Biostratigraphy allowed correlation worldwide. As knowledge of index fossils increased, geologists were able to form a global set of sequence of appearances (and disappearances) of fossils through the geologic strata. These were grouped together into a universal scale: the Geologic Timescale.

Two aspects to the Geologic Timescale:

• Chronostratigraphic: divisions of rock based on time (upper and lower boundaries are isochronous)
• Geochronologic: divisions of time during which each chronostratigraphic unit was formed.

The basic chronostratigraphic unit is the System. Each system is comprised of two or three Series; each Series is comprised of two or more Stages. The boundary between Systems is instantaneous (represents 0 time) and isochronous (the same everywhere).

The basic geochronologic unit is the Period: that chunk of time between the first deposition of a particular System and the start of deposition of the next system. The geochronologic equivalent to the Series is the Epoch, and to the Stage is the Age.

Geologic Timescale Grammar:

• Each System has a formal name (e.g., Devonian, Jurassic, etc.), as does each Stage (Aptian, Messinian, Pliensbachian, etc.).
• The Period and Age names are the same as the System and Stage names: Devonian System & Devonian Period, Aptian Stage and Aptian Age.
• The names of Series typically represent the physical part of the System they are in: Lower, Middle (if any), or Upper: Lower Devonian Series, Middle Devonian Series, Upper Devonian Series
• The names of Epochs represent the part of time of the Period they are in: Early, Middle (if any), or Late: Early Devonian Epoch, Middle Devonian Epoch, Upper Devonian Epoch
  • (A note: some Series/Epochs also have an alternative single names formed in the same manner as other chronostratigraphic unites. for example, the Middle Permian Series is sometimes called the Guadalupian Series, and the Lower Jurassic Series is sometimes called the Liassic Series).
• Not all Systems have three Series: some (such as the Cretaceous) have only two, while the Cambrian and Silurian each have four (that latter are only called by their single word names)
  • And the Carboniferous has six: an Upper, Middle, and Lower Pennsylvanian and an Upper, Middle, and Lower Mississippian Series!
• As the Geologic Timescale was constructed in the 19th Century, knowledge of the absolute dates of the boundaries was not available. Thus, geochronologic units of the same "rank" can have wildly different durations: the Cretaceous is 80 million years long, while the Silurian is 27.7 million.

In 1841 John Phillips grouped Periods into larger units called Eras based on characteristic groups of fossils: the Paleozoic (ancient animal life), Mesozoic (middle animal life), and Cenozoic (recent animal life) Eras. These were based on overall patterns of the changing relative abundance of major types of invertebrates. In particular, the boundaries between the Paleozoic and Mesozoic Eras and the Mesozoic and Cenozoic Eras were major turnovers of the marine realm (as it turns out, major mass extinctions).

Later still, in the 1930s Eras are then grouped into Eons: the largest geochronologic units. Chronostratigraphic equivalents are the Erathem and Eonthem (almost never used professionally: only show up in Historical Geology & Stratigraphy classes).

Here is the hierarchy of units:

Chronostratigraphic Units	Geochronologic Units
(Divisions of Rock)	(Divisions of Time)
Eonthem	Eon
Erathem	Era
System	Period
Series	Epoch
Stage	Age

21st Century Chronostratigraphy: GSSPs and GSSAs
Boundaries between chronostratigraphic (and consquently geochronologic) units have been argued over for scientific (and often personal or political) reasons. A type locality is selected and criteria for the lower boundary of the unit is set, as in lithostratigraphy. In recent years, the type locality concept for chronostratigraphic units have been refined to become the Global Boundary Stratotype Section and Point (abbreviated GSSP) concept. GSSPs are typically defined by faunal changes, although often a geochemical signature is found which is stratigraphically near to the faunal change for additional potential correlation. Ideally, a GSSP should have the following attributes:

• Be easily accessible by public transport and near a major airport
• Be accessible to researchers
• Be extensive enough for long term research and additional sampling
• Contain a radiometrically-datable bed within the outcrop (although not necessarily at the boundary itself)
• Contain a well-documented transition to serve as the basis for the boundary

Stratigraphers are currently engaged in finding GSSPs for previously established Stage boundaries (or, in the case of the Cambrian, replacing old Stages with new ones supportable by GSSPs). About half of the Phanerozoic GSSPs have been assigned already: for an update, go to the GSSP page of the International Commission on Stratigraphy.

In the late 20th and 21st Centuries, official subdivisions of the Precambrian have been proposed. For the youngest two (Ediacaran and Cryogenian Systems) there are plans to find GSSPs for the boundaries. For these, however, boundaries are often arbitrarily selected chronometric dates called Global Standard Stratigraphic Ages (GSSAs).

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Last modified: 3 January 2008