142 
FLORIDA GEOLOGICAL SURVEY—THIRD ANNUAL REPORT. 
foot contour line, and parts of this area may in fact approach or 
exceed the fifty foot contour. Along the east side of the county 
bordering the St. Johns River areas varying in width from 3 to 10 
or more miles lie below the twenty-five foot contour line. 
WATER-BEARING FORMATIONS. 
The Vicksburg Limestone is the chief source of the artesian 
water supply of St. Johns County, although a small flow is prob¬ 
ably obtained before reaching this formation. The Vicksburg 
Limestone consists of alternating hard and soft fossiliferous strata 
and is usually easily recognized. At St. Augustine according to 
determinations made by Dr. W. H. Dali* fossils characteristic 
of this formation were obtained from a depth of 224 feet. At 
Hastings, 17 miles southwest of St. Augustine, well records in¬ 
dicate that a limestone similar in character to the Vicksburg is 
reached at a depth of from 175 to 200 feet. At Orange Mills in 
Putnam County, 3 miles southwest of Hastings, Orbitoides appar¬ 
ently representing some member of the Vicksburg group were ob¬ 
tained at a depth reported at no feet. At the time the sample 
was received the well was drilled to a total depth of only ,130 feet. 
Toward the northern part of St. Johns County the Vicksburg Lime¬ 
stone probably dips deeper, since at Jacksonville this formation is 
first reached at a depth of about 524 feet. 
The superficial material in this county is largely Pleistocene and 
recent sands together with Pleistocene and recent shell deposits. 
Oscillations of level have affected the surface elevation, and con¬ 
sequently the relative extent of land and water area in this county 
within comparatively recent time. That this part of the state stood 
at a lower level during a part of Pleistocene time is evident from 
the occurrence of marine shell deposits of Pleistocene age, at some 
distance inland and at an elevation of several feet above the pre¬ 
sent sea level. Oyster banks, probably of Pleistocene age, are ex¬ 
posed along a small drainage ditch on the farm of A. W. Corbett, 
4 miles southwest of St. Augustine, at an elevation of at least 1^5 to 
20 feet above the present sea level. That this depression during 
Pleistocene time was general for this part of the state is indicated 
by the evidence given elsewhere (pp. 91-93). 
The identification of the formations lying above the Vicksburg 
limestones and beneath the superfical sands, from well records alone 
*U. S. Geol. Surv. Bull. 84, p. 125, 1892. 
