Divisions of Geologic Time—

Major Chronostratigraphic and Geochronologic Units

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3054

August 2018 Printed on recycled paper

Introduction.—Effective communication in the geosciences 

requires a consistent nomenclature for stratigraphic units and, 

especially, for divisions of geologic time. A geologic time scale 

is composed of standard stratigraphic divisions based on rock 

sequences and is calibrated in years (Harland and others, 1982).

Geologists from the U.S. Geological Survey (USGS), State 

geological surveys, academia, and other organizations require a 

consistent time scale to be used in communicating ages of geologic units in the United States. Many international debates have 

occurred over names and boundaries of units, and various time 

scales have been used by the geoscience community.

Updated time scale.—For consistent usage of time terms, 

the USGS Geologic Names Committee (GNC; see box for members) and the Association of American State Geologists developed the Divisions of Geologic Time; the 2018 update shown 

in figure 1 contains the unit names and boundary age estimates 

ratified by the International Commission on Stratigraphy (2018). 

Scientists may use other published time scales, provided that 

these are specified and referenced (for example, Palmer, 1983; 

Harland and others, 1990; Haq and Eysinga, 1998; Gradstein and 

others, 2012; Walker and others, 2012; Ogg and others, 2016). 

Advances in stratigraphy and geochronology require that any 

time scale be periodically updated. Therefore, the Divisions of 

Geologic Time is dynamic and is modified as needed to include 

accepted changes of unit names and boundary age estimates. This 

fact sheet updates the Divisions of Geologic Time released in 

two previous USGS fact sheets (U.S. Geological Survey Geologic 

Names Committee, 2007, 2010).

The Divisions of Geologic Time (fig. 1) shows the major 

chronostratigraphic (position) and geochronologic (time) units; 

that is, from largest to smaller, enonothem/eon to series/epoch 

divisions. The National Geologic Map Database (https://ngmdb.

usgs.gov/Geolex/stratres/timescales) has additional resources 

and information (such as stage/age terms). The systems of the 

Mesozoic are subdivided into formal series designated by the 

terms “Lower,” “Middle,” and “Upper.” The corresponding 

periods are subdivided into formal epochs designated as “Early,” 

“Middle,” and “Late.” Similarly, the Ordovician and Devonian 

Systems and the Mississippian and Pennsylvanian Subsystems 

of the Paleozoic are subdivided into formal series designated 

as “Lower,” “Middle,” and “Upper”; the formal epochs are 

designated as “Early,” “Middle,” and “Late.” The Silurian and 

Permian are divided into series/epochs that have individual 

names. Because some of the series/epoch names for the Cambrian have been set and some have not, the placeholders “Lower/

Early,” “Middle,” and “Upper/Late” may be used. All other uses 

of “lower/early,” “middle,” and “upper/late” are acceptable only 

as informal units (lowercase). The GNC will not include new 

series/epoch names in the Divisions of Geologic Time until all 

are named for a specific system/period.

Cenozoic.—A controversial issue during the first decade of 

the 21st century was the position of the base of the Quaternary 

System/Period and its status as a formal division of time. After 

much debate, the International Union of Geological Sciences 

formally ratified a new definition of the base of the Quaternary 

and the corresponding base of the Pleistocene Series/Epoch, 

changing it from 1.806 Ma to 2.58 Ma (see box for age terms) 

(Gibbard and others, 2010). Although the Tertiary is not recognized by many international time scales, the GNC agrees that it 

is important that it be recognized as a system/period (Orndorff 

and others, 2010); the map symbols “T” (Tertiary) and “Q” 

(Quaternary) have been used on geologic maps for more than a 

century and are widely used today. However, the use of “Paleogene” and “Neogene” is encouraged.

Anthropocene.—The term “Anthropocene” is used by 

scientists and nonscientists to highlight the concept that we are 

living in a time when human activities have significant effects 

on the global environment. The Anthropocene currently has 

no formal status in the Divisions of Geologic Time and is not 

recognized by the GNC. If international agreement is reached, it 

could be become a series/epoch above the Holocene.

Precambrian.—The informal term “Precambrian” lacks a 

specific stratigraphic rank, but it is capitalized because of tradition. For technical discussions, researchers should seek the most 

accurate terms and refer to the Proterozoic Eon or other formal 

divisions. The term “Precambrian” may be used informally when 

communicating with the public and for general discussions.

—By the U.S. Geological Survey Geologic Names Committee

Members of the U.S. Geological Survey Geologic 

Names Committee, 2018

Randall C. Orndorff (chair), Nancy Stamm (recording secretary), 

Lucy Edwards, Julie Herrick, Leslie Ruppert, Janet Slate, David Soller 

(all USGS), and Berry (Nick) Tew, Jr. (State Geologist of Alabama).

Age Terms

The age of a stratigraphic unit or the time of a geologic event 

may be expressed in years before present (before A.D. 1950). The 

“North American Stratigraphic Code” (North American Commission 

on Stratigraphic Nomenclature, 2005) recommends abbreviations 

for ages in SI (International System of Units) prefixes coupled 

with “a” for annum: ka for kilo-annum (103 years); Ma for megaannum (106

 years); and Ga for giga-annum (109 years). Duration of 

time should be expressed in millions of years (m.y.): for example, 

“deposition began at 85 Ma and continued for 2 m.y.”
Figure 1. Chart of the Divisions of Geologic Time approved 

by the U.S. Geological Survey Geologic Names Committee, 

2018. The chart shows major chronostratigraphic and 

geochronologic units. It reflects ratified unit names and 

boundary estimates from the International Commission on 

Stratigraphy (2018). Box heights are scaled to the relative 

duration of time periods named; different scaling factors 

are used for the Phanerozoic column on the left than for the 

column on the right, which represents a longer time period. 

Map symbols are in parentheses. 

EONOTHEM / EONERATHEM / ERA

SYSTEM, SUBSYSTEM / 

PERIOD, SUBPERIOD

SERIES / 

EPOCH

Age estimates of boundaries in mega-annum (Ma)EONOTHEM / EONERATHEM / ERAAge estimates of boundaries in mega-annum (Ma)

SYSTEM / 

PERIOD

Ediacaran

Cryogenian

Tonian

Stenian

Ectasian

Calymmian

Statherian

Orosirian

Rhyacian

Siderian

~635

~720

1,000

1,200

1,400

1,600

1,800

2,050

2,300

2,500

2,800

3,200

3,600

~4,000

~4,600

458.4 ±0.9

Paleocene

Eocene

Oligocene

Miocene

Pliocene

Pleistocene

Holocene

Upper / Late

Lower / Early

Upper / Late

Middle

Lower / Early

Upper / Late

Middle

Lower / Early

Lopingian

Guadalupian

Cisuralian

Upper / Late

Middle

Lower / Early

Upper / Late

Middle

Lower / Early

Upper / Late

Middle

Lower / Early

Pridoli

Ludlow

Wenlock

Llandovery

Upper / Late

Upper / Late

Middle

Middle

Lower / Early

Lower / Early

2.58 

0.0117 

5.33 

23.03 

34.09 

55.9 

66.0 

100.5 

~145

163.5 ±1.1

174.1 ±1.0

201.3 ±0.2

~237

247.2

251.9

259.1 ±0.5

272.95 ±0.11

298.9 ±0.2

307.0 ±0.1

315.2 ±0.2

323.2 ±0.4

330.9 ±0.2

346.7 ±0.4

358.9 ±0.4

382.7 ±1.6

393.3 ±1.2

419.2 ±3.2

423.0 ±2.3

427.4 ±0.5

433.4 ±0.8

443.8 ±1.5

470.0 ±1.4

485.4 ±1.9

~497

~521

541.0 ±1.0

Phanerozoic

Cenozoic ({) Mesozoic (}) Paleozoic (|)

Quaternary (Q) Tertiary (T)Neogene (N) Paleogene (:) Cretaceous (K) Jurassic (J) Triassic (^) Permian (P) Carboniferous (C) Pennsylvanian (*) Mississippian (M) Devonian (D) Silurian (S) Ordovician (O) Cambrian (_)

Proterozoic (<) Archean (A) Hadean (>)

Neoproterozoic (Z) Mesoproterozoic (Y) Paleoproterozoic (X) Neoarchean Mesoarchean Paleoarchean Eoarchean

References Cited

Gibbard, P.L., Head, M.J., Walker, M.J.C., and the Subcommission on 

Quaternary Stratigraphy, 2010, Formal ratification of the Quater-

nary System/Period and the Pleistocene Series/Epoch with a base 

at 2.58 Ma: Journal of Quaternary Science, v. 25, no. 2, p. 96–102.

Gradstein, F.M., Ogg, J.G., Schmitz, M.D., and Ogg, G.M., eds., 

2012, The geologic time scale 2012: Amsterdam, Elsevier, 2 v. 

[v. 1, 1,144 p.; v. 2, 435 p.]

Haq, B.U., and Eysinga, F.W.B. van, eds., 1998, Geological time 

table (5th ed.): Amsterdam, Elsevier, 1 sheet.

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, 

U.K., Cambridge University Press, 263 p.

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A.G., and Walters, R.W., 1982, A geologic time scale: Cambridge, 

U.K., Cambridge University Press, 131 p.

International Commission on Stratigraphy, 2018, International 

chronostratigraphic chart, version 2018/07: International 

Commission on Stratigraphy chart, accessed July 10, 2018, at 

http://www.stratigraphy.org/index.php/ics-chart-timescale.

North American Commission on Stratigraphic Nomenclature, 2005, 

North American stratigraphic code: American Association of Petro-

leum Geologists Bulletin, v. 89, no. 11, p. 1547–1591. [Also avail-

able at https://ngmdb.usgs.gov/Info/NACSN/Code2/code2.html.]

Ogg, J.G., Ogg, G.M., and Gradstein, F.M., 2016, A concise geologic 

time scale—2016: Amsterdam, Elsevier, 234 p.

Orndorff, R.C., Edwards, L.E., Ruppert, L.F., Craigg, S.D., 

Fullerton, D.S., Stamm, N.R., and Soller, D.R., 2010, Gone or 

just forgotten?—A recommendation to recognize the Tertiary as a 

formal period/system of the Cenozoic [abs.]: Geological Society 

of America Abstracts with Programs, v. 42, no. 5, p. 234.

Palmer, A.R., comp., 1983, The Decade of North American Geology 

[DNAG] 1983 geologic time scale: Geology, v. 11, p. 503–504.

U.S. Geological Survey Geologic Names Committee, 2007, Divisions 

of geologic time—Major chronostratigraphic and geochronologic 

units: U.S. Geological Survey Fact Sheet 2007–3015, 2 p.

U.S. Geological Survey Geologic Names Committee, 2010, Divisions 

of geologic time—Major chronostratigraphic and geochronologic 

units: U.S. Geological Survey Fact Sheet 2010–3059, 2 p.

Walker, J.D., Geissman, J.W., Bowring, S.A., and Babcock, 

L.E., comps., 2012, GSA geologic time scale, version 4.0: 

Geological Society of America chart, accessed July 10, 2018, 

at https://doi.org/10.1130/2012.CTS004R3C.

For more information, please contact

Randall C. Orndorff 

U.S. Geological Survey, MS 926A

12201 Sunrise Valley Drive

Reston, VA 20192

Email: rorndorf@usgs.gov

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20183054