Editing Geologic time scale (section)

== Principles ==
{{See also|Age of Earth|History of Earth|Geological history of Earth}}
The geologic time scale is a way of representing [[deep time]] based on events that have occurred through [[History of Earth|Earth's history]], a time span of about [[Age of Earth|4.54&nbsp;±&nbsp;0.05&nbsp;billion years]].<ref name="Dalrymple 2001 AoE">{{cite journal |last=Dalrymple |first=G. Brent |date=2001 |title=The age of the Earth in the twentieth century: a problem (mostly) solved |journal=Special Publications, Geological Society of London |volume=190 |issue=1 |pages=205–221 |bibcode=2001GSLSP.190..205D |doi=10.1144/GSL.SP.2001.190.01.14 |s2cid=130092094}}
</ref> It chronologically organises strata, and subsequently time, by observing fundamental changes in stratigraphy that correspond to major geological or paleontological events. For example, the [[Cretaceous–Paleogene extinction event]], marks the lower boundary of the [[Paleogene]] System/Period and thus the boundary between the [[Cretaceous]] and Paleogene systems/periods. For divisions prior to the [[Cryogenian]], arbitrary numeric boundary definitions ([[Global Standard Stratigraphic Age]]s, GSSAs) are used to divide geologic time. Proposals have been made to better reconcile these divisions with the rock record.<ref name="Shields_2022_pre-Cryogenian">{{Cite journal |last1=Shields |first1=Graham A. |last2=Strachan |first2=Robin A. |last3=Porter |first3=Susannah M. |last4=Halverson |first4=Galen P. |last5=Macdonald |first5=Francis A. |last6=Plumb |first6=Kenneth A. |last7=de Alvarenga |first7=Carlos J. |last8=Banerjee |first8=Dhiraj M. |last9=Bekker |first9=Andrey |last10=Bleeker |first10=Wouter |last11=Brasier |first11=Alexander |date=2022 |title=A template for an improved rock-based subdivision of the pre-Cryogenian timescale |journal=Journal of the Geological Society |language=en |volume=179 |issue=1 |pages=jgs2020–222 |doi=10.1144/jgs2020-222 |bibcode=2022JGSoc.179..222S |s2cid=236285974 |issn=0016-7649|doi-access=free }}</ref><ref name="GTS2012_Precambrian">{{Citation |last1=Van Kranendonk |first1=Martin J. |title=A Chronostratigraphic Division of the Precambrian |date=2012 |work=The Geologic Time Scale |pages=299–392 |url=https://linkinghub.elsevier.com/retrieve/pii/B9780444594259000160 |access-date=2022-04-05 |publisher=Elsevier |language=en |doi=10.1016/b978-0-444-59425-9.00016-0 |isbn=978-0-444-59425-9 |last2=Altermann |first2=Wladyslaw |last3=Beard |first3=Brian L. |last4=Hoffman |first4=Paul F. |last5=Johnson |first5=Clark M. |last6=Kasting |first6=James F. |last7=Melezhik |first7=Victor A. |last8=Nutman |first8=Allen P.}}</ref>

Historically, regional geologic time scales were used<ref name="GTS2012_Precambrian" /> due to the litho- and biostratigraphic differences around the world in time equivalent rocks. The ICS has long worked to reconcile conflicting terminology by standardising globally significant and identifiable stratigraphic [[horizon (geology)|horizons]] that can be used to define the lower boundaries of chronostratigraphic units. Defining chronostratigraphic units in such a manner allows for the use of global, standardised nomenclature. The International Chronostratigraphic Chart represents this ongoing effort.

Several key principles are used to determine the relative relationships of rocks and thus their chronostratigraphic position.<ref name="ICS_chronostratigraphic_units">{{Cite web |title=International Commission on Stratigraphy - Stratigraphic Guide - Chapter 9. Chronostratigraphic Units |url=https://stratigraphy.org/guide/chron |access-date=2024-04-16 |website=stratigraphy.org}}</ref><ref name="Boggs-2011">{{Cite book |last=Boggs |first=Sam |title=Principles of sedimentology and stratigraphy |date=2011 |publisher=Prentice Hall |isbn=978-0-321-74576-7 |edition=5th |location=Boston, Munich}}</ref>

The [[law of superposition]] that states that in undeformed stratigraphic sequences the oldest strata will lie at the bottom of the sequence, while newer material stacks upon the surface.<ref name="Steno_1669" /><ref name="Hutton_1795v1" /><ref name="Lyell_1832v1" /><ref name="Boggs-2011" /> In practice, this means a younger rock will lie on top of an older rock unless there is evidence to suggest otherwise.

The [[principle of original horizontality]] that states layers of sediments will originally be deposited horizontally under the action of gravity.<ref name="Steno_1669" /><ref name="Lyell_1832v1" /><ref name="Boggs-2011" /> However, it is now known that not all sedimentary layers are deposited purely horizontally,<ref name="Boggs-2011" /><ref name=Mehta_et_al_1994>{{Cite journal |last1=Mehta |first1=A |last2=Barker |first2=G C |date=1994-04-01 |title=The dynamics of sand |url=https://iopscience.iop.org/article/10.1088/0034-4885/57/4/002 |journal=Reports on Progress in Physics |volume=57 |issue=4 |pages=383–416 |doi=10.1088/0034-4885/57/4/002 |issn=0034-4885}}</ref> but this principle is still a useful concept.

The [[principle of lateral continuity]] that states layers of sediments extend laterally in all directions until either thinning out or being cut off by a different rock layer, i.e. they are laterally continuous.<ref name="Steno_1669" /> Layers do not extend indefinitely; their limits are controlled by the amount and type of sediment in a [[sedimentary basin]], and the geometry of that basin.

The principle of [[cross-cutting relationships]] that states a rock that cuts across another rock must be younger than the rock it cuts across.<ref name="Steno_1669" /><ref name="Hutton_1795v1" /><ref name="Lyell_1832v1" /><ref name="Boggs-2011" />

The [[law of included fragments]] that states small fragments of one type of rock that are embedded in a second type of rock must have formed first, and were included when the second rock was forming.<ref name="Lyell_1832v1" /><ref name="Boggs-2011" />

The relationships of [[unconformity|unconformities]] which are geologic features representing a gap in the geologic record. Unconformities are formed during periods of erosion or non-deposition, indicating non-continuous sediment deposition.<ref name="Boggs-2011" /> Observing the type and relationships of unconformities in strata allows geologist to understand the relative timing of the strata.

The [[principle of faunal succession]] (where applicable) that states rock strata contain distinctive sets of fossils that succeed each other vertically in a specific and reliable order.<ref name="Smith_1816">{{Cite book |last=Smith |first=William |url=http://www.biodiversitylibrary.org/bibliography/106808 |title=Strata identified by organized fossils, containing prints on colored paper of the most characteristic specimens in each stratum |date=1816-06-01 |publisher=W. Arding |location=London |doi=10.5962/bhl.title.106808}}</ref><ref name="Boggs-2011" /> This allows for a correlation of strata even when the horizon between them is not continuous.