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. 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, or talus slope, a landform composed of talus. 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. Scree is formed by rockfall, which distinguishes it from colluvium. Colluvium is rock fragments or soil deposited by rainwash, sheetwash, or slow downhill creep, usually at the base of gentle slopes or hillsides. However, the terms scree, talus, and sometimes colluvium tend to be used interchangeably. The term talus deposit is sometimes used to distinguish the landform from the material of which it is made. The exact definition of scree in the primary literature is somewhat relaxed, and it often overlaps with both talus and colluvium. Within the outdoor community, talus and scree are often described as two distinct things, with scree being small, loose pieces of rock, no bigger than a fist, while talus is any rock that is bigger, all the way from football sized to boulder sized, which can either be loose or not. One way to tell them apart is that a person can boot-ski scree and boulder hop talus.
Etymology 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.
Description Talus deposits typically have a concave upwards form, where the maximum inclination corresponds to the angle of repose of the mean debris particle size. 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 with large, boulder-sized rock fragments may form talus caves, or human-sized passages formed in-between boulders.
Formation
The formation of scree and talus deposits is the result of physical and chemical weathering acting on a rock face, and erosive processes transporting the material downslope. 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. There are five main stages of scree slope evolution:
accumulation consolidation weathering encroaching vegetation slope degradation. Scree slopes form as a result of accumulated loose, 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 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 cohesive forces between the coarse and fine components, degrading the slope. The predominant processes that 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:
Physical weathering Chemical weathering Biotic processes Thermal stresses Topographic stresses
Physical weathering processes
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.
Chemical weathering processes Phenomena such as acid rain may also contribute to the chemical degradation of rocks and produce more loose sediments.
… excerpt ends here. Continue reading the full article.
