Cretaceous Cretaceous The Cretaceous (, ) is a geologic period and system that spans 79 million years from the end of the Jurassic Period million years ago (mya) to the beginning of the Paleogene Period mya. It is the last period of the Mesozoic Era, and the longest period of the Phanerozoic Eon. The Cretaceous Period is usually abbreviated K, for its German translation "Kreide" (chalk, "creta" in Latin). The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas. These oceans and seas were populated with now-extinct marine reptiles,
Geological period Geological period A geological period is one of several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of divisions into which geologists have split the Earth's history. Eons and eras are larger subdivisions than periods while periods themselves may be divided into epochs and ages. The rocks formed during a period belong to a stratigraphic unit called a system. The twelve currently recognised periods of the present eon – the Phanerozoic – are defined by the International Commission on Stratigraphy (ICS) by reference to the stratigraphy