According to evolutionists, close to 65 million years ago, at the end of the Cretaceous period, and the beginning of what is called the Tertiary period, an event occurred which has come to be known as the K-T Event. This event would have obliterated most life on Earth, plunging the world into something that would now be likened to global nuclear winter, through which few extant species could survive. Although these scientists dispute the nature of the K-T Event (selecting among any number of catastrophes that could have caused the significant global cooling that resulted), most find that the claimed K-T Event was caused by the collision of a massive asteroid with the Earth, the dust and debris from which would have shrouded the sky for thousands of years, cooling Earth considerably. According to this view, the dinosaurs did not survive this cataclysm. A layer of rock containing high concentrations of Iridium, a metal that is extremely rare on earth but common in asteroids, is said to be due to the vaporization and then fall of dust from the meteorite's impact, and its compression within the subsequent geological record.The evidence of a large impact crater can be found in rocks of the Yucatán Peninsula of the supposed age of this layer.  Creationists assert that the assumptions underpinning the methods used by modern geologists are incorrect, and even though the validity of a large impact is accepted, this does not consitute proof that the impact caused the extinction of the dinosaurs—although the abundance of dinosaur fossils in rocks found higher in the rock layers than the impact is drastically less than their abundance in rocks lower down.