THEORIES OF ORIGIN 615 



and the cusps. Beach cusps are clearly the product of on- and off-shore 

 movements of the water. 



ARTIFICIAL BEACH CUSPS 



From the observation of natural beach cusps in process of formation 

 the conclusion was reached that cusps could be formed by a single series 

 of waves advancing parallel with the shore. In order to test the validity 

 of this conclusion the artificial production of cusps was attempted. A 

 sand beach was constructed along one side of a tank 5 feet square and the 

 water in the tank raised until it rested against the beach slope. To make 

 that slope as smooth and gentle as possible^ large waves were washed 

 over the beach until it appeared to the eye as a perfectly uniform, gentle 

 slope of sand. On the opposite side of the tank from the beach was ar- 

 ranged the wave-producing apparatus. This consisted at first of a board 

 which was tipped up and down by hand; later of two boards hinged to- 

 gether, one of which was made stationary on the floor of the tank, while 

 the other could be raised and lowered by a long handle connecting with 

 its free edge. With this simple apparatus it was possible to propel on 

 the beach a series of parallel, straight waves, varying in size and period- 

 icity as the experimenter desired. It was found that beach cusps re- 

 sembling closely those in nature could be artificially produced (plate 42, 

 figure 2). The characteristics of those artificial cusps have been dis- 

 cussed above. 



Theories of Origin 



With the characteristics and variations of beach cusps in mind, we 

 may examine the theories which have been proposed to account for their 

 origin. 



SHALBR'S THEORY 



According to Professor Shaler, "the inner edge of the swash line . . . 

 has a very indented front, due to the fact that it is shaped by a criss- 

 cross action of many different waves." The projecting tongues of water 

 push back the pebbles, leaving indentations or bays, which are then en- 

 larged under the continued wave attack during the rising tide. It should 

 be noted, however, that the indentations of the inner edge of the swash 

 line on a smooth beach are extremely irregular, and vary in position with 

 every wave advance until the development of cusps and intercusp depres- 

 sions affords more definite guidance. That a single advance of the irreg- 

 ular inner edge of the "swash" could develop bays which would there- 

 after control the action of the waves seems doubtful. The inner edge 



