ecological characterization atlas of coastal alabama - Data Center

60 m (100 to 200 ft), and some of the fault movement has occurred recently
enough for the grabens to appear as topographic lows .
The Mobile graben is a complex north-south fault system running from Jackson,
Alabama, south to Mobile Bay . The northernmost fault on the east flank
of the graben system i s the Jac kson faul t . The confl uence of the Al abama and
Tombigbee Rivers and the formation of Mobile Bay is possibly the result of
movement along faults within this system (Chermock 1974) .
Many features of the graben are poorly known .
"The major fault representing the west flank of the graben opposite the
Jackson fault has never been penetrated . Geologists have postulated its
location on the basis of subsurface data but other interpretations are
possible for the data" (Chermock 1974) .
Both the east and west flanks of the graben can be mapped further to the
south, although a relationship between the graben and Mobile Bay is not
obvious . According to well-control data, the graben turns westward north of
the bay and extends to northern Mobile County . To quote Chermock (1974)
again,
"The Mobile graben system has been a highly mobile fault zone since at
least late Mesozoic time, and many structures favorable for petroleum
accumulation have been formed ."
ACTIVE DUNES
For the purposes of this atlas, active dunes are defined as those sand
landforms influenced by wave action and/or eolian (wind) processes that are
in a constant state of change . Typically these dunes support little or no
stabilizing vegetation .
On the 1 :100,000 study maps, areas where active dunes occur have been
appropriately marked . The location of individual dunes that can be considered
currently active is beyond the scale of the atlas maps, although as a
generality the actual number of dunes that are active is small . Those that
are most active are scattered along the gulf side of the western end of
Mobile Point-Fort Morgan Peninsula (Pensacola quadrangle) . A detailed assessment
of dunes in both Mobile and Baldwin Counties was undertaken by the U .S .
Army (U .S . Army Corps of Engineers 1981) as part of the report of damage of
Hurricane Frederic in 1979 . While the report does not deal specifically with
active dunes, it does contain a good description of the existing dunes as
they appeared after the hurricane .
In Bal dwi n County for a di stance of about 6 mi al ong the beach west from
Perdido Bay (Pensacola quadrangle), the dunes were eroded from approximately
3 m (10 ft) in elevation vertically down to 1 .5 m (5 ft) . From Little Lagoon
(Pensacola quadrangle) west for about 10 km (6 mi) the dunes that had
averaged 4 .5 to 6 m(15 to 20 ft) in height were eroded approximately 3 m (10
ft) vertically . For the next 8 km (5 mi) west, to Fort Morgan at Mobile
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