colluvium

Colluvium is unconsolidated sediment deposited at the base of a slope by gravity-driven processes. It forms from weathered rock, rockfall debris, and soil fragments that move downslope through mechanisms such as rockfalls, landslides, slumps, creep, and other downslope movements, sometimes aided by surface runoff. The result is material that accumulates where movement slows, producing colluvial aprons, footslopes, or fans.

Colluvium is typically poorly sorted and heterogeneous, with a wide range of particle sizes from clay to boulders. The fragments are often angular or subangular, reflecting limited transport and short mixing time. Deposits can be thick and extend for considerable distances from the original source areas, but they are prone to reworking by future gravitational events or by water.

In soils terminology, the term colluvium refers to the loose material deposited by gravity at the foot of slopes, while colluvial soils describe soil profiles that develop in place from that material, often with distinctive horizon development and properties influenced by slope processes. By contrast, alluvium refers to sediments deposited by running water, typically in channels and fans, and is usually better sorted and more channel-controlled than colluvium.

Geographically, colluvial deposits are common in mountainous and hilly terrain, along valley margins, and at the toe of slopes where gravity concentrates debris. They play a role in sediment budgets, groundwater recharge, and soil formation, and they can pose hazards when reactivated by heavy rainfall or seismic shaking.