MARYLAND GEOLOGICAL SURVEY 147 



Most of the water of the shallow wells is obtained at the base of 

 the Lafayette or Sunderland deposits, as each of these formations 

 covers large areas in which the streams have not yet cut through to 

 the underlying deposits. There are also a number of shallow wells 

 in the Patuxent River valley that derive their water supply from the 

 base of the Talbot formation. 



The water of the shallow wells usually contains so little mineral 

 matter in solution that it is known as soft water. In many wells, 

 no doubt, it does contain organic matter, yet there is little evidence 

 to show that the water on this account is unfit for drinking purposes. 



ARTESIAN WELLS. 



Since water is so readily procured at shallow depths in almost all 

 sections of the county and few establishments in the region require 

 a large supply, there have not been many attempts to obtain artesian 

 water. The area in which wells may be driven with the expectation 

 of discovering a pressure sufficient to force the water to the surface 

 is restricted to land lying 20 feet or less above tide. In areas above 

 this altitude pump wells can probably be had from the water-bearing 

 strata enumerated in the succeeding paragraphs, the water rising 

 under artesian pressure above the point where it enters the well, but 

 not overflowing. The somewhat meager data obtained in this and 

 adjoining regions indicate the occurrence of water at the horizons 

 described in the following paragraphs: 



Waters of the Crystalline Rochs.— The waters contained in the 

 crystalline rocks of the Piedmont Plateau are not of especial impor- 

 tance in this region, since these rocks occur at or near the surface 

 in a very small area. In the vicinity of Washington some wells 

 obtain water from these rocks, but to the northwest of this county 

 they yield an important water supply. In general water occurs at 

 less definite horizons in the crystalline rocks than in the Coastal 

 Plain deposits, and it is consequently much more difiicult to predict 

 the depth to which wells must be sunk to obtain a good supply. 



Beneath the unconsolidated sedimentary deposits of Prince 

 George's County crystalline rocks similar to those exposed at the 



