View source for Yardang

{{Distinguish|Yadan (surname)}}{{short description|Streamlined aeolian landform}}
[[File:Yardang Lea-Yoakum Dunes.jpg|thumb|right|300px|A yardang near [[Meadow, Texas]] ({{coord|33.321|-102.495|display=inline}}) (USDA photo).]]
[[File:Yardangs in the Tsaidam Desert.jpg|thumb|right|300px|Yardangs in the [[Qaidam]] Desert, [[Qinghai Province]], China.]]

A '''yardang''' is a streamlined protuberance carved from [[bedrock]] or any consolidated or semiconsolidated material by the dual action of wind abrasion by dust and sand and [[Eolian erosion|deflation]] (the removal of loose material by wind turbulence).<ref>[http://www.agc.army.mil/research/products/desert_guide/lsmsheet/lsyard.htm Desert Processes Working Group: Yardangs]  {{webarchive |url=https://web.archive.org/web/20100418201712/http://www.agc.army.mil/research/products/desert_guide/lsmsheet/lsyard.htm |date=April 18, 2010 }}</ref> Yardangs become elongated features typically three or more times longer than wide, and when viewed from above, resemble the [[hull (watercraft)|hull]] of a boat. Facing the wind is a steep, blunt face that gradually gets lower and narrower toward the lee end.<ref>{{cite web | url=http://ougseurope.org/index.php?id=51 | title=Martian Fleets - August 2003}}</ref>  Yardangs are formed by wind erosion, typically of an originally flat surface formed from areas of harder and softer material.  The soft material is eroded and removed by the wind, and the harder material remains.  The resulting pattern of yardangs is therefore a combination of the original rock distribution, and the [[fluid mechanics]] of the air flow and resulting pattern of erosion.

{{anchor|Etymology|Toponymy|Name}}

==Names==
The word itself is of [[Turkic languages|Turkic]] origin,<ref>"yardang." ''Webster's Third New International Dictionary, Unabridged''. Merriam-Webster, 2002. [http://unabridged.merriam-webster.com/ ''web link'']</ref><ref>"yardang." ''Dictionary.com Unabridged (v 1.1)''. Random House, Inc. 12 Mar. 2008. [http://dictionary.reference.com/browse/yardang ''web link'']</ref> meaning 'steep bank', as this type of spectacular landscapes rising {{convert|25-50|feet|0}} are best developed in the interior deserts of this region.<ref name=":0">{{Cite book|last=Leong|first=Goh Cheng|url=https://books.google.com/books?id=XhJ4RAAACAAJ&q=certificate+physical+and+human+geography|title=Certificate Physics And Human Geography; Indian Edition|date=1995-10-27|publisher=Oxford University Press|isbn=978-0-19-562816-6|page=57|language=en}}</ref> And the word was first introduced to the English-speaking world by the Swedish explorer [[Sven Hedin]] in 1903.<ref>Hedin, S.A. (1903)  [https://archive.org/details/centralasiaandt02hedigoog ''Central Asia and Tibet''], 2 vols, pp. 608, New York and London: Charles Scribner and Sons</ref> In China, they are sometimes known as '''yadan''' from the Chinese adaptation of the [[Uyghur language|Uyghur]] form of the same name.<ref>{{citation |contribution=20: Qaidam Basin |contribution-url=https://www.cnpc.com.cn/en/xhtml/pdf/20-Qaidam%20Basin.pdf |publisher=China National Petroleum Corporation |location=Beijing |url=http://www.cnpc.com.cn/en/brochures/brochures_index.shtml |title=Brochures |ref={{harvid|CNPC}} |page=9 }}.</ref>

Other names for them are "mud-lions",<ref>[[Ralph Bagnold]], 1939.{{where?|date=August 2019}}</ref><ref>[[Farouk El-Baz]], in an entry for [https://www.researchgate.net/publication/252454979_Yardangs_of_the_Western_Desert_of_Egypt "Yardangs of the Western Desert of Egypt"] in [https://books.google.com/books?id=FM4UAAAAIAAJ ''The Geology of Egypt: An Annotated Bibliography''] (1984) (p. 374), writes, "Wind has sculpted the exceptional yardang, or mud lion, form."</ref> "mushroom rocks", "sphinx-like hills", "koukour" in Tunisia, and "kalut" ([[Persian language|Persian]] for "ridge") in Iran.<ref>{{cite book|author1= Barich, Barbara E.|author2=Lucarini, Giulio|author3=Hamdan, Mohamed A. |title=From Lake to Sand. The Archaeology of Farafra Oasis Western desert, Egypt|url=https://books.google.com/books?id=Z0rbCwAAQBAJ&pg=PA91|access-date=3 December 2017|date=11 December 2014|publisher=All'Insegna del Giglio|isbn=978-88-7814-520-7|page=91}}</ref> The massive features of mega-yardangs are called "ridges and corridors" (''{{lang|fr|crêtes et couloirs}}'') in [[French language|French]].<ref>{{cite book|author1= Goudie, Andrew S.|author-link=Andrew Goudie (geographer)|author2=Middleton, Nicholas J. |title=Desert Dust in the Global System|url=https://books.google.com/books?id=4m6xempfY1gC&pg=PA43|access-date=3 December 2017|date=5 September 2006|publisher=Springer Science & Business Media|isbn=978-3-540-32355-6|page=43 }}</ref>
[[File:Wusute_Yardang_on_Water_Geological_Park.jpg|thumb|Yardangs on water in the Usut Yardang Geological Park, Qinghai, China]]

==Description==
A yardang is formed in cohesive material.<ref>{{cite journal | last1=Goudie | first1=Andrew S. | title=Mega-Yardangs: A Global Analysis | journal=Geography Compass | date=2007 | volume=1 | issue=1 | pages=65–81 | doi=10.1111/j.1749-8198.2006.00003.x | bibcode=2007GComp...1...65G }}</ref> [[Sven Hedin|Hedin]] first{{cn|date=March 2020}} found the wind-sculptured "clay terraces" or yardangs in the dried up [[Stream bed|riverbed]] of the Kurruk-daria in [[Central Asia]]. However, yardangs can be found in most [[desert]]s across the globe.<ref>Blackwelder, E. 1930. "Yardang and [[zastruga]]". Science 72(1868):396-397.</ref>  Depending upon the winds and the composition of the weakly indurated deposits of silt and sand from which they are carved, yardangs may form very unusual shapes&nbsp;— some resemble various objects, people or even lying lions.<ref name="f418">{{cite journal |last1=Boury |first1=Samuel |last2=Weady |first2=Scott |last3=Ristroph |first3=Leif |date=2024 |title=Yardangs sculpted by erosion of heterogeneous material |journal=Proceedings of the National Academy of Sciences |volume=121 |issue=30 |article-number=e2322411121 |doi=10.1073/pnas.2322411121 |issn=0027-8424|doi-access=free |pmid=38976767 |pmc=11287287 |bibcode=2024PNAS..12122411B }}</ref>

Yardangs come in a large range of sizes, and are divided into three different categories:  mega-yardangs, meso-yardangs, and micro-yardangs. Mega-yardangs can be several kilometers long and hundreds of meters high and are found in arid regions with strong winds; meso-yardangs are generally a few meters high and 10 to 15 meters long and are commonly found carved in semiconsolidated playa sediments and other soft granular materials; and micro-yardangs are only a few centimeters high.

A large concentration of mega-yardangs occurs near the [[Tibesti Mountains]] in the central [[Sahara]]. There is a famous yardang at [[Hole-in-the-Rock (Papago Park)|Hole in the Rock in Papago Park]] in [[Phoenix, Arizona]], a rock formation with a roughly circular hole in it. Another yardang in Arizona is [[Window Rock]], near the town of Window Rock. It is a 60-meter sandstone hill with a very large circular hole in the middle of it.  Some geologists have suggested that the [[Great Sphinx of Giza|Great Sphinx]] of [[Egypt]] is an augmented yardang.<ref>Goodchild, J.G. 1898. Desert conditions in Britain. Transactions of the [[Geological Society of Glasgow]] 11(1):71-104. (See page 77)</ref>

[[File:44777 1725yardangs2.jpg|thumb|right|300px|Yardangs in ''[[Lucus Planum]]'' (Mars)]]
Pictures from [[Mars]] show that the yardang ridges occur on a massive scale there; some individual ridges are tens of kilometers long with intervening valleys nearly 1&nbsp;km wide. [[Yardangs on Mars]] are typically found in the [[Amazonis Planitia|Amazonis]] region but the best ones are found in the equatorial region. Yardangs on Mars demonstrate that much of the eolian erosion is recent since they are sculpted in young geologic units.<ref>
[http://adsabs.harvard.edu/abs/1979JGR....84.8147W SAO/NASA ADS Astronomy Abstract Service: Yardangs on Mars]
</ref>

==Formation==
Explaining the formation of yardangs is challenging. It occurs due to the coupled evolution of complex flow fields and three-dimensional (3D) topographies in the context of harder or less erodible material embedded in softer material. The carving of the geometrical features is generated by differential erosion caused by flow strength, flow funneling, and turbulent wakes.<ref name="f418" />

Yardangs are formed in environments where water is scarce and the prevailing winds are strong, uni-directional, and carry an abrasive sediment load.  The wind cuts down low-lying areas into parallel ridges which gradually erode into separate hills that take on the unique shape of a yardang.  This process yields a field of yardangs of roughly the same size, commonly referred to as a fleet due to their resemblance to the bottoms of ships. Alternatively, one can be formed by the migration of a [[dune]] that leaves behind a cemented core.  As the process of formation continues, typically a trough will form around the base of the yardang. Most yardang fields are in sand-poor areas, but the associated troughs, especially in grooved terrain, may be invaded by sand. Sometimes this sand will accumulate to build shallow moats around the bottom.

They are more commonly created from softer rock types like [[siltstone]], [[sandstone]], [[tuff]] or [[Ignimbrite]], [[shale]], and [[limestone]], but have also been observed in crystalline rocks such as [[schist]] and [[gneiss]].

==See also==
*{{annotated link|Aeolian processes}}
*{{annotated link|Barchan}}
*{{annotated link|Blowout (geomorphology)|Blowout}}
*{{annotated link|Dune}}
*{{annotated link|Dunhuang Yardang National Geopark}}, [[China]]
*{{annotated link|Hoodoo (geology)}}
*{{annotated link|Lop Desert}}
*{{annotated link|Mushroom rock}}
*{{annotated link|Saltation (geology)|Saltation}}
*{{annotated link|Sandhill}}
*{{annotated link|Ventifact}}

==References==
{{reflist}}

==External links==
{{Commons category|Yardangs}}
{{Wiktionary|yardang}}
*[http://hirise.lpl.arizona.edu/PSP_007417_1755 Yardangs in Tithonium Chasma, Mars]
*[http://www.gemland.com/holeinrock.htm Hole in the Rock]
*[http://www.lapahie.com/Window_Rock_Capitol.cfm Window Rock] {{Webarchive|url=https://web.archive.org/web/20060623011535/http://www.lapahie.com/Window_Rock_Capitol.cfm |date=2006-06-23 }}
*[https://archive.today/20121213000344/http://www.tec.army.mil/research/products/desert_guide/lsmsheet/lsyard.htm Yardangs, Desert Processes Working Group; Knowledge Sciences, Inc.]
*{{URL|https://data.mendeley.com/datasets/675gwk5jp7/1|The Bibliography of Aeolian Research}}

[[Category:Aeolian landforms]]
[[Category:Climatic geomorphology]]