Pre-existing topography strongly influenced Sangamonian transgression in the Apalachicola area. Late Pleistocene neritic and estuarine Biloxi and Gulfport barrier-complex deposits indicate that interglacial sea level rose from at least -37 m, relative to present sea level, to above +3 m. The Biloxi Formation provides a Gulf of Mexico-wide stratigraphic marker. Subsequent regression first was accompanied by river-channel incision at a level slightly lower than interglacial- and much higher than full glacial-erosion levels. Large eolian dunes formed over Gulfport barrier surfaces, reflecting the regional extent of a Wisconsinan wind system.

Antecedent Pleistocene topography also greatly influenced Holocene sedimentation and associated landforms. The late Holocene history of St. Vincent and “Little St. George” Islands and St. Joseph barrier spit is characterized by strandplain progradation. Unlike St. George Island, the St. Joseph barrier spit did not form through integration of emerging island cores. Multiple erosional episodes characterize the Quaternary barriers. Conclusive field evidence is lacking for Late Holocene sea-level fluctuations. Massive dilution by Apalachicola River runoff and resuspension/homogenization of the bay deposits by occasional hurricanes tend to diminish lateral and vertical salinity gradients in the record of the late Holocene sedimentary cycle. Unlike certain central Gulf Coast areas, contrasts in vertical salinity between Late Holocene lagoonal and neritic deposits in the Apalachicola are weak to nonexistent.