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 Jack- 

 son, Alabama, south to Mobile Bay. The northernmost fault on the east flank 

 of the graben system is the Jackson fault. The confluence of the Alabama 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 19 74). 



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 consid- 

 ered 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 assess- 

 ment 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 Baldwin County for a distance of about 6 mi along 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 



100 



