SECOND ANNUAL REPORT—rSTRATIGRAPHIC GEOLOGY. 139 
mergence or emergence of the land, the variation in altitude might be 
explained by assuming that they were deposited a\ong the margin of an 
advancing or receding sea. Unfortunately, for this theory, the poor 
assortment of the materials and the absence, locally, of distinct strati¬ 
fication point rather to fluviatile than marginal origin. It is, however, 
probable that some of the deposits were made in the margin of 
estuaries. The presence of such organic remains as bone fragments 
and sharks’ teeth is readily explained by the fact that the phosphatic 
gravels were derived from older formations in which such materials 
occurred. 
Stratigraphic Position:—The Bone Valley gravel rests upon beds 
of Pliocene or older rocks, and the relation is probably unconformable, 
but the poor exposures make it impossible to observe contacts except 
in a few localities. For this reason it may not be safe to postulate an 
unconformity between the Bone Valley gravel and the Underlying- 
Pliocene marls, though the contact with the post-Pliocene beds is cer¬ 
tainly .unconformable. The overlying Pleistocene sands rest upon an 
eroded surface of the Bone Valley gravel, and while the surface of 
the Pliocene beds does not appear to be very irregular, the widespread 
character of the unconformity indicates that the interval between the 
deposition of the Pliocene and Pleistocene was marked by extensive 
erosion. While the exact correlation of the Bone Valley gravel is 
somewhat uncertain, it is believed to be younger than the “Arcadia 
marl,” and older than the upper beds of the Caloosahatchee marl. It 
is probably in part contemporaneous with the Alachua clay. This is 
inferred from the fact that the two formations were deposited under 
similar physiographic conditions. 
Lithologic Character:—The Bone Valley gravel consists of a fine¬ 
grained matrix containing pebbles of phosphate or chert, fragments 
of bone and other organic remains. The matrix is commonly a marly 
clay, though sand is not uncommon, especially in the upper part of 
the formation. The finer grained material is soft and plastic when 
wet, but upon exposure to the air, hardens to a firm mass. The teeth 
and dermal plates of sharks and fish are so common in this formation 
that they may be gathered by the score from the washed material at 
the phosphate mines. 
Thickness:—The Bone Valley gravel is exposed in the land-pebble 
phosphate mines, but the excavations seldom extend far below the 
permanent ground water level and the base of the formation is not 
often reached. The exposures frequently reach a depth of more than 
thirty feet, without encountering the underlying beds, and hence the 
maximum thickness may be safely fixed at more than thirty feet. The 
average thickness, however, is probably lower and it may not exceed 
fifteen to twenty feet. 
