CLASSIFICATION OF SPRINGS 



547 



Overflow springs, like pocket springs, are due to the overflow 

 of a reservoir of porous rock (Fig. 13 6). The underlying impervious 

 bed is not continuous, but percolation through the porous bed is 

 not sufflcient to drain the reservoir. The reservoir is commonly 

 large and of structural origin. These springs are most common in 



A. Pocket springs 



D. Fault-dam springs 



Fig. 13. — -Diagram illustrating four classes of contact springs with underlying 

 bed irregular: a, pocket springs; h, overflow springs; c, rock-dam springs; d, fault- 

 dam springs. 



the collecting area of an artesian system and are distributed along 

 the contacts of the inclosing impervious rocks. An example is 

 found in the North Downs, near London, England (Fig. 14). 

 Many of the springs at this place have ceased to flow because of 

 excessive pumping from wells in the artesian basin to the south. ^ 



^H. B. Woodward, The Geology of Water Supply (London, 1910), pp. 133-35, 

 Figs. 12, 34. 



