76 
FLORIDA STATR GEOLOGICAL SURVEY. 
as belonging to the base of the Miocene, to which the members of the 
Apalachicola group were formerly assigned. 
In Florida the base of the Chattahoochee formation was not seen, 
but there is little doubt that the pronounced unconformity observed 
farther north extends southward into that State. This view is 
strengthened by what is already known of the physical history of the 
State; and by the fact that both the Hawthorne and Tampa forma¬ 
tions, which appear to have been deposited at about the same time as 
the Chattahoochee, rest upon an eroded surface of the limestones of 
the Vicksburg group. 
Lithologic Character:—The Chattahoochee formation consists of 
light-colored limestones and marls, containing some thin beds of chert, 
clay and sand. The colors vary from creamy white to light gray or 
green on recently exposed surfaces to light yellow, brown or more 
rarely, pink, on weathered outcrops. Lithologically, there is a 
gradation from, nearly pure limestone to sands and clays, but, in gen¬ 
eral, the argillaceous and siliceous limestones predominate, forming 
impure limestones or marls. The formation is in part composed of 
semi-crystalline limestone; but soft, loosely coherent rock resembling 
an impure chalk is more common. While chert beds occur at various 
horizons, they are much thinner and less persistent than those of the 
underlying group. At times, organic life appears to have been 
abundant, and hence some layers are very fossiliferous, though the 
fossils are usually preserved in the form of imperfect casts and molds 
which have been left by the solution of the shells. 
Thickness: — Vaughan’s 1 observations along the Apalachicola 
River show that the Chattahoochee attains a considerable thickness 
near the type locality. 
The Chattahoochee limestone at the Old Landing has a thickness of at least 
85 feet and probably is greater because the basal 20 feet of the two sections 
measured on the roads to the water’s edge at the river are not exposed. How¬ 
ever, in all probability the alluvium bottom accompanying the river is underlain 
by this formation, giving it a total thickness of slightly more than 100 feet. 
Well borings from Quincy indicate that the thickness of the Chat¬ 
tahoochee formation at that locality is slightly greater than 100 feet; 
but here, as elsewhere, it is difficult to determine the exact thickness 
of formations from well samples. The maximum thickness of the 
formation is probably double the figure given above and it may even 
be as great as 250 feet. 
Physiographic Expression::—In general, the region underlain by 
the Chattahoochee formation is one of high relief and well developed 
surface drainage. However, this is not always due to the character 
of the rocks of this group, for in the northern part of the State the 
1 Vaughan, T. Wayland. Unpublished notes. 
