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Scree is a collection of broken rock fragments at the base of crags, mountain cliffs, volcanoes or valley shoulders that has accumulated through periodic rockfall from adjacent cliff faces. Landforms associated with these materials are often called talus deposits. Talus deposits typically have a concave upwards form, while the maximum inclination corresponds to the angle of repose of the mean debris size.
Formation of scree or talus deposits results from physical and chemical weathering and erosional processes acting on a rock face. The predominant processes that degrade a rock slope depend largely on the regional climate (temperature, amount of rainfall, etc.). Examples include:
Scree formation is commonly attributed to the formation of ice within mountain rock slopes. During the day, water can flow in joints and discontinuities in the rock wall. If the temperature drops sufficiently, for example with the onset of evening, this water may freeze. Since water expands by 9% when it freezes, it can generate large forces that either creates new cracks or wedges blocks into an unstable position. Special boundary conditions (rapid freezing and water confinement) may be required for this process to be effective. 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 the subject of some debate in the scientific community. Many researchers believe that it is unrealistic to assume that ice formation in large open crack systems can generate large pressures, instead suggesting that the water and ice simply flow out of the cracks as pressure builds. Many argue that a frost heaving process, like that known to act in soil in permafrost areas, may in fact play an important role in cliff degradation in cold environments.
Scree can conceal a glacier. For example, Lech dl Dragon, in the Sella Group of the Dolomites, derives from the melting waters of a glacier, hidden under a thick layer of scree. The melting process of the underlying glacier is slowed by the protective layer of scree.
With sufficient time, a rock slope may become completely covered by its own scree so that production of new material ceases. This slope is said to be mantled with debris.
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