REPORT OF INVESTIGATIONS No. 40


 RELATION OF GEOLOGY TO GROUND WATER
 MOVEMENT OF WATER
 The direction of ground-water flow is determined by the pressure
head from point to point. The head, in turn, is determined by the hy-
drologic, geologic, and topographic conditions between the recharge and
discharge areas. The relative position of rock layers of greatly differing
permeabilities may have an important influence on the direction of
ground-water flow. Owing to the relative impermeable clay unit and
the Bucatunna Clay Member, which dip gently toward the southwest,
one might expect ground water in the Floridan aquifer to move south-
westward in the area. However, the movement of water in the Floridan
aquifer is to the south and southeast. The dip of strata in the sand-and-
gravel aquifer is so slight that ground-water flow in this aquifer is con-
trolled principally by differences in head resulting from local topographic
irregularities.
 The location of four normal faults in the Jay area is shown on figures
9 and 11. These faults are extensions of the fault system around Pollard,
Alabama, where the Pollard oil field is located. Oil is produced in this
field from structural traps along the faults and comes from sands in the
Tuscaloosa Formation of Late Cretaceous age at a depth of approxi-
mately 5,400 to 6,000 feet below sea level.
 Just how faults affect flow of the ground water is not known but
different resistivity readings on opposite sides of faults, shown by elec-
tric logs, suggest that some salty water may move upwards along faults
in the lower part of the lower limestone of the Floridan aquifer. How-
ever, water wells near the faults are not nearly deep enough to verify
this.

 RELATION OF GEOLOGY TO QUALITY OF WATER
 Zones of fresh and salty toater.-Most of the water in the sand-and-
gravel aquifer is fresh. The Floridan aquifer, however, contains sub-
stantial quantities of both fresh and salt water. In the northern part of
the area, the uppermost few hundred feet of the lower limestone of the
Floridan aquifer contains fresh water. At depths greater than about
1,200 feet, the water from this limestone is very salty. In the southern
part of the area, the lower limestone contains only very salty water.
Here the relatively impermeable Bucatunna Clay Member serves to re-
tard the vertical movement of water and thus to prevent salt water in
the lower limestone from moving upward and contaminating the fresher