The time since the Earth was formed 4.6 billion years ago (Ga), and encompasses the entire geologic history of the Earth. It is usually divided chronologically into the following hierarchies of time units eon, era, period, and epoch. The eons are: (1) Hadean (4.6  4 Ga), (2) Archaean (4-2.5 Ga), (3) Proterozoic (2.5- 0.545 Ga), and (4) Phanerozoic (545 million years ago to the present). The Phanerozoic eon is divided into the following eras: (1) Paleozoic (545-245 Ma), (2) Mesozoic (2.45-66.4 Ma), and (3) Cenozoic (66.4-0 Ma). The Cenozoic era is divided into following periods: (1) Tertiary (66.4-1.6 Ma), and (2) Quaternary (1.6 Ma-present). The Quaternary period is divided into the following epochs: (1) Pleistocene (1.6 Ma  10,000 years ago), and (2) Holocene (10,000 years ago to the present). For a detailed discussion of geologic time, See: Eicher (2002), and Hancock and Skinner (2000)