Secondary cratering on Earth: The Wyoming impact crater field. Several dozen small impact craters, 10--70-m in size, have been discovered in southeastern Wyoming. A team of U.S. And German geoscientists found these ancient craters in exposed sedimentary layers from the Permian period (280 million years ago). After discovering the first craters, the team initially suspected that they are a crater-strewn field, formed by the breakup of an asteroid that entered the atmosphere. However, with the discovery of more and more craters over a wide area, this interpretation was ruled out. Source 9z.