.4 Contribution to the Geologic History of the Floridian Plateau. 179 



shoaled and then blown inland by winds. According to this theory the 

 surface sands would belong to no one geological period, but would 

 represent all the periods since the Vicksburg. Some of these surface 

 sands are undoubtedly residual deposits. 



RIVER TERRACES AND OTHER PLEISTOCENE FORMATIONS. 



River terraces are present in Florida, as in the valley of St. John's 

 River, where a well defined one rises 20 to 30 feet above tide,^ and there 

 is evidence of their occurrence along other streams, but in the absence 

 of detailed topographic maps they can not be satisfactorily discriminated. 



Other Pleistocene formations are aggregates of the shells of the 

 fresh-water mollusk Planorhis - and masses of the tubes of the marine 

 mollusk Vermetus negricans.^ 



SUMMARY OF PLEISTOCENE HISTORY. 



In the preceding discussion of the marine Pleistocene formations of 

 the Peninsula of Florida, their salient characteristics, their distribution, 

 and the conditions under which they were formed have been given. 

 These data permit a general statement of the conditions prevalent over 

 the submarine portion of the Plateau, with some deficiency in precision 

 due to a lack of accurate knowledge of succession and synchrony between 

 all of the deposits. There are undoubtedly several horizons in the 

 Pleistocene. The material from west of Fort Lauderdale seems, from 

 the fossils, to be older than the Miami and Key West oolites. The 

 material in the Delray well, depth 118 feet, may be older than that 

 west of Fort Lauderdale. Additional detailed stratigraphic and paleon- 

 tologic w^ork on the Pleistocene deposits is needed. 



The Pleistocene shore-line has already been outlined. Seaward of 

 this margin the entire Peninsula was submerged, but to no great depth ; 

 very likely, unless in channels or entrants, in no place did the depth of 

 the water exceed 50 feet. During a portion of the period of submergence 

 the sea-bottom was gradually lowered and deposition kept pace with the 

 sinking. 



Shell marls were deposited over practically the entire surface of the 

 submerged platform north of the present southern margin of Lake Okee- 

 chobee. The fauna indicates for this region a warm temperature; how^- 

 ever. it was a few degrees cooler than that of the present east coast south 

 of Key Biscayne. Arenaceous sand was brought from the north and carried 

 practically to the southern margin of the Plateau, but in the later Pleis- 

 tocene in much diminished quantities south of Miami. This sand came 

 partly from the mainland to the north and partly by southward-moving 

 shore currents, which must have been warm return-currents, as the fauna 

 was characteristic of warm water. Shell-banks formed by wave-wash 

 accumulated on the east coast from St. Augustine to at least 20 miles 

 below Palm Beach, and are now coquina. These deposits may have been 

 formed along the margin of the bank as it was elevated. Similar 

 coquina beds appear to have been formed on the west coast. 



1 Matson and Clapp, Florida Geol. Siirv., 2d Ann. Report, p. 39, 1910. 

 ^ Matson and Clapp, op. cit., p. 153. 

 ^ Matson and Clapp, op. cit., p. 154. 



