112 



PHYSICAL GEOLOGY 



FIG. 103. Diagram illustrating the conditions 

 necessary for an artesian well. 



Flowing wells. These are artificial springs, and were 

 formerly called artesian wells, from a province in France 



(Artois), where 

 first ones 

 dug. Any 



the 

 were 



very deep well is 

 now called an ar- 

 tesian well, even 

 though its water 

 has to be pumped 

 to the surface. 

 The conditions 

 necessary for the 

 formation of flow- 

 ing wells are shown in Figure 103. They are : (1) a porous, 



water-carrying layer (A), which outcrops at a level (B) higher 



than that of the mouths of the wells ; (2) rainfall sufficient to 



keep the porous layer well filled 



with water; (3) an impervious 



layer (C) above the porous layer. 



This confines to the latter the ma- 

 jor part of the water which enters 



it at B. If under these conditions 



wells are sunk to the porous layer 



as indicated, the water at the 



points tapped, which is under the 



pressure of a sloping stratum of 



water which fills the cavities of 



the porous layer and reaches to 



the water table near B, will be 



forced through the openings to 



the surface, forming flowing wells. 



If a boring is made too far from 



the water table near the outcrop 



of the porous layer, the loss of 



FIG. 104. An artesian well 

 at Lynch, Neb. Flows more 

 than 3000 gallons per min- 

 ute. (Darton, U.S. Geol. 

 Sun.) 



force by friction between the water and the rocks through 



