Shocked quartz is found worldwide, especially in impact craters and in the thin Cretaceous–Paleogene boundary layer. This is at the contact between Cretaceous and Paleogene rocks. It is further evidence (in addition to iridium enrichment) that the transition between the two geologic periods was caused by a large impact. Eugene Shoemaker discovered it in building stones in the Bavarian town of Nördlingen. The stones were from shocked metamorphic rocks, such as breccia, of a meteor crater.