THE WORK OF WAVES A^'D CURRENTS. 89 



portions less finely comminuted fall on the outer slopes of the terrace and 

 serve to broaden it. The coarsest of all the .shore debris usually remains 

 on the terrace and is swept along by the currents until it finds a resting- 

 place in some embankment or wave-built bar. The portion falling on the 

 outer margin of the terrace is frequently consolidated by the precipitation 

 of calcium carbonate in its interstices, thus forming a conglomerate, which 

 adds to the breadth of the structure. In this manner a terrace may become 

 in part a work of destruction and in part a Avork of construction, as indi- 

 cated in the following diagram, whicli.represents a cut-terrace, as in the last 

 figure, with the addition of an accumulation of debris on its outer slope. 



l/lffS- SUfJFACE 



Fig. 9. — Ideal profile of a cut-and-biiilt terrace. 



Observations have shown that this is the most characteristic form of 

 lake terrace, and illustrates the fact that the action of waves and currents 

 in modifying shores may be divided into erosion and deposition, or the 

 processes of destruction and construction. 



SEA-CIilFFS. 



The steep scarps rising above terraces are termed sea-clifFs, whether 

 formed on lake or ocean shores. They occur especially where the borders 

 of a lake are abrupt; but when the bluffs approach the perpendicular and 

 form cliff's with deeply submerged bases, the action of the waves in carving 

 terraces is reduced to a minimum, for the reason that the shore dthris falls 

 into deep water beyond the reach of the waves and can no longer be used 

 as a tool in cutting away the land. On tlie other hand, sea-cliffs are sel- 

 dom formed when the slope of the beach is very gentle. In such localities 

 the waves lose their force before reaching the land, and deposition rather 

 than erosion takes place. Sea-cliffs ai"e most pronounced in rocks of hete- 

 rogeneous composition, which ai-e easily eroded, but yet sufficiently durable 

 to stand in perpendicular escarpments. 



