The periods in the geologic time scale represent distinct intervals of time in Earth's history marked by unique geological, climatic, and biological events. Each period is characterized by particular assemblages of fossils, rock formations, and sometimes significant global changes in climate or in the Earth's surface. Periods are subdivisions of the larger geological epochs, which in turn are subdivisions of the even larger geological eras (such as Paleozoic, Mesozoic, and Cenozoic eras). The geologic time scale is a framework that allows geologists and paleontologists to organize, describe, and date the events, rock strata, and fossils that took place during Earth's 4.6 billion-year-long history.
