162 Papers from the Marine Biological Laboratory at Tortugas. 



initiated that series of countercurrents so important in the subsequent 

 accumulation of sediments on the Floridian Plateau, and the formation 

 of much of the present land surface of Florida. It brought sand and 

 other terrigenous material from the north to be dropped on the Plateau, 

 causing its surface gradually to approach sea-level. The transportation 

 of sediment from the north by a current flowing down the western side 

 of the Peninsula or island of Vicksburg and Apalachicola formations 

 partly explains why the land surface has grown on the east and south 

 and why there has been so little growth on the west. It partly explains 

 the arrangement of the surface outcrop of the later geologic formations 

 with reference to the older eccentric nucleus, and the arrangement of 

 the main drainage lines, described on preceding pages. It also partly 

 explains why there are 400 to 500 feet of Miocene sediments on the east 

 coast and only 25 to 50 feet on the westward extension. The burden 

 of sediment broiight to the ocean by streams in Georgia and the Caro- 

 linas was by the agenc)^ of this current moved southward to the Florida 

 bank. Henceforth, the development of Florida was largely dominated 

 by the southward moving shore currents. 



UPLIFT AT THE CLOSE OF THE MIOCENE. 



Toward the close of Miocene time the Plateau was again subjected 

 to an upward earth-movement, whereby the Suwanee Strait, which, 

 should it have been open during a portion of the period, was definitely 

 closed, and it is probable the Trail Ridge was uplifted. There was more 

 upward movement on the east and south than on the west, for no Mio- 

 cene was brought above the sea-level along the shore from Levy to Pasco 

 counties, while submerged Miocene is apparently present off the mouth 

 of Tampa Bay. 



EVENTS OF PLIOCENE TIME. 



STRATIGRAPHIC RELATIONS OF PLIOCENE TO MIOCENE SEDIMENTS. 



There is a lack of definiteness of information regarding the strati- 

 graphic relations of the Pliocene to the Miocene sediments. It is not 

 positive whether there was a subsidence at the beginning of the Pliocene 

 or whether Pliocene sedimentation took place on areas of the Miocene 

 that remained submerged. Matson and Clapp say concerning the strati- 

 graphic position of the Pliocene Nashua marl of St. John's River valley: 



The Nashua marl is thought to rest unconformably upon the Miocene at De 

 Land, but this opinion lacks confirmation, as the collections from that locahty 

 have not been studied in sufficient detail to determine the exact age of the beds. 

 (Florida Geol. Surv., 2d Ann. Report, p. 128, 1910.) 



The same authors say concerning the Caloosahatchee marl: 



The contact of the Caloosahatchee marl with the underlying Miocene has not 

 been observed, but there is considerable change in fauna between it and the Miocene, 

 which is probably due to phj'siographic changes which may have permitted the 

 erosion of the Miocene beds before the beginning of the Pliocene deposition. 



AREAL DISTRIBUTION OF MARINE PLIOCENE IN FLORIDA. 



There are two important areas of marine Pliocene in Florida. The 

 more northerly, the Nashua marl, occurs along the St. John's River 

 valley from the vicinity of the town of Nashua southward to Enterprise 



