24 



SCIENCE 



[N'. S. Vol. XXXII. No. 809 



SPECIAL ARTICLES 



SKETCH OF THE GEOLOGIC HISTORY OF THE 



FLOKIDIAN PLATEAU ' 



Recently I have endeavored to bring to- 

 gether the data bearing on the geologic his- 

 tory of the Moridian Plateau, and have an 

 essay in press as one of the Contributions 

 from Tortugas Marine Biological Laboratory 

 of the Carnegie Institution of Washing-ton. 

 A summary of the evidence and conclusions 

 vras presented before the Geological Society 

 of Washington, at its meeting on April 27, 

 and a concise statement of the conclusions is 

 :given in the succeeding remarks. 



The agencies which originally shaped, and 

 subsequently dominated, the development of 

 the plateau were of two kinds : (1) those that 

 caused warpings of the earth's crust; (2) those 

 resulting in the deposition of material on the 

 sea-floor, viz., corrosion and erosion of the 

 land surface above sea-level, transportation to 

 the sea, transportation and deposition of land- 

 derived material in the sea, and organisms 

 ■which added their skeletal remains to the 

 material of inorganic origin. 



The plateau existed in Vicksburgian, lower 

 "Oligocene time, forming a projection, as a 

 submarine platform, of the southeastern cor- 

 ner of the continental shelf and extending 

 at least to its present southern limit. This 

 older Oligocene platform was due to a fold of 

 the ocean bottom, perhaps in some way con- 

 nected with the angle of the Piedmont area in 

 central Georgia. During this period the 

 water over this plateau was shallow, perhaps 

 in no place 100 fathoms deep ; the bottom tem- 

 perature was between 70° and 80° F. ; tropical 

 currents passed over its surface; deposits of 

 both terrigenous and organic origin accumu- 

 lated on it in thickness ranging from 100 to 

 200 feet near the northern shore, 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 ac- 

 cumulating on its surface was being deposited. 



At the close of Vicksburgian time the pla- 

 teau was elevated and areas of its surface 

 were subjected to subaerial denudation, as is 



' Published by permission of the president of 

 the Carnegie Institution of Washington and the 

 director of the United States Geological Sur\'ey. 



attested by the erosion unconformity 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 forma- 

 tions; 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 

 Vicksburg deposits, indicating the later was 

 not so extensive as the previous submergence. 

 The northern shore-line was seaward of that 

 of the Vicksburg Group, it seems probable 

 that a small island may have existed in the 

 sea in what is now the northeastern corner of 

 Marion County, and in other areas the sedi- 

 mentation 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 devel- 

 oping southward. The plateau 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 

 temperature 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, predominantly of 

 terrigenous constituents. 



In the later stage of Apalachicolan sedi- 

 mentation, the island of Oligocene lying west 

 of the present longitudinal axis of the penin- 

 sula, 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 the island and be- 

 tween it and the shore of the mainland to the 

 north, permitting additions to the thicknesses 

 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 subaerial erosion, the Apa- 



