THE FORMS CARVED AND MOLDED BY WAVES 237 



FIG. 257. Cut and built terrace with bowlder pave- 

 ment shaped by waves on a steep shore formed of 

 loose materials. 



The cut and built terrace on a steep shore of loose materials. 

 In materials which lack the coherence of firm rock, no vertical 

 cliff can form ; for as fast as undermined by the waves the loose, 

 materials slide down 

 and assume a surface 

 of practically constant 

 slope the " angle of 

 repose" of the mate- 



rials (Fig. 257). The 



terrace below this 



sloping cliff will not 



differ in shape from 



that cut upon a rocky shore; but whenever the materials of the 



shore include disseminated blocks too large for the waves to handle, 



they collect upon the terrace near where they have been exhumed, 



thus forming what' has been called a " bowlder pavement " (Fig. 



258). 



The edge of the cut and built terrace is, as already mentioned, 



maintained at the depth of wave base. If one will study the sub- 

 merged contours of any of our 

 inland lakes, it will be found 

 that these basins are sur- 

 rounded by a gently sloping 

 marginal shelf, the cut and 

 built terrace, and that the 

 depth of this shelf at its outer 

 edge is proportioned to the 

 size of the lake. Upon Lake 

 Mendota at Madison, Wiscon- 

 sin, the large storm waves have 

 a length of about twenty feet, 

 which is the depth of the outer 

 edge of the shore terraces (Fig. 





FIG. 258. Sloping cliff and terrace with 

 bowlder pavement exposed at low tide 

 upon the sea shore at Scituate, Mass- 

 achusetts. 



267, p. 242). The shelf sur- 

 rounding the continents has, 



with few local exceptions, a uniform depth of 100 fathoms, or about 



the wave base of the heaviest storm waves. 



The work of the shore current. In describing the formation 



of the built terrace, it was stated that the greater part of the rock 



