The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields pertaining to geological

stratigraphy Stratigraphy is a branch of geology concerned with the study of rock (geology), rock layers (Stratum, strata) and layering (stratification). It is primarily used in the study of sedimentary rock, sedimentary and layered volcanic rocks. Stratigrap ...

. In its plainest form, it states that in undeformed stratigraphic sequences, the oldest

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

will lie at the bottom of the sequence, while newer material stacks upon the surface to form new deposits over time. This is paramount to stratigraphic dating, which requires a set of assumptions, including that the law of superposition holds true and that an object cannot be older than the materials of which it is composed. To illustrate the practical applications of superposition in scientific inquiry, sedimentary rock that has not been deformed by more than 90° will exhibit the oldest layers on the bottom, thus enabling paleontologists and paleobotanists to identify the relative ages of any fossils found within the strata, with the remains of the most archaic lifeforms confined to the lowest. These findings can inform the community on the fossil record covering the relevant strata, to determine which species coexisted temporally and which species existed successively in perhaps an

evolutionarily Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation t ...

phylogenetically In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

relevant way.

History

The law of superposition was first proposed in 1669 by the Danish scientist

Nicolas Steno Niels Steensen ( da, Niels Steensen; Latinized to ''Nicolaus Steno'' or ''Nicolaus Stenonius''; 1 January 1638 – 25 November 1686William "Strata" Smith, who used it to produce the first geologic map of Britain.Patrick Wyse Jackson, ''The Chronologers' Quest: The Search for the Age of the Earth'', Cambridge University Press, 2006 , pp.127-8. It is the first of

Smith's laws In geology, Smith's laws are two rules, formulated by William Smith (1769–1839), which aid in the determination of geological succession. They are fundamental to the production of geological maps. The laws Both laws were first published b ...

, which were formally published in ''Strata Identified by Fossils'' (1816–1819).

Archaeological considerations

Superposition in archaeology and especially in stratification use during

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is slightly different as the processes involved in laying down archaeological strata are somewhat different from geological processes. Man-made intrusions and activity in the archaeological record need not form chronologically from top to bottom or be deformed from the horizontal as natural strata are by equivalent processes. Some archaeological strata (often termed as

contexts ''Contexts'': ''Understanding People in their Social Worlds'' is a quarterly peer-reviewed academic journal and an official publication of the American Sociological Association. It is designed to be a more accessible source of sociological ideas ...

or layers) are created by undercutting previous strata. An example would be that the silt back-fill of an underground drain would form some time after the ground immediately above it. Other examples of non vertical superposition would be modifications to standing structures such as the creation of new doors and windows in a wall. Superposition in archaeology requires a degree of interpretation to correctly identify chronological sequences and in this sense superposition in archaeology is more dynamic and multi-dimensional.

Other limitations to stratification and superposition

Original stratification induced by natural processes can subsequently be disrupted or permutated by a number of factors, including animal interference and vegetation, as well as limestone crystallization. Stratification behaves in a different manner with surface-formed igneous depositions, such as lava flows and ash falls, and thus superposition may not always successfully apply under certain conditions.

References

General sources

*Hamblin, W.K. ''The Earth's Dynamic Systems: A Textbook in Physical Geology,'' by W. Kenneth Hamblin, BYU, Provo, UT, Illus. William L. Chesser, Dennis Tasa, (Burgess Publishing Company, Minneapolis, Minnesota), c 1978, pg. 115, "The Principle of Superposition and Original Horizontality;" pg. 116: The Law of Faunal Succession, "The Principle of Crosscutting Relations;" pg 116-17: "The Principle of Inclusion," (as in the Steno discussion above). *Principles of Archaeological Stratigraphy. 40 figs. 1 pl. 136 pp. London & New York: Academic Press {{Geologic Principles Dating methods Geological history of Earth Stratigraphy

History

Archaeological considerations

Other limitations to stratification and superposition

See also

References

General sources