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.
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.
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. 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.