ecological characterization atlas of coastal alabama - Data Center

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 overlyi ng mud at 9 to 11 m (30 to 36 ft) bel ow 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
sl owed or sto pped (Otvos 1970a, 1970b) . Otvos al so di sc ussed the devel opment
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
i s accreti ng al ong the western end of the i sl and, i ncreasi ng i ts 1 ength 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 well-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 .
O'Neil and Mettee (1982) state : "Several sets of intersecting dune ridges
indicate a complex depositional history for this spit ."
The only other com ponent 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 considered
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
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