Tributary crevices were subject to the same
action; and there was finally created by each of these water systems a
network of cavities whose ramifications sometimes extend throughout
several townships. In time, sections of the roof, here and there,
became so thin from the combined erosion taking place both above and
below as to be unable to sustain their own weight; the overlying
strata fell into the cave, and the volume of water flowing through it
was augmented by drainage which had previously been disposed of on the
surface. All this had to seek an outlet somewhere, except in those
rare instances where it maintains its downward course until, below the
level of any open stream it can reach, it encounters an impervious
stratum and must lose itself in the deep rocks. Usually, however, it
emerges in the face of a bluff or on the side of a hill; and the
opening becomes "the mouth of a cave." Occasionally, in such
situations, the water continues to flow out; but usually it finds a
way to reach a lower level, and so the cave in time becomes dry
except for such water as seeps through from the earth immediately
above. Sometimes, too, the point of discharge is at or perhaps
somewhat below the level of a stream into which it passes; in the
Ozarks are numerous very large springs or fountains which by inverted
siphon or artesian action are forced up from subterranean streams
lying at a greater depth.

Few large caverns have the floor entirely dry, even when they are well
above the bottom of the valley. Deposits in the front portion may be
dry, perhaps dusty on the surface; but toward the interior moisture
usually accumulates until they are muddy or until the water stands in
pools or puddles. When this is the case there is sometimes a little
stream making its way to the front through a channel which it has cut;
or seepage may dampen, possibly saturate, the lowermost portions of
the otherwise dry earth. These details are controlled principally by
the direction and degree of slopes and by side openings which allow
more or less of the water to escape at some part of its journey.

When a cavern is fairly lighted and has a dry floor, whether of rock
or earth, it forms an excellent abode for a small community unable or
not disposed to construct shelters more comfortable or convenient; and
there is abundant evidence that many caves in the Ozarks were utilized
as habitations by the aborigines. It must be remembered, however, that
in the