Ch. 6] THE GLACIAL ENVIRONMENT 119 



some pre-Cambrian sandstones are usable aquifers; for example, late 

 pre-Cambrian sandstones in the Lake Superior district of northern 

 Wisconsin yield water to wells in several places, though some of the 

 water is highly mineralized. The Paleozoic sediments of the Central 

 States area are important aquifers, and in places some beds of sand- 

 stone are almost completely uncemented. 



CONTINENTAL ENVIRONMENTS— TERRESTRIAL 



The Desert Environment 



True desert deposits play a part in the water supply of desert areas. 

 Wind-blown sand may be a source of usable ground water. Desert 

 sediments in the United States are usually intermingled with piedmont 

 deposits and are not readily distinguished from them. The saline 

 deposits of the desert environment cause the ground water in many 

 places to be highly mineralized and so are a detriment to development 

 of ground-water supplies. 



The Glacial Environment 



Deposits of glacial origin are very important sources of ground 

 water, and in some areas the only source. Though the good aquifers 

 are water-sorted, and so are fluvial, they are normally considered to 

 be of glacial origin. In glaciated areas underlain by crystalline rocks 

 in parts of New England and in most of the North American pre- 

 Cambrian shield area, the glacial deposits are the only source of ground 

 water, or the only source of a sufficient quantity to supply more than 

 small domestic wells. In much of northeastern North Dakota and 

 northeastern Montana, the glacial deposits are the only source of 

 ground water for municipalities and most farms. 



Glacial deposits have a wide range in their capacity as aquifers. 

 Till is so heterogeneous in its composition that normally it has a very 

 low permeability. The gravel and sand deposits of water-sorted glacial 

 material form some of the finest aquifers known. The large quantities 

 of water escaping from melting ice sheets laid down extensive deposits 

 of sand and gravel in front of advancing ice, at the edges of ice sheets, 

 and from streams flowing on and in the ice itself. Many preglacial 

 valleys have been filled with sand and gravel and now form excellent 

 aquifers of great economic importance. The buried valley of the pre- 

 glacial Rock River in southern Wisconsin and northern Illinois is an 

 example. The water supply for Dayton, Ohio (Norris, 1948), is ob- 

 tained from a buried preglacial valley. 



