Earth’s geologic history is divided 
into four Eons, which are further 
divided into smaller and smaller 
sections. From the most amount of 
time to the least, they are Eons, 
Eras, Periods, Epochs, Subepochs, 
and Ages. These sections are not 
divided based on the amount of 
time but on major events or 
changes in the planet. Generally 
though, Eons measure time in 
billions of years, Eras cover 
hundreds of millions of years, 
Periods are tens of millions of 
years, Epochs are millions and 
Ages are thousands. Humans 
currently live in the Phanerozoic 
Eon, Cenozoic Era, Quaternary 
Period, Holocene Epoch, and 

Meghalayan Age.

Geologists learn about the past by 
studying Packages of rocks called 
formations. Formations are usually 
confined to a specific time and 
depositional style. The Hell Creek 
Formation, for instance, is a 
famous formation from the 
American Northwest and records 
the last million years of the 
Maastrichtian Age. It ended with 
the impact event that killed the 
dinosaurs.
