140 
FLORIDA STATE GEOLOGICAL SURVEY. 
Physiographic Expression:—The Bone Valley gravel occupies a 
region of low altitude and the surface presents only slight inequalities. 
The formation has been eroded by some of the larger streams which 
have eroded shallow valleys ; and it sometimes, forms low hills and 
ridges. As in the case of the other Pliocene formations; the minor 
inequalities of the surface are masked by the younger beds of Pleisto¬ 
cene age. 
Paleontologic Characters: — The Bone Valley gravel contains 
abundant organic remains such as fish and shark teeth and worn shells. 
However, it is probable that these fossils were derived from older 
geological formations from which they were freed by weathering. 
They are, therefore, to be regarded as contemporaneous with the 
phosphatic pebbles and older than the gravels in which they are found. 
The Bone Valley gravel appears to contain practically no fossils which 
are contemporaneous with the deposition of the beds. 
Structure: — While the Bone Valley gravel shows no marked 
structural features, it appears to have a gentle seaward inclination. 
From the mode, of origin of this formation it is thought that this dip 
may be original and not the result of tilting subsequent to deposition. 
If the formation has been folded, the deformation is so slight that it 
does not show in the limited exposures which may be observed in the 
pebble phosphate mines. However, it is not improbable that gentle 
arching such as may be seen on the Caloosahatehee River also exists 
in the pebble-phosphate region. 
Local Details:—Partial sections of the Bone Valley gravel may 
be observed in most of the land-pebble phosphate mines. These sec¬ 
tions have certain general characteristics which are worth noting. 
First, there is a deposit of a few inches to several feet of light gray 
to white sand or coarse sandy loam which rests unconformably upon 
the Pliocene beds. The Pliocene beds may be roughly divided be¬ 
tween the non-productive marls and gravels; and those which are 
commercially valuable, on account of the high percentage of phos¬ 
phatic pebbles which they contain. The miners group the Pleistocene 
and the non-productive Bone Valley beds under the term “over¬ 
burden," because they must be removed in order to reach the pro¬ 
ductive beds. The occurrence in the Bone Valley gravel of many 
sharks’ teeth and fragments of bones and shells may readily be ex¬ 
plained by the fact that they could have been derived by weathering 
and erosion from a pre-existing formation. This view receives some 
confirmation from the fact that many of the fragments are rounded 
and they are associated with water-worn pebbles which appear to 
have been given their present form as the result of erosion and trans¬ 
portation. As the percentage of phosphate diminishes toward the top 
of the gravels, the organic remains become less numerous. This 
