Tutorial:Gravity Deposition
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[edit] Mass movement deposits
The deposits of gravity driven, mass movement events tend to accumulate at the base of slopes and upslope of obstructions.
Colluvium is a term frequently used in archaeology to describe the accumulation of poorly sorted, unstratified materials at the base of slopes. The term colluvium is taken to mean two different things. The first is the material that accumulates at the base of slopes as a result of gravity mediated mass movements such as creep and soil creep. The second takes colluvium to mean the material deposited at the base of slopes as a result of all slope processes, except concentrated water run-off, this includes creep, soil creep, sheet flow, and plough erosion. This is generally how the term colluvium is used in archaeology.
Colluvium tends to consist of poorly sorted, weakly stratified or unstratified soil materials, although if there has been a siginificant element of sheet wash deposition the colluvium may be finer-grained than the parent soil materials, however there should be little evidence of rounding compared to the parent soil material. Although generally unstratified, colluvium may contain fossil soil layers relating to periods of relative stability (low erosion), and where colluvial deposits meet overbank floodplain deposits interleaving of the two helps preserve stratigraphy and can be an important archive of natural and human induced catchment wide erosion patterns.
More rapid events such as slides and flows can quickly bury settlements and landscape elements in their path. Flows (including solifluction) containing an appreciable amount of water may produce deposits with weak sorting and stratification, that contain pebbles with a preferred orientation of their long axis parallel to the direction of flow.
