waste.

THE CYCLE OF EROSION. The successive stages through which a land
mass passes while it is being leveled to the sea constitute
together a cycle of erosion. Each stage of the cycle from infancy
to old age leaves, as we have seen, its characteristic records in
the forms sculptured on the land, such as the shapes of valleys
and the contours of hills and plains. The geologist is thus able
to determine by the land forms of any region the stage in the
erosion cycle to which it now belongs, and knowing what are the
earlier stages of the cycle, to read something of the geological
history of the region.

INTERRUPTED CYCLES. So long a time is needed to reduce a land mass
to baselevel that the process is seldom if ever completed during a
single uninterrupted cycle of erosion. Of all the various
interruptions which may occur the most important are gradual
movements of the earth's crust, by which a region is either
depressed or elevated relative to sea level.

The DEPRESSION of a region hastens its old age by decreasing the
gradient of streams, by destroying their power to excavate their
beds and carry their loads to a degree corresponding to the amount
of the depression, and by lessening the amount of work they have
to do. The slackened river currents deposit their waste in Hood
plains which increase in height as the subsidence continues. The
lower courses of the rivers are invaded by the sea and become
estuaries, while the lower tributaries are cut off from the trunk
stream.