Scree, or talus, is accumulation of broken rock fragments at the base of crags, mountain cliffs, or valley shoulders. Landforms associated with these materials are sometimes called scree slopes or talus piles. These deposits typically have a concave upwards form, while the maximum inclination of such deposits corresponds to the angle of repose of the mean debris size.
The term scree comes from the Old Norse term for landslide, skriða,[1] while the term talus is a French word meaning a slope or embankment.[2][3] The two terms are often used interchangeably, though scree commonly refers to smaller material like mixed gravel and loose dirt, talus to rocks larger than scree.[4] Talus is usually the preferred term in scientific writing.
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 create new cracks or wedge blocks into an unstable position. Special boundary conditions (rapid freezing and water confinement) may be required for this process to be effective.[5] 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.[6] 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.[7][8]
Scree drains rapidly. Only in reliably moist climates does talus support mature forest (illustration, right). 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.
Scree can also be the result of human activity, such as the scree beneath the sculpture Mount Rushmore in South Dakota.