REPORT OF INVESTIGATIONS No. 40


10 to 24 inches in diameter. About 99 percent of the wells draw water
from the sand-and-gravel aquifer and the rest draw water from the
Floridan aquifer.
 The larger-diameter wells tapping the sand-and-gravel aquifer are
constructed by drilling an open hole until permeable strata (generally
coarse sand or gravel) are encountered. Screens are then set in these
permeable zones. Almost all of these wells are equipped with screens.
 The wells obtaining water from the Floridan aquifer are constructed
by drilling an open hole into the limestone, then casing the well to the
top of the limestone. The water is obtained from the uncased limestone
section. Sometimes an open hole is drilled to the top of the limestone,
the casing is firmly seated into the limestone, and drilling is continued
into the limestone below the bottom of the casing.
 In 1959 and 1960, the U.S. Geological Survey contracted to have 31
test wells drilled, by the rotary method, in Escambia and Santa Rosa
counties. There were three main purposes for these test wells. First,
they helped to delineate aquifers and aquicludes in parts of the area
where little or no geologic information was available. Geologic logs of
wells were compiled from an examination of rock cuttings that were
collected at intervals of 5 or 10 feet. Fossils were picked from the rock
cuttings and were identified to determine the ages of the geologic for-
mations. Electric logs of the two deepest wells were made to determine
accurately the position of the clay layers and permeable zones. Second,
these test wells were used to establish a grid of water-level observation
wells in areas where information on water levels was needed. Third, 20
of the wells in Santa Rosa County were used to determine the water
budget (water gains and losses) for a small topographic drainage basin.
A total footage of 5,175 feet was drilled, and the depths of the wells
ranged from 32 to 750 feet.


 STRATIGRAPHY
 AQUIFERS
 Sand-and-gravel aquifer.-Virtually all of the wells in Escambia and
 Santa Rosa counties draw their water from the sand-and-gravel aquifer.
 This aquifer extends from the surface to various depths, ranging from
 200 feet in the area 7 miles northwest of Milton to 1,000 feet in the area
 14 miles northwest of Milton (fig. 6). In the northern half of the area,
 the sand-and-gravel aquifer overlies a thin limestone of late Oligocene
 age (the upper limestone of the Floridan aquifer), but in the southern