LAND WATERS STREAMS 129 



way limits the down-cutting of running water. Other barriers, 

 such as lakes, and the outcrops of hard rock in a stream's bed, 

 have a comparable, though more temporary, effect on the develop- 

 ment of valley plains above themselves. Plains thus developed 

 have been called temporary base-levels. 



Stages in a cycle of erosion. Since river valleys have a begin- 

 ning and pass through various stages of- development before the 

 country they drain is base-leveled, it is important to recognize 

 their various stages of advancement. Nor is this difficult. An 

 old valley and a young one have different characteristics, and the 



Fig. 95. A shallow river valley in a plain. Cerro Gordo Co., la. Contrast 

 with Fig. 96. (Calvin.) 



one would no more be mistaken for the other by those who have 

 learned to interpret them, than the face of an aged man would be 

 mistaken for that of a child. 



The cycle begins with the beginning of valley development, 

 and at that stage drainage is in its infancy. The type of the infant 

 valley is the gully or ravine (Fig. 72). It 'has steep slopes and 

 a narrow bottom. Plate III represents somewhat older ravines 

 in contour. The valley is widened, lengthened, and deepened, 

 and passes from infancy to youth. In this stage also the valleys 

 are relatively narrow, and the divides between them broad. They 

 may be deep or shallow, according to the height of the land in 

 which they are cut, and the fall of the water flowing through them ; 

 but in any case the streams flowing through them have done but 

 a small part of the work they are to do before the country they 



