The geologic time scale is used by geologists and other scientists to describe the timing and relationships between events that have occurred during the history of the Earth. The time of Earth's past has been organized into various units according to events which took place in each period, usually delimited by major events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Palaeogene period is defined by the extinction event that marked the demise of the dinosaurs and of many marine species. The largest defined unit of time is the Eon. Eons are divided into Eras, which are in turn divided into Periods, Epochs and Stages.
Source: Wikipedia
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