REVIEW OF THE GEOLOGY OF FLORIDA 
107 
these areas the surface is perhaps but slightly modified from its 
original condition. Of the land area of the State approximately 
one-half lies below the 50-foot contour line. This belt of land be¬ 
low the 50-foot contour includes all of the peninsula south of the 
northern end of Lake Okeechobee, and in addition, a strip of 
varying width bordering the Atlantic and the Gulf coasts. Aside 
from beach deposits and sand-dunes, this belt of country is pre¬ 
vailingly level. The surface deposits are usually sandy, although 
as will be subsequently indicated, limestones underlie considerable 
areas. 
The higher lands of the interior of the State present for the 
most part topographic features that are difficult of description, 
owing to the apparent irregularity and lack of system of the hills 
and valleys. Over much of this area the topographic features are 
the result of the solution of the underlying calcareous deposits, and 
the prevailing surface configuration includes depressions or solu¬ 
tion basins of varying size and depth, and hills of varying size and 
height. Within the area, the topography of which is controlled by 
underlying calcareous deposits, are two subdivisions which are 
worthy of special mention. These are the “sink-hole region” 
which lies chiefly along the west flank of the peninsula, and the 
“lake region,” which lies for the most part near to or somewhat 
east of the center of the peninsula. In the sink hole region the 
limestones lie at no great depth. The sinks which form as a result 
of the solution of the limestones, thus allowing the covering to fall 
in, usually reach through when first formed, to permanent water 
level, thus forming small circular lakes. In the lake region the cal¬ 
careous rocks are covered to a greater depth. The lakes are usually 
circular in outline and are deep with high banks. 
In extreme Western Florida, including Escambia and Santa 
Rosa counties, the calcareous deposits are buried beneath the sur¬ 
face to such a depth as not to affect the topography. A similar 
region is found on the east bank of the Apalachicola River, includ¬ 
ing Gadsden and the northern part of Liberty counties. Under 
these conditions the topography is determined by the normal drain¬ 
age courses. 
