Along the foot of the cliff lies a gently shelving bench of rock, more or less thickly veneered with sand and shingle. At low tide its inner margin is laid bare, but at high tide it is covered wholly, and the sea washes the base of the cliffs. A notch, of which the _sea cliff_ and the _rock bench_ are the two sides, has been cut along the shore (Fig. 132). =Waves.= The position of the rock bench, with its inner margin slightly above low tide, shows that it has been cut by some agent which acts like a horizontal saw set at about sea level. This agent is clearly the surface agitation of the water; it is the wind-raised wave. As a wave comes up the shelving bench the crest topples forward and the wave "breaks," striking a blow whose force is measured by the momentum of all its tons of falling water (Fig. 133). On the coast of Scotland the force of the blows struck by the waves of the heaviest storms has sometimes exceeded three tons to the square foot. But even a calm sea constantly chafes the shore. It heaves in gentle undulations known as the ground swell, the result of storms perhaps a thousand miles distant, and breaks on the shore in surf. [Illustration: Fig. 133. Breaking Wave, Lake Superior] The blows of the waves are not struck with clear water only, else they would have little effect on cliffs of solid rock. Storm waves arm themselves with the sand and gravel, the cobbles, and even the large bowlders which lie at the base of the cliff, and beat against it with these hammers of stone. Where a precipice descends sheer into deep water, waves swash up and down the face of the rocks but cannot break and strike effective blows. They therefore erode but little until the talus fallen from the cliff is gradually built up beneath the sea to the level at which the waves drag bottom upon it and break. Compare the ways in which different agents abrade. The wind lightly brushes sand and dust over exposed surfaces of rock. Running water sweeps fragments of various sizes along its channels, holding them with a loose hand. Glacial ice grinds the stones of its ground moraine against the underlying rock with the pressure of its enormous weight. The wave hurls fragments of rock against the sea cliff, bruising and battering it by the blow. It also rasps the bench as it drags sand and gravel to and fro upon it.