GEOLOGY OF FLORIDA 295 



workable deposits can be reasonably well followed there remains the 

 problem of the origin of the phosphate in the Alum Bluff formation 

 itself. The Eocene and Oligocene formations of peninsular Florida, 

 are calcareous, some of them being very pure limestones. During 

 the Lower Miocene, however, the conditions were changed to such 

 an extent that very considerable quantities of land-derived sedi- 

 ments were carried into northern Florida. This change was perhaps 

 gradual, as the lower part of the Alum Bluff formation is more 

 calcareous than the upper part. Likewise the Alum Bluff formation 

 of southern Florida is more calcareous than the same formation in 

 northern Florida. At Alum Bluff, land plants are preserved in this 

 formation, and in the fuller's earth mines are found land vertebrates. 

 In south central Florida more or less shell marl is found in the Alum- 

 Bluff formation and in southern Florida a deposit of shell marl of 

 great thickness was accumulated during this time. It seems 

 probable that the phosphates of the Alum Bluff formation, from 

 which in later times were formed the workable phosphate beds of 

 Florida, accumulated through chemical or biochemical processes 

 in the warm shallow seas in which were deposited the great marl 

 beds of the formation. 



PLIOCENE 



Four formations in Florida, more or less well differentiated, are 

 referred to the Pliocene. These are the Nashua and Caloosahatchee 

 shell marls, and the Bone Valley and Alachua formations. The two 

 first mentioned are marine formations. The Nashua marls are well 

 developed on the St. Johns River in Putnam and Volusia counties. 

 The Caloosahatchee marls find their typical development on the 

 Caloosahatchee River. Both formations contain an abundant 

 fauna of well-preserved invertebrates. The Bone Valley formation 

 which contains the workable pebble phosphate deposits is well 

 developed in Polk and Hillsboro counties. This formation is 

 evidently of shallow-water origin and is in part at least estuarine. 

 It represents material reaccumulated during Pliocene time, derived 

 chiefly from the disintegration of the nearby Miocene deposits. 

 The Alachua formation likewise represents a re-working of materials 

 derived from the disintegration chiefly of the Miocene deposits and 

 to some extent also of the older formations on which this formation 



