Corresponding to eons, eras, periods, epochs and ages, the terms "eonothem", "erathem", "system", "series", "stage" are used to refer to the layers of rock that belong to these stretches of geologic time in Earth's history.
Geologists qualify these units as "early", "mid", and "late" when referring to time, and "lower", "middle", and "upper" when referring to the corresponding rocks.
For example, the lower Jurassic Series in chronostratigraphy corresponds to the early Jurassic Epoch in geochronology.
Therefore, the second timeline shows an expanded view of the most recent eon.
In a similar way, the most recent era is expanded in the third timeline, and the most recent period is expanded in the fourth timeline.
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).
Leonardo da Vinci (1452–1519) concurred with Aristotle's interpretation that fossils represented the remains of ancient life.
The 11th-century Persian geologist Avicenna (Ibn Sina, died 1037) and the 13th-century Dominican bishop Albertus Magnus (died 1280) extended Aristotle's explanation into a theory of a petrifying fluid.
Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon.
The three million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.
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.