REPORT OF INVESTIGATIONS No. 40


thick. The available data suggest that the clay and sandy clay strata
may range in length from a few feet to several miles.
 Another type of relatively impermeable layer within the sand-and-
gravel aquifer is hardpan. This rock, formed by cementation of sand by
iron oxides precipitated from ground water, occurs extensively through-
out westernmost Florida and southern Alabama. This rock ranges in
thickness from a fraction of an inch to 4 feet. Little is known concerning
the lateral extent of these hardpan layers, but it is unlikely that any
layer extends for more than a few thousand yards. Although the rock is
dense, these layers are sometimes filled with many curiously shaped
cavities of uncertain origin. The rock is rust brown and is generally hard,
although some of it is soft. It is composed of iron oxides in the form of
limonite and goethite. Most "rock" on local drillers' logs is hardpan. It is
the only consolidated rock near the surface in westernmost Florida, and
it is occasionally used in the construction of stone walls and buildings.
 The relatively impermeable layers of clay and hardpan affect ground
water in several ways. First, they reduce the average permeability of
the aquifer. Second, although ground water in the sand-and-gravel aq-
uifer probably is more or less hydraulically connected, owing to the dis-
continuity of the impermeable beds, these layers (assisted by the hy-
draulic gradient) cause the water beneath them to be under artesian
pressure. Third, where these layers lie at or near the ground surface,
they decrease recharge to the aquifer by reducing infiltration rates and
cause water to be retained in depressions, where it is evaporated. Sev-
eral hundred ponds, large enough to be shown on topographic maps,
dot Escambia and Santa Rosa counties. Considerable inconvenience and
damage is caused in some residential areas by ponding of water above
clay or hardpan layers after heavy rains. In some areas these layers
underlie perched water bodies and thus make small or moderate sup-
plies of ground water available at relatively shallow depths. Finally,
these layers are responsible for countless springs, which are typically
found at the heads of gullies and small box canyons called steepheads.
These canyons are notched into the plateau-like areas that are remnants
of marine terraces of Pleistocene age. Excellent examples of such steep-
heads are found on the Eglin Air Force Base, south of the Yellow River.
Here numerous small streams originate as springs that discharge along
clay or hardpan layers at the steepheads of the gullies. As most of these
springs occur at about the same elevation, 50 feet or so above sea level,
it seems likely that they are emerging along the same relatively imper-
meable layer. The gullies were formed by headward erosion from the
edges of the terraces.