Ch. 1] BEACHES 23 



few feet per mile and is characterized by relatively coarse, well-sorted 

 deposits, mostly of sand or coarse silt size. As the water at the outer 

 edge of the shelf is 300 to 600 feet deep, the floor of the shelf is within 

 the reach of wave action during times of heavy storms. Undulations 

 in the continental shelf influence materially the texture of the deposits. 

 The fine material tends to concentrate in depressions on the shelf. 

 Ridges or masses of sediment that rise above the general level of the 

 platform are found on the continental shelf in several places, especially 

 the Gulf of Mexico, the Atlantic Coast, and the North Sea. Some of 

 these ridges may represent former shore lines when the sea stood at a 

 lower level than it now does, but they also may represent submerged 

 masses of sand which migrate across the shelf just as dunes march 

 over the land (Liiders, in Trask, 1939, p. 337). 



Beaches 



Beach deposits are of many different types, depending primarily on 

 the configuration of the land and the strength with which waves can 

 strike the beach. Where the ground slopes very gently both seaward 

 and landward from shore, as in the Gulf of Mexico along the Texas 

 coast, the beach deposits are very fine-grained. Not only is the size of 

 particle supplied the sea of relatively small size, but also the gentle 

 slope of the sea bottom interferes with the progress of large waves 

 toward the beach. However, whether the flat slope is the result or the 

 cause of the distribution of sediments is a question. Geologic text- 

 books speak of a profile of equilibrium, but they do not correlate the 

 angle of slope of the profile with the distribution of energy in the 

 waves, or with the size and amount of material available for transport 

 by the waves. It seems as if the profile depends very much upon the 

 supply of debris, which in turn is influenced by the distribution of 

 currents and the overall configuration of the locus of deposition. Cer- 

 tainly the angle of slope of beaches is related to the size of the con- 

 stituent particles. Steep slopes tend to have coarse particles, and 

 gentle slopes fine particles; so why does not the same relation apply 

 to the shallow water off shore? 



Beaches are influenced greatly by longshore currents, which in turn 

 are influenced by the direction of prevailing wind (Munk and Tray lor, 

 1947). Particles of sand are transported along the shore and form 

 spits and bars (Gilbert, 1890). The waves and currents tend to cause 

 the edge of the beach to migrate shoreward, particularly during time 

 of high water and strong waves. Whether or not a beach erodes or 

 builds seaward depends on whether sand is supplied the beach at a 



