alluvium
{{short description|Loose soil or sediment that is eroded and redeposited in a non-marine setting}}
{{redirect-distinguish|Alluvial|Alluvial (horse)|Alluvial diagram}}
File:Riogranderift sanluisbasin.png
File:South Africa-Eastern Cape-Gamtoos Valley-Alluvial Gravel01.jpg in South Africa]]
File:AlluvialPlain.JPG in Red Rock Canyon State Park (California)]]
File:Amazon alluvium deposit - autazes.jpg, near Autazes, AM, Brazil. The seasonal deposits are extremely fertile and crucial to subsistence farming in the Amazon Basin along the river banks.]]
Alluvium ({{etymology|la|{{wikt-lang|la|alluvius}}|}}, {{etymology||{{wikt-lang|la|alluere}}|to wash against}}) is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings.{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=alluvium}}[https://web.archive.org/web/20110501155938/http://www.geotech.org/survey/geotech/dictiona.html Glossary of Geological Terms]. Geotech.org. Retrieved on 2012-02-12.[http://geology.com/dictionary/glossary-a.shtml Geology Dictionary – Alluvial, Aquiclude, Arkose]. Geology.Com. Retrieved on 2012-02-12. Alluvium is also sometimes called alluvial deposit.{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=alluvial deposit}}{{cite book |last1=Allaby |first1=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=9780199653065 |edition=Fourth |chapter=alluvium}} Alluvium is typically geologically young and is not consolidated into solid rock. Sediments deposited underwater, in seas, estuaries, lakes, or ponds, are not described as alluvium.
Floodplain alluvium can be highly fertile, and supported some of the earliest human civilizations.{{cite journal |last1=Kidder |first1=Tristram |last2=Liu |first2=Haiwang |last3=Xu |first3=Qinghai |last4=Li |first4=Minglin |title=The Alluvial Geoarchaeology of the Sanyangzhuang Site on the Yellow River Floodplain, Henan Province, China: THE YELLOW RIVER FLOODPLAIN, HENAN PROVINCE |journal=Geoarchaeology |date=July 2012 |volume=27 |issue=4 |pages=324–343 |doi=10.1002/gea.21411|s2cid=140587757 }}
Definitions
The present consensus is that "alluvium" refers to loose sediments of all types deposited by running water in floodplains or in alluvial fans or related landforms.{{sfn|Allaby|2013|loc="alluvial"}}{{cite journal |last1=Miller |first1=Bradley A. |last2=Juilleret |first2=Jérôme |title=The colluvium and alluvium problem: Historical review and current state of definitions |journal=Earth-Science Reviews |date=October 2020 |volume=209 |pages=103316 |doi=10.1016/j.earscirev.2020.103316|bibcode=2020ESRv..20903316M |doi-access=free }} However, the meaning of the term has varied considerably since it was first defined in the French dictionary of Antoine Furetière, posthumously published in 1690. Drawing upon concepts from Roman law, Furetière defined alluvion (the French term for alluvium) as new land formed by deposition of sediments along rivers and seas.
By the 19th century, the term had come to mean recent sediments deposited by rivers on top of older diluvium, which was similar in character but interpreted as sediments deposited by Noah's flood. With the rejection by geologists of the concept of a primordial universal flood, the term "diluvium" fell into disfavor and was replaced with "older alluvium". At the same time, the term "alluvium" came to mean all sediment deposits due to running water on plains. The definition gradually expanded to include deposits in estuaries, coasts, and young rock of marine and fluvial origin.
Alluvium and diluvium were grouped as colluvium in the late 19th century. "Colluvium" is now generally understood as sediments produced by gravity-driven transport on steep slopes. At the same time, the definition of "alluvium" has switched back to an emphasis on sediments deposited by river action. There continues to be disagreement over what other sediment deposits should be included under the term "alluvium".
Age
Most alluvium is Quaternary in age and is often referred to as "cover" because these sediments obscure the underlying bedrock. Most sedimentary material that fills a basin ("basin fill") that is not lithified is typically lumped together as "alluvial".{{Cite EB1911|wstitle=Alluvium}} Alluvium of Pliocene age occurs, for example, in parts of Idaho.{{Citation | last = Ames | first = Dan | contribution = Formation of the Soils | year = 1998 | title = Soil Survey of Jerome County and Part of Twin Falls County, Idaho | url = https://archive.org/details/usda-contents/page/238/mode/1up | pages = 238 | publisher = Natural Resources Conservation Service, United States Department of Agriculture | contribution-url = https://web.archive.org/web/20220223221606/https://www.nrcs.usda.gov/Internet/FSE_MANUSCRIPTS/idaho/ID704/0/id704_text.pdf}} Alluvium of late Miocene age occurs, for example, in the valley of the San Joaquin River, California.{{cite book | url=http://pubs.usgs.gov/pp/1197/report.pdf | title=Amount and Timing of Late Cenozoic Uplift and Tilt of the Central Sierra Nevada, California—Evidence from the Upper San Joaquin River Basin (USGS Professional Paper 1197) | publisher=USGS | author=Huber, N. King | year=1981 | location=Washington D.C. | pages=13}}
See also
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References
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