REVIEW OF THE GEOLOGY OF FLORIDA 
II 7 
ern Florida is more calcareous than the same formation in North¬ 
ern Florida. At Alum Bluff land plants are preserved in this for¬ 
mation, and in the fuller’s earth mines are found land vertebrates. 
In south central Florida more or less marl is found in the Alum 
Bluff formation and in Southern Florida a deposit of 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 warm 
shallow seas in which was deposited the great marl bed of the for¬ 
mation. 
PLIOCENE. 
Four formations in Florida, more or less well differentiated, 
are referred to the Pliocene. These are the Nashua and Caloosa- 
hatchee 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 develop¬ 
ment 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 re-accumulated during Pliocene time, de¬ 
rived chiefly from the disintegration of the nearby Miocene de¬ 
posits. The Alachua formation likewise represents a reworking 
of materials derived from the disintegration chiefly of the Mio¬ 
cene deposits, and to some extent also of the older formations on 
which this formation rests. The Alachua formation contains the 
workable deposits of hard rock phosphate. Both the Bone Valley 
and Alachua formations contain a vertebrate fauna on the basis 
of which the formations are referred to the Pliocene. The origin 
and characteristics of these two formations have been more fully 
described in the Fifth and Seventh Annual Reports of the Florida 
Survey. The Nashua and Caloosahatchee formations are de¬ 
scribed in the Second Report of the Survey. The Bone Valley for¬ 
mation rarely exceeds ioo feet in thickness and as a rule is less. 
The Alachua formation likewise is usually less than ioo feet in 
