108 FLORIDA GEOLOGICAL' SURVEY—15TH ANNUAL REPORT 
and Holmes counties. It consists chiefly of a soft, white, granular lime¬ 
stone which is locally silicified. Its thickness rarely exceeds fifty feet. 
A thick mantle of surface sands overlies large areas of this formation 
on its westward margin. 
oligocene 
The Oligocene is represented by three formations : the Marianna, 
the Chattahoochee, and the Tampa. 
The Marianna formation is a soft, light-colored, granular limestone 
occurring only in a small area in the northwestern part of the State 
in the vicinity of Marianna and Cottondale in Jackson County. The for¬ 
mation as it occurs in this area is rather thin, slightly over thirty feet, 
but its upper portion has probably been removed by erosion. 
The Chattahoochee formation is a soft, impure argillaceous or clayey 
limestone extending through portions of Suwannee, Hamilton, Madison 
and Jefferson counties south of the Georgia-Florida boundary and also 
farther westward in Gadsden, Jackson and Holmes counties. In the re¬ 
gion west of the Apalachicola River the Chattahoochee limestone is over- 
lain by a mantle of impure, sandy clay of variable thickness and reddish 
or yellowish in color. This in turn grades, at times sharply, into a gray- 
jointed clay below. These clays are probably residual from the Chatta¬ 
hoochee limestone. The thickness of the Chattahoochee formation ranges 
from 100 to 200 feet. 
The Tampa formation is a hard silicious limestone occurring at the 
head of Tampa Bay and extending northwestward through Hills¬ 
borough, Pasco and Hernando counties. It is probably to some extent 
contemporaneous with the Chattahoochee formation. 
MIOCENE 
The Miocene period in Florida is represented by the Alum Bluff 
formation, the Jacksonville formation and the Choctawhatchee forma¬ 
tion. 
The Alum Bluff formation consists of sands, clays, sandy lime¬ 
stones, etc., deposited under both terrestrial and marine conditions. It 
contains much phosphatic material and perhaps all of the fuller’s earth 
deposits of the State. It extends from the northern side of the Ever¬ 
glades and the Manatee River northward through the central 
portion of the peninsula to the Georgia-Florida boundary, thence west- 
