GEOLOGY BETWEEN APALACHICOLA AND OCKLOCKNEE RIVERS 45 
The maximum thickness of this material at any one exposure 
is found at Alum Bluff, where the. sands lying above the Choctaw- 
hatchee formation, exclusive of the light-colored incoherent sands 
near the surface, reach a thickness of 53 feet. 
The following section at Alum Bluff was made. March 14, 
1918. The place of the section in the bluff is at a landslide about 
one-eighth mile from the north end of the bluff : 
4. Light-colored, incoherent, fine sand 21.14 feet 
3. Prevailingly red sands. This interval includes the following: 
clayey sands at the top which stand vertical, 5.58 feet ; 
coarse sands, sloping, 7.41 feet ; very coarse sands, lami- 
nated, but slightly indurated, consisting of alternating light 
and brown layers, 12.9 feet ; medium coarse sands, white 
and brown layers, 12.64 feet ; largely covered, sloping, but 
consisting as seen elsewhere in the bluff chiefly of dull red, 
medium fine sands, 24.65 feet. Total thickness 53-13 feet 
2. Choctawhatchee formation, consisting of alum-tasting, micaceous, 
drab, sandy clay, 16.4 feet ; very fossiliferous shell marl, 
14.85 feet. Total thickness - 31-25 feet 
1. Alum Bluff formation, including calcareous and phosphide 
sands, 11.7 feet, and a covered, sloping interval, which con- 
sists as seen elsewhere in the bluff of sands and Chipola 
shell marl, 18.07 feet. Total thickness . 29.77 feet 
Total height of bluff 146.56 feet 
The change in color in passing from the Choctawhatchee. for- 
mation to these overlying deposits is abrupt, and the line well 
marked. The change in the materials of the formation, however, 
is not so well marked. The upper part of the Choctawhatchee 
formation consists of drab-colored, sandy clays, which weather on 
exposure to a dull or slightly brownish color. The immediately 
overlying material consists of rather fine, dull red sands or sandy 
clay, which, aside from color, are not entirely unlike the weathered 
product of the Choctawhatchee formation. The contact line is 
exposed at several places in this bluff. Near the. north end of the 
bluff the dividing line between the red sand and the drab, sandy 
clay is 66.8 feet above water level. At an exposure a little farther 
south the dividing line is at 63.2 feet above the river, while at the 
place in the bluff where the section given on the preceding page 
was measured, the dividing line appears to lie 61 feet above the 
river. Near the south end of the bluff the. aluminous clay of the 
