86 FLORIDA GEOLOGICAL SURVEY-THIRD ANNUAL REPORT.
THE AREA TREATED. o
The area considered in etail in this report includes the following counties: Nassau, Duval, Clay, St. Johns, Putnam, Volusia. Orange, Brevard and St. Lucie. This section borders the Atlantic Coast for a distance of 250 miles, and comprises the principal artesian area of the east coast.
While central Florida, described in the preceding paper on water supply* is prevailingly a limestone country, having limestone formations at or near the surface, this eastern section of the State is prevailingly a section free from limestones. These differences, due primarily to differences in geologic structure, have given rise to marked differences in the topography, drainage, soils and water supply of the two sections.
GEOLOGY.
A knowledge of the geologic structure is essential to a clear understanding of the underground water conditions. The prevailingly level country of East Florida renders geologic observations difficult. Some favorable exposures occur, however, and these together with data obtained from well samples and well records permit a reasonably full understanding of the structure of this part of the State.
The geologic periods in eastern Florida in the order of occurrence are: Oligocene, Miocene, Pliocene and Pleistocene. Of these divisions the Oligocene is the oldest; the Pleistocene the most recent.
OLIGOCENE.
VICKSBURG GROUP.
The oldest or deepest formations reached in well drilling in eastern Florida are the Vicksburg limestones. The Vicksburg is an extensive deposit underlying all of Florida and extending into adjacent states. In central peninsular Florida from Columbia to Sumter Counties, as described in the preceding paper on water supply, these limestones are frequently exposed at the surface. Passing to the east from central Florida they dip beneath the surface and while nowhere exposed at the surface in eastern Florida, are reached by all deeper wells. It is in fact from these limestones that the principal water supply of eastern Florida is obtained.
*Fla. State Geol. Surv. Bull. No. I, 1908.