600 D. W. JOHNSON BEACH CUSPS 



the characteristic features of the cusps; to examine the theories which 

 have been proposed to explain their origin, and to offer a theory which 

 differs in some essential particulars from those already advanced. 



Beach cusps are of common occurrence along most shores, but are of 

 no great significance to the geologist or geographer. The writer's apol- 

 ogy for what may seem an over-elaborate treatment of an unimportant 

 topic is that he found the detailed study of the cusps an interesting 

 occupation during vacation hours, and gained from the study some 

 points of more than passing interest. 



Literature 



In an unpublished thesis, "The Geology of Nahant," written by Prof. 

 A. C. Lane about 1887, the cusps on Lynn Beach, Massachusetts, are 

 briefly described and their origin discussed. Lane concluded that cusps 

 are formed by the action of waves parallel to the coast; that they have 

 their beginnings in accidental irregularities on the beach; that they be- 

 come evenly spaced as the result of some process of adjustment not 

 clearly understood, and that the distance between cusps is in some man- 

 ner related to the height of the waves and the breadth of the beach. 

 This brief unpublished account of cusps was not brought to the writer's 

 attention until after the following pages were written. As will appear 

 later, his conclusions agree with those reached by Lane in important par- 

 ticulars. A short abstract of this thesis was published in 1888. 



Prof. N. S. Shaler, in his popular treatise, "Sea and Land'' (1894), 

 gives a clear description of the curious "ridges and furrows" occurring on 

 shores, recognizes their temporary character and the ease with which 

 they are obliterated by wave action, and expresses the opinion that "the 

 origin of these peculiar structures is not 'easily accounted for" (57). A 

 year later Professor Shaler published a somewhat fuller account of beach 

 cusps in his paper, "Beaches and Tidal Marshes of the Atlantic Coast," 

 from which the quotation at the beginning of this article is taken. A 

 theory of origin was proposed in the following words : 



"It seems to the writer that these scallops were formed about as follows : In 

 a time of storm the inner edge of the swash line formed by the body of water 

 which sweeps up and down the beach has a very indented front, due to the 

 fact that it is shaped by a criss-cross action of many waves. As these tongues 

 run up the beach and strike the pebbles, they push them back so as to make a 

 slight indentation where each tongue strikes. As the water goes back, it pulls 

 out the fine material, but does not withdraw the pebbles. The next stroke of 

 the splashing water then finds a small bay, the converging horns of which 

 slightly heap up the fluid, making the stroke a little harder in the center of 



