PEBBLE PHOSPHATE DEPOSITS. 
55 
little doubt, but that at the time of maximum depression the whole 
of the peninsula was submerged. 
POST-OLIGOCENE PERIOD OF EMERGENCE AND EROSION 
The upper Oligocene time was followed by a period of emer¬ 
gence which evidently was very extensive bringing much of the 
present peninsula above water level. Evidence of this emergence is 
found in the fact that the top surface of the upper Oligocene deposit 
is irregular indicating surface erosion. Moreover, the succeeding 
formations without exception where the contact is observed, rest un- 
conformably upon the upper Oligocene. A partial re-submergence of 
Florida occurred during Miocene time as is shown by the fact that 
marine Miocene formations are found over quite extensive areas in 
western Florida, being exposed along the Choctawhatchee, Apalach¬ 
icola and Ocklocknee rivers. Miocene deposits are also found along 
the Atlantic slope at Jacksonville in Duval County and at Rock 
Springs in Orange County, as well as on Black Creek in Clay County. 
As already noted, however, the Miocene formations, unless the Bone 
Valley formation itself is Miocene, are not found in the land pebble 
phosphate section of the State, from which it is inferred that this 
section was dry land during most, if not all of the Miocene period. 
PERIOD OF DEPOSITION OF PLIOCENE SEDIMENTS 
It was not until near the close of the Miocene or perhaps early 
in the Pliocene that the area in southern Florida in which is found 
the pebble phosphate deposits was again submerged. The marine 
formations that accumulated in Florida dufing the Pliocene include 
in addition to the Bone Valley formation, which contains the land 
pebble phosphates, the Caloosahatchee and Nashua marls. That the 
submergence at this time was extensive is shown by the wide distri¬ 
bution of these formations in southern and eastern Florida. 
The Pliocene period was closed in Florida by an uplift, which if 
of no great extent vertically, was at least sufficient to add materially 
to the land area of the peninsula. 
CHANGES IN LEVEL DURING PLEISTOCENE TIME 
With the events of Pleistocene time, we are not so directly con¬ 
cerned in this paper. It is apparent, however, that appreciable 
changes in level occurred during the Pleistocene, to some of which 
