Slopes are not static piles of dirt and rock. In an instant of geological time, such as a human life span, a slope may remain still. Over longer periods, however, all slopes are potentially unstable and some form of downslope movement of material will eventually occur. Mountains, valley walls, hills, and ridges are in a dynamic tug of war between the internal strength of their slopes and the downward pull of gravity. Gravity is the
driving force behind slope failure. Resisting forces oppose gravity and work to maintain slope stability. A slope may become unstable and fail if the driving force exceeds the resisting forces.