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This clock representation shows some of the major units of geological time and definitive events of Earth history.The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga (billion years ago).In the late 17th century Nicholas Steno (1638–1686) pronounced the principles underlying geologic (geological) time scales.
The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic." Evidence from radiometric dating indicates that Earth is about 4.54 billion years old.
For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the non-avian dinosaurs and many other groups of life.
Older time spans, which predate the reliable fossil record (before the Proterozoic eon), are defined by their absolute age.
The geology or deep time of Earth's past has been organized into various units according to events which took place.
Different spans of time on the GTS are usually marked by corresponding changes in the composition of strata which indicate major geological or paleontological events, such as mass extinctions.