46 LAKES OF NORTH AMERICA. 



Reference has already been made to the action of the waves when the 

 wind blows directly on shore. The return current is then an undertow 

 flowing lakeward. When the wind blows against the shore at a low 

 angle, however, currents are established which travel along the lake 

 margin and sweep the loose material on the surface of the terrace with 

 them. These currents have many of the features of streams, and greatly 

 increase the jjower of waves to erode the land. The upward movement 

 of waves tends to lift loose material within their reach and the lateral 

 movement of currents to transport it. The loose material at the base of 

 sea cliffs is thus carried along the beach by shore currents in one direction 

 or another, according to the direction of the wind, and deposited so as to 

 form accumulations of various character. 



When a headland, with a beach at its base, is flanked on either hand 

 by low shores, the debris falling from its face is carried along by the 

 shore currents and built into terraces adjacent to the land or deposited so 

 as to form free embankments or ridges, at some distance from the original 

 shore. That this process is of common occurrence may be shown on 

 many lake margins by examining the material forming rocky headlands 

 and comparing it with the stones on neighljoring beaches. In such in- 

 stances the rock fragments at the base of the cliff will frequently be 

 found to be large and angular and to become smoother and more and 

 more rounded the farther they are traced from their parent ledges. 



Terraces and marginal embankments, built wholly of gravel and sand, 

 may also be formed on low shores by the washing up of loose material 

 from this lakeward margin, thus deepening the water on the outside of 

 the shelf. 



The transportation of debris along the surfaces of terraces by the com- 

 bined action of waves and currents, and its deposition when deep water is 

 reached, leads to the formation of structures of various forms, known as 

 embankments. 



Enibaiikments. — This name has been adopted for free ridges of 

 loose material Imilt by currents about the margins of water-bodies. They 

 have the general form of railroad embankments, and their level crests in 

 most instances rise from a few inches to, perhaps, three or four feet above 

 the calm-weather surfaces of the water in which they occur. The ten- 

 dency of built terraces to change to embankments on low shores has already 

 been noticed, but the most typical examples occur where shore currents, 

 having an abundance of loose material at their command, are deflected 



