spread fans with a radius of as much as forty miles and a slope
too slight to be detected without instruments, where they leave
the rock-cut canyons in the mountains and descend upon the broad
central valley of California.

As a river flows over its fan it commonly divides into a
branchwork of shifting channels called DISTRIBUTARIES, since they
lead off the water from the main stream. In this way each part of
the fan is aggraded and its symmetric form is preserved.

PIEDMONT PLAINS. Mountain streams may build their confluent fans
into widespread piedmont (foot of the mountain) alluvial plains.
These are especially characteristic of arid lands, where the
streams wither as they flow out upon the thirsty lowlands and are
therefore compelled to lay down a large portion of their load. In
humid climates mountain-born streams are usually competent to
carry their loads of waste on to the sea, and have energy to spare
to cut the lower mountain slopes into foothills. In arid regions
foothills are commonly absent and the ranges rise, as from
pedestals, above broad, sloping plains of stream-laid waste.

THE HIGH PLAINS. The rivers which flow eastward from the Rocky
Mountains have united their fans in a continuous sheet of waste
which stretches forward from the base of the mountains for
hundreds of miles and in places is five hundred feet thick (Fig.
80). That the deposit was made in ancient times on land and not in
the sea is proved by the remains which it contains of land animals
and plants of species now extinct. That it was laid by rivers and
not by fresh-water lakes is shown by its structure. Wide stretches
of flat-lying, clays and sands are interrupted by long, narrow