HOW RIVERS AND VALLEYS MAY BE FORMED 369 



stream into 

 empties. When a 



and deepens very much as the gully widened and deepened. 

 Rain, snow, and wind erode the banks, washing away soil and 

 causing the slopes to recede. Stones and pebbles roll along the 

 bottom wearing it down deeper and deeper. Deepening of the 

 bottom usually takes place faster than widening of the banks, 

 and young streams generally 

 flow at the base of steep slopes. 

 But deepening of the river bot- 

 tom does not continue indefi- 

 nitely, because every stream 

 flows into a larger body of 

 water such as a bay or gulf, and 

 cannot cut its channel lower 

 than that of the 

 which it 



stream has cut to this level, 

 that is to base level, it ceases 

 to deepen its valley. 



Although the valley ceases to 

 deepen, it does not cease to 

 widen, indeed it widens more 

 rapidly than before. When 

 water has a small gradient and 

 a low velocity, it deposits sedi- 

 ment. The channel fills up 

 and its water in consequence 

 spreads over the land and forms 

 a wider valley. The valley 

 also widens because the river continues to cut into its banks and 

 to wear them away (Figs. 236 and 237). Slowly moving water 

 does not forge ahead and overcome obstacles, but winds back 

 and forth around them taking the easiest path to be found at 

 the moment. It winds around first one bank and then another, 



FIG. 237. As a river approaches base 

 level, it widens its valley by side 

 cutting. 



