colluvial

Colluvial refers to processes and deposits moved downslope primarily by gravity. The material produced is called colluvium, a gravity-driven sediment that accumulates at the base of slopes or in depressions. In geology and soil science, colluvial deposits are distinguished from alluvial deposits formed by flowing water and from glacial or periglacial deposits.

Colluvial processes include creep, rockfalls, landslides, and sheet or surface wash that transports debris downslope. Material is typically transported short distances and deposited at the toe of a slope or within collapse zones, creating wedges, aprons, or fan-like accumulations. Colluvium is usually poorly sorted and may be highly heterogeneous, with a wide range of grain sizes from clay to boulders, and the grains are often angular or subangular. Stratification, if present, tends to be weak or disrupted by successive failures.

Field characteristics of colluvial deposits include their location on hill slopes or at slope bases, their poor sorting, and their lack of distinctive bedding. They commonly form colluvial wedges or fans and may host soils that develop from colluvial material, known as colluvial soils. These soils often show age-related horizons and differences in drainage and fertility related to the underlying colluvial material.

In geomorphology and soil mapping, distinguishing colluvium from alluvium or glacial deposits is important for understanding landform evolution, slope stability, and groundwater behavior. Colluvial processes contribute to ongoing soil formation and landscape change on inclined terrains.