58 



PHYSICAL* GEOLOGY 



Fig. 33. — Diagram showing the source 

 of well and spring water in fractured rock. 

 (Modified after H. E. Gregory.) 



in southern California one half of the irrigation water and the 

 greater part of the city supplies are drawn from the sands and 



gravels that underlie the val- 

 leys. It is estimated that 

 75 per cent, of the population 

 of the United States depends 

 for its water supply directly 

 upon underground water. 



Movement of Ground Water. — 



Underground water seldom moves 

 in definite channels, except in lime- 

 stone regions, but percolates slowly 

 through the pores and crevices of 

 the rocks. Even in coarse sand- 

 stone the rate of movement may 

 be only one fifth of a mile a year, 

 although in regions of soluble lime- 

 stone it may flow several miles a 

 day in tunnels. In such regions the direction of the underground flow may be 

 opposite to that of the surface streams, since it is determined by the dip of the rock. 



Much of the ground water eventually reaches the surface again 

 unless it enters into chemical combination with minerals of the rocks 

 with which it comes in contact. A large amount is taken up by plants 

 and passes into the atmosphere by evaporation ; some of it is drawn 

 out in wells ; some seeps out, or is discharged in springs, either in 

 river valleys or in lakes and seas. Large springs of fresh water come 

 to the surface of the Mediterranean, the Gulf of Mexico, and other 

 seas at short distances from the shore, and in certain places fresh 

 water is obtained from springs on the ocean bottom by diving. 



The total quantity of mineral matter dissolved by the groundwater 

 is enormous. The greater part of the 4,975,000,000 tons of mineral 

 matter carried to the ocean each year was obtained by the streams 

 from the ground water which escaped through springs and seepage. 



Depth of Ground Water. — We have seen that the rocks of the 

 earth's surface are much broken by cracks of various kinds. This 

 condition holds true of rocks below the earth's surface, down to a 

 depth where the weight above them is greater than their strength 

 to resist pressure. This outer zone is called the zone of fracture. 

 The depth of this zone varies with the strength of the rock. In the 

 case of soft rocks, such as shales, no crack may be found at a depth 

 of 2000 feet, while in the strongest rocks some cracks may possibly 



