166 
FLORIDA STATE GEOLOGICAL SURVEY. 
State and a peninsula which was both shorter and narrower than it is 
at present. During a part of this period the central portion of the 
peninsula may have been separated from the mainland by a shallow 
strait. The exact extent of the encroachment of the sea during Mio¬ 
cene times is difficult to determine because the deposits have been 
partially removed by subsequent erosion and their present extent is 
often obscured by considerable thickness of younger beds. 
During the deposition of the rocks belonging to the Miocene, the 
conditions in East and West Florida appear to have been unlike. In 
West Florida and as far eastward as the St. Johns valley south of 
Palatka, the sediments consisted of soft shell-marls containing a high 
percentage of sand and clay. The character of the materials show 
that this marl was a shallow-water deposit which accumulated in such 
close proximity to the shore that a large proportion of the sediment is 
of land origin. 
In the northern part of the St. Johns valley and extending south¬ 
ward along the east coast to near Fake Worth the Miocene is- repre¬ 
sented by an accumulation of sand and clay interbedded with more or 
less impure limestone. These sediments reach a thickness of nearly 
500 feet at Jacksonville, but decrease toward the south. Limestone 
beds predominate toward the top of this formation, hut they are also 
found at various other horizons. As in the preceding period, there 
appears to have been considerable amorphous silica deposited in the 
Miocene. This silica was subsequently redeposited in the form of 
chert beds replacing limestone, and where the original limestone was 
sandy the resultant rock resembles a coarse quartzite. The marked 
thickening of this formation toward the north is probably due, in part, 
to the proximity of large land areas to supply the sediments, and, in 
part, to deposition upon an uneven surface of the Oligocene limestone. 
During this period the Suwanee Strait became closed, and the 
fion-marine sedimentation extended southward over considerable areas 
of Oligocene rocks. These sediments form part of the thick deposits 
of sand and clay which extend from the northern boundary of the 
State southward nearly to the outcrops of the Miocene marls. This 
period of deposition was terminated by an emergence which was prob¬ 
ably a renewal of the same processes of deformation which had been 
taking place since the early Oligocene. 
The deposition of the rocks of Pliocene age began with an en¬ 
croachment of the sea upon the margin of Florida until it probably 
covered all of the southern end of the peninsula and extended north¬ 
ward beyond the latitude of Lake Okeechobee. On the east coast the 
Pliocene sea occupied all of the St. Johns River valley and it may 
also have covered all of the land east of this river. The margin of 
west Florida was doubtless submerged, but the western margin of the 
