GEOGRAPHY OF CENTRAL FLORIDA 95 



4. THE PENINSULAR LIME-SINK OR HARD-ROCK PHOSPHATE REGION 



(Figs. 8-1 1, 40. Soil analyses 6-9.) 



This extends from a few miles north of the northern boundary 01 

 the state southward through the western half of the peninsula to tHe 

 neighborhood of Tampa. Its southern limits are ill-defined, or ai 

 least insufficiently explored, but there is at least one area of con- 

 siderable size in Hillsborough County, entirely disconnected irow 

 the rest. It reaches the coast in Pinellas County, which seems 10 

 be the only place in peninsular Florida where any high land otiiei 

 than dunes and shell mounds can be seen from the ocean. Its area 

 in central Florida is about 2,400 scjuare miles. 



Geology. The greater part of the area is underlaid at no great 

 depth by a comparatively pure limestone now regarded as of uppei 

 Eocene age, which is practically the oldest rock outcropping m 

 Florida. Toward the southern end of the region this is supposed 

 to dip southward and be overlaid by the Tampa limestone, of 

 Oligocene age. Extending nearly the- whole length of the region arc 

 irregular deposits or pockets of hard-rock phosphate, apparently de- 

 rived mostly from a re- working of the underlying rock by geological 

 processes, but containing many vertebrate fossils of Pliocene age, 

 and designated by geologists as the Alachua formation. Practically 

 the whole surface is covered by several feet of incoherent sana 

 whose age is problematical, and there may be a stratum of clay 

 between the sand and rock in some places, not as extensive in 

 central Florida as farther north, however. 



The underground water, tapped by many artesian wells at depth? 

 usually from 50 to ^.oo feet below the surface, is good to drink, 

 but unsuited for boiler purposes on account of the large amount of 

 limestone dissolved in it. For this reason the Atlantic Coast Line 

 R. R. uses water-softeners at its tanks at Ocala Junction, Dunnellon 

 and Croom, and rain water cisterns are used in some of the towns 



Topography and Drainage. The highest elevations known are a 

 little over 200 feet above sea-level. The topography is everywhere 

 undulating, with many basins of various sizes and shapes, pre- 

 sumably formed by the solution of underlying limestone. Some of 

 these have sinks or caves in their bottoms, some are sandy and al- 

 ways dry, some are inundated part of the time, and some contain 

 permanent water, making ponds or lakes (fig. 10). The dry basins 



