Il6 FLORIDA GEOLOGICAL SURVEY-NINTH ANNUAL REPORT. 
GEOLOGIC HISTORY. 
The geologic history of this region has been indicated in con¬ 
nection with the description of the formations, and may be very 
briefly summarized. During Eocene and Oligocene time marine 
conditions prevailed over this part of the Coastal Plain, which were 
favorable to the accumulation of a great thickness of marine lime¬ 
stones. Early in the Miocene there was a gradual change by which 
much land material was washed into the ocean, thus resulting in the 
accumulation, in shallow waters, of the sand, sandstone and clays 
which make up the Alum Bluff formation. This interval, the early 
Miocene, is also a time, of the extensive accumulation of phosphate 
in the Florida formations. The phosphate, at this time, however, 
was disseminated through the formations and was, perhaps, in no 
case sufficiently concentrated to form workable deposits, the concen¬ 
tration into the., at present known, workable beds having taken 
place subsequently.* 
During the latter part of the Miocene marine shell marls (the 
Choctawhatchee formation) accumulated in the southwestern part 
of this area, indicating an incursion of the sea at that time. While 
minor extensions of the sea over the coastal belt of this area may 
have occurred during the Pliocene and Pleistocene, it would seem 
that much of the higher lands have been above sea level since the 
Miocene, and have thus been subjected through this long period of 
time, to the slow but constantly operating agencies of erosion and 
disintegration. 
TOPOGRAPHIC AND PHYSIOGRAPHIC DEVELOPMENT. 
The area to which this report relates presents unique problems in 
the development of land forms, and affords a chapter of unusual 
interest in topographic and physiographic development. The under¬ 
lying deposits are very largely marine, and we may safely assume 
were deposited continuously over a large extent of the ocean bot¬ 
tom. When the land was first lifted above sea level, some surface 
^Origin of the Hard Rock Phosphate Deposits of Florida, Fifth Annual 
Rept., Fla. Geol. Surv. pp 23-80, 1913; Pebble Phosphates of Florida, Seventh 
Ann. Rept., Fla. Geol. Surv. pp. 25-116, 1915. 
