Il6 FLORIDA GEOLOGICAL SURVEY-TWELFTH ANNUAL REPORT 
The Choctawhatchee formation is later in age and overlies the 
Alum Bluff formation. It is chiefly a loosely cemented shell marl, 
formed in shallow water and often quite sandy. The surface out¬ 
crop of this formation occupies a narrow belt extending in a gen¬ 
eral east-west direction from west of the Choctawhatchee to some¬ 
what east of the Ocklocknee Rivers. In determining the thickness 
of the Miocene it is necessary to rely on well samples. At Jackson¬ 
ville the Miocene apparently extends from near the surface (about 
33 feet) to possibly as deep as 500 feet, giving for the formations 
of this period a thickness approximating 450 feet. Probably at 
least 400 feet of the section of the well at Ft. Myers is likewise to 
be referred to the Miocene, while in the well at Okeechobee the 
Miocene approximates 400 feet in thickness. 
PHOSPHATE IN THE MIOCENE. 
Phosphate rock in considerable quantity appears for the first 
time in the Lower Miocene,* (Alum Bluff formation) the phos¬ 
phates of both the Alachua and Bone Valley formations having 
been derived from this formation; the hard rock chiefly by chemi¬ 
cal, the land pebble phosphate chiefly by mechanical segregation.! 
While the processes of concentration to 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 Mi¬ 
ocene however, the conditions were changed to such an extent that 
very considerable quantities of land-derived sediments were car¬ 
ried 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 south- 
*In samples from a well at Apopka, subsequently described, phosphate 
pebble is found to a depth of 220 feet. In the samples from this well Dr. 
Cushman finds foraminifera indicating the Comanchean at and below 115 feet. 
Whether the pebble phosphate belongs in the rock to this depth or has fallen 
from above remains to be determined. Elsewhere pebble phosphate is not 
present in the Comanchean. 
fSellards, E. H. Origin of the Hard Rock Phosphate Deposits of Florida, 
Florida State Geol. Surv., 5th, Ann. Rept., pp 23-66, 1913; The Pebble Phos¬ 
phates of Florida, ibib, pp 25-116, 1915. 
