mouth of Mobile Bay has tremendous effects upon the salinity, sedimentation, 

 erosion, and water currents within the bay. The exact processes by which 

 these barrier islands and spits are formed are poorly known. 



Whereas some Texas barrier islands rest on Pleistocene deposits, the sand 

 bases of Dauphin Island and the Fort Morgan Peninsula are separated from the 

 Pleistocene sediments by several feet of Holocene mud. Just to the west, in 

 Mississippi Sound, the base of the sand between Cat, Ship, and Horn Islands 

 (Biloxi quadrangle) was found by acoustic reflection to be 9 to 12 m (30 to 

 39 ft) below sea level (Curray and Moore 1963). May and McLain (1970) found 

 barrier sand overlying mud at 9 to 11 m (30 to 36 ft) below sea level in the 

 Bon Secour-Fort Morgan Peninsula area, using both physical probes and 

 acoustic reflection. Kwon (1969) and Otvos (1973) have shown that part of 

 Mobile Point (Biloxi quadrangle) is a relict Pleistocene ridge, and recent 

 work by Otvos has shown that the same is true for at least the eastern end of 

 Dauphin Island (May 1976). 



Curray and Moore (1963) indicated that the barriers started forming near 

 their present position when the postglacial rise of sea levels slowed and was 

 outpaced by the upbuilding of sediments. Most gulf coast barrier islands 

 probably formed about 5,000 to 3,500 years ago when the rise in sea levels 

 slowed or stopped (Otvos 1970a, 1970b). Otvos also discussed the development 

 and migration of barriers formed by aggradation of submerged shoals. 



Although eroding to some extent, the eastern end of Dauphin Island has 

 not migrated westward as has the western end, possibly due to its Pleistocene 

 ridge base. The island is currently 24.35 km (15.13 mi) long, but sediment 

 is accreting along the western end of the island, increasing its length about 

 6.4 km (4 mi) in the last 100 years (May 1971). Dauphin Island has a dune 

 system averaging 3 to 6 m (10 to 20 ft) in altitude, reaching a maximum of 

 12.2 m (40 ft) at the eastern end of the island (Boone 1973). The dunes are 

 fronted on the gulf side by broad wel 1 -developed beaches and on the bay side 

 by beach interspersed with marsh. 



The Fort Morgan Peninsula forms the southern boundary of Mobile Bay and, 

 like Dauphin Island, has broad, well-developed beaches and a dune system 

 reaching a height of 6 m (20 ft) along the western end. The eastern end, 

 connected to the mainland, contains several large lagoons and marshes. 

 0'Neil and Mettee (1982) state: "Several sets of intersecting dune ridges 

 indicate a complex depositional history for this spit." 



The only other component of the barrier island-spit system in Mobile and 

 Baldwin Counties is Perdido Key (Pensacola quadrangle), only a small portion 

 of which is located in Alabama. The central body of the key contains 6-m 

 (20-ft) high dunes which decrease in frequency and height toward the ends of 

 the key (U.S. Army Corps of Engineers 1971). 



Ono Island (Pensacola quadrangle) north of Perdido Key can not be consi- 

 dered a true barrier island since it does not receive the direct wave action 

 of the Gulf of Mexico. The island exhibits a series of parallel stable dune 

 ridges with wet swales in the topographic depression between each pair of 



54 



