deeper water offshore, the character of the sediment indicates that off- 

 shore movement does not occur in Section A. Loss of beach material in 

 the Miami area could 'then only be due to nearshore longshore movement, 

 or solution of shell material. Rusnak (1966) concluded that loss by 

 abrasion is insignificant. Because of the pH and composition of the 

 sea water, loss by solution should be insignificant also. It is judged, 

 therefore, that the observed loss of beach material (less that lost to 

 inlets) is due to movement parallel to shore out of the area, rather 

 than movement directly offshore. 



The gray sand body covering most of the shelf in Section B is prob- 

 ably detrital in origin, but the source of this material is not clear. 

 The black or dark gray coloration of most of the shell material in the 

 sand provides the sand with a distinctive gray color. Such coloration of 

 the shell material has generally been taken to indicate previous burial in 

 a marsh or swamp environment; however, recent studies show that blackened 

 skeletal fragments may form in a subbottom marine environment (Maiklem, 

 1967). Thus, the present color does not necessarily indicate a relict 

 deposit or material eroded from a lagoon bottom or back beach source. 

 In fact, on the Georgia nearshore Continental Shelf, recent sediment is 

 colored gray and contains 25 percent or less of carbonate (Gorsline, 1963), 

 Pilkey and Frankenberg, 1964), not too unlike the sediment in Section B. 



It is unlikely the gray sand comes from offshore for reasons ex- 

 plained above, and a source area. in the high carbonate environment to the 

 south is improbable in view of the high quartz content and the shape and 

 compositional difference of much of the skeletal material here. Also, 

 sediment northward in the Fort Pierce area is dissimilar to sediment in 

 Section B, which would preclude a northward source. One other difficulty 

 with the explanation for a northern source is that within the zone where 

 cores were taken there is a general decrease in average grain size north- 

 ward; the trend, therefore, would be contrary to the premise of a de- 

 crease in grain size downdrift from a source. A possible explanation 

 •may be that the southward moving inshore sediments are washed seaward to 

 deeper water in Section B and then drift back northward because of nor- 

 therly offshore currents. Research presently underway by John Milliman 

 (personal communication) indicates a narrow zone of sediment with rela- 

 tively high quartz content (25-50 percent) extending offshore between Fort 

 Pierce and Jupiter Inlet to approximately the 100-fathom line thence ex- 

 tending north for 150 miles and south for 90 miles. Current measurements 

 near Miami show a northward drift over the shelf of about 0.5 feet per 

 second (House Document 169, 75th Congress, 1937). General velocity of 

 this drift is not competent to move particles greater than silt size 

 (0.062 mm; 4<^) although periodic higher velocity northward currents 

 capable of moving material in the size ranges characteristic of the gray 

 sands of Section B might occur. 



2. Sand Requirements 



At the date of writing of the report. Beach Erosion Control and 

 Hurricane Protection Studies conducted by the Corps on the Florida east 



34 



