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Therefore, the second timeline shows an expanded view of the most recent eon.

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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.

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).

The tables of geologic time spans, presented here, agree with the nomenclature, dates and standard color codes set forth by the International Commission on Stratigraphy (ICS).

The primary defined divisions of time are eons, in sequence the Hadean, the Archean, the Proterozoic and the Phanerozoic.He also formulated the law of superposition, which states that any given stratum is probably older than those above it and younger than those below it.While Steno's principles were simple, applying them proved challenging.The existence, timing, and terrestrial effects of the Late Heavy Bombardment is still debated.In Ancient Greece, Aristotle (384-322 BCE) observed that fossils of seashells in rocks resembled those found on beaches – he inferred that the fossils in rocks were formed by living animals, and he reasoned that the positions of land and sea had changed over long periods of time.In the late 17th century Nicholas Steno (1638–1686) pronounced the principles underlying geologic (geological) time scales.

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