

DEVELOPMENT OF VALLEYS 61 



and if this is not at sea-level, the gully may be lengthening at both 

 ends at the same time. This would have been the case, for exam- 

 ple, had the depression of Fig. 42 been half-way up the slope. Val- 

 leys developed under the control of surface slope are consequent 

 valleys, and their streams are consequent streams. 



2. If the surface material of a slope is of unequal resistance, 

 the water flowing over it will develop irregularities of slope, even 

 if the slope was uniform at the outset. If the material of one 

 part of a slope is less resistant than that elsewhere, the run-off 

 will erode most there. The depression thus started will grow, and, 

 as before, the gully may develop into a valley. In the presence 

 of sufficient rainfall, therefore, either heterogeneity of slope or 

 of material will cause the development of valleys. 



The permanent stream. It appears from the foregoing dis- 

 cussion that a valley may be developed by the run-off of successive 

 showers. If supplied from this source only, surface streams would 

 cease to flow soon after the rain ceased to fall, and a valley might 

 attain considerable size without possessing a permanent stream. 

 The permanent stream is, as a rule, dependent on ground-water. 

 When a valley has been deepened until its bottom is below the 

 ground- water surface (p. 31), water seeps or flows into it from the 

 sides. The valley is then no longer dependent on the run-off of 

 showers for a stream. When the bottom of a valley is below the 

 ground-water level of a wet season, without being below that of a 

 dry one, it will have an intermittent stream. Many valleys are now 

 in that stage of development where their streams are intermittent. 



As the valley of an intermittent stream becomes deeper, the 

 periods when it is dry become shorter, and when it has been sunk 

 below the ground-water level of droughts, it will have a permanent 

 stream (3, Fig. 43). Since a valley normally develops headward, 

 its lower and older portion is likely to have a permanent stream 

 while its upper and younger part has only an intermittent one. So 

 soon as a valley gets a permanent stream, the process of valley- 

 enlargement goes on without the interruption to which it was sub- 

 ject when the supply of water was intermittent. 



In general, a permanent stream at one point in a valley means 

 a continuous stream from that point to the sea or lake to which 

 the valley leads; but to this rule there are many exceptions, as where 

 a stream heads in a region of abundant precipitation, and flows 

 thence through an arid tract where the ground-water level is low 



