When a scientist studied the clay layer in the late Cretaceous strata in Gubio, Italy, he found that the content of trace element iridium increased 30-160 times than that in other periods  Later, people took samples from many places around the world and came to the same conclusion: the abnormal increase of iridium content in the late Cretaceous strata is indeed universal
Rock formations at the end of the Cretaceous are mostly igneous rock formed as lava cooled, while sedimentary rock deposited by dust from asteroid impacts makes up only a tiny fraction of the surface
 The scientists speculated that, in the late Cretaceous strata, a huge asteroid struck the earth, creating dust that blocked out the sun and changed the surface climate, leading to the demise of the dinosaurs