Since the incoming wave affects water which is at the same time under the influence of the undertow, it gives to that current a pulsating character, for the wave-motion sometimes supports and sometimes opposes the undertow, and thus endows it with a higher transporting power than belongs to its mean velocity. Near the breaker-line, the oscillations communicated by the wave may momentarily overcome and even reverse the movement of the undertow. Inside the breaker-line, irregular oscillation only is communicated. The broken wave-crest, dashing forward, overcomes the undertow and throws it back, and the water returns as a simple current descending a slope. The power of the undertow diminishes rapidly from the breaker-line outward as the depth of the water increases. [Illustration: +Fig.+ 300.—Diagram showing relative directions of wave, undertow, and shore-current.] When waves advance on the shore obliquely, a shore-current is developed as illustrated by Fig. 300, where _ab_ represents the direction of the incoming wave, _bc_ the direction of the littoral current, and _bd_ the direction of the undertow. Where they strike the borders of land, the wind-waves, therefore, generate two other movements, the undertow and the littoral current. Any particle of water near shore may be affected by any two or by all three of these movements at the same moment. The effect of littoral current and undertow is to give a particle of water on which both are working a direction between the two, as _be_. The effect of other combinations can be readily inferred. These various combinations are of consequence in the transportation of débris. WORK OF THE WAVES. _Erosion._ The general effects of the waves and the other movements to which they give rise along shores are (1) the wear of the shores; (2) the transportation for greater or less distances of the products of wear; and (3) the deposition of the transported materials. =By waves and undertow.=—In the dash of the waves against the shore, the chief wear is effected by the impact of the water and of the débris which the water carries. Lesser results are accomplished in other ways.