Editing Scree

{{Short description|Broken rock fragments at base of cliff}}
[[File:Yamnuska bottom cliff.jpg|thumb|right|upright|Talus at the bottom of [[Mount Yamnuska]], [[Alberta]], [[Canada]]]]

'''Scree''' is a collection of broken [[Rock (geology) |rock]] fragments at the base of a [[cliff]] or other steep rocky mass that has accumulated through periodic [[rockfall]]. Landforms associated with these materials are often called '''talus deposits'''.

The term ''scree'' is applied both to an unstable steep mountain slope composed of rock fragments and other [[debris]], and to the mixture of rock fragments and debris itself.<ref>{{oed|scree}}</ref><ref name=Jackson1997>{{cite book |editor-last= Jackson |editor-first= Julia A. |chapter= scree |title= Glossary of geology |date=1997 |publisher=[[American Geological Institute]] |edition= 4th |location= Alexandria, Virginia |isbn= 0922152349}}</ref><ref name=Allaby2013>{{cite book |last= Allaby |first= Michael |chapter= scree |title= A dictionary of geology and earth sciences |year= 2013 |publisher=[[Oxford University Press]] |edition= 4th |isbn= 9780199653065}}</ref> It is loosely synonymous with '''talus''', material that accumulates at the base of a projecting mass of rock,<ref name=Jackson1997/>{{sfn|Jackson|1997|loc="talus"}} or '''talus slope''', a landform composed of talus.<ref name=Thornbury66>{{cite book |last1=Thornbury |first1=William D. |title=Principles of geomorphology |date=1969 |publisher=Wiley |location=New York |isbn=0471861979 |edition=2d |page=66}}</ref> The term ''scree'' is sometimes used more broadly for any sheet of loose rock fragments mantling a slope, while ''talus'' is used more narrowly for material that accumulates at the base of a [[cliff]] or other rocky slope from which it has obviously eroded.<ref name=Jackson1997/>

Scree is formed by rockfall,<ref name=Allaby2013/><ref name=BlattEtal1980_176>{{cite book |last1=Blatt |first1=Harvey |last2=Middleton |first2=Gerard |last3=Murray |first3=Raymond |title=Origin of sedimentary rocks |date=1980 |publisher=Prentice-Hall |location=Englewood Cliffs, N.J. |isbn=0136427103 |edition=2d |page=176}}</ref> which distinguishes it from '''[[colluvium]]'''. Colluvium is rock fragments or soil deposited by [[rainwash]], [[sheetwash]], or slow [[downhill creep]], usually at the base of gentle slopes or hillsides.{{sfn|Jackson|1997|loc="colluvium"}} However, the terms ''scree'', ''talus'',<ref name=Jackson1997/><ref name=Allaby2013/> and sometimes ''colluvium''<ref name=TurnerSchuster1996/> tend to be used interchangeably. The term ''talus deposit'' is sometimes used to distinguish the landform from the material of which it is made.<ref>{{cite journal |last1=Brody |first1=A. G. |last2=Pluhar |first2=C. J. |last3=Stock |first3=G. M. |last4=Greenwood |first4=W. J. |title=Near-Surface Geophysical Imaging of a Talus Deposit in Yosemite Valley, California |journal=Environmental & Engineering Geoscience |date=1 May 2015 |volume=21 |issue=2 |pages=111–127 |doi=10.2113/gseegeosci.21.2.111|bibcode=2015EEGeo..21..111B }}</ref> The exact definition of scree in the [[primary literature]] is somewhat relaxed, and it often overlaps with both ''talus'' and ''colluvium''.<ref name=TurnerSchuster1996>{{Cite book |author= Turner, A. Keith |author2= Schuster, Robert L. |title= Landslides: investigation and mitigation |date= 1996 |publisher= National Academy Press |location= Washington, D.C. |isbn=0-309-06208-X |oclc= 33102185 |url=https://www.worldcat.org/oclc/33102185}}</ref>

==Etymology==
The term ''scree'' comes from the [[Old Norse]] term for [[landslide]], ''skriða'',<ref>{{OEtymD|scree|accessdate=2006-04-20}}</ref> while the term ''talus'' is a French word meaning a slope or embankment.<ref>{{OEtymD|talus|accessdate=2008-12-01}}</ref><ref>{{cite web|title=Talus|work=bab.la language portal|url=http://en.bab.la/dictionary/english-french/talus|access-date=2011-12-10}}</ref>

==Description==
Talus deposits typically have a concave upwards form, where the maximum inclination corresponds to the [[angle of repose]] of the mean debris particle size.<ref name=TurnerSchuster1996/>

Scree slopes are often assumed to be close to the angle of repose. This is the slope at which a pile of granular material becomes mechanically unstable. However, careful examination of scree slopes shows that only those that are either rapidly accumulating new material, or are experiencing rapid removal of material from their bases, are close to the angle of repose. Most scree slopes are less steep, and they often show a concave shape, so that the foot of the slope is less steep than the top of the slope.<ref>{{cite journal |last1=Statham |first1=I. |title=Scree Slope Development under Conditions of Surface Particle Movement |journal=Transactions of the Institute of British Geographers |date=July 1973 |issue=59 |pages=41–53 |doi=10.2307/621711|jstor=621711 }}</ref><ref>{{cite journal |last1=Statham |first1=Ian |title=A scree slope rockfall model |journal=Earth Surface Processes |date=January 1976 |volume=1 |issue=1 |pages=43–62 |doi=10.1002/esp.3290010106}}</ref>

Scree with large, boulder-sized rock fragments may form [[talus cave]]s, or human-sized passages formed in-between boulders.<ref>{{cite web |title=Talus Caves - Caves and Karst (U.S. National Park Service) |url=https://www.nps.gov/subjects/caves/talus-caves.htm |website=www.nps.gov|publisher=National Park Service |access-date=16 July 2024 |language=en}}</ref>

==Formation==
[[File:TalusConesIsfjorden.jpg|thumb|right|Talus cones on north shore of [[Isfjord (Svalbard)|Isfjord]], [[Svalbard]], [[Norway]]]]

The formation of scree and talus deposits is the result of physical and chemical [[weathering]] acting on a rock face, and [[Erosion| erosive processes]] transporting the material downslope.{{cn |date= November 2024}} In high-altitude [[arctic]] and [[Subarctic climate|subarctic]] regions, scree slopes and talus deposits are typically adjacent to hills and river valleys. These steep slopes usually originate from late-[[Pleistocene]] [[Periglaciation|periglacial]] processes.<ref>{{Cite journal|last1=Růžička|first1=Vlastimil|last2=Hajer|first2=Jaromír|date=1996-12-01|title=Spiders (Araneae) of stony debris in North Bohemia|journal=Arachnologische Mitteilungen|volume=12|pages=46–56|doi=10.5431/aramit1202|issn=1018-4171|doi-access=free}}</ref>

There are five main stages of scree slope evolution:{{cn |date= November 2024}}
# accumulation
# consolidation
# weathering
# encroaching vegetation
# slope degradation.

Scree slopes form as a result of accumulated loose, [[Granularity |coarse-grained]] material. Within the scree slope itself, however, there is generally good sorting of sediment by size: larger particles accumulate more rapidly at the bottom of the slope.<ref>{{Cite journal |last1= Kirkby |first1= M. J. |last2= Statham |first2= Ian |date= May 1975 |title= Surface Stone Movement and Scree Formation |journal=[[The Journal of Geology]] |volume= 83 |issue= 3 |pages=349–362 |doi=10.1086/628097 |bibcode= 1975JG.....83..349K |s2cid= 129310011 |issn=0022-1376 |url=https://www.journals.uchicago.edu/doi/10.1086/628097|url-access= subscription }}</ref> [[Cementation (geology)|Cementation]] occurs as [[fine-grained]] material fills in gaps between debris. The speed of consolidation depends on the composition of the slope; [[clay]]ey components will bind debris together faster than [[sand]]y ones. Should [[weathering]] outpace the supply of sediment, plants may take root. Plant roots diminish [[Cohesion (geology)|cohesive]] forces between the coarse and fine components, degrading the slope.<ref>{{Cite journal|last1=Gerber|first1=E.|last2=Scheidegger|first2=A. E.|date=May 1974|title=On the dynamics of scree slopes|url=http://link.springer.com/10.1007/BF01238051|journal=Rock Mechanics |language=en|volume=6|issue=1|pages=25–38|doi=10.1007/BF01238051|bibcode=1974RMFMR...6...25G|s2cid=129262031|issn=0035-7448|url-access=subscription}}</ref> The predominant processes that [[Degradation (geology)| degrade]] a rock slope depend largely on the regional [[climate]] (see below), but also on the thermal and topographic stresses governing the parent rock material. Example process domains include:{{cn |date= November 2024}}
* [[Weathering#Physical weathering|Physical weathering]]
* [[Weathering#Chemical weathering|Chemical weathering]]
* [[Biotic component|Biotic]] processes
* [[Thermal stress]]es
* [[Erosion#Mass movement|Topographic stresses]]

===Physical weathering processes===
[[File:DSC 0445 Partie inférieure de la combe de Mai de la montagne d'Aurouze (Hautes-Alpes, France).jpg|thumb|Scree in the lower part of the Mai Valley on the Aurouze mountain (Hautes-Alpes, France)]]

Scree formation is commonly attributed to the formation of ice within mountain rock slopes. The presence of [[Joint (geology) |joints]], [[fracture]]s, and other heterogeneities in the rock wall can allow [[precipitation]], [[groundwater]], and [[surface runoff]] to flow through the rock. If the temperature drops below the freezing point of the fluid contained within the rock, during particularly cold evenings, for example, this water can freeze. Since water expands by 9% when it freezes, it can generate large forces that either create new cracks or wedge blocks into an unstable position. Special boundary conditions (rapid freezing and water confinement) may be required for this to happen.<ref>{{cite book|last=Whalley|first=WB|year=1984|chapter=Rockfalls|title=Slope Instability|editor1-last=Brunsden|editor1-first=D.|editor2-last=Prior|editor2-first=DB|publisher=John Wiley and Sons|location=Chichester|pages=217–256}}</ref> [[Frost weathering|Freeze-thaw]] scree production is thought to be most common during the spring and fall, when the daily temperatures fluctuate around the freezing point of water, and snow melt produces ample free water.

The efficiency of freeze-thaw processes in scree production is a subject of ongoing debate. Many researchers believe that ice formation in large open fracture systems cannot generate high enough pressures to force the fracturing apart of parent rocks, and instead suggest that the water and ice simply flow out of the fractures as pressure builds.<ref>{{cite journal|last=Hallet|first=B|year=2006|title=Why do freezing rocks break?|journal=Science|volume=314|pages=1092–1093|doi=10.1126/science.1135200|pmid=17110559|issue=5802|s2cid=140686582}}</ref> Many argue that [[frost heaving]], like that known to act in soil in [[permafrost]] areas, may play an important role in cliff degradation in cold places.<ref>{{cite journal|last1=Walder|first1=J|last2=Hallet|first2=B|year=1985|title=A theoretical model of the fracture of rock during freezing|journal=Geological Society of America Bulletin|volume=96|pages=336–346|doi=10.1130/0016-7606(1985)96<336:ATMOTF>2.0.CO;2|issue=3|bibcode = 1985GSAB...96..336W }}</ref><ref>{{cite journal|last1=Murton|first1=JB|last2=Peterson|first2=R|last3=Ozouf|first3=J-C|year=2006|title=Bedrock fracture by ice segregation in cold regions|journal=Science|volume=314|pages=1127–1129|doi=10.1126/science.1132127|pmid=17110573|issue=5802|bibcode = 2006Sci...314.1127M |s2cid=37639112}}</ref>

Eventually, a rock slope may be completely covered by its own scree, so that production of new material ceases. The slope is then said to be "mantled" with debris. However, since these deposits are still unconsolidated, there is still a possibility of the deposit slopes themselves failing. If the talus deposit pile shifts and the particles exceed the angle of repose, the scree itself may slide and fail.{{cn |date= November 2024}}

===Chemical weathering processes===
Phenomena such as [[acid rain]] may also contribute to the chemical degradation of rocks and produce more loose sediments.{{cn |date= November 2024}}

===Biotic weathering processes===
Biotic processes often intersect with both physical and chemical weathering regimes, as the organisms that interact with rocks can mechanically or chemically alter them.{{cn |date= November 2024}}

[[Lichen]] frequently grow on the surface of, or within, rocks. Particularly during the initial colonization process, the lichen often inserts its [[hypha]]e into small [[fracture]]s or mineral [[Cleavage (crystal)| cleavage planes]] that exist in the host rock.<ref name=":2">{{Cite journal|last1=Jie|first1=Chen|last2=Blume|first2=Hans-Peter|date=October 2002|title=Rock-weathering by lichens in Antarctic: patterns and mechanisms|url=http://link.springer.com/10.1007/BF02844595|journal=[[Journal of Geographical Sciences]]|volume=12|issue=4|pages=387–396|doi=10.1007/BF02844595|s2cid=128666735|issn=1009-637X|url-access=subscription}}</ref> As the lichen grows, the hyphae expand and force the fractures to widen. This increases the potential of fragmentation, possibly leading to rockfalls. During the growth of the lichen [[thallus]], small fragments of the host rock can be incorporated into the biological structure and weaken the rock.{{cn |date= November 2024}}
[[File:Cliff and forested scree, Paces Lake, Nova Scotia.jpg|thumb|A tall cliff on the eastern shore of Paces Lake, Nova Scotia, with scree at its base. As the rate of erosion is quite slow, the scree has become partially forested.]]

[[Freeze-thaw action]] of the entire lichen body due to microclimatic changes in moisture content can alternately cause thermal contraction and expansion,<ref name=":2" /> which also stresses the host rock. Lichen also produce a number of [[organic acid]]s as metabolic byproducts.<ref name=":2" /> These often react with the host rock, dissolving minerals, and breaking down the substrate into unconsolidated sediments.{{cn |date= November 2024}}

==Interaction with glaciers==
Scree often collects at the base of [[glacier]]s, concealing them from their environment. For example, [[Lech dl Dragon]], in the [[Sella group]] of the [[Dolomites]], is derived from the melting waters of a glacier and is hidden under a thick layer of scree. Debris cover on a glacier affects the energy balance and, therefore, the melting process.<ref>{{Cite book|last1=Benn|first1=D. I.|title=Glaciers and Glaciation, 2nd ed.|last2=Evans|first2=D. J. A|publisher=Hodder-Arnold|year=2010|isbn=9780340905791|location=London}}</ref><ref name=":3">{{Cite journal|last1=Nakawo|first1=M.|last2=Young|first2=G.J.|date=1981|title=Field Experiments to Determine the Effect of a Debris Layer on Ablation of Glacier Ice|journal=Annals of Glaciology|language=en|volume=2|pages=85–91|doi=10.3189/172756481794352432|bibcode=1981AnGla...2...85N|issn=0260-3055|doi-access=free}}</ref> Whether the glacier ice begins melting more rapidly or more slowly is determined by the thickness of the layer of scree on its surface.{{cn |date= November 2024}}

The amount of energy reaching the surface of the ice below the debris can be estimated via the one-dimensional, homogeneous material assumption of [[Fourier's law]]:<ref name=":3" />

<math>Q = -k \left ( \frac{T_s-T_i}{d} \right )</math>,

where ''k'' is the [[thermal conductivity]] of the debris material, ''T<sub>s</sub>'' is the ambient temperature above the debris surface, ''T<sub>i</sub>'' is the temperature at the lower surface of the debris, and ''d'' is the thickness of the debris layer.<ref name=":3" />

[[File:Lech dl Dragon bis.JPG|thumb|right|Scree-covered [[glacier]], [[Lech dl Dragon]], [[Italy]] ]]

Debris with a low thermal conductivity value, or a high [[thermal resistivity]], will not efficiently transfer energy through to the glacier, meaning the amount of heat energy reaching the ice surface is substantially lessened. This can act to [[Thermal insulation|insulate]] the glacier from incoming radiation.{{cn |date= November 2024}}

===Albedo (radiation reflection)===
The [[albedo]], or the ability of a material to reflect incoming radiation energy, is also an important quality to consider. Generally, the debris will have a lower albedo than the glacier ice it covers, and will thus reflect less incoming solar radiation. Instead, the debris will absorb radiation energy and transfer it through the cover layer to the debris-ice interface.{{cn |date= November 2024}}

If the ice is covered by a relatively thin layer of debris (less than around 2 centimeters thick), the albedo effect is most important.<ref name=":4">{{Cite journal|last=östrem|first=Gunnar|date=January 1959|title=Ice Melting under a Thin Layer of Moraine, and the Existence of Ice Cores in Moraine Ridges|url=https://www.tandfonline.com/doi/full/10.1080/20014422.1959.11907953|journal=Geografiska Annaler|language=en|volume=41|issue=4|pages=228–230|doi=10.1080/20014422.1959.11907953|issn=2001-4422|url-access=subscription}}</ref> As scree accumulates atop the glacier, the ice's albedo will begin to decrease. Instead, the glacier ice will absorb incoming solar radiation and transfer it to the upper surface of the ice. Then, the glacier ice begins to absorb the energy and uses it in the process of melting.{{cn |date= November 2024}}

However, once the debris cover reaches 2 or more centimeters in thickness, the albedo effect begins to dissipate.<ref name=":4" /> Instead, the debris blanket will act to insulate the glacier, preventing incoming radiation from penetrating the scree and reaching the ice surface.<ref name=":4" /> In addition to rocky debris, thick snow cover can form an insulating blanket between the cold winter atmosphere and [[Subnivean climate|subnivean]] spaces in screes.<ref>{{Cite journal|last=Wheeler|first=Ralph A.|date=June 1990|title=Spiders Are Spiders…|url=http://dx.doi.org/10.1097/00007611-199006000-00037|journal=Southern Medical Journal|volume=83|issue=6|pages=723|doi=10.1097/00007611-199006000-00037|pmid=2356505|issn=0038-4348|url-access=subscription}}</ref> As a result, soil, bedrock, and also [[Subterranean river| subterranean]] voids in screes do not freeze at high elevations.{{cn |date= November 2024}}

==Microclimates==
A scree has many small interstitial voids, while an [[ice cave]] has a few large hollows. Due to cold air seepage and air circulation, the bottom of scree slopes have a thermal regime similar to ice caves.{{cn |date= November 2024}}

Because subsurface ice is separated from the surface by thin, [[Permeability (Earth sciences)| permeable]] sheets of sediment, screes experience cold air seepage from the bottom of the slope where sediment is thinnest.<ref name=":0">{{Cite journal|last1=Růžička|first1=Vlastimil|last2=Zacharda|first2=Miloslav|last3=Němcová|first3=Lenka|last4=Šmilauer|first4=Petr|last5=Nekola|first5=Jeffrey C.|date=September 2012|title=Periglacial microclimate in low-altitude scree slopes supports relict biodiversity|url=http://www.tandfonline.com/doi/abs/10.1080/00222933.2012.707248|journal=Journal of Natural History|language=en|volume=46|issue=35–36|pages=2145–2157|doi=10.1080/00222933.2012.707248|bibcode=2012JNatH..46.2145R |s2cid=86730753|issn=0022-2933|url-access=subscription}}</ref> This freezing circulating air maintains internal scree temperatures 6.8-9.0&nbsp;°C colder than external scree temperatures.<ref name=":1">{{Cite journal|last1=Zacharda|first1=Miloslav|last2=Gude|first2=Martin|last3=Růžička|first3=Vlastimil|date=July 2007|title=Thermal regime of three low elevation scree slopes in central Europe|url=http://doi.wiley.com/10.1002/ppp.598|journal=Permafrost and Periglacial Processes|language=en|volume=18|issue=3|pages=301–308|doi=10.1002/ppp.598|bibcode=2007PPPr...18..301Z |s2cid=129472548 |url-access=subscription}}</ref> These <0&nbsp;°C thermal anomalies occur up to 1000m below sites with mean annual air temperatures of 0&nbsp;°C.{{cn |date= November 2024}}

Patchy [[permafrost]], which forms under conditions <0&nbsp;°C, probably exists at the bottom of some scree slopes despite mean annual air temperatures of 6.8–7.5&nbsp;°C.<ref name=":1" />

==Biodiversity==


Scree [[microclimate]]s maintained by circulating freezing air create micro[[habitat]]s that support taiga plants and animals that could not otherwise survive regional conditions.<ref name=":0" />

A [[Czech Republic Academy of Sciences]] research team led by [[physical chemist]] Vlastimil Růžička, analyzing 66 scree slopes, published a paper in ''[[Journal of Natural History]]'' in 2012, reporting that: "This microhabitat, as well as interstitial spaces between scree blocks elsewhere on this slope, supports an important assemblage of boreal and [[arctic]] [[bryophyte]]s, [[pteridophyte]]s, and [[arthropod]]s that are disjunct from their normal ranges far to the north. This freezing scree slope represents a classic example of a palaeo [[Refugium (population biology)|refugium]] that significantly contributes to [the] protection and maintenance of regional landscape [[biodiversity]]."<ref name=":0" />

[[Ice Mountain]], a massive scree in [[West Virginia]], supports distinctly different distributions of plant and animal species than northern latitudes.<ref name=":0" />

==Scree running (activity)==
Scree running is the activity of running down a scree slope. This can be very quick, as the scree moves with the runner. Some scree slopes are no longer possible to run, because the stones have been moved towards the bottom.<ref>{{cite web |url=https://teara.govt.nz/en/photograph/9871/scree-running |title=Scree running |website=[[Encyclopaedia of New Zealand]]|last1=Simpson |first1=Peter }}</ref><ref>{{cite magazine |url=https://www.wildernessmag.co.nz/scree-running-madness/ |title=Scree running madness |first=David |last=Short |date=2012-02-01 |magazine=Wilderness |access-date=2020-12-21}}</ref><ref>{{cite web |url=http://www.highfell.org.uk/index.php?option=com_zoo&task=item&item_id=102&category_id=25&Itemid=82 |title= Scree Running |first=John |last=Nettleton |website=[[Wildlife Trust]] |access-date=2020-12-21}}</ref>

== See also ==
* [[Blockfield]] - similar to talus and scree slopes, formed by frost weather instead of mass wastings
*{{annotated link|Fellfield}}
* {{annotated link|Lava stringer}}
* {{annotated link|Mass wasting}}
* {{annotated link|Stratified slope deposit}}
* {{annotated link|Weathering}}
* [[Scree plot]]
* [[Floater (geology)|Floater]]

== References ==
{{reflist|33em}}

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[[Category:Slope landforms]]
[[Category:Montane ecology]]