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Stratigraphic terms—Lower, Middle, and Upper—are capped.  Descriptive time terms—early, middle, and late—are lowercased and are not hyphenated as compound modifiers:
            Lower Jurassic, early Devonian times, Triassic-period rocks, Niagara epoch, Cenozoic era
            Pleistocene or Ice Age or Glacial epoch

New data from rock strata worldwide and continual improvements in methods of determining age mean that no scale is completely definitive. The Geologic Time Scale 2004 (by Gradstein, Ogg, and Smith), the first complete revision of dates and boundaries in 15 years, was a seven-year-long project coordinated by the International Commission on Stratigraphy and has been endorsed by the International Union of Geological Sciences. Although there is still debate about the loss of the Quaternary period, the scale below is the new internationally accepted standard.

Some important aspects of Geologic Time Scale 2004:
The new scale eliminated the most recent period, the Quaternary, and extends the Neogene period up to the present. This is being contested by Quaternary scientists, but there is yet no resolution. Traditionally the Quaternary extended from 1.8 mya to the present; some scientists would like to redefine its span to start at 2.6 mya.

Because scientists in different parts of the world use different geologic formations to establish boundaries between periods, dates can vary. To establish consistency, geologists are now determining "golden spikes," or global standard stratotype sections and points (GSSPs)—internationally accepted representative formations that are dated to establish boundaries between periods.

Using new dating techniques, dates within the Neogene are accurate to within plus or minus 40,000 years.

The earliest time periods are the least defined and most likely to change with more research.

For more information on the 2004 scale, see Geologic Time Scale 2004, published by Cambridge University Press and held by the NG Library (QE508.G395 2004), the website of the International Commission on Stratigraphy, and articles published in scientific journals such as Nature.

The following table is based on Geologic Time Scale 2004, by Gradstein, Ogg, and Smith, Cambridge University Press.

Phanerozoic eon
(542.0 mya - present)
(mya = million years ago)

Cenozoic era
(65.5 mya - present)
Neogene period (23.03 mya - present)
    Holocene epoch (11,500 yrs ago - present)
    Pleistocene epoch (1.8 mya - 11,500 yrs ago)
    Pliocene epoch (5.33 - 1.8 mya)
    Miocene epoch (23.03 - 5.33 mya)
Paleogene period (65.5 - 23.03 mya)
    Oligocene epoch (33.9 - 23.03 mya)
    Eocene epoch (55.8 - 33.9 mya)
    Paleocene (65.5 to 55.8 mya)

  Mesozoic era
(251.0 - 65.5 mya)
Cretaceous period (145.5 - 65.5 mya)
Jurassic period (199.6 - 145.5 mya)
Triassic period (251.0 - 199.6 mya)

  Paleozoic era
(542.0 - 251.0 mya)
Permian period (299.0 - 251.0 mya)
Carboniferous period (359.2 - 299.0 mya)
    Pennsylvanian (318.1 - 299.0 mya)
    Mississippian (359.2 - 318.1 mya)
Devonian period (416.0 - 359.2 mya)
Silurian period (443.7 - 416.0 mya)
Ordovician period (488.3 - 443.7 mya)
Cambrian period (542.0 - 488.3 mya)

Precambrian time
(4,500 - 542 mya)
Proterozoic eon
(2,500 - 542 mya)

Archaean eon
(3,800? - 2,500 mya)

Hadean eon
(? mya)