EFFECTS OF THE MOVEMENTS OF WATER 33 



called strata. In a layer of sand or gravel the water 

 moves easily, because sand and gravel are full of little 

 openings. They are very porous. But in clay water moves 



FIG. ii. Diagram illustrating two types of springs; 

 try to explain them; the springs occur at S; P means 

 porous layer; C is a perpendicular crack in the layers 

 of rock. 



very slowly. Clay packs very closely; it is much less 

 porous than sand and gravel. Then there are layers of 

 solid rock that also obstruct the movement of water in 

 the ground. So ground-water moves laterally, more than 

 it moves vertically. It moves along the more porous layers. 



FIG. 12. Diagram illustrating the artesian well, as ex- 

 plained in the text; P is a porous layer of rock; the 

 well is at W. 



This explains why there are springs. Where porous 

 layers that have water in them come to the surface, the 

 water runs out and forms springs (see Fig. n). This also 

 explains artesian wells. Artesian wells are wells from which 



