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 contotl r 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 probably 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. Dall* fossils characteristic of this formation were obtained from a depth of 224 feet. At Hastings, 17 miles southwest of St. Augustine, well records indicate 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 apparently representing some member of the Vicksburg group were obtained at a depth reported at i1o 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 Limestone'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 consequently 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 present sea level. Oyster banks, probably of Pleistocene age, are exposed along a small drainage ditch on the farm of A. W. Corbett, 4 miles southwest of St. Augustine, at an elevation of at least I5 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.