along with others including the Iowa Geological Survey conducted detailed research in part to test the possible connection of the Manson Crater with the Cretaceous–Paleogene extinction event. The / isotope ratio dating of the core from the impact structure gave an age of about 74 Ma, or about 10 Ma older than the K–T boundary.