Law of superposition
Law stating that newer strata stack above older ones
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History
The law of superposition was first proposed in 1669 by the Danish scientist Nicolas Steno, and is present as one of his major theses in the groundbreaking seminal work Dissertationis prodromus (1669).
In the English-language literature, the law was popularized by William "Strata" Smith, who used it to produce the first geologic map of Britain. It is the first of Smith's laws, which were formally published in Strata Identified by Fossils (1816–1819).
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Archaeological considerations
Superposition in archaeology and especially in stratification use during excavation is slightly different as the processes involved in laying down archaeological strata are somewhat different from geological processes. Human-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 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.