2 8 FLORIDA GEOLOGICAL SURVEY—FOURTH ANNUAL REPORT. 
limestone formations. The importance of limestone formations upon 
the topography is due to the fact that the limestone is more soluble 
and more readily eroded than most other formations, and those sec¬ 
tions of the State that are underlaid at no considerable depth by 
limestones have been more radically affected by erosion than the 
non-limestone sections, and have been affected in a different manner. 
The limestones erode chiefly by underground solution, as a result of 
which sinks, valleys and basins form, giving rise to a characteristic 
topography. 
Although limestone formations underlie the State throughout, 
it is only in limited sections of the State that it is sufficiently near 
the surface to affect the topography. The limestones that exert the 
chief influence on topography in Florida are those of the Oligocene 
period, which are the thickest, purest and most extensive limestones 
found in the State. The Pleistocene limestones cover considerable 
areas along the Atlantic and Gulf Coasts in the southeastern part 
of Florida. Marls and other formations containing more or less 
calcareous material, are found in many parts of the State and in 
many instances affect the soil and the vegetation. 
TOPOGRAPHY CONTROLLED BY OLIGOCENE LIMESTONES. 
That part of the State in which the topography is controlled by 
the Oligocene limestones is indicated on the accompanying map. 
Two areas are shown, which are disconnected by the Apalachicola 
embayment. The smaller area in west Florida includes Jackson and 
Holmes Counties and parts of Washington and Walton Counties. 
The much larger area in central Florida extends from the Ocklock- 
nee Kiver on the west border of Leon County in a southeasterly 
direction to Pasco County. The area extends inland from the Gulf 
from 50 to 75 miles. 
Although controlled by the underlying limestones the topography 
over the large areas outlined above is by no means uniform. Differ 
ences in elevation above sea level, in drainage conditions, and in 
the amount of erosion to which the land has been subjected give rise 
to a variety of topographic types and afford a basis of subdivision 
of the areas. 
THE GULF HAMMOCK BELT. 
Immediately adjacent to the coast and for a few miles inland the 
limestone lies at or very close to the surface. Few lakes exist, as 
