110 FLORIDA GEOLOGICAL SURVEY— 15 TH ANNUAL REPORT 
sibly began to form in Florida during the late Miocene and continued 
actively through the Pliocene and Pleistocene to the present and that 
during the Pliocene the sink-holes, ponds, lakes and surface streams 
reached their abundant and typical development in Florida. Many of 
the sink-holes form lakes which receive sediments carried in from the 
surrounding areas. This sediment settles to the bottom, forming layers 
of clay, sand or an intimate gradation between the two with varying 
amounts of other materials, depending upon the character of the sedi¬ 
ment carried in and the character of the currents within the lake. Clays 
formed in the flood-plain of streams or filling the channels of former 
streams are also of frequent occurrence in Florida. Deposits of these 
types may occupy an area underlain by one formation and except on a 
very detailed or large scale geologic map no differentiation between the 
two horizons will be made. Where the determining features of such de¬ 
posits are evident the clay is termed lacustrine or a flood-plain clay, etc., 
as the case may be. There is moreover in Florida a vast mantle or 
coating of surface sand and sandy clays of variable thickness which 
probably is of residual character resulting from the alteration of surface 
materials. This mantle often masks the true character and extent of 
the underlying formations. Indeed, in some sections of the State, particu¬ 
larly in west Florida, literally hundreds of square miles appear to be 
covered with this material where it caps the hills as well as the lower 
areas. The clay content of this covering material is often quite high, at 
times giving it the appearance of a plastic joint clay. It is, however, un¬ 
suited for the manufacture of clay products. 
The Ocala formation which, in Florida, is the sole representative 
of the Eocene Period, consists primarily of limestone. The limestone 
in places has been altered to flint and in other places has weathered 
into a residual clay. These residual clay deposits, as is characteristic of 
limestone residuals, are of variable depth and limited lateral extent. 
This formation occupies an extensive area on the Gulf side of the north¬ 
ern part of the peninsula extending through portions of Pasco, Hernan¬ 
do, Sumter, Citrus, Marion, Levy, Alachua, Columbia, Lafayette, and 
Suwannee counties. There are within this area numerous small outliers 
of younger surrounding deposits and also local areas of Pliocene to 
Recent lacustrine or fluviatile sediments. The Ocala limestone also 
