Baylor University
Department of Geosciences
College of Arts and Sciences

Baylor > Department of Geosciences > About Us > FAQs about Geology > What is the geologic time scale?

What is the geologic time scale?

Since the Eighteenth Century, geologists have been developing a framework for discussing the history of the Earth. This framework, known as the geologic time scale, has historically been based largely on the fossil record. The vast expanse of geologic time has been subdivided into a hierarchical system of time intervals, each of which is identified by a characteristic assemblage of fossil forms. The boundaries between these intervals are commonly (but not universally) marked by significant extinction events that are recognizable in the fossil record. The modern geologic time scale, in which the interval boundaries are also identified by their age as resolved through radiometric dating, was pioneered by Arthur Holmes.

The hierarchy is currently organized as follows:

  • Eons (Precambrian, Phanerozoic)

    • Eras (Paleozoic, Mesozoic, Cenozoic)

      • Period (Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene)

        • Epoch (e.g., Miocene, Pliocene, Pleistocene, Holocene)

          • Age (e.g., Santonian, Campanian, Maastrichtian)

The Precambrian comprises approximately 8/9 of Earth's history, and spans the time interval from Earth's formation at ~4.55 billion years until ~542 million years. At the beginning of the Phanerozoic ~542 million years ago, the first organisms that contained hard parts (bones, shells, teeth) developed and are represented in the fossil record. We currently live in the Holocene epoch of the Neogene period of the Cenozoic era of the Phanerozoic eon.

Copies of the current geologic time scale, as maintained by the U.S. Geological Survey and the International Commission on Stratigraphy, can be downloaded by clicking on the "Time Scale Charts" button at,

and a PDF file containing a brief explanation of the 2004 timescale by Gradstein and Ogg can be downloaded by clicking

here (

References and suggested reading

Cande, S. C., and Kent, D. V., 1992, A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic: Journal of Geophysical Research, v. 97, p. 13917-13951.

Cande, S.C. and Kent, D.V., 1995, Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and

Cenozoic: Journal of Geophysical Research, v. 100, p. 6093-6095.

Gradstein, F.M., and Ogg, J.G., 2004, Geologic time scale 2004 -- why, how, and where next! , 7 p.

F.M.Gradstein, J.G.Ogg, A.G.Smith, F.P.Agterberg, W.Bleeker, R.A.Cooper, V.Davydov, P.Gibbard, L.Hinnov, M.R.

House, L.Lourens, H-P.Luterbacher, J.McArthur, M.J.Melchin, L.J.Robb, J.Shergold, M.Villeneuve,

B.R.Wardlaw, J.Ali, H.Brinkhuis, F.J.Hilgen, J.Hooker, R.J.Howarth, A.H.Knoll, J.Laskar, S.Monechi, J.Powell,

K.A.Plumb, I.Raffi, U.Rhl, A.Sanfilippo, B.Schmitz, N.J.Shackleton, G.A.Shields, H.Strauss, J.Van Dam, J.Veizer,

Th.van Kolfschoten, and D.Wilson. 2004, A Geologic Time Scale 2004: Cambridge, UK, Cambridge University Press, ~ 500 p.

Harland, W.B., Armstrong, R.L., Cox, A.V., Craig, L.E., Smith, A.G., and Smith, D.G., 1990, A geologic time scale 1989: Cambridge, UK,

Cambridge University Press, 263 p.

Harland, W. B., Cox, A. V., Llewellyn, P. G., Pickton, C. A. G., Smith, A. G., and Walters, R., 1982, A geologic time scale

1982: Cambridge, UK, Cambridge University Press, 131 p.

Holmes, A., 1947, The construction of a geological time-scale: Transactions Geological Society of Glasgow, v. 21, p. 117-


Holmes, A., 1960, A revised geological time-scale: Transactions of the Edinburgh Geological Society, v. 17, p. 183-216.

The information on this page was written and approved by the faculty of the Geology Department at Baylor University.