PEBBLE PHOSPHATE DEPOSITS. 
37 
Above this stratum at Rock Bluff is found gray and blue sands leading up 
to the workable fuller’s earth stratum. At Alum Bluff the fuller’s earth fails to 
appear, and the top surface of the calcareous sand stratum is irregular as if by 
erosion. In some parts of the bluff fossiliferous Miocene marl rests directly 
upon'these sands. Usually, however, the calcareous sands are separated from 
the Miocene by laminated or cross bedded sands and clays often carrying vege¬ 
table remains. At Rock Bluff the calcareous sands have a total thickness of 30 
feet. At Alum Bluff, owing to the irregular top surface, the thickness is vari¬ 
able. At one point near the middle of the bluff this stratum is lacking although 
elsewhere it has a thickness of 9 to 16 feet. 
The cross bedded sands and laminated sands and clays lying above the 
calcareous sands are extremely variable in thickness, being not infrequently 
absent. They seem in fact merely to occupy irregularities in the top surface of 
the calcareous sands. An exposure about 300 feet from the north end of the 
bluff gives the following section: 
Covered to the top of the hill. 
Dark colored alum tasting clays- 10 feet. 
Fossiliferous Miocene marl--- 10 feet. 
Unconformity. 
Laminated clays and sands with plant remains, and blue sands- 7 feet. 
Irregularity or unconformity. 
Calcareous sands - 13 feet. 
Sloping from the water’s edge- 15 feet. 
A small stream enters just below this section. At this point is seen sands 
with buff colored clay partings, and plant remains, grading at the base into blue 
sands, having a total thickness of 8 feet. The greatest thickness observed, 21% 
feet, is at a point near the middle of the bluff. * * * * The pross bedded sands 
here rest directly upon the fossiliferous Chipola beds, the calcareous sand 
stratum being as previously stated absent at this point. 
In Gadsden County where the fuller’s earth beds are worked, 
may be seen numerous partial sections of the Alum Bluff formation. 
The fuller’s earth and the associated gray sands are also seen at 
Jackson Bluff and elsewhere on the Ocklocknee River between Gads¬ 
den and Leon Counties. 
East of the Ocklocknee River in Wakulla County the Alum Bluff 
is believed to be represented by the gray phosphatic and calcareous 
stratum which has received the local name of the Sopchoppy lime¬ 
stone, exposures of which are found near the town of Sopchoppy. 
The red hills of Leon, Jefferson and Madison Counties with little 
doubt represent the Alum Bluff formation. Throughout these coun¬ 
ties remnants of calcareous and phosphatic sandstones are found 
scattered through the partially disintegrated surface materials. 
Samples of this phosphatic sand rock taken near Tallahassee were 
found to contain from 34.3 to 3747 % tri-calcium phosphate. A 
sample taken two miles north of Monticello in Jefferson County was 
