LOCATION AND MOVEMENTS OF UNDERGROUND WATERS 51 



lying far below the ground beneath hilltops while cutting the 

 surface in the valleys, giving rise to a line of springs at about the 

 level of the surface streams. 



The water-table is flattest in porous materials such as sands 

 and gravels and presents the steepest slopes in clays, often fol- 



Flood plain 



FIG. 25. Section showing relation of water-table to surface irregularities (Slichter.) 



lowing, in the latter type of materials, the surface contour with 

 but slight variation. 



It follows from the above that in seeking to locate water the 

 topography and geology should be carefully considered. Supplies 

 are to be expected at shallower depths beneath depressions than 

 beneath the higher lands, and in general will be found nearer the 

 surface in clays and similar materials than in sands or gravels. 

 The waters of the clays, however, are likely to be insufficient for 

 permanent supplies. 



Movements of Ground Waters. - - The motion of the ground 

 water is in the direction of steepest slope of the water-table, as 

 illustrated in Figure 26, and as this roughly coincides with the sur- 

 face slope it follows that the direction of motion of the ground 

 water generally approximates that of the surface drainage. 



From the law of movement thus outlined it appears that in 

 porous materials the points most favorable for obtaining water are 

 where the water movements converge. This is in the valleys or 

 other depressions, where the water, as shown in the preceding 

 section, is also nearest the surface. 



Movements of Shallow Rock Waters. - - The movements of 

 rock waters, since the movement is largely controlled by the 



