86 PHYSICAL GEOLOGY 



Base Level of Erosion. 1 — If a stream is swift, it continues to deepen 

 its valley as it flows from the higher lands to the sea, until at or near 

 the mouth, the bed will be at, or even slightly below, sea level. (The 

 bed of the Mississippi River is locally as much as ioo feet below sea 

 level.) The entire length of the valley, however, will not be deepened 

 to the level of the sea, since as its slope (gradient) is diminished, the 

 ability of the stream to erode its bed also decreases, and before sea 

 level is reached the stream will have ceased to deepen its valley in 

 its upper course. When this condition is attained, the stream is 

 said to be at base level; that is, it has reached the lowest level to 

 which a stream can wear a land surface. As the stream approaches 

 base level, its current becomes less and less rapid, so that the deepening 

 of the last few feet of the valley may take longer than all the rest. 



If the land is raised and the gradients of the streams are increased, 

 they will again cut until a new base level is reached. If on the other 

 hand the land is lowered, base level will be reached more quickly. 



During their histories streams usually reach a number of tem- 

 porary base levels. If, for example, a stream flows into a lake, it 

 cannot cut lower than that level; and if the lake remains in existence 

 for a long time, the stream will excavate a broad valley where it enters 

 the lake. Again, if a stream flows over a stratum of hard rock in its 

 lower course while its upper course is in less resistant rock, the depth 

 to which it cuts in the hard rock will be the temporary base level, and 

 a broad valley will be developed above the resistant rock, while the 

 latter will constitute the steep-sided narrows so characteristic of 

 the scenery of eastern Pennsylvania. 



Effect of Load. — Whether a stream carrying sediment will erode 

 or deposit depends upon its velocity and upon the amount of mate- 

 rial. If its velocity is great, the sand and gravel will be used as tools 

 with which to cut down the stream bed, or widen it. If, however, 

 the velocity is sufficiently decreased, as frequently occurs when a 

 side stream with a steep gradient flows into a master stream with a 



1 The terra base level has been used in several senses, the difficulty arising because of the 

 fact that as commonly used the surface described is a slope and not a level plain. It has been 

 suggested that "base level" be limited to the level base with respect to which normal sub- 

 aSria] erosion proceeds ; to employ the term grade for the balanced condition of a mature or 

 old river ; and to name the geographical surface that is developed near or very near the close of 

 a cycle a "peneplain" or "plain of gradation." (Davis, Wm. M., — Geographical Essays, 

 P- 387.) 



As used in this volume a base level is the lowest possible slope to which a region can be cut 

 by running water. Thus a stream in a canyon may cut its channel to base level ages before it 

 !' .< lopi B plain at that level. A peneplain is any extensive tract of land reduced to essential 

 planene l..i . l.-\ el) by the erosion of running water. 



