OCEANS AND LAKES 



253 



There are several points of similarity between coastal cycles 

 and the erosion cycles of rivers. First a river system roughens 

 the surface of its basin, increasing its relief ; finally it reduces 

 it to a smooth plain, near sea level. As indicated above, 

 waves and currents normally increase to a maximum the irreg- 

 ularities of a coast, and finally reduce them to a minimum. 

 An essential difference is 

 that the irregularities of 

 the river basin are verti- 

 cal irregularities, while 

 those of the shore line are 

 horizontal. In each case 

 the cycle of development 

 is introduced by diastro- 

 phism. Diastrophism in 

 each case frequently ter- 

 minates incompleted 

 cycles. 



From what has been 

 said already, it will be 

 evident that the rate at 

 which a coast develops de- 

 pends on (1) the strength 

 of the waves and currents, 

 and (2) the resistance of 

 the rocks of the coast. The outer coast of the end of Cape 

 Cod is farther advanced than the inner, though the material 

 is the same, because the outer side is exposed to the vigorous 

 waves and currents of the open sea, while the inner side is 

 somewhat protected (Fig. 275). The coast of Maine is in 

 general in a youthful stage of development, though its exposure 

 is comparable to that of the outer "coast of Cape Cod, and 

 waves and currents have worked upon it for nearly as long. 

 The explanation lies in the fact that the material of the Cape 

 Cod coast is loose glacial drift, while most of that of the 

 Maine coast is resistant bedrock. 



FIG. 274. Portion of the Texas coast, 

 showing barrier islands and the tend- 

 ency of shore deposition to simplify the 

 coast line. 



