THE WORK OF GROUND WATER 



6< 



one. The water penetrat- 

 ing the pervious or frac- 

 tured stratum (Fig. 38, 

 A, B, C) is prevented 

 from moving downward 

 through the impervious 

 layer whose slope it fol- 

 lows until it emerges at 

 the contact of the two 

 layers. (2) A second 

 class of springs rise 

 through cracks or fissures 

 (Fig. 40). These are 

 often of great volume 

 and may have a tem- 

 perature higher than the 

 springs of the first type. 

 (3) When the surface of 

 a limestone region is 

 lowered by streams (Fig. 

 39), an underground 

 stream is often encoun- 

 tered and gives rise to 

 the springs of great vol- 

 ume which are so fre- 

 quent in such districts. 

 Silver Spring in Florida 

 forms a navigable stream 

 valleys at, or above, the 

 lowest part of the valley. 



Fig. 38. — Diagrams showing the origin of springs. 

 In A the porous stratum is indicated by dots, the 

 saturated zone being shaded. B is an impervious 

 stratum. A spring (sp) appears at the left, and 

 during wet seasons, when the water table is high, a 

 spring will flow also from the right of the hill. 



In B the impervious stratum is horizontal, and 

 springs will flow from both sides of the hill. If the 

 surface of the saturated zone (shaded) becomes so 

 low that it does not reach the surface, the springs 

 will cease to flow. 



C shows a porous stratum B overlain by an 

 impervious stratum. In such a case the water is 

 derived from the surface at B and appears as a 

 spring (sp) at the left. 



from its source. Springs may flow into 

 thalweg, which is a line following the 



Fig. 39. — Large springs often issue from the base of limestone cliffs. Such springs 

 are frequently contaminated, since their water enters through wide joints and sinks 

 without being filtered by soil. 

 CLELAND GEOL. — Z 



