Осыпи и талусы: формирование и характеристики склонов каменных осыпей.

Broken rock fragments at base of cliff

Scree is a collection of broken 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. Talus deposits typically have a concave upwards form, where the maximum inclination corresponds to the angle of repose of the mean debris particle size. The exact definition of scree in the primary literature is somewhat relaxed, and it often overlaps with both talus and colluvium. The term scree comes from the Old Norse term for landslide, skriða, while the term talus is a French word meaning a slope or embankment. In high altitude arctic and subarctic regions, scree slopes and talus deposits are typically adjacent to hills and river valleys. These steep slopes usually originate from late Pleistocene periglacial processes. Notable scree sites in Eastern North America include the Ice Caves at White Rocks National Recreation Area in southern Vermont and Ice Mountain in eastern West Virginia in the Appalachian Mountains. Screes are most abundant in the Pyrenees, Alps, Variscan, Apennine, Orocantabrian, and Carpathian Mountains, Iberian peninsula, and Northern Europe.

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. It is loosely synonymous with talus, material that accumulates at the base of a projecting mass of rock, 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. which distinguishes it from colluvium. Colluvium is rock fragments or soil that is deposited by rainwash, sheetwash, or slow downhill creep, usually at the base of gentle slopes or hillsides. However, the terms scree, talus,

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.

Scree formation is commonly attributed to the formation of ice within mountain rock slopes. The presence of 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. 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.

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

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 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, 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: 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. As a result, soil, bedrock, and also subterranean voids in screes do not freeze at high elevations.

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, permeable sheets of sediment, screes experience cold air seepage from the bottom of the slope where sediment is thinnest. 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. introducing taiga species to the terrain. These boreal plants and animals still live in modern alpine and subarctic tundra, as well as high altitude coniferous forests and mires. Scree microclimates maintained by circulating freezing air create microhabitats that support taiga plants and animals that could not otherwise survive regional conditions.