Key, Johnson Keys). Over the past 

 50 to 70 years, development through 

 dredge and fill activities has 

 converted many of these mangrove 

 community areas into more habitable 

 environments for man, as evidenced 

 by changes in shoreline habitats 

 (Griswold 1965). The new shorelines 

 are generally characterized by 

 riprap/gravel beaches, mixed sand 

 and gravel beaches, and exposed and 

 sheltered rocky shores and seawalls 

 (Getter et al. 1981). Natural sand 

 beaches are remarkably few and lim- 

 ited in size. Inland, the mangrove 

 communities give way to tropical 

 hardwood hammocks (primarily on the 

 northern Keys and Big Pine Key), and 

 Caribbean slash pine stands ( Pinus 

 eliottii var. densa ) on islands sur- 

 rounding and including Big Pine Key 

 (Alexander and Dickson 1972, CZM 

 1974). 



As evidenced by the extent of 

 mangrove vegetation on the islands, 

 much of the Florida Keys land area 

 lies only .6 to 1 m (2 to 3 ft) 

 above high tide. At only two loca- 

 tions in the island chain (both in 

 the upper Keys) does the elevation 

 reach or exceed 5 m (18 ft) (Hoff- 

 meister and Multer 1968). Where 

 these maximum elevations occur, the 

 island's shape departs from the 

 typically flat character of the 

 majority of the Keys, exhibiting a 

 slightly raised, centrally-elongated 

 axis with the southeast and north- 

 west sides sloping gradually to the 

 Atlantic Ocean and Florida Bay, re- 

 spectively. Beyond the shorelines, 

 intertidal flats border both sides 

 of the islands. These are generally 

 shallow water areas, barely covered 

 at low tide, which gently slope into 

 the deeper water of the surrounding 

 platform. In some places the flats 

 are exposed at low tide and are cov- 

 ered by soft laminated algal crusts 

 (Multer 1977). Florida Bay lies 



beyond the flats to the northwest of 

 the upper Keys. 



Farther southwest of Florida 

 Bay, a great expanse of carbonate 

 sediments exist under a shallow sea 

 with a depth rarely exceeding 18 km 

 (60 ft). This platform bounds the 

 lower and middle Keys to the gulf 

 side and is built upon the south- 

 western submarine extension of the 

 Floridan Plateau (Brooks 1973). 

 Stockman et al. (1967) labeled this 

 current-swept region "The Sluiceway" 

 as it exhibits a scoured seascape 

 with only a thin veneer of recent 

 carbonate sediments a few centi- 

 meters thick. 



Seaward toward the Straits of 

 Florida and paralleling the Keys is 

 an arcuate band of living reefs, 

 linear shoals and depressions de- 

 scribed as the Florida Reef Tract by 

 Vaughn (1916). Living reefs that 

 locally reach to the low water mark 

 are concentrated on the seaward side 

 of this band to form a discontinuous 

 barrier (see Figure 8), e.g., Ameri- 

 can Shoal, Carysfort Reef, Alligator 

 Reef. These are also referred to as 

 the Outer reefs (Ginsburg 1956) or 

 Bank reefs (Shinn 1963). The area 

 between them and the Keys is 4.8 to 

 11 km (3 to 7 mi) wide, and is re- 

 ferred to as the back reef (Ginsburg 

 1956) or inner shelf (Enos 1977). 

 The back reef is characterized by 

 patch reefs (Ginsburg 1956) or reef 

 knolls (Turmel and Swanson 1976), 

 (e.g.. Hen and Chickens Keys, Mos- 

 quito Bank), linear shoals, (e.g.. 

 White Bank, Washer Woman Shoal) and 

 areas of deeper water, exceeding 

 5.5 m (18 ft) (e.g.. Hawk Channel) 

 ranging in depth from 5.5 to 9 m 

 (18 to 30 ft) off Key Largo to 6 to 

 15 m (20 to 50 ft) off Key West 

 (USDC 1962, USGS 1971, Enos 1977). 



The Florida Keys are divided 

 into at least three physiographic 

 zones distinguished by differences 

 in their shape, orientation and li- 

 thology. These are, as illustrated 

 in Figure 8, the upper Keys (Soldier 



23 



