110 FLORIDA GEOLOGICAL SUREY-I5TH 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 sediment carried in and the character of the currents within the lake. Clays formed in the flood-plain of streams or filling the channels of foi-mer 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 deposits 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, particularly 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, unsuited 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 northern part of the peninsula extending through portions of Pasco, Hernando, 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