The direct effect of wave-erosion is restricted to a zone which is narrow both horizontally and vertically. There is no impact of breakers at levels lower than the troughs of the waves, though erosion may extend down to the limit of effective agitation (p. 341). The efficient impact of waves is limited upward by the level of the wave-crests, although the dash of the water produces feebler blows at higher levels. The rise and fall of the water during the flow and ebb of the tides gives the waves a greater vertical range than wind-waves alone would have. The vertical zone of direct wave-work is therefore limited above by the level of wave-crests, and below by the depth of wave-troughs (nearly). The indirect work of waves is limited only by the height of the shore, for as the zone of excavation is carried landward, masses higher up the slope are undermined and fall. The fallen rock temporarily protects the shore against the waves, but are themselves eventually broken up. [Illustration: +Fig.+ 303.—Diagram illustrating high sea-cliffs. It also shows a submerged terrace, due partly to wave-cutting (wave-cut terrace), and partly to building (wave-built terrace). (Gilbert.)] [Illustration: +Fig.+ 304.—Diagram showing a low sea-cliff. (Gilbert.)] The pulsating current of the undertow (p. 341) has both an erosive and a transporting function. It carries the detritus of the shore to and fro, and dragging it over the bottom, continues downward the erosion initiated by the breakers. This downward erosion is the necessary concomitant of the shoreward progress of wave-erosion; for, if the land were merely planed away to the level of the wave-troughs, the incoming waves would break where shoal water was first reached, and become ineffective at the water margin. The rate of erosion by the undertow becomes less and less as the surface it affects is lowered. Littoral currents do little erosive work beyond that inflicted on the material which they transport.