32 PHYSIOGRAPHY 



is lower. In the subsoil or rock, the water spreads more slowly 

 than it would at the surface, because it does not move readily 

 through the small pores and cracks. 



The ground-water surface is not always level, even in a region 

 where the rainfall is uniform. Other things being equal, it is higher 

 beneath high land, and lower beneath low land (Fig. 21), and in 





Fig. 21. Diagram illustrating the position of the ground-water surface (the 

 dotted line) in a region of uneven topography. 



this case, the water under the higher land moves out to lower lands. 

 The tendency is for the water surface below the high land to sink 

 until it is as low as that beneath the low land; but in moist climates 

 it rains so often that the water surface under the hills almost never 

 sinks to the level of the water in the surrounding low lands, before 

 it is raised again by rains. Ground-water is therefore almost 

 always moving out from high lands to low lands. 



While ground-water usually flows in the direction of slope, it is 

 sometimes forced upward. Thus, if water moving down through 

 a porous layer of rock, as b (Fig. 22), between beds, such as d and 

 /. which do not allow it to pass through them, finds an opening, it 

 may escape upward, forming a spring, as at s'. It may even flow 

 out with great force, as shown by some flowing, or artesian, wells 

 (Fig. 1, PI. XII, p. 33). Some ground-water flows underground to 

 the sea or to lakes, and issues as springs beneath them. Some 

 ground-water, too, seeps out in such small quantities that it does 

 not appear to flow, and does not make a spring. 



Ground-water moves to some extent in other ways. Some of it 

 is taken up by roots, and, passing up through the plants, comes out 

 through their leaves into the air. Even in regions where the soil 

 appears to be very dry, evaporation is going on all the time. The 

 pores and cracks of the rock down to the water surface are full of 

 air, and from the water below, vapor passes up into the air in the 

 rock and soil, and thence into the air above. 



