plains of late Holocene oriqin (Fernald 

 1Q81). The linear, qentlv curving beach 

 ridges of the area attest to the changes 

 in orientation of the estuary through 

 geological time in resoonse to wide 

 fluctuations of sea level. The 

 Apalachicola estuary is part of a broad, 

 sandv shore plain, which is constantly 

 being changed by a combination of 

 climatological elements such as wind, 

 rainfall and sea level alterations. The 

 present structure of the bay is around 

 10,000 years old (Tanner 1^83). Sea level 

 reached its modern position about SOOO 

 years ago when the construction of the 

 present barrier island chain was underway. 

 Exceot for the southward migration of the 

 delta front, the general outline of the 

 bay system was established at this time 

 (Tanner 1^83). 



?.!.?. Oeomorphology and Regional Geology 



a. Upland areas . The maior 

 formations in the upper Chattahoochee 

 River system are underlain by igneous 

 rocks and crystalline schists. The area 

 is characterized by Tertiarv limestone 

 outcroppinqs, which add to the habitat 

 diversity of the region (Figure 6). The 

 lower division of Piedmont upland, defined 

 as the Opelika Plateau, is underlain by 

 Archean (i.e., Precambrian) rocks. 

 Tributaries of the Ohattahoochee River 

 have subsequently eroded these formations 

 with some valleys cut approximately 6? m 

 (?00 ft) below the general surface. The 

 rocks of the Appalachian province pass 

 under the Coastal Plain formations. Along 

 the border between the Appalachian 

 province and the Coastal Plain, 

 Appalachian rocks are overlain by 

 Cretaceous formations, 

 more deeply buried 

 Quaternary sediments further north 

 Coastal Plain is covered with a 



These rocks 

 by Tertiary 



are 



and 



The 



thick 



layer of clastic (erosion produced) 

 sediments as well as limestone 

 (nonclastic) sediments, some of which mav 

 be crystalline. 



Adams et al. (1'526) have presented a 

 detailed account of the Paleozoic, 

 Mesozoic, and Cenozoic formations in 

 Alabama, which is generally applicable to 

 the Apalachicola valley. The Cenozoic 

 formations are confined to the Coastal 

 Plain and represent deposits at the bottom 



of an ancient sea, which consist of sand, 

 clay, mud, or calcareous ooze. Fossil 

 marine mollusks and echinoderms are 

 interspersed with remnants of fossil 

 plants from flood plains, marshes, and 

 swamps. Pleistocene marine sands and 

 clays overlie older formations along the 

 coast, and estuarine and fluvial deposits 

 extend up the main river valley. Swamps 

 immediately upland of the Apalachicola 

 estuary are underlain by quartz sand 

 (Brenneman and Tanner 1958). 



Figure 6. Geological features of the 

 Apalachicola drainage system showing (A) a 

 line north and west of which there are 

 thin patches of Tertiary limestone near 

 the land surface and (B) a line beyond 

 which the limestone thickens and is more 

 deeply buried. The top of the Tertiary 

 limestone is shown in feet below sea 

 level, while Tertiary limestone that 

 occurs in or near the land surface is also 

 outlined (modified from Means 1977). 



