SOIL PHYSICS 473 



have their source at a great distance from the well, but also at a 

 higher level. The water-bearing material lies between im- 

 pervious layers of clay or shale, or at least has such a layer 

 overlying it. This overlying layer of clay or shale tends to 

 prevent the water from rising, no matter how great the pres- 



FIG. 287. Geologic section from the Black Hills eastward across South 

 Dakota. The illustration immensely exaggerates the vertical distance as 

 compared with the horizontal distance. 



sure upon it. Figure 287 illustrates the layers of rock, which 

 underlie South Dakota. 



Throughout the eastern half of South Dakota there are 

 numerous artesian wells. These wells receive their supply 

 of water from a layer of sandstone, known to geologists as 

 " Dakota sandstone." This sandstone comes to the surface 

 at the eastern edge of the Black Hills. Much of the water 

 which falls upon these hills soaks into this sandstone and then 

 passes through this porous sandstone down the slope toward 

 the east. Since the sandstone comes to the surface at about 

 3000 ft. above the sea level and since the surface of the land 

 in the eastern portion of the state where the artesian wells 

 are so numerous is less than 2000 ft. above the sea level, we 

 should expect the water to rise above the surface of the land. 

 From many of these wells the water issues with great force, 



