262 PHYSIOGRAPHY AXD GEOLOGY OF THE COASTAL PLAIN PROVINCE. 



UNDERGROUND WATERS. 



This subject is comprehensively treated in the report on "Underground 

 Waters" of the Virginia Coastal Plain, published as Bulletin Y of the 

 Virginia Geological Survey, hence only a brief statement is here made 

 regarding this very important resource. 



The water supply of the Virginia Coastal Plain is secured from both 

 shallow and deep wells, and water from both sources is obtainable in 

 almost all parts of the region. The shallow water is found at the base of the 

 Lafayette and Cohmibia formations at depths ranging from 15 to 60 feet. 

 Most of the region is dependent upon this shallow water, which is generally 

 pure though sometimes contaminated with surface drainage. It rarely 

 contains much mineral matter in solution. The supply of water in these 

 wells is seldom great but is usually sutficient for ordinary domestic pur- 

 poses except in seasons of excessive drought. 



The deep wells of the Coastal Plain furnish uncontaminated water, but 

 much of it is so highly charged with saline materials that it is unserviceable. 

 This is particularly the case in the vicinity of the Bay and Ocean. The 

 deep wells may be divided into two classes, flowing and non-floAving wells. 

 Except in the extreme western portion of the Coastal Plain practically the 

 entire region is underlain by water-bearing strata from which the water 

 will rise from 1 to 20 feet above tide. As a result flowing wells can be 

 usually obtained in the low lands bordering the estuaries. Such wells are 

 very common along the Potomac, Rappahannock, York, and James rivers. 

 The same horizons furnish water for the deep wells over the divides but 

 there the water does not rise to the surface. 



The water-bearing horizons of the deep wells are numerous, the lowest 

 of which occurs at the base of the Patuxent formation where it is in contact 

 with the underlying crystalline floor. These basal beds furnish the water 

 supply for many of the wells in Alexandria, but in the central and eastern 

 portions of the region they have seldom been reached. From the western 

 edge of the Coastal Plain this crystalline floor slopes gently to the eastward, 

 and at Fortress Monroe lies 2246 feet beneath the surface. At that point 

 no water was found at the exact contact of the Patuxent deposits with the 

 crystalline rocks, but a supply of salty water was obtained in a coarse sand 

 bed about 15 feet above. In all probability a good supply of water could 

 be obtained almost everywhere at or near the contact in the central and 

 western portions of the region and would not contain enough salt there to 

 render it objectionable. 



