126 



PHYSICAL GEOLOGY 



single channels throughout their courses, because, as they lose 

 volume, they are unable to carry all of their load and therefore 

 deposit it along the sides of their channels, so narrowing them 

 that the water breaks through the banks and forms other channels. 

 This process may be repeated again and again until at the 

 base of the cone a stream has been divided into a number of 

 distributaries. These distributaries tend to keep the fan or cone 

 symmetrical. The angle of the slope of these accumulations varies 

 (i) with the rapidity with which the velocity of the stream is 



UNCONSOLIDATED SEDIMENTS 



m 



Impervious Porous sand and Porous sand Quartzite 

 C'ay gravel above ground- and gravel below 



water table ground-water table 



Limestone 



Crystalline 

 rock 



Fig. i 16. — Cross section of a typical valley in an arid region. Beneath the alluvial 

 slope gravel predominates, but towards the central flats it gives way to alternate 

 layers of sand and clay. Water is obtained when wells reach the porous sediments, as 

 at c, d, and e. The dotted line shows the base of the alluvial slope. (U. S. Geo).. 

 Surv.) 



diminished; (2) with the kind and amount of the sediment; and 

 (3) with the size of the stream. The slope of cones and fans of large 

 streams usually is less than that of small torrents, which may be as 

 steep as from 5 to 15 degrees. 



An alluvial fan sometimes causes the formation of a lake by build- 

 ing a dam across a river. Where Kings River enters the San Joaquin 

 River of California it has deposited a fan which has dammed the San 

 Joaquin, forming the shallow Tulare Lake (Fig. 115). 



Piedmont or Alluvial Plains are formed by the coalescing of 

 adjoining fans. The slope of such plains may be so uniform that 

 the angle is not easily detected by the naked eye by one traveling 



