54 FLORIDA GEOLOGICAL SURVEY TENTH ANNUAL REPORT. 
exposure of this formation there is evidence of contemporaneous 
erosion probably by stream wash. Land plants likewise were in- 
cluded in considerable numbers indicating that the shore line at that 
time was near to this locality. The presence of land vertebrates at 
places in this formation indicates also the near approach of land. 
Between the deposition of the Alum Bluff Miocene and the Choc- 
tawhatchee Miocene is a time interval, evidence of which is found 
in the eroded top surface of the Alum Bluff formation. This ero- 
sion interval indicates that following the deposition of the. Alum 
Bluff formation this area became dry land. Subsequently the land 
was again depressed and partly submerged. That the submergence 
during the. time of the deposition of the Choctawhatchee formation 
was not complete is shown by the fact that this formation does not 
cover the whole of the area, but extends inland only to near the 
southern line of Gadsden county. 
The Choctawhatchee formation was followed, as has been stated, 
by the accumulation of a considerable thickness of sands and sandy 
clays. These later deposits are cross-bedded, include in places beds 
of gravel and coarse sands indicating that they were accumulated 
in strong currents, either of fresh water streams, or in near-shore 
marine currents. 
SUMMARY OF GEOLOGIC STRUCTURE. 
The data that have been obtained on the structure in this area 
have shown the existence of minor folds, especially in the lime- 
stones, but have not indicated the. presence of any pronounced anti- 
clines such as would suggest favorable structure for the accumula- 
tion of petroleum deposits. The dip of the formations in the main 
is to the south, or southeast, and is more rapid than the gradient 
or fall of larger streams, so that in passing inland from the coast, 
successively older formations are encountered. The average rate 
of dip of the Chattahoochee formation on the Apalachicola river 
from north-northeast to south-southwest has been shown to be about 
7 feet per mile. The dip in this formation from west-northwest to 
east-southeast appears not to be in excess of 4 feet per mile.. The 
rate of dip of the Alum Bluff formation across this area from north 
to south has been shown to be about 3 feet per mile. The. maximum 
dip of the Alum Bluff formation in this area is probably from 
northwest to southeast, and is about 5 *4 feet per mile. 
