Mass movement refers to different types of erosion caused by gravity that make mountains smaller over time. The main agents of erosion are gravity, glaciers, water, and wind. Erosion wears away surface materials and transports them elsewhere via processes like deposition, where sediment is dropped when the agent of erosion loses energy. Types of gravity-caused erosion include slumps, ice wedging, rock falls, rock slides, and mudflows, which typically occur quickly after heavy rain or earthquakes and move materials down steep slopes. Methods to help slow mass movement include planting vegetation, installing drainage pipes, and building retaining walls, but mass movement can never be fully stopped.