70 



PHYSIOGRAPHY 



When a valley has been deepened so that its bottom is below 

 the ground-water surface, the ground-water seeps or flows into the 



Fig. 64. 



Fig. 65. 



Fig. 66. 



Fig. 64. Diagram showing a young valley. If more water enters its head 

 from the direction b than from any other direction, the head of the 

 valley will be carried back toward 6, as shown in Fig. 65. 



Fig. 65. Shows the development suggested in Fig. 64. If more water now 



, enters the head of the valley from the direction c than from any other 

 direction, the growth of the valley will be as shown in Fig. 66. 



Fig. 66. Shows the development suggested by Fig. 65. The head of the 

 gully has advanced from b to c, and it will continue to advance in this 

 direction so long as most water enters the head from the direction c. 



valley, and forms a stream. In Fig. 67, a represents the water 

 surface in wet weather, and b the water surface in dry weather. 

 The valley whose cross-section is shown by 1 would not have a 

 stream derived from ground-water; the valley 2 would have a small 



Fig. 67. Diagram showing ground-water surface; a, the ground- water sur- 

 face at ordinary times, and 6, in times of drought. When a valley has been 

 cut below a, there will be a stream in wet weather, but it wilf go dry in 

 time of drought. . When the valley is below b, the ground- water surface 

 of dry weather, the stream will be permanent. 



stream in wet weather; while the valley 3 would have a permanent 

 stream, because it is below the ground-water level of dry times. 

 Where the ground-water surface is deep, the valley must be deep 

 to get a stream; where the ground-water surface is near the land 

 surface, even shallow valleys have permanent streams. 



