192 
FLORIDA STATE GEOLOGICAL SURVEY. 
shore south of Sanibel Island. On the southwestern side of the Ever¬ 
glades rock comes within a foot of low water level in Rock Creek, 
an arm of one of the series of bays that together make up much of 
the lower twelve miles of Lostmans River. Three miles northeast 
of this .point, rock outcrops in a pine island. Thence northward there 
are many rock exposures scattered through the flat lands. They can 
be seen at the head of Allens River and eleven miles to the east on the 
property of the Deep Take Fruit Company. The belt of country in 
which rock is exposed widens northward and at Naples is fully forty 
miles wide from east to west. This region is monotonously level and 
the rise of the rock surface inland is slight. In short, bed rock out¬ 
crops around the main body of the Everglades from Jupiter River to 
Fort Shackelford, with no important interruption except between Lost- 
mans River and the farthest west of the rocky keys beyond Long Key. 
In this break, rock does not, on the average, lie more than three feet 
below the level of low water. 
Making allowance for the general slope of the water level to¬ 
wards the drainage channels on the east and west coasts, this hydrau¬ 
lic gradient amounting to three-tenths of a foot per mile for thirty 
miles northwest of Miami, it is evident that the actual depth to bed 
rock in the middle of the Everglades is three or four feet more than 
what would be shown by measuring below a level line extending from 
rock rim to rock rim. As these points might be forty miles apart, 
and as the depth below the line at the latitude of Miami would be 
on the average less than five feet, the inappropriateness of the term 
basin for the southern portion of the Everglades is evident. 
The term sink is not accurate because the area is too large; the 
rock floor too flat. Potholes and small sinks are common in the rocky 
prairie south and southeast of the main body of the Everglades, and 
there may be large sinks filled with mud in their main body but the 
existence of such is not proven. It is possible also that there may be 
sinks of some size between the line of rocky keys and the north shore 
of the Bay of Florida. Certain features of the lakes back of Flamingo 
suggest that they are sinks. 
While the rock floor of the southern Everglades is known to be 
almost flat, yet it is altogether possible that farther north there are 
true basins of considerable extent. The preliminary surveys made for 
the drainage work undertaken at New River by the Florida Drainage 
Commission have shown that the bed rock slopes off to the westward 
more steeply there than to the south, and the depth to bed rock five 
miles west of the eastern rim back of Fort Lauderdale is probably 
not less than twenty feet. Surveys to the north seem to indicate a 
flatter slope to the bed rock and there is a report of an east-west rock 
ridge within six feet of the surface' between four and five miles 
