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Stratigraphy is a branch of geology which studies rock layers and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy includes two related subfields: lithologic stratigraphy or lithostratigraphy, and biologic stratigraphy or biostratigraphy.
Historical development 
Nicholas Steno established the theoretical basis for stratigraphy when he reintroduced the law of superposition and introduced the principle of original horizontality and the principle of lateral continuity in a 1669 work on the fossilization of organic remains in layers of sediment.
The first practical large-scale application of stratigraphy was by William Smith in the 1790s and early 19th century. Smith, known as the "father of English geology," created the first geologic map of England and first recognized the significance of strata or rock layering and the importance of fossil markers for correlating strata. Another influential application of stratigraphy in the early 19th century was a study by Georges Cuvier and Alexandre Brongniart of the geology of the region around Paris.
Lithologic stratigraphy 
Lithostratigraphy, or lithologic stratigraphy, provides the most obvious visible layering. It deals with the physical contrasts in lithology, or rock type. Such layers can occur both vertically - in layering or bedding of varying rock type - and laterally - reflecting changing environments of deposition, known as facies change. Key elements of stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries mean in terms of the depositional environment. Stratigraphers have codified a basic concept of their discipline in the Law of Superposition, which simply states that, in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence.
Chemostratigraphy studies the changes in the relative proportions of trace elements and isotopes within and between lithologic units. Carbon and oxygen isotope ratios vary with time, and researchers can use them to map subtle changes in the paleoenvironment. This has led to the specialized field of isotopic stratigraphy.
Biostratigraphy or paleontologic stratigraphy is based on fossil evidence in the rock layers. Strata from widespread locations containing the same fossil fauna and flora are correlatable in time. Biologic stratigraphy was based on William Smith's principle of faunal succession, which predated, and was one of the first and most powerful lines of evidence for, biological evolution. It provides strong evidence for formation (speciation) of and the extinction of species. The geologic time scale was developed during the 19th century, based on the evidence of biologic stratigraphy and faunal succession. This timescale remained a relative scale until the development of radiometric dating, which gave it and the stratigraphy it was based on an absolute time framework, leading to the development of chronostratigraphy.
One important development is the Vail curve, which attempts to define a global historical sea-level curve according to inferences from worldwide stratigraphic patterns. Stratigraphy is also commonly used to delineate the nature and extent of hydrocarbon-bearing reservoir rocks, seals, and traps in petroleum geology.
Chronostratigraphy is the branch of stratigraphy that studies the absolute, not relative, age of rock strata. The branch is based upon deriving geochronological data for rock units, both directly and inferentially, so that a sequence of time-relative events of rocks within a region can be derived. In essence, chronostratigraphy seeks to understand the geologic history of rocks and regions.
The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region and, eventually, the entire geologic record of the Earth.
A gap or lacuna in the geological record may be caused by erosion, in which case it is called a vacuity, or by non-deposition, where is known as a stratigraphic hiatus. It is called a hiatus because deposition was on hold for a period of time. A physical lacuna may represent both a period of non-deposition and a period of erosion. A fault may cause the appearance of a hiatus.
Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout a section. The samples are analyzed to determine their detrital remnant magnetism (DRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. For sedimentary rocks, this is possible because, when very fine-grained magnetic minerals (< 17 micrometres) fall through the water column, they orient themselves with Earth's magnetic field. Upon burial, that orientation is preserved. The minerals behave like tiny compasses. For volcanic rocks, magnetic minerals, which form as the melt cools, orient with the ambient magnetic field.
Oriented paleomagnetic core samples are collected in the field; mudstones, siltstones, and very fine-grained sandstones are the preferred lithologies because the magnetic grains are finer and more likely to orient with the ambient field during deposition. If the ancient magnetic field were oriented similar to today's field (North Magnetic Pole near the North Rotational Pole), the strata would retain a normal polarity. If the data indicate that the North Magnetic Pole were near the South Rotational Pole, the strata would exhibit reversed polarity.
Results of the individual samples are analyzed by removing the natural remanent magnetization (NRM) to reveal the DRM. Following statistical analysis, the results are used to generate a local magnetostratigraphic column that can then be compared against the Global Magnetic Polarity Time Scale.
This technique is used to date sequences that generally lack fossils or interbedded igneous rocks. The continuous nature of the sampling means that it is also a powerful technique for the estimation of sediment-accumulation rates.
Archaeological stratigraphy 
In the field of archaeology, soil stratigraphy is used to better understand the processes that form and protect archaeological sites. The law of superposition holds true, and this can help date finds or features from each context because they can be placed in sequence and the dates interpolated. Phases of activity can also often be seen through stratigraphy, especially when a trench or feature is viewed in section (profile). Because pits and other features can be dug down into earlier levels, not all material at the same absolute depth is necessarily of the same age, but close attention has to be paid to the archeological layers. The Harris-matrix is a tool to depict complex stratigraphic relations when they are found, for example, in the contexts of urban archaeology.
See also 
- Erygmascope (old instrument for studying strata)
- Harris matrix
- Important publications in stratigraphy
- International Commission on Stratigraphy
- Key bed
- Sedimentary basin analysis
- Sequence stratigraphy
- Martinsen, O. J. et al. (1999) "Cenozoic development of the Norwegian margin 60–64N: sequences and sedimentary response to variable basin physiography and tectonic setting" pp. 293–304 In Fleet, A. J. and Boldy, S. A. R. (editors) (1999) Petroleum Geology of Northwest Europe Geological Society, London, page 295, ISBN 978-1-86239-039-3
- Kearey, Philip (2001). Dictionary of Geology (2nd ed.) London, New York, etc.: Penguin Reference, London, p. 123. ISBN 978-0-14-051494-0.
- Chapman, Richard E. (1983) Petroleum Geology Elsevier Scientific, Amsterdam, page 33, ISBN 978-0-444-42165-4
- Christopherson, R. W., 2008. Geosystems: An Introduction to Physical Geography, 7th ed., New York: Pearson Prentice-Hall. ISBN 978-0-13-600598-8
|Wikimedia Commons has media related to: Stratigraphy|
- ICS Subcommission for Stratigraphic Information
- University of South Carolina Sequence Stratigraphy Web
- Front Range stratigraphy
- International Commission on Stratigraphy
- University of Georgia (USA) Stratigraphy Lab
- Stratigraphy.net A stratigraphic data provider.
- Agenames.org A global index of stratigraphic terms