WATER PROVINCES OF THE UNITED STATES ' 61 



surface soil, the glacier began the work of wearing down the solid 

 rocks, plucking off fragments both large and small from the ledge 

 and transporting them southward. This material was also left 

 in part as till, and in part was carried away by the streams. 



The effect of the drift on the water supply of the northern 

 portion of the country is very great. In general, the drift holds 

 very much more water than any of the rocks. This water is 

 yielded readily to shallow wells, and furnishes by far the larger 

 part of the well supplies in the region where it occurs. Water is 

 least abundant in the till and most abundant in the stratified 

 drift. Its occurrence in till and in sand and gravel has already 

 been described (see pp. 40, 42). 



Weathered Rocks. South of the limits of glacial advance the 

 place of the drift is partly taken by the weathered or decomposed 

 rocks. The weathering is deepest in the south where the climate 

 is more humid and, therefore, more favorable to rock decay. 



The soils south of the drift limits consist of small fragments or 

 particles of disintegrated rocks. They are usually colored red and 

 yellow by weathering and are very porous, absorbing much water. 

 Their thickness, however, is not sufficient to make them a good 

 source of water supply, although they yield water to many shallow 

 wells. The water is subjected, as in sands and similar materials, 

 to more or less complete filtration in its passage downward. 



The Atlantic Coastal Plain. - - The Coastal Plain consists of a 

 strip of unconsolidated deposits, extending from Long Island on 

 the north along the Atlantic and Gulf States into Mexico on the 

 south. The width varies from a few miles at the north to several 

 hundred miles in the Mississippi River region. 



The surface of the Coastal Plain is low, usually not exceeding 

 100 to 500 feet above sea level and, where uncut by erosion, is 

 generally flat. Owing to the soft character of the materials, how- 

 ever, the streams have generally cut fairly deep valleys which are 

 separated, where not too close together, by flat- topped ridges 

 marking the original surface. Where the streams are close to- 

 gether the surface is cut into rolling hills. 



