.4 Contribution to the Geologic History of the Floridian Plateau. 157 



The series of Apalachicolan events was terminated by a general 

 ■elevation of the Plateau. 



ABSENCE OF APALACHICOLA SEDIMENTS WEST OF THE VICKSBURG NUCLEUS. 



Before taking up the discussion of the subsequent stages in the 

 histor5' of the Plateau, the very striking peculiarity of the present sur- 

 face distribution of the geologic formations will again be noticed. The 

 Apalachicola Group is not now exposed above sea-level on the seaward 

 side of the Vicksburg exposures in Lafayette, Levy, Citrus, and Her- 

 nando counties; nor does any later geologic formation except a coastal 

 fringe of Pleistocene occur above sea-level in that area. As it dues 

 not seem at all probable that no Apalachicola sediments were laid down 

 in this area, the explanation may be found in erosion during the Apalach- 

 icolan-Miocene uplift, or in a subsequent depression which submerged 

 the Vicksburg-Apalachicola boundary. The growth of the Peninsular 

 land-surface toward the east, southeast, and south, while there has been 

 no addition of importance on the west since Oligocene time, will be con- 

 sidered on later pages. 



APALACHICOLAN-MIOCENE INTERVAL. 



The uplift closing Apalachicolan deposition carried areas of the 

 former sea-bottom above the sea -level, and was followed by the sub- 

 aerial erosion of the Apalachicola sediments. The evidence of the ero- 

 sion of the Apalachicola previous to the deposition of the Miocene is 

 seen at Alum Bluff on the Apalachicola River and at Jackson Bluff on 

 the Ocklockonee River. At both localities the upper surface of the Alum 

 Bluff formation shows distinct erosion furrows and channels, with the 

 Miocene (Choctawhatchee formation) filling and overlying the irregu- 

 larities.' It is difficult to find a gage of the amount of this elevation, 

 but it is evident that extensive areas of the Apalachicola sediments 

 became dry land and the subsequent Miocene depression did not again 

 carry all of them below the ocean level. 



On the east coast, in the vicinity of Jacksonville and St, Augustine, 

 it appears from well-borings that sediments of Apalachicolan age are 

 either very thin or even absent,^ the Miocene apparently resting on the 

 eroded surface of the Vicksburg. It is probable that the rocks of the 

 Apalachicola Group were entirely or almost entirely eroded away over 

 this area during the erosion interval immediately previous to the 

 Miocene depression. 



EVENTS OF MIOCENE TIME. 

 DISTRIBUTION OF MIOCENE SEDIMENTS. 



The Miocene was another period of subsidence and the sea was again 

 admitted over a considerable area of the Apalachicola sediments which 

 had been subjected to subaerial denudation, but not all of the previous 

 land area of those sediments returned to marine conditions. The present 



' Vaughan, in Matson and Clapp's report, Florida Geol. Surv., 2d Ann. Report, 

 pp. 114, 115. 



^ Matson and Clapp, Florida Geol. Surv., 2d Ann. Report, p. 108, lyio. 



