U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3015

March 2007

Divisions of Geologic Time—

Major Chronostratigraphic and Geochronologic Units

Introduction.—Effective communication in the geosciences 

requires consistent uses of stratigraphic nomenclature, especially 

divisions of geologic time. A geologic time scale is composed 

of standard stratigraphic divisions based on rock sequences and 

calibrated in years (Harland and others, 1982). Over the years, the 

development of new dating methods and refinement of previous 

ones have stimulated revisions to geologic time scales.

Since the mid-1990s, geologists from the U.S. Geological 

Survey (USGS), State geological surveys, academia, and other 

organizations have sought 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.

New time scale.—Since the publication by the USGS of the 

7th edition of “Suggestions to Authors” (STA7; Hansen, 1991), 

no other time scale has been officially endorsed by the USGS. 

For consistency purposes, the USGS Geologic Names Committee 

(GNC; see box for members) and the Association of American 

State Geologists (AASG) developed Divisions of Geologic Time

(fig. 1). The Divisions of Geologic Time is based on the time 

scale in STA7 (Hansen, 1991, p. 59) and updates it with the unit 

names and boundary age estimates ratified by the International 

Commission on Stratigraphy (ICS). Scientists should note that 

other published time scales may be used, provided that these are 

specified and referenced (for example, Palmer, 1983; Harland and 

others, 1990; Haq and Eysinga, 1998; Gradstein and others, 2004). 

Advances in stratigraphy and geochronology require that any time 

scale be periodically updated. Therefore, the Divisions of Geologic Time is dynamic and will be modified as needed to include 

accepted changes of unit names and boundary age estimates.

The Divisions of Geologic Time shows the major chronostratigraphic (position) and geochronologic (time) units; that is, 

eonothem/eon to series/epoch divisions. Workers should refer to 

the ICS time scale (Ogg, 2004) for stage/age terms. Most systems 

of the Paleozoic and Mesozoic are subdivided into series utilizing the terms “Lower,” “Middle,” and “Upper.” The geochronologic counterpart terms for subdivisions of periods are “Early,” 

“Middle,” and “Late.” The international geoscience community 

is applying names to these subdivisions based on stratigraphic 

sections at specific localities worldwide. All series/epochs of the 

Silurian and Permian have been named. Although the usage of 

these names is preferred, “lower/early,” “middle,” and “upper/

late” are still acceptable as informal units (lowercase) for these 

two systems/periods. Also the Upper Cambrian has been named 

“Furongian” in the ICS time scale. However, the GNC will not 

recognize this name and include it in the Divisions of Geologic 

Time until all series/epochs of the Cambrian are named.

Cenozoic.—There has been much controversy related to 

subdivisions of the Cenozoic, particularly regarding retention or 

rank of the terms “Tertiary” and “Quaternary.” Although some 

stratigraphers have suggested that these terms be abandoned, 

the issue remains unresolved. If the terms are retained, there 

will need to be agreement on the status of the Quaternary as a 

system/period or subsystem/subperiod. Another controversial 

issue is the position of the base of the Quaternary; is it at the base 

of the Pleistocene or within the upper Pliocene? These positions 

have age estimates of 1.8 Ma and 2.6 Ma, respectively (see box 

for age terms). Until a decision is made on the subdivisions of the 

Cenozoic, the Divisions of Geologic Time will follow the general 

structure of the time scale in STA7 (Hansen, 1991) in accepting 

the use of the terms “Tertiary” and “Quaternary” and the equivalence of the bases of the Quaternary and Pleistocene. The map 

symbols “T” (Tertiary) and “Q” (Quaternary) have been used on 

geologic maps for more than a century and are widely used today.

Precambrian.—For many years, the term “Precambrian” 

was used for the division of time older than the Phanerozoic. 

For consistency with the time scale in STA7 (Hansen, 1991), the 

term “Precambrian” is considered to be informal and without 

specific stratigraphic rank (although it is capitalized).

Map colors.—Geologic maps utilize color schemes based on 

standards that are related to the time scale. Two different schemes 

are used, one by the Commission for the Geologic Map of the 

World (CGMW) and another by the USGS. Colors typically 

shown on USGS geologic maps have been used in a standard 

fashion since the late 1800s and recently have been published in 

the digital cartographic standard for geologic map symbolization 

(Federal Geographic Data Committee, Geologic Data Subcommittee, 2006). The GNC decided in 2006 that the USGS colors 

should be used for large-scale and regional geologic maps of the 

United States. For international maps or small-scale maps (for 

instance, 1:5 million) of the United States or North America, the 

GNC recommends the use of the international colors. Specifications for the USGS colors are in Federal Geographic Data Committee, Geologic Data Subcommittee (2006), and those for the 

CGMW colors are in Gradstein and others (2004).

Acknowledgments.—This Fact Sheet benefited from 

thoughtful reviews by John Repetski (USGS) and Donald E. 

Owen (Lamar University, Beaumont, Tex., and North American 

Commission on Stratigraphic Nomenclature).

By U.S. Geological Survey Geologic Names Committee

Members of the Geologic Names Committee of the 

U.S. Geological Survey, 2006

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

Steven Craigg, Terry D’Erchia, Lucy Edwards, David Fullerton, 

Bonnie Murchey, Leslie Ruppert, David Soller (all of the USGS), 

and Berry (Nick) Tew, Jr. (State Geologist of Alabama).
EONOTHEM / EON

ERATHEM / ERASYSTEM,SUBSYSTEM / PERIOD,SUBPERIOD

SERIES / 

EPOCH

Age estimates of boundaries in mega-annum (Ma) unless otherwise noted

EONOTHEM / EONERATHEM / ERAAge estimates of boundaries in mega-annum (Ma) unless otherwise noted

SYSTEM / 

PERIOD

Holocene

Pleistocene

Pliocene

Neogene

(N)

Quaternary

(Q)

Cretaceous

(K)

Jurassic

(J)

Triassic

(>)

Permian

(P)

Devonian

(D)

Silurian

(S)

Ordovician

(O)

Cambrian

(?)

Tertiary (T)

Cenozoic (

[) Mesozoic (

]) Paleozoic (

\)

Phanerozoic

Paleogene

()

Pennsylvanian

(
)

Carboniferous (C)

Mississippian

(M)

Miocene

Oligocene

Eocene

Paleocene

Upper / Late

Upper / Late

Lower / Early

Middle

Upper / Late

Lower / Early

Middle

Upper / Late

Lower / Early

Middle

Upper / Late

Lower / Early

Middle

Upper / Late

Lower / Early

Middle

Pridoli

Ludlow

Wenlock

Llandovery

Upper / Late

Lower / Early

Middle

Upper / Late

Lower / Early

Middle

Lopingian

Cisuralian

Guadalupian

Lower / Early

11,477 ±85 yr

1.806 ±0.005

5.332 ±0.005

23.03 ±0.05

33.9 ±0.1

55.8 ±0.2

65.5 ±0.3

99.6 ±0.9

145.5 ±4.0

161.2 ±4.0

175.6 ±2.0

199.6 ±0.6

228.0 ±2.0

245.0 ±1.5

251.0 ±0.4

260.4 ±0.7

270.6 ±0.7

299.0 ±0.8

306.5 ±1.0

311.7 ±1.1

318.1 ±1.3

326.4 ±1.6

345.3 ±2.1

359.2 ±2.5

385.3 ±2.6

397.5 ±2.7

416.0 ±2.8

418.7 ±2.7

422.9 ±2.5

428.2 ±2.3

443.7 ±1.5

460.9 ±1.6

471.8 ±1.6

488.3 ±1.7

501.0 ±2.0

513.0 ±2.0

542.0 ±1.0

Ediacaran

Cryogenian

Tonian

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

Proterozoic () Archean (A) Hadean (

")

Stenian

Ectasian

Calymmian

Statherian

Orosirian

Rhyacian

Siderian

630

850

1000

1200

1400

1600

1800

2050

2300

2500

2800

3200

3600

~4000

References Cited

Federal Geographic Data Committee, Geologic Data 

Subcommittee, 2006, FGDC digital cartographic 

standard for geologic map symbolization: Federal 

Geographic Data Committee Document Number 

FGDC–STD–013–2006, 290 p., 2 pls., available online 

at http://ngmdb.usgs.gov/fgdc_gds/.

Gradstein, Felix, Ogg, James, and Smith Alan, eds., 2004, 

A geologic time scale 2004: Cambridge, U.K., Cambridge University Press, 589 p., 1 pl.

Hansen, W.R., ed., 1991, Suggestions to authors of the 

reports of the United States Geological Survey, seventh 

edition [STA7]: Reston, Va., U.S. Geological Survey, 

289 p. (Also available online at 

http://www.nwrc.usgs.gov/lib/lib_sta.htm.)

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.

Harland, W.B., Cox, A.V., Llewellyn, P.G., Picton, 

C.A.G., Smith A.G., and Walters, R.W., 1982, A 

geologic time scale: Cambridge, U.K., Cambridge 

University Press, 131 p.

North American Commission on Stratigraphic Nomenclature, 2005, North American stratigraphic code: American Association of Petroleum Geologists Bulletin, 

v. 89, no. 11, p. 1547–1591. (Also available online at 

http://ngmdb.usgs.gov/Info/NACSN/Code2/

code2.html.)

Ogg, James, comp., 2004, Overview of global boundary 

stratotype sections and points (GSSPs): International 

Commission on Stratigraphy, available online at 

http://www.stratigraphy.org/gssp.htm.

Palmer, A.R., comp., 1983, The Decade of North American Geology [DNAG] 1983 geologic time scale: Geology, v. 11, no. 9, p. 503–504.

For more information, please contact:

Randall C. Orndorff, U.S. Geological Survey

908 National Center, 12201 Sunrise Valley Drive

Reston, VA 20192

E-mail: rorndorf@usgs.gov

Figure 1. Divisions of Geologic 

Time approved by the U.S. Geological Survey Geologic Names 

Committee, 2006. The chart 

shows major chronostratigraphic 

and geochronologic units. It 

reflects ratified unit names and 

boundary age estimates from 

the International Commission on 

Stratigraphy (Ogg, 2004). Map 

symbols are in parentheses.

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 mega-annum, 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.