Colluvium is the name for loose bodies of sediment that have been deposited or built up at the bottom of a low-grade slope or against a barrier on that slope, transported by gravity. The deposits that collect at the foot of a steep slope or cliff are also known by the same name. Colluvium often interfingers with alluvium (deposits transported downslope by water). Coarse deposits due to rockfall at a cliff base are called talus (scree) and if lithified are talus breccias. Avalanches, mudslides, and landslides are processes that deposit colluvium. This build-up process is called colluviation.

Colluvium normally forms humps at the base of mountains or fan-shaped deposits similar in shape to alluvial fans that cover former ground surfaces. This process is an important phenomenon in the fields of archaeology and soil science.

Many colluvial soils tend to have a fragipan associated with them that are a brittle subsoil layer typically high in clay. One theory of fragipan formation is the smearing of soil during the colluvial process causing the clays to seal the surface between the moving portion of soil and the stationary soil on which it slides. Ancient sites can be preserved beneath colluvium if later changes in the landscape such as deforestation encourage a downward movement of material.


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