runoff, and a part may penetrate deeply.

Above the zone of saturation gravity carries the water downward in
devious courses until it reaches the water table. Thereafter its course
is determined largely by the lowest point of escape from the water
table. In other words, the water table is an irregular surface; and
under the influence of gravity the water tends to move from the high to
the low points of this surface. Between the point of entrance and the
point of escape from the water table, the water follows various courses,
depending upon the porosity and the openings in the rocks. In general it
fills all of the available openings, and uses the entire available cross
section in making its progress from one point to another. The difference
in height or the "head" between the point of entrance and the point of
escape, together with the porosity of the rock and other factors,
determine the general speed of its movement (see p. 73). With equal
porosity the flow is at a maximum along a line directly connecting the
two points, and on more devious courses the flow is less.

The surface water first enters the ground through innumerable small
openings. Soon, however, it tends to be concentrated into channels of
easiest flow, with the result that in the later part of its underground
course it may be much concentrated in large trunk channels. These
channels may consist of joints, or frequently of very coarse and
pervious beds. The sedimentary rocks as a whole contain the most voids,
and therefore the largest flow and largest supply of water is often
localized in them. Of the sedimentary rocks, sandstones and limestones
usually contain the largest and most continuous openings, and thus
afford the freest circulation for water. The voids in fine-grained
shales may equal in volume those in sandstones and limestones, but the
openings are so small and discontinuous that the water does not flow
freely. Regardless of total amount of water, unless there are continuous
openings of some size the flow may be small.

The relations of more porous rocks to containing impervious strata also
profoundly affect the flow of underground water. Between impervious
strata the circulation may be concentrated and vigorous within the
porous bed. Where the porous bed is not so contained, the movement may
be more dispersed and less vigorous locally. The inclination of the
beds, of course, also affects the direction and amount of the flow.

The influence