.4 Contribution to the Geologic History of the Floridian Plateau. 181 



of Bahia Honda, perhaps 10 to 20 feet. Along the west coast in places 

 there may have been very slight elevation. 



The details of these differential earth-movements have not been 

 worked out, and until accurate topographic maps are available and 

 more thorough studies of the stratigraphy and structure have been made, 

 it will not be possible to decipher them. 



This uplift may have been intermittent, or there may have been 

 oscillation. The presence of terraces along some stream indicates one or 

 the other. The amount of the elevation closing the main Pleistocene 

 deposition was probably greater than that now shown by the land 

 surface, it being followed by a slight depression, as is evidenced by the 

 drowned valleys of recent times; that of St. John's River is an instance. 1 



RECENT. 



The events closing Pleistocene time gave the Plateau its present 

 configuration, and although no attempt will be made to recount in detail 

 Recent phenomena, the development of both the present coral reefs and 

 the interior swamps of the Everglades may be mentioned. The reefs 

 developed next the pure ocean waters just inside the 10-fathom curve, 

 and the Everglades were formed on the flat, imperfectly drained area 

 south of Lake Okeechobee. 



The geologic history of the Marquesas and Tortugas is reserved for 

 future consideration. The present surface above sea-level of these two 

 groups of islets seems to be geologically Recent. They are mostly or 

 entirely composed of organic detritus and calcareous material drifted 

 westward along the southern margin of the Floridian bank, as both 

 Hunt and A. Agassiz have contended. However, important geologic 

 details remain to be worked out for each group. 



RESUME OF THE GEOLOGIC HISTORY OF THE FLORIDIAN PLATEAU. 



The agencies which originally shaped, and subsequently dominated, 

 the development of the Plateau, were of two kinds: (1) those that cause 

 warpings of the earth 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, pro- 

 jecting as a submarine platform from the southeastern corner of the 

 continental shelf and extending at least to its present southern limit. 

 The forces by which this older Oligocene platform was formed at present 

 can only be the subject of speculation. It was due to some fold of the 

 ocean-bottom, perhaps in some way connected with the angle of the 

 Piedmont area in central Georgia. 



The water over this Plateau was shallow, probably in no place 100 

 fathoms deep; the bottom temperature was between 70 and 8o° F.; 

 tropical currents passed over its surface; deposits of both terrigenous 



1 Matson and Clapp, op. cit., p. 172. 



