OCEANS AND LAKES 239 



(p. 224), and may build islands by depositing drift. Loose- 

 material is often incorporated in ice formed along high- 

 latitude coasts in winter; when the ice breaks up in the 

 spring, this material may be carried away to be dropped 

 where the ice melts. Weathering agents reduce sea cliffs 

 and loosen material along shore, preparing it for removal 

 by other agents. But most important in shaping the details 

 of coast lines is the work of wind waves and of the shore 

 currents which they generate. The features they develop 

 are discussed below. 



EROSION BY THE SEA 



How the sea wears its shores. Clear waves dashing 

 against cliffs of firm, unjointed rock accomplish little or no 

 wear. The inability of waves to erode under these circum- 

 stances recalls the similar dependence of winds, streams, and 

 glaciers upon their rock tools. But the conditions suggested 

 rarely occur. Usually the rocks of the seashore are traversed 

 by joints. If stratified, they contain bedding planes. There 

 are still other openings, and all form weak places. With the 

 impact of strong waves, water is forced into the openings 

 with great pressure. Furthermore, the air in the openings 

 is compressed by the invading water, and then expands with 

 force as the water withdraws. In these ways pieces of rock 

 are broken and sucked off, and the openings enlarged. Ordi- 

 narily, too, the water offshore is sufficiently shallow for the 

 waves to obtain from the bottom sand, stones, and some- 

 times, when very strong, even large bowlders, which are 

 hurled as battering-rams against the shore. Locally, the sea 

 dissolves the rocks of its shore. 



Rate of erosion. The rate at which a given coast is eroded 

 is determined by several factors. (1) Other things being 

 equal, strong waves obviously erode faster than weak ones. 

 The velocity of the winds which generated them, the depth 

 of the water they have traversed, and the distance they have 



