96 FLORIDA GEOLOGICAL SURVEY ELEVENTH ANNUAL REPORT. 
stones project into this area having a general northeast-southwest 
trend. The Apalachicola River skirts the east side of this belt of 
limestones and approximately parallels its structural axis. On the 
west side, the Choctawhatchee river likewise approximately parallels 
the structural axis. For reasons previously given (p. 26) the prin- 
cipal tributaries of the Apalachicola River in Florida enter from the 
west. For similar reasons the principal tributaries of the Choctaw- 
hatchee River in Florida enter from the east. On the west side of the 
Apalachicola, and on the east side of the Choctawhatchee Rivers 
is found a belt of low land. Originally, perhaps the surface eleva- 
tion in the northern and north central part of this area was greater 
than that either to the east or south. This elevation, however, was 
reduced by surface wash by tributaries from the two streams. The 
rate of lowering was increased by subsidence due to solution in the 
underlying limestones. As a result of combined surface wash and 
underground solution the part of this area which originally we may 
well believe was highest in elevation, has at present come to be 
lower than much of the land farther south. By these agencies 
was produced the north facing escarpment which is the most pro- 
nounced topographic feature of this belt of country, marking the 
dividing line between the lower lands of the limestone country and 
the higher lands of the plateau. 
The agencies that have been most active in reducing the land 
level in the limestone areas have been surface wash and under- 
ground solution. On the plateau may be observed the effects of 
solution in the underlying formations. Near the northern margin 
of the plateau are many small round lakes. The basins of these 
lakes have obviously been formed by solution in the Miocene shell 
marls which underlie the uplands. While many of the basins thus 
formed are small, others have been much enlarged by these agen- 
cies. One of the largest of these basins is that known locally 
as the “Deadens” in Washington County. In the sinks in this 
basin in the dry season may be seen exposures of the limestone 
and shell beds to the presence of which the basin owes its origin. 
The streams which flow to this basin disappear through these 
sinks, entering the calcareous formations. In most of the lakes, es- 
pecially the smaller ones, the sides are sloping and sand covered 
