Gravity
Mass movement is a form of erosion in which soil, rock, or sediment move downslope due to gravity. Gravity causes loose rock and soil to move down the hill. A landslide occurs when heavy rains or earthquake and have enough energy to knock over trees or houses.Mudflows sometimes occur in fairly dry areas. Over time, dry sediment build up in deep layers. Trees or utility pole on a hillside sometime lean to one side. This is caused by a slow mass movement called creep. Creep occur where the ground often freezes and thaws.