32 
FLORIDA STATE GEOLOGICAL SURVEY. 
the coast, and they become smaller and less numerous in the high¬ 
lands which border the north line of the State. In the coastal belt 
there are many small lakes and ponds, some of them permanent, but 
most of them lasting only during the rainy season. They seldom ex¬ 
ceed two or three feet in depth and appear to occupy shallow depres¬ 
sions in the surface of the sand. 
In the central part of the peninsula and in some localities near 
the northern boundary of the State there are lakes and swamps which 
appear to be the result either of unequal deposition of the surface 
sands or of solution of the subjacent limestone and consequent lower¬ 
ing of the surface in the manner described under Caverns. Some of 
these lakes are shallow and resemble those of the coastal belt, but 
others are deep basins partially or wholly enclosed by a rim of rock. 
The smaller swamps often contain considerable deposits of peat or 
muck. 
LAKE REGION. 
The lake region occupies a large part of the highland in the central 
part of the peninsula, but similar topography is to be found in both 
North and West Florida. The southern boundary of the lake region 
is not sharply defined and it comprises two more or less distinct areas 
consisting of the lakes in the elevated portion of the peninsula which 
usually have rock basins and those of the coastal and southern low¬ 
lands which occupy shallow depressions in the sand. While the areas 
characterized by these two types are more or less distinct, there are 
many lakes in the highlands which may he due to depressions in the 
sand and there are small lakes in the lowlands which are known to 
occupy rock basins. The highland area of the peninsula where rock 
basins predominate has commonly been known as the Lake Region, 
and for convenience this meaning is retained. 
The Lake Region comprises a type of topography common to all 
limestone areas which have been sufficiently elevated to permit the 
formation of large underground streams. The character of the sur¬ 
face is well shown by the Williston topographic sheet of the U. S. 
Geological Survey. The numerous depressions shown in the accom¬ 
panying text figure, which is taken from this map, are known as 
sink-holes and, in order to understand their origin, it is necessary 
to consider the development of the underground drainage. 
Caverns:—This region is underlain, at no great depth, by several 
hundred feet of porous limestones of Vicksburg age. Where surface 
water hearing carbonic acid, derived from decaying organic matter, 
enters this rock, it gradually dissolves the limestone and thus forms 
underground channels. 
