156 FLORIDA GEOLOGICAL SURVEY-I3TH ANNUAL REPORT

are in most places so nearly horizontal that they make very small angles with the comparatively level surface, so that their edge.s must always be ill-defined.
 The oldest formations known in central Florida appear at the surface in the northwestern quarter, and dip gently southward and eastward from there. The oldest rock is a nearly pure limestone of uppermost Eocene age, known now as the Ocala formation (perhaps a continuation of the. Marianna limestone of West Florida, the St. Stephens limestone of southwestern Alabama, and the Vicksburg and Jackson liniestongs of Mississippi), which is exposed about as far east as Ocala and Sumterville and as far south as Tarpon Springs. Most of the caves in our area are in this formation, because it is almost the only limestone pure enough and thick enough and sufficiently elevated above the ground-water to form caves. It is quarried in several places (fig. 12), either for road-surfacing material, for fertilizing purposes, or for burning into lime. The eastward dip of this formation seems to be very slight, for it has been encountered within 200 feet of the surface in wells drilled near the east coast.
 Next above it is the Tampa limestone, of Oligocene age, in our area principally confined to Hillsborough County. Its exposures are very limited and more or less silicified, so that it is of little economic importance. The Miocene area of central Florida seems to be approximately co-extensive with the lake region, but exposures of the strata are very scarce. Perhaps the best one is the limestone bluff at Rock Spring (fig. 18) in the northern part of Orange County, where. the first Miocene fossils in Florida were found.*
 The Pliocene is represented by the Nashua marl along the St. John's River between Palatka and Sanford, and by the hard rock and pebble phosphate deposits overlying the Eocene and Oligocene in patches west of the lake region. The Pleistocene includes some shell marls near the coast and rivers, and probably much if not most of the peat and surface sand.
 Most of the surface is covered by fairly homogeneous unconsolidated sand averaging several feet in thickness. A generation ago this was commonly regarded as a Pleistocene deposit, and called the Columbia formation; but the trend of opinion in recent years has
 *See references on page 120.