GULF OF MEXICO 



51 



qiiesas is a great lunate key partly closed at the 

 southwest by a series of smaller lunate keys curved 

 oppositely to the major key and built by sec- 

 ondary currents from the west-southwest. The 

 living barrier reef of southern Florida in front of 

 the main Florida Keys stands in about 5 to 7 

 fathoms of water. The Colorados Barrier Reef of 

 western Cuba stands in about 5 to 6 fathoms. 

 The barrier range off northern Yucatdn, however, 

 stands in 20 to 30 fathoms, nearer to the edge of 

 the shelf than to the mainland. 



The Florida Keys are partly coralline, partly 

 of other origin (Cooke 1945, pi. 1, and 1939, pp. 

 68-72). The main eastern Key range is con- 

 sidered to be a former barrier coral reef of the 

 elevated Pleistocene Pamlico (25-foot) shoreline, 

 now emerged and dead. Its highest present 

 natural ground elevations are said to be about 18 

 feet above present mean sea level. This Key 

 range ends to the southwest in the Boot, Mara- 

 thon, and Vaca group of Keys. Westward along 

 the line of the Keys, there is a large emergence of 

 the Miami oolite limestone stratum to the present 

 intertidal zone, somewhat built up, in places, by 

 mangrove peat and marl. Marine carbonate and 

 paralic deposits combine to form the Pine Island 

 group of Keys. This low island mass has been 

 broadly and abundantly channeled in a northwest- 

 southeast direction by the strong tidal currents 

 produced by the regularly recurring tidal differ- 

 ence of 2 to 3 feet between the Gulf and Florida 

 Straits. Key West is the western terminus of 

 this group of channeled-shoal Keys. 



West of Key West and the Pine Islands lie the 

 small Sand Keys (Davis 1942) where the main 

 Miami oolite shoal lies below or mainly below low 

 tide. These Sand Keys only sparingly fill the gap 

 between the Pine Island Keys and the large Mar- 

 quesas atoll. 



Scattered coral patches. — The scattered patches 

 of coral growth mapped by various agencies and 

 persons along the northern coast of the Gulf 

 (fig. 14, Sector 4.3), far out on the shelf are not \vell 

 known. These notations may refer to growths 

 on the tops of small salt-dome-like seamounts 

 found along the edge of the shelf here. Studies 

 by H. C. Stetson show that nodular algal lime- 

 stone balls are common on the tops of some of these 

 small seamounts. Specimens of solitary corals, 

 possibly from the sea areas, are found sparingly 

 upon the beaches. Coral patches occur widely 



259534 0—54 5 



as bottom growths off the central peninsula coast 

 of Florida. 



Paralic, or marine marsh and swamp subdivisional 

 eninronment. — In the biogenous environment, as 

 here defined, grassy to reedy marsh is dominant 

 between the convex areas of drowned karst shore- 

 line (fig. 14, Sector 2.1). It is also scattered among 

 the mangrove swamps. The mangrove swamp 

 forests (Davis 1940, 1942) form a conspicuous 

 marginal coastal belt on the inshore sectors noted 

 (4.1, fig. 14), and occur prominently in thelagoonal 

 habitat on 4.1 and 4.2 Sectors. 



Fresh-water marsh (paludal environment) has 

 some of its most extensive known developments 

 on the broad, very gently sloping coastal plain 

 of southern Florida inland from the marine man- 

 grove shoreline. The paludal areas include the 

 famous Everglades and the almost as well known 

 Big Cypress Swamp. 



Marine marshes (paralic) are conspicuous in 

 places in a relatively narrow zone along the coast 

 of Louisiana in the deltaic alluvial environment. 

 Here salt grasses {Spartina) and reeds have 

 pioneered on deltaic and other shoals. Garden 

 Island Bay, between two mouths of the Mis- 

 sissippi's active bird-foot delta, is reported (Russell 

 1936) to have extended its shoreline materially 

 by the aid of paralic vegetation. Here, again, 

 extensive fresh-water marshes lie inland in a 

 very gently sloping coast from the more notable 

 saline marshes. On the steeper deltaic coast of 

 the western Gulf, shore and coastal marsh are 

 narrow and relatively inconspicuous. 



Mangrove swamp growth. — Charts of the near- 

 tropical coast of Florida (4.1, fig. 14) south of 

 Cape Romano (1113, 1253, 1254) and north of 

 the Bay of Florida, and air photographs of a part 

 of the west coast of the Yucatdn Peninsula (4.1), 

 show a broad, belted disposition of saline man- 

 grove swamp forest with an irregular brackish 

 lagoon or line of lagoons landward from it. This 

 arrangement seems to be unique for North America 

 and for those parts of the Antilles which have been 

 studied by the writer. It depends upon the 

 presence of a broad, shoal continental shelf in a 

 tropical or near-tropical sea. Lesser mangrove 

 growths on lagoonal shores seem to be incomplete 

 approaches to this disposition of the swamp. 



Mangrove swamp forests extend along the 

 coasts of the biogenous sectors (4, fig. 14) with an 

 extension on the drowned karst (2.1), and on the 



