GEOLOGY CHOCTAWHATCHEE AND APALACHICOLA RIVERS. 93 
Chipola River indicate more rapid dip in this formation or rather 
more erosion from the top surface previous to the deposition of 
the Choctawhatchee formation. On the Ocklocknee River, as pre- 
viously recorded the Alum Bluff formation remains above water 
level to tidewater. In the sketch map, page 80, the line AA rep- 
resents approximately the position of tidewater level on this for- 
mation. Owing to the few available exposures this line can be re- 
garded as only approximate. The formation is probably to be 
expected near the surface at the coast line except at the mouth of 
the Apalachicola River where it has been rather deeply buried 
and the coast line built out by sediments carried by the river. 
CHOCTAWHATCHEE FORMATION. 
The Choctawhatchee formation, which includes a highly fos- 
siliferous shell layer at its base and a, clay member above, is well 
exposed on some of the streams and rivers which cross this area. 
On the Apalachicola River all the known exposures are those 
found on the east bank from Watsons Landing to Johnson’s place a 
distance of about io miles. These exposures have been described. 
On the Chipola River good exposures are found on Four Mile 
Creek near Clarksville, and at Darling Slide farther down the 
River. On Econfina Creek at and below Econfina post office are 
exposures of a shell marl which is probably of this formation. 
The same shell marl is found at a locality known as the “Deadens” 
in Washington County, (S18, TiW, R13W), indicating that the 
shell marl underlies the highlands of that county. On the Choctaw- 
hatchee River few if any exposures of this formation are found 
immediately on the river. Back from' the river a short distance 
however, especially on the west side, are numerous exposures of 
the shell marl phase of the formation. According to Matson, the 
shell m'arl phase of the formation on the west side of the Choctaw- 
hatchee River attains a thickness of 30 or more feet.* 
STRUCTURE. 
An estimate of the rate of dip of the Choctawhatchee forma- 
tion on the Apalachicola River has already been given. It is there 
found to dip to the south at a rate apparently not exceeding about 
* Fla. Geol. Surv., 2nd Ann. Report, pp. 117, 1909. 
