The answer is eons, eras, and periods.
A period is a basic unit in geology. Two or more periods form an era. Two or more eras form an eon. For example, Phanerozoic is the eon. It consists of three eras. These eras are Paleozoic, Mesozoic, and Cenozoic. Paleozoic is the era that consists of five periods. These periods are Cambrian, Ordovician, Silurian, Devonian, and Permian.
