536 KIRK BRYAN 



II. SPRINGS DUE TO SHALLOW WATER 



The pore spaces of the upper crust of the earth are filled with 

 water below a certain level called the water table. This zone of 

 saturation generally has an indefinite extension downward, but in 

 relatively few deep wells is much water encountered below 1,500 

 feet. Springs due to these relatively shallow waters may be divided 

 into four large groups, according to the character of the rock in 

 which they occur: (i) springs in porous rock; (2) springs in porous 

 rock overlying impervious rock; (3) springs in porous rock between 

 impervious rock; (4) springs in impervious rock. 



A. SPRINGS IN POROUS ROCK (DEPRESSION SPRINGS) 



Springs in porous rock are formed where the water table or upper 

 surface of the zone of saturation reaches the surface of the ground. 

 Because they are due to the depression of the land surface down 

 to or below the water table, the group may be called depression 

 springs. They are springs whose flow is rather gentle or pools of 

 water that are continually replenished from below. Many are 

 seeps or, if extensive, swamps. They may be divided according 

 to topographic position into four classes, as described in the fol- 

 lowing paragraphs : 



Dimple springs are due to depressions in hillsides which permit 

 the land surface to cut the water table (Fig. 4 a). Such depres- 

 sions or dimples in a sloping surface may arise through erosion by 

 water or wind, through slumping and landslides, through the 

 overturning of trees, or through the operations of burrowing 

 animals, and many are enlarged and deepened by the trampling of 

 larger animals. 



Valley springs are due to the abrupt change in slope at the line 

 between the bounding valley walls and the edge of a flood plain 

 (Fig. 4 h) . Along this line the water table may reach the surface 

 and form seeps or springs. Gullies or low spots between adjacent 

 small alluvial fans may determine the point of emergence. Various 

 causes may enlarge these depressions or may concentrate the flow 

 of the water at specific places, as in the dimple springs. 



Channel springs are due to depressions in flood plains or alluvial 

 plains caused by the channel cutting of streams (Fig. 4 c) . They 



