WATER RESOURCES. 347

is friction. Water under hydrostatic pressure entering the base of a
fine, close sand bed will be hindered by friction from rising as high
as it would were it not thus restrained. When a pipe is sunk to the
base of the bed this friction is removed and the water rises within the
pipe to a higher level than it does in the sand outside the pipe. This
difference may in some cases be sufficient to produce flows at the sur-
face.

Another set of conditions producing effective hydrostatic pressure are
those shown in Fig. 19, p. 348. The figure represents a cross section
taken at right angles to a typical Coastal Plain valley. In the higher
lands shown at either end of the section.the water table stands higher
than in the lower terraces bordering the stream in the center. The rela-
tively impervious stratum a, which serves as the confining agent for
the water-bearing stratum b, permits but little water to pass downward
through it; but the pressure resulting from the weight of the mass of
water above a@ is transmitted downward through it, producing hydro-
static pressure in b. Wells located on the terraces, if sunk deep enough
to penetrate b, will flow. It is believed that many flowing wells in the
Coastal Plain are produced by conditions similar to these.

Other factors than those described, such as temperature and baro-
metric pressure, exert a certain amount of influence upon hydrostatic
pressure, but these are believed to be of minor importance, since they
operate merely to increase or decrease slightly the effectiveness of that
pressure. They are therefore omitted from this discussion.

The upper limit of the saturated portion of the materials making
up a land body has been referred to as the water table. This lies at a
greater or lesser distance below the surface, depending upon a variety
of conditions, such as amount of rainfall, porosity of the materials,
configuration of the surface, etc. In the case of swamps it coincides
with the surface. It is not a level plain, but conforms in its curves to
the curves of the surface, though the degree of curvature is less. In
the higher lands it lies somewhat deeper below the surface than it does
in the lower lands. Neither does it occupy a fixed position, but fluetu-
ates in response to changes in the amount of rainfall. During periods
of excessive rainfall the water table rises; during periods of drouth
it subsides. Its subsidence is due in part to the passage of the water
downward into the underlying beds, whence it moves slowly oceanward
as shown in Fig. 17, p. 344; but in much greater part the subsidence
is due to gravitational movement of the water laterally towards the sur-
- rounding valley and scarp slopes, where it escapes once more to the sur-
face in the form of springs, and eventually finds its way to the ocean
as surface streams.