236 



EARTH FEATURES AND THEIR MEANING 



The broken wave, after rising upon the terrace under the inertia 

 of its motion until all its energy has been dissipated, slides out- 

 ward by gravity, and though 

 checked and overridden by 

 succeeding breakers, it con- 

 tinues its outward slide as 

 the " undertow " until it 

 reaches the end of the ter- 

 race. Here it suddenly en- 

 ters deep water, and losing 

 , its velocity, drops its burden 



FIG. 255. Ideal section of a steep rocky >' * 



shore carved by waves into a notched cliff of rock, and builds the ter- 



and cut terrace, and extended by a built race seaward after the man- 

 ner of construction of an 



embankment. As we are to see, the larger portion of the wave- 

 quarried material is diverted to a different quarter. 



To -gain some conception of the importance of wave cutting 

 as an eroding process, we may consider the late history of Heli- 

 goland, a sandstone island off the mouth of the Elbe in the North 

 Sea (Fig. 256). From a periphery of 120 miles, which it possessed 

 in the ninth cen- 

 tury of the Chris- 

 tian era, the 

 island has reduced 

 its outline to 45 

 miles in the four- 

 teenth century, 8 

 miles in the seven- 

 teenth, and to 

 about 3 miles at 

 the beginning of 

 the twentieth cen- 

 tury. The German 

 government,which 

 recently acquired 



this little remnant FlG ' 256 ' Map snowin g tne outlines of the Island of 

 . Heligoland at different stages in its recent history. The 



from England, has peripheries given are in miles. 



expended large 



sums of money in an effort to save this last relic. 



