REPORT OF INVESTIGATIONS No. 40


half of the area the sand-and-gravel aquifer rests upon a thick clay unit
of Miocene age (fig. 7). The aquifer ranges in age from Miocene to
Recent.
 Abrupt faces changes are characteristic of the sand-and-gravel
aquifer. Although composed predominantly of sand, the aquifer contains
numerous lenses and layers of clay and gravel that are as much as 60
feet thick. The discontinuity of the sediments in the sand-and-gravel
aquifer is shown in figure 8. This is a detailed geologic section of the
uppermost 100 feet of the aquifer along the Perdido River in west-
central Escambia County. The cross section is based on rock cuttings
and electric logs of 20 test wells. These wells were drilled for the St.
Regis Paper Co. to test the infiltration characteristics of the ground
along the Perdido River. The logs were made by the firm of Leggette,
Brashears, and Graham, consulting ground-water geologists. As can be
seen from the cross section, irregular lenses of gravel and clay extend
for short horizontal distances. For example, one gravel lens that is 20
feet thick is only about 200 feet long. Well logs of the sand-and-gravel
aquifer elsewhere indicate that this cross section is fairly representative
of the aquifer throughout the area.
 The uppermost 5-20 feet of the sand-and-gravel aquifer differs
 markedly from the underlying beds. This upper part consists of light
 tan, fine to coarse sand that is soft and loose in contrast to the hard,
 reddish brown, pebbly sand that underlies it. In many places, the light
 tan sand has been removed by erosion, leaving the hard reddish brown
 sand exposed as a flat surface.
 The sand-and-gravel aquifer consists predominantly of quartz sand,
 ranging from white to light brown or reddish brown. Although some
 beds of sand are moderately well sorted, the unit as a whole is generally
 rather poorly sorted. The grains range from very fine to very coarse and
 are commonly mixed with granules and small pebbles of quartz and
 chert. The sand grades laterally into stringers and lenses of gravel which
 are made up chiefly of pea-sized pebbles. In addition to the large lenses
 of clay within the aquifer, small stringers of white to gray clay are
 scattered throughout. Fragments and layers of black lignite are found
 occasionally and at many places throughout both counties layers of black
 carbonaceous sand and gravel, containing twigs and bits of coal, are
 exposed at the surface. These layers range in thickness from a few inches
 to more than 2 feet.
 It seems likely that the materials in the upper part of the sand-and-
 gravel aquifer were deposited in an environment similar to that of the
 present-day Mississippi River delta. This is suggested by the rapid faces