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The following four timelines show the geologic time scale.

The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon.

Older time spans, which predate the reliable fossil record (before the Proterozoic eon), are defined by their absolute age.

Geologic units from the same time but different parts of the world often look different and contain different fossils, so the same time-span was historically given different names in different locales.

For example, in North America, the Lower Cambrian is called the Waucoban series that is then subdivided into zones based on succession of trilobites.