Residual soils on a steep natural slope can move slowly downward, and this is usually referred to as creep. When the downward soil movement is sudden and rapid, it
is called a landslide. The soil deposits formed by landslides are colluvium. Mud flows
are one type of gravity transported soil. In this case, highly saturated, loose sandy
residual soils, on relatively flat slopes, move downward like a viscous liquid and come
to rest in a more dense condition. The soil deposits derived from past mud flows are
highly heterogeneous in composition.