FLORIDA GEOLOGICAL SURVEY


changes, the absence of fossils, and the abundance of sand and gravel.
These sediments were probably deposited by a network of streams
whose channels were constantly shifting back and forth across the sur-
face of the delta. In this environment, clay was deposited in quiet pools
or abandoned channels while gravel was being laid down by swiftly
flowing streams nearby.
 Parts of the sand-and-gravel aquifer have a rather high average
porosity and permeability and are thus excellent reservoirs for ground
water. The aquifer consists principally of relatively insoluble quartz
grains which accounts for the remarkably low mineral content and soft-
ness of this water. In contrast to the rest of Florida, the ground-water
conditions in Escambia and Santa Rosa counties are complicated by the
great lithologic variability of the aquifer. Ground water is under artesian
pressure where lenses and layers of clay, sandy clay, or hardpan overlie
a saturated, permeable bed. Ground water is under non-artesian con-
ditions where such clays and hardpan are absent or where the perme-
able bed is not completely saturated. It is not uncommon for a well to
tap both artesian and non-artesian water. Ground water in the sand-and-
gravel aquifer is derived almost entirely from rain falling in the area.
 Floridan aquifer.-In the northern half of the area, the sand-and-
 gravel aquifer is underlain by a thick sequence of limestones known col-
 lectively as the Floridan aquifer. In the southern half of the area the
 two aquifers are separated by a thick clay unit of Miocene age (fig. 4).
 The Floridan aquifer in Escambia and Santa Rosa counties is divided
 into two parts by an extensive clay bed (Bucatunna Clay Member of
 the Byram Formation) near the top of the aquifer. The part that lies
 above this clay bed was named the upper limestone of the Floridan
 aquifer and the part below the clay was named the lower limestone of
 the Floridan aquifer (Musgrove, Barraclough, and Marsh, 1961).
 The upper limestone of the Floridan aquifer is chiefly the Chicka-
 sawhay Limestone of late Oligocene age. Within the area, this formation
 ranges in thickness from about 30 to 130 feet. Its upper surface is an
 erosional unconformity of low relief which dips gently toward the
 southwest at about 23 feet per mile. The Chickasawhay is typically a
 brown to light-gray hard dolomitic limestone or dolomite with a distinc-
 tive spongy-looking texture. It contains abundant shell fragments. Sev-
 eral wells in the area obtain water from this limestone.
 In the southern part of the area, the Chickasawhay Limestone is
 overlain unconformably by a remnant of the Tampa Limestone of early
 Miocene age. This is a cream-colored to light-gray, soft to hard, sandy