GEOLOGY CHOCTAWHATCHEE AND APALACHICOLA RIVERS. 9 1 
been given, p. 31. On the Chipola River the rocks of this formation 
are exposed at places from some miles south of Marianna to 
Baileys Ferry in Calhoun County. This formation has not been 
identified west of the Chipola River, and apparently does not ex- 
tend to the Choctawhatchee River. 
STRUCTURE. 
The dip of the Chattahoochee formation as measured on the 
Apalachicola River, as already stated, appears to be at an average 
rate of about 7 feet per mile. On the Chipola River the rate of 
dip has not been determined. 
However it is considerably in excess of the gradient of the 
stream as is indicated by the fact that the formation which comes 
into the section somewhat below Marianna passes below water 
level a few miles north of Clarksville. 
MIOCENE. 
ALUM BLUFF FORMATION. 
The Alum Bluff formation which consists largely of sands, clays, 
and shell marls is exposed on all of the larger streams of this area. 
The exposures on the Apalachicola River have been described. 
The Chipola marl at the base of this formation is well exposed on 
the Chipola River and its tributaries, especially on Ten Mile Creek 
north of Clarksville. On Econfina Creek are found exceptionally 
good and complete exposures of the marl phases of the formation. 
On the Choctawhatchee River are found other good .exposures of the 
formation which remains above water level on this river almost 
to tide water. The belt of out cropping of this formation extends 
across this area in a general east-west direction, or from slightly 
south of east to north of west. 
SECTION AT BOYNTON LANDING ON THE CHOCTAWHATCHEE RIVER. 
The following section was observed by the writers at Boynton 
Landing on the Apalachicola River, 35 miles by river from the 
Gulf, and a few miles above the entrance of Holmes Creek : 
