GEOLOGY CHOCTAWHATCHEE AND APALACHICOLA RIVERS. 10 1 
Contours on the Choctawhatchee formation from the Choctaw- 
hatchee River to the Ocklocknee River have the general direction 
of north of west to south of east. Thus the upper Miocene appar- 
ently does not conform in structure to the Alum Bluff formation. 
At Alum Bluff the Choctawhatchee marl rests on the eroded top of 
the Alum Bluff formation at an elevation that is below the. inferred 
level of the fullers earth horizon at that place and within probably 
about 40 feet of the base of the Alum Bluff formation. On the 
Ocklocknee River, on the. other hand, the Choctawhatchee forma- 
tion lies immediately above the fullers earth horizon, probably as 
much as 70 feet or more above the base of the Alum Bluff forma- 
tion. In other words, the Choctawhatchee formation approximately 
parallels the general course of the present gulf border, and in this 
area cuts the Alum Bluff formation at an angle resting at the Apa- 
lachicola River on an older part of that formation than on the Ock- 
locknee River. This lack of conformity in structure is indicated on 
the sketch map, figure 4, page 42. 
Several interesting and important facts are brought out by this 
study. Certain structural features are indicated both by topog- 
raphy and stratigraphy. There is plainly a broad syncline or 
trough which centers between the Ocklocknee and Apalachicola 
Rivers. To the east of the Ocklocknee River the formations which 
enter into the formation of this syncline rise slightly and continue 
to the east and south-east with but slight dip. At the west side of 
the syncline the formations rise to the structurally high area west of 
the Apalachicola River. The formations which enter into the 
structure of this syncline are the Alum Bluff Miocene, the Chatta- 
hoochee Oligocene and presumably the older Oligocene and Eocene 
formation which, however, are concealed except to the west. To 
the south and south-west the syncline gradually flattens out and dis- 
appears probably before reaching the present coast line. 
The upper Miocene deposits, Choctawhatchee formation, which 
rest upon the eroded top surface of the Alum Bluff formation, so far 
as observed have no part in the formation of this syncline. From 
this it is inferred that the earth movements which formed this syn- 
cline took place following the close of the deposition of the Alum 
Bluff formation, and previous to the deposition of the Choctaw- 
hatchee formation. 
