14 PHYSIOGRAPHY AND CxEOLOGY OF THE COASTAL PLAIN PROVIXCE. 



miles, a grade as steep as that often found along the flanks of the greater 

 mountain systems. In striking contrast to this declivity is the comparatively 

 flat ocean bed stretching away to the east, with but slight differences in 

 elevation. Looked at from the base the escarpment would have the appear- 

 ance along its crest of a high mountain range with a very even sky line. 

 Here and there notches would be seen, produced by the streams which once 

 flowed across the continental shelf, but there would be no peaks or serrated 

 ridges. 



The western limit of the Atlantic Coastal Plain is defined by a belt of 

 crystalline rocks, consisting of greatly metamorphosed igneous and sedi- 

 mentary materials, ranging in age from the pre-Cambrian to the Silurian 

 period. These rocks form the eastern portion of the Plateau province. 

 Most of the larger streams and many of the smaller ones, as they cross the 

 western margin of the Coastal Plain, are characterized by falls or rapids and 

 always show a marked decrease in the velocity of their currents, the name 

 "fall-line" being given to this boundary on that account. The position of 

 the "fall-line" near the head of navigation or near the source of water- 

 power has been one of the very important factors in determining the location 

 of many of the towns and cities of the Atlantic coast; Newark, Trenton, 

 Philadelphia, Wilmington, Baltimore, Washington, Fredericksburg, Eich- 

 mond, Petersburg, Ealeigh, Camden, Columbia, Augusta, Macon, and 

 Columbus being located on this boundary. A line drawn through these 

 places would approximately separate the Coastal Plain from the Piedmont 

 Plateau. Along the eastern margin of the Piedmont district outliers of 

 the Coastal Plain are frequent, while along the valleys of the larger streams 

 the crystalline rocks can at times be followed for a mile or more into the 

 body of the Coastal Plain sediments where the mantle of the latter has been 

 cut through. 



The Atlantic Coastal Plain is divided into two parts by the present shore- 

 line, a submerged or submarine portion known as the continental shelf, or 

 continental platform, and an emerged or subaerial portion commonly called 

 the Coastal Plain. In some places the division line is marked by a sea cliff 

 of moderate height, but usually the two grade into each other with scarcely 

 a perceptible change, and the only mark of separation is the shore-line. The 

 areas of the respective portions have changed frequently during past geologic 

 time by the shifting of the shore-line eastward or westward, due to local 

 and general depressions or elevations of greater or less extent, and even at 

 the present time such changes are in progress. Old river valleys, the con- 

 tinuations of channels of existing streams, have been traced entirelv across 



