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 (per- 

 haps a continuation of the Marianna limestone of West Florida, the 

 St. Stephens limestone of southwestern Alabama, and the Vicksburg 

 and Jackson limestones 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, be- 

 cause 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 ec- 

 onomic importance. The Miocene area of central Florida seems 

 to be approximately co-extensive with the lake region, but ex- 

 posures 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 unconsol- 

 idated 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. 



