486 THE JOURNAL OF GEOLOGY. 



carries up material stirred from the bottom. The finer particles 

 are swept back by the undertow, the coarse are placed by the 

 greater waves beyond the reach of the lesser. Thus. waves, con- 

 stantly in advancing, take material from the lower part of the slope 

 to carry it up, and in retreating sweep back more or less of their 

 load with them. If the slope be gentle they thus take from the 

 lower to add to the upper part, and therefore they increase the 

 declivity until the seaward profile becomes so steep that the load 

 carried in retreat balances that advanced. This is the profile of 

 equilibrium, which waves perpendicular to the trend of the beach 

 do not change, unless they are of unusual force. Waves oblique 

 to the beach-slope, scour, transport and deposit the same sands 

 repeatedly, and if the oblique advance be prevailingly from one 

 direction the effect is to move the beach longitudinally. Then 

 the beach, in any one section, continues, while the supply of 

 sand is continuous ; but when the supply ceases the beach is 

 gradually moved onward in the direction of the prevailing wave 

 action, and the material beneath the beach sands is exposed to 

 wave erosion. 



A beach itself is but a narrow zone ; it cannot constitute a 

 wide-spread formation any more than a line can constitute a 

 plane. But if a line be moved in one direction parallel to itself 

 it will develop the plane, and in the same manner if a beach ad- 

 vances landward it may spread a formation. This advance may 

 be a result of wave erosion, which carving a sea cliff on a bold 

 shore planes a surface of marine denudation. The beach deposit 

 is then a basal conglomerate. Or, the land reduced to a low sur- 

 face by subaerial erosion may subside slightly in reference to sea 

 level, and the sea, transgressing, will rearrange the superficial for- 

 mations. If the waves have power to handle the material the sea 

 is margined by beach sands. If they cannot efficiently sort it 

 the land will merge in tide-flats with the water. 



A beach is not only narrow, it is also shallow ; waves build 

 on the surface over which they break, and the height to which 

 they may build does not exceed a few feet. Therefore, beach 

 deposits cannot form thick strata. 



