56 FLORIDA GEOLOGICAL SURVEY-SEVENTH ANNUAL REPORT. 
it will be necessary to refer. Early Pleistocene deposits are found 
fringing the Gulf shore of southern Florida and the Atlantic border 
of eastern Florida. Marine Pleistocene shell beds are found on Six- . 
Mile Creek in Hillsboro County; near Bradentown, and at North 
Creek in Manatee County; on the Caloosahatchee River and gener¬ 
ally over the Everglades, and thence north along the Atlantic Coast. 
Following the deposition of these early Pleistocene shell marls, the 
peninsula was lifted probably by successive minor oscillations to a 
level somewhat above its present elevation. 
Finally, a slight depression probably during the late Pleistocene, 
brought the peninsula to its present level. Undoubted evidence of 
this late Pleistocene depression is found in the fact, long ago noted 
by Shaler,* that the important harbors of Florida, among which 
may be mentioned Charlotte Harbor and Tampa Bay, represent 
flooded river valleys. Additional confirmatory evidence is derived ; 
from many sources. Captain O. N. Bie of the United States Engin¬ 
eering Corps reports that while dredging the channel to Tampa 
there was encountered about seven miles below Tampa at a depth of 
20 feet a mass of cypress limbs and branches, indicating probably the 
location of a Pleistocene cypress swamp which existed at a time 
when the land area stood not less than 25 or 30 feet higher than at 
present. On the Florida Keys Mr. W. J. Krome, Construction En¬ 
gineer of the Florida East Coast Extension, reports the existence of 
swamp and peat deposits at places along the Keys at a depth beneath 
the marls of as much as 20 feet. 
SUMMARY OF THE GEOLOGIC HISTORY 
By way of summary of that part of the geologic history of 
southern Florida which specially concerns the origin of the pebble 
phosphates, it may be well to repeat, that during the late Oligocene 
time there accumulated over a part of Florida the great phosphatic 
marl known to the miners of pebble phosphate as the “bed rock.” 
The phosphate in this marl existed in the form of pebbles of various 
sizes, shapes, and colors, and probably also as soft, or finely divided « 
phosphate. Following the deposition of this marl a very large part 
of Florida became dry land. The marls were thus subjected to sur¬ 
face erosion, by which the top surface came to present the irregulari- 
*Shaler, N. S., The Geological History of Harbors. U. S. Geol. Sur. 13th 
Ann. Rept., pt. 2, pp. 190-192, 1893. 
