THE WORK OF RUNNING WATER 



Borings of this kind form what are called artesian welh. 

 These are of great importance in many regions where it is 

 difficult to obtain sufficient surface water. In some of our 

 western states the water from artesian wells has been ob- 

 tained in sufficient quantity for extensive irrigation. Al- 

 though this water often contains minerals in solution, it 



is free from surface con- 

 tamination and is there- 

 fore usually healthful for 

 drinking. 



In some places the sur- 

 face water penetrates into 

 layers of rock which it 

 can dissolve, such as salt 

 or limestone. Here it 

 forms caves and caverns, 

 the solid material which 

 occupied the place of the 

 cave having been carried 

 away in solution by the 

 water. There are thou- 

 sands of caves of this 

 kind, but perhaps the 

 most noted in this coun- 

 try is Mammoth Cave 



with its nearly 200 miles of underground avenues and 

 grotesquely sculptured halls. 



Sometimes the top of one of these caves is nearly eroded 

 away, leaving a part of its old roof standing as a natural 

 bridge, such as the natural bridge of Virginia or of Utah. 



Supplying Water to Populous Communities. The supply- 

 ing of water to large communities has always been one of 



FLOWING ARTESIAN WELL 



