DRAINAGE. 53 



The upper and lower courses of those streams whose sources are in the 

 Piedmont Plateau but which cross the Coastal Plain are in striking con- 

 trast. In the hard crystalline rocks they have cut narrow steep-sided valleys 

 and their channels are frequently filled with great boulders producing falls 

 and rapids, while in the loose sediments of the Coastal Plain the valleys are 

 wide and open, the streams frequently bordered by extensive flood plains 

 with no obstacles to disturb the quiet sluggish flow of the low-gradient 

 streams. AVithin the Piedmont Plateau and at its margin the large as well 

 as the small streams furnish considerable water power, but in the Coastal 

 Plain all the water power is furnished by the small tributary streams. 



Within the Coastal Plain the streams have series of branches formed at 

 approximately right angles to the next larger stream and producing a stream 

 system somewhat similar to the branchings of a tree and therefore known as 

 dendritic. 



Sfreaiii cdurses. — The drainage of the Virginia Coastal Plain is simple 

 in that neither the geological structure of the region nor the materials 

 tlirnugh which the streams have cut their valleys have had much influence 

 upon the present stream courses. The initial slopes of the region have 

 determined the direction of the streams, and as the general slope is toward 

 the east or southeast all of the leading streams flow in that direction. In 

 the vicinity of the "fall-line," however, many of them are deflected sharply 

 to the right and flow almost due south for a distance of from 5 to 30 miles 

 before finally cutting across the Coastal Plain in their direct course to 

 Chesapeake Bay. It appears as if the superimposed streams of the Pied- 

 mont Plateau, after having maintained their straight courses in the hard 

 crystalline rocks had encountered serious obstacles in the great deposits of 

 unconsolidated materials in the Coastal Plain and had found an easier 

 passage southward. It has been claimed by some investigators that the 

 "fall-line" is a line of fracture and faulting and that the stream courses 

 have been affected in consequence. The few small faults which have been 

 observed in the overlying Coastal Plain deposits along this line and the 

 greater slope of the formations suggest earth movements of greater magni- 

 tude here than elsewhere and these movements may have influenced the 

 present position of the streams. It seems more probable, however, that the 

 main cause is due to the relative ease with which the basal Coastal Plain 

 deposits are eroded in comparison with the overlying beds. Notwithstand- 

 ing the fact that with local exceptions all of the Coastal Plain deposits are 

 unconsolidated, some are much looser and are removed by erosion much 

 more readily. The beds of the Potomac group which outcrop in a narrow 



