Editing Scree (section)

==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}}