64 
FLORIDA GEOLOGICAL SURVEY-I4TH ANNUAL REPORT 
CHOCTAWHATCHEE 
The Choctawhatchee formation has but a limited development in west 
Florida, where it consists of beds of marine shells in a sandy or clayey 
matrix. These marl beds, which seldom exceed 20 to 50 feet in thick¬ 
ness, extend in a narrow belt of occasional outcroppings from Leon 
County east of the Ockloeknee River, west or slightly north of west, to 
a little beyond the Choctawhatchee River. 
PLIOCENE 
The Pliocene of Florida consists of relatively thin marine and estua¬ 
rine or fresh-water sediments. The marine sediments are chiefly shell 
marls with a sandy or clayey matrix. They include the Nashua marls 
of the St. Johns River basin and the Caloosahatchee marls of the 
Caloosahatchee basin, and the Charlton formation of the St. Marys 
River Valley in northeast Florida. 
The sediments, which are of estuarine and fresh-water origin, in¬ 
clude the Bone Valley and Alachua formations. The first named is 
found chiefly in Polk, Manatee and Hillsborough Counties. The de¬ 
posits consist chiefly of sand, clay and pebble phosphate. The Alachua 
formation, probably contemporaneous with the Bone Valley, is found 
in Alachua, Marion and Levy Counties, and contains the hard-rock phos¬ 
phate deposits. In addition, some, of the undifferentiated surface mate¬ 
rials are probably of Pliocene age. 
PLEISTOCENE 
The Pleistocene formations attain their maximum development in 
Florida in the southeastern part of the peninsula, where marine and 
fresh-water limestones cover a considerable area. Even at the extreme 
south end of the peninsula, however, the Pleistocene attains no great 
thickness. In wells on the Florida keys the Pleistocene beds &re be¬ 
lieved not to exceed about 125 feet. Description of these formations in 
some detail will be found in the earlier reports of the Florida Survey. 
Superficial materials, in part, of Pleistocene age are found over large 
areas in the state. 
