182 Papers from the Marine Biological Laboratory at Tortugas. 



and organic origin accumulated on it ranging in thickness from ioo to 

 200 feet near shore to the north to over 1,000 feet near its southern 

 margin. As the water was shallow, the sea-bottom must have been 

 gradually depressed while the material accumulating on its surface was 

 being deposited. 



At the close of Vicksburgian time the Plateau was elevated and 

 areas of its surface were subjected to subaerial denudation, as is attested 

 by the erosion uncomformity along the contact of the basal Apalachicola 

 with the underlying Vicksburg sediments. 



Apalachicolan time needs separation into two stages, an earlier, 

 represented by the Chattahoochee, Hawthorne, and Tampa formations, 

 and a later, represented by the Alum Bluff formation. The areal extent 

 of the deposits of the earlier stage was not so great as that of the Vicks- 

 burg deposits, indicating the later was not so extensive as the previous 

 submergence. The northern shore-line was'seaward of that of the Vicks- 

 burg Group ; it seems probable that a small island existed in the sea in 

 what is now the northeastern corner of Marion County, and in other 

 areas the sedimentation over the Vicksburg deposits was thin. Along 

 the western coast of Florida the Vicksburg formations were being gently 

 folded, and a dome-like structure was developing southward. 



The Plateau, in early Apalachicolan time, had practically the same 

 outline as at present ; the depth of water north of Tampa was probably 

 in no place over 100 feet. Coral reefs were present in southern Georgia, 

 across the base of the present Peninsula, and around Tampa; the tem- 

 perature was tropical, the minimum for the year being at least as high 

 as 70 F. ; the main movement of the ocean water was from the tropics; 

 the sediments consisted to a lesser degree of organic debris, and were 

 predominantly of terrigenous constituents. 



In the later stage of Apalachicolan sedimentation, the island of 

 Oligocene lying west of the present longitudinal axis of the Peninsula, 

 here named Orange Island, had by further uplift increased in size and 

 was separated from the mainland to the north by the Suwanee Strait. 

 There was differential earth-movement, the sea-bottom being depressed 

 around Orange Island and between it and the shore of the mainland to the 

 north, permitting additions to the thickness of Apalachicola sediments. 

 During this later stage of the Apalachicola the oceanic waters of the 

 region gradually cooled and coral reefs disappeared. The sediments were 

 mostly of terrigenous origin and were laid down in shallow water. 



This period of deposition was succeeded by one of uplift and sub- 

 aerial erosion, the Apalachicolan-Miocene Interval, after which was 

 another, the Miocene, subsidence. This subsidence was not so extensive 

 as that of the preceding deposition period, and although it seems prob- 

 able it is not positively proven that the Suwanee Strait was again open 

 water, the Miocene deposits did not extend so far inland as the margin 

 of the Apalachicolan Sea, and there were extensive land areas west of 

 the present longitudinal axis of the Peninsula. The Plateau had approxi- 

 mately its present outline, and thick deposits of arenaceous sands were 

 formed practically to its southern limit, certainly as far south as the 

 locality of Key Vaca; the sea was shallow, perhaps 25 fathoms is a safe 



