96 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



The Blue Ridge geomorphic province terminates southward in northern 

 Georgia, just north of Gainsville, where the Piedmont and the Valley 

 and Ridge provinces seem to close around the Great Smokies. The Blue 

 Ridge structural belt, however, extends on southward into Alabama, 

 where it is buried by the coastal plain sediments; but because it has 

 been eroded down to the level of the Piedmont, it is generally included 

 in the Piedmont province by the geomorphologists. 



The Piedmont province emerges from the Triassic lowlands in New 

 Jersey, where it is known as the Trenton prong (see Fig. 7.3), and extends 

 southwestward to Alabama. It is only a few miles wide in Pennsylvania, 

 Maryland, and northern Virginia, but widens conspicuously to about 170 

 miles in North Carolina, from which place southwestward it continues 

 wide. The surface of the Piedmont rises gradually westward to the foot 

 of the Blue Ridge, where it reaches an altitude of 500 feet at the north 

 and 1500 feet at the south. It is a vast plain along the horizon, but is 

 maturely dissected to a local relief of a few hundred feet in places. 



Numerous hills and ridges rise as monadnocks 200 to 1000 feet above 

 the general plains surface, and are more numerous near the Blue Ridge 

 escarpment. 



The rocks of the Piedmont province are mostly granites, gneisses, and 

 schists, with some belts of marble and quartzite, partly of Paleozoic age 

 but also in part of Precambrian age. A belt of basic rocks containing 

 talc and soapstone is found near the western border. Several elongate 

 basins of Upper Triassic sandstones and shales, cut by diabase dikes and 

 sills, are found in the province. The Richmond basin contains coal, 

 which was the first mined in North America in about 1750. 



The Piedmont crystallines are overlapped on the east by the Cretaceous 

 and Tertiary sediments, and the boundary of the two provinces is called 

 the fall zone. Baltimore, Washington, Fredericksburg, Richmond, Peters- 

 burg, and other cities are located along it, and also mark approximately 

 the points to which the tide extends up the estuaries. 



