OCEANS AND LAKES 241 



that usually deep water fronts high coasts, and relatively 

 shallow water, low coasts. 



On the eastern coast of England, where the rocks are rela- 

 tively weak, entire parishes have been washed away within a 

 few centuries ; in some places the shore line has retreated as 

 much as 15 feet in a single year. The south shore of Nan- 

 tucket Island, Massachusetts, has lost in places as much as 

 6 feet in a year, and as early as 1835 the opinion was expressed 

 that within a few centuries the entire island would be devoured 

 by the sea. 



Sea cliffs and terraces. The chief topographic effects of 

 wave erosion are illustrated by Figure 257. The original slope 

 near the water level is indicated by the dotted line. Erod- 

 ing waves have notched this slope, 

 forming a sea cliff. The develop- 

 ment and recession of a sea cliff 

 involve also the formation and 

 widening at its base of an under- 

 water platform, Called the wave-CUt -p IG 257. Diagram of sea 



terrace. Its surface represents the cliff, wave-cut terrace, and 



i f -j_ t f - L' wave-built terrace. 



lower limit of effective wave action. 



It slopes gently seaward because, as its width increases, the 

 strength of the waves at its inner edge decreases (Why?), and 

 they are accordingly able to cut a less and less distance below 

 sea level. (Should you expect the slope of wave-cut terraces 

 to vary? If so, why?) At first the material worn from the 

 cliffs is swept to the edge of the wave-cut terrace, and de- 

 posited in deeper water. Here it accumulates to form the 

 wave-built terrace, which extends the wave-cut terrace seaward. 

 Later, more or less of the waste of the cliffs remains here and 

 there upon the terrace at their base (Why?) to form a beach. 

 Still later, when the beach is developed more continuously, 

 much of the waste is washed along it by waves and shore cur- 

 rents (p. 246). Wave-formed terraces may become land by 

 lowering of the sea, or by uplift of the coast line (Figs. 258 

 and 259). 



