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Geologic Time Scale

The geologic time scale is what geologists use to measure time and events that have happened in Earth’s history. It’s made up of many different eons, eras, periods, epochs and ages. You may have even heard of a few of these in school, like the Triassic Period, the Jurassic Period, and the Cretaceous Period.An image of layers in the rock that tell us about the geologic time scale.

What is an Eon, and how long are they?

An Eon is a way to measure time and is a very, very, very long time. The only thing longer is a Supereon! Most people say that they are too long to measure. Certainly, no living thing is around for a whole eon. When used in the Geologic Time Scale, scientists say that they are about 1 billion years long.

Eras, Periods, Epochs, and Ages

Eras, Epochs, and Ages are also ways to measure time throughout history. They are much shorter than eons, but still very long. To learn about how they’re used to better understand our earth, check out this video:


Eons divide into eras. There are three eras in the current eon: the Palaeozoic Era, the Mesozoic Era, and the Cenozoic era. We are in the Cenozoic era right now! 

An image of the inside of a cave.

Example of geological growth over time


Eras are further divided into periods. There are three periods in the current era. The Paleogene period was when the dinosaurs died. The Neogene period was when the very first animals evolved in North and South America. We know because we start to find their fossils in the rock formed during the Neogene. And there’s the Quaternary period.

The Quaternary period is the shortest period so far and it is the period we are currently in. This is when more animals like mammoths evolved and when the very first humans evolved! 

A dinosaur fossil in stone.

Photo of a fossilized dinosaur.


You guessed it! Periods divide into what we call Epochs. In the Quaternary Period, there are two Epochs: the Pleistocene and the Holocene.
The Pleistocene epoch ended about 11,700 years ago. This was marked by the Ice Ages and the evolution of Mammoths, Saber Tooth Cats, Dire Wolves, and of course Homo sapiens! (That’s us!)

The Holocene epoch is what you think of when you think of recorded world history. There have been many extinctions in this epoch, and scientists say that this is because of human activity.

An illustration showing prehistoric man hunting a mammoth.

Mammoths and Early Humans

<Mammoths and Early Humans>

Other Great Resources:

Geologic Time Scale – Wikipedia: https://en.wikipedia.org/wiki/Geologic_time_scale

A Walk Through Time – KidsDiscover: https://www.kidsdiscover.com/teacherresources/walk-time/

Written By: Hannah Bertoch