steep slope of earth materials, usually a rock face, that is nearly vertical and may be overhanging. Structural cliffs may form as the result of fault displacement or the resistance of a cap rock to uniform downcutting. Erosional cliffs form along shorelines or valley walls where the most extensive erosion takes place at the base of the slope. Because of their greater gradient, cliffs are subjected to greater erosive action and tend to retreat more rapidly than other slopes.