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Thelaw of superposition is an axiom that forms one of the bases of the sciences ofgeology,archaeology, and other fields pertaining to geologicalstratigraphy. In its plainest form, it states that in undeformed stratigraphic sequences, the oldeststrata will lie at the bottom of the sequence, while newer material stacks upon the surface to form new deposits over time. This is paramount tostratigraphic 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 enablingpaleontologists andpaleobotanists 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 anevolutionarily orphylogenetically relevant way.
The law of superposition was first proposed in 1669 by the Danish scientistNicolas Steno, and is present as one of his major theses in the groundbreaking seminal workDissertationis prodromus (1669).[1]
In the English-language literature, the law was popularized byWilliam "Strata" Smith, who used it to produce the first geologic map of Britain.[2] It is the first ofSmith's laws, which were formally published inStrata Identified by Fossils (1816–1819).
Superposition in archaeology and especially instratification use duringexcavation is slightly different as the processes involved in laying down archaeological strata are somewhat different from geological processes. Human-made intrusions and activity in thearchaeological 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 ascontexts 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 inarchaeology requires a degree of interpretation to correctly identify chronologicalsequences and in this sense superposition in archaeology is more dynamic and multi-dimensional.
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.
Geologic principles and processes | ||
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| Stratigraphic principles |  | |
| Petrologic principles | ||
| Geomorphologic processes | ||
| Sediment transport | ||