54 



GROUND-WATER 



sary, commonly, to take account of the character of the rock be- 

 neath the porous bed which contains the water. 



The bed of porous rock is the "reservoir" of the flowing well. 

 Sand or sandstone, and gravel or conglomerate, most commonly 



Fig. 34. Diagrams illustrating conditions favorable for artesian wells. In A, 

 the porous bed a is in the form of a basin; in B, it merely dips. 



serve as the reservoirs. In order that they may contain abundant 

 water they must have considerable thickness, and their outcropping 



edges must be so situated 

 that water may enter free- 

 ly, and be replenished by 

 rain as the water flows out 

 at the well. 



A relatively imprevious 

 layer of rock above the 

 reservoir (a, Fig. 34) is 

 most important; otherwise 

 the water in the reservoir 

 will leak out, and there 

 will be little or no "head" 

 at the well site. Thus if 

 the rock overlying stratum 

 a were badly broken, the 

 fractures extending up to 

 the surface, the conditions 

 would be unfavorable for 

 flowing wells, for though 

 wells might get abundant 

 water, they would not be 

 (U I.' Geol Sur'T ^ * W nsocket ' S ' D " likely to flow. If the stra- 



turn next below the reser- 

 voir is not impervious, some lower one probably is. No layer of 



