The same thing is made clear in another way. It will be seen (Plate XIV, Figs. 1 and 2) that the tributaries would presently dissect an area of uniform surface, tending to cut it into a series of short ridges or hills. In this way the amount of sloping surface is greatly increased, and as a result, every shower would have much more effect in washing loose materials down to lower levels, whence the streams could carry them to the sea.
[Illustration: Fig. 15.--Cross-sections showing various stages of erosion in one cycle.]
The successive stages in the process of lowering a surface are suggested by Fig. 15, which represents a series of cross-sections of a land mass in process of degradation. The uppermost section represents a level surface crossed by young valleys. The next lower represents the same surface at a later stage, when the valleys have grown larger, while the third and succeeding sections represent still later stages in the process of degradation. Plate XIII, Fig. 3, and Plate XIV, Figs. 1 and 2, represent in another way the successive stages of stream work in the general process of degradation.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XV.
Diagram illustrating how a hard inclined layer of rock becomes a ridge in the process of degradation.]
In this manner a series of rivers, operating for a sufficiently long period of time, might reduce even a high land mass to a low level, scarcely above the sea. The new level would be developed soonest near the sea, and the areas farthest from it would be the last--other things being equal--to be brought low. The time necessary for the development of such a surface is known as a _cycle of erosion_, and the resulting surface is a _base-level plain_, that is, a plain as near sea level as river erosion can bring it. At a stage shortly preceding the base-level stage the surface would be a _peneplain_. A peneplain, therefore, is a surface which has been brought toward, but not to base-level. Land surfaces are often spoken of as young or old in their erosion history according to the stage of advancement which has been made toward base-leveling. Thus the Colorado canyon, deep and impressive as it is, is, in terms of erosion, a young valley, for the river has done but a small part of the work which must be done in order to bring its basin to baselevel.