38 FLORIDA GEOLOGICAL SURVEY TENTH ANNUAL REPORT. 
posure of this formation is found at Rock Bluff on the Apalachi- 
cola river. The following section at Rock Bluff made in 1908 is 
from the Second Annual Report of the Florida Survey, page 273. 
SECTION' AT ROCK BLUFF. 
Rock Bluff lies five and one-half miles in a direct line south of southwest 
of Aspalaga Bluff, or twelve and one-half miles from the State line. It is the 
second point at which the river in Florida strikes the east border of the river 
valley. . That part of Rock Bluff which faces the river lies near the southwest 
corner of Section 17, R. 7 west, T. 2 north. The basal part of thq following 
section is made near the north end where the river channel first strikes the 
bluff. From this point the level was transferred north across a small stream 
to that part of the bluff which does now directly face the river. 
Thickness of stratum. 
11. Covered in the line of the section to the top of the bluff from the 
river, about 14 mile 100 
Fullers earth (exposed) 3 
Ledge with shells 1 
Gray sand , . 5 
Ledge with shells 2 
Gray sand with lime inclusions 5 
Covered 2 
Light gray calcareous sand containing a trace of phosphate (by 
transferring the level across a small branch to, the north the 
section is continued) 30 
Bluish green to gray sands, variable in character. Lime inclusions 
begin to appear in these sands at 20 feet from the base. These 
become more numerous until the material passes gradually into 
the sandy-marl above 34 
Compact sandy marl with concretions near the base and with an 
ostrea layer 6 feet above the base 8 feet 
Chattahoochee limestone above water level 10 feet 
3 - 
feet 
feet 
foot 
feet 
feet 
feet 
feet 
feet 
feet 
Numbers 2 to 10 of this section are believed to represent the 
Alum Bluff formation, which here has a thickness of 90 feet. In 
addition, the uppermost part of the formation, including that part 
which lies above the fullers earth horizon, is not exposed, or is 
wanting. 
The Alum Bluff formation includes, in places, marl beds, con- 
taining a rich and varied invertebrate fauna. The Chipola marl 
at the base of this’ formation at Alum Bluff is one of the localities 
where large collections have been made. In addition to the marine 
invertebrates, both land plants and land animals have been ob- 
