GEOLOGY BETWEEN APALACHICOLA AND OCKLOCKNEE RIVERS. 41 
to Sopchoppy the whole dip amounts to 150 feet in a distance of 
about 50 miles, or an average of 3 feet per mile. Levels on the 
fullers earth horizon of this formation are given on the sketch 
map, figure 3. 
THE CHOCTAWHATCHEE FORMATION. 
The Choctawhatchee formation, which is of upper Miocene 
age, includes marine shell marls and marine sands. The type lo- 
cality of this formation is at Alum Bluff on the Apalachicola river. 
The fossiliferous shell marl of the. Choctawhatchee formation in 
this bluff has a thickness of from 14 to 19 feet. The shell marl 
grades above into blue sandy clay which varies in thickness in this 
bluff from about 16 to 26 feet. The clay contains small flakes of 
mica, and in places tastes of alum. The. Choctawhatchee forma- 
tion at this exposure rests uncon formably upon the Alum Bluff 
formation. 
At the east side of this area the Choctawhatchee marl is well 
shown in the exposure at Jackson Bluff. The following section 
at this bluff was made, by the writers in March, 1918. The water 
level in the river at the. time this section was made was 27.7 feet 
below the floor level at the center of the supporting arch of the 
public road bridge. The section was measured by hand level. 
SECTION AT JACKSON BLUFF. 
i 
6. Sands, rather coarse, in places dark colored 15. 1 feet 
5. Choctawhatchee shell marl, maximum thickness 16.5 feet 
4. Heavy buff-colored clay, resembling fullers earth 1.4 feet 
3. Sandstone, coarse grained 2.0 feet 
2. Heavy buff-colored clay, resembling fullers earth 3.6 feet 
1. Sands, calcareous and slightly phosphatic, few fossils 22.1 feet 
60.7 feet 
Numbers 1 to 4 of this section represent the Alum Bluff forma- 
tion. The two fullers earth layers with the intervening sandstone 
are represented apparently by 2 to 4. The shell marl phase of the 
Choctawhatchee formation is represented by number 5. The top 
surface of this shell marl is irregular, and the overlying aluminous 
clay is wanting. Whether this irregularity represents an uncon- 
