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Scree is a collection of broken
rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition and the way in w ...
fragments at the base of crags, mountain
cliff upEurope's highest cliff, Troll Wall in Norway, a famous BASE jumping location for jumpers from around the world.">BASE_jumping.html" ;"title="Troll Wall in Norway, a famous BASE jumping">Troll Wall in Norway, a famous BASE jumping location ...

cliff
s,
volcano A volcano is a rupture in the crust of a planetary-mass object A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet ar ...

volcano
es or
valley shoulder A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion In earth science, erosion is the action of ...
s that has accumulated through periodic
rockfall A rockfall or rock-fallWhittow, John (1984). ''Dictionary of Physical Geography''. London: Penguin, 1984. . refers to quantities of rock falling freely from a cliff upEurope's highest cliff, Troll Wall in Norway, a famous BASE jumping loc ...
from adjacent cliff faces. Landforms associated with these materials are often called talus deposits or stony accumulations. Talus deposits typically have a concave upwards form, where the maximum inclination corresponds to the
angle of repose The angle of repose, or critical angle of repose, of a granular material is the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping. At this angle, the material on the slope face i ...

angle of repose
of the mean
debris Debris (, ) is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, discarded, or as in geology, large rock fragments left by a melting glacier etc. Depending on context, ''debris'' can ...
particle size. Scree is a subcategory of the broader debris class of
colluvium Colluvium (also colluvial material or colluvial soil) is a general name for loose, unconsolidated sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transpo ...
: any collection of loose, unconsolidated sediments at the base of hillslopes. The exact definition of scree in the primary literature is somewhat relaxed, and it often overlaps with both talus and colluvium. Colluvium refers to sediments produced by nearly any means and transported downslope by gravity; scree refers to larger blocks and fragments of rock transported downslope. The term ''scree'' comes from the
Old Norse Old Norse, Old Nordic, or Old Scandinavian is a stage of development of North Germanic languages, North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants of Scandinavia and th ...
term for
landslide The term landslide or, less frequently, landslip refers to several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a vari ...

landslide
, ''skriða'', while the term ''talus'' is a French word meaning a slope or embankment. In high-altitude
arctic The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Alaska (United States), Canada, Finland, Greenland (Danish Realm, Den ...

arctic
and
subarctic The sub-Arctic zone is a region in the Northern Hemisphere immediately south of the true Arctic and covering much of Alaska Alaska (; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Central Alaskan Yup'ik language, Yup'ik: ''Alaskaq''; tli, An ...
regions, scree slopes and talus deposits are typically adjacent to hills and river valleys. These steep slopes usually originate from late-
Pleistocene The Pleistocene ( , often referred to as the ''Ice Age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the earth’s most recent period of repeated glaciations. Before a change finally c ...
periglacial Example of a periglacial landscape with both polygon wedge ice near Tuktoyaktuk">Ice_wedge.html" ;"title="pingos and Ice wedge">polygon wedge ice near Tuktoyaktuk, Northwest Territories, Canada Periglaciation (adjective: "periglacial", also referr ...
processes. Notable scree sites in North America include the Ice Caves at
White Rocks National Recreation Area Robert T. Stafford White Rocks National Recreation Area is a United States National Recreation Area located in southern Vermont, United States, within the Green Mountain National Forest. Both the Peru Peak Wilderness and the Big Branch Wilderness a ...
in southern Vermont and
Ice Mountain Ice Mountain is a mountain ridge and algific talus slope that is part of a Nature reserve, preserve near the community of North River Mills, West Virginia, North River Mills in Hampshire County, West Virginia, United States. It was designated ...
in eastern West Virginia in the Appalachian Mountains. Screes are most abundant in the Pyrenees, Alps, Variscan orogeny, Variscan, Apennine Mountains, Apennine, Orocantabrian, and Carpathian Mountains, Iberian Peninsula, Iberian peninsula, and Northern Europe.


Formation

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. There are five main stages of scree slope evolution: (1) accumulation, (2) consolidation, (3) weathering, (4) encroaching vegetation, and finally, (5) 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. Cementation (geology), Cementation occurs as fine-grained material fills in gaps between debris. The speed of consolidation depends on the composition of the slope; clayey components will bind debris together faster than sandy 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. 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: * Weathering#Physical weathering, Physical weathering * Weathering#Chemical weathering, Chemical weathering * Biotic component, Biotic processes * Thermal stresses * Erosion#Mass movement, Topographic stresses


Physical weathering processes

Scree formation is commonly attributed to the formation of ice within mountain rock slopes. The presence of Joint (geology), joints, fractures, 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. 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. 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. 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.


Chemical weathering processes

Phenomena such as acid rain may also contribute to the chemical degradation of rocks and produce more loose sediments.


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. Lichen frequently grow on the surface of, or within, rocks. Particularly during the initial colonization process, the lichen often inserts its hyphae into small fractures or mineral Cleavage (crystal), cleavage planes that exist in the host rock. 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. Freeze-thaw action of the entire lichen body due to microclimatic changes in moisture content can alternately cause thermal contraction and expansion, which also stresses the host rock. Lichen also produce a number of organic acids as metabolic byproducts. These often react with the host rock, dissolving minerals, and breaking down the substrate into unconsolidated sediments.


Interactions with surrounding landscape

Scree often collects at the base of glaciers, 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. Whether the glacier ice begins melting more rapidly or more slowly is determined by the thickness of the layer of scree on its surface. 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, Fourier's Law: Q = -k \left ( \frac \right ), where ''k'' is the thermal conductivity of the debris material, ''Ts'' is the ambient temperature above the debris surface, ''Ti'' is the temperature at the lower surface of the debris, and ''d'' is the thickness of the debris layer. Debris with a low thermal conductivity value, or a high Thermal resistance, 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. 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. If the ice is covered by a relatively thin layer of debris (less than around 2 centimeters thick), the albedo effect is most important. 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. However, once the debris cover reaches 2 or more centimeters in thickness, the albedo effect begins to dissipate. Instead, the debris blanket will act to insulate the glacier, preventing incoming radiation from penetrating the scree and reaching the ice surface. 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. As a result, soil, bedrock, and also Subterranean river, subterranean voids in screes do not freeze at high elevations.


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. 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. This freezing circulating air maintains internal scree temperatures 6.8-9.0 °C colder than external scree temperatures. These <0 °C thermal anomalies occur up to 1000m below sites with mean annual air temperatures of 0 °C. Patchy permafrost, which forms under conditions <0 °C, probably exists at the bottom of some scree slopes despite mean annual air temperatures of 6.8–7.5 °C.


Biodiversity

During the Last Glacial Period, last glacial period, a narrow ice-free corridor formed in the Weichselian glaciation, Scandinavian ice sheet, introducing taiga species to the terrain. These Taiga, boreal plants and animals still live in modern Alpine climate, alpine and
subarctic The sub-Arctic zone is a region in the Northern Hemisphere immediately south of the true Arctic and covering much of Alaska Alaska (; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Central Alaskan Yup'ik language, Yup'ik: ''Alaskaq''; tli, An ...
tundra, as well as high-altitude Pinophyta, coniferous forests and mires. Scree microclimates maintained by circulating freezing air create Habitat, microhabitats that support taiga plants and animals that could not otherwise survive regional conditions. A Czech Academy of Sciences, Czech Republic Academy of Sciences research team lead by Physical chemistry, 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 The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Alaska (United States), Canada, Finland, Greenland (Danish Realm, Den ...

arctic
bryophytes, pteridophytes, and arthropods 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."
Ice Mountain Ice Mountain is a mountain ridge and algific talus slope that is part of a Nature reserve, preserve near the community of North River Mills, West Virginia, North River Mills in Hampshire County, West Virginia, United States. It was designated ...
, a massive scree in West Virginia, supports distinctly different distributions of plant and animal species than northern latitudes.


Scree running

Scree running is the activity of running down a scree slope; which 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.


See also

* Blockfield - similar to talus and scree slopes, formed by frost weather instead of mass wastings * * * * * * Scree plot


References

{{reflist, 33em Slope landforms Montane ecology