The process in which material settles out as a moving substance slows down is called sedimentation. Sedimentation is the process by which particles in suspension in a fluid (liquid or gas) are deposited on a surface. This process occurs when the force of gravity acting on the particles is greater than the force of the fluid that is keeping them suspended. Sedimentation is a major process in the formation of sedimentary rocks, as well as in the purification of water and other liquids.