Waves are the most important erosive agent along most coasts but their effect varies with wave energy and characteristics, and with the nature of the material exposed to wave attack. Where a coast is formed by steep cliffs that plunge straight into deep water, waves are not forced to break before they impact. As virtually no forward mass displacement of water exists in such waves, they are reflected with little loss of energy and accomplish negligible geomorphic work. Much more commonly, coasts are subject to breaking waves. These involve significant mass displacement and a considerable loss of kinetic energy as they break on a shoreline. This energy is not dissipated over a short distance where the shore gradient is steep, but over a greater horizontal distance where it is shallow. Of the main types of breaking waves, plunging breakers produce the