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A problem with the impact hypothesis for the Cretaceous/Tertiary (K/T) mass extinction is the apparent absence of an identifiable impact site. The Manson impact structure is a candidate site because of its size (the largest such structure recognized in the United States); in addition, the largest and most abundant shocked quartz grains at the K/T boundary are found relatively close by, and its age is indistinguishable from that of the K/T boundary.

The region of northwest central Iowa that contains the Manson impact structure is covered by Quaternary glacial deposits, which are underlain by Phanerozoic sedimentary rocks (mostly flat-lying carbonates)...

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