land.] As streams grow old they approach more and more closely to baselevel, although they are never able to attain it.  Some slight slope is needed that water may flow and waste be transported over the land.  Meanwhile the relief of the land has ever lessened.  The master streams and their main tributaries now wander with sluggish currents over the broad valley floors which they have planed away; while under the erosion of their innumerable branches and the wear of the weather the divides everywhere are lowered and subdued to more and more gentle slopes.  Mountains and high plateaus are thus reduced to rolling hills, and at last to plains, surmounted only by such hills as may still be unreduced to the common level, because of the harder rocks of which they are composed or because of their distance from the main erosion channels.  Such regions of faint relief, worn down to near base level by subaerial agencies, are known as peneplains (almost plains).  Any residual masses which rise above them are called MONADNOCKS, from the name of a conical peak of New Hampshire which overlooks the now uplifted peneplain of southern New England.

In its old age a region becomes mantled with thick sheets of fine and weathered waste, slowly moving over the faint slopes toward the water ways and unbroken by ledges of bare rock.  In other words, the waste mantle also is now graded, and as waterfalls have been effaced in the river beds, so now any ledges in the wide streams of waste are worn away and covered beneath smooth slopes of fine soil.  Ground water stands high and may exude in areas of swamp.  In youth the land mass was roughhewn and cut deep by stream erosion.  In old age the faint reliefs of the land dissolve away, chiefly under the action of the weather, beneath their cloak of 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.