180 
FLORIDA STATE GEOLOGICAL SURVEY. 
to give streams an opportunity to erode valleys and establish well 
marked drainage systems. 
The shoreline topography has a more varied look, in places its form 
are those of infancy, in places, of youth or adolescence; these differ¬ 
ences are due to the mastery of causes that tend to extend the land’s 
edge irregularly, or to the control of those that smooth shore lines into 
long sweeps and easy curves. 
Shape of the Land Mass/—The relations between land and water 
shown by the maps of any coast are inconstant and ever varying. 
Not only is the shore line a line of battle between the forces that de¬ 
stroy and those that build up the land, but geologic history shows that 
changes of level are the rule, that the lands or the oceans, slowly rise 
or fall during long periods of time, parts of the sea bottom becoming 
dry land, or the oceans invading the land over great areas. In places 
these movements are rapid enough to be proved by human records. 
Where rocky highlands border the ocean, invasion is slow; where 
coasts are low, it is relatively swift. The transitions of coast lines, 
and the changes resulting from slight elevations or depressions of 
coast, are facts of high importance in contemplating the present shape 
of the land mass of southern. Florida. 
As has been pointed out by Dali and others, the present Florida 
mainland is but the top of a vastly greater submarine plateau, the south¬ 
eastern and southern edges of which are near the present shore line, 
the western edge many miles to the west. Hence, we may regard the 
present outline of the Florida mainland as a mere accident. While 
stable enough when measured in terms of human life, it is ephemeral 
compared with the duration of a geologic period. 
A depression of fifty feet would cover all the area considered in 
this report, except the tops of sand hills and ridges; an elevation of 
fifty feet would, from the configuration of the submarine plateau, 
extend the shore line but little on the east, though making dry land 
of Biscayne Bay; on the south it would dry the Bay of Florida; and 
on the west it would extend the land to forty miles west of the entrance 
to Shark River and twenty miles west of Cape Romano. 
FEATURES OF THE MAINLAND. 
While the surface of the south Florida mainland has slight relief 
.yet it shows considerable variety of type. A detailed study of its 
forms is without the province of this paper, which deals primarily with 
geology, but various surface features will be discussed at some length 
because of their intrinsic importance, because of the attention given 
them by previous writers, and because a general understanding of the 
