REPORT OF INVESTIGATIONS No. 40


135 feet below sea level in the area 6 miles northwest of Milton and
1,000 feet below sea level in the southwest corner of Escambia County.
 A few miles north of Cantonment, the clay interfingers with the
sand-and-gravel aquifer (fig. 6). The two clay units are separated by a
bed of sand that ranges from 20 to 160 feet thick.
 The clay is gray to dark gray and contains much silt, very fine to
coarse sand, and some gravel. It is dated as Miocene on the basis of
mollusks and foraminifers. Apparently, this is one of the units that local
drillers sometimes call the "Blue Marl."
 Aquicludes within the Floridan aquifer.-The Bucatunna Clay Mem-
her of the Byram Formation of middle Oligocene age (Marsh, 1962)
separates the upper and lower limestones of the Floridan aquifer and
underlies all of westernmost Florida and parts of Louisiana, Mississippi,
and Alabama. Within the area, the Bucatunna ranges in thickness from
about 45 feet in the northwest corner of Santa Rosa County to 215 feet
just north of Escambia Bay. The Bucatunna rests uncomformably upon
the eroded surface of the lower limestone of the Floridan aquifer and is
overlain conformably by the flat, even base of the upper limestone. The
Bucatunna consists of gray, soft, silty to sand clay containing foramini-
fers, ostracods, and a few mollusks. The unit crops out along a belt that
lies about 10 to 35 miles north of the area in Alabama.
 Although much of the Floridan aquifer is porous, it contains zones of
 dense rock which may have been caused by solution and re-precipita-
 tion calcite. These dense layers serve to prevent or retard movement of
 water and thus may be classed as aquicludes.
 The lower part of the lower limestone of the Floridan aquifer con-
 tains thick but irregular zones of gray, hard, slightly calcareous, silty
 clay-shale as much as 300 feet thick. As these zones are near the base of
 the aquifer and seem to be continuous, they have relatively little effect
 on the water in the limestone. However, they reduce the average trans-
 missibility of the aquifer (see p. 160).
 Aquiclude below the Floridan aquifer.-The lower limestone of the
 Floridan aquifer is underlain everywhere in the area by gray shale and
 clay of middle Eocene age. The top of this shale and clay, although slop-
 ing generally southwestward, undulates broadly implying that these
 rocks were eroded before deposition of the overlying limestone (fig. 4).

 REGIONAL DIP
 The lack of exposures and observable bedding within the sand-and-
 gravel aquifer makes it impossible to obtain the strike and dip of this