140 



PHYSICAL GEOLOGY 



base level. As a stream approaches base level, it flows on a di- 

 minishing slope, and its current therefore becomes less and less 



rapid. In other words, a 

 stream approaches base level 

 more and more slowly as it 

 draws nearer and nearer to it, 

 so that the removal of the last 

 few feet may take longer than 

 all the rest. 



Valley widening. Valleys 

 are widened hi a variety of 

 ways. Relatively sluggish 

 streams are pushed aside by 

 the currents of their tribu- 

 taries or by obstacles. In 

 this way the stream is driven 

 first against one bank, and 

 then against the other, and 

 so undermines each. The 

 points of attack varying from 

 time to time, the valley is 

 opened generally, and a val- 

 ley flat is developed (Figs. 128 

 and 129). Meanwhile, other 

 agencies assist in widening 

 the valley. Rains wash weathered material down its sides, 

 and if the slopes are sufficiently steep, fragments also roll and 

 fall down them. Material works its way down the sides of 

 the valley also by 

 creep and by slump- 

 ing (p. 115), and is re- 

 moved in other less 

 important ways. If 

 the material re- FIG. 130. The divide between the two valleya 

 mninprl at thp bnt- * s being consumed by the side cutting of the 

 rivers. It may be cut away entirely, in which 

 tom, the effect WOllld case the two valleys will become one. 



FIG. 129. Diagrams of a river de- 

 veloping a flat by side cutting. 



