GULF OF MEXICO 



49 



and sandy beaches (Martens 1931) except short, 

 elevated stormbeach ridges and sandy beaches on 

 some of the Cedar Keys archipelago at 29° 10' N. 

 Lat. These latter beaches (Martens 1931) are 

 somewhat muddy and unlike those of glaringly 

 white sand on the front of the Apalachicola delta, 

 Sector 1.2, and westward from it in Florida. 



With this drowned karst coast of Sector 2.1, 

 there are areas of transversely channeled marsh 2 

 to 3 miles wide occupied by grassy vegetation and 

 forested swamp. This swamp is probably mostly 

 saline. Patches of mangrove swamp occur in the 

 southern part of this Sector. 



The scattered mangrove swamps with offshore 

 oyster reefs to be described mark a minor exten- 

 sion of the biogenous environment (Sector 4, fig. 

 14). 



The drowned karst coast is conspicuous for its 

 many and unique marine oyster reefs, located 

 along a shallow-water zone extending outward to 

 a distance of a mile or two from shore. Crassos- 

 trea virginica, the North American oyster of 

 commerce, is notably lagoonal and estuarine, 

 commonly being confined to brackish water en- 

 vironments by its marine-water foes. Only along 

 parts of the Gulf coast are living reefs of this 

 species known in oceanic waters in North America. 

 On Sector 2.1, the highly fractured and channeled 

 limestones of Florida are filled inland with fresh 

 water to a considerable height above sea level. 

 The slope of the groundwater surface (piezomet- 

 ric) toward the coast indicates a movement of 

 underground water in that direction. Also, along 

 much of the coast of Sector 2.1 there is an artesian 

 groundwater head of about 10 feet near and at 

 the shoreline (Cooper and Stringfield 1950, fig. 

 14). This pressure-head forms springs in the 

 stream mouths and stream beds, as well as 

 offshore. '^ The absence of land-derived sedi- 

 ment in these streams during most of the year 

 and the protected nature of the shelf waters leave 

 the water of the Gulf brackish here. Off the 

 mouth of Atchafalaya Baj', Louisiana, oj'ster 

 reefs also grow in the Gulf out to a distance of 

 3 to 5 miles, with the fresh water of the river 

 mixing with Gulf water to produce a brackish 

 environment. 



Beach-bordered karst subsector. — Sector 2.2 (fig. 

 14) is represented both on the central coast of 



" Data on charts and reports of aviators via V. T. Stringfield, letter of 1862. 



peninsular Florida and on the coast of the Yucatiin 

 Peninsula. On Florida, the sector has fairly 

 continuous sandy barrier islands and barrier spits 

 with some mainland beaches. This sector ex- 

 tends from Anclotc Keys near Tampa at the north 

 to Cape Romano at the south. The drowTied 

 karst lies behind the beaches and the coastal 

 lagoons of the sandy barriers. The lagoons are 

 bordered by mangrove swamp and with the karst 

 depressions more or less filled with sediment and 

 marshy growths. 



The beaches of this sector (Martens 1931) 

 have much shell material but also quartz sand. 

 The quartz is derived from elevated sandy 

 Pleistocene beach deposits of the elongated 

 dome-shaped summit (300 feet or more) of the 

 peninsula, which lies immediately inland, and from 

 a sandy limestone formation that has been almost 

 removed by embayment of several streams to 

 form the broadly embayed harbors of Tampa 

 Bay and Charlotte Harbor. These harbors are 

 the only embayed, drowned, stream valleys of 

 the Gulf coast of the peninsula, except the mod- 

 erately widened tidal portion of Caloosahatchee 

 River, nearby. The shelf-bottom slopes more 

 steeply off this sector (2.2 feet per mile, fig. 15, 

 curve 4) than it does farther north on Sector 2.1. 



Cape Sable (fig. 12) protrudes into the Gulf 

 where Florida Bay extends eastward at the end 

 of the mainland of the peninsula. This major 

 shoreline bend produces a convergence zone for 

 waves, swell and currents with the local wave 

 attack necessary to develop a beach, keeping the 

 shore free of mangroves. The beach plain has 

 cuspate points and encloses narrow lagoons 

 behind it. The beach sand is presumably mainly 

 shelly. 



The oval area of plain behind the sandy beaches 

 and the lagoons of the Cape is somewhat marshy. 

 The origin of the broad, irregular lagoon known as 

 Whitewater Bay, lying several miles inland from 

 the beach is linked with the deliverj'^ of a concen- 

 tration of drainage to a marsh. The bay is 

 heavily fringed with mangrove swamp. 



The beach-bordered subsectors (2.2) on the 

 Yucatdn Peninsula include the northern coast 

 and the short Campeche-Champoton sector at 

 the west. The northern coast has barrier islands 

 and a number of slightly disconnected barrier 

 spits which extend westward from moderate pro- 

 jections of the shoreline. Pinnacles of limestone 



