198 
FLORIDA STATE GEOLOGICAL SURVEY. 
Since the keys were elevated to their present height, they have been 
subjected to forces that tend to advance the shore line and to those 
that tend to push it back, and the easterly winds have played an im¬ 
portant part in giving the southern faces of the keys their present 
trend and outlines. Differences in time and height of tides in Florida 
Strait and the Bay of Florida, together with the area of the bay, 
result in strong currents sweeping through the passages between the 
keys, particularly the openings west of Long Key. When northers 
blow, the shallow waters of the Bay are milk white over large areas 
from the limy stuff in suspension. This is deposited, to be picked up 
with a change of wind or tide, or is carried to sea in such quantities 
as to show in the blue waters of the Gulf Stream ten miles outside the 
keys. The bars and banks about the keys and in the Bay of Florida, 
the areas of marl and calcareous sand above sea level show the activity 
of waves and currents and indicate how much material they have re¬ 
cently handled. 
SHORE FINE TOPOGRAPHY. 
The shaping of the shore lines of any region is the joint work of 
tidal and wind-made currents, waves and winds. The share of each 
of these agencies is determined by the efficiency permitted through 
antecedent conditions of coastal topography, the character of the shore 
line materials, and the circumstances controlling the general circula¬ 
tion of ocean currents and winds, and the work being done at any given 
time is in a measure controlled by the rise or fall of the land with re¬ 
ference to tide level. Thus shore line features have the aspects of 
infancy, adolescence, or maturity according to the length of time the 
waves and currents have been working at a certain level and the ef¬ 
fectiveness of their attack on the land. In the same region, as in 
southern Florida, adolescent features may be found where the attack 
is strong and infantile where attack is weak or ineffective. 
The south Florida mainland is low, its coasts dip gently beneath 
the water, the shore line materials are nearly everywhere unconsolidated. 
Under these conditions slight changes of level can swing shore lines 
over long distances, and the effectiveness of wave attack is easily modi¬ 
fied by agencies which tend to build up land, such as corals, or to push 
out the shore, such as mangroves. 
The region has no well-marked valleys, no large rivers, hence 
antecedent drainage has been of minor importance in determining the 
work of waves and currents. Also the streams are clear, they bring 
down little matter in suspension, their waters are not heavily mineral¬ 
ized and they contribute comparatively little to the sea bottom, hence 
delta building by streams is insignificant. At the same time an in> 
