OLD AND YOUNG RIVERS AND VALLEYS 371 



steep valleys are called canons (Fig. 239). The Colorado Canon 

 is the largest in the world. It is many miles long and a mile 

 deep, with almost perpendicular walls. In time canons and 

 gorges change into open and level valleys, because the streams 

 which run through them ultimately cut down to base level. 



FIG. 239. In the Yellowstone Canon. 



When base level has been reached, deepening of the valley 

 ceases, but widening continues. 



In young rivers, waterfalls and rapids are abundant (Fig. 240), 

 since the gradient is steep and irregular; in older rivers, water- 

 falls and rapids are scarce and even lacking. The stage to 

 which a river and a valley have progressed influences the life and 

 customs of the people in the region. A young valley with 

 steep slopes is unsuitable for farming, but its rapidly flowing 



CLARK INTRO. TO SC. 24 



