Building of the Marquesas and Tortugas Atolls, etc. 65 



The conditions under which the Pleistocene barrier reef was formed 

 may be reconstructed by using the results obtained from the recent geologic 

 investigations on the Florida mainland and the records of the wells bored 

 by the Florida East Coast Railway on Key Vaca. Pliocene deposition 

 was followed by uplift, the ensuing Pleistocene deposits being laid down 

 on the eroded surface of those preceding. The Pleistocene reef, which is 

 105 feet thick, was therefore formed on a subsiding sea-bottom succeeding 

 the uplift at the end of Pliocene time. 



The oscillations of southern Florida with reference to coral reefs may 

 be tabularly expressed as follows: 



Oscillations 0} Southern Florida. 



Recent Depression (modern) reefs. 



[Uplift. _ 

 PI • +^ o -:» J Depression (Pleistocene reefs, parts of which now stand as 



i-leistocene < ^^^^^ ^^ jg ^^^^ ^^^^^ sea-level). 



[Uplift. _ 

 Pliocene Depression (some reef corals but no well-developed reefs). 



The results of the recent investigations by Sanford, Drew, Dole, and 

 myself have revealed the salient facts in the geologic history of the tract 

 of the Florida keys and reefs, except the immediate foundation rock of the 

 living barrier reef. Great progress was made by clearly showing the 

 process by which the oolites were formed and by proving that the atoll rims 

 of the Marquesas and Tortugas are not due to the solution of an interior 

 mass of limestone but to constructional geologic processes. The hypoth- 

 esis that Biscayne Bay was formed by marine solution is also definitely 

 disproven. 



This history may be summarized as follows: Pliocene deposition was 

 followed by uplift, which was succeeded by depression; during this Pleisto- 

 cene subsidence along a curve from the eastern side of Biscayne Bay, first 

 trending southward and then bending westward, a barrier coral reef flour- 

 ished, separated by a channel from the main bank on which the Miami 

 oolite was forming or had formed in strongly agitated waters. West of 

 the coral reef, on an extensive flat in shoal water, the Key West oolite 

 was formed, while still farther to the westward the Tortugas were out- 

 lined under the influence of winds and currents. This period of events 

 was succeeded by the elevation of the entire key region to more than 50 

 feet above its previous level. This uplift was succeeded by one of depres- 

 sion, lowering the surface 30 feet or more, establishing practically the 

 same relation of the sea-level to land that now prevails. Subsequent to 

 the beginning of this last depression the present barrier reef has developed 

 seaward of the keys on a platform already prepared for it, the Marquesas 

 have been formed by winds and currents, and coral reefs have reestab- 

 lished themselves in the Tortugas. 



5 



