3 68 



THE WORK OF RUNNING WATER 





of years, but small changes occur daily and can be seen on 

 every hand. This is especially true after a severe rain or 

 wind storm; gullies are formed in field and on hill, banks 

 washed away, rocks thrown upon river banks, and new chan- 

 nels formed by swift waters. The gully developed by a single 

 rain may be the beginning of a ravine, a gorge, a valley, 

 because, although it is small, a mere crack in the soil, it 

 grows longer, wider, and deeper with each successive rain. 

 The rain which runs into a gulley from its head lengthens it, 



FIG. 236. A stream widening its valley by cutting into its bank. 



that which runs down its sides widens it, and that which 

 flows over its bottom deepens it. In time it becomes a ravine 

 (Fig. 23.5), down which a stream courses wildly during rain, but 

 in which there is no trace of a stream in clear weather. A 

 ravine, like a gully, lengthens, widens, deepens with each rain. 

 As it deepens it gets nearer and nearer the ground water level 

 and finally reaches it ; ground water then seeps continually 

 into it and gushes through openings found in weak places. 

 The ravine becomes the bed of a permanent stream. 



The depression in which a permanent stream runs widens 



