Key southwest to New Found Harbor 

 Keys), the lower Keys (East Bahia 

 Honda Key to Key West), and the dis- 

 tal atolls, including the Boca Gran- 

 de Key Group, the Marquesas Keys, 

 and the Dry Tortugas (White 1970). 



The northernmost zone, the up- 

 per Keys, is characterized by long 

 narrow islands, elongated in a 

 northeast to southwest direction 

 that roughly parallels the reef 

 tract described earlier. Based on 

 the slope and orientation to the 

 reef tract, the origin and composi- 

 tion of the upper Keys is easily 

 understood. Named and described by 

 Sanford (1909), the Key Largo Lime- 

 stone is a typical organic reef 

 composed of wave resistant elements, 

 the most important of which are 

 hermatypic corals. These form the 

 framework of the structure and are 

 responsible for trapping large 

 amounts of calcarenite in which they 

 are now embedded (Krawiec 1977). 



The lower Keys form a roughly 

 triangular group of islands which 

 generally elongate at right angles 

 to the northeast-southwesterly ori- 

 entation of the upper Keys. The 

 exposed rock formation here is an 

 extension of the Miami Limestone 

 Formation, oolitic facies, upon 

 which Miami and other southeastern 

 Florida cities have been built 

 (Hoffmeister and Multer 1968). The 

 northwest-southeast elongation of 

 the lower Keys is caused primarily 

 by the direction of movement of the 

 tidal scour produced by differences 

 in time and height of the tides in 

 the Gulf of Mexico and the Straits 

 of Florida. 



The distal atolls form the 

 third physiographic region of the 

 Florida Keys (Puri and Vernon 1964, 

 White 1970). This designation is 

 based, it appears, on little more 

 than the isolated nature of the 

 islands west of Key West, and their 

 general shape. Davis (1942) re- 



ferred to these "atolls" and the 

 scattered islands to the west of Key 

 West as the Sand Keys after an 

 earlier description by Millspaugh 

 (1907). The thirty islands of the 

 Sand Keys fall into three primary 

 groups: two atoll-like groups, the 

 Marquesas and Dry Tortugas Keys , and 

 a loose cluster of small islands 

 just west of Key West, referred to 

 by Millspaugh (1907) as the Boca 

 Grande group. All of these islands 

 fall within an area extending 112 km 

 (70 mi) east and west, and 14.4 km 

 (9 mi) north and south. The Boca 

 Grande group of 14 islands extends 

 17.6 km (11 mi) west of Key West; 

 the elliptical shaped Marquesas Keys 

 lie 27.2 to 32 km (17 to 20 mi) west 

 of Key West; and, the Dry Tortugas 

 are clustered between 104 to 112 km 

 (65 to 70 mi) west of Key West. 

 Davis (1942) concluded that the 

 coarse calcareous sand found in the 

 three island groups are an accumula- 

 ted matrix of unconsolidated detri- 

 tal material of various origins in- 

 cluding calcareous algae, mollusks, 

 foraminifera, echinoids, nullipores 

 (coralline algae - Shrock and Twen- 

 hofel 1953) and coral reef rubble 

 (Dry Tortugas only). The two more 

 easterly groups, the Marquesas Keys 

 and the Boca Grande group, are built 

 up from oolitic limestone banks of 

 the Miami formation and are domina- 

 ted vegetatively by mangrove and 

 beach dune strand communities (Davis 

 1942). Their nearshore marine envi- 

 ronment is distinguished by flat 

 bare and grass covered calcareous 

 sand bottoms, with a notable absence 

 of coral reefs. Seaward of the Boca 

 Grande group's southernmost islands, 

 the Florida Keys reef tract termi- 

 nates. The Dry Tortugas exhibit 

 primarily beach-dune strand communi- 

 ties with the mangroves representing 

 a very small man-introduced percent- 

 age of the land cover. Rockbottoms, 

 shoals, and reefs characterize the 

 nearshore and lagoonal waters 

 (Multer 1977). 



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