202 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The sea also erodes the bottom and supplies material for the bar. 

 The proportion of bottom to side supplied detritus will vary exceedingly. 

 With a deeply dissected, steep coast, the proportion of material from 

 the headland will be large, while in a slightly drowned region, devel- 

 oped to past-maturity in the iirevious cycle, there will be more material 

 under water above wave-base, and therefore a greater proportion of 

 bottom detritus. There are so many variables which enter into this 

 problem, — viz. initial form, prevailing winds, strength of currents, 

 height of tides, radius of curvature of eddies, structure of laud, etc., — 

 that it is difficult to predict where a bar will be built across a bay. 

 It may be said, however, that the sea is not satisfied with an irregular 

 shoreline, and in its attempt to reduce the land to a submarine platform 

 it will straighten the shoreline in order better to attack the land. The 

 curve that a given shore will take depends upon the forces acting at 

 that point. 



In one place wings will extend from the projecting headland, in another 

 the currents will build a bar across the mouth of the bay, in a third 

 the bar will grow from a point between headland and bay-head, while in 

 a fourth [)lace the alongshore action may be so weak or the bay so broad 

 that the sea will begin to fill at the head. In this fourth case any delta 

 filling will go on at the same place as the accumulation by sea action. 



When the bay-bar is completed, and there is transportation of material 

 practically all along the shore, shore-grade is attained, and the period of 

 adolescence in shore evolution is reached. Tlie narrow and broader 

 bays behind the bars are gradually filled by river, tide, and wind. "Where 

 the river activity is strong enough, it pushes a delta beyond the bar. 

 Maturity is reached when the bays are filled and the headlands cut back 

 so that the initial shoreline is lost. From this time forward the sea, 

 satisfied with the shore curves, eats farther and farther into the land with 

 the intention of reducing all that stands above wave-base to a monotonous 

 submarine platform. 



The classification of bay-bars here given is not a satisfactory one. 

 The separation into stages of development is only partial, for more facts 

 of observation are needed. The location of the bar in the bay, which 

 depends upon the ratio of alongshore to on- and offshore currents, as well 

 as upon the form of the bay, has been used to make three types of bay- 

 bars. Under each of these, stages of filling occur, all centring about 

 the attainment of shore-grade, and therefore bay-bars may be limited as a 

 class to a period extending from late youth to early maturity. Bay-bars 

 are characteristic of adolescence. 



