geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...

, a diachronism (

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''dia'', "through" + ''chronos'', "time" + ''-ism''), or diachronous deposit, is a

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

formation in which the material, although of a similar nature, varies in age with the place where it was deposited.Whitten F.G.A & Brooks J.R.V., (1972),''A Dictionary of Geology'', Penguin, Typically this occurs as a result of a

marine transgression A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...

or regression, or the progressive development of a

delta Delta commonly refers to: * Delta (letter) (Δ or δ), a letter of the Greek alphabet * River delta, at a river mouth * D ( NATO phonetic alphabet: "Delta") * Delta Air Lines, US * Delta variant of SARS-CoV-2 that causes COVID-19 Delta may also ...

. As the shoreline advances or retreats, a succession of continuous deposits representing different environments (for example beach, shallow water, deeper water) may be left behind. Although each type of deposit (

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...

) may be continuous over a wide area, its age varies according to the position of the shoreline through time. An example is the sandy beds near the end of the lower

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

of the west of England (the ''Drybrook sandstone'' of the

Forest of Dean The Forest of Dean is a geographical, historical and cultural region in the western part of the county of Gloucestershire, England. It forms a roughly triangular plateau bounded by the River Wye to the west and northwest, Herefordshire to the n ...

). Deposition of this began much later in the

Bristol Bristol () is a city, ceremonial county and unitary authority in England. Situated on the River Avon, it is bordered by the ceremonial counties of Gloucestershire to the north and Somerset to the south. Bristol is the most populous city in ...

area than further north. The detection of diachronous beds can be quite problematic since fossil assemblages tend to migrate geographically with their environment of formation. They are generally revealed by the presence of marker species, fossils which can be dated reliably from other beds.

Other usages

The term may also be applied to other features that vary in age, such as

erosion surface In geology and geomorphology, an erosion surface is a surface of rock or regolith that was formed by erosion and not by construction (e.g. lava flows, sediment deposition) nor fault displacement. Erosional surfaces within the stratigraphic reco ...

s, areas of uplift, etc. It is also sometimes applied to fossils which appear sporadically at different times in different places due to migration, though such usage is regarded by some authors as incorrect. In academic librarianship, the adjectival form, diachronous, is used in the context of "diachronous obsolescence" to describe the reduction of usefulness of a book or journal volume over several years.Line, Maurice B. "Changes in the use of literature with time - obsolescence revisted". Library Trends 41(4), Spring 1993.

References

Sedimentary rocks {{geology-stub