GEOLOGY OF FLORIDA 293 



The Chattahoochee and Tampa formations, which apparently 

 are in part at least contemporaneous in deposition, constitute the 

 upper part of the Oligocene as developed in Florida. The Chatta- 

 hoochee formation is well exposed on the Chattahoochee River from 

 the Georgia-Florida state line to Rock Bluff, a distance of about 

 twelve miles. Another considerable belt of exposures of this 

 formation is found extending from the Ocklocknee River to or 

 somewhat east of the Suwannee River, and from near the Georgia- 

 Florida state line to the Gulf border. The formation as exposed 

 on the Apalachicola River includes impure clayey limestones. A 

 rather harder and perhaps more nearly pure limestone phase 

 of the formation shows between the Ocklocknee and Suwannee 

 rivers. 



The Tampa formation is exposed on the Hillsboro River and on 

 the Manatee River near Tampa. It is likewise a limestone varying 

 in hardness and in purity. The thickness of the Oligocene in 

 Florida is difficult to determine, since there are no surface exposures 

 that afford a measurement of the combined thickness of the forma- 

 tions. The evidence from well records as to the thickness of these 

 beds is at present too indefinite to be of service. 



MIOCENE 



The Miocene of Florida includes the Alum Bluff, Jacksonville, 

 and Choctawhatchee formations. The Alum Bluff formation, 

 formerly referred to the Oligocene, as already noted has been placed 

 in the Miocene on the evidence of the vertebrate and invertebrate 

 fossils. The materials of this formation include calcareous sands 

 and sandstones varying to sandy limestones, calcareous clays, 

 fuller's earth clays, and sands. The conditions under which the 

 formation was deposited were evidently shallow water often in the 

 presence of conflicting currents. This is especially true of the upper 

 part of the formation in which cross-bedding is not uncommon. 

 Fossil plants are found in this formation at the type locality at 

 Alum Bluff. At the fuller's earth mines in Gadsden County there 

 is found a limited although extremely interesting land vertebrate 

 fauna associated with a shallow water invertebrate fauna. Farther 

 to the west on the Choctawhatchee and Yellow rivers the formation 



