For geologists, geoscientists, and those people in touch with the geology of the upper Midwest USA Decorah, Iowa is well known, for it is the type locality of what is known as the Decorah Shale. The Shale is a fossiliferous shale that extends throughout most of the upper Midwest. It was formed during the Ordovician period by deposition of sediments in a shallow tropical sea, but it is generally believed to have been formed 470 million years ago.
But recently Decorah can now lay claim to another page in Earths geological history. Recent studies have confirmed the existence of a 3.5 mile (5.6km) wide crater beneath the city of Decorah. The crater is believed to have formed during the Ordovician period approximately 470 million years ago by a bolide at least 200 meters across and exploded with a force of 1000 megatons of TNT (for comparison, the bomb dropped on Hiroshima was only .02 megatons). It is also possible that this crater is part of a multiple impact event known as the 'Ordovician meteor event', as it is roughly the same age as 3 other craters in North America; the Ames Crater in Oklahoma, the Rock Elm crater in Wisconsin, and the Slate Islands crater in Lake Superior.