WATER SUPPL,Y OP PASTERN AND SOUTHERN FLORIDA. 121 
dunes approach closer to the coast. They recede again, however, 
to the south and at no place directly face the ocean. The dunes 
are now quiescent and are covered with a thick growth of trees, 
indicating that they have been undisturbed for a long time. In 
the same way the coquina rock, found facing the ocean at Anas¬ 
tasia Island, in St. Johns County, falls back from the coast to the 
south, extending at places a few miles inland. The presence of 
this ledge of coquina rock bordering the coast together with the 
sand dunes lying back clearly indicates that the land level former¬ 
ly stood lower than at present, the coquina rock and sand dunes 
having accumulated along what was then the beach. 
Conrad as early as 1846 noted the occurrence of marine shells 
of post-Pliocene age along the bank of the St. Johns River at an 
elevation of from ten to fifteen feet above the present high tide. 
Matson has described* what he believes to be a Pleistocene 
terrace bordering the St. Mary’s River, in Nassau County. A 
similar abrupt rise in passing onto the upland may be observed 
in many places bordering the coast and the valley of the St. Johns 
River. It may be observed that a subsidence of 25 feet would 
submerge the entire St. Johns valley and would allow the sand 
dunes once more to face the ocean. 
TOPOGRAPHY AND DRAINAGE. 
The section of the State to which this report relates borders 
the Atlantic Ocean and the Gulf of Mexico. From sea level the 
rise in elevation is as a rule gradual and the country in general 
level or rolling. It is probable that with the exception of sand 
dunes all of Monroe, Lee, Dade, Palm Beach, St. Lucie and Bre¬ 
vard Counties as well as the eastern one-half or more of Nassau, 
Duval, Clay, Putnam, Volusia and Orange Counties and the en¬ 
tire St. Johns River Valley lie below the 50-foot contour line. 
Elevations exceeding 50 feet occur in the western part of Nassau, 
Duval, Clay, Putnam and Orange Counties and as a ridge ex¬ 
tending from northwest to southeast through Volusia County. 
^Florida Geol. Survey, Second Annual Report, p. 39, 1909. 
