HYDROLOGIC PRINCIPLES 31 



mineralized, many deep sources do not qualify because of non- 

 usability. Admittedly in some areas the depth may be too great 

 for drilling under present economic conditions, but the water 

 remains available whenever it is required. 



In some areas water may be encountered at several depths, 

 perhaps in more than one geological formation, but the map 

 does not discriminate the areas occupied by individual aqui- 

 fers. Instead, the map represents a composite of the impor- 

 tant ground-water reservoirs and includes all areas through 

 which significant quantities of usable water can pass through 

 permeable materials. 



The scale of the map and the extent of knowledge are in- 

 sufficient to show this information in great detail. Successful 

 large wells have been drilled in many small areas that could 

 not be shown, and on the other hand satisfactory wells cannot 

 be obtained at all points within the outlined "ground-water 

 areas." 



Three types of ground-water areas are distinguished on the 

 map: 



1. Watercourses, consisting of a channel occupied by a per- 

 ennial stream, together with the enclosing and underly- 

 ing alluvial material saturated with water that comes 

 from the stream, from infiltration at the surface, or from 

 adjacent water-bearing materials. 



2. Loose water-bearing materials, chiefly gravel and sand, 

 including the productive aquifers of the Coastal Plains, 

 Great Plains, glacial drift and outwash, and western val- 

 leys. Buried glacial valleys not now occupied by peren- 

 nial streams are included in this group. The areas shown 

 include known and potential areas of well development 

 as well as the recharge areas. 



3. Consolidated water-bearing rocks, of which limestone, 

 basalt, and sandstone are the most important. The areas 

 include the recharge areas, which generally coincide with 

 the areas of outcrop of the permeable rocks, as well as 



