Manson crater
The impactor is considered to have been a stony meteorite about  in diameter. The site at the time was the shore of a shallow inland sea. the Western Interior Seaway. The impact disrupted granite, gneiss, and shales of the Precambrian basement as well as sedimentary formations of Paleozoic age, Devonian through Cretaceous. Limestone layers that give the rest of Iowa hard water were instantaneously vaporized down to the basement rocks, giving Manson the anomalous soft water that it has today.