the inner shelf. The local concentrations of the 

 above materials are, therefore, those which one 

 would expect to find in the vicinity of the rocky 

 sponging bars. 



The zone seaward of the inshore or quartz 

 zone is a quartz-shell zone in which the shell 

 material replaces the quartz in making up 

 more than 50 percent of the sediment. This 

 zone also averages about 20 miles in width but 

 may be only a mile or so wide in parts of the 

 sponging ground further northward. Local con- 

 centrations of sediments around the rocky out- 

 croppings are similar to those found in the same 

 areas shoreward. The other constituents of the 

 sediments are similar to those found inshore, 

 with the shell and other sediments of marine 

 origin becoming more predominant seaward. 

 No quartz sand is found seaward of this zone. 



Further seaward from the quartz-shell zone 

 the other zones in order are: broken shell , 

 algal sand, oolite sand , and foraminiferal sand 

 and silt . Within the limits of the previously 

 worked sponging grounds (less than 130 or 140 

 feet in depth) parts of the broken shell and 

 algal sand zones may be found (fig. 12). 



The rocky portion of the sea bottom is com- 

 posed of lithified sediments of cemented lime. 

 These consolidated sediments have been identi- 

 fied as soft marly limestone, sandy limestone, 

 and dense, fine-grainedlimestone. Much of this 

 material is similar to that found on land. Ap- 

 parently all the limestones were deposited 

 during the times of the changing sea level in 

 the Pleistocene. Much of the rock outcropping 

 on the sea bottom has been eroded and worn 

 down by physical and biological means. The 

 edges of the outcroppings are usually rounded, 

 rarely rough or jagged, except where there is 

 a distinct edge exposure of a bed of rock. The 

 divers report that along much of the northern 

 coast between the 60- and 70-foot marks, the 

 outcroppings of rock are very rough and "high" 

 (up to 12 feet), supporting little sponge growth. 



The unconsolidated sediments form a rela- 

 tively thin layer over the surface of the rock. 

 The slope of the bottom is so gradual that the 

 waves have little if any effect in moving the 

 sand in any one direction. There is sufficient 

 evidence to show that occasionally during 



storms wave movements are sufficiently strong 

 to clean off the rocky areas and wash away the 

 sediments from the edges of sponging bars. 



The Sponging Bars 



Clean exposed rock is essential for the 

 attachment of wool and other species of com- 

 mercial sponges. The limestone rock bars 

 supporting sponge growth may have areas of a 

 few square feet to many thousands of square 

 yards. Much of the bar rock is at the same 

 level as the surrounding bottom or only a few 

 inches higher. In many cases also the rock 

 has been fractured in two directions roughly 

 at right angles to each other, so that the bars 

 are frequently composed of rectangular- 

 shaped or diamond-shaped pieces 4 feet by 6 

 feet in size. The appearance depends upon the 

 layer or bed of rock exposed. 



The spongers refer to several different kinds 

 of rock bar. Flat bar refers to flat rock usually 

 covered by a thin layer of sand, high rock to a 

 bar made up of smooth rectangularly shaped 

 pieces as described above, which are 2 or more 

 feet above the lowest level of the bar, and 

 rough bar to rock that is jagged. Any bar may 

 be a combination of these types. 



Along most of the west coast of Florida, rock 

 bars usually do not rise more than 1 or 2 feet 

 above the lowest level of the bar. This lowest 

 level does not necessarily correspond with the 

 level of the surrounding unconsolidated bottom. 

 The highest rocky reef found along the west 

 Florida coast lies between the 40- and 65-foot 

 depths and extends in a northwest direction 

 from Anclote Key to a point southwest of Cedar 

 Keys, an area approximately 8 miles wide and 

 50 miles long. On parts of this reef the rock is 

 as much as 12 feet in height, rough, and pro- 

 vides excellent fishing and sponging. 



In the shallower water area down to depths 

 of 60 feet the sponging bars are normally sur- 

 rounded by a growth of eelgrass or marine 

 flora. Bars are usually bordered by a strip of 

 open sand, 20 or more feet in width, which is 

 free from almost all plant growth. The rocky 

 edges of the bars do not support a heavy growth 

 of marine animals or plants, and there is con- 

 siderable evidence to show that these edges 



31 



