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For example, the lower Jurassic Series in chronostratigraphy corresponds to the early Jurassic Epoch in geochronology.
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 three million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.
The first three of these can be referred to collectively as the Precambrian supereon.
Eons are divided into eras, which are in turn divided into periods, epochs and ages.
In the late 17th century Nicholas Steno (1638–1686) pronounced the principles underlying geologic (geological) time scales.
Steno argued that rock layers (or strata) were laid down in succession, and that each represents a "slice" of time.