GEOLOGY BETWEEN APALACHICOLA AND OCKLOCKNEE RIVERS. 
Sweetwater creek, about 3 miles southeast of Rock Bluff postoffice 
(S33, T2N, R6W), about 80 feet erf sands overlie the Choctaw- 
hatchee marl. The exposure here is limited, and the top of the 
Choctawhatchee. formation cannot be determined, as the hill is 
sloping and covered. This exposure is the last in passing to the 
north and northeast, at which the Choctawhatchee marl is known to 
be present beneath this later formation. Farther to the north, so 
far as known, the Choctawhatchee formation is wanting, the later 
materials resting directly upon the Alum Bluff formation. 
A very instructive section, a part of which is referred to this 
formation, is found in a steephead about 1 mile southeast of Rock 
Bluff. In this section the red sands and clays rest directly upon 
the Alum Bluff formation, the Choctawhatchee formation being 
absent. The actual contact between the two formations, however, 
is concealed. 
The following section is a revision of the section at this place 
made by the writers in 1908. From barometer readings, the top 
of the section at that time, was believed to be about 200 feet above 
water level in the river. However, from levels made in March. 
1918, it is shown that the top of the section is 225 feet above the 
level of the river, or about 267 feet above sea level. 
SECTION ABOUT I MILE SOUTHEAST OF ROCK BLUFF. 
11. Superficial sand, sloping- (about) 5 feet 
10. Reddish, coarse sand, “Alamaha Grit” phase. The surface of the 
ground near the brink of the cliff is profusely covered with 
iron concretions, remaining as residual material from the de- 
cay of the formation. The first 1 to 4 feet of the sand is dis- 
colored and mottled and shows a tendency to the formation 
of iron stained crusts. Iron concretions occur in the sand 
from the surface to a depth of seven feet. These show a ten- 
dency towards arrangement in layers. One such layer is ob- 
served to extend from the mottled and decayed surface ma- 
terial downwards and horizontally into material apparently not 
appreciably affected by decay. The sand is usually cross-bedded, 
containing white siliceous pebbles. Near the base the sands 
are finer than in the upper part of the interval 52 feet 
9. Sandy yellowish laminated clays giving rise to small springs 2 feet 
8. Pink clays, free from sand and very plastic 2 feet 
7. Yellow sandy clays 3 feet 
