The Cretaceous period is part of the geologic system of classifying geologic formations. It is part of the Mesozoic era, and is divided into Early and Late epochs. The Cretaceous is preceded by Jurassic period and followed by the Paleogene period of the Cenozoic era.
The end of the period is associated with a worldwide layer of iridium-rich sediments which geologists have located. This layer is often called the KT Boundary, and has been hypothesized to be associated with a cataclysmic event—such as an asteroid or comet impacting the Earth—that is believed by most evolutionists to have wiped out much of the life on the planet.
Under the dates assigned by uniformitarian geologists, it represents approximately the period of Earth's history from 145 million years ago to 65 million years ago. The worldwide iridium layer provides the approximate date of the end of the period, as it has been dated using radioactive isotopes and uniformitarian assumptions to around 65.5 million years old.